J O U R N A L O F
New routes to poly(4,4-dialkylcyclopentadithiophene-2,6-diyls){
Paolo Coppo,a Domenico C. Cupertino,b Stephen G. Yeatesb and Michael L. Turner*a
aDepartment of Chemistry, University of Sheffield, Dainton Building, Brook Hill, Sheffield,
UK S3 7HF. E-mail: m.l.turner@sheffield.ac.uk
bAvecia Research Centre, Hexagon House, Blackley, Manchester, UK M9 8ZS
C H E M I S T R Y
Received 4th July 2002, Accepted 16th July 2002
First published as an Advance Article on the web 29th July 2002
Two new polymerisation routes to poly(4,4-dioctyl-
cyclopentadithiophene-2,6-diyl) are reported. The polymer
prepared by Kumada type cross-coupling showed
significantly reduced molecular weight over that prepared
by direct oxidative polymerisation using iron(III) chloride.
The higher molecular weight polymer was shown to
have extended conjugation by UV-visible spectroscopy.
involving a metathesis with methylmagnesium bromide to
generate a reactive thienyl Grignard monomer. The resulting
purple polymer (5) was purified by precipitation into methanol
and Soxhlet extraction with acetone. GPC against poly-
styrene standards showed the majority of the material to be
of moderate molecular weight (see Table 1), however, a long
tail to high molecular weight was observed (see Fig. 1). The
molecular weight distribution was reproducible over several
experiments, and is low when compared to the results obtained
by application of the same protocol to the synthesis of poly(3-
The synthesis of poly(3-alkylthiophenes) has been extensively
studied in the last decade and facile synthetic protocols to
obtain high molecular weight, regioregular materials via
Kumada type cross-coupling of heterofunctional monomers
have been developed.1,2 Analogous regiorandom polymers are
readily accessibleby oxidative polymerisation of 3-alkylthiophenes
using iron(III) chloride.3 This widely used procedure gives
excellent yields of high molecular weight material and after
extensive purification procedures iron impurity levels as low as
80 ppm can be achieved.4 Poly(3-alkylthiophenes) show great
potential for use as the charge transporting layer in polymer
based electronic devices such as FETs and PLEDs. However,
it has been shown that the thickness and homogeneity of the
thin films necessary for these devices are enhanced for higher
¯
hexylthiophene) (Mn y 30000).
These results may be due to incomplete Grignard metathesis
of the dibromocyclopentadithiophene and chain capping with
unreacted methylmagnesium bromide. This suggestion is sup-
ported by the observation of several peaks in the 1H NMR
spectrum of 5 around 2.52 ppm (see inset in Fig. 2). This
complex signal has been assigned to methyl groups at the
polymer chain termini, the integration of which is consistent
with the average degree of polymerisation determined by GPC
(y15 units). MALDI-TOF mass spectrometry also confirmed
the presence of these methyl end groups (see Supplementary
Information{). Despite this end-capping process, the Kumada
protocol afforded well-defined, highly soluble material with a
UV-visible absorption in agreement with previous reports (lmax
560 nm, see Table 1 and Fig. 3).6,7
4
5
¯
molecular weight polymers (Mn w 10 ).
Poly(4,4-dialkylcyclopentadithiophene-2,6-diyls) are hetero-
cyclic analogues of poly(dialkylfluorenes), which are currently
the most commercially interesting conjugated polymers.
Poly(cyclopentadithiophenes) exhibit high conductivity in the
doped state6 and the low oxidation potentials (0.1–0.2 V) of
these polymers are compatible with electronic devices where
a low barrier to charge injection is required, such as double
layer LEDs.5d These polymers were initially prepared by
electropolymerisation of 4,4-dialkylcyclopentadithiophenes.6
Only one report of a chemical approach to the synthesis of
poly(dialkylcyclopentadithiophenes) has appeared. Polymers
Direct oxidative polymerisation of 3 was also investigated.
Slow addition of 3 to a suspension of FeCl3 in chloroform
under an atmosphere of dynamic nitrogen gave oxidised
poly(4,4-dioctylcyclopentadithiophene) (6). Polymer 6 was
¯
showing moderate molecular weights (Mn 5000–9000) were
prepared by nickel(0) catalysed coupling of 2,6-dibromo-4,4-
dialkylcyclopentadithiophenes.7 In this report two novel
synthetic approaches to the preparation of poly(cyclopenta-
dithiophenes) are discussed. Polymers with a low degree of
mislinkages, extensive conjugation and high molecular weight
are presented.
The synthetic procedure is shown in Scheme 1. Dioctyl sub-
stituted 4H-cyclopenta[2,1-b:3,4-b’]dithiophene (3) was obtained
by a modified literature procedure, involving a novel Huang–
Minlon reduction of cyclopenta[2,1-b:3,4-b’]dithiophen-4-one
(1).6,8,9 Bromination at positions 2 and 6 occurred almost
quantitatively on treatment of 3 with two equivalents of
N-bromosuccinimide in DMF at room temperature in the dark.
Compound 4 was polymerised by a Kumada type cross-coupling,
{Electronic supplementary information (ESI) available: partial
suppdata/jm/b2/b206477d/
Scheme 1 Synthetic protocol to polymers 5 and 6.
DOI: 10.1039/b206477d
J. Mater. Chem., 2002, 12, 2597–2599
This journal is # The Royal Society of Chemistry 2002
2597