infrared spectra of two sexithiophenes in neutral and doped...
TRANSCRIPT
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.
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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
178 J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189
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.
180 J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189
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).
J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189 181
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.
182 J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189
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.
J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189 183
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.
184 J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189
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).
J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189 185
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).
186 J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189
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.
J. Casado et al. / Vibrational Spectroscopy 30 (2002) 175–189 187
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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