Skabara, P.J. and Mullen, K. (1997) The design and synthesis of a novel TTFthiophene monomer. Synthetic Metals, 84 (1-3). pp. 345-346. ISSN 0379-6779
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The Design and Synthesis of a Novel TTF-Thiophene Monomer
P.J Skabara*†, K. Müllen
Max-Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany.
†New address: Division of Chemistry, School of Science and Mathematics, Sheffield Hallam University, Pond Street, Sheffield S1 1WB,
United Kingdom.
____________________________________________________________________________________________________________
Abstract
The synthesis of a highly -electron rich fused tetrathiafulvalene-thiophene species is reported, together with attempts of polymerisation
under electrochemical and chemical oxidative conditions.
Keywords: Tetrathiafulvalene derivatives; thiophene derivatives; electrochemical polymerisation; low-bandgap conjugated polymers;
organic conductors .
____________________________________________________________________________________________________________
Conjugated polymers are materials which exhibit unusual
electronic and optoelectronic properties; their applications can be
found in energy storage, non-linear optics, sensors and electronic
and electrochromic devices. Varying the bandgap (Eg) of these
polymers has a significant effect on their physical behaviour. By
lowering Eg to a significant degree, the material may establish
intrinsic semiconductivity [1]; in parallel, the value of Eg also
determines the optical properties of the polymer and is thus a
major consideration in applications such as LEDs and non-linear
optics.
One method for lowering Eg in poly(heterocyclopentadiene)
systems, is the introduction of fused benzenoid species at the 3,4
positions of the five-membered rings [2-4]; this strategy leads to
the enhanced stabilisation of the excited quinoid state within the
heterocyclopentadiene
repeating
units.
Indeed,
poly(isothianaphthene) 1 [2] has a bandgap value of 1.1eV [5],
whereas that of poly(thiophene) is approximately 2eV [6].
expected to be beneficial towards the electronic behaviour of the
bulk polymer (TTF derivatives are highly redox-active molecules
which form structurally ordered charge-transfer complexes
possessing high conductivity values) [7]; (ii) stabilisation of the
polymeric quinoid state through the rearomatisation of the TTF
unit; (iii) increased solubility via the two hexylthio chains per
monomer unit.
S
OH
S
S
(i)
OH
S
Br
S
Br
S
5
4
(ii)
S
S
S
(iii)
S
S
S
S
S
6
7
S
S
n
n
(iv)
1
1'
Aromatic
thiophene units
Quinoid
thiophene units
8
We herein report the synthesis of the novel -extended
thiophene monomer 2, which we have designed for the following
reasons: (i) incorporation of the tetrathiafulvalene (TTF) unit 3 is
S
O
S
SC6H13
S
SC6H13
O
S
(v)
2
S
9
R
H13C6S
S
S
S
S
S
S
S
H13C6S
S
S
R
2 R=H
2a R = I
3
Scheme 1
Reagents and conditions: (i) CBr4, PPh3,
dichloromethane; (ii) Na2S.9H2O, ethanol; (iii) 2,3-dichloro5,6-dicyano-1,4-benzoquinone (DDQ), toluene, reflux, 5 h; (iv)
Hg(OAc)2, acetic acid/chloroform; (v) P(OEt)3, 8, 90C, 6 h.
The synthesis of compound 2 is summarised in Scheme 1.
Diol 4 [8] is readily brominated under mild conditions to give 5
in 70% yield. Ring closure of the dihalide via nucleophilic
substitution with sodium sulfide afforded compound 6 in 78%
yield. The sulfide 6 was aromatised to the 3,4-substituted
thiophene 7 with DDQ (96% yield), and in turn oxidised (98%
yield) to compound 9 using mercuric acetate. Cross-coupling of
species 8 and 9 with triethylphosphite gave 2 [9] in 20-30%
yield, together with significant amounts of self-coupled products.
The cyclic voltammogram of compound 2 can be seen in
Figure 1. Two reversible single-electron waves can be observed
at 0.46V and 0.83V, respectively. These values can be attributed
to the TTF portion of the molecule, since TTF 3 itself displays
similar waves at 0.34V and 0.71V. A third single-electron
oxidation wave is also evident in the voltammogram of 2
(2.18V), and can be assigned to the thiophene unit (peak
oxidation potential for unsubstituted thiophene is 2.06V) [10].
tetracyanoquinodimethane 11 in dichloromethane under reflux,
dark solutions formed which on cooling afforded deep red and
dark blue thin platelets, respectively. The conductivities of the
materials vary in the region of 10-3-10-4 Scm-1 (compressed pellet,
two-probe measurements). MALDI-TOF mass spectroscopy
indicated the presence of monomeric units, rather than
oligomeric/polymeric species, whilst the IR spectrum of the 2TCNQ complex showed the expected nitrile shift associated with
TCNQ charge-transfer complexes [14].
R
R
NC
CN
NC
CN
R
R
10 R = H
11 R = F
Due to the inability of the monomer 2 to polymerise under
electrochemical conditions, our current efforts in this field focus
on the chemical polymerisation of the 2,5-diiodothiophene
derivative 2a [15], via standard aryl-aryl coupling methods.
References
0.200
0.600
1.000
1.400
1.800
2.200
E (V)
Figure 1. Cyclic voltammogram of compound 2; Au working
electrode, Ag/AgCl reference electrode, 0.1M TBAPF6, 0.01M 2
in dry dichloromethane under argon at -30 C, with iR
compensation.
Repetitive scans failed to show the formation of a polymer
under electrochemical conditions. Furthermore, an additional
unexpected cathodic peak was observed at 1.36V. This may
suggest that the unusual tricationic species is rapidly reacting
with either the solvent or anion in solution to form by-products;
this phenomenon has been reported to occur with derivatised
thiophene monomers with high oxidation potentials [11].
Conversely, the TTF unit could be responsible for the
stabilisation of the radical cation within the thiophene portion of
the molecule. In this situation the charged species would be able
to diffuse away from the electrode surface and form soluble
oligomers in solution. Indeed, when the solvent system for 3 was
switched to nitrobenzene, a dark green solution formed around
the anode which diffused readily through the solution. For similar
reasons, attempts at the electropolymerisation of 3-(methylthio),
3-(ethylthio) and 3,4-bis(ethylthio)thiophenes have either failed
[12], or resulted in the formation of soluble oligomers [13], due
to the electron donating (and thus stabilising) effects of the (')
alkylthio side chains.
When
compound
2
was
treated
with
tetracyanoquinodimethane (TCNQ) 10 and tetrafluoro-
[1] E.E. Havinga, W. ten Hoeve and H. Wynberg, Synth. Met.,
55-57 (1993) 299.
[2] F. Wudl, M. Kobayashi and A.J. Heeger, J. Org. Chem., 49
(1984) 3382.
[3] G. King and S.J. Higgins, J. Mater. Chem., 5 (1995) 447.
[4] G.M. Brooke, C.J. Drury, D. Bloor and M.J. Swann, J.
Mater. Chem., 5 (1995) 1317.
[5] S.M. Dale, A. Glidle and A.R. Hillman, J. Mater. Chem., 2
(1992) 99.
[6] J.M. Margolis (ed.), Conducting Polymers, Chapman and
Hall, New York, 1989.
[7] M.R. Bryce, Chem. Soc. Rev., 20 (1991) 355.
[8] M.A. Fox and H.-L. Pan, J. Org. Chem., 59 (1994) 6519.
[9] Selected data for 2: M.Pt. 73-74C; C20H28S7 requires C,
48.7%; H, 5.7%; found C, 48.4%; H, 5.7%; m/z (FDMS)
1
492; H NMR (CDCl3)  6.88 (2H, s), 2.83 (4H, t, J =
7.2Hz), 1.65 (4H, m), 1.32 (12H, m) and 0.90 (6H, t, J =
13
6.4Hz); C NMR (CDCl3)  136.0, 127.5, 113.8, 112.1,
112.0, 36.3, 31.3, 29.7, 28.2, 22.5 and 14.0.
[10] R.J. Waltman, J. Bargon and A.F. Diaz, J. Phys. Chem., 87
(1983) 1459.
[11] R.J. Waltman and J. Bargon, Can. J. Chem., 64 (1986) 76.
[12] J.P. Ruiz, K. Nayak, D.S. Marynick and J.R. Reynolds,
Macromolecules, 22 (1989) 1231.
[13] S. Tanaka, M. Sato and K. Kaeriyama, Synth. Met., 25
(1988) 277.
[14] The complex 2-TCNQ gave a sharp signal at 2206 cm-1.
Similarly, the CN peak for the charge-transfer salt of TTFTCNQ is seen at 2202 cm-1; CN stretch for neutral TCNQ is
observed at 2222 cm-1.
[15] Prepared from 2 in 92 % yield, using 2.1 equivalents of LDA
in THF at -78C, followed by the addition of 2.1 equivalents
of perfluorohexyl iodide.