Electronic, redox and charge transport properties of an unusual hybrid structure: A bis(septithiophene) bridged by a fused tetrathiafulvalene (TTF)

Article abstract of DOI:10.1039/c0jm02293d

A hybrid tetrathiafulvalene-oligothiophene compound has been synthesised, in which the fulvalene unit is fused on both sides to an end-capped septithiophene oligomer. The compound (1) has been studied by cyclic voltammetry, UV-vis spectroelectrochemistry and X-ray crystallography. The properties of this material are compared to the half-unit (9), which lacks the TTF core and contains only one septithiophene chain. In the case of the larger molecule, there are multiple and complex redox processes leading to the loss of 6-8 electrons per molecule. Charge generation layer time-of-flight measurements give maximum hole mobilities of ca. 1 × 10^-5 cm² V^-1 s^-1.

Full text of DOI:10.1039/c0jm02293d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Electronic, redox and charge transport properties of an unusual hybrid
structure: a bis(septithiophene) bridged by a fused tetrathiafulvalene (TTF)†‡
a
a
a
a
b
*
ꢀ
Iain A. Wright, Peter J. Skabara, John C. Forgie, Alexander L. Kanibolotsky, Blanca Gonzalez,x
Simon J. Coles,c Salvatore Gambinod and Ifor D. W. Samueld
Received 15th July 2010, Accepted 23rd September 2010
DOI: 10.1039/c0jm02293d
A hybrid tetrathiafulvalene–oligothiophene compound has been synthesised, in which the fulvalene
unit is fused on both sides to an end-capped septithiophene oligomer. The compound (1) has been
studied by cyclic voltammetry, UV-vis spectroelectrochemistry and X-ray crystallography. The
properties of this material are compared to the half-unit (9), which lacks the TTF core and contains
only one septithiophene chain. In the case of the larger molecule, there are multiple and complex redox
processes leading to the loss of 6–8 electrons per molecule. Charge generation layer time-of-flight
measurements give maximum hole mobilities of ca. 1 ꢀ 10ꢁ5 cm2 Vꢁ1 sꢁ1
.
electroactivity of the polymer or participate in a hybrid redox
state.22 End-capped oligothiophenes are particularly attractive
materials to study as they usually give well-defined redox
signatures and stable charged intermediates,23,31 allowing for
more precise analysis of the complex electrochemical behaviour
of these hybrid systems.
Introduction
By incorporating electron rich or deficient units into conjugated
polymers and oligomers we have the potential to manipulate the
HOMO and/or LUMO energy levels of the material, which is key
to the development of semiconductor devices based upon poly-
mer materials.1–3 Tetrathiafulvalene (TTF) derivatives have been
the focus of ongoing research towards the development of new
functional materials, largely due to its reliable redox behaviour
and the metallic conductivity or superconductivity observed in its
charge-transfer salts.4–9 Oligothiophenes are semiconducting
species which have been intensively studied towards the genera-
tion of many new organic electronic devices, including light
emitting diodes, field effect transistors and solar cells.2,10–14
TTF derivatives have been studied as components in field-
effect transistors and there are several examples of materials with
high mobilities, including thiophene–TTF hybrids.15–19 In recent
years we have been investigating oligothiophenes and poly-
thiophenes that possess TTFs20–25 and other redox active
systems26–30 fused directly to the backbone of the conjugated
chain. Our interests arise from the complex redox behaviour
resulting from the merging of two strong redox-active compo-
nents. From these studies, we have found that, depending on the
nature of the polymer, the fulvalene unit can either dominate the
In our previous work, the molecules developed bore TTF units
stemming from the polythiophene chain and the signature of the
TTF redox characteristics was clearly evident in electrochemical
studies. In all these cases, the TTF system was fused directly to
the conjugated chain through only one of the 1,3-dithiole rings,
thereby retaining the signature oxidation of the second dithiole
unit as an independent redox-active moiety. Here we report the
synthesis and electrochemical study of a new hybrid molecule,
featuring a TTF core fused at both ends of the molecule between
two methyl end-capped septithiophene (7T) chains (1). The 7T
chains also bear hexyl groups on the 3-position of the terminal
thiophene rings to provide solubility.
aWestCHEM, Department of Pure and Applied Chemistry, University of
Strathclyde, Glasgow, G1 1XL, UK. E-mail: peter.skabara@strath.ac.uk
b
ꢀ
ꢀ
Departamento de Quımica Inorganica, Facultad de Ciencias, Universidad
Autoꢀnoma de Madrid, Cantoblanco, 28049 Madrid, Espan~a
cSchool of Chemistry, University of Southampton, Highfield, Southampton,
SO17 1BJ, UK
dOrganic Semiconductor Centre, SUPA, School of Physics & Astronomy,
University of St Andrews, St Andrews, KY16 9SS, UK
† Electronic supplementary information (ESI) available: Full
experimental details for the synthesis of all new compounds and
crystallographic data. CCDC reference numbers 783815, 783816. For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0jm02293d
‡ This paper is part of a Journal of Materials Chemistry themed issue in
celebration of the 70th birthday of Professor Fred Wudl.
ꢀ
ꢀ
x Present address: Departamento de Quımica Inorganica
y
Bioinorganica, Facultad de Farmacia, Universidad Complutense de
ꢀ
Madrid, Ciudad Universitaria, Madrid 28040, Espana.
~
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and 9 (Fig. 1 and Table 1). The oxidation of 9 shows three
sequential oxidation waves. The first two occur at half-wave
potentials of +0.44 and +0.64 V and these values are quite close
for the oxidation to the radical cation and to the dication
intermediates, respectively. The third wave appears at +1.02 V,
affording a radical trication. All three oxidation processes are
reversible or quasi-reversible. The electrochemical stability is
a consequence of the a-methyl groups on the terminal thio-
phenes, which impede any coupling reactions to generate
a polymeric species. In comparison to end-capped sexithiophene
derivatives that we have made previously (10 and 11, in which
only dications were accessible),31 the extended septithiophene 9
allows access to an additional redox state.
Results and discussion
Synthesis
The strategy employed for the synthesis of 1 is shown in Scheme
1. 2-Bromo-3-hexyl-5-methylthiophene (2) can be made by
treating 3-hexylthiophene first with NBS to give 2-bromo-3-
hexylthiophene (98% yield),32 followed by methylation using
LDA/iodomethane to give 2 in 91% yield. This procedure is
different to the one reported by Sato and Kamine,33 who per-
formed methylation of 3-hexylthiophene before bromination, to
obtain an overall yield of 36% for compound 2. Negishi coupling
of 2 with 2-thienylzinc bromide affords bithiophene 3 in 67%
yield. Subsequent bromination to generate 4 is achieved easily
and efficiently with NBS (96% yield), prior to Kumada coupling
with thiophen-2-yl magnesium bromide to obtain 5 in 88% yield.
Carbonylation with n-BuLi/DMF generates the desired terthio-
phene aldehyde 6 in 83% yield. Two-fold sequential lithiation of
vinylene trithiocarbonate, followed by the addition of 6, gave the
corresponding diol intermediate, which was oxidised immedi-
ately with MnO2 to afford the 1,3-dithiole-2-thione 7 with an
overall yield of 60%. Transchalcogenation of the 1,3-dithiole-2-
thione to the carbonyl derivative (8) was achieved in almost
quantitative yield with mercuric acetate. Paal–Knorr thiophene
synthesis using P2S5 was applied to yield the TTF precursor 9
(69% yield). Finally, homocoupling of 9 with triethylphosphite at
125ꢂC afforded the TTF compound 1 in 39% yield.
Though the presence of the TTF core in 1 may lead us to
expect two discernible reversible waves in the corresponding
cyclic voltammogram, the oxidation of 1 in fact shows a complex
series of redox processes over a wide potential range. The data
are given in Table 1, but it is difficult to explain and interpret the
results with a high level of confidence. It is reasonable to assume
that the additional peak for 1, seen at the half-wave potential of
Electrochemistry
In order to investigate the electrochemical properties and
consequences of fusing TTF directly between the 7T chains,
cyclic voltammetry experiments were performed on compounds 1
Fig. 1 Cyclic voltammograms of TTF derivative 1 (solid line) and half-
unit 9 (dashed line) for oxidation (top) and reduction (bottom) in
dichloromethane solution, using a carbon working electrode, Ag wire
pseudo-reference electrode, TBAPF6 (0.1 M) as the supporting electro-
lyte, substrate concentration 10ꢁ4 M, scan rate 100 mV sꢁ1. The data are
referenced to the ferrocene/ferrocenium redox couple.
Scheme 1 Synthesis of the TTF–bis(septithiophene) 1 and its 1,3-
dithiole-2-one precursor 9.
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Table 1 Electronic absorption and electrochemical data
Compound 1
Compound 9
lmax/nm (CH2Cl2)
HOMO–LUMO gapa/eV
E1ox/V (CH2Cl2)
E2ox/V (CH2Cl2)
E3ox/V (CH2Cl2)
Ered/V (CH2Cl2)
HOMO/eVb
461
2.20
467
2.29
+0.26/+0.23
+0.66/+0.49d
+0.97/+0.94d
ꢁ1.98e
+0.47/0.41
+0.68/+0.59d
+1.05/+0.98d
ꢁ1.86e
ꢁ4.95
ꢁ5.18
LUMO/eVb
ꢁ3.00
ꢁ3.08
HOMO–LUMO gapc/eV
1.95
2.10
a
Optical HOMO–LUMO gap calculated from the onset of the longest
wavelength absorption maximum. HOMO and LUMO levels are
calculated from the onset of the first peak of the corresponding redox
b
wave and are referenced to ferrocene which has a HOMO of ꢁ4.8 eV.
c
e
d
Electrochemical HOMO–LUMO gap.
Irreversible peak.
Quasi-reversible peak.
Fig. 2 Absorption spectra of compounds 1 (solid line) and 9 (dashed
+0.25 V, suggests the oxidation of the TTF unit. However,
further studies cast a doubt on such an assignment (vide infra). In
general, differential pulse voltammetry normally provides better
resolution of redox processes, but for compound 1 no further
insight into the complex nature of the redox chemistry could be
made. For the purpose of a more accurate electrochemical
analysis, we are currently in the process of synthesising quin-
quethiophene and terthiophene analogues of 1, since these
compounds will serve to simplify the redox characteristics of the
hybrid system sequentially, down to the TTF core.34
line) in dichloromethane solution.
wavelengths is due to the electronic effects of the alkyl and sulfur
substituents in compounds 1 and 9. It is worth noting that the
fused TTF unit does not provide a conjugated link between the
oligothiophene chains in 1. However, the TTF and 1,3-dithiole-
2-one units in 1 and 9 are responsible for the shortest wavelength
peaks at 353 and 338 nm, respectively.24 The onset of the longest
wavelength absorption edge gives the optical HOMO–LUMO
gap of 1 and 9 at 2.20 and 2.29 eV, respectively.
The solid state spectrum of compound 9 (Fig. 3) is very similar
to the end-capped sexithiophenes 10 and 11.31 The absorption
band is red-shifted compared to the solution state spectrum, with
peaks at 495 and 561 nm (lmax for 10 ¼ 481, 510 and 551 nm). As
in 10 and 11, the tail of the absorption band for compound 9
extends over several hundred nm. Solid state UV-vis spec-
troelectrochemistry (SEC) of 9 also follows a similar trend to
those seen for compounds 10 and 11. A broad band forms
between 670 and 930 nm at potentials higher than +0.5 V,
together with a band >1100 nm. Throughout the experiment, the
main p–p* band remains unchanged, but the broad band that
evolves on oxidation superimposes the original band. Notice-
ably, there is a trough in the spectrum of the oxidised compound
at about 930 nm and this is pertinent to the SEC plot for
compound 1.
Cyclic voltammogramms depicting the reduction of 1 and 9
show irreversible peaks at ꢁ2.10 and ꢁ1.80 V respectively, due to
the formation of radical anions in the septithiophene chains. The
small peaks at ca. ꢁ1.3 V are due to oxygen present in the elec-
trolytic solution.
The electrochemical HOMO–LUMO gaps of the materials
were obtained from the difference in the onsets of the first
oxidation and reduction peaks (Table 1). Using data referenced
to the ferrocene/ferrocenium redox couple, HOMO and LUMO
energies were calculated by subtracting the onsets from the
HOMO of ferrocene, which has a known value of ꢁ4.8 eV. The
electrochemical HOMO–LUMO gaps of 1 and 9 were 1.95 and
2.10 eV, respectively, and are a little lower than the optical data
(see the following section).
The absorption spectrum for the film of compound 1 (Fig. 3)
gives a peak at 496 nm, a bathochromic shift of 29 nm compared
to the solution state spectrum. Upon oxidation, a small band
appears at 933 nm at ca. +0.3 V. This peak is typical for the
oxidation of a TTF species to the radical cation and can be
assigned to intermolecular charge transfer between TTFc+
dimers.24 The peak is quickly lost upon further oxidation and is
eclipsed by a band with a peak maximum at 597 nm and a second
band at >1100 nm. The trough observed for compound 9 is not
evident in 1 due to the signature of oxidised TTF over this region
of the spectrum. Furthermore, it is interesting to note that the
p–p* band for the neutral compound diminished upon oxidation
and that there is a clear isosbestic point at 521 nm. For both
compounds, the films dissolve into solution at higher potentials,
due to the solubility of the multi-charged states. These results, in
combination with the cyclic voltammetry data, indicate that the
first site of oxidation in 1 involves the TTF unit.
Spectroelectrochemistry
The absorption spectra of compounds 1 and 9 (Fig. 2) demon-
strate peak maxima at 461 and 467 nm, respectively, due to the
p–p* transitions of the individual septithiophene chains. These
values are slightly red-shifted compared to an unsubstituted
cruciform bis(septithiophene) reported by Scherf et al. (451 nm in
chloroform solution).35 This difference in absorption
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compound 1 is shown in Fig. 4a. The asymmetric unit is defined
by the ring systems labelled A–H and the molecule has an
inversion centre around the central double bond of the TTF unit.
The dithienoTTF fragment is essentially planar, but there is
a significant twisting in one end of the septithiophene chain, most
likely due to packing forces. The dihedral angle between rings A
and B is 33ꢂ and the two thiophenes adopt a syn conformation.
Conjugation is weakest in this portion of the molecule and the
dihedral angles between rings B–C and C–D are 11ꢂ and 16ꢂ,
respectively. In contrast, the dihedral angles between thiophenes
D–G are in the range 4–10ꢂ. The sexithiophene fragment repre-
sented by B–G adopts an anti conformation. The stacked
packing structure of compound 1 is shown in Fig. 4b and the
overlap between chains within each stack, identifying the closest
interactions, is depicted in Fig. 4c. The angle between the two
planes of rings D and F is 5.79(12)ꢂ and the distance between
ꢁ
these planes is 3.738(2) A. There is a larger angle between the
planes of thiophenes C and G (18.36(12)ꢂ), but the two sulfur
atoms of the corresponding rings show a weak non-covalent
ꢁ
interaction with a S/S distance of 3.811(1) A. Given the greater
degree of twist in A and B, it is perhaps not surprising that these
rings do not participate in the stacking mode of compound 1.
Crystals of compound 9 were obtained from a hot dichloro-
methane/hexane solution. Once again, the septithiophene adopts
an all-anti conformation apart from one terminal thiophene ring
(Fig. 5a). The range of dihedral angles between the anti-thio-
phenes is 6–19ꢂ, and the angle between the two syn thiophenes is
26ꢂ. The molecule packs in a stacked herringbone pattern
(Fig. 5b). Within the stacks, the 1,3-dithiole ring is associated
with ring-over-bond overlap with one adjacent molecule (inter-
ꢁ
planar distance of 3.57 A), and p–p overlap with the terminal syn
thiophene ring of the second molecule (interplanar distance of
ꢁ
3.38 A, Fig. 5c).
Fig. 3 Solid state UV-vis spectroelectrochemistry of thin films of the
TTF half unit 9 (top) and fused TTF system 1 (bottom). Thin films were
deposited onto ITO slides via drop-casting. Measurements were per-
formed with a Pt counter electrode and a pseudo-Ag reference in
monomer free acetonitrile solution containing TBAPF6 (0.1 M) as the
supporting electrolyte.
DFT calculations
Density functional theory was applied to 1 to gain a better
understanding of the electronic energy levels within the
compound. The calculations were performed with Spartan 08,
using the B3LYP hybrid functional with a 6-31G* basis set.
Firstly, a single point energy calculation was performed on the
structure obtained by X-ray analysis, with the hexyl groups
replaced by methyl groups. Secondly, the terminal bithiophene
units with the syn conformation were twisted to an anti orien-
tation and this structure was allowed to relax to a local
minimum. The calculated HOMO–LUMO gap for the all-anti
conformer (2.49 eV) was found to be closer to the experimental
HOMO–LUMO gap (Table 1), than the structure obtained from
crystallography (2.70 eV). Also, the all-anti conformer was lower
in energy, showing that this is the preferred geometry in the gas
phase. The orbitals of 1 as the all-anti conformer are depicted in
Fig. 6 and show that the HOMO is derived from the oligothio-
phene chain. The TTF is involved with the HOMO-2 orbital, but
this energy level is only 0.17 eV deeper than the HOMO. It is
entirely feasible that the first oxidation state of this material
involves both components and a more in-depth computational
study will be performed to help elucidate the complex redox
chemistry of this material.
Although the solution and solid state absorption spectra did
not show any signs of doping, we obtained the EPR spectra for
compounds 1 and 9 to investigate the presence of radicals. In
both cases, we obtained peaks at g ¼ 2.0046 (compound 1) and
2.0050 (compound 9), indicating that the materials carry residual
radical cations. In comparison to these values, thiophene radicals
give values in the region of 2.002–2.003, whilst TTF radical
cations have g values of ca. 2.008.36 For hybrid TTF–poly-
thiophene materials, we have observed g ¼ 2.0078 for a polymer
in which the electroactivity is concentrated solely within the TTF
units,24 and values of around 2.005 for structures in which the
radical cation is delocalised over the thiophene–dithiole units.22
Our results for compounds 1 and 9 are commensurate with
hybrid electroactivity, but it is surprising that the dithiole-2-one
heterocycle is involved with the cation radical.
Crystallography
Crystals of the TTF derivative were grown by slow evaporation
from a chloroform–carbon disulfide solution and isolated as
a 1 : 1 carbon disulfide solvate. The molecular structure of
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Fig. 5 (a) Molecular structure of compound 9; (b) herringbone orien-
tation of stacks; (c) overlap of rings within the stacks.
Time-of-flight measurements
Fig. 4 (a) Molecular structure of compound 1; (b) stacking of molecules
through p-interactions; (c) overlap of rings viewed down a stack.
We have seen so far the electronic and redox properties of
compounds 1 and 9. We now move to the study of their charge
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Fig. 7 Hole photocurrent transient on linear (top) and log–log scale
(bottom) for a film of compound 1, at room temperature and for an
Fig. 6 Selected molecular orbitals of compound 1.
applied electric field E ¼ 2.5 ꢀ 105 V cmꢁ1
.
transport properties. We performed charge generation layer
time-of-flight (CGL-TOF) measurements on compounds 1 and 9.
For these measurements, solutions of the two compounds were
made to concentrations of about 40 mg mlꢁ1 in chlor-
obenzene : carbon disulfide (1 : 1). Films were spin-coated onto
an ITO substrate at speeds of 1000 rpm to obtain films around
200–230 nm thick. CGL-TOF experimental details have been
already reported in ref. 37.
the transit time (ttr ¼ 62 ms). The transit time corresponds to
a mobility of 1.4 ꢀ 10ꢁ6 cm2 Vꢁ1 sꢁ1
.
Compound 1 shows hole mobility values three times higher
than compound 9 under the same electric field and this is
consistent with improved molecular orbital overlap between the
larger oligothiophene–TTF molecules.
Several examples of TTF derivatives have been reported so far
in the literature with mobilities up to 3.6 cm2 Vꢁ1 sꢁ1 for OFET
single crystal devices.9,16,17,38 OFET single crystals are found to
give the highest mobilities largely due to their regular molecular
ordering that permits extensive intermolecular overlap to
occur.39 Most attention has been placed so far on improving the
mobility of OFETs based on TTF derivatives, but not so much
has been reported on charge transport measurements in the
direction perpendicular to the substrate. This is a concern
because some important optoelectronic devices, namely solar
cells and organic memory devices based on TTF-based materials,
have been reported already in the literature22,40 and they require
charge transport in the direction perpendicular to the substrate.
Thus an insight into mobility values would be useful for these
applications.
Fig. 7 shows the hole photocurrent transient on linear and log–
log scales for a film of compound 1, at room temperature and for
an applied electric field E ¼ 2.5 ꢀ 105 V cmꢁ1. The absence of
a clear plateau in the photocurrent transient on a linear scale is
indicative of highly dispersive charge transport behaviour. In
order to estimate the transit time (ttr) a log–log plot was neces-
sary, which allowed measurement of the transit time from the
change of slope of the photocurrent transient, ttr ¼ 18 ms. The
transit time corresponds to a mobility of 4.4 ꢀ 10ꢁ6 cm2 Vꢁ1 sꢁ1
.
For a direct comparison, CGL-TOF measurements were per-
formed on compound 9 under the same conditions, such as
temperature and electric field. Fig. 8 shows the hole photocurrent
transient on linear and log–log scales for a film of compound 9, at
room temperature and for an applied electric field E ¼ 2.5 ꢀ 105
V cmꢁ1. Again the absence of a clear plateau in the photocurrent
transient on a linear scale is indicative of highly dispersive charge
transport behaviour and a log–log plot was necessary to estimate
Our measurements give the hole mobility in the direction
perpendicular to the substrate, and we have studied its field
dependence, as shown in Fig. 9. As the electric field is increased
from 2.5 ꢀ 105 to 7.4 ꢀ 105 V cmꢁ1, the hole mobility increases
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direction normal to the substrate instead of parallel to the plane.
It is difficult to compare charge transport properties between the
two methods, because our compounds clearly differ in size and
complexity from the ones previously reported for FET mobility
measurements. Furthermore, as has been reported for P3HT,
TOF mobility values can be orders of magnitude lower than FET
mobilities.41–43
Our mobility values are consistent with others previously
reported using TOF or CELIV techniques on a related family of
thiophene-based conjugated materials.37,44,45 In this work, CGL-
TOF measurements have shown that it is possible to tune the
energy (HOMO–LUMO) levels without inhibiting the charge
transport properties of the thiophene oligomers. In fact, the
thiophene semiconducting properties are similar to previously
reported thiophene-based materials. The OFET characteristics
of these materials will be reported elsewhere.
Summary
A new hybrid TTF-thiophene material (1) has been synthesised
and its electrochemical and electronic properties have been
investigated and compared to the ‘non-TTF’ half-unit. As we
have seen in polymeric systems, a close and intimate association
of the TTF unit with the thiophene chain leads to hybrid elec-
troactivity and complex redox chemistry. The charge transport
properties of compounds 1 and 9 do not differ greatly, but the
HOMO energy level of the TTF compound is closer to vacuum
than the dithiole derivative as the former is a stronger electron
donor. Compound 1 has mobility a factor of three higher than
compound 9 (at a field of 2.5 ꢀ 105 V cmꢁ1) due to improved
molecular orbital overlap. The presence of the TTF unit in 1
affords additional redox states and it is proposed that up to eight
electrons are removed from this material within a narrow
potential window. Such highly redox-active materials are
promising for application in technologies such as solar cells,
batteries or capacitors.
Fig. 8 Hole photocurrent transient on linear (top) and log–log scale
(bottom) for a film of compound 9, at room temperature and for an
applied electric field E ¼ 2.5 ꢀ 105 V cmꢁ1
.
from 1.4 ꢀ 10ꢁ6 to 1.3 ꢀ 10ꢁ5 cm2 Vꢁ1 sꢁ1 for compound 9. For
compound 1 as the electric field is increased from 1 ꢀ 105 to 4 ꢀ
105 V cmꢁ1, the hole mobility increases from 1.4 ꢀ 10ꢁ6 to 1.1 ꢀ
10ꢁ5 cm2 Vꢁ1 sꢁ1. Fig. 9 shows that both materials exhibit similar
electric field mobility dependence.
Acknowledgements
We clearly see that mobility values are several orders of
magnitude less when charge transport is measured in the
ALK, IAW, SG and IDWS thank the EPSRC for funding. IAW
ꢀ
ꢀ
is grateful to Will Skene (Universite de Montreal) for time spent
in his laboratory during the course of this work, funded by the
JCEMolChem programme. PJS thanks Andrey Moiseev
(University of McGill) for conducting EPR measurements.
Dedicated to Fred Wudl for his enthusiasm and encourage-
ment towards complex and unusual electroactive organic struc-
tures.
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