infrared spectra of two sexithiophenes in neutral and doped...

Infrared spectra of two sexithiophenes in neutral and doped states:a theoretical and spectroscopic study

J. Casadoa,c, H.E. Katzb, V. Hernandeza, J.T. Lopez Navarretea,*

aDepartamento de Quımica Fısica, Facultad de Ciencias, Universidad de Malaga, 29071 Malaga, SpainbAT&T Bell Laboratories, 600 Mountain Avenue, Murray Hill, NJ 07974, USA

cDepartment of Chemistry, University of Minnesota, Minneapolis, MN 55455, USA

Received 19 October 2001; received in revised form 21 February 2002; accepted 4 March 2002

Abstract

The FT-infrared spectra of two sexithiophenes having their end a,a0-positions substituted by n-hexyl or -thiohexyl groups, in

neutral and doped states, are studied with the main aim of deriving information about the p-electrons delocalization and about

the electronic structure of the charged defects created upon doping with iodine. The analysis of the experimental data is aided by

Density Functional Theory calculations. The modifications in the electronic structure of the sexithiophene backbone induced

by the n-thiohexyl encapsulation are discussed from the point of view of single molecule interactions in thiol-terminated

p-conjugated oligomers bound to metallic or cluster electrodes.

# 2002 Elsevier Science B.V. All rights reserved.

Keywords: Oligothiophenes; Infrared spectroscopy; p-Electron interactions; Chemical doping; Radical cation; Theoretical calculations

1. Introduction

Vibrational spectroscopy is among the most impor-

tant and promising techniques for the characterization

of organic polyconjugated polymers and oligomers,

both in the undoped and doped states. Vibrational

spectra of p-conjugated materials constitute a very

rich source of information about their molecular

structure, charge distribution and conjugational prop-

erties [1,2]. In particular, infrared and Raman spectra

of polyconjugated chain compounds show peculiar

and characteristic features directly related to the effi-

ciency of the p-electrons delocalization along the

quasi one-dimensional path of alternating C=C/C–C

bonds and also with the different types of charged

defects created upon chemical doping or photoexcita-

tion [3–5]. In this regard, we must stress that the

attainment of detailed information on the microstruc-

ture of the doped materials in terms of bond lengths

and bond angles is hardly accesible by means of other

experimental techniques. An alternative way to obtain

this type of structural information is to combine

vibrational spectroscopies with theoretical calcula-

tions [6–10].

Polythiophene is among the most thoroughly inves-

tigated polyconjugated polymers [11,12]. However,

polythiophene samples synthesized so far have the

traditional complexity of ‘‘real’’ polymers such as

their low solubility, high contents of structural defects,

broad distribution of molecular weights, etc. The

difficulties inherent to the synthesis of any structurally

well-defined p-conjugated polymer led to numerous

Vibrational Spectroscopy 30 (2002) 175–189

* Corresponding author. Tel.: þ34-952-132-081;

fax: þ34-952-132-000.

E-mail address: [email protected] (J.T. Lopez Navarrete).

0924-2031/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 4 - 2 0 3 1 ( 0 2 ) 0 0 0 2 1 - 8

attempts of obtaining their low molecular weight

counterparts [13–16]. Over the last decade, it has been

possible to assess precise relationships between the

physico-chemical properties of these p-conjugated

materials and their chemical architectures, starting

from the systematic study of different series of oligo-

mers with variable chain lengths. This strategy is

commonly known as the ‘‘oligomeric approach’’

[17]. On the other hand, oligothiophenes have been

already used as active components in electronic

devices such as field effect transistors (FETs) and

light emitting diodes (LEDs) [18,19].

Likely, future computers will consist of logic

devices that are ultradense, ultrafast and molecular-

sized [20,21]. The transmision times could be

minimized by using molecular scale electronic inter-

connects, thus resulting in computational systems that

operate at far greater speeds [22]. Alligator clips are

moieties that allow for the connection of single mole-

cules (i.e. oligothiophenes) to a macroscopic interface,

usually a metallic tip or a nanoscale cluster. The

characterization of the tip/molecule interface is of

crucial importance in the design of these molecular

electronic circuits. The majority of systems studied so

far use thiol-terminated molecules, because of sulfur’s

ability to bond to a great variety of metal surfaces

[23,24]. In this context, the a,a0-(n-thiohexyl) end-

capped sexithiophene studied in this paper, referred to

as DHTSxT henceforth, can be viewed as a surface-

bound sexithiophene bearing two end thioether (SR)

substituents, where the alkyl groups play the role of

the tip while the S atoms act as the alligator clips.

Current quantum-chemical methods are in the posi-

tion to give reliable information about the molecular

structure and vibrational properties of the different

classes of polyconjugated materials. Most current

calculations are performed within the ab initio Har-

tree–Fock (HF) scheme. At this level of theory, the

calculated harmonic vibrational frequencies are

usually higher than the corresponding experimental

quantities, due to electron correlation effects and basis

set deficiencies. Density functional theory (DFT) con-

stitutes a non-expensive approach for adding electron

correlation, being its computational requirements

comparable to those of the HF method. DFT studies

have been probed very useful in the study of charged

molecules or ions [25–27]. Recently, the spin-unrest-

ricted DFT methods have been successfully applied to

the study of the polaron to bipolaron transition in

oligophenyls [27]. Our theoretical work is based on

the use of the DFT methodology to calculate ground-

state geometries as well as vibrational frequencies and

intensities for model oligothiophenes.

We have previously reported a spectroelectrochem-

ical Raman and theoretical study of these two end-

capped sexithiophenes, both in their neutral and doped

forms [28]. The doping process was found to generate

two stable oxidized species: a radical cation type

defect at low anodic potentials and a dication type

defect at high potential values. In order to achieve a

more detailed information on the electronic charge

distribution and the effects of the n-hexyl and -thio-

hexyl-substitution, we report here a new theoretical

and infrared spectroscopic study of the above hexam-

ers. The analysis of the experimental spectra will be

guided by means of quantum-chemical calculations

carried out on two quaterthiophenes, a,a0-end-capped

by n–propyl groups, DPQtT, and by n-thiopropyl

groups, DPTQtT, as model systems for DHSxT and

DHTSxT, respectively.

2. Experimental and computational details

The two sexithiophenes were prepared following a

procedure described elsewhere [29]. The chemical

structures of DHTSxT and DHSxT are displayed in

Fig. 1, together with that of the a,a0-dimethyl end-

capped sexithiophene (DMSxT) for comparison pur-

poses. Although the DMSxT compound has been

already studied in depth, it will be referred to as a

model compound bearing a short a-alkyl side chain

[30]. The chemical doping of the compounds was

carried out, under dry atmosphere, by slow in situ

sublimation of iodine at room temperature using a

solid–vapor doping technique.

FT-infrared measurements were made with a Per-

kin-Elmer Model 1760X spectrometer, on the pure and

iodine-doped solid compounds, in the form of KBr

pellets. All spectra were collected using a resolution of

2 cm�1, and the mean of 50 scans was averaged in all

the cases.

A suitable variable temperature cell Specac P/N

21525, with a pair of NaCl windows for transmission

studies, was used to record the FT-infrared spectra

at different temperatures. The cell consisted of a

176 J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189

Fig. 1. Chemical structure of DHTSxT, DHSxT, DMSxT and DPTQtT. Atom numbering corresponds to those appearing in the paper. Bond numbering appears into circles and

correspond to those of Fig. 7.

J.C

asa

do

eta

l./Vib

ratio

na

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pectro

scop

y3

0(2

00

2)

17

5–

18

91

77

surrounding vacuum jacket, with a combination of a

refigerant dewar and a heatable block as the sample

holder. It was fitted to a copper–constanton thermo-

couple for temperature monitoring purposes, and any

temperature ranging from 83 to 523 K could be

achieved.

DFT calculations, for the neutral and doped mole-

cules, were carried out with the Gaussian 98 program

running on a SGI Origin 2000 supercomputer [31].

The standard 3-21G� basis set was used in all the

calculations, as a good compromise between accuracy

and applicability to large systems. The 3-21G� basis

set, which includes a set of d-symmetry polarization

functions for the second-row elements [32], was used

in conjunction with the B3LYP functional. In several

studies, it has been shown that the B3LYP functional

yields similar geometries for medium-sized molecules

as MP2 calculations do with the same basis set [33].

DFT quadratic molecular force fields calculated with

the B3LYP functional yield infrared absorption spec-

tra in very good agreement with experiments [34–36].

Previous geometry optimizations were performed on

isolated entities. Because of the long computing time

of the force field calculations, only the all-anti copla-

nar conformations were evaluated analytically within

the same theoretical scheme used for the geometry

optimizations. No scaling factors of the force con-

stants were used and the theoretical frequencies were

directly compared with the experiments.

3. General considerations

No experimental X-ray or electron diffraction data

are available for DHSxT and DHTSxT. Supposedly, as

shown by the X-ray structures of some related com-

pounds (such as the unsubstituted a-linked oligothio-

henes [37], a,a0-dimethyl end-capped quaterthiophene

[38] and a,a0-dihexyl end-capped quaterthiophene

[39]), it can be assumed that (a) the thienyl sulfur

atoms are located in an all-anti configuration with

respect to the long molecular axis and; (b) the whole

molecule retains a nearly coplanar conformation of the

aromatic units. With such a molecular structure the

two hexamers belong to the C2h symmetry point

group. Nonetheless, the outermost rings of the oli-

gothiophene chain possibly display a slight bent rela-

tive to the inner rings least-square plane, and the strict

C2h symmetry could be partially broken. In what

follows, however, it will be assumed that both mole-

cules present internal symmetry in solid state. As for

DHTSxT, there exist 246 normal vibrational modes,

123 of them are IR-active and the remainder 123

Raman-active, as derived from the optical selection

rules for the C2h symmetry point group.

Although the optical selection rules predict a very

large population of bands, both in the infrared and

Raman spectra, the actual spectral patterns are fairly

simple. This seeming discrepancy between theoretical

predictions and experimental observations needs to be

accounted for:

(i) since the side chains at the end a,a0-positions of

the oligothiophene spine are far apart, no mech-

anical coupling is expected to occur between

their characteristic vibrations, and their in-phase

and out-of-phase motions are expected to be

fully degenerate, thus not showing any splitting

in the spectra;

(ii) it is reasonable to believe that a sexithiophene

chain is not large enough to observe progression

of bands (i.e. set of close bands with frequency

differences of about 4–5 cm�1) associated to the

same type of oscillators but with different phase

angles;

(iii) usually, the infrared and Raman spectra of the

p-conjugated organic materials show for some

few skeletal n(CC) stretching vibrations a selective

intensity enhancement and sizeable frequency

and intensity dispersions with variable number

of units in the chain. This singular spectral

feature has been explained by the existence of a

very large electron–phonon coupling between

the p-electrons system and some molecular

vibrations with a pronounced collective char-

acter [1,2].

4. Infrared spectra of the neutral molecules

The infrared spectra of DHSxT and DHTSxT in the

high and medium low energy region are plotted in

Figs. 2 and 3, respectively (the infrared spectrum of

DMSxT has been also included) [30]. Fig. 4 compares

the theoretical B3LYP/3-21G� infrared spectrum of

DPTQtTwith the experimental one for DHTSxT in the


1600–900 cm�1 frequency range. Fig. 5 shows the

eigenvectors associated to the stronger infrared

absorptions of DPTQtT, while Table 1 summarizes

a correlative analysis of frequencies measured in the

infrared spectra of neutral DMSxT, DHSxT, and

DHTSxT, as solids, together with their tentative

assignment.

The infrared spectra of DHSxT and DHTSxT show

characteristic absorptions around 3080–3060 cm�1

assignable to aromatic n(C–H) stretching vibrations

and four well resolved peaks below 3000 cm�1, cor-

responding to aliphatic n(C–H) stretchings. The broad

features at 3061 cm�1 in DHSxT and at 3065 cm�1 in

DHTSxT are due to stretchings of the C–H bonds

attached at the b-positions of the inner rings [30,40].

On the other hand, the infrared band at 3078 cm�1 in

DHSxT and at 3080 cm�1 in DHTSxT can be assigned

to stretching vibrations of the C–H bonds attached at

the b-positions of the outermost thiophene rings

[30,40]. Absorptions below 3000 cm�1 appear at the

same frequencies in both compounds and have almost

the same relative intensities. Band at 2955 cm�1 arises

from antisymmetric stretching vibrations of the

methylene groups of the hexyl side chains, na(CH2),

probably coupled to some extent with the antisym-

metric stretching of the methyl end group, na(CH3). On

the other hand, bands at 2873 and 2854 cm�1 are

assignable to symmetric stretchings of the methylene

groups, ns(CH2), also coupled with the corresponding

ns(CH3) [30,40].

The spectral region 1550–1350 cm�1 is overwhel-

mingly dominated by the appearance of two or three

bands. The band at 1492 cm�1 in DHTSxT could be

correlated with the theoretical absorption of DPTQtT

Fig. 2. FT-IR spectra over probe energies of 3200–2800 cm�1 of

neutral DHTSxT, DHSxT and DMSxT. Infrared spectrum of

DMSxT has been taken from [30].

Fig. 3. FT-IR spectra over probe energies of 1600–400 cm�1 of

neutral DHTSxT, DHSxT and DMSxT. Infrared spectrum of

DMSxT has been taken from [30].

J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189 179

at 1528 cm�1 (Fig. 4). Its associated eigenvector can

be described as an antisymmetric stretching mode of

the aromatic C=C bonds, na(C=C), spreading over the

whole oligothiophene chain (Fig. 5). The correspond-

ing na(C=C) vibration in DHSxT is measured at

1503 cm�1, on the basis of the B3LYP/3-21G� eigen-

vectors for DPQtT (being calculated at 1545 cm�1).

On the other hand, this na(C=C) vibration is recorded

at the same frequency, 1503 cm�1, both in DHSxT and

DMSxT, thus confirming that the length of the alkyl

side chain has a little influence on the vibrations of the

p-conjugated skeleton [30,40].

Let us pay some attention to the difference in

frequency, Dn ¼ 11 cm�1, for a same type of skeletal

vibration between DHTSxT and DHSxT. Fig. 6 com-

pares the B3LYP/3-21G� Mulliken atomic charges and

bond lengths for the outermost rings of the oligothio-

phene chain in DPQtT and DPTQtT (refer to Fig. 1 for

atom numbering). The C11 atomic charge varies from

�0.21e in DPQtT to �0.46e in DPTQtT, while those

on the C10 and C9 atoms go from þ0.004e and

þ0.003e to �0.005e and þ0.020e, respectively. On

the other hand, the C10–C11 and C9–C10 bonds

partially lose double and single bond character,

respectively, upon attaching sulfur atoms at the end

a,a0-positions of the oligothiophene. These theoretical

data could be explained by the balance between two

resonant structures, mainly located over the outermost

rings of the chain (Scheme 1) [41]. Under this hypoth-

esis, the bond connecting atoms C10 and C11 should

particularly weaken in going from DPQtT to DPTQtT,

thus explaining the downshift by 11 cm�1 of the afore-

mentioned na(C=C) vibration. The balance between

these two resonant structures should also induce certain

degree of polarization of the conjugated bonds, parti-

cularly of those of the outermost molecular domains.

Fig. 4. Comparison of: (a) infrared spectrum of neutral DHTSxT

and (b) theoretical B3LYP/3-21G� infrared spectrum of neutral

DPTQtT.

Table 1

Frequency correlation of the main bands recorded in the infrared

spectra of the neutral DMSxT, DHSxT and DHTSxT together with

their assignment

DMSxT DHSxT DHTSxT Assignmenta

1503 1503 1492 na(C=C)

1466 1466 na(C=C) þ da(CH2)

1457 1455 na(C=C) þ da(CH2)

1443 1441 1444 ns(C=C)

1400 1429 ns(C=C) þ ds(CH2)

1370 1377 1383

1351

1313 n(C–C)intra-ring

1271 1274 1259 da(CH)

1245 1242 da(CH)

1222 1222

1203 1204 1207 n(C–C)inter-ring

1162 1166

da(CH)

1069 1069 1077 da(CH)

1044 1048 1069 da(CH)

989 n(C–S)alkyl

906

875 871 873 na(C–S)

837 840 838 ns(C–S)

795 791 795 g(CH)

683 725

668 668 dring

625

462 463 466 gring

a n, stretching; d, in-plane deformation; g, out-of-plane

deformation.


This fact could justify the appreciable infrared activity

of the aliphatic n(C–S) stretchings at both chain ends,

measured at 989 cm�1 and calculated at 1005 cm�1, in

spite of their low statistical weight as compared with

the very many aromatic n(C=C), n(C–C) and n(C–H)

modes of the inner thiophene units. The eigenvector

for the 1005 cm�1 B3LYP/3-21G�� infrared band of

DPTQtT and the absence of the corresponding coun-

terpart in the experimental spectrum of DHSxT sup-

port the above assignment of the 989 cm�1 band.

Band measured at 1444 cm�1 in DHTSxT may be

correlated with that calculated at 1453 cm�1 for

Fig. 5. Schematic eigenvector for the more relevant infrared active vibrations of neutral DPTQtT calculated at the B3LYP/3-21G� level

(frequency values are given in cm�1).


DPTQtT, being described as a symmetric stretching

vibration of the aromatic C=C bonds, ns(C=C), mostly

localized on the outer rings and where both end rings

vibrate in full out-of-phase (see Fig. 5 for the corre-

sponding eigenvector).

The doublets at 1466 and 1457 cm�1 in DHSxT and

at 1466 and 1455 cm�1 in DHTSxT are new with

respect to DMSxT [30]. These absorptions could be

correlated with the theoretical band for DPTQtT at

1471 cm�1, due to a ns(C=C) mode mainly located on

the inner rings of the chain for which the motions of

the symmetry-equivalent thiophene units also take

place in full out-of-phase (Fig. 5).

The weak band at 1313 cm�1 in DHTSxT could be

assigned to an antisymmetric stretching mode of ring

C–C bonds, na(C–C). Weak bands at 1280–1230 cm�1

are due to antisymmetric in-plane C–H deformations,

da(C–H), [30,40] whereas doublets at 1068 and

1048 cm�1 in DHSxT and 1077 and 1068 cm�1 in

DHTSxT correspond to symmetric in-plane C–H

deformations, ds(C–H), for which the motions of

the symmetry-equivalent oscillators occur in full

out-of-phase. The doublets at 1222 and 1204 cm�1

in DHSxT and the band at 1207 cm�1 in DHTSxT are

mainly due to inter-ring CC stretching vibrations.

In the low energy region, bands at 871 and

840 cm�1 in DHSxT, and 873 and 838 cm�1 in

DHTSxT are due to antisymmetric and symmetric

aromatic n(C–S) stretchings, respectively, while the

characteristic out-of-plane bending vibration of the

2,5-disubstituted thiophenes is easily identified with

the band at 791 cm�1 in DHSxT and 795 cm�1 in

DHTSxT [30,40].

Finally, the bands at 650–750 cm�1 in DHSxT and

DHTSxT could be assigned to in-plane thiophene ring

deformation vibrations, dring, whereas the bands at

463 cm�1 in DHSxT and 468 cm�1 in DHTSxT have

been assigned to out-of-plane thiophene ring folding

modes, gring [30,40].

5. Doped molecules

5.1. Molecular geometry optimizations and charges

Fig. 7 shows the evolution of the calculated B3LYP/

3-21G� and UB3LYP/3-21G� CC bond lengths on

going from the neutral to the radical cationic forms

of DPTQtT (detailed values are given in Table 2).

Table 3 reports the Mulliken atomic charges for

Fig. 6. Relevant Mulliken atomic charges (upper) and bond distances (below) calculated at the B3LYP/3-21G� level for the outermost ring of

neutral DPQtT and DPTQtT.

Scheme 1. Resonant electronic structures in neutral DPTQtT.


Fig. 7. Optimized CC bond lengths of neutral DPTQtT (filled squares) and of DPTQtT as radical cation (open circles). The B3LYP and

UB3LYP methods have been used for the close and open shell systems, respectively. See Fig. 1 for bond numbering.

Table 2

Bond lengths (in A) calculated at the B3LYP/3-21G� and UB3LYP/

3-21G� level for, respectively, the neutral and the radical cation

systems of DPQtT and DPTQtT

DPQtT DPTQtT

Bond Neutral Radical

cation

Bond Neutral Radical

cation

C1–C3 1.440 1.406 C1–C3 1.440 1.410

C3–C4 1.383 1.412 C3–C4 1.383 1.408

C4–C5 1.418 1.388 C4–C5 1.417 1.390

C5–C6 1.383 1.411 C5–C6 1.383 1.408

C6–C8 1.443 1.416 C6–C8 1.441 1.414

C8–C9 1.380 1.398 C8–C9 1.379 1.399

C9–C10 1.427 1.407 C9–C10 1.425 1.402

C10–C11 1.373 1.398 C10–C11 1.377 1.397

C11–S12 1.758 1.732

C11–C13 1.511 1.508 S12–C13 1.839 1.845

C13–C14 1.549 1.550 C13–C14 1.543 1.544

C14–C15 1.549 1.547 C14–C15 1.549 1.549

See Fig. 1 for bond numbering.

Table 3

Mulliken atomic charges calculated at the B3LYP/3-21G� and

UB3LYP/3-21G� level for, respectively, the neutral and the radical

cation systems of DPQtT and DPTQtT

DPQtT DPTQtT

Atom Neutral Radical

cation

Atom Neutral Radical

cation

S2 0.46 0.54 S2 0.46 0.52

C3 �0.25 �0.25 C3 �0.25 �0.25

C4 0.01 0.07 C4 0.01 0.07

C5 0.01 0.07 C5 0.01 0.06

C6 �0.25 �0.24 C6 �0.25 �0.24

S7 0.43 0.51 S7 0.45 0.52

C8 �0.25 �0.26 C8 �0.26 �0.26

C9 0.003 0.006 C9 0.02 0.07

C10 0.004 0.004 C10 �0.005 0.04

C11 �0.21 �0.19 C11 �0.46 �0.45

S12 0.31 0.40

C13 0.003 0.04 C13 �0.09 �0.08

C14 0.02 0.02 C14 0.01 0.03

C15 0.02 0.05 C15 0.04 0.07

See Fig. 1 for atom numbering.


DPQtT and DPTQtT in their neutral forms (B3LYP/3-

21G�) and as radical cations (UB3LYP/3-21G�). The

main geometry changes upon ionization concern the

distorsions of the p-conjugated C=C/C–C bonds,

together with the C11–S12 bond in DPTQtT. The

optimized geometries for both radical cations indicate

the generation of a positive polaron type defect over

the quaterthienyl moiety in DPQtT, which further

extends towards the C11–S12 bonds in DPTQtT. The

amplitude of the structural modifications progres-

sively decrease away from the center of the molecules,

however, in the case of the DPTQtT radical cation the

C11–S12 bond significantly shortens by 0.26 A. This is

a large change as compared with those undergone by

the inner CC bond lengths (center of the charged

defect), whose greatest differences amount 0.30 A.

The analysis of the atomic charges also shows a large

participation of the sulfur atoms in the stabilization of

the positive polaron type defect. Thus, the atomic

charges on the S2 and S7 atoms in DPQtT and DPTQtT

increase by �0.07e, while those on the S12 a-linked

atom increase by �0.09e.

5.2. Infrared spectra

Fig. 8 shows the experimental infrared spectra of

the neutral and iodine-doped forms of DHSxT

together with the UB3LYP/3-21G� infrared spectrum

of DPQtT as radical cation. Fig. 9 displays the same

comparison as Fig. 8, but between the DHTSxT

compound and its DPTQtT model system. Finally,

Table 4 summarizes the frequencies measured in the

spectra of the two iodine-doped samples, and their

tentative assignment.

In general terms, there exists a good agreement

between experiments and calculations, what supports

the reliability of the molecular parameters discussed

along the preceding section. The infrared spectra of

the doped molecules are characterized by the appear-

ance of five intense infrared bands in the 1400–

1000 cm�1 spectral region both in the experimental

as in the theoretical spectra. The injection of a positive

charge in the molecule give rise to strong charge fluxes

along the p-conjugated backbone, generating strong

infrared absorptions.

The infrared spectrum of iodine-doped DHSxT

shows intense bands at 1401 and 1339 cm�1, which

are easily related with the theoretical features at 1437

and 1396 cm�1. In the case of iodine-doped DHTSxT,

experimental bands at 1399, 1365 and 1319 cm�1 are

to be compared with the theoretical features at 1431,

1390 and 1264 cm�1, respectively. Figs. 10 and 11

depict the eigenvectors associated to each of these

theoretical infrared bands. All of these vibrations

correspond to n(CC) stretching modes of the p-con-

jugated backbone, mainly located in the transition

region between the inner part of the chain (i.e. a

molecular domain characterized by a quinonoid sin-

gle–double bond alternation pattern) and the end

thiophene rings (with a typical aromatic single–double

bond alternation pattern) [6,7]. Please, note the sig-

nificant contribution of the a-linked CS bonds to the

molecular vibration associated to the band at

1264 cm�1 in DPTQtT, what could justify for the

different spectral patterns of the iodine-doped DHSxT

and DHTSxT samples.

Fig. 8. Comparision of: (a) infrared spectrum of neutral DHSxT;

(b) theoretical UB3LYP/3-21G� infrared spectrum of DPQtT as

radical cation; (c) infrared spectrum of iodine oxidized DHSxT.


Fig. 9. Comparision of: (a) infrared spectrum of neutral DHTSxT;

(b) theoretical UB3LYP/3-21G� infrared spectrum of DPTQtT as

radical cation; (c) infrared spectrum of iodine oxidized DHTSxT.

Table 4

Correlation between the vibrational frequencies measured in the

infrared spectra of iodine-oxidized DHSxT and of iodine-oxidized

DHTSxT

DHSxT-I2 DHTSxT-I2 Assignment

1453 1455 –

1401 1399 n(CC)

– 1365 n ( C C ) þn(CS)0

1339 – n(CC)

– 1319 n ( C C ) þn(CS)0

– 1249 –

1213 1226 –

1174 1169 –

1110 1106 –

1095 – –

– 1070 –

1041 – –

1014 – –

996 – –

– 955 n(CS)0

892 883 –

842 837 –

788 791 –

724 723 –

670 679 –

455 465 –

n(CS)0: alkyl side CS stretching vibration.

Fig. 10. Schematic eigenvectors for the most intense infrared bands of the theoretical UB3LYP/3-21G� spectrum of DPQtT as radical cation

(all values are in cm�1).


The spectrum of doped DHTSxT shows a strong

band at 955 cm�1, which is missing in the spectrum of

doped DHSxT. This band must be correlated with the

experimental feature at 989 cm�1 in neutral DHTSxT,

being likely associated with an aliphatic n(CS) stretch-

ing mode of the doped material. The reason for the

strong infrared-activity of this vibration can be found

in the polarization of the a-linked C–S bonds, as

shown by the B3LYP/3-21G� Mulliken atomic charges

of the neutral and radical cationic forms of DPTQtT.

Fig. 12 shows the infrared spectra of iodine-doped

DHTSxT at �170 8C and at room temperature. The

band at 1319 cm�1 becomes stronger on lowering

the temperature. Different authors have concluded

that the p-conjugated oligothiophene backbone reaches

a more planar conformation of the thiophene rings at

low temperatures [43]. In this regard, we believe that

the increasing conformational order of the thioalkyl

side chains, at low temperatures, should also lead to a

more favorable overlapping between the d-type orbi-

tals of the a-linked S atoms and the p-conjugated

backbone, thus increasing the participation of the end

thioalkyl groups in the stabilization of the radical

cation (which in its turn is reflected in a stronger

polarization of the C–S bond, and the subsequent

intensification of the infrared band at 1319 cm�1).

The infrared absorptions of doped DHTSxT are

somewhat downshifted with respect to those of doped

DHSxT. These observations can be rationalized within

the framework of the effective conjugation coordinate

Fig. 11. Schematic eigenvectors for the most intense infrared bands of the theoretical UB3LYP/3-21G� spectrum of DPTQtT as radical cation

(all values are in cm�1).


theory (ECC) developed by Zerbi and coworkers to

explain the simple appearance of the Raman spectral

patterns of undoped p-conjugated materials and the

upsurge of strong and broad absorptions in the infrared

spectrum upon chemical doping or photoexcitation

[44]. These authors correlate the doping-induced

infrared bands with initially silent totally symmetric

normal modes with a large contribution of a particular

vibrational coordinate, usually termed as ECC coor-

dinate, which become activated in the infrared due to

the breakdown of the optical selection rules in the

molecular domain perturbed by the doping process.

The ECC coordinate describes a collective vibration of

the p-conjugated path along which all the C=C bonds

lengthen in-phase while all the C–C bonds shrink in-

phase. Thus, the ECC skeletal vibration points in the

direction from a benzenoid structure (usually that of

ground state) to a quinonoid structure (usually that

of the first electronically excited state or of the charged

defect).

During the oxidation of a p-conjugated material,

ring C=C bonds are weakened, while inter- and intra-

ring C–C bonds are strengthened. Therefore, with

respect to the neutral form, normal modes of a doped

or photoexcited species with large contents of n(C=C)

stretchings shift downward due to the softening of the

double C=C bonds (i.e. specially in the case of mole-

cular vibrations with a large contribution of the col-

lective ECC vibrational coordinate). In the Raman

spectrum of neutral DHTSxT the strongest line,

appearing at 1458 cm�1, arises from a totally sym-

metric normal mode whose associated eigenvector

greatly remembers the ECC coordinate [28]. On the

other hand, the strongest infrared absorption of iodine-

doped DHTSxT assignable to a n(C=C) stretching

vibration is that measured at 1319 cm�1. Thus size-

able downshifts (by even more than 100 cm�1) are

observed to take place upon the partial quinoidization

of the sexithiophene spine, in full agreement with the

statements of the ECC theory.

Neutral polythiophene exhibits, in the 800–

1600 cm�1 spectral range, four Raman-active Ag nor-

mal vibrations, which give rise upon chemical doping

or photoexcitation to an infrared absorption pattern

with three main components at 1319, 1195 and 1090–

1060 cm�1 [42,45,46]. These spectroscopic data are

quite similar to those reported in this paper for iodine-

doped DHTSxT. ECC theory states that the strong

doping-induced infrared bands should downshift as

the chain length (or conjugation length) of the oligo-

mers grows up. As aforementioned, the doping-

induced infrared bands of DHTSxT appear at lower

frequencies than in DHSxT, but the oligothiophene

backbone has the same chain length in both com-

pounds. The most feasible explanation is the strong

participation of the a-linked sulfur atoms in the sta-

bilization of the doped species, being the positive

charge delocalization in iodine-doped DHTSxT simi-

lar to that of a photoexcited or doped polythiophene

sample.

6. Conclusions

A comprehensive study of the infrared vibrational

properties of two sexithiophenes, with their end

Fig. 12. Infrared spectrum of iodine-oxidized DHTSxT recorded

at: (a) room temperature; (b) �170 8C.


a,a0-positions capped by n-hexyl or -thiohexyl groups,

in neutral state has been reported. A tentative assign-

ment of the main infrared spectral features of the

corresponding iodine-doped species is also proposed.

The different spectral behavior of the two hexamers

has been interpreted, at the light of the statements

of the ECC theory of Zerbi’s group, in terms of the

role played by the a-linked sulfur atoms in the overall

p-conjugation of the undoped molecule and the

stabilization of the oxidized forms.

The present study shows that upon oxidation of

DHSxT and DHTSxT with iodine vapors, a radical

cation species is generated. The vibrational infrared

features of these radical cation species can be used to

identify prototypes of polaron-like charged defects in

other classes of p-conjugated thiophene-based mole-

cular materials. The comparison of the doping-induced

infrared absorptions of iodine-doped DHTSxT and

DHSxT with the infrared spectra of doped or photo-

excited polythiophene has led to the conclusion that the

electron-donating effect of the end a-thiohexyl groups

improvethedelocalizationalongthechainofthepositive

charge injected in the doping process. The analysis of

the infrared data is consistent with the Raman data

previously reported on neutral and electrochemically

doped DHSxT and DHTSxT.

In terms of molecular electronics, a-linked sulfur

atoms seem to be good candidates to act as alligator

clips, thus preserving the oligothiophene backbone

from electronic interactions with a macroscopic sur-

face. In addition, sulfur atoms facilitate the connec-

tions of this type of p-conjugated molecular material

with a metallic or cluster surface, strongly stabilizing

the radical cations which are likely present in the

charge-separated state of the operating forms of the

molecular electronic devices.

Acknowledgements

The present work was supported in part by the

Direccion General de Ensenanza Superior (DGES,

MEC, Spain) through the research projects

BQU2000–1156 and FD97–1765–C03. We are also

indebted to Junta de Andalucıa (Spain), funding for

our research group (FQM–0159). J.C. is grateful to the

Ministerio de Educacion y Cultura of Spain for a

PostDoctoral fellowship at the Department of Chemistry

of the University of Minnesota (Formacion y Perfec-

cionamiento de Doctores y Tecnologos en el Extranjero,

referencia PF00 25327895).

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