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Molecular engineering of photoinduced charge separation
Citation for published version (APA):Marcos Ramos, A. (2003). Molecular engineering of photoinduced charge separation. Technische UniversiteitEindhoven. https://doi.org/10.6100/IR571230
DOI:10.6100/IR571230
Document status and date:Published: 01/01/2003
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Molecular Engineering of
Photoinduced Charge Separation
Molecular Engineering of
Photoinduced Charge Separation
PROEFSCHRIFT
ter verkrijging van de graad van doctor aan de Technische Universiteit
Eindhoven, op gezag van de Rector Magnificus, prof.dr. R.A. van Santen,
voor een commissie aangewezen door het College voor Promoties in het
openbaar te verdedigen op woensdag 29 oktober 2003 om 16.00 uur
door
Alicia Marcos Ramos
geboren te Reus, Spanje
Dit proefschrift is goedgekeurd door de promotoren:
prof.dr.ir. R.A.J. Janssen
en
prof.dr. J.C. Hummelen
This research has been financially supported by the Dutch Government through the
E.E.T. program (EETK97115), NOVEM (146.120.008.3 and 146.120.003.4), and by the
European Comission (Joule III contract No. JOR3CT9802026).
Omslagontwerp: Isabel Marcos Ramos en Jan-Willem Luiten.
Druk: Universiteitsdrukkerij, Technische Universiteit Eindhoven.
CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN
Marcos Ramos, Alicia Molecular Engineering of Photoinduced Charge Separation / by Alicia Marcos Ramos. – Eindhoven :
Technische Universiteit Eindhoven, 2003.
Proefschrift. – ISBN 90-386-2695-9
NUR 914
Trefwoorden: donor-acceptor systemen / π-geconjugeerde polymeren /supramoleculaire chemie /
electronenoverdracht / energie-overdracht
Subject headings: donor-acceptor systems / π-conjugated polymers /supramolecular chemistry /
electron transfer / energy transfer
A mis padres
Máximo e Isabel
Table of Contents
Chapter 1
Covalently linked donor-acceptor materials 1
1.1 Organic solar cells 2
1.2 Donor-acceptor materials 5
1.2.1 Multichromophoric donor-acceptor materials 5
1.2.2 Donor-acceptor polymers 9
1.2.3 Supramolecular approach 11
1.3 Aim of the thesis 11
1.4 Outline of the thesis 12
1.5 References 13
Chapter 2
Photoinduced multistep energy and electron transfer in an oligoaniline-oligo(p-phenylene
vinylene)-fullerene triad 17
2.1 Introduction 18
2.2 Synthesis 19
2.3 Electronic properties and energetic considerations 21
2.4 Photophysical processes in solution 27
2.4.1 Photoluminescence spectroscopy 27
2.4.2 Near steady state photoinduced absorption (PIA) spectroscopy 29
2.4.3 Femtosecond pump-probe spectroscopy in polar solvents 31
2.4.4 Kinetic considerations 36
2.5 Photophysical processes in the solid state 38
2.5.1 Near steady state PIA spectroscopy 38
2.5.2 Femtosecond pump-probe spectroscopy 39
2.6 Conclusions 40
2.7 Experimental section 42
2.8 References and notes 45
Chapter 3
Photoinduced multistep electron transfer in oligoaniline-oligo(p-phenylene vinylene)-perylene
arrays 51
3.1 Introduction 52
3.2 Synthesis 53
3.3 Electronic properties and energetic considerations 56
3.4 Photophysical processes in solution 61
3.4.1 Photoluminescence spectroscopy 61
3.4.2 Near steady state photoinduced absorption (PIA) spectroscopy 63
3.4.3 Subpicosecond transient pump-probe spectroscopy 64
3.4.3.1 Pentad 1 65
3.4.3.2 Pentad 2 in THF 69
3.5 Kinetic considerations 69
3.6 Conclusions 71
3.7 Experimental section 72
3.8 References and notes 74
Chapter 4
Supramolecular control over donor-acceptor photoinduced charge separation 79
4.1 Introduction 80
4.2 Synthesis 82
4.3 Conformational states of the bridge 84
4.3.1 Folding in chloroform/heptane mixtures 84
4.3.2 Folding in other solvents 87
4.4 Electronic properties and energetic considerations 88
4.5 Photoinduced energy and electron transfer in different conformational states of the bridge 92
4.5.1 Bridge in a random coil conformation 92
4.5.2 Bridge in an folded conformation 95
4.5.2.1 Folded bridge in heptane/chloroform mixtures 95
4.5.2.2 Folded bridge in other solvents 101
4.5 Conclusions 101
4.6 Experimental section 103
4.7 References 104
Chapter 5
Photoinduced electron transfer of conjugated polymers with pendant fullerenes 107
5.1 Introduction 108
5.2 PPV-PPE polymers with pendant fullerenes 109
5.2.1 Synthesis and characterization 109
5.2.2 Photophysical properties 113
5.2.3 Photovoltaic device 116
5.3 Low-bandgap π-conjugated polymers with pendant fullerenes 117
5.4 Conclusions 118
5.5 Experimental section 119
5.6 References 122
Chapter 6
Polyacetylenes with pendant donor-acceptor dyads 125
6.1 Introduction 126
6.2 Design 128
6.3 Synthesis and characterization 129
6.3.1 Ethynyl perylene bisimide 129
6.3.2 Polyacetylene with pendant OPV and PERY chromophores 130
6.3.3 Polyacetylene with pendant OPVE-PERY dyads 131
6.3.4 Polyacetylene with pendant OPE-PERY dyads 133
6.4 UV/Visible absorption and circular discroism spectroscopies 135
6.5 Photophysical properties 136
6.5.1 Solid state 136
6.5.2 Chloroform solution 138
6.6 Conclusions and outlook 140
6.7 Experimental section 141
6.8 References and notes 147
Summary
Resumen
Curriculum Vitae
Acknowledgment
Chapter 1
Covalently linked
donor-acceptor materials
Abstract
Blends of donor and acceptors have successfully been incorporated as the active layer in plastic solar
cells. Improving the performance of such devices implies optimizing the morphology of the bulk-
heterojunction. The covalent linkage between the donor and acceptor materials is the ultimate key to
obtain a well-defined spatial organization. In this chapter a concise overview on covalently linked
donor-acceptor multichromophoric arrays and polymers is given. Well-defined donor-acceptor dyads,
triads and larger arrays give valuable information on the photophysics occurring in plastic solar
cells. The reported donor-acceptor polymers provide an important guideline for design of new
polymers for photovoltaic applications.
Chapter 1
2
1.1 Organic solar cells
Photoinduced charge separation is the primary step in photosynthesis. In natural systems light
induced charge separation is achieved through a unique spatial arrangement of the pigments and
elements of the transport chain. Electronic excitations that reach the reaction centers are converted to
chemical energy in the form of charge separation across the photosynthetic membrane. In these
organized arrays electrons flow rapidly (< ms) over distances as great as 20 Å, with little loss of
energy.1,2 The photosynthesis in plants is a source of inspiration for scientists to engineer non-natural
systems that similarly convert light into chemical potential or electrical energy.
The opto-electronic and semiconducting properties of conjugated organic and polymer
materials has been one of the highlights in chemistry and physics in the past decade and has
developed into an important area of academic and applied research. The breakthrough that enabled
this development was the discovery of the conducting behavior of doped polyacetylene.3 Like for all
π-conjugated polymers (Figure 1.1), the structure of polyacetylene is characterized by the alternation
of single and double bonds that leads to extended π-orbitals. As a result, π-conjugated materials
exhibit a strong optical absorption in the visible region. Furthermore, charges created in these
materials are mobile and can be transported along and between the polymer chains. One of the
important advantages of π-conjugated polymers is that their properties can be widely tuned by
chemical modification. Chemists have been able to adjust the optical band gap, valence and
conduction band energies, charge transport characteristics as well as the solubility and structural
properties by modifying the nature of the polymer backbone or by changing the substituent groups.
**n
* *n
*
*n S* *n
polyacetylene poly-p-phenylene poly(p-phenylene vinylene) polythiophene
Figure 1.1. Chemical structures of some π-conjugated polymers.
The absorption of light and transport of charges are the two important elements that, in
principle, allow conjugated polymers to be used in the solar cells. However, in contrast to inorganic
semiconductors, photoexcitation of conjugated polymers does not (or with low efficiency) result in
the formation of free charge carriers that are required for a photovoltaic effect. Instead, excitons are
formed that represent a bound electron-hole pair that will not spontaneously dissociate into free
charge carriers. In order to be able to generate charge carriers in π-conjugated organic molecules and
polymers using photoexcitation, scientists have used the donor-acceptor concept conceived in natural
photosynthesis. By combining an electron donor (p-type) with an acceptor (n-type) material and
Covalently linked donor-acceptor materials
3
utilizing the different electron affinity and ionization potential of these materials, it is possible to
dissociate excitons created in either material.
The first organic p/n-heterojunction devices based on a donor and acceptor bilayer
configuration reached energy conversion efficiencies of about 1%.4 Such photovoltaic device typically
consists of a bilayer of the organic materials sandwiched between two different conducting contacts,
an optically transparent indium tin oxide (ITO) front electrode and a metal (Au, Al, Ca, Mg) back
electrode (Figure 1.2). One of the disadvantages of such donor-acceptor double layer device is the
limited interfacial area. More importantly, the organic and polymer semiconductors often have an
exciton diffusion range that is limited to 10 nm. As a consequence only the excitons created close to
the donor-acceptor interface contribute to the photocurrent, which strongly limits the performance. A
significant improvement in the performance of polymer photovoltaic devices was achieved by
blending donor and acceptor into a bulk- heterojunction.5,6 Heeger et al. used blends of a substituted
p-phenylene vinylene polymer (MEH-PPV) as the electron donor with a soluble fullerene derivative
(PCBM) as the electron acceptor,5 while Friend et al. used two polymers: MEH-PPV as the donor and
a cyano-substituted PPV (CN-PPV) as acceptor.6 In these bulk-heterojunctions donor and acceptor
materials form an interpenetrating network, resulting in a very high interfacial area between the two
materials and a continuous phase to connect the materials to the electrodes. The large interface is
advantageous for an efficient electron transfer, while the bi-continuity is required for efficient charge
transport. The polymeric nature of these materials gives them good mechanical properties and
processing advantages to fabricate solar cells in a commercially attractive manner.7
Figure 1.2. Schematic representation of an organic photovoltaic device.
One of the most successful bulk-heterojunction photovoltaic devices made up to date are
those based on the MDMO-PPV/PCBM system (Figure 1.3) that reached efficiencies up to 2.5%.8
Recently, these efficiencies have improved to above 3% by utilizing polythiophenes as donor material
in combination with PCBM derivatives9 and by using MDMO-PPV with a C70 PCBM derivative as an
acceptor.10 Perylene diimid dyes (PERY) constitute another promising class of acceptor materials. In
Glass
PEDOT:PSSActive layer
Al SMU+-
Illumination
ITOGlass
PEDOT:PSSActive layer
Al SMU+-
Illumination
ITO
Chapter 1
4
contrast to the fullerene molecules, the PERY acceptor has a strong absorption in the visible, and
plays an important role in the collection of light. Tang incorporated this acceptor in his double-layer
device4 and Friend et al. have shown that it is possible to use perylene in combination with π-
conjugated polymers in bulk-heterojunction devices by processing from solution.11
C12H25
C12H25
C12H25
C12H25
C12H25
C12H25
NN
O
O
O
O
O
O
OOMe
HBC-PhC12
PERY
+
n
+
MDMO-PPV PCBM
Figure 1.3. Components of the most successful organic solar cells up to date.
Because of their difference in chemical structure, donors and acceptors are commonly not
miscible and tend to macro-phase segregate. The resulting morphology of the donor/acceptor blend is
crucial because it affects several aspects of the photovoltaic performance. High interfacial areas are
beneficial for efficient charge separation and interconnection between domains of equal electron
affinity is essential for the transport of opposite charges to opposite electrodes. These factors need to
be optimized in the existing bulk-heterojunction devices, though remarkable improvements in
morphology have been achieved. In polymer/fullerene blends an increased efficiency has been
achieved by optimizing on the processing conditions.8-10 For perylene-based devices the highest
efficiency has been achieved by the use of components that can preorganize via π-π interactions and
via liquid crystallinity (hexaphenyl-substituted hexabenzocoronene, HBC-PhC12, Figure 1.3).12
The development of the plastic solar cell has revealed that it is possible to artificially convert
visible light into photoinduced charge separation and use this state to generate electrical energy.7 So
far the design of plastic solar cells has not been using the sophisticated supramolecular organization as
found in natural photosynthesis. Implementing design elements of the natural system such as multi-
chromophoric arrays and supramolecular organization into the design of donor/acceptor materials will
help in the understanding the photophysical process and might increase the performance of the solar
cells.
Covalently linked donor-acceptor materials
5
1.2 Donor-acceptor materials
Control over the morphology of donor/acceptor bulk-heterojunctions can be pursued at
different dimensional levels. The smallest dimension of control is the molecular scale. The discipline
of molecular engineering gives the opportunity to generate donor and acceptor systems with an almost
unlimited control over their size, shape, and intermolecular interactions on this molecular scale. By
design, synthesis and photophysical evaluation the processes occurring at the molecular level can be
understood and improved. Systems with the donor and acceptor elements connected, either covalently
or via a supramolecular interaction, represent the ultimate bottom-up approach to gain control over the
morphology of donor-acceptor bulkheterojunctions. As an example, the design and synthesis of
covalently linked donor–acceptor dyads, triads and multichromophoric arrays in general, has helped
to understand the photophysical phenomena occurring in photosynthesis.13 The design principles
derived from this approach can be incorporated into the design of polymers containing both
components in a well-defined arrangement. This should allow for the creation of bicontinuous
networks of donor and acceptor materials. Moreover, by judicious choice and use of supramolecular
interactions, donor-acceptor dyads and triads can be brought to preorganize in the solid state into
well-ordered architectures.
1.2.1 Multichromophoric donor-acceptor arrays
Molecular dyads. Donor-acceptor dyads constitute the most elemental molecularly
engineered multichromophoric arrays with a high interfacial area between donor and acceptor. They
can be used as model systems to understand the intricate photophysical processes occurring in
polymer donor/acceptor devices. For photovoltaic applications the donor-acceptor dyads have to be
designed to undergo a fast electron transfer and generate long-lived charge-separated states.
The progress in synthetic methods for the functionalization of fullerene14 and perylene dye15
electron acceptors has allowed the preparation and subsequent photophysical study of a number of
donor-acceptor materials. A large number of fullerene-porphyrin dyads in a variety of arrangements
(for example dyads 1 to 3 in Figure 1.4) have been synthesized in order to gain control over the
electronic coupling, geometrical overlap, and nature of the intervening spacer in the photophysical
processes between the donor and acceptor.16 The use of perylene in multichromophoric arrays has a
more recent history, and it is mainly found in the pioneering work of Wasielewski17 et al. and Lindsay
et al.18 As in the case of fullerene dyads, mostly porphyrins have been used as donor chromophore in
junction with perylene dyes (for example compounds 4 and 5 in Figure 1.4). Lindsay et al. have
shown that arrays containing perylenes and porphyrins can be designed to select for excited-state
energy or charge transfer by tuning the redox and photophysical properties of the components. In
particular, the use of perylene diimid dyes over perylene monoimid dyes results in a photoinduced
charge separation process that prevails over the energy transfer reaction. These comprehensive studies
Chapter 1
6
on fullerene/perylene and porphyrin based donor-acceptor dyads have laid down some of the
important basic rules for the design of novel donor-acceptor multichromophoric arrays with desired
photophysical properties.
Ar
NN
NNAr
Ar
N
O
H
M
O
OO
O
HO
ZnNN
NN
Ph
Ph
O
OO
OOO O
OMeMeO
O
O
NN
NNH
H
Ph
Ph
PhO
OO
O
1 2 3
M
4
NN
NN
Ar
Ar
N
OR
N
OR
O
O
M
N
N
O
O
O
OOAr
ArO
N
N
O
O
O
OOAr
ArO
N
N
O
O
O
O
OAr
ArO
N
N
O
OO
O
OAr
ArO
N
N N
N
5
Figure 1.4. Several examples of donor-acceptor dyads based on porphyrin as donor and fullerene or
perylene as acceptor.
The success of π-conjugated polymers in photovoltaic applications, together with the
development of strategies to synthesize functional well-defined oligomers,19 have motivated in the late
90’s the study of photoinduced charge generation in covalently linked dyads, consisting of conjugated
oligomers and fullerenes20-24 or perylenes.25 This type of donor-acceptor dyad is of high interest
because of the remarkable advantages of the oligomer approach. In contrast to π-conjugated polymers,
the oligomers are monodisperse and well defined. This allows the study of oligomer based donor-
acceptor dyads in an isolated form in solution with the possibility to discern the intramolecular
photophysical processes from the intermolecular ones occurring in the solid state. When of sufficient
length, the oligomers feature the essential electrical and optical properties of the corresponding
polymers. Because of this, oligomer-acceptor dyads are excellent models to understand the
photophysics in polymer/acceptor bulk-heterojunction devices. Another interesting characteristic,
derived from their well-defined nature, is the capability of oligomers to organize into crystalline
domains26 or pack efficiently into highly ordered nano-aggregates,27 properties that are crucial in the
use of self-organization of donor-acceptor materials via π-π interaction.
Covalently linked donor-acceptor materials
7
In the design of oligomer-acceptor dyads, the size and functionalization of the oligomer has to
be carefully considered. By going from polymers to shorter oligomers a reduction of the donor
capabilities occurs, to which the fullerene molecule is especially sensitive. In systems using short
donor oligomers the energy transfer process might compete with and even overrule the photoinduced
charge separation. The importance of oligomer length has been manifested in two homologous series
of OPV-C60 dyads, 6-8 and 9-12 (Figure 1.5), synthesized by Nierengarten et al.20 a,b and Janssen et
al., 20d respectively. For dyads 6-8 no significant charge separation has been observed in solution.
Although devices made with these compounds show photovoltaic effect, indicating that electron
transfer occurs in the solid state, the performance is limited by the competing energy transfer
processes. In contrast, the study of the analogous oligomers 9-12 reveals that the dyads undergo
electron transfer in polar solvents when the oligomer exceeds a critical length (three or more phenyl
rings). In this case the oligomers have been heavily decorated with alkoxy side chains. The side
chains not only ensure solubility of the ensembles but also confer a higher electron donating character
to the oligomers. An important conclusion of this study is that in the solid state the charge separation
is much faster than in solution, and that the photoinduced charges are much longer lived, both
implying that in bulk-heterojunctions made out of dyads, the intermolecular processes prevail over the
intramolecular ones.
N
OO
OO
n
N
C12H25
C12H25On
6 n = 07 n = 28 n = 3
9 n = 010 n = 111 n = 212 n = 3
Figure 1.5. Chemical structures of two analogous sets of OPV-C60 dyads.
Another important class of oligomer based donor-acceptor dyads that have attracted attention
are those based on oligothiophenes (nT) and C60. As in OPV-C60 dyads, the length and chemical
modification of the donor oligomer play an important role in determining the outcome of each
photoexcitation. Janssen et al. have studied a series of symmetrical C60-nT-C60 triads (compounds 13-
15, Figure 1.6).21f,g The longer triads 14 and 15 exhibit photoinduced electron transfer in polar
solution and in the solid state and for the shorter triad 13 the same photophysical process occurs,
however only in a small extend. Investigations on a set of analogous dyads with oligomer lengths
ranging from four to sixteen thiophene rings (compounds 16-19, Figure 1.6) show that charge
Chapter 1
8
separation occurs for all dyads, in polar solvents as in the solid state. The combined results of 13-19
reveal that four thiophene rings is the critical minimum size for efficient charge separation to occur in
this class of oligothiophene-C60 systems.21d,e,h,i Dyads 16 to 19 have been incorporated in photovoltaic
cells with a incident photon-to-current efficiency that systematically increases with oligothiophene
length.21k
NN
S S
S n
C12H25n
S NS S
S
H
C6H13
C6H13
13 n = 114 n = 215 n = 3
16 n = 117 n = 218 n = 319 n = 4
Figure 1.6. Chemical structures of two analogous sets of nT-C60 dyads and symmetrical triads.
Triads and larger arrays. Extending the lifetime of the charge-separated state and avoiding
the undesired back electron transfer can be achieved using a multistep electron transfer as occurring in
photosynthesis. For a sequential charge transfer a gradient of potentials within a multichromophoric
array is needed. In that regard, a number of multi-site covalently linked porphyrin-fullerene
combinations (triads, tetrads, and pentads) have been described.28,29 The design of molecular triads
and larger arrays that efficiently accomplish the purpose of multistep electron transfer is far from
trivial because of the many possible (and competitive) photophysical processes in these
multichromophoric arrays. In fact, for the first C60-based reported triad, made up of a carotenoid
polyene, a diaryl-porphyrin and a fullerene (C-P-C60), the overall quantum yield for the formation of
the C•+-P-C60•–
charge-separated state was only 0.14.29a In the mean time, a few examples have been
described that reach efficiencies for the charge separation of unity.29i,o The record for the longest lived
charge-separated state with 380 ms has been reported for a tetrad incorporating sequentially one
ferrocene (Fc), one zinc 5,10,15,20-tetraphenylporphyrin (ZnTPP), one free base porphyrin (TPP) and
a fullerene molecule (Fc-ZnTPP-TPP-C60, compound 20, Figure 1.7).29p Imahori et al. have used Fc-
H2TPP-C60 and Fc-ZnTPP-C60 triads (compounds 21 and 22, Figure 1.7), to create photoactive self-
assembled monolayers on Au(111) surfaces. These systems feature the highest efficiency for
photocurrent generation of monolayer-modified electrodes and show that the application of molecular
triads as such in photovoltaic applications is viable.29h
Covalently linked donor-acceptor materials
9
Fe
NO
NONN
NNN
O NOS
Fe
NO
NONN
NN
NN
NNZn N
O
HH
N
Au(111)M
20
21 M = H2
22 M = Zn
Figure 1.7. Multichromophoric donor-acceptor arrays.
1.2.2 Donor-acceptor polymers
Incorporation of donor and acceptor within one polymer chain using an appropriate structural
design is the most convenient way to obtain bicontinuous networks. In these polymers, donors and
acceptors are enforced to occupy predefined positions by the covalent connection. Two main designs
of donor-acceptor polymers have been employed up to date. One approach consists of making π-
conjugated polymers with pendant acceptors. 30 This idea has come to be known as the ‘double-cable’
approach. In the double-cable polymer the photoinduced generated charges are expected to diffuse
away from each other: the hole by an intrachain diffusion process and the electron by hopping from
acceptor to acceptor. The first double-cables ever made (for example polymer 23, Figure 1.8) met the
requirements for photoinduced charge separation, however they were insoluble and have not been
incorporated in photovoltaic devices. 31-33 The synthesis of processable double-cables has been
achieved by the use of multiple solubilizing side chains in the polymer backbone. The first of such
polymers that has been reported is a hybrid polymer of poly(p-phenylene vinylene) (PPV) and poly(p-
phenylene ethynylene) (PPE) with pendant methanofullerenes, which shows good photophysical
properties and was incorporated as the active layer in photovoltaic device. A more detailed description
of this polymer is given in chapter 5 of this thesis. Two following examples of soluble double-cables
are polymers 24 and 25 (Figure 1.8).34,35 Polymer 24 has been synthesized by random
copolymerization of thiophenes bearing the fullerene with thiophenes destined to ensure solubility,
and polymer 25 by grafting the fullerene molecule to an existing functionalized polymer backbone.
Polymer 24 has been successfully used in a photovoltaic device. Furthermore, the organization of the
polymer chain was not disturbed by the presence of side-bonded fullerenes. Apart from fullerenes,
Chapter 1
10
other acceptors such as tetracyanoanthraquinodimethane (TCAQ) have been used in double-cable
polymers.36 Following the same approach a polyfluorene with pendant perylenes has been
synthesized, though in this case the polymer is meant to be used in light emitting diodes (LEDs).37
(OCH2CH2)3ON
SS n
N
N
N
OC5H11
C5H11Ox y n
x = 0.2 y = 0.8
23 24 25
OC8H16N
n mSS
OC2H4OC2H4OCH3
C8H17O
Figure 1.8. Examples of double-cable polymers.
Another macromolecular strategy to gain order within the bulkheterojunctions is the
incorporation of donor and acceptors in blockcopolymers. In these class of polymers self-assembly of
the different components takes place through phase segregation at the nanometer scale. Hadziioannou
et al. have synthesized the first donor-acceptor diblock copolymer (polymer 26, figure 1.9), by
growing a flexible poly(styrene-stat-chloromethylstyrene) (PS) coil from an end-functionalized rigid
PPV block and subsequent functionalization of the flexible PS block with C60.38 In this polymer an
efficient electron transfer was inferred from photoluminescence quenching of the PPV block.
Morphology studies reveal that the film of the polymer exhibits a honeycomb structuring at the
micrometer scale. As the active layer in a device, the donor-acceptor diblock copolymer 26 shows
enhanced photovoltaic response, specially a much higher short-circuit current, relative to a blend of its
constituent polymers.39 Another example of donor-acceptor blockcopymer is that reported by Janssen
et al. consisting of alternating OPV and perylene diimid (PERY) segments connected via saturated
spacers, polymer 27 (Figure 1.9). 40 In this example π-π interactions are added as structuring elements
to the blockcopolymer approach. Photophysical studies of this polymer in solution reveal that the
photoinduced charge separated state is long-lived owing to the long saturated spacer between the
donor and acceptor. However, face-to-face orientations of OPV and PERY segments diminish the
lifetime of the charge-separated state in the film and limit the photovoltaic performance.
Covalently linked donor-acceptor materials
11
OOO
O
O
N
O
N
OO
OO
OO
O
O
n
RORO
OROR
ONO
7n m
H
26
27
Figure 1.9. Examples of donor-acceptor blockcopolymers.
1.2.3 Supramolecular approach
Proteins acquire their three-dimensional architecture (tertiary and quaternary structures) by
folding from an unordered chain using a variety of supramolecular interactions. By analogy, an
optimal morphology of the donor-acceptor materials in the solid state could be achieved through the
wise choice of such supramolecular interactions. These interactions can be used to bring donor and
acceptors in close proximity and to induce the self-organization of donor-acceptor multichromophoric
arrays or donor-acceptor blockcopolymers into highly ordered assemblies. Among the many possible
supramolecular interactions, π-π stacking and hydrogen bonding have already resulted in some
remarkable examples of organized donor-acceptor nano-aggregates. Wasieleswki et al.17 have shown
that a system, made of four perylene diimides connected to a central zinc 5,10,15,20-
tetraphenylporphyrin (ZnTPP) electron donor (compound 5, Figure 1.4), self-assembles into ordered
nanoparticles, both in solution and in the solid state, driven by the π-π stacking of the perylene dyes.
These nanoparticles exhibit photoinduced charge separation and charge transport, which demonstrates
the feasibility of the supramolecular approach. Another interesting example of self-organization is
given by Meijer et al. combining both hydrogen-bonding and π-π interactions to give well-defined
chiral self-assemblies having OPV donors and perylene dyes as acceptor chromophores. 41
1.3 Aim of the thesis
The aim of this thesis is to use molecular engineering to study fundamental issues of
photoinduced charge separation on novel well-defined multichromophoric systems and to develop
polymeric donor-acceptor materials with unprecedented architectures. The study of
multichromophoric donor-acceptor systems and of the role of supramolecular chemistry in directing
the interaction between donors and acceptors will assist in finding the optimal means to control
photoinduced charge separation. The generation of novel donor-acceptor polymers, based on
Chapter 1
12
knowledge gained from the single molecule models, may lead the way to processable materials with
optimal properties for photovoltaic devices.
As building blocks for the synthesis of donor-acceptor materials well-defined π-conjugated
oligomers as well as fullerenes and perylenes will be used. The well-defined nature of the oligomers
in combination with the acceptor molecules facilitates the identification of the different photophysical
intermediates. In addition, the planar structure of the oligomers in combination with that of the
perylene acceptor allows for the use of π-π interactions as a structural element that can aid in self-
assembly in the solid state.
1.4 Outline of the thesis
Two donor-donor-acceptor (D(1)-D(2)-A) multichromophoric arrays with a gradient of
potentials have been synthesized by connecting an oligoaniline (OAn), an oligo(p-phenylene
vinylene) (OPV) and a fullerene (C60) (OAn-OPV-C60, chapter 2) or a perylene diimid (PERY), (OAn-
OPV-PERY, chapter 3) in a linear array. In these ensembles photoexcitation of any of the
chromphores generates, after a sequential event that involves multiple energy and charge transfers, the
D(1) •+-D(2)-A•– charge separated state. The overall efficiency for the formation of this D(1) •+-D(2)-
A•– charge-separated state depends on the ratio of rates for forward electron transfer and charge
recombination to generate the ground state. In turn, this ratio can be controlled by changing the
polarity of the solvent. The D(1) •+-D(2)-A•– charge-separated state has a much longer lifetime than
that of the associated D(2) -A dyad, because of the weak electronic coupling between D(1) •+ and A•–.
The results of the photophysical studies on both triads are rationalized in terms of the Marcus theory
for electron transfer.
Chapter 4 deals with the use of supramolecular chemistry to modulate donor and acceptor
interactions. An OPV donor and a PERY acceptor are linked at the opposite ends of a long foldable
cross-conjugated oligomer. The conformation of the bridge between donor and acceptor can be varied
in a controlled way between random coil and helically folded states, by external stimuli such as
solvent polarity. This property of the bridge provides a tool to govern the interaction and, thus, the
photophysical processes occurring between the two redox centers. By going from the random coil
conformation to the collapsed state the distance between the two chromophores is drastically
decreased which allows for electron transfer to occur.
In the last part of the thesis, the structure and photophysical properties of well-defined dyads
are transferred to the polymeric level. In chapter 5, a new double-cable polymer is discussed. Its
synthesis is based on an A-B type Sonogashira copolymerization to ensure the alternation of both
monomers and to provide the opportunity to synthesize a well-defined donor-acceptor polymer. One
monomer bears the fullerene acceptor, the other monomer is a bifunctional OPV consisting of three
Covalently linked donor-acceptor materials
13
phenyl units, which confers donor capabilities to the polymer backbone. The photoinduced electron
transfer is studied by means of photoluminescence and photoinduced absorption spectroscopies and
the photovoltaic properties of the polymer have been tested. In chapter 6, the design and synthesis of a
new donor-acceptor polymer is explored: a π-conjugated polyacetylene with pendant donor-acceptor
dyads. The dyads consist of an OPV and a PERY connected via a saturated spacer. In this new design
the covalent and supramolecular approaches are combined in order to guarantee donor-donor and
acceptor-acceptor selective interaction patterns.
1.5 References
1 (a) The Photosynthetic Reaction Center, Deisenhofer, J., Norris, J. R., Eds.; Academic Press: New York,
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Kluwer Academic Publishers: Dordrecht, 1995.
2 Electron Transfer in Chemistry Vol. I-IV, Balzani, V., Wiley-VCH, Weinheim, 2001.
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5 Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A.J Science, 1995, 270, 1789.
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A. B. Nature 1995, 376, 498.
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Adv. Funct. Mater. 2001, 11, 15. c) Nelson, Current Opinion in Solid State & Materials Science 2002, 6,
87. d) Nunzi, J.-M. C. R. Physique 2002, 3, 523.
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Lett. 2001, 78, 841.
9 a) Schilinsky, P.; Waldauf, C.; Brabec, C. J. Appl. Phys. Lett. 2002, 81, 3885. b) Padinger, F.; Rittberger,
R. S.; Sariciftci, N.S. Adv. Funct. Mater. 2003, 13, 85.
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J. Angew. Chem. Int. Ed. 2003¸ 42, 3371
11 Dittmer, J. J.; Marseglia, E. A.; Friend, R. H. Adv. Mater. 2000, 12, 1270.
12 Schmidt-Mende L.; Frechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science
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13 Wasielewski, M. R. Chem. Rev. 1992, 92, 435-461.
14 (a) Fullerenes: Chemistry, Physics, and Technology, Kadish, K. M.; Ruoff, R. S., Eds.; J. Wiley, New
York, 2000. (b) Fullerenes: From Synthesis to Optoelectronic Properties, Guldi, D. M.; Martín, N., Eds.
Kluwer Academic Publishers, Dordrech, 2002.
15 Langhals, H. Heterocycles 1995, 40, 477.
Chapter 1
14
16 For reviews see: (a) Imahori, H.; Sakata, Y. Adv. Mater. 1997, 119, 5744. (b) Imahori, H.; Sakata, Y. Eur.
J. Org. Chem. 1999, 2445. (a) Martín, N.; Sánchez, L.; Llescas, B.; Pérez, I. Chem. Rev. 1998, 98, 2527.
(d) Guldi, D. M. Chem. Comm. 2000, 321. (e) Guldi, D. M. Chem. Soc. Rev. 2002, 31, 22.
17 For example: Van der Boom, T.; Hayes, R. T.; Zhao, Y.; Bushard, P. J.; Weiss, E. A.; Wasielewski, M.
R. J. Am. Chem. Soc. 2002, 124, 9582.
18 For example: (a) Kirmaier, C.; Hindin, E.; Schwartz, J. K.; Sazanovich, I. V.; Diers, J. R.;
Muthukumaran, K.; Taniguchi, M.; Bocian, D. F.; Lindsey, J. S.; Holten, D. J. Phys. Chem. B 2003, 107,
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Diers, J. R.; Bocian, D. F.; Holten, D.; Lindsey, J. S. J. Phys. Chem. B 2003, 107, 3431.
19 (a) Tour, J. M. Chem. Rev. 1996, 96, 537. (b) Martin, R. E.; Diederich, F. Angew. Chem. Int. Ed. 1999,
38, 1350. (b) Segura, J. L.; Martín, N. J. Mater. Chem. 2000, 10, 2403.
20 Oligo(p-phenylene vinylene)-fullerene dyads: (a) Nierengarten, J. F.; Eckert, J. F.; Nicoud, J. F.; Ouali,
L.; Krasnikov, V.; Hadziioannou, G. Chem. Commun. 1999, 617. (b) Eckert, J. F.; Nicoud, J. F.;
Nierengarten, J. F.; Liu, S. G.; Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov, V. V.;
Hadziioannnou, G. J. Am. Chem. Soc. 2000, 122, 7467. (c) Armaroli, N.; Barigelletti, F.; Ceroni, P.;
Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F. Chem. Commun. 2000, 599. (d) Peeters, E.; van Hal, P.
A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C.; Janssen, R. A. J. J. Phys. Chem. B 2000,
104, 10174. (e) Segura, J. L.; Gómez, R.; Martín, N.; Luo, C. P.; Swartz, A.; Guldi, D. M. Chem.
Commun. 2001, 707. (f) van Hal, P. A.; Janssen, R. A. J.; Lanzani, G.; Cerullo, G.; Zavelani-Rossi, M.;
De Silvestri, S. Phys. Rev. B 2001, 64, 075206. (g) Nierengarten, J.-F.; Armaroli, N.; Accorsi, G.; Rio, Y;
Eckert, J.-F. Chem. Eur. J. 2003, 9, 37.
21 Oligothiophene-fullerene dyads: (a) Benincori, T.; Brenna, E.; Sannicolo, F.; Trimarco, L.; Zotti, G.;
Sozzani, P. Angew. Chem., Int. Ed. Engl. 1996, 35, 648. (b) Effenberger; F.; Grube ,G. Synthesis 1998,
1372. (c) Knorr, S.; Grupp, A.; Mehring, M.; Grube, G.; Effenberger, F. J. Chem. Phys. 1999, 110, 3502.
(d) Yamashiro, T.; Aso, Y.; Otsubo, T.; Tang, H.; Harima, Y.; Yamashita, K. Chem. Lett. 1999, 443. (e)
Fujitsuka, M.; Ito, O.; Yamashiro, T.; Aso, Y.; Otsubo, Y. J. Phys. Chem. A 2000, 104, 4876. (f) van Hal,
P. A.; Knol, J.; Langeveld-Voss, B. M. W.; Meskers, S. C. J.; Hummelen, J. C.; Janssen, R. A. J. J. Phys.
Chem. A 2000, 104, 5974. (g) van Hal, P. A.; Janssen, R. A. J.; Lanzani, G.; Cerullo, G.; Zavelani-Rossi,
M.; De Silvestri, S. Chem. Phys. Lett. 2001, 345, 33. (h) Fujitsuka, M.; Masahura, A.; Kasai, H.; Oikawa,
H.; Nakanishi, H.; Ito, O.; Yamashiro, T.; Aso, Y.; Otsubo, T. J. Phys. Chem. B. 2001, 105, 9930. (i)
Fujitsuka, M.; Matsumoto, K.; Ito, O.; Yamashiro, T.; Aso, Y.; Otsubo, T. Res. Chem. Intermed. 2001,
27, 73. (j) Hirayama, D.; Takimiya, K.; Aso, Y.; Otsubo, T. Hasobe, T.; Yamada, H. Imahori, H.;
Fukuzumi, S.; Sakata, Y. J. Am. Chem. Soc. 2002, 124, 532. (k) Negishi, N.; Yamada, K.; Takimiya, K.;
Aso, Y.; Otsubo, T.; Harima, Y. Chem. Lett. 2003, 32, 404-405.
22 Oligo(thienylene vinylene)-fullerene dyads: (a) Liu, S.-G.; Shu, L.; Rivera, J.; Liu, H.; Raimundo, J.-M.;
Roncali, J.; Gorgues, A.; Echegoyen, L. J. Org. Chem. 1999, 64, 4884. (b) Liu, S.-G.; Martineau, C.;
Raimundo, J.-M.; Roncali, J.; Echegoyen, L. Chem. Commun. 2001, 913. (c) Martineau, C.; Blanchard,
Covalently linked donor-acceptor materials
15
P.; Rondeau, D.; Delaunay, J.; Roncali, J. Adv. Mater. 2002, 14, 283. (d) Apperloo, J. J.; Martineau, C.;
van Hal, P. A.; Roncali, J.; Janssen, R. A. J. J. Phys. Chem. A 2002, 106, 21.
23 Oligoene-fullerene dyads: (a) Imahori, H.; Cardoso, S.; Tatman, D.; Lin, S.; Noss, L.; Seely, G. R.;
Sereno, L.; Chessa de Silber, J.; Moore, T. A.; Moore, A. L.; Gust, D. Photochem. Photobiol. 1995, 62,
1009. (b) Yamazaki, M.; Araki, Y.; Fuijtsuha, M.; Ito, O. J. Phys. Chem. A. 2001, 105, 8615.
24 Miscellaneous: oligomer-fullerene dyads: (a) Segura, J. L.; Gómez, R.; Martín, N.; Luo, C.; Guldi, D. M.
Chem. Commun. 2000, 701. (b) Guldi, D. M.; Swartz, Luo, C.; Gómez, R.; Segura, J. L.; Martín, N. J.
Am. Chem. Soc. 2002, 124, 10875. (c) Guldi, D. M.; Luo, C.; Schwartz, A.; Gómez, R.; Segura, J. L.;
Martin, N.; Brabec, C. J.; Sariciftci, N. S. J. Org. Chem. 2002, 67, 1141. (d) Gu, T.; Tsamouras, D.;
Melzer, C.; Krasnikov, V.; Gisselbrecht, J.-P.; Gross, M.; Hadziioannou, G.; Nierengarten, J.-F. Chem.
Phys. Chem. 2002, 124;
25 (a) Peeters, E.; van Hal, P. A.; Meskers, S. C. J.; Janssen, R. A. J.; Meijer, E. W. Chem. Eur. J. 2002,
8(19), 4470. (b) Asha S., Schenning, A. P. H. J.; Meijer, E. W.,Chem. Eur. J. 2002, 8, 15, 3353-61.
26 Gill, R. E.; Meetsma, A.; Hadziioannou, G. Adv. Mater. 1996, 8, 212.
27 (a) Peeters, E.; Marcos Ramos, A.; Meskers, S. C. J.; Janssen, R. A. J. J. Chem. Phys. 2000, 112, 9445.
(b) Schenning, A. P. H. J.; Kilbinger, A. F. M.; Biscarini, F.; Cavallini, M.; Cooper, H. J.; Derrick, P. J.;
Feast, W. J.; Lazzaroni, R.; Leclere, Ph.; McDonell, L. A.; Meijer, E. W.; Meskers, S. C. J. J. Am. Chem.
Soc. 2002, 124, 1269.
28 For recent reviews see: (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (b) Guldi,
D. M. Chem. Soc. Rev. 2002, 31, 22.
29 (a) Liddell, P. A.; Kuciauskas, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore, A. L.; Moore, T. A.;
Gust, D. J. Amer. Chem. Soc. 1997, 119, 1400. (b) Imahori, H.; Yamada, K.; Hasegawa, M.; Taniguchi,
S.; Okada, T.; Sakata, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 2626. (c) Carbonera, D.; Di Valentin,
M.; Corvaja, C.; Agostini, G.; Giacometti, G.; Liddell, P. A.; Kuciauskas, D.; Moore, A. L.; Moore, T.
A.; Gust, D. J. Amer. Chem. Soc. 1998, 120, 4398. (d) Kuciauskas, D.; Liddell, P. A.; Moore, A. L.;
Moore, T. A.; Gust, D. J. Amer. Chem. Soc. 1998, 120, 10880. (e) Tamaki, K.; Imahori, H.; Sakata, Y.;
Nishimura, Y.; Yamazaki, I. Chem. Commun. 1999, 625. (f) Imahori, H.; Yamada, H.; Ozawa, S.; Sakata,
Y.; Ushida, K. Chem. Commun. 1999, 1165. (g) Fujitsuka, M.; Ito, O.; Imahori, H.; Yamada, K.;
Yamada, H.; Sakata, Y. Chem. Lett. 1999, 721. (h) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki,
I.; Sakata, Y. J. Phys. Chem. B 2000, 104, 2099. (i) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Stone, S. G.;
Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. B 2000, 104, 4307. (j) Luo, C.; Guldi, D. M.;
Imahori, H.; Tamaki, K.; Sakata, Y J. Amer. Chem. Soc. 2000, 122, 6535. (k) Imahori, H.; Tamaki, K.;
Yamada, H.; Yamada, K.; Sakata, Y.; Nishimura, Y.; Yamazaki, I.; Fujitsuka, M.; Ito, O. Carbon 2000,
38, 1599. (l) Bahr, J. L.; Kuciauskas, D.; Liddell, P. A.; Moore, A. L.; Moore, T. A.; Gust, D. Photochem.
Photobiol. 2000, 72, 598. (m) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.;
Sakata, Y.; Fukuzumi, S. J. Amer. Chem. Soc. 2001, 123, 100. (n) Fukuzumi, S.; Imahori, H.; Yamada,
H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Guldi, D. M. J. Amer. Chem. Soc. 2001, 123, 2571. (o)
Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Amer.
Chapter 1
16
Chem. Soc. 2001, 123, 2607. (p) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.;
Fukuzumi, S. J. Amer. Chem. Soc. 2001, 123, 6617. (q) Fukuzumi, S.; Imahori, H.; Okamoto, K.;
Yamada, H.; Fujitsuka, M.; Ito, O.; Guldi, D. M. J. Phys. Chem. A 2002, 106, 1903. (r) Liddell, P. A.;
Kodis, G.; De la Garza, L.; Bahr, J. L.; Moore, A. L.; Moore, T. A.; Gust, D. Helv. Chim. Acta 2001, 84,
2765. (s) Ikemoto, J.; Takimiya, K.; Aso, Y.; Otsubo, T.; Fujitsuka, M.; Ito, O. Org. Lett. 2002, 4, 309. (t)
Imahori, H.; Tamaki, K.; Araki, Y.; Hasobe, T.; Ito, O.; Shimomura, A.; Kundu, S.; Okada, T.; Sakata,
Y.; Fukuzumi, S. J. Phys. Chem. A 2002, 106, 2803. (u) Imahori, H.; Tamaki, K.; Araki, Y.; Sekiguchi,
Y.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Amer. Chem. Soc. 2002, 124, 5165. (v) D'Souza, F.; Deviprasad,
G. R.; Zandler, M. E.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O. J. Phys. Chem. B 2002, 106, 4952. (w)
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Kodis, G.; Liddell, P. A.; de la Garza, L.; Moore, A. L.; Moore, T. A.; Gust, D. J. Mater. Chem. 2002, 12,
2100. (y) Sánchez, L.; Pérez, I.; Martín, N.; Guldi, D. M. Chem. Eur. J. 2003, 9, 2457.
30 Cravino, A.; Sariciftci, N. S. J. Mater. Chem. 2002, 12, 1931.
31 Benincori, T.; Brenna, E.; Sannicoló, F.; Trimarco, L.; Zotti, G. Angew. Chem. 1996, 108, 718.
32 Ferraris, J. P.; Yassar, A.; Loveday, D.; Hmyene, M. Opt. Mat. 1998, 9, 34.
33 (a) Cravino, A.; Zerza, G.; Maggini, M.; Bucella, D.; Svensson, M.; Andersson, M. R.; Neugebauer, H.;
Sariciftci, N. S. Chem. Commun. 2000, 2487. (b) Cravino, A.; Zerza, G.; Neugebauer, H.; Maggini, M.;
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2002, 106, 70.
34 Zhang, F.; Svensson, M.; Andersson, M. R.; Maggini, M.; Bucella, S.; Menna, E.; Inaganäs, O. Adv.
Mater. 2001, 13, 1871.
35 Xiao, S.; Wang, S.; Fang, H.; Li, Y.; Shi, Z.; Du, C.; Zhu, D. Macromol. Rapid. Commun. 2001, 22,
1313.
36 (a) Zerza, G.; Cravino, A.; Neugebauer, H.; Sariciftci, N. S.; Gómez, R.; Segura, J. L.; Martín, N.;
Svensson, M.; Anderson, M. R. J. Phys. Chem. A 2001, 105, 4172. (b) Giacalone, F.; Segura, J. L.;
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37 Ego, C.; Marsitzky, D.; Becker, S.; Zhang, J.; Crimsdale, A.; Müllen, K.; MacKenzie, J. D.; Silva, C.;
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38 Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten P. F.; Melzer, C.; Krasnikov V. V.; Hadziioannou, G.
J. Am. Chem. Soc. 2000, 122, 5464.
39 de Boer, B.; Stalmach, U.; van Hutten P. F.; Melzer, C.; Krasnikov V. V.; Hadziioannou, G. Polym..
2001, 42, 9097.
40 Neuteboom, E. E.; Meskers, S. C. J.; van Hal, P. A.; van Duren, J. K. J.; Meijer, E. W.; Dupin, H.;
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41 Schenning, A. P. H. J.; van Herrikhuyzen, J.; Jonkheijm, P.; Chen, Z.; Würthner, F.; Meijer, E. W. J. Am.
Chem. Soc. 2002, 124, 10252.
Chapter 2
Photoinduced multistep energy and
electron transfer in an oligoaniline –
oligo(p-phenylene vinylene) – fullerene
triad
Abstract
A donor-donor-acceptor triad, OAn-OPV-C60, with a redox gradient has been synthesized by
covalently linking an oligoaniline (OAn), an oligo(p-phenylene vinylene) (OPV), and a fullerene (C60)
in a non-conjugated linear array. Photoluminescence and femtosecond pump-probe spectroscopy
studies reveal that photoexcitation of any of the three chromophores of this triad in a polar solvent
results in formation the OAn-OPV•+-C60 •– charge-separated state, subsequent to an efficient ultrafast
(< 190 fs) singlet-energy transfer to the fullerene singlet-excited state. The initial OAn-OPV•+-C60•–
state can rearrange to the low-energy OAn•+-OPV-C60•– charge-separated state via an intramolecular
redox reaction. Because the competing charge recombination of the OAn-OPV•+-C60•– state to the
ground state is fast (≤ 65 ps) and increases with increasing polarity of the solvent, the quantum yield
for this charge shift is the highest (~0.4) in weakly polar solvents such as chlorobenzene. Once
formed, the OAn•+-OPV-C60•– state has a long lifetime (> 1 ns) due to weak electronic coupling
between the distant redox sites in the excited state. The stabilization gained is more than one order of
magnitude. The experimental results are found to be in qualitative agreement with Marcus theory. In
thin films, the OAn•+-OPV-C60•– state is formed at a higher rate and in higher quantum yield than in
solution.
Chapter 2
18
2.1 Introduction
Photoinduced energy and electron transfer reactions are the key steps in natural
photosynthesis and the elucidation of their mechanism continues to attract considerable interest.1
Similar processes occur in artificial photoactive and redoxactive molecular donor entities linked to
acceptors. These systems are considered as promising for the application in molecular and
supramolecular electronics, light harvesting, and photocatalysis.2 Molecular donor-acceptor
combinations also find application in organic and polymer photovoltaic cells to convert sunlight into
electrical energy.3,4 One of the potentially promising materials for photovoltaic cells is a blend of a
conjugated polymer as a donor and a methanofullerene derivative as acceptor.5 In these polymer/C60
bulkheterojunctions, the forward electron transfer is extremely fast (<< 1 ps),6 while the electron
recombination extends to the millisecond time domain.7 The large difference between the forward and
backward transfer rates ensures efficient charge generation and gives the opportunity to transport and
collect the photogenerated charges at the electrodes, and both processes have been studied extensively
in recent years.
In this context there is a considerable interest in the study of photoinduced charge generation
in covalently linked dyads, consisting of linear conjugated oligomers and fullerenes.8-12 Compared to
the polymer/C60 blends, these covalent molecular dyads are much more well-defined and allow to
study the charge transfer reactions in different media. Using this approach, it has been shown that
photoexcitation of the oligo(p-phenylene vinylene) unit of an OPV-C60 dyad (Figure 1) in a polar
organic solvent results in an ultrafast (< 190 fs) singlet-energy transfer (1OPV*-C60 → OPV-1C60*),
followed by a much slower (~10 ps) electron transfer reaction (OPV-1C60*→ OPV•+-C60•–) that
produces a charge-separated state with a lifetime of 50 ps.9e More recent studies revealed that the rate
of the forward electron transfer strongly depends on the relative orientation of the two moieties.13 For
an end-to-end substitution of donor and acceptor the electron transfer is much slower (~10 ps) than for
a face-to-face orientation (<< 1 ps), suggesting that the latter configuration explains the fast forward
reaction observed in polymer/C60 blends. With respect to charge recombination, however, there
remains a substantial discrepancy between the lifetimes of oligomer-C60 dyads in solution, which are
typically less than 100 ps, and the long-lived charges in polymer/C60 films. Nature solved the problem
of fast charge recombination in photosynthesis by creating a multi-step electron transfer to increase
the distance between the charges and slow down recombination.1 In mimicking natural
photosynthesis, a number of multi-site covalently linked porphyrin-fullerene combinations (triads,
tetrads, and pentads) have been described,14,15 showing that also in artificial fullerene systems multi-
step charge transfer results in an increase in lifetime of the charge-separated state. Hence, inspired by
nature, a tentative explanation for the long lifetime in the polymer/C60 films is the diffusion of charges
to different sites in the blend.
Photoinduced multistep energy and electron transfer in a molecular triad
19
In this contribution, the complementary views that originate from mimicking natural
photosynthesis and polymer/C60 solar cells are connected by extending the OPV-C60 dyad to include a
p-oligoaniline (OAn) moiety as an additional donor, to create a donor-donor-acceptor triad (OAn-
OPV-C60, Figure 2.1). By using a meta-substituted phenylene ring in OAn-OPV-C60, the OAn and
OPV parts are electronically decoupled in the ground state and operate as isolated redox active
segments. In OAn-OPV-C60, the oxidation potential decreases from C60, via OPV, to OAn, while at
the same time the reduction potential increases (vide infra). By introducing this redox gradient it is
expected that the energetically most favorable charge-separated state corresponds to OAn•+-OPV-
C60•– and that the lifetime of this state is enhanced as a result of the larger distance between the centers
of positive and negative charge density. The detailed analysis of the photophysical processes in OAn-
OPV-C60 has been performed using photoluminescence and femtosecond pump-probe spectroscopy in
solvents of different polarity and in the solid state, and by comparing the results with those of the
model compounds OAn-OPV, OPV-C60, OPV, OAn, and MP-C60 (Figure 2.1) that have only one or
two chromophores.
N N
NO
OO
O
OO
NO
OO
O
OO
O
O
N N
OO
O
O
OO
D
N N
OO
O
O
OO
O
O
N
OAn-OPV-C60
OPV-C60
OPV
OAn-OPVMP-C60
OAn
Figure 2.1. Structure of OAn-OPV-C60 triad and reference compounds.
2.2 Synthesis
The synthesis of OAn,16 OPV, MP-C60, and OPV-C609d have been described elsewhere. The
synthesis of the triad OAn-OPV-C60 (7, Scheme 2.1) starts from OPV aldehyde 1, which has been
described previously.9d The aldehyde functionality of 1 was protected as a dimethyl acetal 2.
Aldehyde 4 was obtained after reacting amine 3 with 3-bromobenzaldehyde in a palladium-catalyzed
Chapter 2
20
reaction. Aldehyde 4 was subsequently converted into the Schiff base 5 after refluxing with aniline in
ethanol. N-phenylaldimine 5 was then reacted with the methyl group of 2 in a Siegriest reaction17
affording, after acidic work up, aldehyde 6. A chlorobenzene solution of aldehyde 6, N-
methylglycine, and C60 was stirred for 16 h in the dark at reflux temperature to yield a mixture of C60,
the desired monoadduct, and higher adducts. The triad 7 was isolated after extensive column
chromatography in a 43% yield. This last synthetic step was done in collaboration with Joop Knol
(University of Groningen).
OR*OR*
OR*
R*OR*O
R*OO
O
OR*OR*
OR*
R*OR*O
R*OO
a
HN N
bN N
O
N N
N
N NOR*
OR*OR*
R*OR*O
R*OO
N NOR*
OR*OR*
R*OR*O
R*ON
c
d
e
OR*=O
1
6
2
3 4 5
7
Scheme 2.1. Synthesis of OAn-OPV-C60 (7). a. Amberlite IR 120, trimethyl orthoformate, methanol,
70 °C, 2 h, 91%; b. 3-bromobenzaldehyde, Pd2(dba)3, BINAP, Cs2CO3, toluene, 100° C, 5 days, 62%;
c. aniline, ethanol, 85 °C, 4 h, 79%; d. 1. Compound 2, t-BuOK, DMF, 80° C, 3 h, 72%; 2. HCl; e. N-
methylglycine, C60, chlorobenzene, reflux, 18 h, 43%.
Photoinduced multistep energy and electron transfer in a molecular triad
21
For the synthesis of the OAn-OPV dyad (10, Scheme 2.2), the bromine atom in 8 was
exchanged for a deuterium atom18 by lithiation and subsequent deuteration with D2O to yield 9. A
Siegriest reaction of 9 with N-phenylaldimine 5 afforded 10. All compounds used in the photophysical
investigations were fully characterized using 1H and 13C NMR spectroscopy, mass spectrometry, FT-
IR, and elemental analysis.
OR*OR*
OR*
R*OR*O
R*O Br
OR*OR*
OR*
R*OR*O
R*O D
N N
D
OR*OR*
OR*
R*OR*O
R*O
O
a
OR*=
8 9
10
b
Scheme 2.2. Synthesis of OAn-OPV (10). a. 1. n-BuLi, diethyl ether, -10 °C; 2. D2O, room
temperature, 41%; b. 1. Compound 5, t-BuOK, DMF, 80° C, 5 h, 50%; 2. HCl.
2.3 Electronic properties and energetic considerations
UV/Visible absorption. The absorption spectrum of OAn-OPV-C60 in toluene solution
(Figure 2.2) exhibits two strong absorption bands at 327 and 440 nm and a weak absorption at 705
nm. Whereas each of the three chromophores contributes to the absorption band at 327 nm, the
absorption at 440 nm is dominated by the π−π* transition of the OPV segment. The absorption at 705
nm is characteristic for fulleropyrrolidines.9d The absorption spectrum of OAn-OPV-C60 is a near
superposition of the absorption spectra of the different components of the triad; only at high energies
there is a slight deviation of the linear combination (Figure 2.2). This is likely due to the fact that the
OAn-OPV-C60 triad has one less alkoxy substituted phenylene group compared to combined
chromophores (OAn + OPV + MP-C60)
Chapter 2
22
650 700 750
0
500
300 400 500 600 700
0.0
2.5
5.0
7.5
10.0
ε (l/
mol
.cm
) x
10-4
Wavelength (nm)
Figure 2.2. UV/Visible absorption spectra of the OAn-OPV-C60 triad (solid line) and model
compounds OAn (dotted line), OPV (dashed line), and MP-C60 (dashed-dotted line) recorded in
toluene solution, and the summation of the spectra of all three reference compounds (squares). Inset:
Magnification of the 705 nm absorptions.
Electrochemistry. The OAn-OPV-C60 triad exhibits a reversible first reduction wave at –0.70
V, corresponding to the fullerene moiety, and three reversible oxidation waves due to the OAn (+0.55
V and +1.03 V) and the OPV (+0.83 V) moieties (potentials are given vs. SCE, calibrated against
Fc/Fc+, recorded in dichloromethane with 0.1 M TBAPF6) (Table 2.1). These oxidation potentials are
in close agreement with the values established for the reference compounds (Table 2.1). 9d The small
difference in oxidation potential between the OPV moiety in triad OAn-OPV-C60 (+0.83 V) and the
OPV chromophore (+0.78 V)9d is attributed to the smaller number of electron donating alkoxy
substituents of the former. The reduction potentials of OPV and OAn-OPV (-1.91 and –1.87 V) were
measured in tetrahydrofuran (THF) and are much more negative than that of the fullerene (Table 2.1).
Photoinduced multistep energy and electron transfer in a molecular triad
23
Table 2.1. One-electron redox potentials (E0) of OAn, OPV, MP-C60, OAn-OPV, and OAn-OPV-C60
(vs. SCE) calibrated with Fc/Fc+ (in dichloromethane with 0.1 M TBAPF6).
Compound E0red (V) E0
ox (V)
OAn 0.53 / 1.02
OPV -1.91 a 0.78
MP-C60 -0.70
OAn-OPV -1.87 a 0.53
OAn-OPV-C60 -0.70 0.53 / 0.83 / 1.03
a Measured in THF
Absorption spectra of redox states. UV/visible/near-IR spectroscopy enables to monitor the
electronic transitions of the donor-acceptor arrays during a stepwise oxidation process. Quantitative
chemical oxidation of the OAn-OPV dyad in dichloromethane solution was achieved by the addition
of thianthrenium perchlorate19 (Figure 2.3). After the addition of one equivalent of this oxidizing
agent, the intensity of the absorption band at 3.79 eV decreases and two absorption bands emerge in
the spectrum, one at 1.44 eV, and the other overlapping with the absorption band of the OPV unit at
2.83 eV. Comparison with the electronic transitions of the N,N,N´,N´-tetraphenyl-1,4-benzenediamine
radical cation (1.44 and 3.05 eV),20 demonstrates that the absorption bands are associated with the
formation of an OAn•+ radical cation, while the disappearing band is that of the neutral OAn moiety.
When a second equivalent of thianthrenium perchlorate is added, the band of the neutral OPV unit, at
2.83 eV, decreases and two absorption bands with vibronic fine structure, at 0.74 and 1.64 eV, related
to an OPV•+ radical cation,21 appear in the spectrum. At the same time the absorption of OAn•+ at 1.44
eV remains and the second band of OAn•+ at 3.04 eV is now clearly observable. After two equivalents
of oxidizing agent the absorption spectrum exhibits the characteristics of both OAn•+ and OPV•+
radical cations and therefore corresponds to that of the OAn•+-OPV•+ dication diradical.
Chapter 2
24
1 2 3 4
0.00
0.25
0.50
0.75
Abs
orba
nce
(O. D
.)
Energy (eV)
0.00
0.25
0.50
0.75
1.00
b
a
OPV+OPV+
OPV
OAn+
OAn
OAn+
Figure 2.3. UV/visible/near-IR spectra recorded during the conversion of OAn-OPV by stepwise
oxidation using thianthrenium perchlorate19 in CH2Cl2: (a) before (solid line) and after (dashed line)
adding 1 equivalent; (b) after adding 1 equivalent (dashed line) and 2 equivalents (solid line).
Energetic considerations. In the two dyads (OAn-OPV, OPV-C60) and triad (OAn-OPV-C60)
numerous processes may occur after photoexcitation. Apart from the intrinsic decay of the singlet-
excited state of the individual chromphores (photoluminescence, intersystem crossing, and thermal
decay), energy and electron transfer reactions involving more than one chromophore (or redox active
group) are possible. Whether these reactions occur depends, amongst others, on whether these
processes are exergonic. The absorption spectra (Figure 2.2) reveal that the lowest singlet-excited
state is located on the fullerene at 1.76 eV, followed by the singlet state of the OPV at 2.48 eV, while
that of the OAn segment is positioned at approximately 3.40 eV.
The energies of the charge-separated states can be estimated by calculating the change in free
energy (∆G0) for charge separation using a continuum model 22:
( ) ( )( )
−
+−−−−=∆ −+
sref0
2
ccs0
2
00redox0 1111
84AD
εεπεεπε rr
e
R
eEEEeG (2.1)
Photoinduced multistep energy and electron transfer in a molecular triad
25
In this equation, Eox(D) and Ered(A), are the oxidation and reduction potentials of the donor
and acceptor molecules or moieties measured in a solvent with relative permittivity εref, E00 is the
energy of the excited state from which the electron transfer occurs, and Rcc is the center-to-center
distance of the positive and negative charges in the charge separated state. The radii of the positive
and negative ions are given by r+ and r– and εs is the relative permittivity of the solvent, -e is the
elemental charge, and ε0 is the vacuum permittivity.
Table 2.2. Change in free energy (∆G0) with reference to the lowest singlet state, reorganization
energy (λ), and barrier (∆G‡) for charge separation (CS), charge recombination (CR1), charge shift
(CSH), and charge recombination (CR2) in toluene (TOL), chlorobenzene (CB), o-dichlorobenzene
(ODCB), and benzonitrile (BZN) as determined using Eqs 2.1 and 2.3.
reaction solvent ∆G0
(eV) λ
(eV) ∆G‡ (eV)
OAn-OPV-1C60* → OAn-OPV+-C60– TOL 0.21 0.35 0.224
CS CB -0.22 0.75 0.093 ODCB -0.36 0.86 0.073 BZN -0.46 0.99 0.070
OAn-OPV•+-C60•–→ OAn-OPV-C60 TOL -1.97 0.35 1.905
CR1 CB -1.54 0.75 0.204 ODCB -1.40 0.86 0.088 BZN -1.30 0.99 0.024
OAn-OPV•+-C60•–→ OAn•+-OPV-C60
•– TOL -0.08 0.35 0.051 CSH CB -0.21 0.80 0.106
ODCB -0.25 0.91 0.119 BZN -0.29 1.06 0.140
OAn•+-OPV-C60• –→ OAn-OPV-C60 TOL -1.89 0.36 1.618
CR2 CB -1.32 0.89 0.053 ODCB -1.15 1.03 0.004 BZN -1.01 1.20 0.007
OAn-1OPV* → OAn•+-OPV•– TOL 0.41 0.35 0.407 CS CB -0.07 0.80 0.165
ODCB -0.22 0.91 0.133 BZN -0.34 1.06 0.123
The change in free energy for charge separation was calculated using Eq. 2.1 for four solvents
of interest with increasing polarity: toluene (ε = 2.38), chlorobenzene (ε = 5.71), o-dichlorobenzene (ε
= 9.93), and benzonitrile (ε = 25.2) (Table 2.2). For the calculations, the Rcc distances were
determined assuming that the charges are located at the centers of the OAn, OPV and C60 moieties.23
These distances were 15 and 30 Å for the OAn-OPV•+-C60•– and OAn•+-OPV-C60
•– charge-separated
states respectively. The radius of the negative ion is reported for C60 to be r– = 5.6 Å24 and that of the
Chapter 2
26
ions of OPV to r+/ r– = 5.1 Å 9d The radius of the positive charge of OAn was set to r+ = 4.8 Å, as
calculated from molecular modeling. Small variations of the approximated Rcc and r+/ r– values did
not significantly alter the outcome of the calculations for the change in free energy. According to Eq.
2.1 charge separation (CS: OAn-OPV-1C60* → OAn-OPV•+-C60
•–) and the charge shift (CSH: OAn-
OPV•+-C60•– → OAn•+-OPV-C60
•–) are energetically feasible in the three polar solvents (Table 2.2).
The experimental and estimated energies of the various neutral and charge-separated states of
the OAn-OPV-C60 triad are depicted in Figure 2.4, assuming chlorobenzene as the medium. Figure 2.4
also shows the photophysical reactions that have been identified in the triad or its model compounds,
with the prevailing processes highlighted in black, vide infra.
1OAn*-OPV-C601OAn*-OPV-C60
OAn-1OPV*-C60
OAn-OPV-C60
OAn-1OPV*-C60OAn-1OPV*-C60
OAn-OPV-C60OAn-OPV-C60
OAn-OPV•+-C60• -
OAn•+-OPV-C60• -
OAn-OPV•+-C60• -OAn-OPV•+-C60• -
OAn•+-OPV-C60• -OAn•+-OPV-C60• -
OAn•+-OPV•--C60
OAn-OPV-1C60*OAn-OPV-1C60*
OAn-OPV-3C60*OAn-OPV-3C60*
kET
kCS
kCSH
kPLOPV
k´ET
kISCC60
OAn-3OPV*-C60
kISCOPV
kPLC60
Energy
k´CS
k´CR
kCR1
kCR2
Figure 2.4. Schematic energy levels (in chlorobenzene) and photoinduced processes in OAn-OPV-
C60. The rate constants are collected in Table 2.3. The fastest processes have been highlighted in
black (vide infra).
Photoinduced multistep energy and electron transfer in a molecular triad
27
2.4 Photophysical processes in solution
2.4.1 Photoluminescence spectroscopy
Photoluminescence (PL) experiments on the triad, dyads, and model compounds, were used to
study the energy and electron transfer reactions that occur in these molecules. These transfer
relaxation pathways are expected to quench the PL of the chromophores, especially when their rate
constants are higher than that of the intrinsic decay.
500 600 700 8000
3
6
300 400 500 600 7000.0
0.5
1.0b
a
PL
Inte
nsity
(a.
u.)
Wavelength (nm)
Nor
mal
ized
PL
Inte
nsity
Wavelength (nm)
Figure 2.5. (a) PL spectra of OPV-C60 in toluene (dashed line) and of OAn-OPV-C60 in toluene (solid
line) and o-dichlorobenzene (dotted line). (b) Excitation spectra of the 710 nm emission of OPV-C60
(dashed line) and OAn-OPV-C60 (solid line) compared to the absorption spectrum of OAn-OPV-C60
(dashed line), all in toluene.
The PL at 499 nm of the OPV moiety of the OAn-OPV-C60 triad, dissolved in toluene, is
highly quenched (quenching factor Q > 4000) compared to the PL of the corresponding OPV
molecule. Apart from a residual emission at 499 nm,25 photoexcitation of the OPV moiety of OAn-
OPV-C60 results in a weak PL signal at 715 nm (Figure 2.5a), characteristic of the fluorescence
emission band of fulleropyrrolidines.9d In toluene the quantum yield of this emission is nearly
identical to that of the reference compound MP-C60. The same result is observed for the OPV-C60
dyad, although in this case the PL quenching of the OPV is somewhat less (Q ≈ 1500).25 The strong
Chapter 2
28
quenching of the 1OPV* singlet-excited state (S1) in the OPV-C60 dyad has previously been studied in
detail and was found to involve an ultrafast photoinduced intramolecular singlet-energy transfer (ET)
towards the fullerene moiety (1OPV*-C60 → OPV-1C60*), which occurs with a time constant of less
than 190 fs.9e,26 By analogy, the same ET process must occur in the OAn-OPV-C60 triad. In
accordance with this proposition, the excitation spectrum of the fullerene fluorescence of OAn-OPV-
C60 coincides with the corresponding absorption spectrum of OAn-OPV-C60 (Figure 2.5b). It is
interesting to note that the fullerene excitation spectra of OPV-C60 and OAn-OPV-C60 differ
appreciably at lower wavelengths, where the OAn moiety absorbs. The excellent agreement between
the absorption spectrum of the OAn-OPV-C60 triad and the fullerene excitation spectrum implies that
not only excitation of OPV, but also excitation of OAn is responsible for the emission of the fullerene.
This points to an efficient sequential singlet-energy transfer in the triad (Figure 2.4), that starts with
the formation of the 1OAn*-OPV-C60 singlet-excited state and ends at the OAn-OPV-1C60* fullerene
singlet-excited state.
In a more polar solvent, like o-dichlorobenzene (ε = 9.93), the PL of the OPV unit of the
OAn-OPV-C60 triad is quenched to a similar extent as in toluene (Figure 2.5a). In contrast, the PL of
the fullerene moiety at 715 nm is significantly quenched in this more polar solvent as compared to
toluene (Q ≈ 400, Figure 2.5a). The same result has previously been observed for the OPV-C60 dyad9d,e
and gives evidence of a photoinduced charge separation (CS) that occurs from the fulleropyrrolidine
singlet-excited state and produces the OPV•+-C60•– charge-separated state. For the OAn-OPV-C60 triad,
a photoinduced CS will initially produce a similar state (OAn-OPV•+-C60•–), which may then go
through a charge shift (CSH) to generate the energetically more favorable OAn•+-OPV-C60•– state
(Figure 2.4).
For a further comparison, we have studied the PL of the OAn-OPV dyad in solvents of
different polarity. Figure 2.6a shows that the emission of 1(OAn-OPV)*, dissolved in toluene, stems
exclusively from the OPV moiety, irrespective of the excitation wavelength (330 nm (OAn) or 444
nm (OPV)). This implies that an efficient singlet-energy transfer occurs from the excited 1OAn* state
to OPV (1OAn*-OPV → OAn-1OPV*). In toluene, the fluorescence quantum yield of OAn-OPV is
slightly (~10%) higher than that of OPV, but the PL is progressively quenched with increasing solvent
polarity (Figure 2.6a), providing quenching factors of Q ≈ 2, 9, and 22 for chlorobenzene, o-
dichlorobenzene, and benzonitrile, respectively. The quenching of OAn-OPV PL after excitation at
440 nm in more polar solvents is attributed to an intramolecular photoinduced electron transfer
reaction in the excited state to produce the OAn•+-OPV•– state. The excitation spectrum of the residual
emission of OAn-OPV recorded in chlorobenzene (Figure 2.6b) closely corresponds to the absorption
spectrum of OAn-OPV. This indicates that the 1OAn*-OPV → OAn-1OPV* singlet-energy transfer is
significantly faster than an electron transfer from the same state (1OAn*-OPV → OAn•+-OPV•–).
Photoinduced multistep energy and electron transfer in a molecular triad
29
Hence, OAn•+-OPV•– is primarily formed via the OAn-1OPV* singlet state, irrespective of the
excitation wavelength.
400 500 600 700
0
20
40
60
80
100
120
300 400 500 600
0.0
0.5
1.0 b
a
Cou
nts
x 10
-4
Wavelength (nm)
Nor
mal
ized
inte
sity
Wavelength (nm)
Figure 2.6. (a) PL spectra of OAn-OPV in toluene (solid squares), chlorobenzene (open squares), o-
dichlorobenzene (closed circles), and benzonitrile (open circles) solutions with excitation at 444 and
330 nm (for toluene only, dashed line). (b) Excitation spectrum of the OPV emission of OAn-OPV in
chlorobenzene (dashed line) and compared to the absorption spectrum (solid line).
2.4.2 Near steady state photoinduced absorption (PIA) spectroscopy
Near steady state PIA spectroscopy in the microsecond and millisecond time domain is a very
sensitive technique (detection limit ∆T/T ~ 10-6) to probe small concentrations of long-lived
photoexcitations such as triplet states and intermolecular charge-separated states. The PIA spectrum
of OAn-OPV-C60 in toluene solution, recorded with excitation at 458 nm, exhibits a band at 1.78 eV
with a shoulder at 1.52 eV, characteristic of the long-lived (~40 µs) triplet state of the
fulleropyrrolidine moiety (OAn-OPV-3C60*) formed via intersystem crossing from the fullerene
singlet-excited state. 9d
The photoinduced electron transfer in OAn-OPV-C60 in o-dichlorobenzene and subsequent
charge shift or charge recombination will likely occur in the picosecond to nanosecond time regime
and cannot be resolved with the near-steady state PIA technique. However, absorptions of the charged
Chapter 2
30
states can be observed in mixtures of the different model compounds of the triad in o-dichlorobenzene
solution, by using the redox activity of the corresponding triplet states. Photoexcitation of OPV (at
458 nm) or MP-C60 (at 528 nm) will result in the formation of the corresponding excited triplet states.
These triplet states (3OPV* and MP-3C60*) can undergo an electron transfer to one of the other redox
active chromophores present in solution to produce an intermolecularly charge-separated state, which
is long-lived because the formed cation and anion radicals diffuse away in solution.
0.5 1.0 1.5 2.00.0
0.5
1.0
75 Hz 2500 Hz
Energy (eV)
0.0
2.5
c
-∆T
/T x
104
b
a
0.0
0.5
1.0
1.5
Figure 2.7. (a) Normalized photoinduced absorption spectra of the mixtures OPV/MP-C60 (1:1) (solid
line) and OAn/MP-C60 (1:1) (dashed-line) in o-dichlorobenzene (excitation at 528 nm with 25 mW
and modulation frequency of 275 Hz). (b) Photoinduced absorption spectra of the mixtures OAn-
OPV/MP-C60 (1:1) (solid line) or OAn/OPV/MP-C60 (1:1:1) (dashed line) in o-dichlorobenzene
(excitation at 528 nm with 25 mW and modulation frequency of 275 Hz). (c) Normalized photoinduced
absorption spectra of the mixture OAn/OPV/MP-C60 (1:4:4) in o-dichlorobenzene recorded at
modulation frequencies of 75 Hz (solid line) and 2500 Hz (dashed line) (excitation at 528 nm with 25
mW).
Photoinduced multistep energy and electron transfer in a molecular triad
31
Accordingly, selective photoexcitation of MP-C60 at 528 nm in mixtures with OPV or OAn in
o-dichlorobenzene solution produces the charge-separated states OPV•+/MP-C60•– and OAn•+/MP-C60
•–
respectively, which are characterized by the absorptions of OPV•+ (at 0.68 and 1.52 eV), OAn•+ (at
1.40 eV), and MP-C60•– (at 1.24 eV) (Figure 2.7a). Likewise, photoexcitation of MP-C60 in a mixture
with OAn-OPV in o-dichlorobenzene results in a band at 1.40 eV (Figure 2.7b) attributed to OAn•+-
OPV radical cation. Although in this mixture the OAn-OPV•+ radical cation can also be formed, it will
probably rearrange by an intramolecular redox reaction to the OAn•+-OPV state within the timescale
of the experiment. In an equimolar mixture of OAn, OPV, and MP-C60, charge transfer from the
positively charged OPV+ onto the OAn becomes intermolecular, i.e. diffusion limited and slower.
This allows for the detection of a weak residual absorption band characteristic of the OPV•+ radical
cation in the PIA spectrum at 0.68 eV (Figure 2.7b).
Even in a mixture where both OPV and MP-C60 are present in fourfold excess with respect to
OAn, the PIA spectrum is still dominated by the absorption of the OAn•+ radical cation (Figure 2.7c).
At high modulation frequency (2500 Hz) the relative intensity of the OPV•+ radical cation band at
0.68 eV increases compared to low modulation frequency (75 Hz) (when normalized at 1.24 eV),
indicating that in this mixture OPV•+ has a shorter lifetime than the OAn•+ radical cation, consistent
with the proposed OPV•+ + OAn → OPV + OAn•+ redox reaction.
2.4.3 Femtosecond pump-probe spectroscopy in polar solvents
The formation and decay of the transient charged species generated after photoexcitation of
the OPV moiety in OAn-OPV-C60 and the reference compounds OPV-C60 and OAn-OPV have been
investigated with sub-picosecond pump-probe spectroscopy in solvents of different polarity.
Upon excitation at 450 nm of OPV-C60 or OAn-OPV-C60, the transient absorption at 1450 nm
(0.85 eV) that is associated with the OPV•+ radical cation, i.e. the OPV•+-C60•– and OAn-OPV•+-C60
•–
states, exhibits a rise and a decay component (Figure 2.8). It is important to note that the charge
separation occurs subsequent to the ultrafast singlet-energy transfer: OAn-1OPV*-C60 → OAn-OPV-1C60* (kET ≥ 5.3 × 1012
s-1).9e Fitting of the temporal differential absorption data at 1450 nm to a
biexponential function27 provides the rate constants for forward and backward electron transfer
reactions (Table 2.3). As can be seen in Figure 2.8 (and Table 2.3), the rate constants for the charge
separation (kCS) are equal in the dyad and triad within experimental error. The rates for charge
separation in the dyad and triad increase significantly with solvent polarity from kCS = 4.3 × 1010 s
-1 in
chlorobenzene to kCS = 2.0 × 1011 s-1 in benzonitrile. A similar increase is found for the charge
recombination. This effect of the medium on the rate constant has often been observed in donor-
acceptor dyads and is in agreement with Marcus theory, vide infra.28
Chapter 2
32
-60
-40
-20
0
0 50 100 150 200 250
-60
-40
-20
0
-60
-40
-20
0
a
b
c
∆T/T
(a.
u.)
Time delay (ps)
Figure 2.8. Differential transmission dynamics of the OPV•+ transition at 1450 nm for OAn-OPV•+-
C60•–
(solid line) and OPV•+-C60•– (dashed line) in (a) chlorobenzene, (b) o-dichlorobenzene, and (c)
benzonitrile after excitation at 450 nm.
In comparison to the dyad, the charge shift (CSH) from OAn-OPV•+-C60•– to OAn•+-OPV-
C60•– provides an extra pathway in the triad for the decay of the OPV•+ radical other than charge
recombination (CR1, Figure 2.4) to the ground state. Hence, the OPV•+ radical cation absorption may
disappear faster in the triad than in the dyad. The experimental data in Figure 2.8 show, however, that
this difference is only discernable in chlorobenzene, i.e. the least polar solvent. This is a direct
consequence of the increased rate for charge recombination in more polar solvents (Figure 2.8), which
implies that the probability for the competing (slower) charge shift is strongly reduced. Non-linear
least squares analysis of the experimental OPV•+ traces (Figure 2.8) does not give a clear indication of
the magnitude of the rate of charge shift; it yields values of kCR1 and (kCR1 + kCSH) that are essentially
the same (see table 3). This may be explained in part by a difference in kCR1 for the dyad and the triad
and in part by the difficulties in the data analysis itself. Due to the fact that the rates for formation and
decay of the OPV•+ species are very similar, it is difficult to estimate the rate. Non-linear-least-
Photoinduced multistep energy and electron transfer in a molecular triad
33
squares-fit procedures invariably yield rate parameters for formation and decay that are strongly
correlated.
Table 2.3. Rate constants for intrinsic decay of the lowest-energy singlet-excited chromophore (k0OPV,
k0C60), singlet-energy transfer (kET), charge separation (kCS), charge recombination (kCR1), and charge
shift (kCSH) in the studied compounds in toluene (TOL), chlorobenzene (CB), o-dichlorobenzene
(ODCB), benzonitrile (BZN).
Compound TOL
k (ns-1)
CB
k (ns-1)
ODCB
k (ns-1)
BZN
k (ns-1) OPV k0
OPV 0.83 0.79 0.74 0.69
MP-C60 k0C60 0.68 0.72 0.75 0.68
OPV-C60 kET ≥ 5300 n.d. ≥ 5300 n.d.
kCS - 40±2 50±9 250±40
kCR1 - 12±1 19±3 37±8
OAn-OPV-C60 kET ≥ 5300 n.d. ≥ 5300 n.d.
kCS - 48±5 71±12 201±54
kCR1+ kCSH - 11±1 16±3 61±19
OAn-OPV k´CS - 16.4 15.6 40.8
k´CR - 1.0 1.4 4.1
Additional information on the CSH reaction can be obtained by measuring the photoinduced
absorption at 1030 nm (1.20 eV). At this probe wavelength the S1 states 1OPV* and 1MP-C60*, as well
as the radical ions MP-C60•–, and OAn•+ contribute to the induced absorption (Figure 2.7). The
differential transmission at 1030 nm of OPV-C60 and OAn-OPV-C60 in chlorobenzene undergoes a
strong rise and drop within 2 ps (Figure 2.9). This transient signal is associated with the formation and
decay of the 1OPV* singlet-excited state and involves the Sn←S1 transition of the OPV unit.9e The
short lifetime of the 1OPV* state is due to the ultrafast ET onto the fullerene moiety as described
above. After the ET, the 1030 nm signal for the OAn-OPV-C60 triad remains constant over the
timescale of the experiment (1 ns), while for the dyad the signal decays to zero within 200 ps (Figure
2.9). It is proposed that the remaining signal is due to absorption by C60•– and OAn•+ radical ions, and
hence characteristic of the OAn•+-OPV-C60•– charge separated state. Using the following extinction
coefficients for these species (7.7 × 104 M-1cm-1 (OPV(S1), estimate), 7 × 103 M-1cm-1 (MP-C60(S1) 29),
7 × 103 M-1cm-1 (MP-C60•– 29), 8.2 × 103 M-1cm-1 (OAn•+ 20)) the time profile for the absorption at 1030
nm (Figure 2.9) has been modeled for both the OPV-C60 dyad and the OAn-OPV-C60 triad. Rate
Chapter 2
34
constants were taken from the transient absorption measurements at 1450 nm probe wavelength
(Table 2.3) and the rate for charge shift (kCSH) is used as an adjustable parameter.30 The results show a
large induced absorption due to the OPV(S1) species. The duration of this transient is mainly
determined by the width of the laser pulses used. For the modeling a cross-correlation of 500 fs
(FWHM) for the pump and probe pulse was assumed. For the dyad, a smooth trace is observed after
the initial contribution of the excited OPV moiety. This is consistent with the fact that the C60(S1) and
the C60•– groups have almost equal extinction coefficients. For the triad, a long-lived signal is
observed which can be modeled taking kCSH = 5 × 109 s-1 and kCR1 = 6 × 109 s-1. Taking a higher
(lower) value for kCSH results in a curve that bends downward (upward) after 60 ps. Using kCSH = 5 ×
109 s-1 results in a calculated yield of formation for the OAn•+-OPV-C60•– species of about 0.4 per
absorbed photon. This result can be compared with the following crude estimate. At 30 ps mainly the
Oan-OPV•+-C60•– state is present of which only the C60
•– makes a major contribution to the absorption
at 1030 nm. At t= 200 ns only the OAn•+-OPV-C60•– state is present and OAn•+ and C60
•–contribute
almost equally to the transient absorption. The experimental observation that the induced absorption
at 1030 nm hardly changes between 30 and 200 ps thus indicates that the probability of formation of
OAn•+-OPV-C60•– out of OAn-OPV•+-C60
•– is roughly one half, in good agreement with the estimate
above. Figure 2.9 also shows the induced absorption signal at 1450 nm as observed for the triad. This
latter signal has been modeled using the same parameters as mentioned above.
0.1 1 10 100 1000
-100
-50
0
∆T/T
(a.
u.)
Time delay -1 (ps)
Figure 2.9. Differential transmission dynamics of OAn-OPV-C60 (open circles) and OPV-C60 (closed
squares) in chlorobenzene monitored at 1030 nm with excitation at 450 nm. The induced absorption
signal at 1450 nm as observed for the triad is shown with open squares. The lines correspond to a
numerical simulation based on the model described in the text. For the simulation the rate constants
from Table 3 are used and the rate for charge shift is used as an adjustable parameter. The time delay
has been shifted by 1 ps to show the signals on a logarithmic plot.
Photoinduced multistep energy and electron transfer in a molecular triad
35
We also studied the reference compound OAn-OPV with pump-probe spectroscopy in
solution. In particular, it is of interest to investigate whether an OAn-1OPV*-C60 → OAn•+-OPV•–-C60
electron transfer reaction can compete with the OAn-1OPV*-C60 → OAn-OPV-1C60* energy transfer.
As a result of the electron-hole symmetry in conjugated oligomers, the optical spectra of OPV
radical cations and OPV radical anions are expected to be very similar and, hence, we probe the
formation of OPV•– at 1450 nm.31 The transient differential transmission recorded for OAn-OPV,
dissolved in solvents with increasing polarity, at 1450 nm after excitation at 455 nm of the OPV
chromophore is shown in Figure 2.10. In full agreement with the PL quenching of the OAn-OPV
fluorescence (Figure 2.6), the OAn•+-OPV•– charge-separated state is only formed in the three polar
solvents (chlorobenzene, o-dichlorobenzene, and benzonitrile) but not in (less polar) toluene (Figure
2.10). The rate constants for charge separation and charge recombination (k´CS and k´CR, Table 2.3)
increase with increasing polarity. Table 2.3 reveals that the rate for the OAn-1OPV* → OAn•+-OPV•–
charge separation (k´CS = 1.6 - 4.1 × 1010 s
-1) is significantly less than the rate of the 1OPV*-C60 →
OPV-1C60* energy transfer (kET ≥ 5.3 × 1012 s
-1) and as a consequence the OAn•+-OPV•–-C60 state is
unlikely be formed in a significant yield from OAn-1OPV*-C60. Hence, excitation of OAn-OPV-C60 in
solution will always provide OAn-OPV-1C60* as an intermediate state, irrespective of the excitation
wavelength and solvent polarity.
0 200 400 600 800 1000-0.3
-0.2
-0.1
0.0
∆T/T
(a.
u.)
Time delay (ps)
Figure 2.10. Differential transmission dynamics of OAn•+-OPV•– in toluene (closed squares),
chlorobenzene (closed circles), o-dichlorobenzene (open circles), and benzonitrile (open squares)
monitored at 1450 nm with excitation at 455 nm.
Charge recombination in OAn•+-OPV•– is almost an order of magnitude slower than in
OPV•+-C60•– and occurs in the nanosecond regime. In this respect, a remarkable phenomenon was
observed when the fluorescence lifetimes of OAn-OPV were recorded. In the most polar solvents o-
Chapter 2
36
dichlorobenzene and benzonitrile the lifetime of the OAn-OPV emission is significantly reduced to
~140 and ~100 ps, compared to the OPV model compound (1.36 and 1.44 ns, respectively). This is in
accordance with the proposed OAn-1OPV* → OAn•+-OPV•– electron transfer reaction, which reduces
the lifetime of the 1OPV* state. The lifetimes of 1OPV* and OAn-1OPV* in toluene are very similar
(1.20 and 1.38 ns, Figure 2.9) as expected because electron transfer does not occur here. However,
and surprisingly, a significant increase of fluorescence lifetime was observed for OAn-1OPV* in
chlorobenzene (2.65 ns) compared to OPV (1.27 ns) (Figure 2.11). At first glance this increase in
fluorescence lifetime seems to contradict the observed OAn-1OPV* → OAn•+-OPV•– charge
separation reaction, which is approximately 20 times faster than the intrinsic decay (Figure 2.10,
Table 2.3). Instead of an increase in fluorescence lifetime, a decrease would be expected. Note that
except for a loss in intensity (only a factor of 2), the fluorescence spectra of OPV and OAn-OPV are
identical (Figure 2.6), and that the emission is thus from the OAn-1OPV* state. These experimental
observations lead to the conclusion that in chlorobenzene the singlet OAn-1OPV* state and the
OAn•+-OPV•– charge-separated state are nearly degenerate and that back electron transfer from
OAn•+-OPV•– reproduces the OAn-1OPV* singlet state. The estimated change in free energy for OAn-1OPV* → OAn•+-OPV•– of only ∆G0 = – 0.07 eV (Table 2.2) supports this suggestion.
0 5 10
10
100
1000
10000
OAn-OPV: TOL
OAn-OPV: CB
OPV: CB
OPV: TOL
Cou
nts
Time (ns)
Figure 2.11. Time-resolved fluorescence of OPV and OAn-OPV in toluene (TOL) and chlorobenzene
(CB) recorded with excitation at 400 nm.
2.4.4 Kinetic considerations
The final outcome of a photoexcitation not only depends on the energetics of the reaction, but
also on the kinetics. Marcus theory provides an estimate for the free energy barrier (∆G‡) for electron
transfer reactions based on the change in free energy (∆G0) and the reorganization energy (λ) via:
Photoinduced multistep energy and electron transfer in a molecular triad
37
( )
λλ
4
20‡ +∆=∆ G
G (2.2)
The reorganization energy consists of an internal contribution (λ i) and a solvent term (λs),
which can be approximated via the Born-Hush approach to give after summation:
−
−
++=+= −+
s2
0
2
isi11111
2
1
4 επελλλλ
nRrr
e
cc
(2.3)
The rate constants for the different processes, are not only a function of the energy barrier
∆G‡, but also of the reorganization energy (λ) and the electronic coupling (V) between donor and
acceptor in the excited state according to the equation:
( )
+∆−
=
Tk
GV
Tkhk
B
202
21
B2
2
4exp
4
λλ
λπ
(2.4)
The values of ∆G0, λ, and ∆G‡ calculated on the basis of Eqs. 2.1 to 2.4 collected in Table
2.2, show that the initial charge separation (kCS) is in the Marcus normal region (–∆G0 < λ). As the
polarity of the solvent increases, the OAn-OPV•+-C60•– charge-separated state is stabilized with a
concomitant increase of the reorganization energy. The combination of these trends results in
reduction of the barrier for charge separation in more polar solvents (Table 2.2). As a consequence,
the rate for charge separation (kCS) is expected to increase with polarity as has been found
experimentally (Figure 2.8, Table 2.3). Charge recombination in OAn-OPV•+-C60•– is in the Marcus
inverted region (–∆G0 > λ). The use of Eq. (2.4) in the inverted region often underestimates the true
rate constant because of nuclear tunneling,32 but will be used here for qualitative comparison. We find
that the barrier for charge recombination is strongly reduced in more polar solvents, consistent with
the higher recombination rate (kCR1, Table 2.3). Apart from recombination to the ground state, the
OAn-OPV•+-C60•– state may undergo a charge shift to form OAn•+-OPV-C60
•–. The charge shift occurs
in the normal region (–∆G0 < λ) and is energetically less favorable than the recombination (Table 2.3).
However, in contrast to the recombination, the barrier for charge shift is reduced with decreasing
polarity. Hence, the balance between charge recombination and the competing charge shift, will move
towards the latter with decreasing polarity. As a result, the barrier for the charge shift in OAn-OPV•+-
C60•– is lower than that for charge recombination in chlorobenzene, but not in o-dichlorobenzene and
benzonitrile. This is in full agreement with the experimental result that the charge shift was more
easily observed in chlorobenzene (Figure 2.8 and 2.9).
Chapter 2
38
The energy barriers for relaxation to the ground state in OAn•+-OPV-C60•– are small.
However, besides the energy barrier, the rate constant (Eq. 2.4) is also determined by the electronic
coupling V. This electronic coupling depends exponentially on the center-to-center distance between
the donor and acceptor via ))(exp()( 0cc02
02 RRRVV −−= β , with R0 the contact distance. Hence, V
is orders of magnitude less for OAn•+-OPV-C60•– (Rcc = 30.0 Å) than for OAn-OPV•+-C60
•– (Rcc = 15.4
Å).33 The reduction of the electronic coupling V, caused by the longer distance between the centers of
positive and negative charge density in OAn•+-OPV-C60•–, is the origin of the increase in lifetime for
OAn•+-OPV-C60•– compared to OAn-OPV•+-C60
•–.
The rate constants of the various photoinduced processes in solvents of different polarity have
been calculated using Eq. 2.4 relative to the corresponding rate constant in chlorobenzene by
assuming that V is independent on the solvent, and are collected in Table 2.4.
Table 2.4. Rate constants for the charge separation (CS), charge recombination (CR1), charge shift
(CSH), and charge recombination (CR2) in o-dichlorobenzene (ODCB) and benzonitrile (BZN) with
relative to the corresponding rate constant in chlorobenzene (CB), calculated using Eq. 2.4.
rate constant CS CR1 CSH CR2
k(CB) 1 1 1 1
k(ODCB) 2.00 89 0.58 6.25
k(BZN) 2.16 980 0.23 5.06
2.5 Photophysical processes in the solid state
2.5.1 Near steady state PIA spectroscopy
Near steady state PIA spectra of thin films of OPV-C60 and OAn-OPV-C60 were recorded at
80 K with excitation at 458 nm. The PIA spectrum of the OPV-C60 film (Figure 2.12) exhibits the
signals of OPV•+ at 0.68 and 1.52 eV and that of C60•– at 1.24 eV, characteristic of a charge-separated
state. The PIA spectrum of a film of OAn-OPV-C60 (Figure 2.12) lacks the bands at 0.68 and 1.52 eV
and exhibits only one broad band that peaks at 1.25 eV. This signal is attributed to the overlapping
transitions of the OAn•+ and C60•– radical ions and gives evidence for the formation of an
intramolecular (OAn-OPV•+-C60•–) or intermolecular (OAn•+-OPV-C60 / OAn-OPV-C60
•–) charge-
separated state. The charge-separated state in the OPV-C60 and OAn-OPV-C60 films, measured with
this PIA technique, extend into the millisecond time domain. The PIA band at 1.25 eV increases with
the pump intensity (I) following a square-root power law (–∆T/T ~ I0.5). This suggests a non-geminate
bimolecular recombination of the photoinduced charges. We propose that the long-lived (ms domain)
Photoinduced multistep energy and electron transfer in a molecular triad
39
charges observed with near steady state PIA in films of OPV-C60 and OAn-OPV-C60 at 80 K, are
associated with a small fraction of positive and negative charges that have escaped from geminate
recombination by charge migration to other sites in the film where they became trapped and are thus
associated with different molecules in the film (i.e. OAn•+-OPV-C60 and OAn-OPV-C60•–).
0.5 1.0 1.5 2.0
0.0
0.5
1.0
1.5
2.0
∆T
/T x
104
Energy (eV)
Figure 2.12. Photoinduced absorption spectra of OAn-OPV-C60 (solid line) and OPV-C60 (dotted
line) in thin films. Recorded at 80 K with excitation at 458 nm (25 mW) and a modulation frequency
of 275 Hz.
2.5.2 Femtosecond pump-probe spectroscopy
Femtosecond spectroscopy shows that in films of OPV-C60 and OAn-OPV-C60 at room
temperature a charge-separated state is formed within 0.5 ps, as evidenced by the instantaneous rise of
the 1450 nm differential transmission (Figure 2.13a) associated with OPV•+ radical cations. Although
a lack of higher time-resolution precludes an unambiguous conclusion, we have no evidence that a
singlet-energy transfer precedes the electron transfer reaction in the solid state. Possibly, the
photoinduced electron transfer in the solid state is predominantly intermolecular. The OPV•+-C60•–
state is much longer lived in the film than in solution (Table 2.3). This phenomenon has also been
observed in other donor-acceptor dyads and has been attributed to the migration of opposite charges to
different sites in the film.8h,9d,34 In contrast, the OPV•+ signal in the film of the triad rapidly decays,
with a time constant of 20 ps over the first 100 ps. This is interpreted to result from an intermolecular
or intramolecular charge shift (CSH) from OPV•+ to OAn•+. The time profile of the 1030 nm
differential transmission (Figure 2.13b) is consistent with the formation of an OAn•+-OPV-C60•– state
in the triad and explains the short lifetime of the OAn-OPV•+-C60•– state (Figure 2.13a). This time
profile cannot be fitted to monoexponential decay, and suggests several lifetimes. This fact can be
rationalized by a combination of a direct charge recombination and an indirect charge recombination
Chapter 2
40
after migration of the charges in the film.8h,9d It should be noticed however, that contrary to what is
observed for the OPV-C60 dyad, the OAn•+-OPV-C60•– state seems to be longer lived in solution than
the OAn•+-OPV-C60 / OAn-OPV-C60•– state in the film. As a tentative explanation for this difference
we propose that in solution the triads are isolated from each other such that the weak electronic
coupling between the OAn and the C60 decelerates the intramolecular charge recombination in OAn•+-
OPV-C60•–. In the film the triads are in intimate contact with each other, and intermolecular charge
recombination between OAn•+-OPV-C60 and OAn-OPV-C60•– can occur.
0 100 200 300 400 500 600-120
-80
-40
0
0 200 400 600 800 1000-100
-80
-60
-40
-20
0b
a
Time delay (ps)
∆T/T
(a.
u.)
∆T/T
(a.
u.)
Time delay (ps)
Figure 2.13. (a) Differential transmission dynamics at 1450 nm of thin films of OAn-OPV-C60 (open
circles) and OPV-C60 (closed circles) at 298 K with excitation at 450 nm. (b) Differential transmission
dynamics at 1030 nm of a thin film of OAn-OPV-C60 at 298 K with excitation at 450 nm.
2.6 Conclusions
A molecular triad, OAn-OPV-C60, with a redox gradient in a linear array has been
synthesized. The photophysical processes that may occur in this system are schematically depicted in
Figure 2.4 and have been investigated in solution and in thin films with photoluminescence and
transient absorption spectroscopy.
Photoinduced multistep energy and electron transfer in a molecular triad
41
In solution, photoexcitation of either chromophore of the OAn-OPV-C60 triad results in an
ultrafast (sequence of) singlet-energy transfer (ET, Figure 2.4) and provides a singlet state on the
fulleropyrrolidine unit (OAn-OPV-1C60*), irrespective of the polarity of the solvent. The competitive
process of charge separation from the primary 1OAn*-OPV-C60 or OAn-1OPV*-C60 states are more
than one order of magnitude slower and were not observed in the triad. In toluene, the singlet OAn-
OPV-1C60* state decays via intersystem crossing (ISC) to the OAn-OPV-3C60* triplet state and via PL
to the ground state. In more polar solvents, the singlet OAn-OPV-1C60* state gives rise to an
intramolecular charge separation reaction (CS) that generates the OAn-OPV•+-C60•– state. The rate for
this forward electron transfer reaction increases with the polarity of the solvent from kCS = 4.8 × 1010 s
-1
in chlorobenzene to kCS = 2.0 × 1011 s
-1 in benzonitrile, in qualitative agreement with Marcus theory.
Because the oxidation potential of the OAn segment is below that of the OPV unit, the primary OAn-
OPV•+-C60•– charge-separated state may undergo an intramolecular redox reaction, or charge shift
(CSH), to form OAn•+-OPV-C60•–. The charge recombination in OAn-OPV•+-C60
•– (CR1), however,
competes with the charge shift. Because the charge recombination is slowed down in less polar
solvents, the quantum yield for the charge shift is the highest (~0.4) in the least polar solvent in which
electron transfer occurs, i.e. chlorobenzene. The charge recombination in the secondary OAn•+-OPV-
C60•– charge-separated state (CR2) is significantly slower (kCR2 < 1 × 109
s-1) than that of the primary
OAn-OPV•+-C60•– state (kCR1 = 1.1 × 1010
s-1). The observed trends in the various rate constants (kCS,
kCSH, kCR1, kCR2, k´SC and k´CR) with changing solvent polarity are in qualitative agreement with Marcus
theory when the free energies of the charge-separated states are determined using a continuum model
(Eq. 2.1).
In thin films, charge generation on the OPV unit of OAn-OPV-C60 is much faster (kCS ≥ 3.0 ×
1012 s
-1) and likely predominantly intermolecular. In the films a subsequent charge-shift occurs from
the primary OPV•+ radical cation to an OAn•+ radical cation with a rate close to kCSH = 5.0 × 1010 s
-1.
Because we consider it likely that in the film the primary charge-separated state involves two
molecules, also the charge shift probably involves an intermolecular OAn-OPV•+-C60 → OAn•+-OPV-
C60 reaction, with the negative charge located on the fullerene unit of a third molecule (OAn-OPV-
C60•–). The lifetime of the charges formed in the film (OAn•+-OPV-C60 / OAn-OPV-C60
•–) is somewhat
less as compared to the solution.
The experiments on OAn-OPV-C60 demonstrate that in solution the OAn-OPV•+-C60•– →
OAn•+-OPV-C60•– charge shift and the resulting spatial extension of the charges increase the lifetime
of the charge-separated state compared to OAn-OPV•+-C60•–, because the electronic coupling between
the redox active groups is strongly reduced. This provides a rationale to explain the long-lifetime of
the charge-separated state in conjugated polymer:C60 blends in terms of charge migration. The major
differences in the kinetics of the electron transfer reactions observed after photoexcitation of OAn-
Chapter 2
42
OPV-C60 in solution or in thin films, further demonstrate that intermolecular interactions are of crucial
significance in this respect. Creating and investigating well-defined multichromophoric
supramolecular donor-acceptor assemblies, consisting of many judiciously positioned chromophores,
will enable a more detailed understanding of photoinduced charge-separation processes in natural and
artificial systems.
2.7 Experimental section
All reagents and solvents were used as received or purified using standard procedures. C60 was
purchased from BuckyUSA. NMR spectra were recorded on a Varian Unity Inova and a Varian Unity Plus at frequencies of 500 and 125 MHz for 1H and 13C nuclei; a Varian Mercury Vx at frequencies of 400 and 100 MHz for 1H and 13C nuclei or Varian Gemini 2000 at frequencies of 300 and 75 MHz for 1H and 13C nuclei, respectively. Tetramethylsilane (TMS) was used as an internal standard for 1H NMR and CDCl3, CD3COCD3 or CS2 for 13C NMR. Infrared (FT-IR) spectra were recorded on a Perkin-Elmer Spectrum One UATR FT-IR. Elemental analyses were preformed on a Perkin Elmer 2400 series II CHN Analyzer. Matrix assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was performed on a Perseptive DE PRO Voyager MALDI-TOF mass spectrometer using a dithranol matrix. All HPLC analyses were performed on a Hewlett Packard HP LC-Chemstation 3D (HP 1100 Series) with DAD detection using an Inertsil® 5 Si column (250x3 mm). A Shimadzu LC-10AT system combined with a Polymer Laboratories MIXED-D column (Particle size: 5µm; Length/I.D. (mm): 300 × 7.5) and UV detection was employed for size exclusion chromatography (SEC), using CHCl3 as an eluent (1 mL/min). (E,E)-4-{4-(4-methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy}-5-benzaldehyde-dimethylacetal (2). Amberlite IR 120 (1.5 g), trimethyl orthoformate (20 mL) and 1 (1.2 g, 1.78 mmol) where added to 100 mL methanol. The suspension was stirred under an argon atmosphere at 70 °C for 2 h. The reaction mixture was cooled to room temperature and 1.5 g of Na2CO3 was added. The suspension was filtered and the solvent was removed in vacuo to yield 1.38 g (91%) of 2, which was used without further purification. 1H NMR (CDCl3, 400 MHz): δ (ppm) 7.50 (d, 1H),7.49 (s, 2H), 7.44 (d, 1H), 7.18 (s, 1H), 7.17 (s, 1H), 7.16 (s, 1H), 7.10 (s, 1H), 7.08 (s, 1H), 6.73 (s, 1H), 3.92-3.74 (m, 12H), 3.42 (s, 6H), 2.24 (s, 3H), 1.98-1.88 (m, 6H), 1.69-1.54 (m, 6H), 1.39-1.26 (m, 6H), 1.10-0.96 (m, 36 H); 13C NMR (CDCl3, 100 MHz): δ (ppm) 151.69, 151.10, 150.96, 150.67, 150.45, 128.17, 127.71, 127.57, 126.98, 126.56, 125.21, 123.19, 123.06, 122.54, 121.71, 116.32, 111.80, 110.14, 109.94, 109.26, 108.39, 99.75, 74.69, 74.35, 74.28, 74.23, 73.73, 73.38, 54.34, 35.11, 35.08, 35.05, 34.96, 34.89, 26.36, 26.26, 26.21, 16.79, 16.70, 16.40, 11.46, 11.37, 11.33. N-(4-Diphenylaminophenyl)-N-phenyl-3-aminobenzaldehyde (4). To a tube fitted with a magnetic stirrer was added 3 (0.8 g, 2.38 mmol), 3-bromobenzaldehyde (1.32 g, 7.14 mmol), Pd2(dba)3 (0.022g, 0.024 mmol), BINAP (0.044 g, 0.071 mmol) and Cs2CO3 (1.16 g, 3.57 mmol). After purging with argon, freshly distilled toluene (11.9 mL) was added. The reaction mixture was heated at 100 °C under Ar atmosphere. After 48 h Pd2(dba)3 (0.022g, 0.024 mmol), BINAP (0.044 g, 0.071 mmol) and Cs2CO3 (1.16 g, 3.57 mmol) were added and the mixture was heated for another 72 h. After cooling to room temperature, the reaction mixture was filtered over Celite 545 and concentrated in vacuo. Column chromatography (SiO2, heptane/CH2Cl2 1:1, Rf = 0.4) and evaporation to dryness from heptane yielded 0.643 g (62 %) of a yellow powder. IR (UATR) ν (cm-1) 3034, 2923, 2851, 1698, 1583, 1485, 1277, 1263, 749, 690, 626. 1H NMR (CDCl3, 400 MHz): δ (ppm) 9.92 (s, 1H), 7.56 (t, 1H), 7.43(dt, 1H), 7.37 (t, 1H), 7.34-7.23 (m, 7H), 7.13-7.11 (m, 6H), 7.08-6.96 (m, 7H); 13C NMR (CDCl3, 100 MHz): δ (ppm) 192.24, 149.14, 148.00, 147.45, 144.17, 142.11, 137.91, 129.98, 129.70, 129.45, 128.18, 126.37, 125.41, 124.62, 124.20, 123.67, 123.07, 122.91, 122.66. Anal. Cald for C31H24N2O: C, 84.5; H, 5.5; N, 6.4. Found: C, 84.1; H, 5.1; N, 6.1. (E)-N,N’-(Diphenyl)-N’-(4-diphenylaminophenyl)-3-aminobenzaldimine (5). To a suspension of 4 (0.55 g, 1.24 mmol) in ethanol (50 mL), was added aniline (0.14g, 1.50 mmol). The reaction mixture was heated at 85 °C for 4 h. After cooling to room temperature the product precipitated slowly
Photoinduced multistep energy and electron transfer in a molecular triad
43
from the ethanol. The product was obtained as 0.508 g (79%) of a yellow powder after washing with ethanol. IR (UATR) ν (cm-1) 3034, 1628, 1589, 1486, 1267, 751, 693. 1H NMR (CDCl3, 400 MHz): δ (ppm) 8.34 (s, 1H), 7.61 (t, 1H), 7.53 (dt, 1H), 7.38-7.31(m, 3H), 7.28-7.10 (m, 16H), 7.03-6.97 (m, 7H); 13C NMR (CDCl3, 100 MHz): δ (ppm) 160.31, 151.99, 148.47, 147.78, 147.56, 143.29, 142.34, 137.40, 129.62, 129.33, 129.19, 129.09, 126.32, 125.91, 125.66, 125.26, 123.95, 123.88, 123.53, 122.82, 122.52, 122.44, 120.85. Anal. Cald for C37H29N3: C, 86.2; H, 5.7; N, 8.1. Found: C, 85.8; H, 5.3; N, 8.0. MALDI-TOF MS (Mw = 515.65) m/z = 515.23 [M]+. (E,E,E)-4-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-3-aminostyryl]2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]benzaldehyde (6). Schiff base 5 (0.450 g, 0.873 mmol) and acetal 2 (0.775 g, 0.873 mmol) were dissolved in DMF (5 mL). The mixture was heated to 80 °C under an argon atmosphere and potassium tert-butoxide (0.352 g, 3.142 mmol) was added. The reaction mixture was stirred for 3 h. After cooling to room temperature, the reaction mixture was poured on ice, washed with HCl 3N and brine, dried over MgSO4 and concentrated in vacuo. Column chromatography (SiO2, toluene/cyclohexane 7:3 Rf = 0.4, and heptane/CH2Cl2 6:2, Rf = 0.2) and evaporation to dryness from heptane yielded 0.795 g (72%) of an orange powder. IR (UATR) ν (cm-1) 3062, 2960, 2917, 2873, 1675, 1589, 1504, 1490, 1422, 1262, 1200, 1039, 968, 744, 693. 1H NMR (CDCl3, 500 MHz): δ (ppm) 10.43(s, 1H), 7.64 (d, 1H), 7.54 (d, 1H), 7.51 (d, 1H), 7.50 (d, 1H), 7.40 (d, 1H), 7.33 (s, 1H), 7.28-6.98 (m, 29H), 3.79 3.98 (m, 12H), 1.98-1.91 (m, 6H), 1.70-1.57 (m, 6H), 1.39-1.31 (m, 6H), 1.13-0.94 (m, 36H); 13C NMR (CDCl3, 100 MHz): δ (ppm) 189.06, 156.43, 151.44, 151.26, 151.04, 150.64, 148.21, 147.84, 142.91, 142.70, 139.13, 135.22, 129.45, 129.19, 129.18, 128.50, 128.45, 127.45, 126.74, 126.48, 126.42, 125.46, 125.30, 123.97, 123.79, 123.74, 123.56, 123.19, 122.98, 122.56, 122.43, 122.40, 122.21, 121.90, 120.29, 110.64, 110.29, 110.17, 109.82, 109.74, 109.64, 74.39, 74.25, 74.11, 74.05, 73.91, 73.69, 35.13, 35.01, 34.93, 34.88, 34.84, 26.38, 26.34, 26.33, 26.18, 16.88, 16.86, 16.81, 16.76, 16.63, 11.52, 11.46, 11.38, 11.32. Analysis Calcd for C85H102N2O7 C, 80.8; H, 8.1; N, 2.2. Found: C, 80.7; H, 7.6; N, 2.2. MALDI-TOF MS (Mw = 1263.75) m/z = 1263.71 [M]+. N-Methyl-2<4-[4-{4-[N’-(4-Diphenylaminophenyl)-N’-(phenyl)]3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-methylbutoxy]>-3,4-fulleropyrrolidine (7). A solution of aldehyde 6 (0.2 g, 0.158 mmol), finely ground N-methylglycine (84.4 mg, 0.949 mmol) and C60 (227 mg, 0.316 mmol) in chlorobenzene (60 mL) was stirred and refluxed in the dark under an atmosphere of dry nitrogen for 18 h. After cooling to room temperature the solvent was removed in vacuo and the remaining residue was purified by column chromatography on silica gel (toluene/cyclohexane: 2/1, Rf = 0.4) to afford triad 7 as a ~1/1 mixture of diastereomers according to HPLC (eluent: toluene/cyclohexane 50/50 v/v; flow: 1 mL/min.; peaks at tr = 9.1 and 10.3 min.). Traces of impurities were effectively removed after a 2nd chromatographic purification on silica gel (CS2/toluene: 1/0 to 7/3 Rf = 0.3) to afford analytically pure material (assay >99.5%) according to HPLC and GPC analysis. The product was precipitated from a concentrated toluene solution with methanol (100 mL) and the resulting solid was washed with methanol (2 × 100 mL) and finally dried in vacuo at 55 ºC. The triad was obtained as a light brown powder (137 mg, 43%). IR (UATR) ν (cm-1) 2957, 2914, 2872, 1589, 1502, 1491, 1264, 1190, 965, 750, 694, 526. 1H NMR (CS2, 500 MHz): δ (ppm) 7.61-6.96 (m, 35H), 5.62 (s, 1H), 5.05 (d, 1H), 4.41 (d, 1H), 4.15-3.70 (m, 12H), 2.92 (s, 3H), 2.16-1.26 (m, 18H), 1.02-1.26 (m, 36H). 13C NMR (CS2, 125 MHz): δ (ppm) 156.50, 154.91, 154.84, 154.18, 154.15, 153.53, 151.61, 150.93, 150.76, 150.71, 150.67, 150.63,147.79, 147.40, 147.06, 146.60, 146.57, 146.52, 146.13, 146.07, 146.03, 146.00, 145.93, 145.89, 145.85, 145.75, 145.50, 145.47, 145.37, 145.26, 145.19, 145.07, 145.04, 145.01, 144.98, 144.91, 144.50, 144.37, 144.31, 144.15, 142.92, 142.86, 142.51, 142.46, 142.43, 142.40, 142.35, 142.12, 142.10, 142.06, 141.99, 141.98, 141.95, 141.92, 141.79, 141.63, 141.53, 141.51, 140.06, 139.98, 139.69, 139.51, 139.07, 136.35, 135.99, 135.88, 135.86, 134.49, 134.44, 129.42, 129.20, 129.17, 127.99, 127.68, 127.66, 127.40, 127.27, 126.88, 126.30, 125.24, 125.14, 124.85, 124.80, 123.79, 123.70, 123.61, 123.29, 122.69, 122.56, 122.52, 122.47, 122.29, 121.96, 120.39, 114.29, 114.16, 110.04, 109.63, 109.40, 109.22, 108.71, 108.61, 76.53, 75.43, 75.40, 73.97, 73.91, 73.69, 73.59, 73.52, 73.44, 72.95, 72.91, 69.70, 68.98, 40.02, 35.41, 35.31, 35.29, 35.25, 35.20, 35.18, 35.17, 26.86, 26.84, 26.80, 26.49, 17.14, 17.09, 17.06, 17.02, 17.00, 16.80, 12.04, 12.03, 12.00, 11.94, 11.89, 11.85. Analysis Calcd for C147H107N3O6 C, 87.7; H, 5.4; N, 2.1. Found: C, 87.8; H, 5.0; N, 2.1. MALDI-TOF MS (Mw = 2011.44) m/z = 2011.18 [M]+. (E,E)-2-{4-(4-methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-2,5-bis[(S)-2-methylbutoxy]styryl}-1,4-bis[(S)-2-methylbutoxy]-5-deuteriobenzene (9). n-BuLi 1.6 M (0.13 mL, 0.21 mmol) was added dropwise to a solution of 8 (0.15 g, 0.16 mmol) in freshly distilled diethyl ether (3 mL) at –10° C. The reaction mixture was stirred for 5 minutes. After the addition of
Chapter 2
44
D2O (0.5 mL) the cooling bath was removed and the reaction mixture was stirred for 2 h at room temperature. The mixture was estracted with diethyl ether dried over MgSO4 and concentrated in vacuo. Column (SiO2; heptane/CH2Cl2: 1/1, Rf = 0.3) and recrystallization from ethanol yielded 54 mg (41%) of the product as yellow crystals. 1H NMR (CDCl3, 400 MHz): δ (ppm) 7.49 (d, 1H), 7.48 (s, 2H), 7.44 (d, 1H), 7.19(s, 1H), 7.18 (s, 1H), 7.17 (s, 1H), 7.10 (s, 1H), 6.82 (s, 1H), 6.72(s, 1H), 3.92-3.71 (m, 12H), 1.99-1.82 (m, 6H), 1.68-1.54 (m, 6H), 1.38-1.23 (m, 6H), 1.20-0.86 (m, 36H); 13C NMR (CDCl3, 100 MHz): δ (ppm) 151.68, 151.14, 150.94, 150.90, 150.44, 128.21, 127.61, 127.54, 127.10, 125.23, 123.24, 122.98, 122.82, 121.74, 116.31, 113.96, 111.58, 110.36, 109.85, 108.34, 74.68, 74.46, 74.30, 74.26, 73.59, 73.38, 35.11, 35.06, 35.04, 34.97, 34.80, 26.36, 26.34, 26.26, 26.19, 16.80, 16.70, 16.56, 16.40, 11.46, 11.40, 11.37, 11.31. Anal. Cald for C53H79DO6: C, 77.8; H, 10.2. Found: C, 77.5; H, 9.1. MALDI-TOF MS (Mw = 813.96) m/z = 813.53[M]+.
(E,E,E)-2-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl]-1,4-bis[(S)-2-methylbutoxy]-5-deuteriobenzene (10). Schiff base 5 (0.025 g, 0.049 mmol) and 9 (0.40 g, 0.049 mmol) were dissolved in DMF (5 mL). The mixture was heated at 80 °C under an argon atmosphere and potassium tert-butoxide (0.020 g, 0.176 mmol) was added. The reaction mixture was stirred for 5 h at 80 °C. After cooling to room temperature, the reaction mixture was poured onto ice and washed with HCl 3N and brine. The organic layer was dried over MgSO4 and concentrated in vacuo. Column chromatography (SiO2, toluene/cyclohexane 7:3, Rf = 0.6) and recyrstallization from hexane, heptane and a few drops of CH2Cl2 yielded 30 mg of 10 (50%) as a yellow powder. IR (UATR) ν (cm-1) 2960, 2917, 2873, 1588, 1504, 1489, 1255, 1205, 1042, 970, 744, 693. 1H NMR (CD3COCD3, 400 MHz): δ (ppm) 7.61 (s, 2H), 7.60 (d, 1H), 7.55 (d, 1H), 7.47(d, 1H), 7.35-6.95 (m, 30H), 4.01-3.76 (m, 12H), 2.09-1.83 (m, 6H), 1.74-1.55 (m, 6H), 1.45-1.26 (m, 6H), 0.95-1.15 (m, 36H); 13C NMR (100 MHz): δ (ppm) 155.25, 152.89, 152.84, 152.72, 150.10, 149.58, 149.54, 144.87, 144.58, 141.06, 131.32, 131.03, 130.98, 130.19, 129.47, 129.13, 128.96, 128.91, 128.15, 127.30, 127.04, 125.39, 125.35, 125.16, 124.93, 124.87, 124.73, 124.68, 124.37, 124.34, 124.30, 123.17, 122.16, 115.71, 113.14, 112.34, 111.95, 111.58, 111.39, 75.61, 75.56, 75.53, 75.36, 74.72, 36.76, 36.68, 36.63, 36.44, 27.85, 27.81, 27.60, 18.04, 17.98, 17.95, 17.92, 17.60, 12.63, 12.60, 12.58, 12.52, 12.39. Anal. Cald for C85H102N2O7: C,81.6; H, 8.2; N, 2.3. Found: C, 81.2; H, 7.7; N, 2.2. MALDI-TOF MS (Mw = 1236.74) m/z = 1235.62 [M]+.
Electrochemistry. Cyclic voltammograms were measured in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6) as a supporting electrolyte in dichloromethane (or THF) using a Potentioscan Wenking POS73 potentiostat. The working electrode was a Pt disk (0.2 cm2), the counter electrode was a Pt plate (0.5 cm2), and a saturated calomel electrode (SCE) was used as reference electrode, calibrated against Fc/Fc+ (+0.43 V).
Absorption and Photoluminescence. UV/visible/near-IR absorption spectra were recorded on a Perkin Elmer Lambda 900 spectrophotometer. Fluorescence spectra were recorded on an Edinburgh Instruments FS920 double-monochromator spectrometer and a Peltier-cooled red-sensitive photomultiplier. Time-correlated single photon counting. Time-correlated single photon counting fluorescence studies were performed using an Edinburgh Instruments LifeSpec-PS spectrometer. The LifeSpec-PS comprises a 400 nm picosecond laser (PicoQuant PDL 800B) operated at 2.5 MHz and a Peltier-cooled Hamamatsu micro-channel plate photomultiplier (R3809U-50). Lifetimes were determined from the data using the Edinburgh Instruments software package.
Near steady state PIA. Solutions were prepared in a nitrogen-filled glove box in order to exclude interference of oxygen during measurements. The PIA spectra were recorded between 0.5 and 3.0 eV by exciting with a mechanically modulated cw Ar ion laser (λ = 458 or 528 nm, 275 Hz) pump beam and monitoring the resulting change in transmission of a tungsten-halogen probe light through the sample (∆T) with a phase-sensitive lock-in amplifier after dispersion by a grating monochromator and detection, using Si, InGaAs, and cooled InSb detectors. The pump power incident on the sample was typically 25 mW with a beam diameter of 2 mm. The PIA (-∆T/T ≈ ∆αd) was directly calculated from the change in transmission after correction for the PL, which was recorded in a separate experiment. PIA and PL spectra were recorded with the pump beam in a direction almost parallel to the direction of the probe beam. The solutions were studied in a 1 mm near-IR grade quartz cell at room temperature. Solvents for PIA measurements were distilled under nitrogen before use. The solid-state measurements were performed on films, drop cast from chloroform solution, on quartz substrate and held at 80 K in an Oxford continuous flow cryostat.
Photoinduced multistep energy and electron transfer in a molecular triad
45
Transient subpicosecond photoinduced absorption. The femtosecond laser system used for pump-probe experiments consists of an amplified Ti/sapphire laser (Spectra Physics Hurricane). The single pulses from a cw mode-locked Ti/sapphire laser were amplified by a Nd-YLF laser using chirped pulse amplification, providing 150 fs pulses at 800 nm with an energy of 750 µJ and a repetition rate of 1 kHz. The pump pulses at 450 nm were created via optical parametric amplification (OPA) of the 800 nm pulse by a BBO crystal into infrared pulses which were then two times frequency doubled via BBO crystals. The probe beam was generated in a separate optical parametric amplification set-up in which 1030 and 1450 nm pulses were created. The pump beam was focused to a spot size of about 1 mm2 with an excitation flux of 1 mJ cm-2 per pulse. For the 1030 and 1450 nm pulses a RG 850 nm cut-off filter was used to avoid contributions of residual probe light (800 nm) from the OPA. The probe beam was reduced in intensity compared to the pump beam by using neutral density filters of OD = 2. The pump beam was linearly polarized at the magic angle of 54.7° with respect to the probe, to cancel out orientation effects in the measured dynamics. The temporal evolution of the differential transmission was recorded using Si or an InGaAs detector by a standard lock-in technique at 500 Hz. Solutions in the order of 2-5 × 10-4 M were excited at 450 nm, i.e. providing primarily excitation of the OPV part within the molecules. 2.8 References and notes 1 (a) The Photosynthetic Reaction Center, Deisenhofer, J., Norris, J. R., Eds.; Academic Press: New York,
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9 Oligo(p-phenylene vinylene)-fullerene dyads: (a) Nierengarten, J. F.; Eckert, J. F.; Nicoud, J. F.; Ouali,
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Hadziioannnou, G. J. Am. Chem. Soc. 2000, 122, 7467. (c) Armaroli, N.; Barigelletti, F.; Ceroni, P.;
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Raimundo, J.-M.; Roncali, J.; Echegoyen, L. Chem. Commun. 2001, 913. (c) Martineau, C.; Blanchard,
P.; Rondeau, D.; Delaunay, J.; Roncali, J. Adv. Mater. 2002, 14, 283. (d) Apperloo, J. J.; Martineau, C.;
van Hal, P. A.; Roncali, J.; Janssen, R. A. J. J. Phys. Chem. A 2002, 106, 21.
11 Oligoene-fullerene dyads: (a) Imahori, H.; Cardoso, S.; Tatman, D.; Lin, S.; Noss, L.; Seely, G. R.;
Sereno, L.; Chessa de Silber, J.; Moore, T. A.; Moore, A. L.; Gust, D. Photochem. Photobiol. 1995, 62,
1009. (b) Yamazaki, M.; Araki, Y.; Fuijtsuha, M.; Ito, O. J. Phys. Chem. A. 2001, 105, 8615.
12 Miscellaneous: oligomer-fullerene dyads: (a) Segura, J. L.; Gómez, R.; Martín, N.; Luo, C.; Guldi, D. M.
Chem. Commun. 2000, 701. (b) Guldi, D. M.; Swartz, Luo, C.; Gómez, R.; Segura, J. L.; Martín, N. J.
Am. Chem. Soc. 2002, 124, 10875. (c) Guldi, D. M.; Luo, C.; Schwartz, A.; Gómez, R.; Segura, J. L.;
Martin, N.; Brabec, C. J.; Sariciftci, N. S. J. Org. Chem. 2002, 67, 1141. (d) Gu, T.; Tsamouras, D.;
Melzer, C.; Krasnikov, V.; Gisselbrecht, J.-P.; Gross, M.; Hadziioannou, G.; Nierengarten, J.-F. Chem.
Phys. Chem. 2002, 124;
13 van Hal, P. A.; Beckers, E. H. A.; Meskers, S. C. J.; Janssen, R. A. J.; Jousselme, B.; Blanchard, P.;
Roncali, J. Chem. Eur. J. 2002, 8, 5415.
14 For recent reviews see: (a) Gust, D.; Moore, T. A.; Moore, A. L. Acc. Chem. Res. 2001, 34, 40. (b) Guldi,
D. M. Chem. Soc. Rev. 2002, 31, 22.
15 (a) Liddell, P. A.; Kuciauskas, D.; Sumida, J. P.; Nash, B.; Nguyen, D.; Moore, A. L.; Moore, T. A.;
Gust, D. J. Amer. Chem. Soc. 1997, 119, 1400. (b) Imahori, H.; Yamada, K.; Hasegawa, M.; Taniguchi,
S.; Okada, T.; Sakata, Y. Angew. Chem., Int. Ed. Engl. 1997, 36, 2626. (c) Carbonera, D.; Di Valentin,
M.; Corvaja, C.; Agostini, G.; Giacometti, G.; Liddell, P. A.; Kuciauskas, D.; Moore, A. L.; Moore, T.
A.; Gust, D. J. Amer. Chem. Soc. 1998, 120, 4398. (d) Kuciauskas, D.; Liddell, P. A.; Moore, A. L.;
Moore, T. A.; Gust, D. J. Amer. Chem. Soc. 1998, 120, 10880. (e) Tamaki, K.; Imahori, H.; Sakata, Y.;
Nishimura, Y.; Yamazaki, I. Chem. Commun. 1999, 625. (f) Imahori, H.; Yamada, H.; Ozawa, S.; Sakata,
Y.; Ushida, K. Chem. Commun. 1999, 1165. (g) Fujitsuka, M.; Ito, O.; Imahori, H.; Yamada, K.;
Photoinduced multistep energy and electron transfer in a molecular triad
47
Yamada, H.; Sakata, Y. Chem. Lett. 1999, 721. (h) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki,
I.; Sakata, Y. J. Phys. Chem. B 2000, 104, 2099. (i) Kuciauskas, D.; Liddell, P. A.; Lin, S.; Stone, S. G.;
Moore, A. L.; Moore, T. A.; Gust, D. J. Phys. Chem. B 2000, 104, 4307. (j) Luo, C.; Guldi, D. M.;
Imahori, H.; Tamaki, K.; Sakata, Y J. Amer. Chem. Soc. 2000, 122, 6535. (k) Imahori, H.; Tamaki, K.;
Yamada, H.; Yamada, K.; Sakata, Y.; Nishimura, Y.; Yamazaki, I.; Fujitsuka, M.; Ito, O. Carbon 2000,
38, 1599. (l) Bahr, J. L.; Kuciauskas, D.; Liddell, P. A.; Moore, A. L.; Moore, T. A.; Gust, D. Photochem.
Photobiol. 2000, 72, 598. (m) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.;
Sakata, Y.; Fukuzumi, S. J. Amer. Chem. Soc. 2001, 123, 100. (n) Fukuzumi, S.; Imahori, H.; Yamada,
H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Guldi, D. M. J. Amer. Chem. Soc. 2001, 123, 2571. (o)
Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Amer.
Chem. Soc. 2001, 123, 2607. (p) Imahori, H.; Guldi, D. M.; Tamaki, K.; Yoshida, Y.; Luo, C.; Sakata, Y.;
Fukuzumi, S. J. Amer. Chem. Soc. 2001, 123, 6617. (q) Fukuzumi, S.; Imahori, H.; Okamoto, K.;
Yamada, H.; Fujitsuka, M.; Ito, O.; Guldi, D. M. J. Phys. Chem. A 2002, 106, 1903. (r) Liddell, P. A.;
Kodis, G.; De la Garza, L.; Bahr, J. L.; Moore, A. L.; Moore, T. A.; Gust, D. Helv. Chim. Acta 2001, 84,
2765. (s) Ikemoto, J.; Takimiya, K.; Aso, Y.; Otsubo, T.; Fujitsuka, M.; Ito, O. Org. Lett. 2002, 4, 309. (t)
Imahori, H.; Tamaki, K.; Araki, Y.; Hasobe, T.; Ito, O.; Shimomura, A.; Kundu, S.; Okada, T.; Sakata,
Y.; Fukuzumi, S. J. Phys. Chem. A 2002, 106, 2803. (u) Imahori, H.; Tamaki, K.; Araki, Y.; Sekiguchi,
Y.; Ito, O.; Sakata, Y.; Fukuzumi, S. J. Amer. Chem. Soc. 2002, 124, 5165. (v) D'Souza, F.; Deviprasad,
G. R.; Zandler, M. E.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O. J. Phys. Chem. B 2002, 106, 4952. (w)
Liddell, Paul A.; Kodis, Gerdenis; Moore, Ana L.; Moore, Thomas A.; Gust, Devens. J. Amer. Chem.
Soc. 2002, 124, 7668. (x) Kodis, G.; Liddell, P. A.; de la Garza, L.; Moore, A. L.; Moore, T. A.; Gust, D.
J. Mater. Chem. 2002, 12, 2100. (y) Sánchez, L.; Pérez, I.; Martín, N.; Guldi, D. M. Chem. Eur. J. 2003,
9, 2457.
16 Strohrieghl, P.; Jesberger, G.; Heinze, J.; Moll, T. Makromol. Chem. 1992, 193, 909.
17 (a) Siegrist, A. E. Helv. Chim. Acta 1967, 50, 906. (b) Siegrist, A. E.; Meyer, H. R.; Weber, K. Helv.
Chim, Acta 1969, 52, 2521.
18 The bromine has been replaced by a deuterium atom in order to simplify the 1H-NMR spectrum of the
molecule.
19 Murata, Y.; Shine, H. J. J. Org. Chem. 1969, 34, 3368.
20 van Meurs, P. J. High-Spin Molecules of p-Phenylenediamnine Radical Cations, PhD Thesis, Eindhoven
University of Technology, 2002, ISBN 90-386-2574-X.
21 van Hal, P. A.; Beckers, E. H. A.; Peeters, E.; Apperloo, J. J.; Janssen, R. A. J. Chem. Phys. Lett. 2000,
328, 403.
22 Weller, A. Z. Phys. Chem. Neue Folge 1982, 133, 93.
23 The assumption that charges are located at the centers, is of course a simplification of the actual situation
in which charges are delocalized. Especially for C60, the charge is not expected to be at the center, but
rather at the outer surface.
24 Williams, R. M.; Zwier, J. M.; Verhoeven, J. W. J. Am. Chem. Soc. 1995, 117, 4093.
Chapter 2
48
25 The PL lifetime of the residual emission at 510 nm contained a contribution of long lifetime (~ 1 ns) and
is in part attributed to residual PL emission by minor impurities in the sample.
26 In toluene the energy level of initial charge separated state [OAn-OPV•+-C60•–] is higher than that of
OAn-OPV-1C60* due to the destabilization in the nonpolar solvent. In such case one can also expect that
initial electron transfer from 1OPV* to C60 occurs to yield OAn-OPV•+-C60•–, which subsequently
recombines to generate OAn-OPV-1C60*. The stepwise pathway would compete with the direct energy
transfer pathway from 1OPV* to C60 to produce the same state. However, in view of the high rate of the
energy transfer (190 fs) it is very unlikely that this process occurs and the same rate of 190 fs has been
found in ODCB, where the OAn-OPV•+-C60•– state is below the OAn-OPV-1C60* state (Ref 9e).
27 The function f(t)= a0(exp(-a1(t-a3) - exp(-a2(t-a3)) + a4) was used to fit the data and all 5 parameters (a0 –
a4) were optimized.
28 (a) Wasielewski, M. R.; Niemczyk, M.P.; Svec, W. A.: Pewitt, E. B. J. Am. Chem. Soc. 1985, 107, 1080.
(b) Asahi, T.; Ohkohchi, M.; Matsusaka, R.; Mataga, N.; Zhang, R. P.; Osuka, A.; Maruyama, K. J. Am.
Chem. Soc. 1993, 115, 5665. (c) Macpherson, A. N.; Liddell, P. A.; Lin, S.; Noss, L.; Seely, G. R.;
DeGraziano, J. M.; Moore, A. L.; Moore, T. A.; Gust, D. J. Am. Chem. Soc. 1995, 117, 7202. (d) Kroon,
K.; Verhoeven, J. W.; Paddon-Row, M. N.; Oliver, A. M. Angew. Chem Int. Ed. Engl. 1991, 30, 1358. (e)
Imahori, H.; El-Khouly, M. E.; Fujitsuka, M.; Ito, O.; Sakata, Y.; Fukuzumi S. J. Phys. Chem. A 2001,
105, 325.
29 Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695.
30 The following kinetic scheme has been used to model the photoinduced absorption at 1030 nm. In this
scheme, the intramolecular recombination of charges in OAn+-OPV-C60– and also charge generation from
electronic states other than the singlet-excited state of the C60 moiety have been neglected.
CRCOPV
kkk
kkk
CSHCSET
↓↓↓
−− →−−→→ −•+•−•+•
6000
60601601 COPVOAnCOPVOAn)S(C)OPV(S
For this model the following set of coupled differential equations can be written describing the change in
the concentration of each of the four species involved over time.
−−−−
+−+−+
+−
=
−−−−
−•+•
−•+•
−•+•
−•+•
]COPV[OAn
]COPV[OAn
)]S([C
)][OPV(S
000
0)(0
00)(
000)(
]COPV[OAn
]COPV[OAn
)]S([C
)][OPV(S
60
60
160
1
1
1
0
0
60
60
160
1
60
CR
CSHCRCS
CSC
ET
ETOPV
k
kkk
kkk
kk
dt
d
The singlet-excited state of the OPV is assumed to be generated by photon absorption at time t = 0 and
thus the relevant boundary value condition is:
Photoinduced multistep energy and electron transfer in a molecular triad
49
==
−−−−
−•+•
−•+•
0
0
0
)](0)[OPV(S
)0(
]COPV[OAn
]COPV[OAn
)]S([C
)][OPV(S 1
60
60
160
1
t
A solution to the above set of equations is:
))(exp()0]()OPV(S[))](OPV(S[ 011 tkkt ETOPV +−=
[ ]))(exp())(exp()0]()OPV(S[)]()(SC[ 60
6000
001160 tkktkk
kkkk
kt C
CSETOPV
CCSET
OPVET +−++−−
−−+=
[ ]
[ ]
[ ]
+−
−−+−−+
++−−−+−−+
−
+
+−
−−+−−+
=−− −•+•
))(exp())((
))(exp())((
))(exp())((
)0)](OPV(S[
)](COPVOAn[
1
1010
0
1000
0
1000
1
60
60
60
6060
60
tkkkkkkkkkk
kk
tkkkkkkkkkk
kk
tkkkkkkkkkk
kk
t
CSHCR
CSHCRETOPV
CSHCRC
CS
CSET
CCS
CSHCRC
CSC
CSETOPV
CSET
ETOPV
CSHCRETOPVC
CSETOPV
CSET
[ ]
[ ]
[ ]
+−−+−−+
++−−+−−+
−
+
+−−+−−+
++−
+−−+−−+−
++−+−−+−−+
+
+−
+−−+−−+−
×=−− −•+•
))()((
))()((
))()((
))(exp())()((
)(exp())()((
))(exp())()((
)0)](OPV(S[)](COPVOAn[
11000
01000
11000
1
11000
0
01000
0
1000
160
60
606060
60
60
60
606060
60
CSHCRCSHCRETOPVC
CSETOPV
CSHCSET
CCSCSHCR
CCS
CCSET
OPVCSHCSET
CSHCRCSHCRETOPVC
CSETOPV
CSHCSET
CSHCR
CSHCRCSHCRETOPVC
CSETOPV
CSHCSET
CCSC
CSCSHCRC
CSC
CSETOPV
CSHCSET
ETOPV
CSHCRCSHCRETOPVC
CSETOPV
CSHCSET
kkkkkkkkkk
kkk
kkkkkkkkkk
kkk
kkkkkkkkkk
kkk
tkkkkkkkkkkkk
kkk
tkkkkkkkkkkkk
kkk
tkkkkkkkkkkkk
kkk
t
In these equations it can be verified quite easily that they satisfy the boundary condition at t = 0.
Furthermore, setting kCS = 0, the yield of C60(S1) (abbrv. φ(C60(S1))) is given by kET/( kET + k0OPV).
Similarly, setting kCR1 = 0, the yield of OAn-OPV•+-C60•– is φ(C60(S1)) kCS /( kCS + k0
C60). Finally, the yield
of OAn• +-OPV-C60•– is: φ (C60(S1)) φ(OAn-OPV•+-C60
•–) kCSH /( kCSH + kCR1).
Chapter 2
50
31 Deussen, M; Bässler, H. Chem. Phys. 1992, 164, 247.
32 Jortner, J. J. Chem. Phys. 1976, 64, 4860.
33 Imahori et. al. reported that the electronic coupling in a triad (0.019 cm-1) is much less compared to that
in a dyad (3.9 cm-1), see ref. 15o.
34 (a) Thomas, K. G.; Biju, V.; Guldi, D. M.; Kamat, P. V. George, M. V. J. Phys. Chem. B 1999, 103,
8864. (b) Imahori, H.; Hasobe, T.; Yamada, H.; Kamat, P. V.; Barazzouk, S.; Fujitsuka, M.; Ito, O.;
Fukuzumi, S. Chem. Lett. 2001, 784.
Chapter 3
Photoinduced multistep electron transfer in
oligoaniline - oligo(p-phenylene vinylene) -
perylene arrays
Abstract
Multistep energy and electron transfer have been studied in solutions containing two different OAn-
OPV-PERY-OPV-OAn pentads, corresponding to a donor(1)-donor(2)-acceptor-donor(2)-donor(1)
symmetrical arrangement. The pentads have been synthesized by double coupling of an oligoaniline-
oligo(p-phenylene vinylene) (OAn-OPV) dyad to a central perylene diimide (PERY) segment, directly
connected (pentad 1) and via a saturated spacer (pentad 2). Photoexcitation of any of the
chromophores yields in first instance the OAn-OPV•+-PERY•—-OPV-OAn charge separated state. This
state can relax to the ground state via charge recombination or generate the OAn•+-OPV-PERY•–-
OPV-OAn state via a charge shift. In pentad 1, the intramolecular charge separation and charge
recombination are extremely fast. The same processes are slowed down in pentad 2, because the
saturated spacer reduces the electronic coupling between the OPV donor and PERY acceptor. For
pentad 1 in toluene, the OAn-OPV•+-PERY•–-OPV-OAn and the OAn•+-OPV-PERY•–-OPV-OAn states
are nearly isoenergetic and thus there is no driving force for the charge shift process in this apolar
solvent. In more polar media, the charge shift competes with the fast charge recombination to the
ground state with an estimated efficiency of 0.22. The slower charge recombination of OAn-OPV•+-
PERY•–-OPV-OAn to the ground state in pentad 2, with respect to that in pentad 1, is beneficial for
the charge shift process that occurs with an efficiency of 0.28 in polar media. The low electronic
coupling between the OAn and PERY chromophores accounts for the remarkably long-lived OAn•+-
OPV-PERY•–-OPV-OAn geminate ion pair (τ > 1 ns).
Chapter 3
52
3. 1 Introduction
Fuelled by intriguing applications in molecular electronics, light harvesting, photocatalysis,
and artificial photosynthesis, photoinduced energy and electron transfer in covalently linked donor-
acceptor molecules attract enormous attention in recent years.1 For practical applications it is often of
interest to combine a high rate for charge separation with a low rate for charge recombination. For this
purpose, multichromophoric arrays have been designed in which the initial photoinduced charge
separation reaction is followed by charge migration reactions along a well-defined redox gradient that
ultimately provides the spatial separation of the photogenerated charges, which is essential to lower
the rate for charge recombination. Various elegantly designed triads, tetrads, pentads, etc. have been
synthesized and investigated in detail.2,3
Multistep electron transfer and associated charge migration is possibly also the origin of the
longevity of charges in solid state blends of donor and acceptor materials used in organic and polymer
solar cells.4 In these cells, excitons, created by the absorption of light, dissociate at the donor-acceptor
interface forming an electron-hole pair. When this electron and hole escape from geminate
recombination and diffuse away from the interface, they can be collected at electrodes as a
photocurrent.
Recent studies on covalently linked oligo(p-phenylene vinylene) (OPV) donors and perylene
diimide (PERY) acceptors in liquid crystalline OPV-PERY-OPV triads,5,6 π-stacks of hydrogen-
bonded OPV-PERY-OPV trimers,7 and alternating (OPV-PERY)n copolymers,8 have revealed that
photoinduced electron transfer can be extremely fast in solution (< 1 ps) and that the lifetime is
generally rather short (<1 ns). Because of their high absorption coefficients in the visible region and
their charge transport properties, OPV9-14 and PERY15-19 chromophores have also attracted interest for
organic solar cells.
With the aim to extend the lifetime of the charge-separated state, two new multichromophoric
molecular arrays incorporating a central OPV-PERY-OPV triad augmented with two p-oligoaniline
(OAn) moieties as additional donor, have been designed (1 and 2 in Figure 3.1). In both donor(1)-
donor(2)-acceptor-donor(2)-donor(1) pentads, the OAn and OPV donors are electronically decoupled
in the ground state by an m-phenylene linkage and operate essentially as isolated redox active
segments. The two pentads differ in the connectivity between the central PERY segment and the OPV
donor. In pentad 1 the OPV and PERY units are directly connected providing a close proximity of the
chromophores and possibly some overlap of the π-electronic system, even though the two
chromophores are not coplanar. In 2 the OPV-PERY distances are increased and their π-overlap is
fully interrupted by saturated bonds. The redox potentials of the OAn, OPV, and PERY segments
favor the OAn•+-OPV-PERY•–-OPV-OAn state as the lowest-energy charge-separated state after
photoexcitation. To elucidate the mechanism of multistep photoinduced electron transfer in these
pentads and to assess the lifetime of the charge separated state, the photophysical properties of 1 and 2
Photoinduced multistep electron transfer in multichromophoric arrays
53
have been studied using photoluminescence and transient photoinduced absorption spectroscopy in
solvents of different polarity in comparison to those of the isolated chromophores and dyad
combinations. The rates for charge separation, migration, and recombination are rationalized using
Marcus theory and a continuum model for describing the free energy.
OO
O
OO
OOO
O
OO
ONNN N
NN
O
O
O
O
1
OO
O
OO
O
NN
OO
O
OO
ON N
NO
ONN
N O
O
O
O
O
O
2
N N NN
O
O
O
O
OO
O
OO
ON N
O
O
O
O
OO
O
OO
O
OAn OPV PERY
OAn-OPV
Figure 3.1 Chemical structures of pentads 1 and 2 and reference compounds.
3.2 Synthesis
The synthesis of pentads 1 and 2 and the reference compound OAn-OPV are depicted in
schemes 3.1 to 3.3. The synthesis of pentad 1 (Scheme 3.1) starts with the coupling of aldehyde 3,
whose synthesis has been described in chapter 2, to diethyl 4-nitrobenzyl phosphonate 4 via a Wittig-
Horner reaction, affording nitro compound 5. The reduction of the nitro group to the terminal amine 6
was achieved using stannous dichloride and ethanol in ethyl acetate. Condensation of 6 with 3,4,9,10-
perylenetetracarboxy-dianhydride in imidazole and catalytic amounts of Zn(OAc)2 afforded pentad
OAn-OPV-PERY-OPV-OAn 1. Purification of 1 was achieved via extensive column chromatography
and preparative size exclusion chromatography, which resulted in a low yield for this last step. For the
Chapter 3
54
synthesis of reference dyad OAn-OPV 8, aldehyde 3 was reacted with diethyl benzylphosphonate 7 in
a Wittig-Horner reaction (Scheme 3.2).
N NOR*
OR*OR*
R*OR*O
R*OO
N NOR*
OR*OR*
R*OR*O
R*OR
N
N NOR*
OR*OR*
R*OR*O
R*OO
O
NO2(EtO)2P
O
O
6. R = NH2
5. R = NO2
1
a
b
c
2
OR* =
43
+
Scheme 3.1. Synthesis of OAn-OPV-PERY-OPV-OAn (1). a. t-BuOK, DMF/THF (2/1), r.t., 2 h, 86%;
b. SnCl2·H2O, EtOH, EtOAc, 95° C, 5 h, 70%; c. Imidazole, Zn(OAc)2, 160 °C, 4 h, 15%.
N NOR*
OR*OR*
R*OR*O
R*OO
N NOR*
OR*OR*
R*OR*O
R*O
(OEt)2P
O
O
3
8
a
OR* =
7
+
Scheme 3.2. Synthesis of OAn-OPV (8). a. t-BuOK, DMF/THF (2/1), r.t., 2.5 h, 73%.
Photoinduced multistep electron transfer in multichromophoric arrays
55
In pentad 2 (Scheme 3.3) a saturated spacer has been intercalated between the OAn-OPV
segments and the perylene diimide central unit. This saturated spacer was first introduced on the
perylene unit by a double condensation of (S)-(+)-leucinol with 3,4,9,10-
tetracarboxyperyleneanhydride, affording the dihydroxy terminated perylene diimide 10. The amino
function in OAn-OPV 6 was reacted with phosgene in toluene to yield isocyanate 9. Pentad 2 was
synthesized by double reaction of isocyanate 9 with the hydroxyl functions of 10 using dibutyltin
dilaurate as catalyst.
N NOR*
OR*OR*
R*OR*O
R*ONCO
OHNN
OH
O
O
O
O
N NOR*
OR*OR*
R*OR*O
R*ONH2
O
N
O
O
O
N NOR*
OR*OR*
R*OR*O
R*ON
H
O
6
9 10
2
a
b
OR* =
+
2
Scheme 3.3. Synthesis of OAn-OPV-PERY-OPV-OAn (10). a. Phosgene, toluene, 95 °C, 16 h, 100%;
b. Dibutylin dilaurate, CH2Cl2, reflux, 20 h, 47%.
All final compounds were characterized using 1H-NMR spectroscopy, mass spectrometry, and
size exclusion chromatography.
In analogy with a previously studied analogous molecular triad based on OPV-PERY-OPV,6
the pentads and reference dyad were studied with 1H-NMR spectroscopy. Pentad 1 and reference dyad
8 feature the spectral characteristics of molecularly dissolved species when measured in deuterated
dichloromethane. However, pentad 2 features four clearly separated and well-resolved signals for the
perylene protons, whereas in a molecular dissolved state only two different signals are expected
(Figure 3.2a). This difference between the aromatic protons points to the formation of aggregates via
Chapter 3
56
intermolecular hydrogen bonding as has been established for the analogous triads.6 The spectrum of
pentad 2 recorded in THF (Figure 3.2b) shows the signals typical for molecular dissolved species and
indicates that intermolecular hydrogen bonding is much less present in THF.
7.88.28.69.09.49.8
N-H
N-H
a
b
1 2 3 4
1-4
7.88.28.69.09.49.8
N-H
N-H
a
b
1 2 3 4
1-4O RORNN
O
O
O
ONH O
NO H
2 1
13
3
4
4
2
R = OPV-OAn
Figure 3.2. 1H-NMR spectra of pentad 2 in dichloromethane (a) and THF (b).
The 1H-NMR results indicate that pentad 1 and reference dyad 8 are molecular dissolved in
most organic solvents of intermediate polarity. Pentad 2, however, will dimerize or aggregate in a
variety of solvents due to intermolecular hydrogen bonding of the urethane functionalities. This has
implications for the opto-electronic behavior of pentad 2 and therefore the solvent for 2 must be
carefully selected. Tetrahydrofuran seems to be a solvent that allows for the study of pentad 2 in its
molecularly dissolved state.
3.3 Electronic properties and energetic considerations
Absorption spectroscopy. The absorption spectrum of pentad 1 in toluene solution (Figure
3.3 left) exhibits two strong absorption bands, one centered at 327 nm and the other featuring vibronic
fine structure with maxima at 464, 494 and 532 nm. For comparison, the absorption spectra of the
corresponding individual chromophores OAn, OPV and PERY as well as that of dyad OAn-OPV are
also plotted in Figure 3.3. Whereas all chromophores contribute to the absorption of the UV region of
the pentad, the absorption in the visible region is dominated by the π-π* transitions of the OPV and
PERY chromophores. The vibronic fine structure typical for the PERY chromophore is still present in
the visible region.
Photoinduced multistep electron transfer in multichromophoric arrays
57
300 400 500 6000.0
0.5
1.0
1.5N
orm
aliz
ed A
bsor
banc
es (
O.D
.)
Wavelength (nm)300 400 500 600
0.0
0.3
0.6
Abs
orba
nce
(O. D
.)
Wavelength (nm)
Figure 3.3. Left: UV/Visible absorption spectra of the OAn-OPV-PERY-OPV-OAn pentad 1 (solid
line) and model compounds OAn-OPV (solid squares), OAn (open squares), OPV (open triangles),
and PERY (open circles) recorded in toluene solution. Right: UV-Visible absorption spectra of the
OAn-OPV-PERY-OPV-OAn pentad 2 in tetrahydrofuran (solid line), dichloromethane (dashed line)
and toluene (dotted line) solutions.
The UV-visible absorption spectrum of pentad 2 (Figure 3.3 right) recorded in
tetrahydrofuran, consists of the overlapping absorptions of the individual chromophores OAn, OPV
and PERY. However, the low energy absorption band of the PERY at 532 nm exhibits a lower
intensity in pentad 2 with respect to pentad 1, suggesting aggregation, most probably via π-π
interactions. In toluene and dichloromethane the absorption spectra show a further reduction of the
intensity of the 532 nm peak and a weak contribution above 550 nm, which indicates aggregation of
the pentads. In these solvents, a combination of intermolecular hydrogen bonding and π-π interactions
is probably responsible for the aggregation.6
Electrochemistry. The electrochemical properties of pentads 1 and 2 and of the reference
compounds OAn, OPV, and PERY were investigated using cyclic voltammetry. The redox potentials
are collected in Table 3.1. The cyclic voltammogram of pentad 1 shows two reversible reduction
waves at –0.53 and –0.76 V, corresponding to the reduction of the PERY acceptor, and four reversible
oxidation waves of the OAn (+0.53 V and +1.03 V) and the OPV (+0.76 and 0.96 V) donor moieties
(potentials are given vs. SCE, calibrated against Fc/Fc+, recorded in dichloromethane with 0.1 M
TBAPF6) (Table 3.1). In dichloromethane pentad 2 also exhibits four oxidation waves at +0.53 and
+1.07 V associated with the OAn segment and at +0.74 and +0.90 V corresponding to the OPV
segment. In this solvent no reduction waves could be observed for pentad 2. The measurement of the
reduction potentials in dichloromethane was most likely hampered by the aggregation via hydrogen
Chapter 3
58
bonding. In THF, however, the cyclic voltammogram of pentad 2 exhibits the two reduction waves of
the PERY moiety at -0.60 and –0.89 V.
Table 3.1 reveals that the redox potentials of the OAn and OPV donors are hardly affected by
their linkage to the other units. In contrast, the reduction potentials of the PERY acceptor depend on
the substitution of the imide functionalities. The reduction potentials of 1 and 2 are shifted by +0.12
and +0.05 V respectively with respect to that of the PERY reference compound (-0.65 V). For pentad
2 the shift is less, owing to the similar alkyl substitution as in the PERY reference. In pentad 1, the
overlap of the π-electronic clouds of the OPV and PERY chromophores is possible, although the first
phenyl ring of the OPV moiety will not be coplanar with the PERY unit.
Table 3.1. One-electron redox potentials (E0) of OAn, OPV, PERY, OAn-OPV, and OAn-OPV-PERY-
OPV-OAn (vs. SCE) calibrated with Fc/Fc+ (in dichloromethane with 0.1 M TBAPF6).
Compound E0red (V) E0
ox (V)
OAn 0.53 / 1.02
OPV 0.73/ 0.80
PERY -0.65/-0.85
Pentad 1 -0.53/-0.76 0.53 / 0.76/ 0.96/ 1.07
Pentad 2 -0.60/-0.89a 0.53 / 0.74/ 0.90/ 1.07
a Measured in THF.
Energetic considerations. The scheme in Figure 3.4 illustrates the various photophysical
processes that may occur in the pentads upon illumination. Excitation of one of the redox-active
chromophores generates locally excited states, e.g. the OAn-OPV-1PERY*-OPV-OAn singlet excited
state. Exergonic processes such as energy transfer (ET) and charge separation (CS) might compete
with intrinsic decay processes generating cor OAn•+-OPV•–-PERY-OPV-OAn charge-separated states.
These charge-separated states either decay to the ground state via charge recombination (CR) or
evolve via charge shift (CSH) to the more stable OAn•+-OPV-PERY•–-OPV-OAn state.
Photoinduced multistep electron transfer in multichromophoric arrays
59
1OAn*-OPV-PERY1OAn*-OPV-PERY
OAn-1OPV*-PERYOAn-1OPV*-PERY
OAn-OPV-PERYOAn-OPV-PERY
OAn-OPV•+-PERY• -
OAn•+-OPV-PERY• -
OAn•+-OPV•--PERYOAn-OPV-1PERY*OAn-OPV-1PERY*kET2
kCS1
kCR1
kCR2
kCSH1
kPLOPV
kET1
kISCOPV
kPLPERY
Energy
kCS3
kCR3
kCS2
OAn-3OPV*-PERY
kCSH2
kCS4
kPLOAn
Figure 3.4. Schematic energy levels (in THF) and photoinduced processes in OAn-OPV-PERY-OPV-
OAn pentads 1 and 2. (In the scheme the pentads are defined as OAn-OPV-PERY for simplification)
The rate constants are collected in Table 3.2. The fastest processes have been highlighted in black.
The energies of the singlet-excited states are at 3.40, 2.39 and 2.33 eV for the OAn, OPV and
PERY chromophores respectively, as extracted from absorption spectroscopy. In contrast to the
singlet- excited states, the energy of the charged separated states is strongly influenced by the polarity
of the medium. The Weller equation (Eq. 3.1) provides a means of calculating the energy of the
charged states in function of the polarity of the solvent and distance between chromophores. 20
( ) ( )( )
−
+−−−−=∆ −+
sref0
2
ccs0
2
00redox0 1111
84AD
εεπεεπε rr
e
R
eEEEeG (3.1)
In this equation, Eox(D) and Ered(A) are the oxidation and reduction potentials of the donor
and acceptor molecules or moieties measured in a solvent with relative permittivity εref, E00 is the
Chapter 3
60
energy of the excited state from which the electron transfer occurs, and Rcc is the center-to-center
distance of the positive and negative charges in the charge separated state. The radii of the positive
and negative ions are given by r+ and r– and εs is the relative permittivity of the solvent, -e is the
elemental charge, and ε0 is the vacuum permittivity.
Using equation 3.1 the change in free energy of the different processes of pentads 1 and 2 in
solutions of low to high polarity, i.e. toluene (ε = 2.38), chlorobenzene (ε = 5.72), THF (ε = 7.51), and
o-dichlorobenzene (ε = 9.93), have been calculated (Table 3.2). For the calculation the radius of the
negative perylene radical anion, PERY•–, was set to r– = 4.7 Å8 and that of the radical cations of OPV
and OAn to r+ = 5.4521 and 4.8 Å (Chapter 2) respectively. The Rcc distances were determined,
assuming that the charges are located at the centers of the OAn, OPV, and PERY moieties.22 The Rcc
distances were, in pentad 1, 21 and 41 Å for the OAn-OPV•+-PERY•–-OPV-OAn and OAn•+-OPV-
PERY•–-OPV-OAn charge-separated states respectively. In pentad 2 the relative position between the
OAn-OPV segment and the PERY central unit is not constant due to the relatively flexible character
of the spacer separating them. The upper limit for the Rcc distances in pentad 2 (27 and 44 Å for OAn-
OPV•+-PERY•–-OPV-OAn and OAn•+-OPV-PERY•–-OPV-OAn respectively) have been determined
in a conformation in which all chromophores are coplanar.
The experimental and estimated energies of the various neutral and charge-separated states of
the pentads are depicted in Figure 3.4, assuming THF as the medium. In the energetic scheme, the
dominant photophysical processes are depicted in black to highlight them from the slower and
therefore less probable processes, which are depicted in gray. The slow processes are the intrinsic
decay to the ground state of the individual chromophores (kPLOAn, kPL
OPV and kPLPERY), the intersystem
crossing to the triplet state for the OPV chromophore (kISCOPV), and the formation of the OAn•+-OPV•–
-PERY-OPV-OAn charge-separated state either from the 1OAn*-OPV-PERY-OPV-OAn or OAn-1OPV*-PERY-OPV-OAn singlet excited states.23 The change in free energy for the fast processes is
collected in Table 3.2. Here, the charge separation (CS1) process refers to the reaction taking place
from the lowest singlet excited state, i.e. OAn-OPV-1PERY*-OPV-OAn → OAn-OPV•+-PERY•–-
OPV-OAn. This separated state can also originate directly from the OPV singlet excited state (kCS2),
however, energy transfer to generate the PERY singlet excited (kET2) state is likely to compete with
this process.
The charge separation process, charge shift and charge recombination (CS1, CSH1, CR1, and
CR2) are exergonic in all solvents, with the exception that the driving force for charge shift is zero in
toluene. Similar energy values are obtained for pentads 1 and 2 because the only difference taken in
consideration was the Rcc parameter.
Photoinduced multistep electron transfer in multichromophoric arrays
61
Table 3.2. Change in free energy (∆G0) with reference to the lowest singlet excited state,
reorganization energy (λ), and barrier (∆G‡) for charge separation (CS1, OAn-OPV-1PERY*-OPV-
OAn → OAn-OPV•+-PERY•–-OPV-OAn), charge recombination (CR1, OAn-OPV•+-PERY•–-OPV-
OAn→ OAn-OPV-PERY-OPV-OAn), charge shift (CSH1, OAn-OPV•+-PERY•–-OPV-OAn→ OAn•+-
OPV-PERY•–-OPV-OAn), and charge recombination (CR2, OAn•+-OPV-PERY•–-OPV-OAn→ OAn-
OPV-PERY-OPV-OAn) in toluene (TOL), chlorobenzene (CB), tetrahydrofuran (THF), and o-
dichlorobenzene (ODCB) as determined using Eq. 3.1 and 3.3.
Pentad 1 Pentad 2
Reaction Solvent ∆G0
(eV) λ
(eV) ∆G‡
(eV) ∆G0
(eV)λ
(eV) ∆G‡
(eV) CS1 TOL -0.45 0.36 0.006 -0.34 0.36 0.000
CB -0.98 0.85 0.005 -0.90 0.89 0.000 THF -1.07 0.94 0.004 -1.00 0.99 0.000 ODCB -1.14 0.98 0.007 -1.08 1.03 0.001
CR1 TOL -1.88 0.36 1.625 -1.99 0.36 1.846 CB -1.35 0.85 0.072 -1.43 0.89 0.080 THF -1.26 0.94 0.026 -1.33 0.99 0.029 ODCB -1.19 0.98 0.011 -1.25 1.03 0.013
CSH1 TOL 0.00 0.35 0.073 0.00 0.35 0.060 CB -0.16 0.83 0.135 -0.16 0.83 0.135 THF -0.18 0.92 0.148 -0.18 0.83 0.128 ODCB -0.20 0.95 0.149 -0.19 0.95 0.152
CR2 TOL -1.85 0.37 1.474 -1.93 0.37 1.636 CB -1.19 0.98 0.011 -1.26 0.99 0.019 THF -1.08 1.10 0.000 -1.15 1.11 0.000 ODCB -0.99 1.18 0.008 -1.06 0.99 0.001
3.4 Photophysical processes in solution
3.4.1 Photoluminescence spectroscopy
The quenching of the photoluminescence (PL) of any of the chromophores indicates that
faster processes are occurring than the intrinsic decay to the ground state from the singlet-excited
state. As shown in Figure 3.4, the competitive processes are energy and charge transfer. Whereas the
quenching of the PL of OAn and OPV chromophores can be caused by energy and electron transfer
(ET1, CS4, ET2, CS3, and CS2, Figure 3.4), the quenching of the PERY acceptor emission gives
direct evidence of charge separation taking place (CS1), because the PERY S1 state is the lowest-
energy singlet-excited state.
In pentad 1, photoexcitation of either the OAn or OPV chromophore in toluene solution
results in a very weak emission that is associated with the OPV chromophore (Figure 3.5, right).
When compared to the reference OPV compound, the PL of the OPV in the pentad is quenched by a
Chapter 3
62
factor of Q = 50. Selective photoexcitation of the PERY chromophore at 530 nm in pentad 1 gives
also a very weak emission, that corresponds to the PERY chromophore and that is quenched by a
factor of Q = 3000 with respect to the PERY reference (Figure 3.5, right). The excitation spectra of
the observed residual photoluminescence at 519 and 540 nm do not superimpose with the absorption
spectrum of the pentad but instead coincide with the absorption spectrum of the OAn-OPV dyad
(Figure 3.5, left). The features corresponding to the PERY chromophore are absent in the excitation
spectra meaning that the contribution of acceptor excitation to the residual fluorescence is negligible.
The fact that the excitation spectrum of the residual OPV emission corresponds to the OAn-OPV
segments gives evidence that energy transfer occurs from the OAn to the OPV segment (ET1, Figure
3.4). Similar quenching factors have been measured in solvents of higher polarity like o-
dichlorobenzene. The photoluminescence of the PERY chromophore is nearly completely quenched
in pentad 1, meaning that the quantum yield for charge separation to generate the OAn-OPV•+-
PERY•–-OPV-OAn charge-separated state is close to unity and that this photophysical process is
extremely fast in all solvents. The OAn-OPV•+-PERY•–-OPV-OAn in itself may subsequently go
through a charge shift (CSH) to generate the energetically more favorable OAn•+-OPV-PERY•–-OPV-
OAn state.
300 400 500
0
1
Nor
mal
ized
inte
nsity
Wavelength (nm)500 600
0
100
200
300
x 200x 20
Inte
nsity
(a.
u.)
/ 10
4
Wavelength (nm)
Figure 3.5. Left: Absorption (solid line) and excitation spectra of the 519 (open circles) and 540 nm
emissions (open squares) in toluene solution. Right: PL spectra of the OPV (solid circles) and PERY
(solid squares) reference compounds and of pentad 1 after selective excitation of the OPV (open
circles) and the PERY (open squares) chromophores at 400 and 530 nm, in toluene solution.
For pentad 2 the PL of the OPV and PERY chromophores are also strongly reduced, with
quenching factors of Q = 66 and 300 respectively (Figure 3.6, right). Photoexcitation of OAn or OPV
results in a weak fluorescence that can be attributed to the sum of the OPV and PERY emissions. This
result is an indication that energy transfer is occurring from the OAn, to the PERY via the OPV
chromophore (ET1 and ET2, Figure 3.4). In pentad 2 the emission of the PERY acceptor is 10 times
Photoinduced multistep electron transfer in multichromophoric arrays
63
more intense than in pentad 1, indicating that the charge separation process (CS1, Figure 3.4) must be
10 times slower for pentad 2 than for pentad 1, due to the saturated spacer that separate donor and
acceptor.8 The excitation spectra of both the OPV donor and PERY acceptor coincide with the
absorption spectrum of pentad 2 (Figure 3.6, left), consistent with the multistep energy transfer
mentioned above (processes ET1 and ET2, Figure 3.4).
300 400 500 6000
1
Nor
mal
ized
inte
nsity
Wavelength (nm)
500 6000
100
200
x 20
Inte
nsity
(co
unts
) / 1
04
Wavelength (nm)
Figure 3.6. Left: Absorption (solid line) and excitation of the 534 nm emission (open circles) spectra
of pentad 2 in THF solution. Right: PL spectra of the OPV (solid circles) and PERY (solid squares)
reference compounds and of pentad 2 after selective excitation of the OPV (open circles) and the
PERY (open squares) chromophores at 400 and 530 nm, in THF solution.
3.4.2. Near steady state photoinduced absorption (PIA) spectroscopy
By measuring the PIA spectrum of a 1:1 mixture of the OAn-OPV dyad and the PERY
reference compound in a o-dichlorobenzene solution, the feasibility of a charge shift occurring in the
triad (CSH1, OAn-OPV•+-PERY•–-OPV-OAn → OAn•+-OPV-PERY•–-OPV-OAn) can be assessed.
Illumination of this mixture with monochromatic light of 458 nm results in the almost selective
excitation of the OPV chromophore.24 The OPV singlet excited state can evolve into the OAn•+-OPV•–
charged separated state (Figure 3.7, path a) or via intersystem crossing to the 3OPV* triplet excited
state (Figure 3.7, path b). While the charge-state decays to the ground state within a few nanoseconds
with no further consequences,25 the triplet state is characterized by a very long lifetime (τ = 25 µs).21
This long-lived triplet state can encounter a PERY chromophore in solution by diffusion and undergo
intermolecular charge transfer to generate the OAn-OPV•+ and PERY•– charged molecules (Figure
3.7, path d). These charged species will diffuse apart resulting in a long lifetime. During its lifetime,
the photogenerated positive charge of OAn-OPV•+ will most likely shift to the OAn segment to yield
OAn•+-OPV, because OAn has a lower oxidation potential than OPV (Figure 3.7, path e). The
experimental PIA spectrum (Figure 3.8) is in agreement with this sequence of reactions (paths b to e
Chapter 3
64
in Figure 3.7). It consists of a structured absorption band with maxima at 1.28, 1.54 and 1.72 eV that
are characteristic for the PERY•– radical anion.26 The absorption of the OAn•+ radical cation cannot be
distinguished because the electronic transitions of this charged species coincide in energy with those
of the PERY radical anion and are less intense (ε (1.44 eV) =15 × 103 M-1cm-1 for OAn•+ vs. ε (1.72
eV) ∼ 100 × 103 M-1cm-1 for PERY•–).26 Nevertheless, the absence of the typical polaronic absorptions
corresponding to the OPV radical cation, at 0.59 and 1.43 eV,21 in particular the one at low energy,
gives evidence that the positive charge has shifted from the OPV to the OAn segment.
OAn - 1 OPV * + PERY
OAn •+
+ - OPV •−
- + PERY
OAn - 3 OPV * + PERY OAn - OPV •+ + PERY •−-
a
b
c
d OAn •+
+ - OPV + PERY •− e
OAn - OPV + PERY
Figure 3.7. Probable photophysical reactions in a mixture of OAn-OPV PERY 1:1 after excitation at
458 nm.
Figure 3.8. Photoinduced absorption spectra of the mixture OAn-OPV/PERY (1:1) (solid line) in o-
dichlorobenzene solution (excitation at 458 nm with 25 mW and modulation frequency of 275 Hz).
3.4.3. Subpicosecond transient pump-probe spectroscopy
To unravel the different photophysical processes occurring on the picosecond to nanosecond
time domain, subpicosecond transient pump-probe spectroscopy has been performed at room
temperature on solutions of pentad 1 in solvents of different polarity. For pentad 2 the study has been
restricted to THF, because in this solvent aggregation is less pronounced. The experiments have been
performed by selective excitation of the OPV or PERY chromophores at 455 or 520 nm, respectively,
and by monitoring the transient absorptions at 1450, 900, and 700 nm. The absorption at 1450 nm is
0.5 1.0 1.5 2.0 2.5-1
0
1
2
3
4
-∆T
/T x
104
Energy (eV)
Photoinduced multistep electron transfer in multichromophoric arrays
65
caused by the low-energy transition of the OPV•+ radical cation21 and, hence, this transient signal
gives direct information on the rates of formation (kCS = kCS1+kCS2) and decay (kdecay) of the OAn-
OPV•+-PERY•–-OPV-OAn charge separated state. The decay of this charge-separated state is the sum
of the rate constants for charge recombination to the ground state and charge shift to the OAn•+-OPV-
PERY•–-OPV-OAn state (kdecay = kCR1 + kCSH1). The radical ions of all chromophores absorb at 700
nm, with estimated molar absorption coefficients of 7 × 103, 15 × 103 and ~80 × 103 M-1cm-1 for
OAn•+, OPV•+,27 and PERY•–26 respectively. The 1OPV* and 1PERY* states dominate the transient
absorption at 900 nm. Also at this wavelength all radical ions absorb, though with much lower molar
absorption coefficients for OPV•+ and PERY•– than at 700 nm. On the basis of the molar absorption
coefficients of the radical ions a OAn-OPV•+-PERY•–-OPV-OAn to a OAn•+-OPV-PERY•–-OPV-OAn
charge shift can be identified by comparing the intensities of transient absorptions at 700 nm
(dominated by the PERY•– radical anion), 900 nm (dominated by the S1 states of the OPV or PERY
chromophores), and 1450 nm (dominated by the OPV•+ radical cation).
3.4.3.1 Pentad 1
Apolar medium (toluene solution). The 1450 nm transient absorption of pentad 1 in toluene
solution after excitation at 450 nm exhibits a fast rise and a slow decrease of the (negative) intensity
(Figure 3.9). The rise occurs within 1 ps, which corresponds to a rate constant for charge separation
(kCS) of > 1000 ns-1. This fast formation of the OAn-OPV•+-PERY•–-OPV-OAn charge-separated state
is in agreement with the complete quenching of the PERY emission (Figure 3.5, right). The slow
decay (3×109 s-1) points to a long-lived charge-separated state (~ 366 ps) in toluene.
The transient absorption recorded at 700 nm essentially superimposes with the absorption at
1450 nm (Figure 3.9). This strongly suggests that both absorptions originate from the same charged
species, i.e. the OAn-OPV•+-PERY•–-OPV-OAn and, hence, no charge shift is taking place in toluene
solution.
The 900 nm differential transmission, after photoexcitation at 450 nm (OPV segment),
exhibits in the first picosecond an abrupt rise and decay of the intensity corresponding to the
formation and decay of the OPV singlet excited state (Figure 3.9). A similar feature is observed if the
PERY chromophore is excited instead, although in this case the short-lived absorption corresponds to
the PERY singlet excited state. Both S1 states evolve within 1 ps into the OAn-OPV•+-PERY•–-OPV-
OAn charge-separated state, as evidenced by the 1450 and 700 nm transient absorptions. Apart from
this initial transient feature, the absorption at 900 nm exhibits the same temporal evolution as the 1450
and 700 nm differential transmissions, which is associated with the OAn-OPV•+-PERY•–-OPV-OAn
charge-separated state.
Chapter 3
66
0 100 200 300 400 500 600
-6
-5
-4
-3
-2
-1
0
-4 -2 0 2 4 6
-5
0
∆T (
a. u
.)
Time (ps)
∆T (
a. u
.)
Time (ps)
Figure 3.9. Differential transmission dynamics of pentad 1 in toluene monitored at 1450 (open
circles), 900 nm (closed squares) and 700 (open triangles) with excitation at 450 nm. The inset shows
the differential transmissions on shorter time scale.
Polar media. The 1450 nm transient absorption reveals that the formation of the OAn-OPV•+-
PERY•–-OPV-OAn charge-separated state in pentad 1 in THF solution is as fast as in toluene, but that
the lifetime of the charged state is dramatically shorter (~ 12 vs. 366 ps) (Figure 3.10). Again, the
same kinetics are observed regardless which chromophore is excited, OPV or PERY.
In the differential transmission at 700 nm of pentad 1 in THF two different regimes can be
distinguished (Figure 3.10). The first regime consists of a fast rise and decay of the signal occurring in
the initial 30 ps. This initial feature is associated with the formation and decay of the OAn-OPV•+-
PERY•–-OPV-OAn charge-separated and, thus, the OPV•+ and PERY•– radical ions account for the
absorption in this first regime. After 30 ps most of the absorption has decayed but a less intense long-
lived signal remains, which constitutes the second regime. Because the 1450 nm absorption of the
OAn-OPV•+-PERY•–-OPV-OAn state has disappeared after 30 ps, the remaining signal at 700 nm
corresponds to the OAn•+-OPV-PERY•–-OPV-OAn charge separated state. In the first picosecond
after photoexcitation, the differential transmission at 900 nm features the formation and decay of the
singlet excited state of the OPV and PERY chromophores (depending on which has been excited). As
in toluene, this initial singlet excited state is immediately quenched by the formation of the OAn-
OPV•+-PERY•–-OPV-OAn charge-separated state. After 20 ps an almost constant signal remains that
corresponds to the OAn•+-OPV-PERY•–-OPV-OAn charge separated state.
Photoinduced multistep electron transfer in multichromophoric arrays
67
0 200 400 600 800 1000
-1
0
∆T (
a. u
.)∆T
(a.
u.)
∆T (
a. u
.)
Time (ps)
0 20 40 60 80 100
-1
0
Time (ps)
0 2 4 6 8
-1
0
Time (ps)
Figure 3.10. Differential transmission dynamics of pentad 1 in THF monitored at 1450 (open circles),
900 (solid squares) and 700 nm (open triangles) with excitation at 455 nm, measured on different time
scales.
Even though the rate for charge shift cannot be extracted from the data, an efficiency of 22%
for the process in THF can be estimated from the 700 nm transient absorption, using the molar
absorption coefficients of the different radical ions at this wavelength and assuming that the
maximum intensity at 3 ps corresponds to the OAn-OPV•+-PERY•– charge separated state, while at 50
ps the remaining absorption corresponds to the OAn•+-OPV-PERY•– charged state only.
Chapter 3
68
In chlorobenzene and o-dichlorobenzene, solvents of lower and higher polarity than THF, the
differential transmissions at 1450, 900 and 700 nm of pentad 1 exhibit similar time profiles as
observed in THF. Also the efficiencies for charge shift are similar in all the polar solvents.
0 200 400 600 800 1000
-1
0
∆T (
a. u
.)
Time (ps)
0 100 200 300 400
-1
0
∆T (
a. u
.)
Time (ps)
0 2 4 6 8 10 12
-1
0
∆T (
a. u
.)
Time (ps)
Figure 3.11. Differential transmission dynamics of pentad 2 in THF monitored at 1450 (open circles),
900 (solid squares) and 700 nm (open triangles) with excitation at 455 nm, measured on different time
scales.
Photoinduced multistep electron transfer in multichromophoric arrays
69
3.4.3.2. Pentad 2 in THF
The differential transmission at 1450 nm of pentad 2 dissolved in THF (Figure 3.11) reveals
that the charge separation process to the OAn-OPV•+-PERY•–-OPV-OAn charge-separated state is
slowed down to kCS1 = 285 ns-1, compared to 1000 ns–1 for the same process in pentad 1. The reduced
rate of charge formation is consistent with the lower photoluminescence quenching of the PERY
chromophore in pentad 2 compared to 1. Moreover, the lifetime of the charge-separated state is
influenced by the longer and non-conjugated connection between donor and acceptor. As can be
expected, the rate for charge recombination has decreased resulting in a longer lifetime of the primary
charge-separated state (OAn-OPV•+-PERY•–-OPV-OAn) in pentad 2 (100 ps) than in pentad 1 (12 ps).
The efficiency of the charge shift process to generate the OAn•+-OPV-PERY•–-OPV-OAn charge-
separated state has improved to 28%, judging from the 700 nm transient absorption.
3.5. Kinetic considerations
Marcus theory provides an estimate for the free energy barrier (∆G‡) for electron transfer
reactions based on the change in free energy (∆G0) and the reorganization energy (λ) via:
( )
λλ
4
20‡ +∆=∆ G
G (3.2)
The reorganization energy consists of an internal contribution (λ i) and a solvent term (λs),
which can be approximated via the Born-Hush approach to give after summation, with n the index of
refraction:
−
−
++=+= −+
s2
0
2
isi11111
2
1
4 επελλλλ
nRrr
e
cc
(3.3)
The rate constants for the different processes, are not only a function of the energy barrier
∆G‡, but also of the reorganization energy (λ) and the electronic coupling (V) between donor and
acceptor in the excited state according to:
( )
+∆−
=
Tk
GV
Tkhk
B
202
21
B2
2
4exp
4
λλ
λπ
(3.4)
Chapter 3
70
The values of ∆G0, λ, and ∆G‡ calculated on the basis of Eqs. 3.1 to 3.4 are collected in Table
3.2. For the initial charge separation (kCS1) the values of free energy variation and reorganization
energies are comparable (–∆G0 ~ λ), close to the Marcus optimal region. In such cases, the reaction
rate is governed mainly by the electronic coupling V between the donor and acceptor and the polarity
of the solvent becomes less important. The electronic coupling depends on the nature of the spacer
and on the separation of donor and acceptor via ))(exp()( 0cc02
02 RRRVV −−= β , with R0 the
contact distance. The electronic coupling in pentad 1 is probably high due to the semi-conjugated
fashion by which the OPV and the PERY chromophores are linked. This explains the extraordinarily
high rate constants for the process observed for pentad 1 in all solvents (kCS > 1000 ns-1). Charge
recombination in OAn-OPV•+-PERY•–-OPV-OAn is in the Marcus inverted region (–∆G0 > λ) for all
solvents (Table 3.2). The use of Eq. 3.4 in the inverted region often underestimates the true rate
constant because of nuclear tunneling,28 but will be used here for qualitative comparison. In general
the barrier for charge recombination is reduced upon increasing the polarity of the medium. In
toluene, the most apolar solvent, the energy barrier is remarkably high, due to the large difference
between the free and reorganization energies. This is in agreement with the rather long-lived (366 ps)
OAn-OPV•+-PERY•–-OPV-OAn charge-separated stated observed for pentad 1 in toluene. In the more
polar solvents, the energy barrier is up to two orders of magnitude less than in toluene and, hence
charge recombination is expected to be much faster than in toluene. This explains, to some extend, the
12 ps lifetime of the OAn-OPV•+-PERY•–-OPV-OAn charge-separated state in THF. The charge shift
occurs in the normal region (–∆G0 < λ) and is energetically less favorable than the recombination
process (Table 3.2). In addition, the energy barrier of this process is higher than that of charge
recombination. These two facts imply that the charge shift is probably a slower process than the
charge recombination, although the influence of V is not accounted for. The study on the analogous
OAn-OPV-C60 molecular triad reported in chapter 2, has shown that increasing the lifetime of the
OAn-OPV•+-PERY•–-OPV-OAn charge-separated state is beneficial for the slower charge shift
process to compete. In principle this condition is nicely fulfilled for pentad 1, in toluene solution.
However, the driving force for the charge shift is close to zero in toluene and, hence, the charge shift
does not occur. However, in the polar solvents, where charge recombination is much faster, the charge
shift can be observed. The efficiency of the charge shift is similar in all polar solvents, consistent with
the small variation in energies reported in Table 3.2.
For pentad 2 the rate for charge separation (kCS) is less than for pentad 1, even though the
energy values depicted in Table 3.2 predict a similarly low barrier. Whereas in pentad 1, the electron
can be transferred from the OPV donor, through a ‘conjugated’ pathway to the PERY acceptor, the
electron transfer in pentad 2 occurs through space or through the saturated spacer by superexchange
mechanism. Moreover, the Rcc distance is higher in pentad 2. Both effects result in a lower electronic
Photoinduced multistep electron transfer in multichromophoric arrays
71
coupling in the excited state between the OPV donor and the PERY acceptor in pentad 2, and thus a
slower charge separation process. The same explanation holds for the charge recombination process
and explains the longer lifetime of the OAn-OPV•+-PERY•–-OPV-OAn state of pentad 2 in THF
solution (100 ps) compared to that of pentad 1 (12 ps). As a result of the slower charge recombination
to the ground state a slightly higher efficiency for charge shift is found for pentad 2 than for pentad 1.
The electronic coupling between the OAn and the PERY moieties is very weak.
Consequently, the charge recombination process is very slow and a long lifetime is observed for the
OAn•+-OPV-PERY•–-OPV-OAn charge-separated state.
3.6 Conclusions
Two multichromophoric arrays have been synthesized by covalently linking OAn, OPV and
PERY chromophores in an OAn-OPV-PERY-OPV-OAn symmetrical arrangement. In pentad 2 all
chromophores are decoupled in the ground state while in pentad 1 some π-overlap is possible between
the OPV and the PERY chromophores. Because of its urethane functionality, pentad 2 dimerizes or
aggregates in a variety of solvents (except THF) due to intermolecular hydrogen bonding.
Upon illumination of the pentad a number of sequential photophysical events occur. These
processes are mainly multistep energy and electron transfers. Even though no unambiguous evidence
has been established for an ultrafast multistep singlet-energy transfer process (ET1 & ET2, Figure
3.4) resulting in the OAn-OPV-1PERY*-OPV-OAn state prior to the charge transfer and irrespective
of the excitation wavelength, some facts seem to indicate that such sequential event indeed occurs.
First, the excitation spectrum of the residual PERY emission in pentad 2, superimposes with the
absorption spectrum of the pentad (Figure 3.6, left). Second, the rate for intramolecular charge
separation that generates the OAn-OPV•+-PERY•–-OPV-OAn state is independent of the chromophore
that has been excited, strongly suggesting that they originate from the same state.
The sequential electron transfer starts by the formation of OAn-OPV•+-PERY•–-OPV-OAn.
This state can relax to the ground state via charge recombination or generate via a charge shift the
long-lived OAn•+-OPV-PERY•–-OPV-OAn charge-separated state (CR1 and CSH1, Figure 3.4). In
pentad 1, intramolecular charge separation to generate the OAn-OPV•+-PERY• –-OPV-OAn state is
extremely fast (kCS > 1000 ns-1), regardless the polarity of the solvent, because it occurs under optimal
conditions (–∆G0 ~ λ). The charge recombination of OAn-OPV•+-PERY•–-OPV-OAn to the ground
state occurs in the Marcus inverted region (–∆G0 > λ) and is therefore very fast in the polar solvents
and remarkably slow in toluene. The charge separation and recombination are slowed down in pentad
2, because the saturated spacer reduces the electronic coupling between the OPV donor and PERY
acceptor. In pentad 1, the charge shift process that generates OAn•+-OPV-PERY•–-OPV-OAn
competes with the faster charge recombination with an efficiency of 0.22. The slower charge
Chapter 3
72
recombination of OAn-OPV•+-PERY•–-OPV-OAn to the ground state in pentad 2, with respect to that
in pentad 1, is beneficial for the charge shift process that occurs with an efficiency of 0.28 in polar
media. Thus placing the OPV and PERY chromophores at longer distances seems to be the key to
improve the quantum yield for charge shift. As a matter of fact, the steady state PIA experiment on
the OAn-OPV and PERY mixture is an extreme situation of such a large distance, resulting in a high
efficiency because for every OAn-OPV•+/PERY•– the OAn•+-OPV/PERY•– is formed.
Pentads 1 and 2 show a few differences with respect to the OAn-OPV-C60 molecular triad
presented in chapter 2. In contrast to the fullerene-based triad, charge separation is possible in apolar
media for the pentads owing to the lower reduction potential of the perylene diimid acceptor.
Furthermore, in the pentads the various photophysical processes are less sensitive to the solvent
polarity (within the polar solvents) than in the OAn-OPV-C60 triad. This means that even though
similar trends are followed (for example, faster charge recombination in polar solvents), these changes
are almost negligible in the pentads. The most striking difference between the two multichromophoric
arrays originates from the fact that the charge shift process to the OAn•+-OPV-PERY•–-OPV-OAn is
able to compete with the charge recombination to the ground state in the pentads, even though the
latter process seems to be even much faster than in the OAn-OPV-C60 triad (Chapter 2).
In this chapter, the photophysical phenomena following photoexcitation have been
investigated in pentads 1 and 2 in an isolated form (molecularly dissolved). These molecules have the
potential to form ordered molecular aggregates, because of the strong and directing π−π interactions
of the perylene unit and additionally for pentad 2 the possibility of hydrogen bonding via the urethane
linkage. Future studies on the aggregated and solid states of the pentads might reveal promising bulk
photophysical properties.
3.7 Experimental section
For general methods and materials the reader is referred to chapter 2 of this thesis. Diethyl(4-nitrobenzyl) phosphonate (4). Triethyl phosphite (2.30 g, 13.88 mmol) and 4-nitrobenzyl bromide (2g, 9.25 mmol) were heated to 160 °C and stirred for 2 h. Subsequently, the mixture was cooled to 70 °C and the formed ethyl bromide and the excess of triethyl phosphite were distilled under reduced pressure. The residue was dissolved in ethyl acetate and filtrated over silica gel. The solvent was removed in vacuo to yield 2.2 g of diethyl 4-nitrobenzyl phosphonate. 1H NMR (CDCl3, 300 MHz): δ 8.18 (d, 2H), 7.5 (d, 2H), 4.07 (m, 4H), 3.27 (d, 2H), 1.28 (t, 6H); 13C NMR (CDCl3, 75 MHz): δ 146.78, 139.62 (d), 130.45(d), 123.44 (d), 62.20 (d), 34.61, 32.79, 16.15 (d). (E,E,E,E)-4-<4-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl>nitrobenzene (5).
Photoinduced multistep electron transfer in multichromophoric arrays
73
Diethyl(4-nitrobenzyl) phosphonate (73 mg, 0.27 mmol), was dissolved in anhydrous DMF (6 mL) under an argon atmosphere and KtBuO (35 mg, 0.32 mmol) was added to the solution at room temperature. After 15 minutes a solution of aldehyde compound 3 (307 mg, 0.243 mmol) in 3 mL DMF/THF (2/1) was added dropwise to the reaction mixture. Twenty minutes after the addition was completed, the product starts precipitating from solution. The reaction mixture was stirred for 2 h more. The product was isolated by filtration and washed with ethanol and methanol as a red powder (0.290 g, 86%). 1H NMR (CDCl3, 300 MHz): δ 8.22 (d, 2H), 7.65 (d, 1H), 7.63(d, 2H), 7.57 (d, 1H), 7.52 (s, 2H), 7.51 (d, 1H), 7.40 (d, 1H), 7.31-6.96 (m, 31H), 4.05-3.72 (m, 12H), 2.10-1.85(m, 6H), 1.70-1.45 (m, 6H ), 1.45-1.2 (m, 6H), 1.2-0.85 (m, 36H); 13C NMR (CDCl3, 75 MHz): δ 151.81, 151.25, 151.01, 148.21, 147.84, 146.43, 144.73, 142.90, 142.70, 139.15, 129.45, 129.18, 128.42, 128.31, 127.75, 127.59, 127.07, 126.63, 126.04, 125.45, 125.30, 125.11, 124.18, 123.78, 122.96, 122.79, 122.68, 122.42, 122.24, 120.29, 111.15, 110.67, 110.01, 109.89, 109.70, 109.53, 74.47, 74.25, 74.16, 74.04, 73.94, 35.13, 35.05, 34.94, 26.37, 16.85, 11.52, 11.38. MALDI-TOF MS (Mw =1382.87) m/z = 1382.0 [M]+; Anal. Calc for C92H107N3O8: C, 79.9; H, 7.8; N, 3.0. Found: C, 79.7; H, 7.8; N, 2.7. (E,E,E,E)-4-<4-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl>aniline (6). Under an Ar atmosphere nitro compound 5 (0.28 g, 0.2 mmol) was suspended in EtOAc (15 mL). SnCl2.H2O (0.36 g, 1.62 mmol) and EtOH (2 mL) were added and the mixture was heated to 85 °C. The reaction mixture was stirred for 5 h at 95 °C and subsequently cooled to room temperature. After cooling to room temperature the reaction mixture was poured into crushed ice. The aqueous phase was slightly basified by the addition of NaOH 0.1 M and was subsequently extracted three times with diethyl ether. The collected organic fractions were dried over MgSO4, and the solvent removed in vacuo. Purification by column chromatography (silica gel, pentane/CH2Cl2 1:1, Rf = 0.4), yielded 6 (0.19 g, 70%) as an orange solid. 1H NMR (CDCl3, 400 MHz): δ 7.52 (s, 4H), 7.42 (d, 1H), 7.36 (d, 2H), 7.31 (d, 1H), 7.27-6.97 (m, 31H), 6.69 (d, 2H), 3.93-3.89 (m, 12H), 1.70-1.45 (m, 6H), 1.45-1.2 (m, 6H), 1.2-0.85 (m, 36H); 13C NMR (CDCl3, 75 MHz): δ 151.23, 151.08, 151.02, 150.95, 148.17, 147.81, 145.94, 142.86, 142.69, 139.15, 129.43, 129.17, 128.77, 128.67, 128.30, 127.72, 127.53, 127.36, 127.21, 126.79, 126.43, 125.44, 125.30, 123.77, 123.71, 123.59, 122.68, 122.41, 122.25, 120.26, 119.88, 115.22, 110.62, 110.41, 109.85, 109.65, 74.44, 74.28, 74.06, 73.99, 35.13, 35.04, 34.98, 34.92, 26.37, 26.29, 16.89, 16.87, 16.82, 16.79, 11.54, 11.48, 11.41, 11.37. MALDI-TOF MS (Mw =1352.89) m/z = 1352.92 [M]+.
N,N’-Bis[(E,E,E,E)-4-<4-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl>phenyl]-3,4,9,10-perylene bis(dicarboximide) (1). Amine compound 6 (55 mg, 0.04 mmol), 3,4,9,10-perylenetetracarboxylic dianhydride (8 mg, 0.02 mmol), imidazole (0.5g, 7.3 mmol), and a catalytic amount of Zn(AcO)2 were mixed and heated till 160 °C. After stirring for 4 h, the reaction mixture was cooled to room temperature. The residue was purified by column chromatography (silica gel, ethyl acetate/ CHCl3 1:0 to 1:4, Rf = 0 to 0.4) and repetitive preparative size exclusion chromatography (Bio Beads S-X1 and S-X3, CH2Cl2) to yield 9 mg (15 %) of 1 as a dark red solid. 1H NMR (CD2Cl2, 400 MHz): δ 8.61-8.52 (m, 8H), 7.61 (d, 4H), 7.51-6.89 (m, 77H), 3.90-3.70 (m, 24H), 1.94-1.82 (m, 12H), 1.65-1.52 (m, 12H), 1.36-1.25 (m, 12H), 1.07-0.90 (m, 64H). MALDI-TOF MS (Mw =3062.08) m/z = 3060.64 [M]+. Diethyl benzylphosphonate (7). Triethyl phosphite (1.45 g, 8.77 mmol) and benzyl bromide (1 g, 5.84 mmol) were heated to 160 °C and stirred for 3 h. Subsequently, the mixture was cooled to 90 °C and the formed ethyl bromide and the excess of triethyl phosphite were distilled under reduced pressure. The residue was dissolved in ethyl acetate and filtrated over silica gel. The solvent was removed in vacuo to yield 1.05 g (79 %)of diethyl benzylphosphonate. 1H NMR (CDCl3, 300 MHz): δ 7.34 (m, 5H), 4.06-3.95 (m, 4H), 3.15 (dd, 2H), 1.23 (td, 6H); 13C NMR (CDCl3, 75 MHz): δ 131.55 (d), 129.70 (d), 128.45 (d), 126.78 (d), 62.02 (d), 34.63, 32.80, 16.29 (d). (E,E,E,E)-4-<4-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl>benzene (8). Phosphonate 7 (17 mg, 0.074 mmol) was dissolved in anhydrous DMF (1 mL) under an argon atmosphere and KtBuO (18 mg, 0.016 mmol) was added to the solution at room temperature. After 15 minutes, a solution of aldehyde compound 3 (54 mg, 0.042 mmol) in 2.2 mL DMF/THF (2/1) was added dropwise to the reaction mixture. The reaction mixture was stirred for 2.5 h. Evaporation of the solvent and precipitation in methanol yielded 7 (42 mg, 73 %) as an orange solid. 1H NMR (CD2Cl2, 400 MHz): δ 7.56-6.98 (m, 42 H), 3.98-3.81 (m,
Chapter 3
74
12H), 2.04-1.92 (m, 6H), 1.73-1.59 (m, 6H), 1.43-1.32 (m, 6H), 1.14-0.96 (m, 36H); 13C NMR (CDCl3, 100 MHz): δ 151.65, 151.50, 151.41, 148.73, 148.27, 143.46, 143.21, 139.54, 138.39, 129.84, 129.55, 129.06, 128.93, 128.71, 127.86, 127.80, 127.65, 126.91, 126.77, 126.02, 125.81, 124.11, 123.78, 123.68, 123.07, 122.92, 122.83, 122.25, 120.61, 110.96, 110.78, 110.15, 109.93, 74.76, 74.56, 74.42, 35.56, 35.49, 35.43, 26.77, 17.00, 11.70, 11.56. MALDI-TOF MS (Mw =1337.90) m/z = 1337.74 [M]+. (E,E,E,E)-4-<4-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl>phenylisocyanate (9). Amine compound 6 (50 mg, 0.04 mmol) was suspended in 20% phosgene solution in toluene (2.4 mL) and stirred at 95 °C for 16 h under an inert atmosphere. The reaction mixture was cooled to room temperature and the solvent removed in vacuo. The complete conversion of the amine to the isocyanate was monitored by IR spectroscopy by observing the disappearance of the amine peak at 3364 cm-1 and the formation of the isocyanate peak at 2264 cm-1 Compound 9 was used without further purification. MALDI-TOF MS (Mw =1378.91) m/z = 1378.64 [M]+. N,N’-Di-((S)-1-isobutyl-2-hydroxyethyl)-3,4,9,10-perylene bis(dicarboximide) (10). (S)-(+)-Leucinol (0.455 g, 3.88 mmol), 3,4,9,10-perylenetetracarboxylic dianhydride (0.63 g, 1.61 mmol), imidazole (10 g), and catalytic amounts of Zn(OAc)2 were heated to 160 °C and stirred for 1.5 h. After cooling to room temperature the reaction mixture was dissolved in methylene chloride and washed 2 times with HCl 1M, brine, and dried over MgSO4. The solvent was removed in vacuo to yield 0.72g (75%) of a dark red solid. 1H NMR (CD2Cl2, 400 MHz): δ 8.40-7.52 (m, 8H), 5.54-5.44 (m, broad signal, 2H), 4.60-4.48 (m, broad signal, 2H), 3.98-3.94 (m, 2H), 2.10-2.01 (m, 2H), 1.80-1.46 (m, broad signal, 4H), 1.10-0.92 (m, 12). MALDI-TOF MS (Mw = 590.2) m/z = 590.1 [M]+. N,N’-Bis[(S)-1-Isobutyl-2-[(E,E,E,E)-4-<4-[4-{4-[N-(4-Diphenylaminophenyl)-N-(phenyl)-3-aminostyryl]-2,5-bis[(S)-2-methylbutoxy]styryl]-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]styryl>phenyl carbamicacid]ethyl] 3,4,9,10-perylene bis(dicarboximide) (2). A solution of the isocyanate (38 mg, 0.027 mmol) in dry methylene chloride was added to a solution of 6 (7.4 mg, 0.012 mmol) in dry methylene chloride. Dibutylin dilaurate (3.22 mg) was added as a catalyst, and the reaction mixture was heated at reflux for 20 h under an argon atmosphere. The crude mixture was cooled to room temperature, the solvent was removed in vacuo. The residue was purified by silica gel chromatography (CH2Cl2/pentane; 4:1, Rf = 0.15) and preparative size exclusion chromatography (Bio Beads S-X3 , CH2Cl2) to yield 20 mg (47 %) of 7 as a red solid. 1H NMR (CD2Cl2, 400 MHz): δ 9.38 (br s, 2H), 8.57, 8.21, 8.12, 7.79 (4 x d, 8H), 7.58-7.06 (m, 82H), 5.94-5.78 (br m, 2H), 5.35-5.18 (br m, 2H), 4.76-4.57 (br. m, 2H), 3.98-3.83 (m, 24H), 2.37-2.22 (m, 2H), 2.02-1.89 (m, 14H), 1.75-1.60 (m, 16H), 1.52-1.32 (m, 12H), 1.16-0.99 (m, 84H); 13C NMR (THF, 100 MHz): δ 165.00 (broad signal), 154.83, 153.07, 152.95, 150.14, 149.76, 144.83, 144.73, 141.33, 140.59, 136.06, 134.32, 132.50, 131.21, 131.09, 130.82, 130.60, 129.89, 129.79, 129.36, 129.18, 129.04, 128.89, 128.65, 128.34, 128.26, 127.74, 127.09, 126.99, 126.79, 126.79, 125.45, 125.06, 124.49, 124.37, 124.22, 124.12, 123.97, 123.28, 123.09, 121.83, 119.90, 112.15, 112.02, 111.47, 111.20, 111.13, 80.28, 75.64, 75.46, 53.07, 40.05, 37.07, 36.97, 36.89, 28.11, 27.26, 24.44, 23.65, 18.08, 12.73, 12.61.MALDI-TOF MS (Mw =3348.49) m/z = 3347.85 [M]+. 3.8 References and notes 1 (a) Molecular Electronics, Jortner, J., Ratner, M. Eds.; Blackwell: London, 1997. (b) Lehn, J.-M.,
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Photoinduced multistep electron transfer in multichromophoric arrays
75
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2002, 124, 7668. (x) Kodis, G.; Liddell, P. A.; de la Garza, L.; Moore, A. L.; Moore, T. A.; Gust, D. J.
Mater. Chem. 2002, 12, 2100. y) Sánchez, L.; Pérez, I.; Martín, N.; Guldi, D. M. Chem. Eur. J. 2003, 9,
2457.
4 (a) Kraabel, B.; McBranch, D.; Sariciftci, N. S.; Moses, D.; Heeger, A. J. Phys. Rev. B 1994, 50, 18543.
(b) Kraabel, B.; Hummelen, J. C.; Vacar, D.; Moses, D.; Sariciftci, N. S.; Heeger, A. J. J. Chem. Phys.
1996, 104, 4267. (c) Meskers, S. C.J.; van Hal, P. A.; Spiering, A. J. H.; Hummelen, J. C.; van der Meer,
A. F.G.; Janssen, R. A.J. Phys. Rev. B 2000, 61, 9917. (d) Nogueira, A. F.; Montanari, I.; Nogueira, A. F.;
Nelson, J.; Durrant, J. R.; Winder, C.; Loi, M. A.; Sariciftci, N. S.; Brabec, C. J. Phys. Chem. B 2003,
107, 1567
5 Peeters, E.; van Hal, P. A.; Meskers, S. C. J.; Janssen R. A. J.; Meijer, E. W. Chem. Eur. J., 2002, 8, 4470
Chapter 3
76
6 Asha, S.; Schenning, A. P. H. J.; Meijer, E. W.,Chem. Eur. J. 2002, 8, 3353.
7 Schenning, A. P. H. J.; van Herrikhuyzen, J.; Jonkheijm, P.; Chen, Z.; Würthner F.; Meijer, E. W. J. Am.
Chem. Soc., 2002, 124, 10252.
8 Neuteboom, E. E.; Meskers, S. C. J.; van Hal, P. A.; van Duren, J. K. J.; Meijer, E. W.; Dupin, H.;
Pourtois, G.; Cornil, J.; Lazzaroni, R.; Brédas, J.-L.; Beljonne, D.; Janssen, R. A. J. J. Am. Chem. Soc.,
2003, 125, 8625
9 Nierengarten, J.-F.; Eckert, J.-F.; Nicoud, J.-F.; Ouali, L.; Krasnikov, V.; Hadziioannou, G. Chem.
Commun. 1999, 617.
10 Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.; Liu, S.-G.; Echegoyen, L.; Barigelletti, F.; Armaroli, N.;
Ouali, L.; Krasnikov, V.; Hadziioannou, G. J. Am. Chem. Soc. 2000, 122, 7467
11 Peeters, E.; van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C.; Janssen, R. A. J. J.
Phys. Chem. B 2000, 104, 10174.
12 El-ghayoury, A.; Schenning, A. P. J. H.; van Hal, P. A.; Van Duren, J. K. J.; Janssen, R. A. J.; Meijer, E.
W. Angew. Chem. Int. Ed. 2001, 40, 3660.
13 Marcos Ramos, A.; Rispens, M.T.; van Duren, J. K. J.; Hummelen, J. C.; Janssen, R. A. J. J. Am. Chem.
Soc. 2001, 123, 6714.
14 De Boer, B.; Stalmach, U.; Van Hutten, P. F.; Melzer, C.; Krasnikov, V. V.; Hadziioannou, G. Polymer
2001, 42, 9097.
15 (a) Wohrle, D.; Meissner, D. Adv. Mater. 1991, 3, 129. (b) Schlettwein, D.; Wohrle, D.; Karmann, E.;
Melville, U. Chem. Mater., 1994, 6, 3. (c) Ferrere, S.; Zaban, A.; Gregg, B. A. J. Phys. Chem. B, 1997,
101, 4490.
16 Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.; Friend, R. H.; MacKenzie, J. D. Science
2001, 293, 1119.
17 Dittmer, J. J.; Marseglia, E. A.; Friend, R. H. Adv. Mater. 2000, 12, 1270.
18 Dittmer, J. J.; Petritsch, K.; Marseglia, E. A.; Friend, R. H.; Rost, H.; Holmes, A. B. Synth. Met. 1999,
102, 879.
19 Angadi, M. A.; Gosztola, D.; Wasielewski, M. R. J. Appl. Phys.1998, 83, 6187.
20 Weller, A. Z. Phys. Chem. Neue Folge 1982, 133, 93.
21 Van Hal, P. A.; Beckers, E. H. A.; Peeters, E.; Apperloo, J. J., Janssen, R. A. J. Chem. Phys. Lett. 2000,
328, 403.
22 The assumption that charges are located at the centers of the chromophores is of course a simplification
of the actual situation in which charges are likely to be strongly delocalized.
23 This assumption is based on the studies of the analogous OAn-OPV-C60 triad, described in chapter 2.
24 The small amount of PERY chromophore that might be excited will decay to the ground state by
fluorescence before encountering any other chromophore.
25 As reported in chapter 2, charge recombination in an analogous dyad, OAn+-OPV– with the OPV
featuring four phenyl rings instead of five, occurs in the nanosecond regime.
26 Salbeck, J. J. Electroanal. Chem. 1992, 340, 169.
Photoinduced multistep electron transfer in multichromophoric arrays
77
27 The molar absorption coefficient of the OPV chromophore at 700 nm has been extracted from the
photoinduced absorption spectrum of a OPV/MP-C60 (1:1) mixture in o-dichlorobenzene reported in
reference 21, using the extinction coefficients of the MP-C60 reported in Guldi, D. M; Prato, M. Acc.
Chem. Res. 2000, 33, 695.
28 Jortner, J. J. Chem. Phys. 1976, 64, 4860.
Chapter 4
Supramolecular control over
donor-acceptor photoinduced charge
separation
Abstract
A novel donor-bridge-acceptor system has been synthesized by covalently linking a p-phenylene
vinylene oligomer (OPV) and a perylene diimid (PERY) at opposite ends of a m-phenylene ethynylene
oligomer (FOLD) of twelve phenyl rings, containing non-polar, (S)-3,7-dimethyl-1-octanoxy side
chains. For comparison, also model compounds have been prepared in which either the donor or
acceptor is absent. In chloroform solution, the oligomeric bridge is in a random coil conformation.
Upon addition of a poor solvent (heptane) the oligomeric bridge first folds into a helical stack and
subsequently intermolecular aggregation of the stacks into columnar architectures occurs. In almost
pure heptane, the compounds bearing PERY chromophores exhibit higher degrees of aggregation. In
a random coil conformation, the small interaction between the donor and acceptor chromophores
allows for energy transfer from the OPV singlet-excited state to yield the PERY singlet-excited state.
In the folded and aggregated states of the bridge, donor and acceptor are in a favorable orientation
to generate the OPV•+-FOLD-PERY•– charge transfer product upon photoexcitation. The quantum
yield for charge separation depends on the degree of folding and aggregation of the bridge between
donor and acceptor and therefore on the apolar nature of the medium. As a consequence, and
contrary to conventional photoinduced charge separation processes, the formation of the OPV•+-
FOLD-PERY•– charge-separated state is more favored in apolar media.
Chapter 4
80
4.1 Introduction
Distance and orientation between covalently linked electron donors and acceptors are key
factors that determine the nature and kinetics of the photophysical processes that occur upon
illumination. Photoinduced charge separation is a reaction that is much more sensitive to the distance
between the chromophores than long-range energy transfer. In particular, for oligo(p-phenylene
vinylene)-perylene diimide (OPV-PERY) donor(D)-acceptor(A) dyads, direct connection of the redox
centers leads to a very fast charge separation, but also very fast charge recombination.1 When a
saturated spacer is interposed between the chromophores both processes are slowed down2 and if the
spacer is rigid and long, the OPV and PERY only exchange energy.3 Much effort has been given to
study the effects of distance and orientation in donor-acceptor dyads that are either directly linked4 or
connected via rigid, or flexible spacers.5
On/off switchability of intramolecular photophysical reactions can be achieved by the use of
supramolecular connections between donor and acceptor, such as hydrogen bonding3,6 and metal
coordination,7 or, in covalent D-bridge-A systems, by photoisomerization of the bridge.8 It is also
possible to modify the distance of covalently connected D-A dyads by modifying the conformation of
the flexible bridge between them. Two examples of that are complexation of the bridge with metal
cations9 and denaturation of the helical structure of peptide bridges.10
In the present work an OPV (donor) and a PERY (acceptor) have been linked at opposite ends
of a m-phenylene ethynylene oligomer made of twelve phenyl rings and containing non-polar (S)-3,7-
dimethyl-1-octanoxy side chains (FOLD) (Figure 4.1).
The type of oligomer that has been used as a bridge in OPV-FOLD-PERY, can go through
conformational changes in solution by varying the polarity of the media as well as the temperature,
when it is of sufficient length (n > 10, n being the number of phenyl rings).11 This behavior arises
from the difference in polarity between the aromatic hydrocarbon backbone and the side chains. In a
good solvent the oligomer adopts a random coil conformation and when a poor solvent (for the
backbone) is added, the oligomer folds into a helical stack, keeping the side chains exposed to the
solution. As a second step, aggregation of the helical stacks into larger stacks can also occur (Figure
4.2). A powerful technique to monitor the conformational changes is UV/Visible absorption
spectroscopy, as the different conformational states feature different absorption spectra. This has been
shown for oligomers featuring polar12 and apolar13 side chains. In both cases, the side chains contain a
chiral center, and this small perturbation is enough to bias the helical twist and as a result solutions of
the folded state show circular dichroism (CD) in the electronic absorption of light by the π system of
the backbone. For apolar side chain oligomers, the twist sense bias is only expressed when
intermolecular aggregation of the stacks into columnar architectures takes place.
Supramolecular control over donor-acceptor photoinduced charge separation
81
R*'O
R*'O
R*'O OR*'
OR*'
OR*'
N
OOR*
R*O OR*
OOR*
OR*
OR*
R*O
N
O
O
O
O
O
OR*O
O
OR*
OO
O
OOR*
O
OR*
OR*O
O
O
O
R*O =
R*'O =
Figure 4.1. Chemical structure of the OPV-FOLD-PERY dyad.
Figure 4.2. Schematic representations of the possible conformations of the FOLD oligomer upon
increasing the volume percentage of poor solvent in good solvent (from left to right).
The control over the conformation of these oligomers provides an unprecedented means to
influence the relative spatial orientation between donor and acceptor when FOLD is used as a bridge
in a dyad (OPV-FOLD-PERY, Figure 4.1). In a random coil conformation the OPV and PERY
chromophores are presumably too far away to interact and in the folded conformation or stacked state
they are expected to neighbor each other and to lead to high photophysical activity. In this chapter, the
synthesis of the OPV-FOLD-PERY dyad, with FOLD featuring apolar side chains, as well as the
synthesis of model compounds, in which either the donor or acceptor are missing, i.e. OPV-FOLD
and FOLD-PERY, are described. The conformational changes that the FOLD bridge undergoes by the
folding experiments are monitored with absorption and CD spectroscopy. Photoluminescence studies
of the different conformational states provide a measure of the interaction between donor and acceptor
in the excited state.
Chapter 4
82
4.2 Synthesis
The OPV-FOLD-PERY dyad 10 and its model compounds FOLD-PERY 7 and OPV-FOLD 9
have been synthesized following the synthetic approach previously developed for the m-phenylene
ethynylene oligomers. This synthetic strategy is based on the palladium-catalyzed Sonogashira
coupling, which allows for an orthogonal protecting group procedure and thus enables a divergent
modification of the folding oligomer. The monofunctional model compounds were obtained by
coupling either a PERY or an OPV moiety to one of the bridge oligomers, and the dyad was obtained
by selectively coupling of PERY and OPV at either side of the bridge. The synthesis of the bridged
donor-acceptor system and the model compounds is depicted in Scheme 4.1. To allow for the donor
and acceptor chromophores to be linked to the foldamer via the palladium-catalyzed cross-coupling
reaction, the OPV and the PERY chromophores had to be functionalized with ethynyl and iodine,
respectively. The building blocks for these functionalized chromophores are perylene monoanhydride
monoimide 114 and OPV 3,15 whose syntheses have been described elsewhere. Iodine-functionalized
perylene diimide 2 was obtained after reaction of 4-iodoaniline with the perylene monoanhydride
monoimide 1. Coupling of trimethylsilylacetylene with bromine 3 and subsequent deprotection with
tetrabutylammonium fluoride (TBAF) afforded the ethynyl end-capped OPV 4. Model compound 7
(FOLD-PERY) was obtained by coupling of iodoperylene diimide 2 with the trimethylsilylethynyl
end-capped dodecamer 6, in a Sonogashira reaction with in situ deprotection of the ethynyl functional
group.16 Reaction of 2 with previously deprotected dodecamer 6 resulted only in low yields due to the
occurrence of a considerable amount of homocoupled 6. Model compound 9 (OPV-FOLD) was
obtained via reaction of ethynyl OPV 5 with the iodine functionalized foldamer 8. This OPV-FOLD
was further reacted with 2 under similar conditions as used to obtain PERY-FOLD 7, utilizing the
ethynyl functionality at the other end of the chain, affording the OPV-FOLD-PERY bridged dyad 10
in low (9%) yield.
All compounds used in the photophysical investigations were characterized using 1H-NMR
spectroscopy, mass spectrometry, and size exclusion chromatography. Mass spectrometry established
the correct mass of the compounds and showed additional masses corresponding to extra methylene
groups. These additional signals arise due to the fact that the (S)-3,7-dimethyl-1-octanol used for the
m-phenylene ethynylene bridge is not completely pure but contained some alcohols of higher mass.
For each of the compounds a single, symmetrical peak was obtained by size exclusion
chromatography, revealing the absence of starting material or undesired longer adducts and that the
compounds are highly monodisperse.
Supramolecular control over donor-acceptor photoinduced charge separation
83
N
N
O
O
O
O
OR*O
OR*'
OR*'
OR*'
OR*'
OR*'
OR*'
OR*'
OR*'
OR*'
OR*'
OR*'
OR*'
Br
OR*'
OR*'
OR*'
OR*'
OR*'
OR*'
X
NN
O
O O
O
ION
O
O O
O
NN
O
O O
OOR*
O
H
OR*O
tms
H
OR*O
I
tms
OR*
O
tms
OR*'
OR*'
OR*'
OR*'
OR*'
OR*'
OR* = O
OR*' = O
12
10
f
3
b
c4. X = TMS
5. X = H
1 2
a
d
67
1212
12
12
e
98
Scheme 4.1. a. 4-Iodoaniline, imidazole, Zn(OAc)2, 160 °C, 1 h, 92%; b. Pd(PPh3)2Cl2, PPh3, CuI,
Et3N, 80 °C, 16 h, 45%; c. TBAF, THF, 1 min, r.t., 95%; d. 2, Pd(PPh3)4, KOAc, DMF, toluene, 100
°C, 16 h, 22%; e. 5, Pd(PPh3)2Cl2, PPh3, CuI, Et3N, 70 °C, 16 h, 48%; f. 2, 9, Pd(PPh3)4, KOAc,
DMF, toluene, 100 °C, 5 h, 9%.
Chapter 4
84
4.3 Conformational states of the bridge
4.3.1 Folding in chloroform/heptane mixtures
The UV/Visible absorption spectrum of OPV-FOLD-PERY in chloroform solution (Figure
4.3) shows the superposition of the individual electronic transitions of the FOLD11 bridge (288 and
304 nm), the OPV donor17 (430 nm), and the PERY acceptor1 (454, 490 and 526 nm) chromophores.
For OPV-FOLD and FOLD-PERY the absorption spectra display the related chromophore transitions.
300 400 500 6000.0
0.2
0.4
0.6
0.8
1.0
A
bsor
banc
e (O
. D.)
Wavelength (nm)
Figure 4.3. UV/Visible absorption spectra of OPV-FOLD-PERY (solid line), OPV-FOLD (dashed
line), and FOLD-PERY (dotted line) in chloroform solution.
The absorption spectrum of the FOLD bridge exhibits a shoulder at 304 and a peak at 288 nm
in chloroform solution. The ratio of the absorbance of the shoulder (As) and the peak (Ap) is
characteristic of the conformational state of the FOLD bridge.13 Transoidal conformations result in a
high As/Ap ratio, while cisoidal conformations lower the As/Ap ratio. The random-coil conformation,
in which transoidal conformations persist, is therefore characterized by a high As/Ap ratio, whereas
cisoidal conformations account for the formation of helices and a significant lower As/Ap ratio.
The high As/Ap absorption ratio observed in chloroform solution indicates that in the FOLD
bridge transoidal conformations dominate for OPV-FOLD, FOLD-PERY, and OPV-FOLD-PERY
and that the bridge is in a random coil conformation (Figure 4.3).13
In order to establish the conditions at which the bridge folds, solutions of OPV-FOLD-PERY
in chloroform/heptane mixtures of different composition were prepared and studied by means of
UV/Visible absorption and CD spectroscopy. With increasing amount of heptane in chloroform a
decrease of the As/Ap ratio of OPV-FOLD-PERY (Figure 4.4) is observed, characteristic of a helical
folding of the bridge.13 In addition, the shoulder at 304 nm and the peak at 288 nm observed in
chloroform, undergo small shifts with increasing the heptane content. Starting at 80% heptane, a
Cotton effect is observed in the UV region of the CD spectrum (Figure 4.4, inset). The appearance of
optical activity is accompanied by a small blue shift of the FOLD absorption band. In general, a CD
Supramolecular control over donor-acceptor photoinduced charge separation
85
effect in these m-phenylene ethynylene oligomers is only observed when intermolecular aggregation
of the molecular helices into columnar architectures occurs.13 As the relative amount of heptane
increases further, the overall FOLD absorption becomes less intense (hypochromicity) while the CD
effect grows. Above 95% heptane, an unexpected inversion of the chirality in the region of the FOLD
bridge occurs. In the absorption spectrum, the FOLD features a red–shifted broader absorption band
and a sudden small increase of the As/Ap ratio is observed.
300 400 500 6000,0
0,4
0,8
1,2
300 400 500 600
-60
-30
0
30
60
0%
80% 93%
96%
99%
CD
(mde
g)
Wavelength (nm)
Abs
orba
nce
(O. D
.)
Wavelength (nm)
Figure 4.4. UV/Visible spectra of OPV-FOLD-PERY in 0% (solid star), 60% (diamond), 80%
(triangle), 96% (circle) and 99% (solid square) volume heptane in chloroform. Inset: CD spectra of
OPV-FOLD-PERY in heptane/chloroform solutions of different composition (the numbers denote
volume percentage heptane in chloroform).
The absorption bands associated with the PERY chromophore also undergo some changes
upon the addition of heptane, which are a direct consequence of the conformational changes occurring
in the FOLD bridge. Figure 4.5 shows that the optical density of the lowest energy PERY absorption
band decreases considerably above 80% heptane content for both the OPV-FOLD-PERY dyad and the
FOLD-PERY model compound. This indicates aggregation of the PERY chromophores under these
conditions.
Chapter 4
86
0 20 40 60 80 100
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Nor
mal
ized
abs
orba
nces
Volume % heptane in chloroform
Figure 4.5. The absorbance due to the S1 ← S0 transition the PERY at 528 nm of OPV-FOLD-PERY
(solid squares) and FOLD-PERY (open squares) in chloroform/heptane solutions of different
composition.
The As/Ap ratios of the OPV-FOLD-PERY dyad, the OPV-FOLD and the FOLD-PERY
model compounds and the unsubstituted FOLD bridge are plotted against the volume percentage of
chloroform in heptane in Figure 4.6. This plot shows that all compounds possess a similar folding and
aggregation behavior with decreasing polarity of the medium, up to a heptane content of 95%, where
the compounds with a PERY chromophore start to deviate because of aggregation.
0 20 40 60 80 100
0.6
0.7
0.8
0.9
As/A
p
Volume % heptane in chloroform
Figure 4.6. Ap/As ratios of OPV-FOLD-PERY (solid star), FOLD-PERY (open triangle), OPV-FOLD
(open circle) and FOLD (open square) in chloroform/heptane solutions of different composition.
The inversion of chirality together with the loss of the isodichroic points observed for the
compounds bearing PERY, i.e. OPV-FOLD-PERY and FOLD-PERY, at high percentage of heptane,
Supramolecular control over donor-acceptor photoinduced charge separation
87
(Figure 4.4, inset) suggests that several columns could be interacting with each other laterally
resulting in multi-columnar architectures with a different overall chirality. The formation of this
highly collapsed state is driven by the high tendency of perylene diimid chromophores to aggregate.18
In summary, absorption and CD spectroscopy show that OPV-FOLD-PERY adopts different
conformational states depending on the volume percentage of heptane (Cartoon 4.1). In pure
chloroform the OPV-FOLD-PERY dyad is in a random coil conformation (state A) and upon the
addition of heptane it folds into the well-known helical conformation (state B). Above 70% heptane,
intermolecular aggregation of the helical stacks into columnar architectures (state C) occurs. Above
95% heptane, several columns may interact with each other laterally resulting in a highly compact
conformational state (state D) that resembles the solid state but still is soluble. In accordance with
previous studies on the oligomeric bridge, it is assumed that the bridge chain is in one of the proposed
states and the observed UV/Vis and CD spectra are linear combinations of the spectra of these four
conformations.11-13
A B C D
Donor AcceptorBridge
A B C DA B C D
Donor AcceptorBridgeDonor AcceptorBridge
Cartoon 4.1. Schematic drawing of the different conformational states of OPV-FOLD-PERY with
decreasing polarity of the medium (from left to right). A: random coil; B: folded (helical)
conformation; C: columnar stacks; D: aggregated state.
4.3.2 Folding in other solvents
The number of solvents in which the bridge of OPV-FOLD-PERY can fold is rather high.19
Different As/Ap ratios are observed in different solvents (Table 4.1) and indicate that OPV-FOLD-
PERY adopts a varying degree of intramolecular folding and intermolecular aggregation. In general,
the As/Ap ratio is lower for less polar solvents. The As/Ap ratio increases with Reichardt’s ENT index.20
The ENT index has found acceptance in the literature as a reliable and convenient measurement of
solvochromatic effects of solvents of different polarity.
Chapter 4
88
Table 4.1. As/Ap absorption ratios observed for OPV-FOLD-PERY dissolved in solvents of different
ENT.
solvent ENT As/Ap
heptane - 0.65
cyclohexane 0.006 0.61
carbon tetrachloride 0.052 0.76
dioxane 0.164 0.74
tetrahydrofuran 0.207 0.83
chloroform 0.259 0.90
The lowest As/Ap ratio is observed for the most apolar solvents, heptane and cyclohexane.
Only in these two solvents a Cotton effect is observed (Figure 4.7). The absence of optical activity
together with a low As/Ap absorption ratio in solvents like tetrahydrofuran, dioxane, and carbon
tetrachloride, indicates that the conformation of the bridge in solvents of medium polarity is restricted
to random coil or helical conformations of the bridge, and that no aggregation occurs (only
conformational states A and B are present).
250 300 350 400 450 500 550
-80
-60
-40
-20
0
20
40
60
CD
(m
deg)
Wavelength (nm)
THF Dioxane CCl
4
Cyclohexane Heptane
Figure 4.7. CD spectra of OPV-FOLD-PERY in different solvents.
4.4 Electronic properties and energetic considerations
To establish whether energy or electron transfer reactions between FOLD, OPV, and PERY
chromophores can be expected after photoexcitation, it is of interest to determine the energy of the
Supramolecular control over donor-acceptor photoinduced charge separation
89
various electronic states. Absorption and fluorescence spectroscopy in combination with cyclic
voltammetry have been used to obtain these parameters.
Absorption and photoluminescence spectroscopy. The UV/Visible absorption spectrum of
OPV-FOLD-PERY in chloroform solution (Figure 4.3) is a near superposition of the spectra of the
individual chormophores. The para connectivity between the OPV and the FOLD bridge results in the
extension of the conjugation length of the OPV moiety to at least the first phenyl ring of the bridge.
The meta linkage of the next phenyl ring disrupts further conjugation. Thus, although the OPV only
contains two vinylene bonds, the observed absorption resembles that of an OPV with three vinylene
bonds.17 The PERY chromophore preserves its spectroscopic identity after being connected to the
bridge and the observed absorption spectra corresponds to the individual perylene diimid
chromophore.1,2
The photoluminescence spectra of the donor and acceptor moieties have been measured in
chloroform solution after selective excitation of the OPV and PERY chromophores in the OPV-FOLD
and FOLD-PERY model compounds respectively (Figure 4.8). The emission of the OPV
chromophore maximizes at 502 nm. The photoluminescence of the PERY consists of an emission
band with vibronic fine structure and has a maximum intensity at 535 nm.
Hence, absorption and photoluminescence spectroscopy reveal that in OPV-FOLD-PERY the
optical bandgap, and hence the energy of the excited singlet state, decreases from the FOLD bridge
(4.55 eV), via OPV (2.47 eV), to the PERY (2.32 eV) chromophore. As a consequence singlet-energy
transfer may occur in the series: 1FOLD* → 1OPV* → 1PERY* (Figure 4.9).
450 500 550 600 650 7000
50
100
150
Inte
nsity
(co
unts
/104 )
Wavelength (nm)
Figure 4.8. Fluorescence spectra of OPV-FOLD (dashed line) and FOLD-PERY (dotted line) after
photoexcitation of the OPV and PERY chromophores respectively.
Chapter 4
90
Cyclic voltammetry. The oxidation and reduction potentials of the redox centers have been
measured for the donor OPV-FOLD and acceptor FOLD-PERY reference compounds and are
assumed to be identical in the OPV-FOLD-PERY dyad. The cyclic voltammogram of OPV-FOLD
exhibits two reversible oxidation waves at 0.83 and 1.05 V, while for FOLD-PERY two reversible
reduction waves are found at –0.60 and –0.82 V (potentials are given vs. SCE, recorded in
dichloromethane with 0.1 M TBAPF6). In the –1.0 to +1.0 V range no redox reactions occur that are
associated with the FOLD bridge.
Energetic considerations. The most probable photophysical processes after photoexcitation
of either donor or acceptor in dyad OPV-FOLD-PERY are schematically represented in Figure 4.9.
Photoinduced electron transfer from the OPV donor to the PERY acceptor can only occur
when the OPV•+-FOLD-PERY•– charge-separated state is lower in energy than the PERY singlet-
excited state (OPV-FOLD-1PERY*) and competes with the intrinsic relaxation processes (mainly
fluorescence) of the 1PERY* state. Contributions of the FOLD bridge in charge transfer processes,
either in the random coil or helical stacked conformations, are not likely because of the high redox
potential of the FOLD segment.
OPV-FOLD-PERY
kET
kCR
kPLOPV
kPLPERY
Energy
1OPV*-FOLD-PERY
OPV-FOLD-1PERY*
OPV+-FOLD-PERY-
kCSPERY
kCSOPV
OPV-1FOLD*-PERY
kPLFOLD
kET
kET
OPV-FOLD-PERY
kET
kCR
kPLOPV
kPLPERY
EnergyEnergy
1OPV*-FOLD-PERY
OPV-FOLD-1PERY*
OPV+-FOLD-PERY-
kCSPERY
kCSOPV
OPV-1FOLD*-PERY
kPLFOLD
kET
kET
Figure 4.9. Schematic diagram describing the energy levels of the singlet and charge-separated states
of OPV-FOLD-PERY.
Supramolecular control over donor-acceptor photoinduced charge separation
91
Most of the folding experiments of the bridge in OPV-FOLD-PERY presented in this chapter
are performed by the addition of heptane to a chloroform solution of the dyad. Two parameters with
opposite effects on the charge separation reaction change as the amount of heptane is increased. On
one hand, at high volume percentages of heptane, folding of the bridge occurs which reduces the
distance between donor and acceptor and, thereby, favors the charge transfer. On the other hand, the
low polarity of heptane compared to chloroform, makes this solvent less effective in screening the
photogenerated charges and thereby will increase the energy of the OPV•+-FOLD-PERY•– charge-
separated state.
An estimate of the change in Gibbs free energy for photoinduced electron transfer in solution
for the OPV-FOLD-PERY dyad can be obtained from the Weller equation: 21
( ) ( )( )
−
+−−−−=∆ −+
sref0
2
0
2
00redox0 1111
84AD
εεπεεπε rr
e
R
eEEEeG
ccsCS (4.1)
In this equation, Eox(D) and Ered(A), are the oxidation and reduction potentials of the donor
and acceptor molecules or moieties respectively measured in a solvent with relative permittivity εref,
E00 is the energy of the excited state from which the electron transfer occurs (PERY singlet-excited
state), and Rcc is the center-to-center distance of the positive and negative charges in the charge-
separated state. The radii of the positive and negative ions are given by r+ and r- and εs is the relative
permittivity of the solvent, -e is the elemental charge, and ε0 is the vacuum permittivity.
The limiting distance at which charge separation becomes no longer exergonic in heptane is
21 Å (Table 4.2), as calculated from equation 4.1 taking Eox(D) = 0.83 V, Ered(A) = -0.60 V and
setting the ionic radii to r– = 4.7 Å2 and r+ = 5.6 Å.15 Larger distances will result in an OPV•+-FOLD-
PERY•– charge-separated state that is less stable than the PERY singlet excited state.
Table 4.2. Change in Gibbs free energy change for intramolecular and intermolecular electron
transfer in OPV-FOLD-PERY dyad in chloroform and heptane calculated from equation 4.1.
Solvent Rcc
(Å) CSG∆ (PERY (S1))
(eV)
CSG∆ (OPV (S1))
(eV)
Heptane 106 0.28 0.11
21 0.00 -0.17
Chloroform 106 -0.61 -0.78
21 -0.73 -0.90
Chapter 4
92
In the more polar solvent chloroform, charge separation is in principle feasible ( CSG∆ <0)
even when the molecule is in a completely extended configuration (distance ~106 Å). However,
kinetic factors will impede the intramolecular charge transfer at such large distances, because the
electronic coupling between donor and acceptor in the excited state is negligible at this distance.
4.5 Photoinduced energy and electron transfer in different conformational states of the bridge
The interaction between donor and acceptor in the excited state can be investigated with
photoluminescence and photoinduced absorption spectroscopies. Quenching of either donor or
acceptor fluorescence indicates an energy or charge transfer process. Photoinduced absorption
spectroscopy (PIA) can be used to monitor the formation and decay of the OPV•+-FOLD-PERY•–
charged separated state. In the following paragraphs the donor acceptor interaction in the excited state
is first described for a random coil conformation followed by the results for folded conformations and
aggregated states.
4.5.1 Bridge in a random coil conformation
In chloroform, the bridge is in a random-coil conformation. Hence, the relative orientation of
donor and acceptor chromophores for OPV-FOLD-PERY is not well defined and probably numerous
conformers exist.
500 600 7000
50
100
150
OPV-FOLD-PERY OPV-FOLD
Inte
nsity
(co
unts
/104 )
Wavelength (nm)
500 600 7000
50
100
150
OPV-FOLD-PERY FOLD-PERY
Inte
nsity
(co
unts
/104 )
Wavelength (nm)
Figure 4.10. Left: Photoluminescence spectra of OPV-FOLD-PERY and OPV-FOLD in chloroform
solution after selective photoexcitation of the OPV moiety at 380 nm. Right: Photoluminescence
spectra of OPV-FOLD-PERY and FOLD-PERY in chloroform solution after selective photoexcitation
of the PERY moiety at 512 nm.
Photoluminescence spectroscopy reveals that, in chloroform, the two chromophores interact
in the excited state. Selective photoexcitation of the OPV unit in OPV-FOLD-PERY at 380 nm results
in emission of the PERY part (λem, max = 537 nm), concomitant with a reduction of the fluorescence
Supramolecular control over donor-acceptor photoinduced charge separation
93
intensity of the OPV chromophore (λem, max = 504 nm) compared with the fluorescence of the OPV-
FOLD compound (Figure 4.10, left).
In contrast, the quantum yield of the PERY emission, after selective photoexcitation of the
PERY moiety in OPV-FOLD-PERY at 512 nm, approaches that of the FOLD-PERY reference
compound (Figure 4.10, right). According to the same diagram in Figure 4.9, a quenching of the
PERY emission would indicate the formation of the OPV•+-FOLD-PERY•– charge-separated state.
Because the PERY emission is not significantly quenched, photoinduced charge separation is
negligible in chloroform. In a random coil conformation, the OPV-FOLD-PERY dyad resembles D-
bridge-A systems with a saturated hydrocarbon chain as a bridge, albeit with less conformational
freedom. Out of all possible conformations in chloroform, barely a few will provide sufficient
closeness between the OPV donor and the PERY acceptor to allow for charge transfer to occur.
The excitation spectrum of the PERY emission nearly coincides with the absorption spectrum
of OPV-FOLD-PERY (Figure 4.11). The overlap is particularly good in the region of the PERY and
OPV absorption. This implies that singlet-energy transfer to the PERY chromophore occurs after
photoexcitation of OPV chromophore, and to a considerable extent also after exciting the FOLD
(Figure 4.9). However, there is a small mismatch between the absorption and excitation spectra in the
region of the FOLD absorption, indicating that energy transfer from the bridge is not complete. The
excitation spectrum of the OPV emission also reveals that energy transfer occurs from the FOLD
bridge onto the OPV chromophore. The difference between the absorption and excitation spectra is
larger for the OPV moiety. This indicates that some, but not all, of the photoexcitations of the FOLD
oligomer transfer their excited state energy first to the OPV chromophore, before ending up at the
PERY moiety.
300 400 500 6000
1
2
3
4
Inte
nsity
(a.
u.)
Wavelength (nm)
Figure 4.11. Excitation spectra of the 496 nm (solid triangles) and 535 nm (solid squares) emission
and absorption spectrum of OPV-FOLD-PERY, in chloroform.
Chapter 4
94
Because the excitation spectrum of OPV-FOLD-PERY coincides with the absorption
spectrum above 350 nm, the quenching of the OPV fluorescence is mainly due to a intramolecular
photoinduced singlet-energy transfer from the 1OPV* towards the more stable PERY the singlet-
excited state (Figure 4.9). The quenching of the OPV emission can be used to estimate an average
distance between the donor and acceptor using the Förster model for energy transfer.22 This model,
which is based on a dipole-dipole interaction mechanism, provides an expression for the rate for the
energy transfer, FETk , via:
6
1
=
d
Rk cF
ET τ (4.2)
In this equation τ is the lifetime of the donor chromophore, cR is the critical transfer radius
and d the center-to-center distance between the redox centers, the parameter that we wish to
determine. Using the OPV fluorescence quenching (Q = 4.16) and the fluorescence lifetime of the
OPV segment in OPV-FOLD-PERY (τ = 1.41 ns) the experimental rate constant for energy transfer
of kET = 2 ns-1 can be obtained via the expression:
τ
1−= maz
ETQ
k (4.3)
cR can be calculated using the Förster equations:
45
6
1283
2)10ln(9000
nN
JR
F
c⋅⋅⋅
⋅⋅⋅⋅=
π
φ (4.4)
∫
∫=
dvvf
dvv
vvf
J F )(
))(
).((4
ε
(4.5)
In expressions 4.4 and 4.5, φ is the luminescence quantum yield of OPV(S1) (φ = 0.84, for
OPV-FOLD), N the Avogadro constant, and n the refractive index of the solvent (1.444). FJ is the
overlap integral of the luminescence spectrum of the donor ( )(vf of OPV) on an energy scale (cm-1)
and the absorption spectrum of the acceptor ( )(vε of PERY). For OPV-FOLD-PERY the overlap
integral is FJ = 2.5 × 10-13 cm6/mol. The average distance between the chromophores is then d = 5.5
nm. This implies that the foldamer does not adopt an extended configuration, in which the center-to-
center distance is of roughly 10 nm, but as expected, a random conformation (Cartoon 4.2).
Supramolecular control over donor-acceptor photoinduced charge separation
95
5.5 nm
Cartoon 4.2. Chemical structure of one of the many possible random coil conformations of OPV-
FOLD-PERY in chloroform solution.
4.5.2 Bridge in a folded conformation
4.5.2.1 Folded bridge in heptane/chloroform mixtures
Steady-state and time-resolved photoluminescence spectroscopy. To study the influence
of the decreasing polarity and the associated folding and intermolecular aggregation of the FOLD on
the emission of the individual chromophores, experiments were performed on the OPV-FOLD and
FOLD-PERY model compounds in solvents with different heptane/chloroform content.
The fluorescence intensity of OPV-FOLD increases upon addition of heptane (Figure 4.12).
The absence of any quenching in OPV-FOLD with decreasing polarity excludes the possibility of
OPV-OPV aggregates in any heptane/chloroform mixture.
500 600 7000
20
40
60
80
100
120 Heptane
CHCl3
Inte
nsity
(co
unts
/104 )
Wavelength (nm)
Figure 4.12. Steady-state photoluminescence spectra of OPV-FOLD in chloroform/heptane mixtures
of different composition.
Chapter 4
96
0 20 40 60 80 100
50
100
150
500 550 600 650 700 7500
50
100
150
Inte
nsity
(co
unts
/104 )
Wavelength (nm)
Inte
nsity
(co
unts
/104 )
Volume % heptane in chloroform
0 50 100 1501
10
100
1000
10000
99% Heptane85% Heptane
80% Heptane70% Heptane
CHCl3
Inte
nsity
(co
unts
)
Time (ns)
Figure 4.13. Left: Steady-state photoluminescence spectra of FOLD-PERY model compound in
solutions of chloroform/heptane mixtures of different composition. Inset: Maximum fluorescence
intensity of FOLD-PERY in chloroform/heptane mixtures of different composition (The fluorescence
intensities have been corrected for small deviations from O.D. = 0.1 at the excitation wavelength).
Right: Time profiles of the fluorescence at 600 nm of FOLD-PERY model compound in solutions of
chloroform/heptane mixtures of different composition.
Quite on the contrary, the photoluminescence of the acceptor chromophore in FOLD-PERY
undergoes a dramatic quenching starting from 80% of heptane in chloroform (Figure 4.13, left and
inset therein). This quenching coincides with a decreased intensity of the lowest energy PERY
absorption band (Figure 4.5) and is consistent with aggregation of the PERY chromophores and with
intermolecular aggregation of the helixes (state C) above 80% heptane. Moreover, at high heptane
contents, the photoluminescence spectra of FOLD-PERY feature an additional emission centered at
approximately 630 nm, associated with perylene excimers.23 Photoluminescence lifetime
measurements of the PERY emission at 600 nm reveal that the perylene aggregates (with an excited-
state lifetime of τ = 30 ns vs. ~ 4 ns of molecularly dissolved PERY chromophores) start to appear
already at 70 volume percent heptane and become the predominant species in almost pure heptane
(Figure 4.13, right).
For OPV-FOLD-PERY, an increase of the percentage of heptane results in a quenching of the
PERY emission, regardless which of the two chromophores is excited (Figure 4.14, left and right). In
such apolar environment, the folding of the bridge (state B) and the formation of intermolecular stacks
(states C and D) decreases the distance between donor and acceptor. When the two chromophores are
in close proximity, charge separation may occur in the excited state. The lack of excimer emission at
630 nm for OPV-FOLD-PERY at all chloroform/heptane ratios, indicates the absence of perylene
diimid excimers and leaves charge separation as the most likely fluorescence quenching process
Supramolecular control over donor-acceptor photoinduced charge separation
97
(Figure 4.9). The absence of a red-shifted emission also implies that donor and acceptor do not form
an emissive exciplex prior to electron transfer.
450 500 550 600 6500
40
80
120
99% Heptane
CHCl3
Inte
nsity
(co
unts
/104 )
Wavelength (nm)550 600 650 700
0
40
80
120
99% Heptane
CHCl3
Inte
nsity
(co
unts
/104 )
Wavelength (nm)
Figure 4.14. Steady-state photoluminescence spectra of OPV-FOLD-PERY in chloroform/heptane
solutions of different composition after selective excitation of the OPV at 380 nm (left) and the PERY
at 510 nm (right).
Time-resolved fluorescence of OPV-FOLD-PERY measured at 600 nm (Figure 4.15) reveals
that the lifetime of the PERY emissions is ~ 4 ns up to 80% heptane, while above 80% slightly shorter
lifetimes are observed. The absence of a 30 ns lifetime component in the fluorescence decay curves
upon the addition of heptane supports the absence of perylene diimid excimers in the aggregated
states in OPV-FOLD-PERY (states B to D, Cartoon 4.1). On the other hand, the absence of a
reduction of the lifetime above 80% heptane is surprising, because a reduction of the PERY emission
is observed in steady-state fluorescence (Figure 4.14, right). This suggests that the electron transfer
reaction that causes the PL quenching is so fast in state B that that the corresponding lifetime is
shorter than the time resolution of the experimental set up and is not detected by time-resolved
photoluminescence. Such a fast formation of the charge-separated state is rarely observed in solution,
unless donor and acceptor are directly connected1 or in a face-to-face orientation,5h,k,o especially when
taking into account that the decreasing polarity of the medium usually slows down the rate for the
charge separation. Hence, within the folded state (state B) only in conformations where donor and
acceptor are placed in a face-to-face orientation, charge transfer may take place as a predominant
decay route, while for less favorably folded conformers radiative decay of 1PERY* will occur
(Cartoon 4.3).
Chapter 4
98
0 20 40 60 80 1001
10
100
1000
10000 100 % Heptane 80% Heptane 70% Heptane 60% Heptane 30% Heptane 0% Heptane
Inte
nsity
(co
unts
)
Time (ns)
Figure 4.15. Time-resolved photoluminescence spectra of OPV-FOLD-PERY in chloroform/heptane
solutions of different composition.
-+-+-+
Cartoon 4.3. Conformational states B of OPV-FOLD-PERY. Only those molecules with appropriate
orientation of donor and acceptor yield the OPV+-FOLD-PERY– charge-separated state after
photoexcitation. In other conformations, relaxation to the ground state takes place via PERY
emission.
In the region above 80% heptane, where intermolecular and inter-columnar aggregation
occurs (states C and D, Cartoon 4.1), more donor and acceptors are brought in close proximity and
intermolecular charge separation can occur in addition to the intramolecular process.
A comparison of the steady-state photoluminescence of PERY-FOLD and OPV-FOLD-PERY
gives an estimate of the extent of charge transfer in the dyad. The significant quenching of the PERY
photoluminescence upon addition of heptane to chloroform solutions of OPV-FOLD-PERY strongly
suggests that charge separation occurs to yield the OPV•+-FOLD-PERY•– charge-separated state. For
low amounts of heptane the charge separation takes place only to a small extent and upon folding and
intermolecular aggregation in OPV-FOLD-PERY the quantum yield for charge transfer increases.
Above 80% heptane the comparison of the two intensities is no longer valid, because the excimer
Supramolecular control over donor-acceptor photoinduced charge separation
99
formation of FOLD-PERY results in a quenching of the fluorescence and that is not associated with
charge separation. The observed photoluminescence quenching supports the proposed conformational
exchanges in OPV-FOLD-PERY inferred from absorption and CD spectroscopy.
0 20 40 60 80 100
0
4
8
τ (n
s)
Volume % heptane in chloroform
50
100
150
CS
Inte
nsity
(co
unts
/104 )
Figure 4.16. Photoluminescence intensities (above) and lifetimes (below) of the PERY emission in
OPV-FOLD-PERY (solid squares) and FOLD-PERY (open squares) in chloroform/heptane mixtures
of different composition. The extent of charge separation can be estimated from the difference
between the two emission intensities. The arrows denote the amount of the charge separation (CS).
Femtosecond pump-probe spectroscopy. The formation and decay of the photoinduced
charges have been followed by means of transient photoinduced absorption spectroscopy. In this
experiment, the OPV chromophore is excited at 450 nm and the absorption at 1450 nm associated
with the OPV•+ radical cation is monitored in time.2
At early stages of intermolecular aggregation of the bridge in OPV-FOLD-PERY (60%
volume heptane in chloroform, state B), the differential transmission at 1450 nm is not very intense
(Figure 4.17, left). The small signal is in agreement with the low quenching of the PERY emission
observed at 60% heptane (Q ≈ 2, Figure 4.16). The time profile of the 1450 nm transient absorption
indicates that the charges are formed within 1 ps. This rapid formation of the OPV•+-FOLD-PERY•–
charge-separated state is consistent with the absence of short lifetimes in the time-resolved
photoluminescence of the PERY chromophore and is attributed to occur for conformations of OPV-
FOLD-PERY with a face-to-face orientation of donor and acceptor moieties (Cartoon 4.3). The decay
of the signal cannot be fitted to a single exponential function, suggesting that several charged species
are involved in the measurement. A feasible explanation for this result could be the conformational
Chapter 4
100
heterogeneity of the system: in the folded conformation of the bridge (state B), the position between
donor and acceptor is dynamic and not all conformers provide the face-to-face orientation between
donor and acceptor required for immediate charge transfer. The fact that the signal is low suggests
that few systems have the required face-to-face orientation, and the fact that the signal does not grow
with time implies that the conformational changes are slow on the time scale of the experiment.
-20 0 20 40 60 80 100-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
1.0
∆T
(a.
u.)
Time (ps)
0 100 200 300 400 500-120
-100
-80
-60
-40
-20
0
∆T (
a. u
.)
Time (ps)
Figure 4.17. Differential transmission dynamics for OPV-FOLD-PERY in mixtures of
chloroform/heptane of 60:40 (left) and 99:1 (right) at the radical cation absorption of OPV at 1450
nm with excitation of primarily OPV at 450 nm from –20 to 100 ps (left) and –100 to 500 ps (right).
At almost 100% volume heptane, the transient absorption is very intense (Figure 4.17, right).
This is consistent with the much higher PL quenching and attributed to intermolecular photoinduced
electron transfer in the aggregated state. The helical conformations have folded into columnar
architectures, and even several columns may be interacting with each other (state D, Cartoon 4.1).
More donor and acceptors are brought in close proximity and apart from intramolecular processes,
also intermolecular charge separation takes place. Under these conditions, the formation of the
charges is immediate, even though the heptane content is the highest and the medium is strongly
apolar. In strongly aggregated states the polarity of the solvent is less relevant, because charge transfer
takes place as it would do in a solid-state blend of the donor and acceptor. Paddon-Row et al.
observed that in a face-to-face orientated donor-acceptor dyad, photoinduced electron transfer took
place within 1 ps, irrespective of the polarity of the medium.5b Again the decay of the 1450 nm signal
cannot be fitted to a single exponential function. However, the charged-separated states are much
longer lived at high percentage of heptane, than in 60% heptane. These results can be rationalized by
migration of the charges to different parts of the aggregates.
Supramolecular control over donor-acceptor photoinduced charge separation
101
4.5.2.2 Folded bridge in other solvents
Analogous to the behavior in different heptane/chloroform mixtures, a decrease of the solvent
polarity results in an increasing quenching of the PERY photoluminescence in OPV-FOLD-PERY
(Figure 4.18, left). The quenching in these solvents is also attributed to photoinduced charge
separation from the OPV to the PERY chromophore caused by the folding of the bridge. There seems
to be a direct relation between the As/Ap ratio and fluorescence quenching in different solvents of
different polarity (Figure 4.18, right). For example, the observed As/Ap ratio and the quenching of the
PERY emission are comparable in carbon tetrachloride and a mixture of heptane/chloroform (60:40)
(Figure 4.16 and 4.18 left).
500 600 7000
50
100
150
200
Inte
nsity
(co
unts
/104 )
Wavelength (nm)
CHCl3
THF Dioxane CCl
4
Cyclohexane Heptane
-5 0 5 10 15 20 25 30 350.5
0.6
0.7
0.8
0.9
1.0
AS/A
P
Quenching
Figure 4.18. CD and PL spectra of OPV-FOLD-PERY in different solvents (left) and quenching ratio
vs. the AS/AP ratio in chloroform/heptane mixtures of different composition (open squares) and in pure
solvents of different polarity (solid squares) (right).
These results show that the conformational folding of OPV-FOLD-PERY and the resulting
enhancement of the photoinduced charge transfer is a general phenomenon that can be achieved and
controlled by modifying the polarity of the solvent, either in solvent mixtures of by varying the nature
of the pure solvent.
4.5 Conclusions
A novel donor-bridge-acceptor system has been synthesized by connecting OPV donor and
PERY acceptor chromophores at the opposite ends of a long foldable cross-conjugated oligomer,
FOLD. The conformation of this bridge can be controlled between random coil and helically folded
states, by changing the polarity of the medium.
In chloroform solution the bridge is in a random coil conformation. As a consequence most
donor and acceptor chromophores are too far apart to undergo a photoinduced charge transfer
reaction, and only energy transfer from the 1OPV* singlet excited state to the PERY occurs (Figure
Chapter 4
102
4.9), as inferred from photoluminescence spectroscopy (Figure 4.10). The average distance between
OPV and PERY chromophores in chloroform, as estimated by the Förster equations for energy
transfer, is 5.5 nm. This distance supports the idea that the FOLD oligomer in chloroform is not in a
completely extended conformation (~ 10 nm) but adopts a random coil conformation.
Upon addition of heptane to chloroform solutions of OPV-FOLD-PERY, the bridge folds up
into a helix (state B, Cartoon 4.1) and, with increasing amount of heptane, these helices aggregate into
columnar architectures (state C, Cartoon 4.1). In almost pure heptane (starting from 80% heptane in
chloroform), an additional higher degree of aggregation has been inferred from further changes in
absorption and CD spectra (Figure 4.4). The observed inversion of the chirality suggests that several
columns might interact with each other laterally, resulting in multi-columnar architectures with a
different overall chirality (state D, Cartoon 4.1). A comparison of the spectra of the OPV-FOLD-
PERY dyad with the model compounds OPV-FOLD, FOLD-PERY, and the unsubstituted bridge
FOLD shows that state D only forms in compounds containing the PERY chromophore. Possibly, the
multi-columnar architectures in state D originate from the tendency of the PERY chromophore to
aggregate. In the collapsed states of OPV-FOLD-PERY, OPV and PERY are prone to be involved in
a photoinduced charge transfer, because of a decreased distance. In state B, intramolecular
photoinduced charge transfer occurs within 1 ps. The high speed suggests that electron transfer only
occurs in (helical) conformers that provide a face-to-face orientation of donor and acceptor (Cartoon
4.3). In states C and D, two phenomena favor photoinduced electron transfer. On one hand, more
chromophores are placed in close proximity. On the other hand, under aggregated conditions, the
chromphores experience a ‘solid state environment’ and, hence, the solvent is less important with
respect to screening of charges and solvating ions.
The folding of the bridge and the associated changes in photoinduced reactions of OPV-
FOLD-PERY not only occurs for high heptane/chloroform ratios, but also is a general phenomenon in
less polar solvents.
In conclusion, it has been shown that the bridge in OPV-FOLD-PERY provides a unique
means to change the distance and orientation, and thereby the photophysical interaction between
donor and acceptor. Full control over the hierarchical build-up of a supramolecular D-A system has
been obtained, starting from two semi-isolated chromophores showing only energy transfer, via
intermediates capable of rendering electron transfer, into an aggregated state featuring, in addition to
electron transfer, charge migration. This donor-foldamer-acceptor dyad can be considered as a first
step towards the supramolecular construction of synthetic architectures that are able to perform
complicated photophysical and photochemical processes, similar to those observed in the
photosynthetic reaction center. Such appealing synthetic analogues may find future application in
photocatalytic reactions and solar light energy conversion.
Supramolecular control over donor-acceptor photoinduced charge separation
103
4.6 Experimental section
For general methods and materials the reader is referred to chapter 2 of this thesis. N-(1-Ethylpropyl)-N’-(4-iodophenyl) perylene diimide (2). Perylene monoanhydride monoimide 1 (38.6 mg, 0.08 mmol), 4-iodoaniline (182 mg, 0.83 mmol), imidazole (0.53g, 7.8 mmol), and catalytic amounts of Zn(AcO)2 were mixed and stirred for 1 h at 160 °C. After cooling to room temperature the solid reaction mixture was extensively washed with methanol to yield 49 mg of 2 (92%) as a dark red powder. 1H NMR (CDCl3, 300 MHz): δ 8.66 (t, 4H), 8.56 (d, 4H), 7.90 (d, 2H), 7.12 (d, 2H), 5.11-5.00 (m, 1H), 2.35-2.17 (m, 2H), 2.03-1.89 (m, 2H), 0.95 (t, 6H); 13C NMR (CDCl3, 100 MHz): δ 138.65, 135.17, 134.77, 134.09, 131.81, 131.38 (broad signal), 130.60, 129.72, 129.46, 126.57, 126.29, 123.32, 123.00, 94.66, 57.81, 25.01, 11.38. MALDI-TOF MS (Mw =662.49) m/z=662.19 [M]+. (E,E)-4-{4-(4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]-4-trimethylsylilethynylbenzene (4). Bromide 3 (0.2 g, 0.22 mmol), PdCl2 (3.97 mg, 0.02 mmol), PPh3 (17.6 mg, 0.07 mmol) and Cu(AcO)2 (4.47 mg, 0.02 mmol) were dissolved in 5 mL anhydrous triethylamine. Argon was purged trough the solution for 30 minutes after which (trimethylsilyl)acetylene (44 mg, 0.44 mmol) was added. The reaction mixture was heated at 80 °C for 16 h. The solvent was removed in vacuo, the residue dissolved in methylene chloride and washed with NH4Cl, water and dried over MgSO4. Purification by column chromatography (chloroform:pentane, 1:1, Rf= 0.2) afforded 92 mg (45%) of compound 4. 1H NMR (CDCl3, 300 MHz): δ 7.52 (d, 1H), 7.50 (d, 1H), 7.45 (d, 1H). 7.44 (d, 1H), 7.18 (s, 1H), 7.16 (s, 1H); 7.12 (s, 1H), 7.10 (s, 1H), 6.94 (s, 1H), 6.72 (s, 1H), 3.92-3.67 (m, 12H), 2.22 (s, 3H), 2.00-1.84 (m, 6H), 1.70-1.55 (m, 6H), 1.40-1.25 (m, 6H). 1.11-1.02 (m, 18H), 1.02-0.95 (m, 18H), 0.27 (s, 9H); 13C NMR (CDCl3, 100 MHz): δ 154.91, 151.67, 151.20, 150.93, 150.46, 150.20, 129.03, 127.95, 127.62, 126.76, 125.15, 123.97, 123.20, 122.16, 121.64, 117.38, 116.28, 112.06, 110.14, 109.92, 109.60, 108.38, 101.79, 98.91, 74.66, 74.25, 74.14, 73.38, 35.10, 35.04, 35.01, 34.98, 34.95, 26.36, 26.32, 26.25, 26.14, 16.78, 16.70, 16.52, 16.40, 11.44, 11.36, 0.01. MALDI-TOF MS (Mw = 909.37) m/z=908.35 [M]+. (E,E)-4-{4-(4-Methyl-2,5-bis[(S)-2-methylbutoxy]styryl)-2,5-bis[(S)-2-methylbutoxy]styryl}-2,5-bis[(S)-2-methylbutoxy]-4-ethynylbenzene (5). To a solution of 4 (36.7 mg, 0.04 mmol) in dry THF was added 1 M tetrabutylammonium fluoride in THF (0.044 mL). The reaction mixture was stirred for 1 min and subsequently filtered over silica gel using chloroform as eluent. The solvent was removed in vacuo yielding 32 mg (95%) of a yellow solid which was used without further purification. 1H NMR (CDCl3, 300 MHz): δ 7.53 (d, 1H), 7.51 (d, 1H), 7.45 (d, 1H), 7.44 (d, 1H), 7.18 (s, 1H), 7.16 (s, 1H); 7.15 (s, 1H), 7.10 (s, 1H), 6.97 (s, 1H), 6.72 (s, 1H), 3.92-3.67 (m, 12H), 3.31 (s, 1H), 2.22 (s, 3H), 2.00-1.84 (m, 6H), 1.70-1.55 (m, 6H), 1.40-1.25 (m, 6H). 1.11-1.02 (m, 18H), 1.02-0.95 (m, 18H). MALDI-TOF MS (Mw = 837.25) m/z = 836.51 [M]+. PERY-FOLD (7). A mixture of 2 (11 mg, 0.016 mmol), 5 (39.8mg, 0.0014 mmol), Pd(PPh3)4 (1.06 mg, 0.0009 mmol) and potassium acetate (2 mg, 0.020 mmol) in dry toluene (1 mL) and N,N-dimethylformamide (1 mL) was heated to 100 °C for 24 hours, and subsequently the solvent was evaporated in vacuo. The residue was triturated with water and extracted with methylene chloride. The methylene chloride extract was worked up to give crude products, which were purified by flash column chromatography (CH2Cl2/pentane to 9:1, Rf = 0.4) to yield 10 mg (22 %) of 4 as a light red solid. 1H NMR (CDCl3, 400 MHz): δ 8.77-8.67 (m, 8H), 8.20-8.17 (m, 22 H), 8.03 (ddd, 1H), 7.92-7.87 (m, 11H), 7.75 (d, 2H), 7.70 (ddd, 1H), 7.45 (t, 1H), 7.38 (d, 2H), 5.10-5.02 (m, 1H), 4.50-4.32 (m, 24 H), 2.33-2.22 (m, 2H), 2.00-1.90 (m, 2H), 1.90-1.78, (m, 12 H), 1.70-1.47 (m, 36H), 1.40-1.12(m, 72 H), 1.00-0.90 (m, 42 H), 0.88-0.84 (m, 72H); 13C NMR (CDCl3, 100 MHz): δ 165.23, 165.18, 165.12, 163.43, 138.24, 135.70, 135.34, 134.31, 132.74, 132.67, 132.42, 131.97, 131.44, 131.37, 130.95, 129.90, 129.71, 129.58, 128.93, 128.56, 126.77, 126.46, 124.06, 123.89, 123.58, 123.49, 123.42, 123.12, 122.99, 90.34, 89.18, 89.11, 89.02, 88.92, 64.30, 63.96, 57.76, 50.89, 39.18, 37.10, 35.51, 29.94, 29.70, 27.94, 25.01, 24.59, 22.69, 22.59, 19.66, 11.35. MALDI-TOF MS (Mw =3949.19) m/z= 3948.18 [M]-. OPV-FOLD (9). A mixture of 8 (38.7 mg, 0.01 mmol), 5 (11.7 mg, 0.012 mmol), Pd3(dba)2 (2.4 mg, 0.003 mmol), PPh3 (7mg, 0.03 mmol), CuI (1mg, 0.005 mmol) dissolved in dry triethylamine (1 mL) was purged with Ar for 15 min and subsequently stirred and heated at 70 °C for 16 h. The solution was filtered over celite to remove the precipitate
Chapter 4
104
and concentrated in vacuo. The crude product was purified by pressure column chromatography (CH2Cl2/pentane 1:1, Rf = 0.1 and CHCl3/pentane 3:2 to 4:1, Rf = 0.1) to yield 23 mg (48%) of a yellow waxy solid. 1H NMR (CDCl3, 400 MHz): δ 8.22 -8.14 (m, 21 H), 8.38-8.18 (m, 2H), 8.08 (t, 1H), 7.89-7.87 (m, 12 H), 7.86-7.84 (m, 2H), 7.81(t, 1H), 7.42-7.59 (m, 4H), 7.19 (s, 2H), 7.17 (s, 1H), 7.10 (s, 1H), 7.02 (s, 1H), 6.73 (s, 1H), 4.48-4.32 (m, 24H), 4.00-3.74 (m, 12H), 2.25 (s, 3H), 2.05-1.78 (m, 18H), 1.70-1.47 (m, 42H), 1.40-0.9 (m,150H), 0.90-0.81 (m, 72H), 0.26 (s, 9H); 13C NMR (CDCl3, 75 MHz): δ 165.30, 165.09, 154.58, 151.67, 151.24, 150.93, 150.47, 150.36, 138.68, 138.23, 138.01, 132.67, 131.44, 131.26, 131.18, 129.46, 128.08, 127.64, 126.65, 125.13, 124.85, 124.22, 124.10, 123.59, 123.27, 122.04, 121.62, 117.07, 116.27, 111.49, 110.18, 109.87, 109.65, 108.36, 92.09, 89.38, 89.03, 88.77, 88.62, 74.64, 74.28, 73.37, 64.28, 39.18, 37.10, 35.51, 35.11, 34.99, 29.93, 27.93, 26.34, 26.28, 24.59, 22.68, 22.59, 19.65, 16.80, 16.69, 11.47, 11.37, -0.22. MALDI-TOF MS (Mw = 4321.78) m/ z= 4320.17 [M]+. OPV-FOLD-PERY (10) Similar to the procedure for the synthesis of PERY-FOLD a mixture of 2 (6 mg, 0.009 mmol), 9 (31 mg, 0.0072mmol), Pd(PPh3)4 (1 mg, 0.0008 mmol) and potassium acetate (1 mg, 0.01 mmol) in toluene (1 mL) and N,N-dimethylformamide (1 mL) was heated to 100 °C for 5 hours, the solvent was evaporated in vacuo. The residue was triturated with water and extracted with methylene chloride. The methylene chloride extract was worked up to give crude products, which were purified by pressure column chromatography (CH2Cl2/pentane to 9:1, Rf = 0.4) to yield 3 mg (9 %) of 10 as a light red solid. 1H NMR (CDCl3, 400 MHz): δ 8.76-8.65 (m, 8H), 8.20-8.15 (m, 23H), 8.12 (t, 1H), 7.91 (t, 1H), 7.90-7.87 (m, 10H), 7.84 (t, 1H), 7.74 (d, 2H), 7.56-7.41 (m, 4H), 7.38 (d, 2H), 7.18 (s, 2H), 7.16 (s, 1H), 7.09 (s, 1H), 7.01 (s, 1H), 6.72 (s, 1H), 5.11-5.02 (m, 1H), 4.45-4.35 (m, 24H), 3.97-3.74 (m, 12H), 2.31-2.24 (m, 5H), 2.05-1.78 (m, 20H), 1.70-1.47 (m, 42H), 1.40-0.95 (m, 156H), 0.88-0.84 (m, 72H).MALDI-TOF MS (Mw = 4784.90) m/ z= 4783.38 [M]+. Transient subpicosecond photoinduced absorption. Solutions in the order of 10-5 M were excited at 450 nm, i.e. providing primarily excitation of the OPV part within the molecules. 4.7 References
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106
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Chapter 5
Photoinduced electron transfer of
conjugated polymers with pendant
fullerenes
Abstract
A processable π-conjugated polymer bearing covalently linked methano-fullerenes has been
synthesized using the Sonogashira polycondensation reaction between a diiodoaryl functionalized
fullerene and a bisethynylterminated p-phenylene vinylene oligomer. The photophysical properties of
this donor-acceptor polymer have been studied and compared to an analogous polymer lacking the
pendant fullerenes. Photoexcitation of the donor-acceptor polymer results in a photoinduced charge
transfer reaction from the conjugated backbone to the pendant C60 moieties. This novel polymer was
applied via spin coating to form the active layer of the first polymer solar cell based on a covalently
linked donor-acceptor bulk-herojunction. The synthesis of analogous polymers with low-bandgap
characteristics was investigated using the Suzuki polycondensation.
Chapter 5
108
5.1 Introduction
Photoinduced electron transfer from a donor to an acceptor is widely studied to mimic the
natural photosynthetic reaction center and to investigate the prospects of molecular materials in
photovoltaic energy conversion.1 Promising energy conversion efficiencies have been obtained in so-
called bulk-heterojunction solar cells in which the active layer is a composite film of a conjugated
donor polymer and an acceptor polymer or a fullerene derivative.2,3 In these blends charges are
preferentially formed at the donor-acceptor interface and intimate mixing of donor and acceptor is
therefore beneficial for charge generation. For efficient transport of the positive charge carriers
through the donor phase and of electrons via the acceptor phase to the electrodes, a phase-segregated
bicontinuous network is required. A convenient route to obtain a predefined nanoscopic phase-
segregated network is linking donor and acceptor via a covalent bond. As a first step towards the
making covalently linked donor-acceptor materials, small fragments of π-conjugated polymers have
been connected to fullerene moieties in well-defined donor-acceptor molecular dyads.4,5 The same
concept can be extended to macromolecules by synthesizing π-conjugated polymers with pendant
fullerenes (Figure 5.1).
hυ
ITOMetal
electrode
e-e-
h+
h+
hυ
ITOMetal
electrode
e-e-
h+
h+
Figure 5.1. Cartoon of the working principle of a π-conjugated polymer with pendant fullerene
moieties in a solar cell.
The preparation of well-defined polymers incorporating fullerenes has remained a challenge
over the years and only few synthetic routes have ensured full structural homogeneity of the final
polymer.6 Conjugated polymers incorporating fullerenes have previously been prepared by
electrochemical polymerization of oligothiophene-fullerene dyads or by grafting C60 on precursor
polymers but these materials have not been incorporated in electro-optical devices because of
insolubility of the resulting polymers.7 Processable polymers with pendant fullerenes have been
prepared by random copolymerization of thiophenes bearing fulleropyrrolidine moieties with
thiophenes carrying solubilising agents using oxidative coupling with FeCl3.8 These polymers, that
Photoinduced electron transfer of conjugated polymers with pendant fullerenes
109
were only soluble if the percentage of fullerene monomer in the feed was not very high, have
successfully been used as the active layer in photovoltaic devices. Examples of soluble polymers
having a backbone containing p-phenylene vinylene and carbazole units, and also triphenylamine
moieties have been reported.9
In this chapter, the synthesis of a novel processable π-conjugated polymer with covalently
linked methanofullerenes is described. The π-conjugated backbone is based on short p-phenylene
vinylene segments that are linked via p-phenylene ethynyl units. The polymer has been synthesized
using the Sonogashira palladium-catalyzed cross-coupling reaction. This polymerization can be
performed under mild conditions and is one of the reactions developed in recent years for the
synthesis of electronic polymer materials, which ensures the alternation of the two monomers. A
similar polymer that lacks the pendant methanofullerenes has also been synthesized and is used as a
reference. The photophysical processes occurring after photoexcitation of the polymer backbone have
been studied by means of photoluminescence and photoinduced absorption spectroscopies. The
polymer has been tested as the active layer of a polymer solar cell. Moreover, the synthesis of an
analogous polymer with low-bandgap properties has been explored using the Suzuki
polycondensation reaction.
5.2 PPV-PPE polymers with pendant fullerenes 5.2.1 Synthesis and characterization
Polymer 3 was synthesized using the Sonogashira palladium catalyzed cross-coupling
reaction between a diiodinated aryl compound 1 and an oligo(p-phenylene vinylene) with terminal
ethynyls 2 (Scheme 5.2). The pendant fullerenes were incorporated into the polymer via the
diiodinated monomer 1. Because of the well-known poor solubility of fullerene molecules, both
monomers have been decorated with numerous branched side chains in order to ensure solubility of
the resulting polymer, required for processability from solution.
Monomer 1, bearing the fullerene, was synthesized by Minze Rispens (University of
Groningen).10 Diethynylene monomer 2 was readily synthesized starting from methylhydroquinone
(6) (Scheme 5.1). Etherification of 6 with 2-ethylhexyl-p-toluenesulfonate, followed by radical
bromination using NBS in the presence of AIBN and ionic bromination with NBS, gave 8.
Phosphonate 9 was obtained by treatment of 8 with triethylphosphite. For the central unit of 1
etherification of 4-methoxyphenol with 3,7-dimethyloctyl-p-toluenesulfonate gave 11, which was then
brominated to give 12, followed by bis-formylation using butyllithium and N,N-dimethylformamide to
yield dialdehyde 13. A double Wittig Horner coupling of 9 and 13 gave 14 which was reacted with
Chapter 5
110
(trimethylsilyl)acetylene using a palladium-catalyzed coupling to afford monomer 2 after
deprotection.
OR
RO
OR
ROBr
Br
OR
ROP
Br
O
OEt
OEt
OR'
MeO
OR'
MeO
Br Br
OR'
MeO
O
O
OR'
MeO
X
X
OR
OR
RO
RO
O
O
O
H
H
O
O
O
6: R = H7: R = CH2CH(C2H5)(CH2)3CH3
8 9
10: R' = H11: R' = CH2CH2CH(CH3)(CH2)3CH(CH3)2
12 13
14: X = Br 15: X = C≡C-TMS
a
b
d
e f g
h
i
2
R = CH2CH(C2H5)(CH2)3CH3 R' = CH2CH2CH(CH3)(CH2)3CH(CH3)2
c
Scheme 5.1. a. CH3(CH2)3CH(C2H5)CH2OTs, K2CO3, TBAC, MEK, 93%; b. 1. NBS, AIBN, CCl4; 2.
NBS, THF, 24%; c. P(OEt)3, 160 °C, 1.5 h. 100%; d. (CH3)2CH(CH2)3CH(CH3)CH2CH2OTs; e. Br2,
HOAc, 65-116 °C, 2 h. 75%; f. 1. BuLi, DMF, Et2O, -10 °C, 56%; g. 9, KtBuO, DMF, 35%; p. TMS-
CCH, NEt3, PdCl2, PPh3, Cu(OAc)2, 50%; i. TBAF, THF, 100%
Polymer 3 was synthesized according to Scheme 5.2 under inert conditions using Pd(PPh3)4
and CuI as catalysts, in a mixture of 1,2-dichlorobenzene/triethylamine (7:3 v/v). The polymerization
reaction was followed by absorption spectroscopy, as a red shift in time of the π-π* transition of the
polymer with respect to that of monomer 2 (λmax= 428 nm) (Figure 5.1, left). Polymer 5, which is
similar to 3 but lacks the pendant methanofullerenes was synthesized by reaction of 2 and 4 under the
same conditions used for 3. The three hexyloxy chains ensure that 1 is a highly soluble C60 derivative,
but its reactivity towards 2 under these conditions is less than that of 1,4-diiodo-2,5-bis(2-
ethylhexyloxy)benzene (4). After 24 h of polymerization, the effective conjugation length of 3, as
inferred from λmax = 468 nm, approaches that of 5 (λmax= 474 nm) (Figure 5.1, left). The polymers
Photoinduced electron transfer of conjugated polymers with pendant fullerenes
111
were isolated by precipitation into methanol. Polymer 3 consisted of a soluble brown powder and an
insoluble fraction. The partial insolubility is probably due to high molecular weight chains.8
O O
O
O
O
O
O
O
O
O n
O
O
O
O
O
O
H
H
O O
O
O
II
O
O
O
O
O
O
O
O n
O
O
IIPd(PPh3)4
CuI, NEt3ODCB
2
3
1
4
5
Scheme 5.2. Synthesis of polymers 3 and 5.
The molecular weight of the polymers determined by size-exclusion chromatography (SEC,
Figure 5.1, right) shows that the soluble fraction of 3 (Mw =16.2 kg/mol, PDI = 2.82) has a lower
degree of polymerization than 5 (Mw =35.8 kg/mol, PDI = 2.32), consistent with the small 6 nm
hypsochromic shift (Figure 5.1, left). The difference in SEC molecular weights might not reflect the
actual situation accurately. For monomer 2 the molecular weight determined by SEC corresponds to
the actual value (SEC: Mw = 1172 g/mol; MALDI-TOF Mw = 1029.46 g/mol) but there is a significant
underestimation for 1 by a factor of three (SEC: Mw = 555 g/mol; MALDI-TOF Mw = 1510.6 g/mol)
(Figure 5.1 right).
Chapter 5
112
300 400 500 600 7000
1
2 3 5
Nor
mal
ized
Abs
orba
nce
Wavelength (nm)
300 400 500 600
1235
Nor
mal
ized
Abs
orba
nce
(a.u
.)
Time (s)
Figure 5.1. Left: UV-Vis spectra of 2, 3 and 5 in chloroform. Right: SEC traces of monomers 1 and 2,
and polymers 3 and 5 in chloroform.
The IR-spectrum of 3 clearly shows an absorption at 526 cm-1, characteristic of the fullerene
moiety. This peak is absent in the IR-spectrum of the reference 5.
The 1H-NMR spectrum of 3 features the characteristic signals of the polymer backbone and
additional absorptions corresponding to the pendant moieties (Figure 5.2). The polymeric nature of
the sample broadens the signals. The broad multiplet at 3 ppm corresponds to the -CH2- closest to the
fullerene. A comparison of the integrals of this multiplet with that of the peaks at around 4 ppm,
which are characteristic for the –OCH2- and –OCH3 protons, gives an estimate of the relative content
of the monomers in the polymer. According to this quantitative analysis the ratio of the monomers 1
and 2 is 0.7 to 1. The ratio of the monomer content can be explained by polymer chains that are
mainly terminated with monomer 2. Actually, it is also possible to observe a small signal
corresponding to ethynyl terminated chains at 3.5 ppm.
Photoinduced electron transfer of conjugated polymers with pendant fullerenes
113
C≡CH end group
CHCl3
C≡
Ha
0.51.01.02.02.02.53.03.03.54.04.04.55.05.05.56.06.06.57.07.07.58.08.0
O
O
OO
O
O
O
OO
O
Ha Ha
-O-CH2-and –O-CH3
C≡CH end group
CHCl3
C≡
Ha
0.51.01.02.02.02.53.03.03.54.04.04.55.05.05.56.06.06.57.07.07.58.08.0
O
O
OO
O
O
O
OO
O
Ha Ha
-O-CH2-and –O-CH3
Figure 5.2. 1H-NMR spectrum of polymer 3 in deuterated chloroform solution.
5.2.2 Photophysical properties
UV/Visible absorption spectroscopy. The absorption spectrum of polymer 3 features the
absorption of the π-conjugated backbone at 468 nm plus an additional absorption in the UV region,
corresponding to the pendant fullerene moiety (Figure 5.1 left). The optical bandgap of polymers 3
and 5 is approximately 2.35 eV, as estimated from the onset of the absorption spectra. The presence of
both double and triple bonds in the backbone make 3 and 5 hybrid polymers of poly(p-phenylene
vinylene) (PPV) and poly(p-phenylene ethynylene) (PPE), similar to those recently reported.11 As a
consequence, the bandgap of 3 and 5 is in between the typical values of the bandgaps for alkoxy
substituted PPV and PPE, which are 2.1 and 2.5 eV respectively.
Photoluminescence spectroscopy in solution. Excitation of 3 in dilute toluene solution at
486 nm reveals that the photoluminescence (PL) of 3 is quenched by two orders of magnitude in
comparison with the emission of 5 (Figure 5.3). Also a weak PL signal at 720 nm is observed after
photoexcitation of the polymer backbone, characteristic of the fluorescence emission band of
fulleropyrrolidines (Figure 5.3 right). The same result has been observed in well-defined molecular
donor-acceptor dyads based on fullerene as acceptor after photoexcitation of the donor, and attributed
Chapter 5
114
to a singlet-singlet energy transfer from the π-conjugated segment to the fullerene moiety in apolar
medium.5,12,13 In Figure 5.3 the photoluminescence of PCBM is also plotted. PCBM is a molecule that
has a structure close to the pendant methanofullerenes in polymer 3. The emission of the fullerene in
polymer 3 is slightly less intense than the emission of the corresponding reference PCBM. This small
quenching can be attributed to photoinduced charge separation to generate the polymer•+-C60•– charge-
separated state. This charge-separated state is only energetically feasible in such an apolar enviroment
for C60 donor-acceptor systems when donor and acceptor are in a face-to-face orientation.13 This can
happen in this polymer via folding of the pendant C60 onto the polymer chain, enabled by the flexible
nature of the spacer between the polymer backbone and the fullerene moiety. In more polar solvents
like benzonitrile, the additional fullerene emission is completely quenched implying a much more
efficient charge separation.
PL quenching in solution confirms unambiguously the covalent linkage of the C60 moieties to
the polymer backbone in 3.
500 600 700 8000
100
200
300
Inte
nsity
(a.
u.)
Wavelength (nm)700 750 800
0
1
2
Inte
nsity
(a.
u.)
Wavelength (nm)
Figure 5.3. Left: PL spectra of polymers 3 (solid circles) and 5 (open circles) in toluene solution.
Right: Fullerene emission of PCBM in chloroform solution (solid line) and of polymer 3 in toluene
(open squares) and benzonitrile (open circles) solutions.
Photoinduced absorption spectroscopy of the film. The PIA spectrum of a thin film of 3
exhibits a characteristic band at 1.20 eV of the methanofullerene radical anion and the two distinctive
strong bands of cation radicals (polarons) generated on the conjugated polymer at 0.62 and 1.53 eV
together with a bleaching band at 2.45 eV (Figure 5.4). The PIA spectrum of a film of reference
polymer 5 exhibits a single band at 1.55 eV of the triplet excited state. The near coincidence of the
triplet absorption of 5 and the high-energy radical cation absorption of 3 at 1.53 eV, is often
encountered in π-conjugated polymers.14 The low-energy radical cation band at 0.62 eV and the
methanofullerene anion band at 1.20 eV for 3, and their absence in the PIA spectrum of 5, give direct
Photoinduced electron transfer of conjugated polymers with pendant fullerenes
115
spectral evidence of a photoinduced electron transfer in 3 between the conjugated chain and the
pendant methanofullerene.
0.5 1.0 1.5 2.0 2.5-1.5
-1.0
-0.5
0.0
0.5
1.0
Nor
mal
ized
-∆T
/T
Energy (eV)
Figure 5.4. Photoinduced absorption spectra of thin films of polymer 3 (solid line) and 5 (dashed
line) on quartz recorded at 80 K.
All PIA bands of 3 increase with the pump intensity (I) following a square-root power law (–
∆T~I0.5) (Figure 5.5, left). This suggests non-geminate bimolecular recombination of the photoinduced
charges, which indicates the migration of opposite charges to different sites in the film.
0 25 50 75 1000
25
50
75
100
125
0.62 eV 1.20 eV 1.53 eV
-∆T
(µV
)
Laser power (mW)100 1000
1
Polymer 3 0.62 eV 1.20 eV 1.53 eV
Polymer 5 1.56 eV
Nor
mal
ized
-∆T
Modulation frequency (Hz)
Figure 5.5. Intensity (left) and frequency (right) dependence of the PIA absorption bands at 0.62,
1.20 and 1.53 eV of polymer 3 and at 1.56 eV of polymer 5.
The lifetime of the photoinduced generated species can be estimated from the frequency
depence of the intensity of the photoinduced absorption bands (Figure 5.5, right). For the triplet state
in 5 the lifetime is 5.4 ms. The lifetime of the absorptions at 0.62 and 1.20 eV, associated with the
photoinduced generated charges in 3, also extend into the millisecond time domain. Figure 5.5 (right)
Chapter 5
116
reveals that the high energy polaronic absorption band of 3 peaking at 1.53 eV has a frequency
dependence that is a combination of the frequency dependence of the triplet state of 5 and that of the
charged species at 3. This implies that some triplet state is formed together with charge separation in 3
after photoexcitation of the backbone.
5.2.3 Photovoltaic device
Photovoltaic cells were prepared by spin coating 3 from chloroform onto a transparent ITO
front electrode covered with a conducting layer of polyethylenedioxythiophene polystyrenesulfonate
(PEDOT:PSS) and depositing an aluminum back electrode in vacuum. The film thickness was around
30 nm with a surface roughness of less than 5 nm. The dark current and photocurrent of the device
under white-light illumination (100 mW/cm2, AM1.5) reveal promising characteristics (Figure 5.6,
left). A short circuit current of Isc = 0.42 mA/cm2, an open circuit voltage of Voc = 0.83 V, and a fill
factor of 0.29 characterize the depicted cell. The rather low rectification ratio of 36 of the cell at ±2 V
in the dark is related to the low film thickness.
The incident monochromatic photon-to-current efficiency (IPCE) has an onset at 550 nm and
exhibits a maximum of 6% at 480 nm (Figure 5.6, right).
-2 -1 0 1 21E-5
1E-3
0.1
10
I (m
A/c
m2 )
Bias (V)400 500 600 700
0
2
4
6
8
10
IPC
E (
%)
Wavelength (nm)
Figure 5.6. Left: I/V curves of an ITO/PEDOT:PSS/3/Al device in the dark (dashed line) and under
white-light illumination (solid line). Right: The incident photo-to-current efficiency (IPCE, solid
squares) and absorption spectrum (solid line) of an ITO/PEDOT:PSS/3/Al device.
Photoinduced electron transfer of conjugated polymers with pendant fullerenes
117
5.3. Low-bandgap π-conjugated polymers with pendant fullerenes
With the solar spectrum peaking at 700 nm, a mismatch occurs between the absorption
spectrum of polymer 3 and the solar emission. In order to improve the absorption of photons to create
charges, a backbone with low bandgap properties would be beneficial. Low-bandgap polymers are
typically obtained by alternating electron rich and electron deficient conjugated units along the
polymer chain. Another aspect of polymer 3 that needs to be addressed is its solubility. A less rigid
backbone might aid in solubilizing the donor-acceptor polymer.
BO
O
BO
OS S
NS
N
RR
R R
OC6H13
OC6H13
OC6H13
O*
S S
NS
N* n
RR
R R
+ 1
16. R = OC8H17
17. R = OC8H17
Scheme 5.3. Synthesis of low-bandgap polymers with pendant fullerenes.
These issues are taken in consideration in polymer 17, prepared by copolymerization of
boronic ester 16 with diiodo functionalized fullerene 1 using the Suzuki polycondensation reaction
(Scheme 5.3). 15 The alternation between thiophenes and benzothiodiazoles in monomers 16 confers
the low-bandgap character to the corresponding polymer, together with a less rigid structure.
The optical bandgap of polymer 17 is 1.7 eV, lower than that of polymer 3. The polymer is
also highly soluble in most common solvents and, according to SEC analysis, of rather high molecular
weight (Mw = 25.2 kg/mol, PDI =2.3). IR and UV-Visible absorption spectroscopies exhibit the
typical features of the C60 fragments that are evidence of the incorporation of the fullerene molecules
in the polymer chains. However, several characterization techniques seem to indicate that the polymer
does not correspond with the proposed structure. The SEC-trace of polymers 17 shows a bimodal
distribution, which indicates that likely more than one reaction mechanism occurs in the
polymerization. The bimodality is not observed in a reference polymer without the fullerene moieties
that has been made by reaction of 16 with 4. The resolution of the 1H-NMR spectrum is extremely
low due to peak broadening and the characteristic multiplet at 3 ppm of the –CH2– closest to the
fullerene moiety is not distinguishable.
Chapter 5
118
Photoinduced absorption spectroscopy of 17 reveals that charge separation occurs after
photoexcitation of the backbone. The PIA spectrum of 17 shows two bands at 0.48 and 1.08 eV of the
backbone radical cation with a residual triplet signal (Figure 5.7, left). The signal of the C60 radical
anion at 1.24 eV cannot be distinguished because of the radical cation absorptions present in the same
spectral region. As expected, the PIA spectrum of the corresponding reference polymer exhibits a
single photoinduced absorption attributed to the formation of the triplet state.
0.5 1.0 1.5 2.0-10
-5
0
5
10
15
Nor
mal
ized
-∆T
/T
Energy (eV)
-2 -1 0 1 21E-5
1E-3
0.1
10
I (m
A/c
m2 )
Bias (V)
Figure 5.7. Left: Photoinduced absorption spectra of polymer 17 (solid line) and its corresponding
reference polymer (dashed line). Right: I/V curves of an ITO/PEDOT:PSS/17/Al device in the dark
(dashed line) and under white-light illumination (solid line).
Following a procedure similar as for polymer 3, a photovoltaic device of 17 has been prepared
spincoating from a toluene solution. The semilogaritmic plot of the I/V characteristics in the dark and
under white-light illumination of the device are shown in Figure 5.7 (right). The I/V characteristics of
the cell show a diode behavior with a rectification ratio (RR) of 54.7 at ± 2 V. This rather low value is
related to low film thickness (~ 40 nm). Under white-light illumination (100 mW/cm2, AM1.5) an
open-circuit voltage of Voc= 0.75 V and a short circuit current of Isc = 0.31 mA/cm2 are obtained. The
fill-factor (FF) is 0.23. This low fill-factor can be attributed to a small parallel resistance caused by
the low film thickness.
5.4. Conclusions
Two novel π-conjugated polymers with pendant methanofullerenes have been synthesized
using palladium catalyzed cross-coupling reactions.
The first one, a hybrid polymer between p-phenylene vinylenes and p-phenylene ethynylenes
bearing fullerenes, 3, has been synthesized using the Sonogashira coupling between dihalogenated
and bisethynyl terminated aryl compounds 1 and 2. SEC reveals that polymer 3 has a lower molecular
Photoinduced electron transfer of conjugated polymers with pendant fullerenes
119
weight than its reference polymer 5. Higher molecular weights result in insolubility of the polymer
chains. Photoluminescence and PIA spectroscopies reveal that photoexcitation of 3 results in a
electron transfer from the polymer backbone to the pendant fullerenes. A device with the structure
ITO/PEDOT:PSS/3/Al has been prepared by spincoating from a chloroform solution. The values of Isc
and Voc have improved compared to previously reported π-conjugated polymer/fullerene solar cells.16
Moreover, this performance is given by a polymer with a fullerene loading of 31 wt %, a value
considerably smaller than that used commonly in ‘bulk-heterojunction’ solar cells (about 75%). The
performance of the device of 3 could be improved by increasing the solubility of the polymer, which
would enable the preparation of thicker active layers. Also, there is a mismatch between the solar
spectrum and the absorption spectrum of 3, meaning that the solar spectrum is not used in an optimal
way to generate photoinduced charges.
In an attempt to increase the processability and the absorption of light of polymer 3, a
covalently linked donor-acceptor polymer with low-bangap characteristics, 17, has been synthesized
using the Suzuki polycondensation. Characterization of these polymers reveals that although the
polymers contain fullerene molecules, the polymer does not correspond to a perfect alternation of the
comonomers. This is attributed to a significant amount of side reactions taking place, apart from the
Suzuki cross-coupling reactions. Although this polymer shows some interesting photophysical and
photovoltaic properties, the Suzuki polycondensation is not an ideal alternative to improve this type of
polymers.
Overall, the results indicate that a bicontinuous network of donor and acceptors, confined to a
molecular scale, is an attractive approach to new materials for photovoltaic applications, although
much has to be improved to bring the performance of these novel polymers to that of the best bulk-
heterojunction cells.
5.5. Experimental section
For general methods and materials the reader is referred to chapter 2 of this thesis. 1,4-Bis(2-ethylhexyloxy)-2-methylbenzene (7). Under an argon atmosphere methylhydroquinone (10 g, 80.5 mmol), 2-ethylhexyl-p-toluenesulfonate (48 g, 169 mmol) and tetrabutylammonium chloride (2.66 g, 9.6 mmol) were added to a suspension of K2CO3 (66.3 g, 480 mmol) in dry 2-butanone (160 mL). The reaction mixture was stirred for 16 h at reflux temperature. After cooling, the suspension was filtered and the solvent was removed in vacuo. The resulting crude product was purified by column chromatography (silica gel, hexane/CHCl3 2:1). Evaporation of the solvent yielded 28.3 g (93%) of 7 as a pure colorless oil: 1H NMR (CDCl3) δ 6.79 (d, 1H), 6.77 (d, 1H), 6.71 (dd, 1H), 3.83 (m, 4H), 2.25 (s, 3H), 1.7 (m, 2H), 1.48 (m, 16H), 0.98 (m, 12H); 13C NMR (CDCl3) δ 152.95, 151.50, 127.99, 117.62, 111.66, 111.35, 70.94, 70.79, 39.62, 39.45, 30.68, 30.52, 29.11, 29.07, 24.05, 23.83, 23.08, 16.38, 14.09, 11.21, 11.09; GC-MS (Mw = 348.56) m/z = 348 [M]+. 1-Bromo-2,5-bis(2-ethylhexyloxy)-4-bromomethylbenzene (8). Under an argon atmosphere, NBS (6.12 g, 34.4 mmol) and AIBN (1.72 g, 10.3 mmol) were added to a solution of 7 (10 g, 28 mmol) in dry CCl4 (28 mL). After stirring for 1h under reflux, the reaction mixture was cooled to
Chapter 5
120
room temperature. The mixture was filtered and the solvent evaporated. To remove the last traces of AIBN and NBS, hexane was added to the residue, followed by filtration and evaporation of the solvent. Subsequently, dry THF (28 mL) and NBS (6.63 g, 37 mmol) were added and the reaction mixture was stirred at reflux temperature for 1h. After evaporation of the solvent, hexane was added. The solution was filtered and the solvent removed in vacuo. After column chromatography (silica gel, hexane) and evaporation of the solvents, 3.5 g of 8 (24%) was obtained as a colorless oil. 1H NMR (CDCl3) δ 7.06 (s, 1H), 6.80 (s, 1H), 4.48 (s, 2H), 3.84 (m, 4H), 1.76 (m, 2H), 1.4 (m, 16H), 0.92 (m, 12H); 13C NMR (CDCl3) δ 151.20, 149.55, 125.87, 116.98, 115.61, 113.15, 72.35, 71.04, 39.50, 30.57, 30.46, 29.05, 23.97, 23.86, 23.01, 14.05, 11.16. Diethyl[2,5-bis(2-ethylhexyloxy)-4-bromo-benzyl]phosphonate (9). Triethyl phosphite (1.55 g, 9.33 mmol) and 8 (3.15 g, 6.22 mmol) were stirred at 160 oC for 1.5 h. The reaction mixture was cooled to 75 oC and the ethyl bromide, formed during the reaction, and the excess of triethyl phosphite were distilled under reduced pressure. The product 9 was a light yellow oil. Yield 3.30 g (100%). 1H NMR (CDCl3) δ 7.02 (d, 1H), 6.86 (d, 1H), 4.03 (m, 4H), 3.79 (m, 4H), 3.16 (d, 2H), 1.70 (m, 2H), 1.36 (m, 22H), 0.90 (m, 12H); 13C NMR (CDCl3) δ 150.94 (d), 149.34 (d), 120.08 (d), 116.55 (d), 116.25 (d), 110.50 (d), 72.17, 71.27, 61.76 (d), 39.39 (d), 30.38 (d), 28.93 (d), 23.74 (d), 22.88, 16.22 (d), 13.91, 11.01. 1,4-Dibromo-2-(3,7-dimethyloctyloxy)-5-methoxybenzene (12). A solution of Br2 (11.78 g, 73.74 mmol) in glacial acetic acid (30 mL) was added dropwise to a solution of 1-(3,7-methyloctoxy)-4-methoxybenzene (11) (10 g, 37.82 mmol) in glacial acetic acid (45 mL) at 65 oC. After stirring during 45 min at 65 oC, the temperature was raised to reflux during 1h. The solution was then cooled to room temperature and subsequently poured on water (560 mL) and made alkaline with 2N NaOH (660 mL). The aqueous phase was extracted with CH2Cl2 (3 x 200 mL). The combined organic layers were washed with brine and dried over MgSO4. The resultant crude product was purified by column chromatography (silica gel, hexane/CHCl3 2:1) Evaporation of the solvent yielded 12 g (75%) of 12 as a colorless oil: 1H NMR (CDCl3) δ 7.09 (s, 1 2H), 1.52 (m, 1H), 1.32 (m, 2H), 1.68 (m, 4H), 0.94 (d, 3H), 0.87 (d, 6H); 13C NMR (CDCl3) δ 150.34, 150.09, 118.38, 116.91, 111.11, 110.13, 68.57, 56.90, 39.13, 37.15, 35.97, 29.70, 27.92, 24.61, 22.66, 22.56, 19.64; GC-MS (Mw = 422.20) m/z= 422 [M]+. 2-(3,7-Dimethyloctyloxy)-5-methoxybezene-1,4-dialdehyde (13). Dibromide 12 (7 g, 16.58 mmol) was dissolved in dry diethyl ether (135 mL). The solution was cooled to –10 oC and 1.6 M n-buthyllithium hexane solution (24.87 mL) was added slowly. The reaction mixture was stirred for 5 min., then, the cooling bath was removed and dry DMF (3.23 mL) was added dropwise. The mixture was stirred for another hour at room temperature. After addition of 6 M HCl (30 mL), the organic layer was washed with water (2 x 100 ml) a saturated NaHCO3 solution (100 mL) and again water (100 mL). The organic layer was dried over MgSO4 and the solvent was evaporated. The resultant crude product was purified by column chromatography (silica gel, hexane/toluene 2:1, Rf = 0.2). Evaporation of the solvent yielded 3.02 g (56%) of 13 as a light yellow solid. 1H NMR (CDCl3) δ10.54 (s, 1H), 10.49 (s, 1H), 7.44 (s, 2H), 4.13 (m, 2H), 3.94 (s, 3H), 1.88 (m, 1H), 1.66 (m, 2H), 1.53 (m, 1H), 1.25 (m, 6H), 0.96 (d, 3H), 0.87 (d, 6H); 13C NMR (CDCl3) δ189.20, 155.47, 155.28, 129.08, 111.70, 110.58, 67.53, 56.10, 39.11, 37.13, 35.89, 29.83, 27.88, 24.60, 22.61, 22.51, 19.57. (E,E)-1,4-Bis[4-bromo-2,5-bis(2-ethylhexyloxy)styryl]-2-(3,7-dimethyloctyloxy)-5-methoxybezene (14). Phosphonate 9 (1.82 g, 3.23 mmol) was dissolved in dry DMF (10 mL) under an argon atmosphere and 0.43 g (3.9 mmol) of KtBuO were added to the solution. After 15 min, a solution of dialdehyde 13 (0.5 g, 1.56 mmol) in dry DMF (12 mL) was added dropwise and the reaction mixture was stirred for 3 h. The solution was poured on crushed ice and of 6 M HCl (200 mL) was added. The aqueous phase was extracted twice with diethyl ether and the combined organic layers were subsequently washed with 3 M HCl, water, a saturated aqueous solution of Na2CO3 and dried over MgSO4. The solvent was removed in vauo. Column chromatography (silica gel, hexane/CHCl3 4:1, Rf = 0.6) and posterior evaporation of the solvent afforded 0.7g (35%) of 14 as a greenish oil: 1H NMR (CDCl3) δ 7.5 (d, 1H), 7.49 (d, 1H), 7.45(d, 1H), 7.44 (d, 1H), 7.18 (s, 2H), 7.17 (s, 2H), 7.10 (s, 1H), 7.10 (s, 1H), 4.10(t, 2H), 3.92 (m, 11H), 1.9-0.8 (m, 79H); 13C NMR (CDCl3) δ 151.37, 151.08, 150.98, 150.90, 149.92, 149.88, 127.10, 127.02, 126.90, 126.84, 123.41, 123.35, 123.22, 122.63, 117.55, 117.41, 111.59, 111.55, 111.08, 110.72, 110.35, 108.45, 72.34, 72.25, 71.79, 71.50, 67.17, 55.91, 39.61, 39.53, 39.21, 37.29, 36.46, 30.72, 30.73, 30.50, 30.21, 29.10, 29.06, 27.92, 24.70, 24.15, 23.89, 23.87, 23.05, 22.63, 22.54, 19.88, 14.11, 14.08, 14.06, 11.32, 11.26, 11.23, 11.20; MALDI-TOF MS (Mw = 1139.32) m/z = 1138.59 [M]+.
Photoinduced electron transfer of conjugated polymers with pendant fullerenes
121
(E,E)-1,4-Bis[4-trimethylsilylethynyl-2,5-bis(2-ethylhexyloxy)styryl]-2-(3,7-dimethyloctyloxy)-5-methoxybezene (15). (Trimethylsilyl)acetylene (0.114 g, 1.17 mmol) and dibromide 14 (0.5 g, 0.39 mmol) were dissolved in anhydrous triethylamine (8 mL). Argon was purged trough the solution for 15 min and the temperature was raised to 80 oC. Then PdCl2 (6.98 mg, 0.04 mmol), triphenylphosphine (31 mg, 0.11 mmol) and copper(II) acetate (7.8 mg, 0.04 mmol) were added to the solution. The reaction mixture was stirred for 16 h. The solvent was removed in vacuo, the crude solid was dissolved in diethyl ether. The organic layer was washed with a saturated aqueous solution of NH4Cl and brine. After the organic layer was dried over MgSO4, the solvent was removed in vacuo. The residue was purified by column chromatography (silica gel, hexane/CHCl3 85:15, Rf = 0.4) and recrystalization from ethanol yielded 230 mg (50%) of 15 as yellow crystals: 1H NMR (CDCl3) δ7.48 (s, 2H), 7.47 (s, 2H), 7.15 (s, 2H), 7.11 (s, 1H), 7.10 (s, 1H), 6.94 (s, 2H), 4.07 (t, 2H), 3.90 (m, 11H), 1.89 (m, 1H), 1.77 (m, 5H), 1.51 (m, 16), 1.34 (m, 20H), 1.14 (m, 4H), 0.92 (m, 33H), 0.26 (s, 18H); 13C NMR (CDCl3) δ 154.90, 151.49, 151.01,150.40, 150.29, 128.72, 128.58, 127.26, 127.06, 123.94, 123.84, 123.48, 122.83, 117.07, 116.93, 112.17, 110.47, 110.05, 109,62, 108.47, 101.84, 98,86, 71.81, 71.70, 71.64, 71.35, 67.80, 55.94, 39.73, 39.94, 39.63, 39.24, 37.30, 36.47, 30.79, 30.76, 30.54, 30.24, 29.19, 29.15, 27.93, 24.69, 24.21, 23.92, 23.91, 23.09, 23.07, 22.64, 22.55, 19.90, 14.12, 14.08, 14.06, 11.35, 11.32, 11.28, 0.02; MALDI-TOF MS (Mw =1173.94) m/z = 1173.84 [M]+; Anal. Cald for C75H120O6Si2: C, 76.7; H, 10.3. Found: C, 76.64; H, 10.33. (E,E)-1,4-bis[4-ethynyl-2,5-bis(2-ethylhexyloxy)styryl]-2-(3,7-dimethyloctyloxy)-5-methoxybezene (2). To a solution of 15 (27 mg, 0.02 mmol) in dry THF was added 1 M tetrabuthylammonium fluoride in THF (0.023 mL). The reaction mixture was stirred for 1 min and subsequently filtrated over silica gel using chloroform as eluent. The solvent was removed in vacuo yielding 22.6 mg (100%) of a yellow solid which was used without further purification: 1H NMR (CDCl3) δ7.49 (s, 1H), 7.49 (s, 1H), 7.48 (s, 2H), 7.15 (s, 2H), 7.14 (s, 1H), 7.13 (s, 1H), 6.97 (s, 1H), 6.97 (s, 1H), 4.08 (t, 2H), 3.90 (m, 11H), 3.29 (s, 2H), 0.91 (m, 33H); 13C NMR (CDCl3) δ 154.87, 151.46, 150.99, 150.39, 150.28, 128.97, 128.81, 127.18, 127.01, 124.11, 124.03, 123.33, 122.73, 117.43, 117.27, 111.13, 110.38, 110.28, 109.88, 108.47, 81.41, 80.53, 80.50, 72.15, 72.05, 71.62, 71.34, 67.73, 55.93, 39.63, 38.57, 39.43, 39.22, 37.29, 36.45, 30.93, 30.76, 30.74, 30.52, 30.22, 29.68, 29.10, 29.06, 27.92, 24.69, 24.18, 23.89, 23.87, 23.05, 22.64, 22.54, 19.88, 14.10, 14.06, 11.31, 11.26, 11.23, 11.20. MALDI-TOF MS (Mw =1029.58) m/z = 1029.45 [M]+. Reference polymer (5). To a sealed tube fitted with a magnetic stirrer was added diiodo monomer 417 (14.85 mg, 0.025 mmol), diethylnyl monomer 2 (25 mg, 0.024 mmol), Pd(PPh3)4 (1.16 mg, 0.001 mmol), CuI (0.19 mg, 0.001 mmol), dry Et3N (0.3 mL) and dry ortodichlorobenzene (0.7 mL). The reaction mixture was degassed using freeze-pump-thaw cycles and heated at 75 °C under Ar atmosphere for 24 h. After cooling it to room temperature, the reaction mixture was added dropwise to rapidly stirred EtOH (30 mL). After stirring for 2 h, the precipitate was collected and dried under vacuum overnight. Polymer 5 was obtained as 25 mg (75%) of an orange solid. SEC (chloroform): Mw =35.8 kg/mol, PDI = 2.82. Polymer 3. This polymer was prepared by a procedure identical polymer 5 using diethylnyl monomer 2 (22 mg, 0.02 mmol), diiodo monomer 1 (27mg, 0.018mmol), Pd(PPh3)4 (0.92 mg, 0.0008 mmol), CuI (0.15 mg, 0.0008mmol), dry Et3N (0.3 mL) and dry orthodichlorobenzene (0.7 mL ). Polymer 3 was obtained as 34 mg (76%) of a brown solid. SEC (chloroform): Mw =16.2 kg/mol, PDI = 2.32. Photoinduced absorption. PIA measurements were performed between 0.25 eV and 3.0 eV by exciting thin films on quartz with a mechanically modulated (275 Hz, 25 mW 2 mm diameter, 488 nm) beam from a continuous wave argon ion laser (Spectra physics 2025). The change in transmission of probe light (∆T) was monitored with a phase-sensitive lock-in amplifier using Si, InGaAs, and cooled InSb detectors after dispersion by a triple grating monochromator. The photoinduced absorption, -∆T/T ≅ ∆αd, is directly calculated from the change in transmission after correction for fluorescence, which is recorded in a separate experiment. The lifetime of the photoexcitations has been determined by recording the intensity of the PIA bands as a function of the modulation frequency (ω) in the range of 30-4000 Hz. Photovoltaic cells. For photovoltaic cells, polyethylenedioxythiophene polystyrenesulfonate (PEDOT:PSS, Bayer AG) (90 nm) was spin coated on UV-ozone cleaned glass substrates covered with indium tin oxide (ITO) (140 nm), followed by spin coating a solution of 3 in chloroform to form the active layer of 30 nm as determined with a Tencor P-10 surface profiler. Finally, an aluminum back electrode (100 nm) was deposited in
Chapter 5
122
vacuum to give an active area of 4 mm2. I/V characteristics were measured under 100 mW/cm2 AM1.5 white-light illumination from a Steuernagel Solarconstant 1200 solar simulator with a Keithley 2400 Source Meter in inert nitrogen atmosphere at room temperature. Surface roughness was determined by AFM. IPCE measurements. Spectrally resolved photocurrent measurements were performed by illuminating the device with 1 mW/cm2 monochromatized light with a FWHM of ~6 nm from a Xe arc lamp. No noticeable degradation of the devices was observed during the measurement cycles. Light intensities were measured by a calibrated Si photodiode. The incident photon to current efficiency (IPCE) or external quantum efficiency was calculated from the photocurrent and light intensity as:
IPCE [%] =[ A / cm
[nm] [W / m ]
2
2
1240 ⋅
⋅
I
I
sc µλ
]
5.6 References 1 (a) Tang, C. W. Appl. Phys. Lett. 1986, 48, 183. (b) Granström, M.; Petrisch, K.; Arias, A. C.; Lux, A.;
Lux, M.; Andersson. M. R.; Friend, R. H. Nature 1998, 395, 257.
2 (a) Halls J. J. M.; Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H.; Moratti, S. C.;
Holmes, A. B. Nature 1995, 376, 498. (b) Yu, G.; Gao, Y.; Hummelen, J. C.; Wudl, F.; Heeger, A. J.
Science 1995, 270, 1789.
3 Shaheen, S. E.; Brabec, J. C.; Padinger, F.; Fromherz, T.; Hummelen, J. C.; Sariciftci, N. S. Appl. Phys.
Lett. 2001, 78, 841.
4 (a) Nierengarten, J.-F.; Eckert, J.-F.; Nicoud, J.-F.; Ouali, L.; Krasnikov, V. V.; Hadziioannou, G. Chem.
Commun. 1999, 617. (b) Stalmach, U.; de Boer, B.; Videlot, C.; van Hutten, P. F.; Hadziioannou, G. J.
Am. Chem. Soc. 2000, 122, 5464. (c) Eckert, J.-F.; Nicoud, J.-F.; Nierengarten, J.-F.; Liu, S.-G.;
Echegoyen, L.; Barigelletti, F.; Armaroli, N.; Ouali, L.; Krasnikov, V.; Hadziioannou, G. J. Am. Chem.
Soc. 2000, 122, 7467.
5 Peeters, E.; van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C.; Janssen. R. A. J. J.
Phys. Chem. B 2000, 104, 10174.
6 (a) Shi, S.; Khemani, K. C.; Li, C.; Wudl, F. J. Am. Chem. Soc. 1992, 114, 10656. (b) Sun, Y.-P.; Liu, B.;
Moton, D. K. Chem. Commun. 1996, 2699. (c) Zhang, N.; Schricker, S. R.; Wudl, F.; Prato, M.; Maggini,
M.; Scorrano, G. Chem. Mater. 1995, 7, 441. (d) Gügel, A.; Belik, P.; Walter, M.; Kraus, A.; Harth, E.;
Wagner, M.; Spickermann, J.; Müllen, K. Tetrahedron 1996, 52, 5007. (e) Kraus, A.; Müllen, K.
Macromolecules, 1999, 32, 4241. (f) Ilhan, F.; Rotello, V. M. J. Org. Chem. 1999, 64, 1455. (g) Xiao, L.;
Shimotani, H.; Ozawa, M.; Li, J.; Dragoe, N.; Saigo, K.; Kitazawa, K. J. Polym. Sci. A 1999, 37, 3632.
(h) Sun, Y.-P.; Lawson, G. E.; Huang, W.; Wright, A. D.; Moton, D. K. Macromolecules, 1999, 32, 8747.
(i) Okamura, H.; Miyazono, K.; Minoda, M.; Komatsu, K.; Fukuda, T.; Miyamoto, T. J. Polym. Sci. A
2000, 38, 3578.
7 (a) Benincori, T.; Brenna, E.; Sannicoló,F.; Trimarco, L.; Zotti, G.; Sozzani, P. Angew. Chem., Int. Ed.
Eng. 1996, 35, 648. (b) Ferraris, J. P.; Yassar, A.; Loveday, D. C.; Hmyene, M. Opt. Mater. 1998, 9, 34.
Photoinduced electron transfer of conjugated polymers with pendant fullerenes
123
(c) Cravino, A.; Zerza, G.; Maggini, M.; Bucella, S.; Svensson, M.; Andersson, M. R.; Neugebauer, H.;
Sariciftci, N. S. Chem. Commun. 2000, 2487.
8 Zhang, F., Svensson, M; Andersson, M. R.; Maggini, M.; Bucella, S.; Menna, E.; Inganäs, O. Adv. Mater.
2001, 13, 171.
9 (a) Xiao, S; Wang, S.; Fang, H.; Li, Y.; Shi, Z.; Du, C.; Zhu, D. Macromol. Rapid. Commun. 2001, 22,
1313. (b) Wang, S.; Xiao, S.; Li, Y.; Shi, Z.; Du, C.; Fang, H.; Zhu, D. Polymer, 2002, 43, 2049.
10 Marcos Ramos, A.; Rispens, M. T.; van Duren, J. K. J.; Hummelen, J. C.; Janssen, R. A. J. J. Am. Chem.
Soc., 2001, 123, 6714.
11 Brizius, G.; Pschirer, N. G.; Steffen, W.; Stitzer, K.; Zur Loye, H.-C.; Bunz, U. H. F. J. Am. Chem. Soc.,
2000, 122, 12435.
12 Van Hal, P. A.; Knol, J.; Langeveld-Voss, B. M. W.; Meskers, S. C. J.; Hummelen, J. C.; Janssen, R. A.
J. J. Phys. Chem. A 2000, 104, 5964.
13 van Hal, P. A.; Beckers, E. H. A.; Meskers, S. C. J.; Janssen, R. A. J.; Jousselme, B.; Blanchard, P.;
Roncali, J. Chem. Eur. J. 2002, 8, 5415.
14 Lane, P. A.; Wei, X.; Vardeny, Z. V. Phys. Rev. B 1997, 56, 4626.
15 For a full account on the synthesis of polymer 17 see: Hamelinck, P. Novel Double Cable Materials for
Polymer Solar Cells, 2003, research report Eindhoven University of Technology.
16 (a) Roman, L. S.; Andersson, M. R.; Yohannes, T.; Inganäs, O. Adv. Mater. 1997, 9, 1164. (b) Mattoussi,
H.; Rubner, M. F.; Zhou, F.; Kumar, J.; Tripathy, S. K.; Chiang, L. Y. Appl. Phys. Lett. 2000, 77, 1540.
17 Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157.
Chapter 6
Polyacetylenes with pendant
donor-acceptor dyads
Abstract
The synthesis of polyacetylenes functionalized with donors and acceptors in a variety of arrangements
has been investigated. The acceptor is a perylene diimide chromophore (PERY) and the donors are an
oligo(p-phenylene vinylene) (OPV), an oligo(p-phenylene ethynylene) (OPE) and a hybrid of OPV
and OPE, an OPVE. The polymers were prepared either by the copolymerization of OPV and PERY
functionalized acetylenes or by the polymerization of homologous OPVE-PERY and OPE-PERY
dyads. Absorption spectroscopy reveals that preorganization of the donor and acceptor might take
place in solution. Photoexcitation of the donor or acceptor results in charge separation in all
polymers. Even though the copolymerization approach represents the fastest and most facile method
to obtain donor-acceptor functionalized polyacetylenes, the alternating arrangement of donors and
acceptors results in very short-lived charged-separated states. In the polymers resulting from the
polymerization of the dyads, the lifetime of the photoinduced charge separated state is increased by
more than one order of magnitude with respect to the copolymer. The improved photophysical
properties are the result of a more refined architecture.
Chapter 6
126
6.1 Introduction
Promising power conversion efficiencies have been reported for spontaneously formed
interpenetrating networks of electron- and hole-accepting organic semiconductors.1 However, the poor
miscibility of donor and acceptor and the disordered nature of the resulting networks are thought to
limit the performance of these devices. In chapter 5, the advantage of having donors and acceptors
within one polymer chain has already been addressed. A large interfacial area is beneficial for
efficient charge generation and with an appropriate organization of the chromophores transport of the
charges to opposite electrodes can be expected. Here, in pursue of additional ordering and
consequently enhanced charge mobility, a new design for a donor-acceptor polymer is proposed. This
consists of well-defined donor-acceptor dyads that are covalently linked to a polyacetylene backbone
(Figure 6.1).
Donor Acceptor
Poly acetylene backbone
Donor Acceptor
Poly acetylene backbone
Donor Acceptor
Poly acetylene backbone
Donor Acceptor
Poly acetylene backbone
Figure 6.1. Schematic representation of a polyacetylene with pendant donor-acceptor dyads.
An important aspect of polyacetylenes to take into consideration is the stereoregularity of the
polymer backbone. Four conformers are possible, i.e., cis-cisoidal, cis-transoidal, trans-cisoidal, and
trans-transoidal. When the backbone mainly consists of the trans-transoidal conformer, it will be
stretched into a linear arrangement. If the backbone mainly consists of the cis-transoidal conformer, it
generally exhibits a helical arrangement. The handedness of this helix can be controlled via chiral
centers on the side-chains or via external stimuli.2 Furthermore, the stiffness of the polyacetylene
backbone and the tightness of the helical conformation highly depend on the side-chains. Cis-
transoidal polyacetylenes can be prepared in a selective manner by using [Rh(nbd)Cl]2
(nbd=norbornadiene) as a catalyst. This rhodium catalyst has been reported to be effective in the
polymerization of mono-substituted acetylenes, especially phenyl-substituted acetylenes.
There are a few examples of polyacetylenes with pendant redox-active components, yet none
of them contains both donor and acceptor elements. Schenning et al. have previously shown a
polyacetylene functionalized with oligo(p-phenylene vinylene) (OPV)3 and Vohlidal et al. have
prepared analogous polymers with pendant oligo(p-phenylene ethynylene) (OPEs).4 Yashima et al.
have incorporated acceptor fullerene moieties in a polyphenyl acetylene polymer.5
Polyacetylenes with pendant donor-acceptor dyads
127
Here, a perylene diimide chromophore (PERY) is used as an acceptor. This type of molecule
is potentially a better candidate for the acceptor role in plastic solar cells than fullerene molecules,
because of their lower reduction potential (~ -0.55 vs. ~ -0.7 V) and their strongly enhanced
absorption in the visible region. Furthermore, perylene diimide dyes readily stack via π-π interactions
and have the ability to form crystals that exhibit remarkable exciton diffusion ranges and charge
mobility.6 In the proposed design (Figure 6.1) the strong π-π interactions characteristic of perylene
chromophores are used in order to gain dimensional control over the phase separation. By having the
perylene at the periphery and the polyacetylene backbone connecting the dyads, donor-donor and
acceptor-acceptor interactions might be favored.
In this chapter the synthesis of different polyacetylenes functionalized with donors and
acceptors (polymers 1, 2 and 3, Figure 6.2) is described. Whereas the acceptor moiety is constant for
all polymers, the donor ranges from an oligo(p-phenylene vynylene) (OPV) in 1, to a oligo(p-
phenylene ethynylene) (OPE) in 3. In 2 the donor is a mixed OPV and OPE (OPVE). The
photophysical properties of the polymers are investigated in chloroform solution and in the solid state
by means of photoluminescence and photoinduced absorption spectroscopy.
NNO
O
O
O
O
O
H
n
O
OOO
O O ONN
O
O
O
O
OH
n
O
O
O
O
O
O
NNO
O
O
O
O
H
H
n
m
1
2
3
n
Figure 6.2. Chemical structure of polyacetylenes 1, 2 and 3.
Chapter 6
128
6.2 Design
Several design criteria have been formulated in order to screen for the optimal synthesis of the
polymers and to gain knowledge on how to obtain the most favorable photophysical properties.
A first consideration concerns the catalytic system to be employed for the polymerization.
The rhodium chlorine bridge catalyst [Rh(nbd)Cl]2 has extensively proven to be effective for the
polymerization of mono-substituted phenylacetylenes. The catalyst accounts for high molecular
weights and stereoregular control, yielding cis-transoidal polyphenylacetylenes.7 This rhodium
catalyst polymerizes selectively terminal ethynyl groups, allowing for the use of internal triple bonds
in the monomers as conjugating elements.4 The solvent system consisting of toluene and Et3N as a
base ensures sufficient solubility of the conjugated monomers used in the reaction.
A disadvantage of the rhodium catalyst is its tendency to generate cyclic trimers, consisting of
a benzene core made up out of three acetylenes. This side reaction may overrule the polymerization,
especially in cases where the terminal phenyl ring bearing the acetylene also carries alkoxy
substituents. Thus, in order to favor polymerization over the formation of cyclic trimers, the absence
of substituents on the terminal phenyl ring is required for all monomers.
An intrinsic feature of conjugated polymers is their poor solubility in conventional organic
solvents, due to their rigid backbone and tendency to aggregate. The designed polymers feature an
additional perylene function that is known to disfavor solubility. These features, lowering the
solubility, need to be compensated by flexible, preferably branched, side chains. The alkoxy side
chains on the donor segments and alkyl chains on the PERY have proven to greatly enhance
solubility. An extra requirement is the need for branching at the first carbon of the alkyl chains on the
PERY.8
An element that strongly influences the lifetime of the donor(+)-acceptor(-) charged separated
state is the connection of the donor and acceptor. Previous studies have shown that a direct connection
of these two chromophores will result in a very fast recombination of the created charges, whereas a
connection via a saturated spacer enhances the lifetime.9 As a long-lived charge separated state is
beneficial for charge transport through the system, the use of a flexible saturated spacer has been
included in the design of the polymers.
The synthesis of the monomers for polymers 2 and 3 requires the coupling of the perylene
functionality to only one side of the symmetrical donor functionality. Two approaches can be
envisaged to achieve these asymmetric monomers. The first approach, though not being very elegant,
is a fast one and consists of the unselective reaction of the phenyl acetylene modified perylene with a
symmetrical donor diiodide. This procedure will yield a mixture of non-reacted, mono-functionalized
and difunctionalized donor, thus limiting the best possible yield to 50 %. The second approach
consists of the total synthesis of a selectively addressable donor compound, featuring one halogenated
function and one protected acetylene function. The advantage of this approach is the intrinsic high
Polyacetylenes with pendant donor-acceptor dyads
129
yield of the coupling reaction at a crucial stage in the synthesis. The major drawback of this approach
is the extended synthetic pathway to the bifunctional donor, compared to that for the symmetrical
donor functionality.
The detailed synthesis of all monomers and polymers are described in the following section.
6.3 Synthesis and characterization
6.3.1 Ethynyl perylene bisimide (10)
Perylene diimid 10 (scheme 6.1) is a common element in all three polymers. It is one of the
comonomers of polymer 1 and a building block of the dyads in polymers 2 and 3. As 10 features a
terminal acetylene, it was also subjected to polymerization.
The synthesis of 10 starts with the protection of the amine function in (S)-(+)-leucinol.9b The
BOC-protected (S)-(+)-leucinol 4 was reacted with 4-iodophenol using Mitsonobu conditions,
affording, after deprotection of the amine with TFA, compound 6. Iodophenyl functionalized perylene
diimide 8 was obtained after reaction of amine 6 with the perylene monoanhydride monoimide 7.
Subsequent coupling of trimethylsilylacetylene (TMA), using a palladium-catalyzed coupling, and
desilylation with TBAF yielded monomer 10.
N O
O
OO
O
NH
O
OOH
NH
OO
OI
NH2
O I
N NO I
O
O
O
O
N NO
O
O
O
OR
NNO
O
O
O
O
*
*n
c
d
4 65
a b
7
89. R = TMS10. R = H
11
e
f
Scheme 6.1. a. 4-Iodophenol, PPh3, DEAD, toluene, r.t., 16 h, 40%; b. TFA, CH2Cl2, r.t., 16 h, 86%;
c. Imidazole, Zn(OAc)2,1 h, 160 °C, 67%; d. TMA, Pd(PPh3)2Cl2, CuI, Et3N, toluene, 50°C, 4 h, 84%;
e. TBAF, THF, 1 min, 81%; f. [Rh(nbd)Cl]2, Et3N, toluene, r.t., 24 h.
Chapter 6
130
Polymerization of monomer 10 results in a red solid, polymer 11, which is scarcely soluble in
any solvent. The SEC trace of the soluble fraction of 11 in chloroform reveals the presence of some
starting monomer, the formation of cyclotrimer, soluble polymer and mainly aggregated polymer. The
low solubility of this polymer is caused by the high content of perylene. Soluble acetylenes with
pendant perylene chromophores may be achievable by the copolymerization of 10 with a non-
perylene based comonomer.5
6.3.2 Polyacetylene with pendant OPV and PERY chromophores (polymer 1).
As a first facile synthesis of a polyacetylene incorporating pendant donor and acceptors,
perylene 10 and OPV 12,10 both featuring a terminal acetylene, were copolymerized to afford
copolymer 1 (Scheme 6.2) as a very soluble red solid. After work up, the SEC analysis of 1 revealed
the presence of cyclic trimers, which were removed from the polymer by preparative size exclusion
chromatography.
H
N NO
O
O
O
OH
R*O
OR*
R*OH
OR*
O
R*O
OR*
R*O
OR*
N NO
O
O
O
O
H
10
12
+
OR* = 1
n
ma
Scheme 6.2. Synthesis of copolymer 1: a.[Rh(nbd)Cl]2, Et3N, toluene, r.t. 16 h, 36%.
The size exclusion chromatography (SEC) trace of the polymer in chloroform solution does
not correspond to a perfect Gaussian distribution, which suggests aggregation of the polymer in
chloroform solution. Addition of 1% methanol does not alter the shape of the SEC trace (Figure 6.3).
The molecular weight of the polymer is Mw= 14.6 kg/mol with PDI = 1.6. The molecular weight has
been determined using polystyrene standards, which gives a rough indication, but is not quantitative
due to the different shape of the polymer with regard to polystyrene. In contrast to polystyrene, which
is in a random coil conformation under these conditions, the polyacetylene is most probably in a rigid
rod conformation. The IR spectrum of polymer 1 does not feature the characteristic terminal acetylene
absorption, present in the starting monomers at around 3300 cm-1. This indicates that the
polymerization reaction has proceeded at the terminal ethynyl position and that no starting monomer
is left in the polymers. The 1H-NMR spectrum of polymer 1 shows very broad resonances. Generally,
Polyacetylenes with pendant donor-acceptor dyads
131
polyphenylacetylenes prepared using [Rh(nbd)Cl]2 exhibit sharp line widths, because of the high
stereoregularity of these polymers.7 However, polyacetylene 1 bears donor and acceptor
chromophores that are highly prone to aggregate. The mobility of the polymer backbone is most
probably restricted by this aggregation and peak broadening of the proton ressonances occurs.11
Consequently, the characteristic vinylic proton corresponding to the acetylene backbone could not be
distinguished and therefore, the regularity of the polymer backbone could not be determined. The
good solubility of copolymer 1 with respect to polymer 11 shows however that random incorporation
of non-perylene oligomers, disrupts the perylene aggregation by which the processability is greatly
enhanced.
300 400 500 600 700
0
1
Nor
mal
ized
res
pons
e
Time (sec)
Figure 6.3. SEC traces of polymers 1 (solid line), 2 (open squares) and 3 (open triangles) measured
in chloroform with 1% methanol. Inset: SEC traces of polymer 2 recorded in chloroform (solid
squares) and in chloroform with 1% methanol (open squares).
6.3.3 Polyacetylene with pendant OPVE-PERY dyads (polymer 2)
Polymer 2 has been synthesized by polymerization of the terminal acetylene OPVE-PERY
dyad 20 (Scheme 6.3). This dyad has been assembled by the non-selective reaction of diiodinated
OPV 17 with ethynyl perylene 10.
The synthesis of the OPV chromophore started by the radical bromination of 4-iodotoluene
giving 14. Treatment of 14 with triethylphosphite afforded phosphonate 15. Diiodinated OPV 17 was
obtained after a double Wittig-Horner reaction of phosphonate 15 with dialdehyde 16.19 The coupling
between the OPV and the PERY was done using the palladium catalyzed Sonogashira cross-coupling
of ethynyl compound 10 with diiodinated OPV 17 affording 19. An excess of 17 was used in order to
minimize the double coupling reaction, and the dyad was obtained as the main product of a statistical
Chapter 6
132
mixture. Due to the different nature of the two alkoxy side chains on the central ring, 17 is not
symmetrical and 19 therefore consists of a mixture of two regioisomers (only one of which has been
represented in Scheme 6.3). Coupling of 19 with TMS-acetylene and subsequent desilylation gave
functional monomer 20.
Monomer 20 was polymerized for 16 hours. Already after 20 minutes of polymerization a red
solid starts precipitating out of solution. The polymer was worked up by precipitation from the
reaction mixture with methanol. Removal of the cyclic trimers was achieved by reprecipitation from
THF. The polymer is slightly soluble in chloroform and o-dichlorobenzene, but the solubility strongly
increases with a small percentage of ethanol in the chloroform.
IR
OO O
O O
O
I
I
NNO
O
O
O
O
O
O
R
tmsH
NNO
O
O
O
O
O
O
*
*
n
16 17
+
2
18. R = I19. R = 20. R =
13. R = H14. R = Br15. R = P(OEt)2O
a
b
c
d
e
f
g
Scheme 6.3. a. NBS, benzoyl peroxide, CCl4, reflux, 3.5 h, 25%; b. Triethylphosphite, 160 °C, 1.5 h,
100 %; c. t-BuOK, DMF, r.t., 3 h, 64 %; d. 10, Pd(PPh3)2Cl2, CuI, Et3N, toluene, 50 °C, 16 h, 47%; e.
TMA, Pd(PPh3)2Cl2, CuI, Et3N, toluene, 50 °C, 16 h, 80%; f. TBAF/Silica, THF, r.t., 30 min, 97%; g.
[Rh(nbd)Cl]2, Et3N, toluene, r.t. 16h, 60%.
In contrast to polymer 1, the SEC trace of 2 is enormously influenced by the addition of a
small percentage of methanol (Figure 6.3, and inset). The SEC trace in chloroform points to very
Polyacetylenes with pendant donor-acceptor dyads
133
strong aggregation, compared to polymer 1, but this is largely overcome by the addition of methanol.
But even methanol does not allow for complete disruption of the aggregation, which makes it difficult
to estimate an accurate molecular weight against polystyrene standards (Mw= 45.8 kg/mol with PDI
=1.9, using 1% methanol in chloroform as eluent). As for polymer 1, the 1H-NMR spectrum of
polymer 2 shows extremely broad resonances.
6.3.4 Polyacetylene with pendant OPE-PERY dyads (polymer 3)
For polymer 3 an acetylene terminated OPE-PERY dyad was synthesized. In this case, the
OPE donor segment has first been end-capped with two different functionalities (a TMS-ethynylene
group and a bromine) to facilitate a selective monocoupling to the phenylethynyl terminated perylene
acceptor 10.
For the synthesis of the bromine-ethynyl bifunctional donor OPE (Scheme 6.4) a repetitive
strategy developed by Ziener et al. was followed.12 The synthesis started by bisbromination and
subsequent bisalkylation of hydroquinone that gave dibromide 23. The exchange of one bromine for
one iodine to yield 24 was achieved by reaction of 23 with n-BuLi followed by treatment with 1,2-
diiodoethane, while working at temperatures below –80 °C. Selective coupling of TMS-acetylene
with 24 at the iodo-phenyl position yielded 25. In contrast to literature,12 iodo-selectivity was only
achieved by keeping the temperature of the reaction at 0 °C and by using exact amounts of TMS-
acetylene. One part of 25 was treated with TBAF to give acetylene 26. The other part was converted
into the iodo compound 27. The yield of the halogen exchange was only 77%, and several attempts of
separating 27 from 25 using column chromatography were unsuccessful. Nevertheless, the synthetic
procedure was continued and removal of 25 was achieved in the following synthetic step. An iodo-
bromo-selective cross-coupling of 26 with 27 gave dimer 28. Tetramer 31 was derived from 28 after
another cycle of desilylation, iodine-bromine exchange and a palladium catalyzed cross-coupling.
Although tetramer 31 was obtained in reasonable overall yield, the synthesis of the donor fragment is
hampered by difficult and tedious purification of some of the intermediates.
Coupling of the donor tetramer 31 with ethynyl terminated perylene 10, followed by treatment
with TBAF gave acetylene 33. An attempt to polymerize 33, resulted selectively in cyclic trimers due
to the alkoxy substituents on the first phenyl ring. Therefore 33 was extended by coupling it to 4-
iodotrimethylsilylethynylbenzene to afford monomer 35 after deprotection of the ethynyl function.
Polymer 3 was prepared using the usual polymerization conditions for 16 hours. The reaction
mixture was precipitated into methanol and THF to yield a red solid, which is highly soluble in
solvents of intermediate polarity such as chloroform, dichloromethane and o-dichlorobenzene.
Chapter 6
134
OH
OH
OH
OH
BrBr
OR
RO
BrBr
OR
RO
IBr
OR
RO
Br tms
OR
RO
Br H
OR
RO
tmsI
OR
RO
Br tms
OR
RO
Br H
OR
RO
I tms
OR
RO
Br tms
ONN
O
O O
O
OR
RO
R'
ONN
O
O O
O
OR
RO
R'
OR = O
ONN
O
O O
O
OR
RO
H
*
*n
2
2
2
4
a b c
de
f
g
h
i
j
4
4
l
m
32. R' = tms33. R' = H
n
o
34. R' = tms35. R' = H
21 22 23 24
25
26
27
28
29
30
31
4
3
k
Scheme 6.4. a. HBr, Br2, 100 °C, 24 h, 49%; b. ROTs, KOH, EtOH, reflux, 24 h, 57%; `c. n-BuLi,
1,2-diodoethane, THF, -60 °C, 72%; d. TMA, Pd(PPh3)2Cl2, CuI, Et3N, 0 °C, 1 h, 82%; e. TBAF,
THF, r.t.,1 min, 100%; f. n-BuLi, 1,2-diodoethane, THF, -60 °C, 77%; g. Pd(PPh3)2Cl2, CuI, Et3N, 0
°C, 3.5 h, 67%; h. TBAF, THF, r.t., 1 min, 100%; i. n-BuLi, 1,2-diodoethane, THF, -60 °C, 60%; j.
Pd(PPh3)2Cl2, CuI, Et3N, 0 °C, 3.5 h, 56%; k. 10, Pd(PPh3)2Cl2, CuI, Et3N, toluene, 50 °C, 16 h, 34%
; l. TBAF/Silica, THF, r.t., 30 min, 100% ; m. 4-Iodotrimethylsylilphenylacetylene Pd(PPh3)2Cl2, CuI,
Et3N, toluene, 50 °C, 16 h, 40% ; n. BAF/Silica, THF, r.t., 30 min, 99% ; o. [Rh(nbd)Cl]2, Et3N,
toluene, r.t. 16h, 30%.
Polyacetylenes with pendant donor-acceptor dyads
135
The molecular weight of the polymer according to SEC is Mw = 53.5 kg/mol with PDI= 1.5
(Figure 6.3). The high molecular weight, especially with respect to polymer 1, is partly due to the high
molecular weight of the monomers, but most probably to the good solubility of polymer 3.
Additionally, the different hydrodynamic volume caused by the large pendant dyads might contribute
to the high molecular weight. In the 1H-NMR spectrum of polymer 3, peak broadening is somewhat
less prominent than for polymers 1 and 2. The OPE segment is heavily substituted with alkoxy chains,
which may hamper aggregation and allow for a better resolution in the NMR spectrum.
6.4 UV/Visible absorption and circular dicroism spectroscopies
The absorption spectra of polymers 1, 2 and 3 in chloroform solution exhibit the electronic
transitions associated with their donor and acceptor chromophores (Figure 6.4). All spectra feature the
well-resolved vibronic structure absorption characteristic of the PERY chromophore with maxima at
490 and 530 nm, approximately. The donor moieties dominate the rest of the absorption spectrum.
Monosubstituted polyacetylenes mainly absorb in the UV region and have very low extinction
coefficients. 13 Therefore the contribution of the backbone to the absorption spectra can be considered
negligible.
In Figure 6.4 the absorption spectra of all monomers in chloroform have been plotted together
with those of the respective polymers. The polymers exhibit a red-shifted onset with respect to their
corresponding monomers. Also the normalized spectra of the monomers and the related polymers do
not superimpose. These differences in the spectra suggest that at least the perylene chromophore is
aggregating when incorporated in the polymeric chain. In the solid state the onset is even more red-
shifted and the absorption bands even broader. By comparing the absorption spectra of the polymers
in chloroform solution with those of the monomeric species and with that in the solid state it can be
concluded that already in chloroform solution the pendant chromophores and dyads are interacting
with each other and are preorganizing the polymer into its solid state conformation. Of all, 2 is the
polymer that experiences the highest aggregation in solution as the absorption spectra in solution
highly resembles the one in solid state.
None of the polymers showed a Cotton effect in chloroform solution, even though they are
decorated with stereocenters. The absence of optical activity might stem either from aggregation
phenomena that disrupt the helical arrangement or from the presence of racemic side-chains or
mixtures of side-chains with different chiral centers, resulting in non-preferential twist senses of the
helices.
Chapter 6
136
300 400 500 6000
1
Abs
orba
nce
(O.D
.)
Wavelength (nm)
0
1
0
1
b
c
a
Figure 6.4. Absorption spectra of the different polyacetylenes and monomers.(a) Polymer 1 in
solution (solid line), and solid state (doted line) and monomer 10 (dashed-dotted line) and 12 (dashed
line). (b) Polymer 2 in solution (solid line) and thin film (dotted line) and monomer 20 (dashed line).
(c) Polymer 3 in solution (solid line) and in solid state (dotted line) and monomer 35 (dashed line).
6.5 Photophysical properties
6.5.1 Solid state
Near steady-state photoinduced absorption (PIA) spectroscopy of thin films of polymers 1, 2
and 3 recorded at 80 K revealed the occurrence of charge separation upon illumination (Figure 6.5 left
and right). All PIA spectra exhibit the absorptions characteristic of the perylene bisimide radical anion
at 1.28, 1.54 and 1.73 eV.14 For polymer 3 two additional absorptions centered at 0.5 and at 2.0 eV are
observed. A similar PIA spectrum is obtained from a blend of a p-phenylene ethynylene polymer
(PPE)15 with a soluble perylene dye PERY (Figure 6.5 right), meaning that the additional absorption
bands observed in the PIA spectrum of polymer 3 correspond to the OPE•+ radical cations. The
Polyacetylenes with pendant donor-acceptor dyads
137
absorptions of the positively charged donor moieties in polymers 1 and 2 are expected at similar
energies as observed for the OPE segments.16 However, the PIA spectrum of polymers 1 and 2 display
very weak ill-defined absorptions at low energies. Similar low absorption of the OPV•+ radical cation
have been previously observed in the PIA spectrum of a molecular triad based on comparable
chromophores as in polymers 1 and 2, OPV3-PERY-OPV3 (Figure 6.5 left).17
0.5 1.0 1.5 2.0 2.5
-2
0
2
4
6
∆T/T
x 1
04
Energy (eV)
0.5 1.0 1.5 2.0 2.5
0
2
4
∆T/T
x 1
04
Energy (eV)
Figure 6.5. PIA spectra of thin films of the polyacetylene polymers. Left: polymers 1(solid line), 2
(dashed line) and OPV3-PERY-OPV3 (solid squares). Right: polymer 3 (solid line) and PPE/PERY
solid state blend (solid squares).
NN
O
O
O
O
* *
O
O
n
OO
OO
OO
OONN
O
O
O
O
OPV3-PERY-OPV3
PERY
PPE
Figure 6.6. Chemical structure of reference compounds OPV3-PERY-OPV3, PPE and PERY.
The rates of formation and decay of the charge-separated states in thin films of 2 and 3 have
been inferred from sub-picosecond pump-probe spectroscopy (Figure 6.7 and Table 6.1). The time
profile of the 1450 nm absorption, corresponding to the donor radical cation, has been measured after
exciting the polymers at 450 nm. Fitting of the temporal transient absorption to exponential functions
reveals that the charges are formed within 1 ps and that their lifetimes range from 300 to 400 ps. For
polymer 1 only extremely weak signals could be measured.
Chapter 6
138
0 200 400 600 800 1000-70
-60
-50
-40
-30
-20
-10
0
Time (ps)
∆T (
a. u
.)
Time (ps)
-3 0 3-60
-30
0
Figure 6.7. Differential transmission dynamics of the 1450 nm absorption of polymers 2 (solid
squares) and 3 (open squares) in a thin film, after excitation at 450 nm. The inset shows the 1450 nm
signals on short timescales.
Table 6.1. Lifetimes and rates for the charge separation (CS) and charge recombination (CR) of
polymers 1, 2 and 3 in the solid state and in chloroform solution.
Solid state CHCl3 solution
CS CR CS CR
τ (ps) k (s-1) τ (ps) k (s-1) τ (ps) k (s-1) τ (ps) k (s-1)
Polymer 1 n.d. n.d. <1 >1012 13 8·1010
Polymer 2 <1 >1012 340 3·109 2.2 5·1011 634 2·109
Polymer 3 <1 >1012 425 2·109 6 2·1011 400 2·109
6.5.2.Chloroform solution
The photoluminescence spectra of polymers 1, 2 and 3, recorded in chloroform solution after
excitation of either donor or acceptor, exhibit very weak emission intensities. In particular, the
fluorescence emission of the OPV chromophore is 60 times lower than that of the starting monomer
12. The PL of the PERY chromophore is also quenched in polymer 1, by a factor of 20 with respect to
the PL of the PERY model compound (Figure 6.6). For polymers 2 and 3 the PL quenching factor of
the PERY chromophore scales to 450 times. The modest PL quenching in copolymer 1, with respect
to 2 and 3, can be attributed to a less efficient charge transfer in the copolymer caused by the presence
of some domains of only OPV or PERY chromophores within the polymer chains.
Polyacetylenes with pendant donor-acceptor dyads
139
500 600 7000
50
Inte
nsity
(a.
u.)
Wavelength (nm)
Figure 6.8. PL spectra of monomers 10 (solid squares) and 12 (solid circles) and polymer 1 in
chloroform solution, after selective excitation of the OPV (open circles) and of the PERY (open
squares) chromophores at 400 and 520 nm respectively.
According to sub-picosecond pump-probe spectroscopy, charge separation occurs
immediately in copolymer 1 (Figure 6.9 left and Table 6.1), which is consistent with a face-to-face
orientation of donor and acceptor, the most probable orientation of two neighboring pendant
chromophores in the copolymer. The face-to-face orientation of the chromophores is also responsible
for a very fast recombination of the charges. Additional to the fast decay of the majority of the
charges, a weak signal remains that only decays very slowly. This residual signal most probably
corresponds to trapped charges in small domains of donor or acceptor units. The presence of small
domains of chromophore units with equal signature has also been inferred from fluorescence
spectroscopy.
In polymers 2 and 3 the charges are generated more slowly than in polymer 1, but what is
more important, their lifetimes have been increased by more than one order of magnitude (Figure 6.9
and Table 6.1). This is the result of the different design of the last two polymers. The spacer placed
between the donor and the acceptor moieties slows down the charge separation and recombination
processes by intercalating a distance between the redox centers and, because of its semirigid nature,
by preventing the folding of the PERY donor on top of the donor segment to a face-to-face
orientation.9b
Both for polymer 2 and for polymer 3 the formation of the charges in the solid state is faster
than in solution (Table 6.1). However, for polymer 2 the formation of the charges is increased by only
a factor of 2 upon going from solution to the solid state, whereas for polymer 3 this increases by a
factor of 6. This observation is in line with the results obtained by absorption spectroscopy, which
show that the conformation and aggregational state of polymer 2 in solution closely resembles that in
Chapter 6
140
the solid state. For polymer 3 this relationship was not manifested as strongly as for polymer 2.
Polymer 3 was found to be much better dissolved in chloroform and hence features a much larger
conformational flexibility. The decrease in conformational flexibility for polymer 3 upon going to the
solid state accounts for the chromophores lining up in close proximity, thus very rapidly generating
the charges.
0 200 400 600 800 1000
-5
0
∆T (
a. u
.)
Time (ps)
∆T (
a. u
.)
Time (ps)
-3 0 3
-5
0
0 200 400 600 800 1000
-100
-50
0
Time (ps)
∆T (
a. u
.)
Time (ps)
-5 0 5 10 15-100
-50
0
Figure 6.9. Differential transmission dynamics of the 1450 nm absorption of polymers 1 (left, solid
line), 2 (right, solid squares) and 3 (right, hollow squares) in chloroform solution, after excitation at
450 nm. The insets show the 1450 nm signals on short timescales.
Similar as for the charge separation in polymer 2 also the lifetime of the charges is decreased
by a factor of 2 upon going from solution to the solid state. This reduction of the lifetime results
probably from the combination of denser packing of the chromophores in the solid state and increased
interchain interactions, both speeding up the charge recombination in polymer 2 in the solid state.
Surprisingly, for polymer 3 a different behavior of the lifetime of the charges is observed. The
lifetime of the charges is similar in solution and in the solid state. This phenomenon might be the
result of the alkoxy substitution of the OPE element. Although a denser packing might also occur for
polymer 3 upon going to the solid state, the alkoxy side chains might hamper lateral interaction of the
OPE chromophores with acceptors belonging to other polymer chains, preventing ‘interchain
recombination’, and thus allowing longer lifetimes. The alkoxy side chains might play the same role
as the spacer between donor and acceptor in a single dyad.
6.6 Conclusions and outlook
Several polyacetylenes substituted with a OPV or OPE donor and a PERY acceptor have been
synthesized for the first time. By designing different architectures, the strengths and weaknesses of
the placement of the functional groups on processability and photophysical properties have been
explored.
Polyacetylenes with pendant donor-acceptor dyads
141
The polymers were prepared either by the copolymerization of OPV and PERY terminal
acetylenes or by the polymerization of homologous OPVE-PERY and OPE-PERY dyads. SEC
analyses exhibit the typical distribution corresponding to polymeric systems. The molecular weights
of the polymers are in the range of 15-55 kDa giving especially polymers 2 and 3 good processing
properties, due to their higher molecular weight and good solubility in common organic solvents.
Nevertheless, the low solubility of especially polymers 1 and 2 in toluene, used in the polymeriztion,
limits the chain length. Although the polydispersity of the polymers is good, the degree of
polymerization might eventually be optimized by a different choice of the polymerization solvent.
Even though the copolymerization approach as applied for polymer 1 offers the fastest
synthetic access to donor-acceptor functionalized polyacetylenes, the alternating arrangement of
donors and acceptors results in very short-lived charged-separated states. It remains to be seen if this
can be overcome by another design of the two monomers. On the contrary, polymers 2 and 3 show
promising photophysical properties owing to their more refined architecture, avoiding donor-acceptor
stacking and favoring donor-donor and acceptor-acceptor interactions. In these polymers the lifetime
of the photoinduced charge separated state is increased by at least more than one order of magnitude
with respect to polymer 1.
These results, together with the optimization of the polymerization conditions, make these
polyacetylenes an attractive system to explore in future experiments.
6.7 Experimental Section
For general methods the reader is referred to chapter 2 of this thesis. [(S)-1-Isobutyl-2-(4-iodo-phenyloxy)ethyl]carbamic acid tert-butyl ester (5). Iodophenol (4.34g, 19 mmol), 49 (4.25 g, 19 mmol) and triphenylphosphine (7.77 g, 29 mmol) were dissolved in toluene (60 mL). Diethyl azodicarboxylate (4.66 ml, 29 mmol) in toluene (30 mL) was slowly added while keeping the temperature below 35 °C. The reaction mixture was stirred for 16 h at room temperature and subsequently washed with 1N aqueous HCl. The organic phase was dried over MgSO4 and evaporated in vacuo. Column chromatography (n-heptane/EtOAc, 4/1, Rf = 0.3) and crystallization from hexane afforded the product (3.2 g, 40%) as white crystals.1HNMR (CDCl3, 300 MHz): δ 7.54 (d, 2H), 6.67 (d, 2H), 4.75-4.65 (m, 1H), 4.10-3.8 (m, 3H), 1.80-1.65 (m, 1H), 1.60-1.40 (m, 11 H), 1.00-0.88 (m, 6H); 13C NMR (CDCl3, 75 MHz): δ .158.50, 155.18, 138.04, 116.80, 82.91, 79.36, 70.25, 48.15, 41.00, 28.48, 24.94, 23.12, 22.38; Anal. Calc for C17H26INO3: C, 48.7; H, 6.2; N, 3.3. Found: C, 48.9; H, 6.1; N, 3.28. (S)-1-Isobutyl-2-(4-iodo-phenyloxy)-ethylamine (6). TFA (12 mL) was added to a solution of 5 (3 g, 7.3 mmol) in methylene chloride (12 mL). After stirring the reaction mixture for 16 h at room temperature under an argon atmosphere, NaHCO3 was added. The reaction mixture was subsequently washed with water, NaOH 1 N, water, brine and dried over Na2SO4.The product (2 g, 86%) was obtained after evaporation of the solvent as a colorless oil.1H NMR (CDCl3, 300 MHz): δ 7.54 (d, 2H), 6.68 (d, 2H), 3.86 (dd, 1H), 3.64 (dd, 1H), 3.22 (s, 1 H), 1.81-1.74 (m, 1 H), 1.25 (t, 2H), 0.97-0.92 (m, 6H); 13C NMR (CDCl3, 75 MHz): δ .158.64, 137.99, 116.76, 82.66, 73.69, 48.27, 42.98, 24.46, 23.27, 21.91.
Chapter 6
142
N-[(S)-1-Isobutyl-2-(4-iodo-phenyloxy)-ethyl]-N’-(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (8). N-(1-ethylpropyl)-3,4,9,10-perylenetetracarboxylic monoanhydride monoimide 518 (1.91 g, 4.17 mmol), amine 4 (2 g, 6.26 mmol), imidazol (30 g, mmol), and catalytic amounts of Zn(AcO)2 were mixed and stirred for 2.5 h at 160 °C. After cooling to room temperature the solid reaction mixture was dissolved in CH2Cl2 and washed with HCl 1N, water, brine and dried over MgSO4. Purification by column chromatography (flash silica, CH2Cl2, Rf = 0.36) yielded 1.97 g (67%) of as a dark red solid.1HNMR (CDCl3, 300 MHz): δ 8.76-8.59 (m, 8H), 7.46 (d, 2H), 6.65(d, 2H), 5.77-5.69 (m, 1H), 5.11-5.03 (m, 1H), 4.72 (dd, 1H), 4.33-4.30 (dd, 1H), 2.33-2.21 (m, 3H), 2.00-1.90 (m, 2H), 1.81- 1.75 (m, 1H), 1.67-1.59 (m, 1H) 1.02 (d, 3H), 098 (d, 3H), 0.94 (t, 6H); 13C NMR (CDCl3, 100 MHz): δ 163.16, 158.20, 137.87, 133.58, 133.22, 131.15, 130.61, 128.72, 125.35, 123.18, 122.32, 122.12, 117.02, 82.98, 68.78, 57.77, 54.41, 38.63, 25.64, 25.04, 23.14, 22.60, 11.62. Anal. Calc for C41H35IN2O5: C, 64.6; H, 4.6; N, 3.7. Found: C, 64.3; H, 4.4; N, 3.5. MALDI-TOF MS (Mw = 762.65) m/z= 762.08 [M]+. N-[(S)-1-Isobutyl-2-(4-trimethylsylylethynyl-phenyloxy)-ethyl]-N’-(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (9). A mixture of 8 (1.42 g, 1.86 mmol), Pd(PPh3)2Cl2 (0.104 g, 0.148 mmol), CuI ( 0.021 g, 0.107 mmol) was dissolved in anhydrous toluene/triethylamine (320 mL, 5:1). After argon was purged trough the solution for 15 min trimethylsylylacetylene (0.32 mL, 2.23 mmol) was added. The reaction mixture was heated at 50 oC for 4 h. The crude reaction mixture was filtered through Celite, evaporated to dryness, and chromatographied (flash silica, CH2Cl2, Rf = 0.35) to afford a red solid (1.2 g, 84 %). 1H NMR (CDCl3, 300 MHz): δ 8.56-8.36 (m, 8H), 7.34 (d, 2H), 6.84 (d, 2H), 5.76-5.70 (m, 1H), 5.10-5.03 (m, 1H), 4.80 (dd, 1H), 4.39-4.34 (dd, 1H), 2.33-2.20 (m, 3H), 2.04-1.90 (m, 2H), 1.87-1.75 (m, 1H), 1.68-1.61 (m, 1H), 1.07-0.94 (m, 12H), 0.18 (s, 9H); 13C NMR (CDCl3, 100 MHz): δ 163.80 (broad signal), 158.78, 134.36, 134.01, 133.37, 131.20, 129.33, 126.08, 123.47, 122.88, 122.71, 115.41, 114.61, 105.10, 92.43, 68.64, 57.72, 53.40, 51.40, 38.53, 25.55, 24.98, 23.10, 22.42, 11.38, -0.03. Anal. Cald for C41H35IN2O5: C, 64.6; H, 4.6, N, 3.4. Found: C, 64.3; H, 4.4, N, 3.4. MALDI-TOF MS (Mw = 732.96) m/z= 732.30 [M]+. N-[(S)-1-Isobutyl-2-(4-ethynyl-phenyloxy)-ethyl]-N’-(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (10). To a solution of 9 (0.4 g, 0.56 mmol) and tetrabutylammonium fluoride on silica (0.177 g, 0.68 mmol) in 70 mL THF was stirred for 0.5 h under argon. The reaction mixture was filtered through Celite and evaporated to dryness. The product was obtained as a red powder (0.3 g, 81%) and was used without further purification. 1H NMR (CDCl3, 300 MHz): δ 8.63-8.50 (m, 8H), 7.35 (d, 2H), 6.84 (d, 2H), 5.76-5.70 (m, 1H), 5.11-5.02 (m, 1H), 4.80 (dd, 1H), 4.39-4.34 (dd, 1H), 2.94 (s, 1H), 2.33-2.20 (m, 3H), 2.04-1.90 (m, 2H), 1.87-1.75 (m, 1H), 1.68-1.61 (m, 1H), 1.07-0.94 (m, 12H); 13C NMR (CD2Cl2, 100 MHz): δ.163.81, 159.55, 134.15, 133.89, 133.75, 131.56, 131.01, 129.25, 125.87, 123.72, 122.90, 122.70, 115.16, 114.65, 83.76, 76.14, 69.31, 58.40, 51.64, 38.99, 25.93, 25.36, 23.23, 22.63, 11.71. MALDI-TOF MS (Mw =660.78) m/z = 660.38 [M]+. 1-Bromomethyl-4-iodobenzene (14). A mixture of 4-iodotoluene (10 g, 458.8 mmol), N-bromosuccinimide (9.79 g, 55 mmol) and benzoyl peroxide (0.44 g, 1.8 mmol) in dry carbon tetrachloride (14 mL) was stirred and heated to reflux for 3.5 h and then cooled and filtered. The red filtrate was washed with saturated sodium thiosulfate solution (10 mL), dried and filtered. The solvent was removed in vacuo and the solid residue was further purified by recrystalization in hexane. The product was obtained as an off-white solid (3.5 g, 25%). 1H NMR (CDCl3, 300 MHz): δ 7.67 (d, 2H), 7.15(d, 2H), 4.41 (s, 1H); 13C NMR (CDCl3, 100 MHz): δ 137.94, 137.40, 130.82, 94.12, 32.45. Diethyl(4-iodo-benzyl)phosphonate (15). Triethyl phosphite (2.28 g, 13 mmol) and 14 (3.40 g, 11 mmol) were stirred at 160 oC for 1.5 h under an argon atmosphere. The reaction mixture was cooled to 75 oC and the ethyl bromide, formed during the reaction, and the excess of triethyl phosphite were distilled under reduced pressure. The product was obtained as a light yellow oil. Yield 3.89 g (100%). 1H NMR (CDCl3) δ 7.54(d, 2H), 6.97 (dd, 2H), 4.00- 3.84(m, 4H), 2.98 (d, 2H), 1.17(t, 6H); 13C NMR (CDCl3) δ 137.32 (d), 131.48 (d), 131.20 (d), 92.09 (d), 61.93 (d), 34.00, 32.17, 16.16 (d). (E,E)-1,4-Bis(4-iodo-styryl)-2-(3,7-dimethyloctyloxy)-5-methoxybenzene (17). Phosphonate 15 (0.8 g, 2.25 mmol) was dissolved in dry DMF (9 mL) under an argon atmosphere and 0.32 g (2.8 mmol) of KtBuO were added to the solution. After 15 min, a solution of dialdehyde 1619 (0.29 g, 0.9 mmol) in dry DMF (6 mL) was added dropwise and the reaction mixture was stirred for 3.5 h. The solution was poured on crushed ice and of 6 M HCl (200 mL) was added. The aqueous phase was extracted twice with diethyl ether
Polyacetylenes with pendant donor-acceptor dyads
143
and the combined organic layers were subsequently washed with 3 M HCl, water, a saturated aqueous solution of Na2CO3 and dried over MgSO4. The solvent was removed in vauo. Column chromatography (flush silica gel, CH2Cl2/pentane, 4:1, Rf = 0.3) and evaporation of the solvent afforded 0.42 g (64%) of the product as a yellow solid: 1H NMR (CDCl3; 300 MHz): δ 7.67 (d, 2H), 7.67 (d, 2H), 7.47(d, 1H), 7.46 (d, 1H), 7.27 (d, 2H), 7.26 (d, 2H), 7.11 (s, 1H), 7.09 (s, 1H), 7.06 (d, 1H), 7.02 (d, 1H), 4.09 (t, 2H), 3.91 (s, 3H), 1.97-1.86 (m, 1H), 1.82-1.45 (m, 4H), 1.45-1.05 (m, 6H), 0.99 (d, 3H), 0.87 (d, 3H). 13C NMR (CD2Cl3, 100 MHz) δ 151.54, 151.19, 137.76, 137.54, 137.48, 128.25, 128.20, 127.73, 127.69, 126.58, 126.35, 124.20, 123.98, 110.35, 109.09, 92.46, 92.42, 67.75, 56.19, 39.27, 37.41, 36.49, 30.14, 28.03, 24.85, 22.49, 22.39, 19.57. MALDI-TOF MS (Mw =720.48) m/z= 719.95[M]+. N-[(S)-1-Isobutyl-2-<4-{(E,E)-4-[4-(4-iodostyryl)-2-(3,7-dimethyloctyloxy)-5-methoxy)styryl]phenylethynyl}phenyloxyl>ethyl]-N’-(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (18). A degassed solution of 17 (0.19 g, 0.26 mmol), 10 (0.12 g, 0.18 mmol), Pd(PPh3)2Cl2 (0.01 g, 0.014 mmol), CuI (0.002 g, 0.01 mmol) in anhydrous toluene/triethylamine (30 mL, 5:1) was heated at 50 oC for 16 h. The crude reaction mixture was filtered through Celite, evaporated to dryness, and chromatographied (flash silica, CHCl3/pentane, 9:1, Rf = 0.5) to afford a red solid (0.102 g, 47 %). 1H NMR (CDCl3, 400 MHz): δ 8.56-8.36 (m, 8H), 7.64 (d, 1H), 7.62 (d, 1H), 7.44-7.38 (m, 8H), 7.22 (d, 1H), 7.21 (d, 1H), 7.06-6.97 (m, 4H), 7.01 (d, 1H), 7.00(d, 1H), 6.99 (d, 1H), 6.93-6.90 (m, 2 H), 5.81-5.74 (m, 1H), 5.11-5.03 (m, 1H), 4.82 (dd, 1H), 4.41-4.38 (dd, 1H), 4.08-3.99 (m, 2H), 3.87 (s, 3H), 2.34-2.22 (m, 3H), 2.03-1.81 (m, 4H), 1.78-1.13 (m, 4 H),1.58-1.16 (m, 6H), 1.06 (d, 3H), 1.02 (d, 3H), 0.98 (d, 3H), 0.96 (t, 6H), 0.86 (d, 3H), 0.85 (d, 3H); 13C NMR (CDCl3, 100 MHz): δ 163.80 (broad signal), 158.66, 151.35, 151.37, 151.04, 137.65, 137.58, 137.36, 137.30, 134.39, 134.04, 132.93, 131.62, 131.56, 131.18, 129.34, 129.30, 128.24, 128.18, 128.11, 127.64, 127.61, 126.77, 126.52, 126.41, 126.34, 126.29, 126.14, 126.08, 124.17, 123.99, 123.91, 123.72, 122.87, 122.74, 122.31, 115.58, 114.85, 110.19, 108.93, 92.54, 92.49, 90.28, 88.40, 68.69, 67.63, 57.70, 56.13, 51.45, 39.19, 38.58, 37.36, 36.43, 30.06, 29.66, 27.93, 25.58, 24.98, 24.80, 24.78, 23.12, 22.67, 22.58, 22.48, 19.74, 11.41. MALDI-TOF MS (Mw =1253.34) m/z= 1252.23[M]+. N-[(S)-1-Isobutyl-2-<4-{(E,E)-4-[4-(4-trimethylsylylethynyl-styryl)-2-(3,7-dimethyloctyloxy)-5-methoxy)styryl]phenylethynyl}phenyloxy>ethyl]-N’-(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (19). A mixture of 18 (0.073 g, 0.058 mmol), Pd(PPh3)2Cl2 (0.005 g, 0.007 mmol), CuI ( 0.001 g, 0.005 mmol) was dissolved in anhydrous toluene/triethylamine (18 mL, 5:1). After argon was purged trough the solution for 15 min trimethylsylylacetylene (0.1 mL, 0.707 mmol) was added. The reaction mixture was heated at 50 oC for 16 h. The crude reaction mixture was filtered through Celite, evaporated to dryness, and chromatographied (flash silica, CH2Cl2, Rf = 0.5) to afford a red solid (57 mg, 80%). 1H NMR (CDCl3, 300 MHz): δ 8.56-8.37 (m, 8H), 7.45-7.37 (m, 12H), 7.06 (d, 1H), 7.05, 7.04, 7.03, 7.03 (4xs, 2H), 7.02 (d, 1H), 6.92-6.89 (m, 2H), 5.80-5.73 (m, 1H), 5.09-5.03 (m, 1H), 4.82 (dd, 1H), 4.41-4.37 (dd, 1H), 4.05-4.01 (m, 2H), 3.82 (s, 3H2.34-2.22 (m, 3H), 2.03-1.81 (m, 4H), 1.78-1.13 (m, 4 H), 1.58-1.16 (m, 6H), 1.06 (d, 3H), 1.02 (d, 3H), 0.98 (d, 3H), 0.96 (t, 6H), 0.86 (d, 3H), 0.85 (d, 3H) ), 0.18 (s, 9H); 13C NMR (CDCl3, 100 MHz): δ 163.77 (broad signal), 158.66, 151.38, 151.07, 137.99, 137.41, 137.33, 134.39, 134.04, 132.91, 132.21, 131.59, 131.55, 131.13, 130.72, 129.30, 128.19, 128.06, 126.72, 126.56, 126.47, 126.33, 126.27, 126.18, 126.08, 124.33, 124.06, 123.75, 123.38, 122.87, 122.71, 122.28, 121.80, 115.58, 114.84, 110.21, 108.92, 105.26, 94.99, 90.25, 88.39, 68.68, 67.65, 57.69, 56.11, 53.39, 51.45, 39.19, 38.56, 37.34, 36.41, 30.05, 27.93, 25.57, 27.98, 24.77, 23.09, 22.65, 22.55, 22.45, 19.71, 11.38, -0.054. MALDI-TOF MS (Mw =1223.65) m/z = 1222.36 [M]+. N-[(S)-1-Isobutyl-2-<4-{(E,E)-4-[4-(4-Ethynyl-styryl)-2-(3,7-dimethyloctyloxy)-5-(methoxy)styryl]phenylethynyl}phenyloxy>ethyl]-N’-(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (20). A solution of 19 (50 mg, 0.041 mmol) and tetrabutylammonium fluoride on silica (0.05 mL of a 1M solution, 0.05 mmols) in 5 mL THF was stirred for 0.5 h under argon. The reaction mixture was filtered through Celite and evaporated to dryness. The product was obtained as a red powder (46 mg, 97 %) and was used without further purification. The complete deprotection of the ethynyl bond was confirmed by 1H-NMR spectroscopy as the disappearance of the singlet at 0.18 ppm corresponding to the TMS end group and the appearance of the signal for the ethynylinic proton at 3.10 ppm. 2,5-Dibromo-hydroquinone (22). p-Benzoquinone (15 gr, 136 mmol) was dissolved in concentrated HBr (320 mL). Bromine (22.5 gr, 140 mmol) was added slowly to the solution while stirring. After 7 h the reaction mixture was heated to 100 °C and stirred
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for another 12 h. An additional amount of bromine (7 g, 43 mmol) was added and reacted at 100 °C for another 3 h. The reaction mixture was cooled to room temperature and the precipitated solid washed with water. The product was purified by repetitive recrystalization in water with addition of charcoal and obtained as white crystals (17.98 g, 49%). 1H NMR (CD3CO CD3, 400 MHz): δ 8.58 (s, 2H), 7.16(s, 2H); 13C NMR (CDCl3, 100 MHz): δ 147.98, 120.05, 108.90. 1,4-Dibromo-2,5-bis(2-ethylhexyloxy)benzene (23). Dibromohydroquinone 22 (26.27 g, 98 mmol) was dissolved in a solution of potassium hydroxide (21.99 g, 392 mmol) in ethanol (1250 mL) and heated to reflux, after what 2-ethylhexyl-p-toluenesulfonate (97.56 g, 343 mmol) was added. The reaction mixture was cooled to room temperature after 16 h. The precipitate formed was filtered off and washed with methanol. The solvent of the liquid phase was removed in vacuo. The resulting crude product was purified by column chromatography ( silica gel, hexane, Rf = 0.35). Evaporation of the solvent yielded 27.81 g (57 %) of a colorless oil. 1H NMR (CD3CO CD3, 300 MHz): δ 7.08 (s, 2H), 3.83 (d, 4H), 1.80-1.68 (m, 2H), 1.60-1.26 (m, 18 H), 0.96-0.90 (m, 12H); 13C NMR (CDCl3, 75 MHz): δ 150.28, 118.29, 111.15, 72.64, 39.52, 30.53, 29.11, 23.96, 23.10, 14.14, 11.23. 2,5-Bis(2-ethylhexyloxy)-4-bromo-iodobenzene (24). A solution of 23 (28 g, 56 mmol), in THF (225 mL) was cooled to –90 °C. n-BuLi in hexane (22.74 mL, 2.5 M) was added at such a rate that the internal temperature did not exceed –80 °C. This cold solution was added via a cannula to 1,2-diiodoethane (17.05 g, 60.5 mmol) in THF (113 mL) at –60 °C. After addition, the cooling bath was removed, and the reaction mixture was stirred for 30 min. The color changed from yellowish to brown. The reaction mixture was washed with saturated aqueous Na2S2O5 untill it lost the color. The water phase was extracted three times with diethyl ether. The combined organic phases were dried over MgSO4 and the solvent removed in vacuo. The crude product was purified by flash chromatography (silica gel, hexane, Rf = 0.5). Evaporation of the solvent yielded 22.10 g (72%) of a colorless oil. 1H NMR (CDCl3; 400 MHz): δ 7.27 (s, 1H), 6.98 (s, 1H), 3.85-3.81 (m, 4H), 1.80-1.70 (m, 2H), 1.60-1.48 (m, 16H), 0.96-0.90 (m, 12H); 13C NMR (CDCl3, 100 MHz): δ 152.04, 150.06, 123.52, 116.25, 112.01, 84.22, 72.24, 72.01, 39.22, 39.20, 30.30, 30.24, 28.84, 28.90, 23.75, 23.68, 22.84, 13.93, 13.90, 11.03, 11.01. 2,5-Bis(2-ethylhexyloxy)-4-trimethylsilylethynyl-bromobenzene (25). To a degassed solution of 24 (22 g, 40.77 mmol) in Et3N (123 mL), Pd(PPh3)2Cl2 (0.28 g, 0.4 mmol) and CuI (0.15 g, 0.8 mmol) were added, while cooling in an ice bath. (Trimethylsilyl)acetylene (5.76 mL, 40.97 mmol) was added and the mixture was stirred at 0 °C. After 50 min the reaction was quantitative according to GC-MS. The solvent was removed in vacuo and the residue was dissolved in water and diethyl ether. The water phase was extracted three times with diethyl ether. The combined organic phases were washed with saturated aqueous NH4Cl and dried over MgSO4. The solvent was removed and the residue was flash chromatographed (silica gel, hexane, Rf = 0.25). The product was obtained as a colorless oil (17.11 g, 82%). 1H NMR (CDCl3; 300 MHz): δ 7.03 (s, 1H), 6.93 (s, 1H), 3.88-3.77 (m, 4H), 1.77-1.69 (m, 2H), 1.61-1.25 (m, 16H), 0.96-0.90 (m, 12H), 0.24 (s, 9H); 13C NMR (CDCl3, 75 MHz): δ 154.89, 149.40, 117.62, 117.55, 113.52, 112.26, 100.68, 98.99, 72.35, 71.94, 39.61, 39.47, 30.47, 29.10, 23.89, 23.05, 14.06, 11.25, 11.14,-0.07. 2,5-Bis(2-ethylhexyloxy)-4-ethynyl-bromobenzene (26). To a solution of 25 (5.49 g, 10.7 mmol) in dry THF was added 1 M tetrabuthylammonium fluoride in THF (10.79 mL). The reaction mixture was stirred for 1 min and subsequently filtrated over silica gel using chloroform as eluent. The solvent was removed in vacuo yielding 4.68 g (100%) of a slightly colored oil which was used without further purification: 1H NMR (CDCl3; 300 MHz): δ 7.07 (s, 1H), 6.96 (s, 1H), 3.85-3.82 (m, 4H), 3.26 (s, 1H), 1.77-1.69 (m, 2H), 1.61-1.25 (m, 16H), 0.96-0.90 (m, 12H). 2,5-Bis(2-ethylhexyloxy)-4-trimethylsilylethynyl-iodobenzene (27). Following a similar procedure as for the synthesis of compound 24, the reaction of 25 (8.5 g, 16.6 mmol) in THF (100 mL) with 2.5 M BuLi in hexane (6.7 mL) and 1,2-diiodoethane (5.52 g, 19.6 mmol) in THF (50 mL) gave, after chromatography (flash silica gel, hexane, Rf = 0.23), 27 (7.2 g, 77%, purity~ 90%). 1H NMR (CDCl3; 400 MHz): δ 7.24 (s, 1H), 6.93 (s, 1H), 3.77-3.50 (m, 4H), 1.77-1.69 (m, 2H), 1.61-1.25 (m, 16H), 0.96-0.90 (m, 12H), 0.24 (s, 9H); 13C NMR (CDCl3, 75 MHz): δ 155.03, 151.70, 123.38, 115.88, 113.23, 100.85, 99.21, 87.65, 72.12, 71.95, 39.62, 39.46, 30.53, 3047, 29.10, 29.06, 23.96, 23.50, 14.09, 11.27, 11.18, -0.08.
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4-[4-Trimethylsilylethynyl-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy)bromobenzene (28). To a degassed solution of 26 (4.71 g, 10.78 mmol) and 27 (5.5 g, 10.78 mmol) in Et3N (48 mL), Pd(PPh3)2Cl2 (76 mg, 0.1 mmol) and CuI (41 mg, 0.21 mmol) were added, while cooling in an ice bath. The mixture was stirred at 0 °C for 3.5 h. The solvent was removed in vacuo and the residue was dissolved in water and diethyl ether. The water phase was extracted three times with diethyl ether. The combined organic phases were washed with saturated aqueous NH4Cl, dried over MgSO4 and the solvent evaporated in vacuo. The residue was extensively purified by flash column chromatography (silica gel, pentane/diethyl ether, 100:0 to 99:1 Rf = 0.1-0.5). The product was obtained as a light yellow oil (5.9 g, 67). 1H NMR (CDCl3; 300 MHz): δ 7.09 (s, 1H), 6.98 (s, 1H), 6.93 (s, 1H) 6.92 (s, 1H), 3.92-3.78 (m, 8H), 1.79-1.72 (m, 4H), 1.63-1.37 (m, 16H), 1.32-1.30 (m, 16H), 0.97-0.84 (m, 24H), 0.26 (s, 9H); 13C NMR (CDCl3, 100 MHz): δ 154.35, 154.20, 153.44, 149.57, 117.82, 117.20, 117.02, 116.33, 114.47, 113.40, 113.12, 112.90, 101.22, 99.84, 90.81, 90.47, 72.28, 72.20, 71.88, 71.64, 39.63, 39.56, 39.54, 39.46, 30.60, 30.56, 30.50, 30.47, 29.09, 29.08, 29.02, 24.07, 24.03, 23.87, 23.07, 23.05, 14.09, 14.06, 11.30, 11.25, 11.11, -0.05. MALDI-TOF MS (Mw = 866.20) m/z = 866.38 [M]+. 4-[4-Ethynyl-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy)bromobenzene (29). To a solution of 28 (1.76 g, 2 mmol) in dry THF was added 1 M tetrabuthylammonium fluoride in THF (2 mL). The reaction mixture was stirred for 1 min and subsequently filtrated over silica gel using chloroform as eluent. The solvent was removed in vacuo. The product was purified by column chromatogragphy (flush silica gel, hexane/toluene, 4:1, Rf = 0.2). The product (1.55 g, 95%) is light yellow oil: 1H NMR (CDCl3; 300 MHz): δ 7.09 (s, 1H), 6.98 (s, 1H), 6.96 (s, 2H), 3.90-3.82 (m, 8H), 1.79-1.73 (m, 4H), 1.60-1.26 (m, 32H), 0.98-0.84 (m, 24H); 13C NMR (CDCl3, 75 MHz): δ 154.44, 154.24, 153.50, 149.61, 117.86, 117.54, 117.26, 116.70, 114.90, 113.22, 112.85, 112.48, 90.87, 90.26, 82.14, 80.05, 72.32, 72.20, 72.07, 71.97, 39.54, 39.41, 30.58, 30.48, 29.08, 29.03, 24.04, 23.87, 23.03, 14.03, 11.28, 11.10. 4-[4-Trimethylsilylethynyl-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy)iodobenzene (30). Following a similar procedure as for the synthesis of compound 24, the reaction of 28 (3.14 g, 3.55 mmol) in THF (100 mL) with 2.5 M BuLi in hexane (1.56 mL) and 1,2-diiodoethane (1.10 g, 3.91 mmol) in THF (50 mL) gave, after chromatography (flash silica gel, toluene/pentane, 1:4, Rf = 0.42), 30 (2.1 g, 63%, purity~85%). 1H NMR (CDCl3; 400 MHz): δ 7.29 (s, 1H), 6.93 (s, 1H), 6.92 (s, 1H), 6.87 (s, 1H), 3.90-3.81 (m, 8H), 1.79-1.71 (m, 4H), 1.54-1.26 (m, 32H), 0.97-0.85 (m, 24H), 0.26 (s, 9H); 13C NMR (CDCl3, 75 MHz): δ 154.34, 153.44, 151.89, 123.68, 117.01, 116.34, 115.50, 114.48, 113.90, 113.45, 101.22, 99.81, 91.00, 90.67, 87.18, 72.22, 72.05, 71.85, 71.62, 39.63, 39.56, 39.47, 30.57, 30.51, 30.49, 29.08, 29.03, 24.03, 23.93, 23.88, 23.03, 14.06, 11.30, 11.24, 11.14, -0.07. MALDI-TOF MS (Mw = 913.20) m/z = 912.47 [M]+. 4-<4-{4-[4-Trimethylsilylethynyl-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy) phenylethynyl}-2,5-bis(2-ethylhexyloxy)phenylethynyl >-2,5- bis(2-ethylhexyloxy)bromobenzene (31). To a degassed solution of 29 (1.20 g, 1.5 mmol) and 30 (1.4 g, 1.5 mmol) in Et3N (15 mL), Pd(PPh3)2Cl2 (52 mg, 0.075 mmol) and CuI (14 mg, 0.075 mmol) were added, while cooling in an ice bath. The mixture was stirred at 0 °C for 2.5 h. The solvent was removed in vacuo and the residue was dissolved in water and diethyl ether. The water phase was extracted three times with diethyl ether. The combined organic phases were washed with saturated aqueous NH4Cl, dried over MgSO4and the solvent evaporated in vacuo. The residue was purified with flash column chromatography (silica gel, toluene/hexane, 3:7 Rf = 0.25). The product was obtained as a greenish fluorescent oil (1.4 g, 60%). 1H NMR (CDCl3; 400 MHz): δ 7.12 (s, 1H), 7.01 (s,3H), 7.01 (s, 2H) 6.96 (s, 1H), 3.94-3.83 (m, 16H), 1.88-1.72 (m, 8H), 1.70-1.40 (m, 32H), 1.40-1.25 (m, 32H), 1.01-0.88 (m, 48H), 0.27 (s, 9H); 13C NMR (CDCl3, 100 MHz): δ154.33, 154.18, 153.67, 153.64, 1253.45, 149.56, 117.79, 117.17, 117.00, 116.62, 116.33, 114.57, 114.22, 114.15, 114.10, 113.39, 113.07, 112.94, 101.23, 99.81, 91.52, 91.41, 90.83, 90.60, 72.22, 72.14, 71.87, 71.81, 71.59, 39.62, 39.57, 39.53, 39.44, 30.59, 30.48, 30.45, 29.07, 28.99, 24.04, 23.86, 23.05, 23.00, 14.06, 14.02, 11.27, 11.22, 11.08, -0.09. MALDI-TOF MS (Mw = 1579.31) m/z = 1579.12 [M]+. N-[(S)-1-Isobutyl-2-<4-[4-{4-[4-Trimethylsilylethynyl-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy)phenylethynyl}-2,5-bis(2-ethylhexyloxy)phenylethynyl>-2,5-bis(2-ethylhexyloxy)phenylethynyl]phenyloxy>-ethyl]-N’-(1-ethylpropyl)- 3,4,9,10-perylenebis(dicarboximide) (32). A degassed solution of 31 (0.3 g, 0.19 mmol), 10 (0.12 g, 0.19 mmol), Pd(PPh3)2Cl2 (0.01 g, 0.014 mmol), CuI (0.002 g, 0.01 mmol) in anhydrous toluene/triethylamine (30 mL, 5:1) was heated at 50 oC for 16 h. The crude
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reaction mixture was filtered through Celite, evaporated to dryness, and chromatographied (flash silica, CH2Cl2/pentane, 3:2 to 4:1, Rf = 0.23-0.4) to afford a red solid (0.144 g, 34 %). 1H NMR (CDCl3, 400 MHz): δ 8.64-8.45 (m, 8H), 7.41 (d, 2H), 6.99 (s, 4H), 6.96 (s, 1H), 6.94 (s, 3H), 6.91 (d, 2H), 5.81-5.73 (m, 1H), 5.10-5.03 (m, 1H), 4.82 (dd, 1H), 4.40 (dd, 1H), 3.94-3.80 (m, 16H), 2.32-2.45 (m, 3H), 2.01-1.94 (m, 2H), 1.86-1.25 (m, 66H), 1.06-0.82 (m, 60H), 0.26 (s, 9H); 13C NMR (CDCl3, 100 MHz): δ 163.51, 158.66, 153.65, 151.32, 153.45, 131.48, 131.11, 132.88, 131.21, 129.43, 126.18, 122.95, 116.99, 116.62, 115.90, 114.78, 114.57, 114.24, 114.10, 113.84, 113.34, 101.23, 99.82, 94.82, 91.54, 91.37, 84.49, 71.86, 71.59, 68.69, 57.71, 51.45, 39.55, 38.57, 30.74, 30.57, 30.47, 29.06, 25.57, 24.98, 24.03, 23.94, 2.85, 23.04, 22.45, 14.034, 11.36, 11.27, -0.069 MALDI-TOF MS (Mw = 2159.17) m/z = 2158.31[M]+. N-[(S)-1-Isobutyl-2-<4-[4-{4-[4-Ethynyl-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy)phenylethynyl}-2,5-bis(2-ethylhexyloxy)phenylethynyl>-2,5-bis(2-ethylhexyloxy)phenylethynyl]phenyloxy>-ethyl]-N’-(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (33). To a solution of 32 (76 mg, 0.035 mmol) and tetrabutylammonium fluoride impregnated on silica (0.042 mL of a 1M solution, 0.042 mmol) in 10 mL THF was stirred for 0.5 h under argon. The reaction mixture was filtered through Celite and evaporated to dryness. The product was obtained as a red powder (70 mg, 96%) and was used without further purification. The complete deprotection of the ethynyl bond was confirmed by 1H-NMR spectroscopy as the disappearance of the singlet at 0.2 ppm and the appearance of a singlet for the ethynylinic proton at 3.2 ppm. N-[(S)-1-Isobutyl-2-<4-[4-{4-[4-(4-Trimethylsilylethynyl-phenylethynyl)-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy)phenylethynyl}-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy)phenylethynyl>phenyloxy>-ethyl]-N’-(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (34). A degassed solution of 33 (70 mg, 0.035 mmol), 4-iodotrimethylsylilphenylacetylene (34 mg, 0.123 mmol), Pd(PPh3)2Cl2 (2 mg, 0.003 mmol), CuI ( 0.3 mg, 0.002 mmol) in anhydrous toluene/triethylamine (6 mL, 5:1) was heated at 50 oC for 16 h. The crude reaction mixture was filtered through Celite, evaporated to dryness, and chromatographied (flash silica, CH2Cl2/pentane, 1:1 to 2:1, Rf = 0.1-0.3) to afford a red solid (30 mg, 40 %). 1H NMR (CDCl3, 400 MHz):δ 8.72-8.58 (m, 8H), 7.44 (s, 4H), 7.39 (d, 2H), 6.99 (s, 4H), 6.98 (s, 2H), 6.96 (s, 1H), 6.95 (s, 1H), 6.87 (d, 2H), 5.90-5.70 (m, 1H), 5.11-5.01 (m, 1H), 4.78 (dd, 1H), 4.38 (dd, 1H), 3.96-3.82 (m, 16H), 2.32-2.23 (m, 3H), 1.99-1.92 (m, 2H), 1.86-1.73 (m, 8H), 1.68-1.25 (m, 66H), 1.05-0.84 (m, 60H), 0.26 (s, 9H); 13C NMR (CDCl3, 100 MHz): δ 163.51 (broad signal), 158.66, 153.87, 153.67, 153.60, 134.72, 134.34, 132.88, 132.04, 131.85, 131.24, 129.61, 129.55, 126.43, 126.38, 123.64, 123.12, 122.97, 122.74, 116.63, 116.59, 116.54, 116.51, 116.47, 115.88, 114.78, 114.65, 114.31, 114.26, 114.13, 114.09, 113.86, 113.37, 104.71, 96.22, 94.83, 94.38, 91.74, 91.62, 91.58, 91.52, 91.42, 91.37, 88.16, 84.84, 71.96, 71.88, 68.65, 57.71, 51.45, 39.58, 38.55, 30.65, 30.59, 29.08, 25.57, 24.99, 54.03, 23.97, 23.95, 23.14, 23.06, 22.44, 14.06, 11.34, 11.31, 11.28, 11.23, 11.21, -0.10. MALDI-TOF MS (Mw = 2259.29) m/z = 2258.69[M]+. N-[(S)-1-Isobutyl-2-<4-[4-{4-[4-(4-Ethynyl-phenylethynyl)-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy)phenylethynyl}-2,5-bis(2-ethylhexyloxy)phenylethynyl]-2,5-bis(2-ethylhexyloxy)phenylethynyl]phenyloxy>-ethyl]-N’-(1-ethylpropyl)-3,4,9,10-perylenebis(dicarboximide) (35). To a solution of 34 (28 mg, 0.012 mmol) and tetrabutylammonium fluoride impregnated on silica (0.013 mL of a 1M solution, 0.013 mmol) in 5 mL THF was stirred for 0.5 h under argon. The reaction mixture was filtered through Celite and evaporated to dryness. The product was obtained as a red powder (26 mg, 99%) and was used without further purification. The complete deprotection of the ethynyl bond was confirmed by 1H-NMR spectroscopy as the disappearance of the singlet at 0.26 ppm and the arising of the signal for the ethynylinic proton at 3.2 ppm. Synthesis of the polymers: Polyphenylacetylene with pendant perylenes (11). To a mixture of the ethynyl terminated compound 10 (37 mg, 0.06mmol) and triethylamine (0.8mL) in toluene (20 mL) at room temperature [Rh(nbd)Cl]2 (2.6 mg, 0.006 mmol) was added under an argon atmosphere. The reaction mixture was stirred for 24 h and the polymer was isolated by precipitation into methanol as a red polymer. The polymer was insoluble in any common solvent.
Polyacetylenes with pendant donor-acceptor dyads
147
Polyphenylacetylene with pendant OPVs and perylenes (1). To a mixture of monomers 10 (29 mg, 0.044 mmol) and 12 (30 mg, 0.045 mmol) in triethylamine (0.8 mL) and toluene (20 mL), [Rh(nbd)Cl]2 (2.6 mg, 0.006 mmol) was added under an argon atmosphere. The reaction was stirred for 24 h and subsequently poured into stirring methanol. The polymer was isolated from the cyclic trimers side products using preparative size exclusion chromatography (Biobeads, SXIII, CH2Cl2) and obtained as a red solid (21 mg, 36%). SEC (chloroform/methanol, 99/1): Mw =14.6 kg/mol, PDI = 1.6. Polyphenylacetylene with pendant OPV-PERY dyads (2). To a solution of ethynyl monomer 20 (44 mg, 0.038 mmol) dissolved in toluene (13 mL) and triethylamine (0.6 mL), [Rh(nbd)Cl]2 (2 mg, 0.004 mmol) was added. The reaction was stirred for 16 h and subsequently poured into stirring methanol. The cyclic trimers were removed by precipitation in THF. The polymer was obtained as a red solid (26 mg, 60 %). SEC (chloroform/methanol, 99/1): Mw =45.8 kg/mol, PDI = 1.9. Polyphenylacetylenes with pendant OPE-PERY dyads (3). To a solution of ethynyl monomer 35 (26 mg, 0.012 mmo) dissolved in toluene (4 mL) and triethylamine (0.15 mL), [Rh(nbd)Cl]2 (0.55 mg, 0.001 mmol) was added. The reaction was stirred for 16 h and subsequently poured into stirring methanol. The cyclic trimers were removed by precipitation in THF. The polymer was obtained as a red solid (7 mg, 30 %). SEC (chloroform/methanol, 99/1): Mw = 53.5 kg/mol, PDI = 1.5. Absorption and Photoluminescence. UV/visible/near-IR absorption spectra were recorded on a Perkin Elmer Lambda 900 spectrophotometer. Fluorescence spectra were recorded on a Perkin Elmer LS 50B spectrometer, using a 4 nm bandwidth and optical densities of the solutions of 0.1 at the excitation wavelength. Transient subpicosecond photoinduced absorption. Solutions in the order of 10-5 M were excited at 450 nm, i.e. providing primarily excitation of the donor part within the molecules. 6.8 References and notes 1 (a) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789; (b) Halls, J. J. M.;
Walsh, C. A.; Greenham, N. C.; Marseglia, E. A.; Friend, R. H., Moratti, S. C.; Holmes, A. B. Nature
1995, 376, 498; (c) Shaheen, S. E.; Brabec, C. J.; Padinger, F.; Fromherz, T.; Hummelen, J. C.; Sariciftci,
N. S. Appl. Phys. Lett. 2001, 78, 841; (d) Schmidt-Mende, L.; Fechtenkötter, A.; Müllen, K.; Moons, E.;
Friend, R. H.; MacKenzie, J. D. Science, 2001, 293, 1119.
2 (a) Shinohara, K.; Yasuda, S.; Kato, G.; Fujita, M.; Shigekawa, H. J. Am. Chem. Soc. 2001, 123, 3619.
(b) Yashima, E.; Katsuhiro, M.; Okamoto, Y. Nature 1999, 399, 449.
3 Schenning, A. P. H. J.; Fransen, M.; van Duren, J. K. L.; van Hal, P. A., Janssen, R. A. J.; Meijer, E. W.
Macromol. Rapid Commun. 2002, 23, 271.
4 Vohlidal, J.; Sedlacek, J.; Patev, N.; Lavastre, O.; Dixneuf, P. H.; Cabioch, S.; Balcar, H.; Pfleger, J.;
Blechta, V. Macromolecules 1999, 32, 6439.
5 Nishimura, T.; Takatani, K.; Sakurai, S.; Maeda, K.; Yashima, E. Angew. Chem. Int. Ed. 2002, 41, 3602.
6 Dittmer, J. J.; Marseglia, E. A.; Friend, R. H. Adv. Mater. 2000, 12, 1270 and references therein.
7 (a) Masuda, T.; Higashimura, T. Adv. Polym. Sci. 1987, 81, 121. (b)Yashima, E.; Maeda, Y.;
Matsushima, T.; Okamato, Y. Chirality 1997, 9, 593. (c)Tabata, M.; Sone, T.; Sadahiro, Y. Macromol.
Chem. Phys. 1999, 200, 265.
8 Demmig, S.; Langhals, H. Chem. Ber. 1988, 121, 225.
9 (a) Peeters, E.; van Hal, P. A.; Meskers, S. C. J.; Janssen, R. A. J.; Meijer, E. W. Chem. Eur. J. 2002, 8,
4470. (b) Neuteboom, E. E.; Meskers, S. C. J.; van Hal, P. A.; van Duren, J. K. J.; Meijer, E. W.; Dupin,
Chapter 6
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H.; Pourtois, G.; Cornil, J.; Lazzaroni, R.; Brédas, J.-L.; Beljonne, D.; Janssen, R. A. J. J. Am. Chem. Soc.
2003, 125, 8625.
10 Albert Schenning, unpublished results.
11 Peak broadening has been reported by Okamoto in polyphenylacetylenes which mobility was hampered
by complexation of the pendant carboxylic acid groups with optically active aminoalcohols. Yashima, E.;
Matsushima, T.; Okamoto, Y. J. Am. Chem. Soc. 1997, 119, 6345.
12 Ziener, U.; Godt, A. J. Org. Chem. 1997, 62, 6137.
13 Fujita, Y.; Misumi, Y.; Tabata, M.; Masuda, T. J. Pol. Sci. A: Pol. Chem. 1998, 36, 3157.
14 Salbeck, J. J. Electroanal. Chem. 1992, 340, 169.
15 Polymer PPE was synthesized following a method as reported in Weder, C.; Wrighton, M. S.
Macromolecules 1996, 29, 5157.
16 Peeters, E.; van Hal, P. A.; Knol, J.; Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C.; Janssen, R. A. J. J.
Phys. Chem. B 2000, 104, 10174.
17 Peeters, E. Mesoscopic Order in π-conjugated Materials, Ph.D. Thesis, Eindhoven University of
Technology, 2000, ISBN 90-386-2951-6.
18 Nagao, Y.; Naito, T.; Abe, Y.; Misono, T. Dyes and Pigments 1996, 32, 71.
19 Marcos Ramos, A.; Rispens, M. T.; van Duren, J. K. J.; Hummelen, J. C.; Janssen, R. A. J. J. Am. Chem.
Soc., 2001, 123, 6714-6715.
Summary
Combinations of π-conjugated materials with electron donor and electron acceptor properties
have been used successfully as the active layer in the so-called plastic solar cells. The efficiency of
these devices, however, still needs to be improved and much remains to be learned about the
photophysical processes occurring in the photoactive materials. This multidisciplinary problem can be
tackled from many points of view. Molecular engineering provides a unique means for determining
the role of the individual components in π-conjugated systems consisting of many more components
at the molecular to multi-molecular level. This approach might aid in understanding and optimising
the sequence of events that result in solar energy conversion.
One of the major goals of molecular engineering is to increase the lifetime of the charge-
separated states, as it allows the generated charges to diffuse away from each other, before the non-
desired charge recombination can occur. This can be achieved establishing a redox gradient within a
multichromophoric array, similar as how it occurs in photosynthesis. A number of multichromophoric
arrays with donor(1)-donor(2)-acceptor arrangements were synthesized, studied, and compared to
reference compounds made of the combination of the individual components. The synthesized
systems were the molecular triad oligoaniline-oligo(p-phenylene vinylene)-fullerene (OAn-OPV-C60)
and two symmetrical molecular pentads OAn-OPV-PERY-OPV-OAn (PERY meaning perylene
diimid) with different connection between the OPV and PERY chromophores. The photophysical
studies of these multichromophoric arrays in solution revealed that sequential intramolecular energy
and electron transfer leads to the formation of the donor(1)•+-donor(2)-acceptor•– charge-separated
state. The lifetime of this charged state extended in all cases to the nanosecond time-regime. The
quantum yield for the formation of this state in solution is a result of the interplay between the redox
potential of the different chromophores, nature of the linkage between them and polarity of the
medium. Although, the results could be rationalized in terms of the Marcus theory for charge transfer,
the theory does not suffice to predict or design a system with an efficient sequential electron transfer.
By going to the solid state, intermolecular interactions that might influence the processes occurring at
the molecular level have to be taken into consideration. As observed for the triad Oan-OPV-C60, in the
solid state the formation of donor(1)•+-donor(2)-acceptor•– charge separated state is favored with
respect to the solution, but also an increase of the recombination rate of the charges occurs. In the
bulk, control over these processes is difficult, unless structural tools such as supramolecular chemistry
are used to control the arrangement of the chromophores.
The additional use of supramolecular chemistry in the molecular engineering approach was
achieved with a donor-bridge-acceptor system in which secondary interactions were employed to
control the distance and orientation of the chromophores and thus to govern their photophysical
interaction. Such a system has been created by covalently linking an OPV and a PERY at opposite
ends of a long foldable m-phenylene ethynylene oligomer (FOLD), OPV-FOLD-PERY. The
conformation of the bridge can be varied from a random coil to a folded helical state by decreasing
the polarity of the medium. This conformational change affects the distance between the
chromophores, and thus allows for tuning the system to undergo energy or electron transfer. In very
apolar media, additional intermolecular interactions arise bringing more than one acceptor and donor
in close proximity. This creates a ‘microsolid’ state situation, increasing the overall yield of charge
generation.
Judicious engineering of polymers provides another means of obtaining spatial organization.
The most convenient way to obtain the highest contact between donors and acceptors, as obtained in
solid films of donor-acceptor dyads, but in a less chaotic manner, can be achieved by connecting
molecular dyads with each other into a polymeric chain. Pursuing this idea, a possible design consists
of only linking the donors elements to each other in a linear array. This results in a π-conjugated
polymer with dangling acceptors. Such a polymer has been made with the conjugated backbone being
a hybrid between poly(p-phenylene vinylene) and poly(p-phenylene ethynylene) (PPVE) and
methanofullerenes as the pendant acceptors. The photophysical studies on this donor-acceptor
polymer reveal that photoexcitation of the polymer results in a photoinduced electron transfer reaction
from the conjugated backbone to the pendant C60 moieties. This novel polymer has been applied via
spin coating to form the active layer of the first polymer solar cell based on a covalently linked donor-
acceptor polymer. This first approach has yielded promising photovoltaic properties, with an
exceptional low acceptor loading.
A second type of donor-acceptor polymer was engineered based on OPV and/or OPE donors
and PERY acceptors. A polyacetylene backbone was decorated with dangling donor-acceptor dyads
made up out of these elements. With this last structural design both the covalent and supramolecular
approach have been implemented, pursuing not only intimate contact between donors and acceptors
but also spatial organization. As a direct result of the higher degree of order, in these polymers,
photoexcitation of the donor or acceptor generates a long-lived charge separation in solution as well
as in the solid state. This long-lived charge-separated state makes these polymers ideal candidates for
the active layer in photovoltaic devices.
The systems studied show that molecular engineering is a powerful tool for optimising the
photophysical processes in donor-acceptor systems and for the creation of promising materials for
photovoltaic devices. The ability to control the spatial organization of the donor and acceptor
materials and to direct both the intra- and intermolecular interactions provides a level of control not
achievable by other techniques. The more refined architectures obtained via molecular engineering
not only help in understanding the photophysical processes but are also the key to the photovoltaic
devices of tomorrow.
Resumen
Combinaciones de materiales conjugados con propiedades de dador y aceptor de electrones
han sido utilizadas con éxito en la elaboración de las llamadas ‘células solares de plástico’. No
obstante, la eficacia de dichos dispositivos no es todavía óptima y poco se conoce sobre los procesos
fotofísicos que ocurren en los materials activos que los componen. Éste es un problema
multidisciplinar que se puede abordar desde muchos puntos de vista. La ingeniería molecular
proporciona una herramienta única mediante la cual se puede determinar el papel que juega cada uno
de los componentes activos en sistemas conjugados complejos a nivel molecular o multimolecular.
Esta aproximación al problema puede ayudar a entender y optimizar la secuencia de sucesos que
resulta en la conversión de la energía solar.
Uno de los mayores objetivos de la ingeniería molecular es incrementar la vida media de los
estados de separación de carga para permitir la difusión de las cargas generadas, evitando así la
recombinación no deseada de las mismas. Esto se puede lograr estableciendo un gradiente de
potenciales redox en un sistema consistente en varios cromóforos enlazados, al igual que ocurre en la
fotosíntesis. Diversos sistemas multicromofóricos que se ajustan al patrón dador(1)-dador(2)-aceptor
han sido sintetizados, estudiados y comparados con compuestos de referencia resultantes de la
combinación de los cromóforos individuales. Los sistemas preparados son la tríada molecular
oligoanilina-oligo(p-fenilenvinileno)-fulereno (OAn-OPV- C60) y dos péntadas moleculares simétricas
OAn-OPV-PERY-OPV-OAn (PERY significando perylendiimida), en las cuales únicamente se varió
el enlace entre los cromóforos OPV y PERY. Los estudios fotofísicos de estos sistemas
multicromóforos realizados en disolución revelaron que una transferencia secuencial de energía y
carga conduce a la formación del estado cargado de separación dador(1)•+-dador(2)-aceptor•–. La vida
media de dicho estado es, en todos los casos, superior a 1 nanosegundo debido a la considerable
distancia que separa las cargas. El rendimiento de separación de carga en disolución depende de los
potenciales redox de los diferentes cromóforos, de la naturaleza del enlace que los une y de la
polaridad del medio. En estado sólido se tienen que considerar interacciones intermoleculares que
pueden influir en los procesos que occurren a nivel molecular. Para la triada OAn-OPV-C60 en estado
sólido la generación del estado de separación de carga OAn •+-OPV-C60•– resulta, por una parte, más
favorable que en solución, pero, por otra, su vida media es más corta. En estado sólido el control
sobre todos estos procesos es sumamente complicado. Herramientas estructurales como la química
supramolecular pueden asistir en regular la disposición relativa de los diferentes cromóforos.
El uso de la química supramolecular como elemento en la estrategia de la ingeniería
molecular se consiguió con un sistema dador-puente-aceptor en el que interacciones secundarias
fueron empleadas para controlar la distancia y orientación de los cromóforos, siendo así posible
gobernar su interacción fotofísica. Dicho sistema fue creado mediante la unión de un OPV y un PERY
a ambos extremos de un oligómero m-fenilenetinileno (FOLD) de longitud considerable el cual se
puede plegar sobre si mismo adoptando una conformación helicoidal, OPV-FOLD-PERY. El cambio
conformacional entre ‘ovillo estadístico’ y ‘hélice’ ocurre en el puente FOLD cuando la polaridad del
medio decrece. Esta transformación resulta en un acortamiento de la distancia entre los cromóforos
OPV y PERY proporcionando la manera de modular la actividad fotofísica del sistema. En un medio
extremadamente apolar, interacciones intermoleculares se suman a las intramoleculares, acercando
más de un dador y un aceptor. Se crea así una situación de ‘estado microsólido’ en el que el
rendimiento total de la transferencia de carga se ve incrementado.
Otro medio con el que lograr organización de los cromóforos en el espacio es mediante un
juicioso diseño del polímero. La manera más conveniente de obtener el máximo contacto entre
dadores y aceptores en estado sólido, similar al obtenido con las díadas dador-aceptor, aunque de
manera menos caótica, es mediante la incorporación de dichas díadas en una misma cadena
polimérica. Un diseño fiel a esta idea consiste en unir sólo los elementos dadores en una disposición
lineal, resultando en un polímero conjugado dador con aceptores pendiendo de él. Un sistema así,
consistiendo en un polímero híbrido de poli(p-fenilenvinileno) y poli(p-fenilenetinileno) (PPVE) con
metanofulerenos colgando de la cadena principal, se ha sintetizado. Estudios fotofísicos mostraron
que la fotoexcitación de dicho polímero resulta en una transferencia de electrón fotoinducida de la
cadena principal al los fulerenos laterales. Este polímero ha sido usado como la parte activa de la
primera célula solar basada en polímeros de tipo dador-aceptor. Este primer diseño se caracteriza por
unas propiedades fotovoltaicas prometedoras, si bien con un bajo contenido en material aceptor.
Un segundo tipo de polímero dador-aceptor fue diseñado usando OPV y/o OPE (oligo(p-
fenilenetinileno)) como dadores y PERY como aceptores. Un poliacetileno fue decorado con díadas
dador-aceptor basadas en dichos cromóforos. En este último diseño estructural, interacciones
covalentes y supremoleculares han sido combinadas con el fin de conseguir no sólo un contacto
íntimo entre dadores y aceptores, sino también una mayor organización espacial. Como resultado
directo del orden en estos polímeros la fotoexcitación de cualquiera de los cromóforos genera un
estado de separación de carga de larga vida media, tanto en disolución como en estado sólido. La
longevidad de las cargas en dicho estado cargado hace de estos polímeros candidatos ideales como la
parte activa de células solares.
El trabajo descrito en esta tesis demuestra que la ingeniería molecular es una potente
herramienta con la que se pueden optimizar los procesos fotofísicos en sistemas dador-aceptor y con
la que se puede generar materiales prometedores para la fabricación de células solares. La habilidad
de dominar la organización de los materials dadores y aceptores y de gobernar las interacciones intra-
e intermoleculares proporciona un nivel de control no asequible con ninguna otra técnica. Las
refinadas arquitecturas obtenidas mediante la ingeniería molecular no sólo ayudan a entender los
complejos procesos fotofísicos que puedan ocurrir entre diferentes cromóforos, sino que también son
la clave para las células solares del futuro.
Curriculum Vitae
Alicia Marcos Ramos was born on December 11th 1973 in Reus (Spain). Her secondary
school studies took place at the I. B. “Gaudí” in Reus. She studied Chemistry at the ‘Rovira i Virgili”
University in Tarragona, Spain. During her studies she visited the laboratory of Marcromolecular and
Organic Chemistry at the Eindhoven University of Technology, the Netherlands (Prof. Dr. E.W.
Meijer) with an Erasmus grant. After working as a research assistant in the same group she started as
a Ph.D. student in the same laboratories under the supervision of Professor Dr. R.A.J. Janssen. The
most important results of the investigations are described in this thesis. As of September 2003 she will
be working as a post-doctoral research fellow at TNO-Industrial Technology in Eindhoven, the
Netherlands.
Acknowledgments
The Spanish popular saying ‘nunca te acostarás sin saber una cosa más’ has been more than
ever before true during my Ph.D. period. Four years as a Ph.D. student in Eindhoven have enriched
me not only at the scientific and but also at the personal level, sometimes step by step, often with
gigantic leaps. My promotor Professor René Janssen has been responsible for initiating most of these
steps. He has provided me with freedom to embark on my scientific journey and supported this with
stimulating ideas and criticism. For this ideal working atmosphere and his contagious optimism, I
would like to thank him. Working from day to day in a stimulating environment has truly been an
‘onbetaalbare’ experience. Concerning this I also would like to thank Professor Bert Meijer. Dr.
Stefan Meskers has been a beacon in the dark on many problems of physical organic origin and
furthermore it has always been a pleasure to listen to what he had to say in matters of very different
nature.
I would like to thank Paul van Hal, Edwin Beckers, and Jeroen van Duren for their important
contribution to this work. Working together with them not only gave nice results but also taught me a
lot about the disciplines of photophysics and photovoltaics. Dr. Jef Vekemans and dr. Albert
Schenning were always enthusiastic in discussing synthetic and conceptual issues, my sincere
appreciation for that. For the fullerene chemistry presented in chapters 2 and 5, I would like to thank
Professor Kees Hummelen, dr. Minze Rispens, and dr. Joop Knol. They have also been magnificent
hosts during the week I spent in the ‘extremely cold’ Groningen. I would like to thank Professor
Jeffrey Moore, dr. David Hill and dr. Ryan Prince for our cooperation on chapter 4. I want to thank
Paul Hamelinck for his hard work and enthusiasm in the difficult subject described in chapter 5.
Quisiera agradecer al Profesor Nazario Martín por corregir esta memoria y por formar parte del
tribunal de mi defensa. También quisiera agradecerle su grata hospitalidad durante el encuentro que
tuvo lugar en Madrid
During my stay in the SMO group I have had the pleasure to discuss with many people about
science and about daily things. I would like to thank everybody who has contributed in such a way to
this thesis and to make of it an enjoyable four years. I would like to thank the members of the
‘Janssen-group’ for their many suggestions and critics during the lunch-meetings. For help and advice
with characterization I would like to thank Joost van Dongen, Xianwen Lou, Ralf Bovee, and Henk
Eding. For facilitating matters outside chemistry I would like to thank Hans Damen, Hanneke
Veldhoen, Ingrid Dirkx, Emma Eltink, Carine van der Vaart, Joke Rediker, and Hanny van der Lee. I
want to thank Koen Pieterse for his help with computers and for being there in desperate situations.
My former roommates Jan-Willem, Jack, Fiorella, and Hinke, I would like to thank them for
the entertaining conversations, resulting in laughter that sometimes even managed to shield us from
the music coming out of lab 3. Fiorella, voglio ringraziarti anche per avermi insignato l’arte delle
buone maniere e la gentilezza. Agradezco a Cristina, Elena, Marcela, Joaquín, Laia, Jorge y Marga,
los españoles a los que he tenido el placer de conocer aquí, porque reirse en castellano no es lo mismo
que reirse en holandés o inglés. For the good moments I also want to thank Ky, Francesca, Michel,
Edda, Mitsutoshi, Daniela, Dodo, Corinne…
Quisiera agradecer a mis padres y hermanos por su cariño y apoyo todo este tiempo desde la
distancia. Y a Luc, por hacer de la realidad un sueño maravilloso.
Alicia.
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