Thioethyl Porphyrazines: Attractive Chromophores for

Second Order Nonlinear Optics and DSSCs

Sandra Belviso,*,a Ernesto Santoro,a Marta Penconi,b Stefania Righetto,c Francesca Tessore*,c

aUniversità della Basilicata, Dipartimento di Scienze, via dell’Ateneo Lucano, 10; 85100 Potenza,

ITALY.bIstituto di Scienze e Tecnologie Molecolari - CNR, via C. Golgi 19, 20133 Milano, Italy and

SmartMatLab Center, Via C. Golgi 19, 20133 Milano, ITALY.cUniversità di Milano, Dipartimento di Chimica, Unità di Ricerca dell’INSTM, via C. Golgi 19;

20133 Milano, ITALY.

Running title: Nonlinear optical properties of thioalkyl porphyrazines

Keywords: thioalkyl porphyrazines, nonlinear optics, unconventional push-pull systems, organic

photovoltaics, dye-synthesized solar cells, pyrene porphyrazines.

Authors to whom correspondence should be addressed:

Dr. Sandra BelvisoDipartimento di ScienzeUniversità della BasilicataVia dell’Ateneo Lucano, 10I-85100 Potenza, ITALYtel: +39-0971-205937e-mail: sandra.belviso@unibas.it

Dr. Francesca TessoreDipartimento di ChimicaUniversità di Milano Via C. Golgi, 19 I-20133 Milano, ITALYtel: +39-02-50314397e-mail: francesca.tessore@unimi.it

1

ABSTRACT

The first study on the second order NLO properties of unconventional push-pull systems constituted

by asymmetrically mono-arylsubstituted thioalkyl porphyrazines, carried out by EFISH

measurements and DFT calculations, is here reported. The results obtained show that the

porphyrazine macrocycle behaves as both a better electron donor and acceptor in respect to

structurally similar porphyrin and phthalocyanine tetrapyrroles. Noteworthy, the highest quadratic

hyperpolarizability (1907) values are displayed by the pyrene substituted thioalkyl porphyrazines,

both as ‘free base’ and as PdII complex, without the presence of either electron withdrawing of

electron acceptor groups. In these cases, the pyrene moiety behaves as an electron donating group,

giving rise to charge transfer HOMO-LUMO electronic transitions. These outcomes show, for the

first time, the great potential of the thioalkyl porphyrazine macrocycles for second order NLO

applications. Moreover, the hydroxyphenyl substituted porphyrazine 2 dye has been also tested as a

sensitizer in a TiO2-DSSC, representing the second example reported so far of a DSSC with

alkylthio porphyrazines.

2

1. INTRODUCTION

Organic push-pull -conjugated chromophores have been widely investigated for their nonlinear

optical (NLO) properties.1-3 Among them, a prominent role is played by tetrapyrrole macrocycles

which, displaying highly delocalized -electron systems, may provide an ideal structural framework

from which to elaborate molecules endowed with NLO features.4-6 In particular, push-pull systems

constituted by porphyrins7-13 and phthalocyanines14-17non-symmetrically substituted at the periphery

with suitable donor (D) and acceptor (A) groups exhibit second order nonlinear optical responses.

Moreover, porphyrins and phthalocyanines with similar donor-bridge-acceptor structure constitute

some of the most successful dyes employed in organic photovoltaic (OPV) devices.18 In these

systems the HOMO and the LUMO should reside on the donor and the acceptor portion,

respectively, and push-pull excitation creates an electron-hole pair at opposite sides of the molecule.

The porphyrazine macrocycle is structurally related to porphyrins and phthalocyanines.19,20 Indeed,

it can be considered as a porphyrin in which the four meso carbons have been replaced by aza

linkages or as a phthalocyanine lacking its fused benzene units. While the porphyrin core can be

functionalized in both electron-deficient meso and electron-rich -pyrrolic position and the

phthalocyanine core in the benzo-ring of the isoindole unit, in porphyrazines only the -positions of

the pyrroles are available for chemical modification, and the peripheral substituents can then

strongly couple to the macrocycle. However, surprisingly, porphyrazines have been much less

studied for optoelectronic applications and only a few of these systems have been investigated for

their NLO properties.21-26 In particular, to the best of our knowledge, up to now porphyrazines and

especially non-symmetrically substituted thioalkyl-porphyrazines,27-30 have never been investigated

as chromophores for second order NLO by means of the Electric Field Induced Second Harmonic

Generation (EFISH) technique.31-33

Recently, Belviso et al. have shown that in non-symmetrically substituted34,35mono -aryl or -

arylethynyl (alkylsulfanyl)porphyrazines and in the corresponding NiII complexes an efficient

electron transfer between the aryl and macrocycle moieties occurs.36 From DFT calculations, the

highest perturbation of the porphyrazine -electron core is provided by strong electron-donating

(NMe2) and electron withdrawing (NO2) aryl substituents, which increase and decrease the

macrocycle electron density, respectively. Whereas in many porphyrazine compounds the HOMOs

and LUMOs orbitals are centred on the core, in these -NMe2 substituted porphyrazines the HOMO

is on the peripheral aryl moiety, and the LUMO on the macrocycle, giving rise to a HOMO-LUMO

charge transfer (CT) transition upon excitation. Conversely, in the -NO2 substituted porphyrazines a

3

CT transition occurs from the HOMO orbital centred on the macrocycle to the LUMO+2 orbital

localized on the nitroaryl unit.36 Therefore, the porphyrazine ring displays an ambivalent behavior,

acting as an electron acceptor in the case of the NMe2-substituted compounds and as an electron

donor in the NO2-substituted derivatives, providing an “unconventional” push-pull systems suitable

for second order NLO, without a classic electron-withdrawing and electron-donating group couple.

A similar behavior was reported some years ago by some of us for tetraphenylporphyrins (both as a

free-base and as a ZnII complex) carrying a push or a pull group in the -pyrrolic position.11 The

concept of “push-no-pull” and “pull-no-push” was recently invoked also to explain the large

hyperpolarizabilities (measured by Hyper-Rayleigh Scattering) of meso-dialkynyl porphyrins,37 and

is supported from a theoretical point of view by a pioneering investigation on the effect of single

donors and single acceptors appended to heterocyclic rings.38,39

Hereafter we describe an experimental investigation of the second order NLO response of some

unconventional push-pull porphyrazinic chromophores by the EFISH technique. The study has been

carried out on several classes of thioethyl-porphyrazines non-symmetrically -substituted with

different systems extending macrocycle conjugation (Figure 1).

First, we studied the two arylethynyl NiII-porphyrazines 1a,b, para-phenyl substituted with either an

electron-donating NMe2 or an electron withdrawing NO2 group, respectively, following the

outcomes of our previous paper36 in which the spectroscopic properties of this class of molecules

have been investigated and their NLO properties have been envisaged. Second, the 4-hydroxyphenyl

substituted NiII complex 2 has been prepared and analyzed as a representative example of simpler

aryl substitution. In this case the para-hydroxy substitution makes this compound also a suitable

dye for application in Dye-Sensitized Solar Cells (DSSCs), allowing to widen the exploration of its

optoelectronic applications. Finally, the pyrene-substituted ‘free-base’ porphyrazine 3a,40 recently

described by some of us as a promising dye for organic photovoltaics, and its PdII complex 3b41

have been investigated. The choice of these compounds has been driven by the fact that pyrene

derivatives have demonstrated to exhibit excellent chromophore features, thanks to their extended

-electron delocalized systems.42 Moreover, the PdII complex 3b was chosen taking into account that

porphyrins and phthalocyanines palladium complexes display fluorescence and phosphorescence

and these properties can have a role on optoelectronic applications, influencing the excited states

and the charge transfer processes.43,44

4

Figure 1. Structure of studied aryl-substituted thioalkyl porphyrazines.

2. RESULTS AND DISCUSSION

Second order Non Linear Optical properties.

Compounds 1a,b36 and 3a,b40,41 have been prepared, as already described, from the corresponding

mono-brominated ’free-base’ or NiII and PdII thioethyl porphyrazine by either Sonogashira45 or

Suzuki-Miyaura46 type cross-coupling reactions, respectively. The brominated porphyrazines have

been prepared, in turn, by a unique a posteriori asymmetrization process34,35 of the symmetric parent

macrocycle,47,48 which allows to obtain non symmetrical porphyrazines avoiding the commonest

low yielding statistical macrocyclization. The new compound 2 has been also synthesized by

Suzuki-Miyaura cross-coupling49 following the procedure described for its thiooctyl analogue.36

The molecular second order response figure of merit (or quadratic hyperpolarizability) of 1-3

have been measured by the EFISH technique, which gives the scalar product of the dipole moment

() and the projection along the dipole moment direction of the vectorial component of the third

rank tensor working with an incident wavelength λ (see Experimental for the details). The 1907

EFISH values of 1-3 have been recorded at 5×10-4 M concentration in CH2Cl2 and the ground state

molecular dipole moments () (Figure S18-S20 in SI) have been obtained by DFT quantum

mechanical calculations both in gas phase and in CH2Cl2 as solvent. These data and the electronic

absorption maxima in CH2Cl2 are reported in Table 1 together with the HOMO-LUMO transitions

oscillator strength.

5

Table 1. Electronic absorption maxima, computed dipole moments, HOMO-LUMO transitions oscillator strength and EFISH data for 1-3.

Compd MAXa (nm) b (D) 1907

a,c

(×10-48 esu)1907

d

(×10-30 esu) f

1ae

329 (Soret)511(Q band)621(shoulder)665(Q band)

5.6830f

7.9165g 720h 91h 0.5429i

1be

344 (Soret)381(shoulder)486 (Q band)668 (Q band)

11.2850f

14.9817g 842j 56j 0.4412i

2328 (Soret)478 (Q band)656 (Q band)

4.1751f

5.4379g 507 93 0.3780i

3ak

277350 (Soret)513 (Q band)640 (Q band)709 (Q band)

1.8470f

2.9930g 780 261 0.2024l

3bm

273346 (Soret)493 (Q band)653 (Q band)

2.1119f,n

2.8309g,n 960 339 0.0761o

aMeasured in CH2Cl2. bComputed at DFT/BP86/TZVP level of theory. cConcentration c = 5×10-4 M in CH2Cl2. dObtained using the values calculated in CH2Cl2. eAbsorption data from ref. 36. fComputed in gas phase. gComputed in CH2Cl2.

hAt c = 10-3 M 1907 = 525×10-48 esu (1907 = 66×10-30 esu) and at c = 10-4 M1907 = 1370×10-48 esu (1907 = 173×10-30 esu). iComputed at TDDFT/M06/6-31G(d) level of theory in CH2Cl2. jAt c = 10-4 M1907 = 1770×10-48 esu (1907 = 118×10-30 esu). kAbsorption data from ref. 40.. lComputed at TDDFT/M06/6-311G(d,p) level of theory in CH2Cl2. mAbsorption data from ref. 41. nComputed at DFT/BP86/Lanl2dz level of theory in CH2Cl2. oComputed at TDDFT/M06/Lanl2dz level of theory in CH2Cl2.

According to the phenomenological “two-state” model developed by Oudar,50,51 the quadratic

hyperpolarizability of a molecule is related to the mobility of polarizable electrons under a strong

electric field (for example the one provided by a laser source) and depends on CT transitions. Under

the assumption that the second order NLO response is mainly related to only one CT transition

involving the ground state and the first excited state, the component of the tensor along the CT

direction (CT) is given by eq. 1:

βCT=3e2ℏ2

2mW

[W 2−(2 ωℏ )2 ] [W 2−( ωℏ )2 ]f Δ μeg (eq. 1)

6

In this expression, W and f are the energy and the oscillator strength of the first electronic

absorption band, ħ is the energy of the incident photon and eg is the difference between excited

and ground state dipole moments. Therefore, an effective way to enhance the second order response

is to have low lying transitions with high intensity and high difference between the excited and the

ground state dipole moments, as it occurs for push-pull D--A dipolar molecules, in which

increasing the strength of the donor and of the acceptor group and/or the length of the -delocalized

bridge between them a high CT value can be obtained.52 Even if the two-level model was proved to

be inherently unable to account for the NLO response of 2D structures, due to the presence of a

possibly complex pattern of CT directions,53 it can be safely used for our discussion, since in 1-3 the

presence of a substituent in -pyrrolic position produces a distortion of the electronic density of the

porphyrazine ring along one main direction (associated to the HOMO-LUMO transition), as it

occurs in linear push-pull structures (Figure 2 and Figures S20-S23 in SI).

Figure 2. Frontier MO calculated at M06/6-31G(d)/IEFPCM(CH2Cl2) level of theory for the major conformer of 2.

The results of our experimental EFISH investigation on 1-3 (Table 1) are in agreement with the

theoretical suggestion of an ambivalent (acceptor/donor) role of the porphyrazine macrocycle. In

fact, the 1907 values of 1-3 are all positive and quite high. According to eq. 1, the sign of CT

depends only on eg, and it is positive, as in 1-3, when the molecular ground state is less polar than

the excited one. Considering the series of NiII complexes 1 in Figure 1, the electron density of the

porphyrazine core appears to be more affected by the presence of a -NO2 acceptor group than by a -

NMe2 donor group, in fact, the dipole moment value of 1b is twice that of 1a (Figure S24 in SI).

Some interesting observations can be inferred from the 1907 value comparison of 1a (1907 = 9×10-30

esu) and 1b (1907 = 56×10-30 esu) with the structurally related porphyrins 4 and 5 and

phthalocyanines 6 and 7 (Figure 3). As far as the substituent effect is concerned, NR2 substituted

derivatives show a stronger second order NLO response than the NO2 ones in both porphyrazine

7

and porphyrin macrocycles (i.e. the 1907 value trend is 1a>1b and 4>>5),11 while an opposite trend

is observed in phthalocyanines (6<7). Therefore, as it happens for porphyrins, also in thioethyl

porphyrazines the linking of an electron donor substituent to the electron-excessive -pyrrolic

position of the macrocycle through an ethynylphenyl moiety is an effective way to enhance the

second order NLO response, due to the increased donor properties of the substituent induced by

conjugation with the extended -system of the macrocycle.11

Taking into account the macrocycle nature it can be noticed that the porphyrazine and porphyrin

macrocycles give rise to a stronger NLO response than the phthalocyanines with both electron

donating and electron withdrawing substituents. In fact, the trends 4>1a>6 and 1b>7>5 are

observed (Figure 3). In particular, complexes 1a and 4, in which the tetrapyrrolic macrocycle acts as

the electron acceptor part of the unconventional push-pull system, show comparable 1907 values

within the experimental error associated to the EFISH measurement (±15%) and taking into account

the different metal (NiII for 1a and ZnII for 4), the different solvent used in the measure (CH2Cl2 for

1a and CHCl3 for 4) and the different spacer connecting the donor dialkylamino moiety to the

macrocycle (ethynylphenyl for 1a and ethenylphenyl for 4). In both cases 1907 is much higher than

in the phthalocyanine 6. Conversely, for complexes where the macrocycle plays the role of the

donor part of the push-pull system, the 1907 value of the thioethyl porphyrazine 1b is 2.7 times

higher than that of porphyrin 5 and slightly higher than that of phthalocyanine 7.16

Figure 3. Structure of literature aryl-substituted porphyrins 4 (1907 = 127.5×10-30 esu)11 and 5 (1907

= 20.4×10-30 esu)11 and phthalocyanines 6 (1907 = -25.6×10-30 esu)16 and 7 (1907 = 41.1×10-30 esu).16

Neglecting the electronic effect that the different metal (NiII for 1b and ZnII for 5 and 7) can exert

on tetrapyrrolic macrocycles and that was investigated by a theoretical study some years ago,54 our

8

EFISH data suggest that in these non-classical push-pull systems the thioethyl porphyrazine ring

behaves as a better electron donor than the phthalocyanine and even more of the porphyrin. Indeed,

the porphyrazine is isoelectronic with the porphyrin, but the replacement of four methine bridge

carbon atoms by nitrogen atoms leads to a higher electronegativity and to a more rigid skeleton for

the former macrocycle, favoring the extension of the -conjugation with the 4-nitrophenylethynyl

fragment. The meso-tetraaza substitution significantly reduces the metal-nitrogen distance,

enhancing the bond energy between the metal and the core and thus lowering the size of the

macrocycle,54 so that the porphyrazine ring is a stronger donor than the porphyrin, as confirmed

by electrochemical measurements.55 Recent studies have also shown that the porphyrazine

macrocycle is both a better -donor and a better -acceptor than the porphyrin macrocycle.56

Condensation of benzene rings to the pyrroles in phthalocyanines enhance the structure flatness, but

produces a strong destabilizing effect on the metal-macrocycle bonding.54 Therefore, the trend of the

1907 of 1b, 7 and 5, i.e. metal-porphyrazine > metal-phthalocyanine > metal-porphyrin, is nicely

opposite to that of the metal-nitrogen bond length of the macrocycle.

The linking on the -pyrrolic position of the thioethyl porphyrazine 2 of a 4-hydroxyphenyl moiety

by a C-C single bond induces a lower asymmetry in the rings -conjugation than the linking of an

ethynylaryl fragment. Indeed the HOMO and LUMO orbitals have a * character and are both

located on the planar part of the molecule (Figure 2), while the aryl ring directly attached to the -

position is twisted with respect to the porphyrazine macrocycle of about 36.5° (Figure 2).36

However, such torsional distortion and the lack of the acetylenic spacer do not impair the electronic

delocalization between the donor-substituted aryl ring and the core, as confirmed by the fair dipole

moment and 1907 values of 2 which are comparable to those of 1a, notwithstanding the –OH

substituent is a less efficient electron donor than –NMe2 (Hammett p = -0.37 and -0.83,

respectively). The porphyrazine ring of 2 then behaves as the electron acceptor part of the

unconventional push-pull system as in 1a.

The most unexpected results of our investigation are the high 1907 values of thioethyl porphyrazines

3, with a 1.3-fold increase of 1907 by PdII complexation going from 3a to 3b, an effect also observed

in the comparison of some free-base porphyrins with their ZnII complexes.11,57 Compounds 3 are

different from 1-2, since they are a porphyrazine-pyrene dyad in which the two extended -

conjugate systems are connected through a C-C single bond, with no other attached electron donor

or electron acceptor substituent. Therefore, we can considered them as a “super” unconventional

“no push-no pull” system. DFT calculations show for 3a a twist of the pyrene moiety from the plane

of the macrocycle (dihedral angles of 57° and 65° for the two most stable conformers),40 not

9

affected by metal complexation with PdII.41 Moreover, in both 3a and 3b the HOMO is on the

pyrene moiety and the LUMO on the porphyrazine core (Figures S22 and S23 in SI). Therefore, as

evidenced by UV-vis spectroscopy,40 a well-defined HOMO-LUMO charge transfer from the pyrene

moiety to the porphyrazine occurs and the macrocycle plays as the acceptor part of the push-pull

system. The dipole moments of 3a and 3b (Table 1 and Figure S26 in SI) are significantly lower

than those of 1a and 1b, supporting a lower electronic asymmetry for the former compounds, as

expected from the lack of electron-donor or electron-acceptor groups attached on the aromatic

moiety. Computations in dichloromethane (Table 1) show a slight decrease of the computed dipole

momentwhen the free-base 3a is coordinated to PdII, in agreement with an increased electron

density of the core induced by the presence of the metal and, therefore, with a decrease of the

acceptor properties of the macrocycle.58 Pyrene has been exploited in second order NLO through

C=C double bond connection to the strong electron-acceptor 2-dicyanomethylidene-3-cyano-4,5,5-

trimethyl-2,5-dihydrofuran (TCF)59 or to position 4 of a pyridine ring.60 In both these literature

examples DFT and TDDFT calculations evidenced a HOMO-LUMO CT transition emanating from

pyrene to the TCF59 or the pyridine moiety,60 respectively, and remarkable 1907 values were

reported (for pyrene-TCF59 -600×10-48esu and -1700×10-48esu at 10-3M concentration in CHCl3 and

CHCl3+py, respectively; for pyrene-pyridine60 1500×10-48 esu at 10-4M concentration in CHCl3). In

particular, for the pyrene-pyridine compound a dipole moment value of 4.4 D was computed,60

giving rise to a 1907 value of 341×10-30 esu, comparable to that of 3b.

As inferred from eq. 1, the quadratic hyperpolarizability along the CT direction is CT ∝

(feg/Eeg2), where f and eg are defined above and Eeg is the excitation energy of the HOMO-

LUMO transition. Notably, in the 1-3 series, where the lowest energy transitions have

approximately the same energy (see Table 1), the CT values appears uncorrelated to the oscillator

strengths, being the lowest f associated to 3a,b having the highest 1907. Therefore, in these

compounds the high 1907 shall be ascribed to a much higher eg value. This is consistent with the

small ground state dipole moments calculated for 3a,b (see Table 1 and Figures S24-S26 in SI) and,

conversely, with the large excited state dipole moments expected as a consequence of the large

charge separation observed in the LUMOs (Figures S22 and S23 in SI).

Application in Dye-Sensitized Solar cells.

10

As reported in the introduction, the para-hydroxy substitution makes porphyrazine 2 a suitable dye

for application in DSSCs. In fact, the acidic phenolic group can allow anchoring of the dye on the

TiO2 electrode surface.61 First of all, the electronic properties of 2 were investigated by experimental

analysis of its UV-vis spectrum (Figure 4). The UV-vis spectrum of 2 shows the typical features of

non-aggregated metal complex thioalkyl-porphyrazines, displaying a broad Q band at 656 nm, a

Soret band near 328 nm and in between a band at 478 nm associated to nsulfur→* transitions.36,49,62,63

Figure 4. UV-vis spectrum of 2 in CH2Cl2.

Because of the importance of the electrochemical properties for photoactive compounds employed

in OPV, an electrochemical analysis of 2 was also carried out. The cyclic voltammetry (CV) and

differential pulse voltammetry (DPV) measurements on 2 in CH2Cl2 are reported in Figure 5 and the

electrochemical data are collected in Table 2. The redox behavior of this compound is typical of

thioalkyl-porphyrazines. The cathodic curve is characterized by two sequential one electron

reduction processes and their anodic counterparts displaying half-wave potentials E1/2(ΔEp) =

−1.010 V (0.117) and E1/2(ΔEp) = −1.394 (0.105) (vs Fc/Fc+, used as internal standard), respectively.

Both such processes can be considered as quasi-reversible because the conditions for the

reversibility were not rigorously fulfilled (as inferred from ΔEp = |Ea − Ec| values). These two

reduction processes can be assigned to the formation of a porphyrazine -anion radical and a porphyrazine dianion, respectively.36,64 The oxidation process has been clearly

detected only by the DPV experiments at ca. 0.74V.

11

Figure 5. Cyclic voltammogramm and differential pulse voltammogramms of 2 in CH2Cl2.

Table 2. Summary of the peak potentials of 2 E1/2 (Ep = |Ea − Ec|) (Volts vs Fc/Fc+).

Techniquea Oxidation Reduction EHOMO ELUMO

I II Exptlb Comptc Exptlb Comptc

(CV) -1.010 (0.117)

-1.394(0.105) -3.790

(DPV)red 0.736 -1.024 -1408 -5.536 -5.708 -3.776 -3.229

(DPV)ox 0.740 -1.026 -1.408 -5.540 -3.774

a Measured in CH2Cl2 10-3 M solution at a glassy carbon working electrode. bValues (eV) referred to first oxidation and first reduction, and calculated assuming the energy level for the Ferrocene at − 4.8 eV [see ref.65]. cIn CH2Cl2 IEFPCM-based computations as described in Experimental section in SI .

The energies of the HOMO and LUMO orbitals were then achieved experimentally, from

electrochemical data and, computationally, by TDDFT calculations. The energy of the HOMO and

the LUMO measured for compound 2 nicely match the required values for the fabrication of a

working DSSC cell. In fact, the ELUMO value of -3.776 eV (-3.229 eV by computations) indicates

12

that efficient electron injection into the TiO2 conduction band (CB) (ETiO2 = -4.00 eV) is

energetically possible, while the EHOMO value of -5.536 eV (-5.708 eV by computations), makes the

regeneration of the dye energetically feasible by the I-/I3- (Eredox = -4.75 eV) red/ox couple.66

Moreover, the 1.76 eV bandgap found is fully compatible with an efficient sunlight harvesting.

Therefore complex 2 has been tested in a TiO2-DSSC, under full sun illumination (AM1.5G), using

I−¿/ I3−¿ ¿¿ as the redox mediator in the electrolyte. The uptake of the dye on TiO2 was made by a 0.2

mM solution in EtOH/toluene = 1/1 after 24 hours of dipping. Although a modest power conversion

efficiency (PCE, see eq. 3 in the Experimental section) was measured for our DSSC (Figure S11 in

ESI), having a Jsc = 0.18 mA/cm2, a Voc = 486 mV and a FF = 0.604 (PCE = 0.052%), this PCE

value is 2.6 times higher than that recently reported for a similar benzofused thioethyl porphyrazine

with a –COOH anchoring group,67 and also higher than the PCEs of other alkyl, amine and alkoxy

substituted porphyrazines investigated in the same work.67 Despite the presence of a less efficient –

OH anchoring group, our chromophore produces higher Jsc and Voc values than its counterpart, with

comparable FF (Table S1 and Figure S27 in SI). We can tentatively explain the better performances

of our dye by its structural flexibility, due to the C-C single bond connecting the 4-hydroxyphenyl

moiety to the porphyrazine core. This feature could allow our molecule to approach the surface of

TiO2 without the structural constraints of the isoindole unit of its counterpart,67 thus allowing a

higher electron injection in the conduction band of the semiconductor, as reflected by the higher Jsc

value. In support of this hypothesis, in our DSSC no addition of chenodeoxycholic acid as a

disaggregating agent68,69 was required, which on the other hand was present in 9 mM concentration

in its counterpart.67

The PCEs reported so far for TiO2-DSSCs with porphyrazine-based dyes67 are very low in

comparison to that of benchmark RuII complexes (11%)70 and also of porphyrin-based

chromophores (13%).71,72 However, this merely reflects that porphyrazines have been in general

much less studied than other tetrapyrrolic macrocycles, mainly due to synthetic issues. Therefore

plenty of room for the improvement of the photovoltaic performances of TiO2-DSSCs based on

these dyes is left.

Therefore, further experiments are currently underway to study in detail the photochemical features

of our DSSC, hand in hand with the synthesis of other thioethyl porphyrazines carrying more

suitable anchoring groups to improve the electron injection, with the goal to reach at least the value

reported for the benchmark compound TT112.73

3. CONCLUSIONS

13

In conclusion, we report here the first study on the second order NLO properties of asymmetrically

mono-arylsubstituted thioalkyl porphyrazines, carried out by EFISH measurements. Notably,

arylethynyl porphyrazines 1a,b display higher 1907 values than structurally similar phthalocyanines

in the presence of both electron donating and electron withdrawing substituents. Comparing the

porphyrazines with porphyrins, comparable results were observed with NR2 substitution, while

higher 1907 was measured with electron withdrawing NO2 groups. These results show that the

porphyrazine macrocycle behaves as both a better electron donor and acceptor in respect to the

other two tetrapyrroles classes in mono-substituted unconventional push-pull systems. The quite

high 1907 values observed in pyrene substituted thioalkyl porphyrazines 3a,b both as ‘free base’ and

as PdII complex, are very remarkable. In these cases, the pyrene moiety, although cannot be

considered as a “classical” electron donating group, gives rise to charge transfer HOMO-LUMO

electronic transitions which provide a strong dipole moment change between the ground and excited

state. These outcomes show, for the first time, the great potential of the thioalkyl porphyrazine

macrocycles for second order NLO applications.

Moreover, the photovoltaic performance of the porphyrazine 2 dye was tested in TiO2-DSSC,

representing the second example reported so far of DSSC with alkylthio porphyrazines. However,

our chromophore produces a PCE value 2.6 times higher than that recently reported for a similar

benzofused thioethyl porphyrazine.67 Further experiments to study the photochemical features of

DSSC and particularly to improve the electron injection by using thioethyl porphyrazines carrying

other suitable anchoring groups are in progress.

ASSOCIATED CONTENT

Supporting Information

Experimental procedures and spectral characterization data, computational procedures, EFISH

measurements and DSSC preparation. The Supporting Information is available….

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