Functionalised 9-(1,3-dithiol-2-ylidene)thioxanthene derivatives: A C 60 conjugate as an ambipolar organic field effect transistor (OFET)

Article abstract of DOI:10.1039/b500500k

Thioxanthene has been converted into 9-(1,3-dithiol-2-ylidene) -thloxanthene derivatives which are new two-electron n-donors; an ambipolar OFET has been fabricated from the covalent 9-(1,3-dithiol-2-ylidene)thioxanthene-C₆₀ diad system 6 The Ro

Full text of DOI:10.1039/b500500k

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COMMUNICATION
www.rsc.org/materials | Journal of Materials Chemistry
Functionalised 9-(1,3-dithiol-2-ylidene)thioxanthene derivatives: a C60
conjugate as an ambipolar organic field effect transistor (OFET){
Samia Amriou, Aravinda Mehta and Martin R. Bryce*
Received 11th January 2005, Accepted 17th February 2005
First published as an Advance Article on the web 28th February 2005
DOI: 10.1039/b500500k
the mono- and bis-adducts 6 and 7 in 16% and 9% yields,
respectively, after clean chromatographic separation. Their
structures were assigned by MALDI MS, 1H and 13C NMR
spectra and electronic absorption spectra.{
Thioxanthene has been converted into 9-(1,3-dithiol-2-ylidene)-
thioxanthene derivatives which are new two-electron p-donors;
an ambipolar OFET has been fabricated from the covalent
9-(1,3-dithiol-2-ylidene)thioxanthene–C60 diad system 6.
The redox properties of the diad 6 and triad 7 were studied by
cyclic voltammetry (CV) along with compounds 5, C60 and 811a
(Table 1). The 9-(1,3-dithiol-2-ylidene)thioxanthene donor system
There is considerable interest in organic molecules with tuneable
p-electron-donor and -acceptor characteristics for applications
in molecular electronics, notably display technologies, organic
field effect transistors (OFETs) and photovoltaic devices.1 A range
of electron donors with varying redox potentials have been
exploited as p-type semiconductors in OFETs.2 Materials
include polycyclic aromatics3 (pentacene is the benchmark),4
oligothiophenes,5 and tetrathiafulvalene (TTF) derivatives.6
However, there have been fewer studies on n-channel systems:2b
specific examples include C60,7 fluorinated naphthalenebisimides,8
and electron-deficient thiophene-based oligomers.9 From these
viewpoints it is important to explore new redox-active building
blocks which are processable and can serve as active components
in devices.
In this communication we introduce the 9-(1,3-dithiol-2-
ylidene)thioxanthene framework 3 as a new p-electron donor
system and describe its functionalisation to afford the covalent
donor–acceptor compounds 6 and 7. An unusual property of diad
6 is its ability to function as an active component in bipolar (both
p- and n-channel) OFETs, of which there are very few examples,
all involving C60 derivatives.10
Quinones can be functionalised with the 1,3-dithiol-2-ylidene
moiety via Horner–Wadsworth–Emmons methodology to provide
efficient p-electron donors.11 We identified the 9-(1,3-dithiol-2-
ylidene)thioxanthene system
3 as an interesting target, as
heteroaromaticity could be gained in both the dithiolium (6p)
and thianthrenium (14p) cations, suggesting that 3 might be an
efficient two-electron donor. Thioxanthene 1 reacted with the
anion generated from reagent 211c to afford compound 3 (48%
yield) (Scheme 1). Lithiation of 3 followed by electrophilic
iodination using perfluorohexyliodide12 gave compound 4 (85%
yield). Palladium-mediated cross-coupling of 4 with 4-formyl-
benzeneboronic acid under standard Suzuki–Miyaura conditions
gave product 5 in 73% yield. The aldehyde group of 5 served
for attachment to C60 via 1,3-dipolar cycloaddition with an
azomethine ylide generated in situ.13 Thus a mixture of compound
5, C60 and sarcosine was refluxed in dry toluene for 48 h to afford
Scheme 1 Reagents and conditions: i, Reagent 2, LDA, THF, 278 uC;
ii, LDA, THF, 278 uC, C6F13I; iii, 4-formylbenzeneboronic acid,
[Pd(PPh3)2Cl2], THF, Na2CO3 (aq), reflux; iv, C60, sarcosine, toluene,
reflux, 48 h.
Table 1 Cyclic voltammetric dataa
Compound
Eoxpa/V
Eoxpc/V
E1red/V
E2red/V
E3red/V
5
0.88 (2e)
0.78
C60
6
7
20.62
20.73
20.75
21.01
21.11
21.14b
21.48
21.63
21.66b
0.86 (2e)
0.88 (4e)
0.45 (2e)
0.74
0.72
0.13
8
a
2
Compound 2 6 1023 M, versus Ag/AgCl, electrolyte Bu4N+PF6
{ Electronic supplementary information (ESI) available: Characterisation
data for 6; details of OFET device fabrication procedures; figures of the
UPM3 optimised geometry of 6, the localisation of HOMO and LUMO
orbitals of 6, and the cyclic voltammogram of 6. See http://www.rsc.org/
suppdata/jm/b5/b500500k/
(0.1 M), 1,2-dichlorobenzene : acetonitrile (10 : 1 v/v), 20 uC, scan
pa
rate 100 mV s21. Eox is the oxidation peak potential on the first
anodic scan; Eox is the coupled reduction peak potential on the
cathodic scan. An ill-defined wave; the intensity varied with the
scan rate.
pa
b
*m.r.bryce@durham.ac.uk
1232 | J. Mater. Chem., 2005, 15, 1232–1234
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5 undergoes a single, two-electron oxidation process,14 similar to
system 8,11 although the oxidation process for 5 is anodically
shifted compared to 8 (less gain in aromaticity in 52+, compared
to 82+) and is electrochemically more reversible than for 8 (cf. DEox
values from Table 1). The first oxidation potential of 5 is,
therefore, more comparable to that of a short oligo(phenyl-
enevinylene)15 or an oligo(alkylthiophene) fragment,16 than TTF
or compound 8. As expected, the fullerenopyrrolidine reduction
waves of 6 and 7 are cathodically shifted (by ca. 100 mV) in
comparison with the parent C60 due to saturation of one and two
double bonds, respectively. The electrochemical data for 6 and 7
suggest that there is no significant interaction between the donor
and acceptor moieties in the ground state. The HOMO–LUMO
gap for 6 calculated from the CV data (1.59 eV) is close to the
value obtained from PM3 calculations (1.52 eV).{
Macroscopic transport properties depend on the efficiency of
intermolecular hopping mechanisms within the 3D organisation.
There are two notable features of compound 6 in this regard. (i)
The C60 moiety appears to play a key role in determining the
supramolecular structure within the film, as good quality spin-cast
films could not be obtained for other derivatives of system 3 which
lacked a C60 substituent, e.g. compound 5. (ii) The 9-(1,3-dithiol-2-
ylidene)thioxanthene moiety is considerably non-planar in both the
neutral and oxidised redox states,14 unlike the oligothiophene10b
and oligophenyleneethynylene systems10c considered above.
These studies have demonstrated that system 3 is an interesting
and versatile p-electron donor. New derivatives will be reported in
due course. Current work is aimed at optimising OFET behaviour
while maintaining the benefits of solution processing.
We have fabricated OFETs using 6 as the active layer using
Acknowledgements
spin-coating techniques and
a standard top-contact device
We thank EPSRC and Avecia for funding, Dr D. Kreher for
assistance with the C60 reactions, and Dr I. F. Perepichka for
the PM3 calculations.
configuration.{ X-Ray diffraction experiments in reflection mode
showed evidence of polycrystallinity: the layer spacing from the
˚
first weak reflection peak (22 A) corresponds to the extended
molecular length of 6 calculated by PM3 methods. Transistor
responses were observed at both positive and negative bias which
means that compound 6 acts as a bipolar semiconductor (Fig. 1).
The field effect mobilities of holes and electrons calculated for
the film of compound 6 in the saturation regime2b are 1026 and
4 6 1025 cm2 V21 s21, respectively, with on/off current ratios of
ca. 102 for both p- and n-channels at Vds = 240 and +40 V,
respectively. These values compare favourably with other con-
temporary ambipolar systems.10 For examples, Kunugi et al.
reported 1.1 6 1025 and 4.3 6 1025 cm2 V21 s21, for
an oligothiophene–(C60)2 system;10b Nierengarten et al. reported
9.2 6 1028 and 7.0 6 1027 for an oligophenyleneethynylene–C60
system.10c Notably, the electron mobility is higher than the hole
mobility for all of these covalent compounds, although this is not
always the case with (non-covalent) blends.10a (For comparison,
pure C60 has a field effect mobility of up to 0.12 cm2 V21 s21 after
annealing,7b with on/off ratios as high as 106).7a
Samia Amriou, Aravinda Mehta and Martin R. Bryce*
Department of Chemistry and Centre for Molecular and Nanoscale
Electronics, University of Durham, Durham, UK DH1 3LE.
E-mail: m.r.bryce@durham.ac.uk
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Fig. 1 Drain current (ID) vs. drain voltage (VD) at different gate voltages
(VG) for OFETs with 6 as the active layer.
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