Hexyl-substituted oligothiophenes with a central tetrafluorophenylene unit: Crystal engineering of planar structures for p-type organic semiconductors

Article abstract of DOI:10.1039/b417642a

Rigidification has been achieved in thiophene-tetrafluoro phenylene architectures through strong S...F and H...F intramolecular interactions; the resulting materials are promising candidates for p-type organic field effect transistors. The Royal Society o

Full text of DOI:10.1039/b417642a

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Hexyl-substituted oligothiophenes with a central tetrafluorophenylene
unit: crystal engineering of planar structures for p-type organic
semiconductors{
David J. Crouch,a Peter J. Skabara,*a Martin Heeney,b Iain McCulloch,b Simon J. Colesc and
Michael B. Hursthousec
Received (in Cambridge, UK) 23rd November 2004, Accepted 22nd December 2004
First published as an Advance Article on the web 24th January 2005
DOI: 10.1039/b417642a
Rigidification has been achieved in thiophene–tetrafluoro-
…
…
phenylene architectures through strong S F and H F intra-
molecular interactions; the resulting materials are promising
candidates for p-type organic field effect transistors.
Organic conjugated oligomers and polymers are an important class
of semiconductor, attracting great interest in applications such as
light emitting diodes,1 photovoltaics2 and thin film transistors
(TFTs).3 Achieving efficient device performance (approaching that
of amorphous silicon) is the major challenge for organic
semiconductors. For TFTs, the drive is to design and synthesise
materials that possess a low threshold voltage, a high on/off ratio,
high mobility and stability under ambient and operating condi-
tions. Several strategies have been investigated to promote the self-
assembly of conjugated systems in order to increase non-covalent
interactions between chains and maximise the efficiency of charge
transport in the bulk material. Exploiting liquid crystal properties4
or using inherently ‘flat’ molecules, such as pentacene, are good
examples in which co-facial packing is achieved. In the latter case,
ribbon or ladder type structures can suffer from poor solubility
and processability. Materials that give a planar conformation
through intrachain non-bonding interactions can provide a high
degree of co-planarity in the solid state.5 At the same time, the
materials possess good solubility due to the rotational freedom of
the main chain in solution.
Compounds 4–6 were prepared according to Scheme 1. The
reaction of 2-thienyllithium and 4-hexyl-2-thienyllithium with
hexafluorobenzene gave the triaryl derivatives (4a10 and 4b) in
65 and 66% yield, respectively. The procedure is noteworthy for
two reasons: (i) ease of synthesis compared to the Stille route used
for compounds 2 and 4a; (ii) the lithiation of 3-hexylthiophene is
regiospecific and results in the isolation of 4b as a single isomer.
Bromination of 4a and 4b was achieved in ca. 90% yield using
N-bromosuccinimide, whilst the pentaaryl derivatives, 5a–d, 6a
and 6b, were obtained by palladium catalysed coupling with the
corresponding stannylated thiophenes 9a–c (45–70% yield).
With respect to the above, fluorinated conjugated oligomers are
of recent topical interest.6–8 Katz and co-workers have found that
structures such as 1–39 are essentially p-type. Incorporating the
fluorinated units serves to lower both the LUMO and HOMO
energies of the materials and also facilitates the planarization of 2.
Lowering of the HOMO energy level is a prerequisite in obtaining
oxidative stability in thiophene containing polymers. Within this
theme, we present a series of compounds (4–6) bearing a central
tetrafluorophenylene unit. The non-covalent interactions of the
fluorine atoms with components on adjacent thiophene rings result
in highly planar structures in the solid state, together with excellent
solubility of the oligomers and corresponding polymers in
common organic solvents.
{ Electronic supplementary information (ESI) available: X-ray structure
of 5b, UV-vis absorption data and experimental conditions for X-ray
crystallography, cyclic voltammetry and device fabrication. See http://
www.rsc.org/suppdata/cc/b4/b417642a/
Scheme 1 Reagents and conditions: (i) n-BuLi, THF, 278 uC, then
hexafluorobenzene, reflux; (ii) NBS, CHCl3, AcOH; (iii) Pd(PPh3)4,
toluene.
*peter.skabara@manchester.ac.uk
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The structures of compounds 5b and 5d have been determined
by X-ray crystallography.{ In both cases, the common central unit
of the conjugated oligomers is held in good planarity by strong
… …
F and S F non-covalent intramolecular contacts. For 5d the
H
…
…
contact distances are 2.210 s (H F) and 2.704 s (S F) and the
maximum torsion angle between the thiophene and benzene ring is
1.99u (sum of the van der Waals radii: H + F 5 2.67 s, S + F 5
3.27 s).11 Compound 5d is more co-planar throughout the chain
than 5b, with maximum torsion angles between adjacent thiophene
rings at 4.08u for the former and 7.05u for 5b. Oligomer 5d forms
…
chains of molecules (Fig. 1) through intermolecular S S contacts
(3.408 s, van der Waals radii for S + S 5 3.60 s). The molecules
are arranged in sheets in which parallel chains are insulated by the
alkyl groups. In the third dimension, the pseudo-chains form
slipped stacks (ESI{); the mean co-facial distance between the
chains is 3.557 s. The interactions described above give the
material a unique two-dimensional assembly of strong intermole-
cular contacts.
Scheme 2
Compounds 5a–d and 6a display absorption maxima in the
range 378–394 nm (hexane), significantly higher than the values for
4a and 4b (310 and 332 nm, respectively, in CH2Cl2), due to an
increase in conjugation length. As expected, the absorption
maximum for 6b is at a shorter wavelength compared to those
of its direct analogues (353 nm, hexane), due to the steric effect of
the head–head hexyl thiophene units. In the solid state (evaporated
from hexane as a film), the absorption maximum of 5d is shifted
bathochromically by only 11 nm to 396 nm, indicating that the
planarity of the material is retained in solution state. Compound
5d was extended by two thiophene units via the reaction of 5d with
NBS to give 7 (60%), followed by Stille coupling with 4-hexyl-2-
tributylstannylthiophene to afford 8 (25%) (Scheme 2). The
absorption maximum of 8 is red-shifted to 405 nm (hexane) and
433 nm (film), indicative of an increase in conjugation length. In
order to assess the degree of self-assembly in polymeric analogues,
we prepared polymer 9 via Pd catalysed coupling of the dibromo
derivative 7 and 2,5-bis(tributylstannyl)thiophene. The resulting
polymer was extracted using Soxhlet procedures with a range of
solvents (firstly acetone, methanol, then dichloromethane and
chloroform). The chloroform fraction contained the highest
molecular weight, Mn 5 10.5 6 103; Mw 5 21.5 6 103, equating
to about 70 aryl units in the polymer chain. The absorption
maximum for this material was found to be 420 nm (chloroform),
only slightly higher than that of oligomer 8. However, a drop
cast chloroform solution of 9 gave absorption maxima at 508
and 566 nm. These large bathochromic shifts, together with
the detailed structure of the absorption band (ESI{), is
indicative of high ordering in the solid state. In addition to the
above, we have also prepared poly(5d) by electrochemical
polymerisation (ESI{). Absorption maxima appear at 492, 526
and 569 nm and the absorption characteristics are very similar to
those of polymer 9. This is quite surprising, since one would expect
that the head–head coupling inherent in poly(5d) would be
detrimental to the planarity and effective conjugation length of the
polymer.
In conclusion, we have prepared a series of oligothiophenes
bearing a central tetrafluorophenylene unit and shown the effect of
…
…
planarisation through H F and S F contacts by X-ray crystal-
lographic and absorption studies. Polymeric materials have been
obtained by chemical and electrochemical methods and show good
evidence for high order in the solid state. The introduction of hexyl
chains provides excellent solubility: all materials are soluble in
common organic solvents. Moreover, the out-of-plane extension of
the hexyl chains observed in 5d encourages interplane interdigitisa-
tion of the alkyl groups and hence stabilises the crystal phase.
Compounds 5b and 5d show a tendency to form highly ordered,
two-dimensional materials in the crystalline state with several non-
covalent intermolecular contacts. The deposition of active films by
cheap, solution processing techniques is highly desirable.
Preliminary results on hole mobility measurements for spin coated
polymer 9 give values ca. 2 6 1023 cm2 V21 s21 (ESI{). These
results were obtained without any transistor fabrication optimisa-
tion, which would be expected to significantly improve charge
carrier mobility,12 and are encouraging for materials incorporating
Fig. 1 Packing diagram of compound 5d, viewed along the a axis,
showing intra- and inter-molecular contacts.
1466 | Chem. Commun., 2005, 1465–1467
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b = 11.3722(8), c = 102.556(4)u, V = 1043.26(11) s3, Z = 1, Dc
=
1.297 Mg m23, m = 0.278 mm21, hmax = 27.50u, 13381 measured,
4477 unique (Rint = 0.1914) and 3138 (I . 2s(I)) reflections, R1 (obs.) =
the dithienyl tetrafluorophenylene core. Further work is under way
to optimise the mobilities of these materials.
0.0839 and wR2 (all data) = 0.2350, rmax/rmin = 0.919/21.500 e s23
.
CCDC 245178 (5b) and 245177 (5d). See http://www.rsc.org/suppdata/cc/
b4/b417642a/ for crystallographic data in .cif or other electronic format.
The authors wish to thank David Sparrowe and Kristijonas
Genevicius (MERCK Chemicals) for their assistance with
electrical measurements.
David J. Crouch,a Peter J. Skabara,*a Martin Heeney,b
Iain McCulloch,b Simon J. Colesc and Michael B. Hursthousec
aSchool of Chemistry, University of Manchester, Oxford Road,
Manchester, UK M13 9PL. E-mail: peter.skabara@manchester.ac.uk;
Tel: 44 161 275 4781
1 A. Kraft, A. C. Grimsdale and A. B. Holmes, Angew. Chem., Int. Ed.,
1998, 37, 402.
2 C. J. Brabec, N. S. Sariciftci and J. C. Hummelen, Adv. Funct. Mater,
2001, 11, 27.
3 C. D. Dimitrakopoulos and P. R. L. Malenfant, Adv. Mater., 2002, 14,
99.
4 M. O’Neill and S. M. Kelly, Adv. Mater., 2003, 15, 1135.
5 T. Khan, J. J. W. McDouall, E. J. L. McInnes, P. J. Skabara, P. Fre`re,
S. J. Coles and M. B. Hursthouse, J. Mater. Chem., 2003, 13, 2490.
6 S. W. Watt, C. Dai, A. J. Scott, J. M. Burke, R. Ll. Thomas,
J. C. Collings, C. Viney, W. Clegg and T. B. Marder, Angew. Chem., Int.
Ed., 2004, 43, 3061.
bMERCK Chemicals, Chilworth Science Park, Southampton, UK
SO16 7QD
cDepartment of Chemistry, University of Southampton, Highfield,
Southampton, UK SO17 1BJ
Notes and references
7 Y. Sakamoto, S. Komatsu and T. Suzuki, J. Am. Chem. Soc., 2001, 123,
4643.
8 Y. Sakamoto, T. Suzuki, M. Kobayashi, Y. Gao, Y. Sakamoto,
T. Suzuki, M. Kobayashi, Y. Gao, Y. Fukai, Y. Inoue, F. Sato and
S. Tokito, J. Am. Chem. Soc., 2004, 126, 8138.
9 A. Facchetti, M.-H. Yoon, C. L. Stern, H. E. Katz and T. J. Marks,
Angew. Chem., Int. Ed., 2003, 42, 3900.
10 H. Sarker, Y. Gofer, J. G. Killian, T. O. Poehler and P. C. Searson,
Synth. Met., 1998, 97, 1.
11 A. Bondi, J. Phys. Chem., 1964, 68, 441.
{ Crystal data: Data were collected at 120 K on a Nonius KappaCCD area
detector situated at the window of a rotating anode (l(Mo-Ka) =
0.71073 s). The structures were solved by direct methods, SHELXS-97,
and refined using SHELXL-97. Hydrogen atoms were included in the
refinement, but thermal parameters and geometry were constrained to ride
on the atom to which they are bonded. The data were corrected for
absorption effects using SORTAV. 5b: C34H34F4S4, tetragonal, P41212, a =
20.5113(12), c = 14.2496(11) s, V = 5995(7) s3, Z = 8, Dc = 1.433 Mg m23
m = 0.366 mm21, hmax = 27.98u, 59088 measured, 6585 unique (Rint
0.2283) and 4093 (I . 2s(I)) reflections, R1 (obs.) = 0.1388 and wR2 (all
,
=
data) = 0.3407, rmax/rmin = 0.734/20.828 e s23. 5d: C46H58F4S4, triclinic,
12 J.-F. Chang, B. Sun, D. W. Breiby, M. M. Nielsen, T. I. Soelling,
M. Giles, I. McCulloch and H. Sirringhaus, Chem. Mater., 2004, 4772.
¯
P1, a = 6.1497(2), b = 11.3722(8), c = 15.4457(10) s, a = 91.678(3),
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