Comparison of thiophene-pyrrole oligomers with oligothiophenes: A joint experimental and theoretical investigation of their structural and spectroscopic properties

Article abstract of DOI:10.1002/chem.201000143

We have prepared a new series of mixed thiophene-pyrrole oligomers to investigate the electronic benefits arising from the combination of these two heterocycles. The oligomers are functionalized with several hexyl and aryl groups to improve both processability and chemical robustness. An analysis of their spectroscopic (absorption and emission), photophysical, electrochemical, solid state, and vibrational properties is performed in combination with quantum-chemical calculations. This analysis provides relevant information regarding the use of these materials as organic semiconductors. The balance between the high aromatic character of pyrrole and the moderate aromaticity of thiophene allows us to address the impact of the coupling of these heterocycles in conjugated systems. The data are interpreted on the basis of the aromaticity, molecular conformations, ground and excited electronic state structures, frontier orbital topologies and energies, oxidative states, and quinoidal versus aromatic competition.

Full text of DOI:10.1002/chem.201000143

DOI: 10.1002/chem.201000143
Comparison of Thiophene–Pyrrole Oligomers with Oligothiophenes: A Joint
Experimental and Theoretical Investigation of Their Structural and
Spectroscopic Properties
Marꢀa Moreno Oliva,[a] Ted M. Pappenfus,*[b] Jacob H. Melby,[b]
Kathryn M. Schwaderer,[b] Jared C. Johnson,[b] Kari A. McGee,[c]
Demetrio A. da Silva Filho,[d] Jean-Luc Bredas,[d] Juan Casado,*[a] and
Juan T. Lꢁpez Navarrete*[a]
Abstract: We have prepared a new
series of mixed thiophene–pyrrole olig-
omers to investigate the electronic ben-
efits arising from the combination of
these two heterocycles. The oligomers
are functionalized with several hexyl
and aryl groups to improve both proc-
essability and chemical robustness. An
analysis of their spectroscopic (absorp-
tion and emission), photophysical, elec-
trochemical, solid state, and vibrational
properties is performed in combination
with quantum-chemical calculations.
This analysis provides relevant infor-
mation regarding the use of these ma-
terials as organic semiconductors. The
balance between the high aromatic
character of pyrrole and the moderate
aromaticity of thiophene allows us to
address the impact of the coupling of
these heterocycles in conjugated sys-
tems. The data are interpreted on the
basis of the aromaticity, molecular con-
formations, ground and excited elec-
tronic state structures, frontier orbital
topologies and energies, oxidative
states, and quinoidal versus aromatic
competition.
Keywords: heterocycles
·
oligo-
mers · oligothiophenes · quantum
chemistry · semiconductors
Introduction
[a] M. M. Oliva, Dr. J. Casado, Prof. J. T. Lꢀpez Navarrete
Department of Physical Chemistry
University of Mꢁlaga
An important research avenue to develop new organic ma-
terials acting as semiconductors in organic electronics is the
investigation of molecules addressing a variety of forms of
conjugation.[1] Classic examples include acenes[2] and oligo-
thiophenes.[1a,3] In these two groups, several strategies have
been used to search for improved materials. In oligothio-
phenes, for instance, many chemical modifications, such as
ring fusion,[4] chain-length elongation[5] (i.e., up to the 96-
mer), replacement of sulfur by other heteroatoms,[6] substi-
tution of the a-terminal and b-inner positions with alkyl and
electroactive groups,[7] combination with acenes,[8] selective
sulfur oxidation,[9] and so on, have been reported. In partic-
ular, incorporation of different heteroatoms targets the
modification/improvement of the p-conjugation, whereas in-
clusion of alkyl chains at the a- and b- positions leads to
processability benefits and p-stacking in the solid state.[10]
Pyrrole-based oligomers have been less intensively stud-
ied for organic device applications relative to their oligo-
thienyl homologues.[11–13] This fact may be due, in part, to
the limited synthetic routes available to oligopyrroles.[14] Re-
cently, however, Fujii et al.[14b] have reported the use of thio-
phene–pyrrole mixed oligomers in organic field-effect tran-
Campus de Teatinos s/n, Mꢁlaga 29071 (Spain)
Fax : (+34)952-132000
E-mail: casado@uma.es
teodomiro@uma.es
[b] Dr. T. M. Pappenfus, J. H. Melby, K. M. Schwaderer, J. C. Johnson
Division of Science and Mathematics
University of Minnesota Morris
MN 56267 (USA)
Fax : (+1)320-589-6371
E-mail: pappe001@morris.umn.edu
[c] Dr. K. A. McGee
Department of Chemistry
University of Minnesota Minneapolis
MN 55455 (USA)
[d] Dr. D. A. da Silva Filho, Prof. J.-L. Bredas
School of Chemistry and Biochemistry
Georgia Institute of Technology
Atlanta, GA 30332-0400 (USA)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000143. General synthetic
considerations, new calculation data together with additional X-ray
measurement details. CCDC-06127x contains the supplementary crys-
tallographic data for Hex2-TPT. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif for are provided.
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FULL PAPER
sistors (OFETs) with mobilities exceeding values of
10ꢀ2 cm2 Vꢀ1 sꢀ1. Given the renewed interest in these materi-
als and the fact that pyrrole–thiophene co-oligomers have
not been thoroughly investigated,[11ꢀ13] our goal in this work
is to shed additional light on the physical properties such
oligomers exhibit.
Homogeneous oligothiophenes have been synthesized
with a multitude of chemical variants for use in device appli-
cations, such as OFETs,[1,3,4,9,15] OLEDs[1,3,9,16] (organic light-
emitting diodes), organic photovoltaics,[17] and optically
pumped organic lasers.[18] Given this diverse spectrum of ap-
plications for the related oligothiophenes, here we report
the study of a new series of thiophene–pyrrole co-oligomers
(3T, Hex2-3T, Hex2-5T, TPT, Hex2-TPT, Hex2-TTPTT,
TPT-p(HP), Hex2-TPT-p(HP), Hex2-TTPTT-p(HP)) with
brational Raman spectra in combination with their theoreti-
cal estimation. In all cases, we discuss in depth the compari-
son between the co-oligomers and the homogeneous oligo-
thiophenes of the same length and substitution pattern. Our
study includes both neutral and oxidized species. We believe
that this new series of materials together with the analysis
of their electronic and structural properties will be of value
to the chemical community in general and the organic elec-
tronics community in particular.
Experimental Section and Theoretical Details
Synthesis: The hexyl-capped trimers were prepared according to the pro-
cedures outlined in Scheme 1. The previously reported[19] Hex2–3T was
prepared by an alternative procedure
using Stille coupling.[20] The reaction
of 2,5-dibromothiophene with excess
2-hexyl-5-(tributylstannyl)thiophene
afforded Hex2–3T in 54% yield. Al-
though 1,4-bis(2-thienyl)-1,4-butane-
dione can be prepared in high yield (>
80%) by the Friedel–Crafts reac-
tion,[21] preparation of analogous dike-
tones with 2-alkyl-substituted thio-
phenes has been reported to proceed
at much lower yields.[22] We obtained
similar results for the reaction of 2-
hexylthiophene with succinyl chloride
to produce the targeted diketone
1
(Scheme 1). Paal–Knorr condensa-
tion[21,23] of 1 with the appropriate am-
monium salt or amine afforded the de-
sired dithienylpyrroles Hex2-TPT and
Hex2-TPT-(pHP) in good yields.
The hexyl-capped pentamers were pre-
pared according to the procedures in
Scheme 2. The reaction of 1,4-bis(5-
bromo-2-thienyl)-1,4-butanedione with
excess 2-hexyl-5-(tributylstannyl)thiophene afforded the intermediate di-
ketone 2 in 79% yield. Paal–Knorr condensation of 2 with the appropri-
ate ammonium salt or amine afforded the desired dithienylpyrroles
Hex2-TTPTT and Hex2-TTPTT-(pHP) in modest yields. Dihexylquin-
the following features: 1) The nitrogen on the pyrrole ring
either carries a proton or is functionalized with an aryl
group. 2) The molecules are capped with hexyl groups at the
a-terminal positions to enhance both electronic and solid-
state properties; such hexyl substitutions have been shown
to promote favorable crystalline arrangements.[10] 3) These
modifications are applied to various chain lengths and
number of conjugating units in the main chain.
In the design of new p-conjugated molecules, one of the
key parameters to optimize is the band gap, by which we
define here: 1) the absolute energy difference between the
HOMO and LUMO orbitals, 2) the optical gap, and 3) the
electrochemical (transport) gap. These energy differences
critically decide many of the electronic properties important
for engineering of the semiconducting properties, such as
charge injection ability, p- and n-doping (ambipolar behav-
ior), photoluminescence, etc. In this work, we focus on some
of these properties by relating the spectroscopy of these
molecules in the context of the band-gap properties directly
related to the linear inter-ring p-electron conjugation. The
approach includes studying the electronic absorption and
emission spectra, the electrochemical properties, and the vi-
Scheme 1. Synthesis of the trimers investigated in this study.
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6867

T. M. Pappenfus, J. Casado, J. T. Lꢀpez Navarrete et al.
of 3486 strong reflections from the
actual data collection after integration
(SAINT).[26] The structure was solved
by using SHELXS-97 (Sheldrick,
1990)[26] and refined by using
SHELXL-97 (Sheldrick, 1997).[27] See
the Supporting Information for addi-
tional details.
Theoretical calculations: Density func-
tional theory has a good track record
as far as predicting the electronic
structure of neutral and charged oligo-
thiophene molecules.[28] DFT has been
used for the ground-state properties
(i.e., optimized geometries, vibrational
spectra, etc.) and time-dependent
DFT, or TD-DFT, for the estimation
of vertical-adiabatic excited-state tran-
Scheme 2. Synthesis of the pentamers in this study.
sitions (i.e., energies and oscillator
strengths).[29] The geometries of the
relevant excited states were optimized
quethiophene Hex2–5T was prepared in a similar manner as previously
reported.[24] Detailed synthetic procedures and analytical data are found
in the Supporting Information section of this paper.
by taking advantage of the restricted single excited configuration interac-
tion approach (CIS) within the Hartree–Fock (HF) approximation, or
RCIS/HF, meaning that the single-determinant RHF wavefunction repre-
sents the reference determinant in a CIS calculation of excited states.[30]
Spectroscopic measurements: UV/Vis absorption spectra were recorded
on an Agilent 8453 instrument equipped with a diode array detection
system. Emission spectra were measured by using a spectrofluorimeter
Most of the calculations were performed with the Gaussian 03 suite of
programs.[31] For the DFT calculations, Beckeꢄs three-parameter (B3) gra-
dient-corrected exchange functional combined with the Lee–Yang–Parr
(LYP) correlation functional was used as implemented in Gaussian 03.[32]
The 6-31G** basis set was taken in the DFT and RCIS/HF calcula-
tions.[33] Methyl groups were considered instead of hexyl groups in all
molecules to reduce the computational cost. No constraints were imposed
during the geometry optimizations. In the TD-DFT calculations, an eval-
uation of at least the 10 lowest-energy vertical electronic excited states
was carried out. TD-DFT calculations were performed by using the same
functional (B3LYP, UB3LYP for the closed and open shell systems) and
basis set (6-31G**). Theoretical Raman spectra were obtained for the op-
timized ground-state geometries; the harmonic vibrational frequencies
and Raman intensities were calculated analytically and numerically, re-
spectively.
from Edinburgh Analytical Instruments (FLS920P) equipped with
a
pulsed xenon flash-lamp. Fluorescence decays were measured by using a
single-photon photomultiplier detection system (S900) with picosecond
pulsed diode laser (PDL 800-B), from Edinburgh Instruments. All sol-
vents used were of spectroscopic grade from Aldrich. Fluorescence quan-
tum yields, fF, were measured for all the solutions by using 1ꢃ
10ꢀ7 molLꢀ1 quinine sulfate in 0.1 molLꢀ1 H2SO4 as the standard (fF =
0.546).
1064 nm FT-Raman spectra were measured by using an FT-Raman acces-
sory kit (FRA/106-S) of a Bruker Equinox 55 FT-IR interferometer. A
continuous-wave Nd-YAG laser working at 1064 nm was used for excita-
tion along with a germanium detector operating at liquid nitrogen tem-
perature. Raman scattering radiation was collected in a back-scattering
configuration with standard spectral resolutions of 4 and 1 cmꢀ1. 1000–
3000 scans were averaged for each spectrum.
UV/Vis-NIR and Raman spectrochemistry was done by chemical oxida-
tion by using FeCl3 as the oxidant in anhydrous dichloromethane. This
strong oxidizing agent was chosen according to the redox potential ob-
tained electrochemically.
Results and Discussion
Absorption spectra: Figure 1 displays the absorption spectra
of the target molecules in dichloromethane solution. All the
spectra are broad and without defined vibrational structure,
which indicates the conformational flexibility of both series
of molecules. In all of the oligomer series, substituting a
thiophene ring with a pyrrole ring results in blueshifted ab-
Electrochemical measurements: Room-temperature electrochemical
measurements were performed with a BAS 100B electrochemical ana-
lyzer and C3 cell stand in a three-electrode configuration with a glassy
carbon working electrode (A=0.07 cm2), a platinum counter electrode,
and a standard AgjAgCljKCl (1.0m) reference electrode. A single com-
partment, low volume cell was used for all measurements. Tetrabutylam-
monium hexafluorphosphate electrolyte solution was added to the cell
(5 mL, 0.1m/CH2Cl2) and background cyclic voltammograms of the elec-
trolyte solution were recorded prior to the addition of the sample. Suita-
ble amounts of sample were added to create 0.5 mm solutions. The E0
values for the ferrocenium/ferrocene couple for concentrations similar to
those used in this study were 0.43 V for dichloromethane solutions at a
glassy carbon electrode. Anodic–cathodic peak separations were typically
80–90 mV for this redox couple.
sorptions (average shift=11 nm/0.1 eV).
A more pro-
nounced blueshift is seen when thiophene is substituted with
an N-aryl pyrrole (average shift=20 nm/0.2 eV). On the
other hand, the incorporation of hexyl groups at the a-posi-
tions promotes a significant optical gap reduction (on the
order of 0.07 eV) due the positive inductive or electron-don-
ation effect of these saturated groups.
TD-DFT calculations at the B3LYP/6-31G** level have
been carried out for all molecules under investigation. The
results indicate that the low-lying absorption in the electron-
ic spectra can be assigned to a HOMO!LUMO transition,
which allows us to relate the experimental trends to the
energy and topology of these two orbitals defining the
Single-crystal X-ray analysis of Hex2-TPT: Crystals of Hex2-TPT were
grown from slow evaporation of a CH2Cl2/hexane solution. A crystal (ap-
proximate dimensions 0.45ꢃ0.15ꢃ0.10 mm3) was placed onto the tip of a
0.1 mm diameter glass capillary and mounted on a Siemens SMART Plat-
form CCD diffractometer for a data collection at 173(2) K. The data col-
lection was carried out by using MoKa radiation (graphite monochroma-
tor). The intensity data were corrected for absorption and decay
(SADABS).[25] Final cell constants were calculated from the xyz centroids
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Comparison of Thiophene–Pyrrole Oligomers with Oligothiophenes
FULL PAPER
ty than polythiophene. For example, the LUMO orbital in
TPT relative to 3T is destabilized significantly by 0.51 eV
(0.29 eV for the HOMO). This feature is also seen in the
case of TPT-p(PH), the HOMO and LUMO of which are
also displaced to higher energies relative to TPT.
The increase of the chain length, either Hex2–3T!Hex2–
5T, Hex2-TPT!Hex2-TTPTT, or Hex2-TPT-p(HP)!Hex2-
TTPTT-p(HP), always produces a destabilization of the
HOMO and stabilization of the LUMO as expected from
the increase in conjugation in larger linearly connected mol-
ecules that overall causes reduction of the HOMO–LUMO
and optical gaps.
Fluorescence spectra and lifetimes: Figure 3 compares the
absorption and emission spectra of the oligomers. Contrary
to the absorption spectra, the emission profiles display a vi-
bronic structure independent of the molecular length and
chemical substitution pattern. This feature is due to the qui-
noidization and subsequent planarization of the molecular
backbone in the first singlet excited state (see Figure 4 for
Figure 1. Left: UV/Vis electronic absorption spectra in dichloromethane
of a) 3T, b) TPT, and c) TPT-p(HP). Middle: As above for a) Hex2–3T,
b) Hex2-TPT, and c) Hex2-TPT-p(HP). Right: As above for a) Hex2–5T,
b) Hex2-TTPTT, and c) Hex2-TTPTT-p(HP).
energy gap (Figure 2). On the basis of DFT/B3LYP/6-31G**
calculations, substituting a thiophene with a pyrrole ring
leads to a destabilization of both HOMO and LUMO fron-
tier orbitals. This effect is more pronounced for the LUMO,
resulting in an enlargement of the HOMO–LUMO and op-
tical gaps for pyrrole-containing oligomers. This finding can
be seen as a consequence of the greater aromatic character
of pyrrole relative to thiophene.[34] Due to this increased ar-
omaticity, one would expect some disruption of the conjuga-
tion between the two external thiophenes through the pyr-
role. This p-electron decoupling has been investigated previ-
ously in TPT[35] and is also supported by the molecular ge-
ometry, more planar in 3T than in TPT, which means great-
er conjugation in the former (see the section entitled
Vibrational Raman spectra), a conjugation dephasing that is
insufficient to provoke a donor-to-acceptor charge transfer
property.
Figure 3. Left: Absorption (c) and emission (a) spectra in dichloro-
methane of a) 3T, b) TPT, and c) TPT-p(HP). Middle: As above for
a) Hex2–3T, b) Hex2-TPT, and c) Hex2-TPT-p(HP). Right: As above for
a) Hex2–5T, b) Hex2-TTPTT, and c) Hex2-TTPTT-p(HP).
The higher HOMO/LUMO energies in TPT versus 3T are
in line with the fact that polypyrrole presents both a lower
ionization potential and a significantly lower electron affini-
Figure 2. Orbital energies and topologies of the HOMO (top) and LUMO (bottom) according to DFT/B3LYP/6-31G** calculations.
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T. M. Pappenfus, J. Casado, J. T. Lꢀpez Navarrete et al.
agreement with a less effective heavy-atom effect and inter-
system crossing.[36]
Inclusion of an aryl group on the nitrogen atom increases
F owing to the more distorted system. However, hexyl in-
corporation negatively affects F since the radiationless
channels are enhanced. As for chain elongation, F increases
as commonly found in homogeneous oligothiophenes due to
a partial decrease in intersystem crossing in longer oligo-
mers.[37]
Figure 4. Front view of a) HF/6-31G**-optimized geometry of Hex2-
TTPTT-p(HP) in the ground electronic state (S0) and b) HF/RCIS/6-
31G**-optimized geometry in the first singlet excited state (S1).
Electrochemical data: Figure 5 displays the cyclic voltammo-
grams of the molecules and Table 1 also summarizes the
redox potentials. Stable and reversible oxidations are ob-
the optimized geometries in the
Table 1. Photophysical[a] and electrochemical[b] data for oligomers.
absorbing S0 and in the emitting
S1 singlet states), a phenomen-
on related to the bonding/anti-
bonding pattern between the
successive five-membered rings
in the HOMO and LUMO or-
bitals.
Oligomer
labs
lem
FF
tF
kF
kNR
Eo
Eo
2
1
[d]
3T
TPT
354
345
339
366
352
345
428
418
404
411, 431
397, 414
400, 417
423, 446
406, 425
414, 428
496, 525
487, 511
492, 519
0.07
0.18
0.25
0.09
0.10
0.25
0.22
0.43
0.17
0.18
0.44
0.61
0.20
0.33
0.67
0.81
1.22
1.18
0.39
0.41
0.41
0.45
0.30
0.37
0.27
0.35
0.14
5.17
1.86
1.23
4.55
2.73
1.12
0.96
0.47
0.70
1.15[c]
0.72[c]
0.74[c]
0.92
0.55
0.53
0.78
0.48
0.52
–
–
–
[d]
[d]
TPT-(pHP)
Hex2-3T
Hex2-TPT
Hex2-TPT-(pHP)
Hex2-5T
1.37[d]
1.18[c]
1.14[c]
1.01
The Stokes shifts (evaluated
here as the energy difference in
eV between the maximum in
absorption and the first peak in
emission) for the series 3T!
TPT!TPT-p(HP) evolve as
0.48!0.47!0.56 eV. Two fac-
Hex2-TTPTT
Hex2-TTPTT-(pHP)
0.89
0.86
[a] Wavelength (l [nm]) of the absorption and emission maxima. Radiative lifetimes (tF) in nanoseconds [ns].
Kinetic constants in nsꢀ1. [b] Potentials in Volts [V] vs. Ag/AgCl in 0.1m TBAPF6/CH2Cl2 solution. [c] Irrever-
sible process; Epa value provided. [d] Additional oxidation processes not investigated.
tors are expected to account for these displacements: 1) the
overall electronic reorganization upon excitation, which is
distinct in the two molecules given the different contribution
served only for the hexyl-capped molecules since the non-
capped oligomers undergo radical oligomerization/polymeri-
zation. The hexyl-capped compounds display reversible one-
electron oxidations generating the radical cation species.
The electrochemical formation of these monooxidized spe-
cies is significantly displaced to lower potentials on going
from Hex2–3T to the pyrrole-containing oligomers. Within
the pyrrole trimers, the oxidation potential of Hex2-TPT-
p(HP) is 20 mV lower than that of Hex2-TPT. It is possible
to relate these one-electron oxidation potentials with the ab-
solute HOMO energies according to Koopmansꢄ theorem.[38]
For the series Me2–3T!Me2-TPT!Me2-TPT-p(HP), the
HOMO evolves as ꢀ4.98!ꢀ4.72!ꢀ4.70 eV, in good agree-
ment with the trend in the measured potentials of 0.92!
0.55!0.53 V. It is pertinent to mention that in nonconjugat-
ed amine and thio compounds, the nitrogen-containing com-
pounds always show easier oxidations, a chemical feature
exported to the pyrrole and thiophene derivatives, which
means that oxidation potentials are not only dictated by the
p-valence electrons. This would explain that a more aromat-
ic TPT compound shows more accessible oxidations than
the more conjugated 3T. The oxidation potentials to gener-
ate some of the dications also deviate from the behavior ex-
pected simply from the p-valence electrons, as we will see
next.
ꢀ
to the LUMO of N R compared to S and 2) the planariza-
tion of the molecule in the excited state. In 3T and TPT,
these two factors appear to be similar (although one has to
remain cautious due to the uncertainty in the Stokes shift
estimates in spectra with relatively low vibronic resolution).
The same happens in Hex-5T (0.40 eV) and Hex-TTPTT
(0.42 eV). For TPT-p(HP), the deviation from planarity in
the ground state is larger due to molecular congestion in-
duced by the phenyl group; thus, the vibrational dissipation
of energy to reach the excited-state planarization, and con-
sequently the Stokes shift, are greater. Inclusion of hexyl
groups at the terminal a-positions minimally alters the
Stokes shift (i.e., 0.47 in TPT and 0.47 eV in Hex2-TPT), in
agreement with the p–p* character of the transitions. Final-
ly, chain elongation leads to an evolution from a shift of
0.60 in Hex2-TPT-p(HP) to 0.55 eV in Hex2-TTPTT-p(HP),
in agreement with the latter being a more conjugated
system.
Fluorescence quantum yields were measured for these
molecules and are summarized in Table 1 together with the
lifetimes and kinetic constants for the radiative and nonra-
diative processes. Lifetimes are in the nanosecond scale,
which indicates the p–p* character of the radiative electron-
ic transitions. Except for Hex2-TTPTT-(pHP), the quantum
yields increase when switching from thiophene to pyrrole, in
On going from trimers to pentamers, the first oxidation
potentials decrease at the same time that a second reversible
one-electron oxidation wave appears due to the formation
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Comparison of Thiophene–Pyrrole Oligomers with Oligothiophenes
FULL PAPER
DFT/B3LYP/6-31G** calcula-
tion. Figure 6 also compares the
experimental and theoretical
ꢀ
C C bond distances; the results
are consistent with the tenden-
cy of the DFT methodology to
ꢀ
provide somewhat too long C
C bond lengths in conjugated
molecules, though in general
the agreement is good. The
packing
in
Hex2-TPT
(Figure 7) consists of layers of
molecules in parallel planes
that overlap with the hexyl
groups of each molecule over
the ring systems of the next.
These pairs of interacting mole-
cules then repeat down the
layer in a ladder motif. This
layer of interacting molecules
in parallel planes then interacts
with a symmetry-related layer
on each side in an edge-to-face
Figure 5. Left: Cyclic voltammograms for Hex2-3T (top), Hex2-TPT (middle) and Hex2-TPT-(pHP) (bottom).
Right: Cyclic voltammograms for Hex2-5T (top), Hex2-TTPTT (middle), and Hex2-TTPTT-(pHP) (bottom).
of a dication state. These two
processes are again more ener-
getically accessible in the pyr-
role pentamers than in Hex2–
5T. However, the inclusion of
the hexyl-phenyl group in
Hex2-TTPTT-p(HP)
has
a
rather different influence in the
formation of this bipolaron-like
state: whereas in the radical
cations it promotes a stabiliza-
tion effect (i.e. from [Hex2-
TTPTT]·+ to [Hex2-TTPTT-
Figure 6. Comparison between the experimental X-ray diffraction molecular structure (top) and the DFT/
B3LYP/6-31G**-optimized structure (bottom) for Hex2-TPT (Me2-TPT in the calculations; distances in ꢅ).
p(HP)] +), the aryl group destabilizes the second electron
extraction by 40 mV from [Hex2-TTPTT]2+ to [Hex2-
TTPTT-p(HP)]2+ (this is also seen in the irreversible oxida-
tion of the trimer to the dication). A tentative explanation
could be that the more demanding effect of the doubly
charged molecule over the nitrogen atom results in an ex-
ceeding electrodeficient situation in N that competes with
the s-electron inductive effect towards the aryl group.
C
Solid-state properties: The crystal structure of Hex2-TPT
has been resolved by X-ray diffraction of a crystal grown
from a CH2Cl2/hexane solution. Noteworthy is the fact that
the crystal structure of Hex2–3T has not been reported yet.
Figure 6 illustrates the deviation from planarity of the
three-ring system in both the experimental and calculated
structures. In the experimental structure, the pyrrole ring is
somewhat out of plane from the thiophene rings. A least-
squares plane made from the pyrrole ring is at an angle of
8.98 to a least-squares plane made from the thiophene rings.
This feature is also present in the structure predicted by the
Figure 7. Packing in Hex2-TPT: View of the interaction of the hexyl
chain with aromatic backbone between two molecules that are in parallel
planes (top) and a view of the edge-to-face interaction between mole-
cules (bottom).
fashion. As a result, p-stacking is not present in this struc-
ture.
Vibrational Raman spectra: Figure 8 compares the FT-
Raman spectrum of Hex2-TPT with the theoretical Raman
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T. M. Pappenfus, J. Casado, J. T. Lꢀpez Navarrete et al.
Scheme 3. DFT/B3LYP/6-31G**-optimized geometries of the trimer com-
pounds (methyl instead of hexyls). Only half the molecule is shown.
follows: 1) the more aromatic character of the nitrogen-
based five-membered ring reinforces the five-membered
structure, which thus requires more energy to carry out the
ring stretches and 2) the larger distortion of the molecular
backbone in the pyrrole derivative, which negatively affects
the molecular conjugation (we recall that an increase in con-
jugation is accompanied by a frequency downshift[39]). On
the other hand, frequencies in TPT-p(HP) are slightly
higher relative to TPT, which can be understood on the
basis of the larger oligomer distortion caused by this bulky
group (i.e., 128 in Scheme 3).
The Raman spectra also reflect the inclusion of the hexyl
groups at the a-positions. It is observed that the main
Raman frequencies move to higher values with a,aꢄ-alkyla-
tion. The case of the frequency behaviour is more complex
than the optical spectral trend (which can be accounted for
on the basis of a positive inductive effect alone) since there
are two possible effects acting over the vibrational force
field: 1) the same inductive effect that alters the geometries
and the frequencies (i.e., upshift) and 2) a mechanical effect
since the movement of the external C=C stretches is signifi-
cantly modified by attaching the more heavy hexyl groups
(relative to hydrogen in the uncapped molecules).[40]
Figure 8. Comparison between a) the theoretical DFT/B3LYP/6-31G**
spectrum of Me2-TPT and b) the experimental 1064 nm FT-Raman spec-
trum of Hex2-TPT.
spectrum of Me2-TPT. There is good agreement between
the experimental and theoretical data, which allows for an
accurate assignment of the main Raman lines to normal
modes, and an understanding of the changes in the spectra
on the basis of the molecular structures. The most intense
experimental Raman bands at 1558 and 1512 cmꢀ1 are re-
produced at 1552 and 1499 cmꢀ1, respectively, and can be as-
signed to the antisymmetric C=C stretching vibration of the
external thiophene rings and to the symmetric C=C stretch-
ing vibration of these rings, which is more delocalized
toward the central pyrrole ring.[39]
Figure 9 displays the FT-Raman spectra of the trimer
compounds, whereas Scheme 3 shows the DFT/B3LYP/6-
ꢀ
31G** optimized geometries. Substitution of S by N R pro-
duces a net displacement of the intense Raman bands to
higher frequencies, an effect that can be accounted for as
Figure 10 compares the FT-Raman spectra of the penta-
ACHTUNGTRENmNGNU ers, in which two main trends are deduced: 1) the main
Raman lines are displaced to lower frequencies with respect
to the homologous trimers, by
ꢀ11 cmꢀ1 from Hex2–3T!
Hex2–5T, ꢀ11 cmꢀ1 from Hex2-
TPT!Hex2-TTPTT,
and
ꢀ15 cmꢀ1
from Hex2-TPT-
p(HP) !Hex2-TTPTT-p(HP).
This feature is due to the better
conjugation in a larger p-elec-
tron system, a phenomenon re-
flected in the calculated CC
bond lengths and dihedral
angles in Hex2-TPT and in
Hex2-TTPTT that display
a
slight quinodization and plana-
rization, respectively, in the
pentamer due to conjugation
(see Figure S1 in the Support-
ing Information).
The introduction of the aryl
groups connected to the nitro-
gen atom induces a shortening
of the C=C pyrrole bond
Figure 9. Left: Solid state 1064 FT-Raman spectra of a) 3T, b) TPT, and c) TPT-p(HP). Right: Solid state 1064
FT-Raman spectra of a) Hex2–3T, b) Hex2-TPT, and c) Hex2-TPT-p(HP).
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Comparison of Thiophene–Pyrrole Oligomers with Oligothiophenes
FULL PAPER
charge carriers associated with hole mobility in such molec-
ular-based semiconductors. Investigation of the optical and
vibrational properties of these oxidized species will allow us
to find new insights on the molecular and electronic struc-
tures of the charged species, and to study in the cations the
changes imparted by thiophene!pyrrole substitution and
chain elongation. Chemical oxidation with FeCl3 has been
carried out in dichloromethane up to the radical cation and
the UV/Vis–NIR spectra are shown in Figure 11.
Figure 10. Left: Solid state 1064 FT-Raman spectra of a) Hex2–5T,
b) Hex2-TTPTT, and c) Hex2-TTPTT-p(HP). Right: As above for a) a
thiophene–phosphole trimer (see insert) and b) TPT-p(HP). The insert
shows the chemical structures denoting in bold the case of cyclic and
linear conjugation discussed in the text.
lengths (Scheme 3) from 1.393 in Hex2-TTPTT to 1.391 ꢅ
in Hex2-TTPTT-p(HP) and also in the thiophene bond
lengths due to the molecular distortion and p-electron de-
localization disruption, all together inducing the frequency
upshift.
We conclude this section by addressing the aromaticity
versus conjugation question and providing another perspec-
tive related to the substitution of the nitrogen atom with
phosphorus. The nonaromatic character of phosphole pro-
vides us with the opportunity of comparing the impact in
the vibrational spectra of the replacement of an aromatic
five-membered ring (i.e., pyrrole) with a nonaromatic one
(i.e., phosphole).[41] The spectra of the thiophene–phosphole
trimer in Figure 10 (see the insert for the complete chemical
structure) and that of TPT-p(HP) are compared.[41] Interest-
ingly, the Raman line most associated with the stretching
movement of the central ring appears at similar frequencies
but reflects two competing effects with three characteristics:
1) in the case of TPT-p(HP), this frequency is related to its
aromatic character, which is also referred to as ring or cyclic
conjugation;[41] 2) in the case of the phosphole derivative,
the frequency position is due to the increased diene charac-
ter (i.e., double bonds strengthened, nonaromaticity) of the
phosphole moiety; and 3) to distinguish the two effects in
the spectra, the key bands are those at a higher frequency
due to the thiophenes: in the case of pyrrole, linear inter-
ring conjugation is impeded and the thiophenes are “isolat-
ed” showing the Raman band at 1548 cmꢀ1. Instead, for the
phosphole system, the diene character (resulting from its
nonaromaticity) facilitates interthiophene electronic com-
munication causing the typical frequency downshift of the
Raman bands, down to 1512 cmꢀ1 in the thiophene–phos-
Figure 11. UV/Vis-NIR spectra of the radical cations of a) Hex2–3T,
b) Hex2-TPT, c) Hex2-TPT-p(HP), d) Hex2–5T, e) Hex2-TTPTT, and
f) Hex2-TTPTT-p(HP) obtained after oxidation with FeCl3 in CH2Cl2.
The spectra of the radical cations show the typical two-
band pattern corresponding to the intragap transitions gen-
erated after one-electron extraction from the HOMO
(SOMO!LUMO and HOMO!SOMO transitions). On
the basis of previous works on these oxidized species,[43] we
assign the absorption band at lowest energies, for example,
at 772 nm in Hex2-TPT, to the HOMO!SOMO one-elec-
tron transition, and the intense absorption at 513 nm in
Hex2-TPT to the SOMO!LUMO one-electron excitation.
Both bands are of p–p* character and polarized along the
ꢀ
main molecular axis defined by the alternating C=C/C C
bond sequence, thus resulting in intense electronic absorp-
tions the wavelength position of which strongly depends on
the conjugation extension or efficiency. Again, as in the neu-
ꢀ
tral molecules, the S!N R replacement in the trimer and
pentamer series produces a blueshift of these two absorp-
tions. The lesser contributions from the strong electron-do-
nating effect of sulfur, much more accentuated in the case
of the electron-deficient cations relative to nitrogen, causes
the blueshift in the mixed oligomers. The displacement in
the two thienyl–pyrrole compounds is very small, which is in
contrast with the behavior of the neutral molecules. Oxida-
tion causes the quinoidization of the conjugated path and its
planarization thus minimizing the distorting effect of the
phenyl group attached to the nitrogen atoms.
ACHTUNGTRENNUNGphole trimer, which indicates more effective linear interan-
nular conjugation.[42]
Oxidized species: This section is devoted to the spectroscop-
ic properties of the oxidized species generated in the elec-
trochemical processes, which would correspond to the main
There are a couple of distinctive features in the absorp-
tion spectrum of the Hex2-TTPTT radical cation at 600 and
943 nm, both displaced to the blue with respect to the main
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6873

T. M. Pappenfus, J. Casado, J. T. Lꢀpez Navarrete et al.
absorptions at 662 and 1125 nm. These blueshifted bands
are due to the p-dimer species formed by through-space p-
bonding with the two radical cations situated in a parallel,
face-to-face configuration.[44] These species have been well
characterized in homogeneous oligothiophenes and are in-
teresting species to address the question of aggregation and
intermolecular charge migration in crystals and solid stacks
of p-conjugated molecules. In the trimers, no evidence of
the formation of p-dimers is observed; this is likely due to
the presence of the hexyl groups, which hinders a close in-
teraction between the p-clouds, and to the low polarity of
the solvent. No clear bands associated to p-dimers are re-
corded in Hex2–5T, which highlights, for this particular
issue, the effect of the inclusion of the pyrrole ring. Owing
to the requirement of close contact between the conjugated
chains the existence of the hexyl–aryl groups in Hex2-
TTPTT-p(HP) clearly limits the formation of these species
due to steric congestion.
cal cation formation[45] in contrast with the ꢁ100 cmꢀ1 shift
here (i.e., 1505!1411 cmꢀ1) revealing the strong skeletal
quinoidization of the CC conjugated chain (most of the ex-
ꢀ
tracted electron is released from the C=C/C C path) to bal-
ance the lower electron-donating effect of nitrogen relative
to sulfur.
Conclusion
The investigation of thiophene–pyrrole mixed oligomers and
the comparison of their properties to those of the analogous
oligothiophenes had been scarcely explored. In this work,
we have synthesized a new family of pyrrole–thiophene olig-
omers with different numbers of conjugation units and with
different lateral substitutions of the main conjugated back-
bone, either with hexyl groups (better processability and
solid-state properties) or hexyl–phenyl groups (chemical ro-
bustness and stability). We have presented a thorough study
of their electronic and molecular properties to better under-
stand the role of a more aromatic ring (i.e., pyrrole) in con-
nection with moderate aromatic rings (i.e., thiophenes). In
this context, a series of optical (absorption and emission),
electrochemical, solid-state, and vibrational studies have
been used to provide the physicochemical data required for
the discussion of their electro-optical features. The pyrrole
inclusion improves both the oxidation capacity and the lumi-
nescence properties relative to their homogeneous oligothio-
phenes. On the other hand, the addition of hexyl and hexyl–
phenyl groups might improve the processability and chemi-
cal stability, two key requirements for the successful imple-
mentation in useful devices. The inclusion of five-membered
rings of a different nature causes the molecular p-conjugat-
ed backbone to be slightly distorted, whereas conserving
good redox and luminescence responses. Overall, this paper
sheds light on the potential of mixed thiophene–pyrrole olig-
omers for use in organic electronics.
The quinoidal sequence in these oxidized conjugated olig-
omers can also be investigated with Raman spectroscopy. To
this end, Figure 12 shows the Raman spectra, as a represen-
Figure 12. 1064 nm FT-Raman spectra of the neutral (a) and radical cat-
ACHTUNGTRENNUNGion (b) of Hex2-TPT-p(HP). The solvent band is denoted in grey.
tative example, of Hex2-TPT-p(HP) in its neutral and radi-
cal cation states. The neutral bands in dichloromethane at
1558 and 1505 cmꢀ1 evolve to lower frequencies at 1539 and
1411 cmꢀ1 in the oxidized species. Following the correlation
and vibrational description above, these two bands corre-
spond to ring CC stretching modes located at the terminal
thiophenes and in the central pyrrole, respectively. The fre-
quency downshift is due to the ring relaxation caused by
Acknowledgements
The present work was supported in part by the Direcciꢀn General de En-
seÇanza Superior (DGES, MEC, Spain) through research project
CTQ2006-14987-C02-01 and CT2007-65683. The authors are also indebt-
ed to Junta de Andalucꢆa for the project FQM1678/2006. J.C. is grateful
to the MEC of Spain for I3 professorship. M.M.O is indebted to the
MEC for a predoctoral fellowship. T.M.P. and J.H.M. acknowledge the
Morris Academic Partnership Program funded by the University of Min-
nesota, Morris (UMM) Deanꢄs Office. T.M.P. acknowledges UMM Facul-
ty Research Enhancement Funds supported by the University of Minne-
sota Office of the Vice President for Research and the UMM Division of
Science and Mathematics for financial assistance. K.A.M. acknowledges
the University of Minnesota X-ray lab in the Department of Chemistry.
K.M.S. acknowledges the University of Minnesota Undergraduate Re-
search Opportunities Program. D.A.S.F. and J.L.B. acknowledge primary
support from the MRSEC Program of the National Science Foundation
under Award DMR-0819885.
ꢀ
quinoidization, which reinforces the inter-ring C C bonds at
the expense of the C=C ring bonds. The ꢀ94 cmꢀ1 displace-
ment of the pyrrole band indicates that the oxidation affects
mainly the center of the molecules and the effect relaxes
toward the two thiophene ends in agreement with the role
played by the pyrrole group in the electrochemical and UV/
Vis–NIR data described above.
Typical downshifts of 40–60 cmꢀ1 are observed in the
Raman spectra of homogeneous oligothiophenes upon radi-
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Comparison of Thiophene–Pyrrole Oligomers with Oligothiophenes
FULL PAPER
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