A new family of conjugated metallopolymers from electropolymerised bis[(terthiophene)dithiolene] complexes

Article abstract of DOI:10.1039/b206243g

Bis(dithiolene) metal complexes incorporating fused terthiophene units have been prepared; the nickel analogue undergoes electropolymerisation to afford a low bandgap material with very broad absorption characteristics.

Full text of DOI:10.1039/b206243g

A new family of conjugated metallopolymers from electropolymerised
bis[(terthiophene)dithiolene] complexes
Cristina Pozo-Gonzalo,a Rory Berridge,a Peter J. Skabara,*a Elena Cerrada,b Mariano Laguna,b
Simon J. Colesc and Michael B. Hursthousec
a Department of Chemistry, University of Manchester, Oxford Road, Manchester, UK M13 9PL.
E-mail: peter.skabara@man.ac.uk
b Departamento de Química Inorgánica, Instituto de Ciencia de Materiales de Aragón, Universidad de
Zaragoza-C.S.I.C, 50009 Zaragoza, Spain
c
Department of Chemistry, University of Southampton, Highfield, Southampton, UK SO17 1BJ
Received (in Cambridge, UK) 28th June 2002, Accepted 5th September 2002
First published as an Advance Article on the web 20th September 2002
Bis(dithiolene) metal complexes incorporating fused terthio-
phene units have been prepared; the nickel analogue
undergoes electropolymerisation to afford a low bandgap
material with very broad absorption characteristics.
equivalents of sodium ethoxide under reflux to generate the
dithiolate intermediate 2. Addition of NiCl2 and n-Bu4NBr gave
the corresponding nickel bis(dithiolene) complex 3 (50% yield),
after precipitation with petroleum ether. Similarly, the Pd(II
)
analogue (4) was obtained from 2 by addition of Na2[PdCl4]–
Et4NBr (49% yield). Gold(III) complexes, 5 and 6, were isolated
from the reaction of 2 with PPN[AuCl4] (38% yield) and n-
Bu4N[AuBr4] (55% yield), respectively. Recrystallisation of 3
from THF–hexane afforded deep red crystals and the X-ray
structure† of the complex can be seen in Fig 1. The asymmetric
unit shows a mirror plane bisecting the metal centre of the
bis(dithiolene) unit. Short intramolecular S…S contacts are
observed between S(4)–S(5) and S(3)–S(1A), which are shorter
than the sum of the van der Waals radii for two sulfur atoms
(3.60 Å). Disorder in one of the peripheral thiophene rings leads
to an equal distribution of all-anti and anti–syn conformations
within the terthiophene unit. This disorder, together with the
weakened S…S interaction, is responsible for the larger
dihedral angle observed within [S(2)–C(5)–C(4)–S(1A)] com-
pared to [S(5)–C(9)–C(8)–S(2)], at 26.2 and 9.2°, respectively.
No significant intermolecular interactions were observed be-
tween the nickel bis(dithiolene) units.
The redox properties of nickel bis(dithiolene) 3 have been
studied by cyclic voltammetry (Fig. 2). Three electrochemical
processes are observed, however, the nature of these waves are
somewhat different to those reported for a related bis(dithie-
nyldithiolene) complex.3 The reversible redox wave at 20.13 V
is due to oxidation of the metal dithiolene unit (3 Ô 3+·);
analogous peaks were observed for the remaining complexes in
the series, albeit at higher potentials (+0.13 V, 4; +0.64 V, 5;
+0.65 V, 6). The peak at +0.46 V for 3 corresponds to the second
oxidation of the dithiolene unit to the charge neutral complex
and is irreversible. This process is not observed for the gold
complexes 5 and 6 but occurs at a lower potential for 4 (+0.40
V) and is quasi-reversible. The irreversible reduction process at
21.16 V is due to the reduction of the terthiophene unit. Similar
peaks are observed for the remainder of the series at 20.90 V
The role of conjugated polymers with the structural or
functional capacity for coordinating to metals is diverse. The
incorporation of the metal can be accomplished at the monomer
stage or post polymerisation and can influence the electronic
properties of the uncomplexed polymer by perturbation of
conductivity, structure and optical and electronic properties.
Consequently, these materials have varied applications. For
example, metal-free polymers can behave as selective and
highly responsive sensors towards metal-containing analytes,1
whereas metallopolymers can be used in catalysis2 or as low
bandgap materials.3
Electrogenerated metallopolymers have received particular
attention over recent years, with pyrrole and thiophene
derivatives as frequent monomers of choice.2,4 Very recently,
conjugated polymers have been prepared electrochemically, in
which [Ru(terpy)2]5 and [Ni(dithiolene)2]3 units are present in
the main chain of a polythiophene based material. As part of our
studies into 3,4-dithio-substituted terthiophene species,6 we
now report the synthesis, electropolymerisation and character-
isation of a new cross-linked polythiophene bearing nickel
bis(dithiolene) units fused to the main chain.
The synthesis of the dithiolene monomer is shown in Scheme
1. Terthiophene 16 was dissolved in THF and treated with 2.2
Scheme 1 Reagents and conditions: (i) THF, NaOEt, reflux, 15 min; (ii)
metal salt (and R4NBr for 3 and 4).
Fig. 1 X-Ray crystal structure of the nickel bis(dithiolene) complex 3.
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Fig. 2 Cyclic voltammogram of 3 (solid line) and poly(3) (dashed line) on
glassy carbon working electrode, using Ag/AgCl reference and Pt counter
electrodes, in CH2Cl2 for 3 (substrate ca. 1023 M) and MeCN (monomer
free) for poly(3) (n-Bu4NPF6 0.1 M as supporting electrolyte).
Fig. 4 Electronic absorption spectra for 3 in MeCN and poly(3) as a thin film
on ITO glass.
3 at 20.4 V (spectroelectrochemistry was conducted using an
OTTLE cell). The complex oxidises after several days under
ambient conditions, giving a spectrum that can be reproduced
spectroelectrochemically at +0.25 V and which features an
additional absorption peak at 917 nm, corresponding to the
oxidised nickel dithiolene monoanion species.8 In both oxida-
(4), 21.09 V (5) and 21.07 V (6). Unequivocal proof that this
reduction process is independent of the metal centre comes from
the CV (under identical experimental conditions) of 2,5-di(2-
thienyl)-3,4-bis(methylsulfanyl)thiophene,6 which displays a
single electron irreversible reduction at 21.17 V.
*
tion states, the peaks at 335 and 342 nm correspond to a p–p
Electropolymerisation for all complexes was conducted by
repetitive cycling over the range 0.00 to +1.30 V on glassy
carbon or ITO glass working electrodes; a typical experiment is
shown in Fig. 3. During polymerisation, the successive
oxidation of the monomer is accompanied by the growth of two
new redox waves at ca. +0.5 and +1.0 V, which represents the
electroactivity of a new material (i.e. the polymer). A plot of
scan rate vs. peak current of Eox for the polymer in monomer
free solution (Fig. 3, inset), gives a linear fit (R > 0.998),
confirming that charge transport through the film is not
diffusion limited.7
transition within the triaryl units, as seen for similar terthio-
phene systems.6 For the polymer film, this transition is shifted
bathochromically by ca. 150 nm, indicative of an increase in
conjugation and providing further proof for the generation of a
polymeric material. The peak for the monoanion dithiolene
moiety remains at 908 nm, indicating that, under the electro-
chemical conditions employed, this is the preferred ground state
of the polymer. The most remarkable feature of poly(3) is the
extremely broad absorption of the material between 400–1000
nm. Conjugated polymers that cover such a vast range are
extremely rare; since the maximum of the photon flux of the sun
peaks around 700 nm (just out of the range of most conjugated
polymers such as simple PPVs and polythiophenes), low band-
gap polymers such as poly(3) could be extremely useful
materials as light-harvesting components in plastic photovoltaic
devices.9
We wish to thank the EPSRC for a grant to R. B. (GR/
R23053). M. L. and E. C. thank the DGES (PB98-0542) for
financial support.
Notes and references
† Crystal data. C56H84N2NiS10, M = 1164.56, monoclinic, a = 8.5643(6),
b = 20.0991(6), c = 17.8440(11) Å, b = 101.9540(10)°, U = 3005.0(3)
Fig. 3 Polymerisation of 3 on glassy carbon working electrode, under
identical conditions as those for Fig. 2 in CH2Cl2. Inset: plot of scan rate vs.
peak current from E2ox for poly(3) in monomer free MeCN solution.
Å3, T = 120(2) K, space group P21/n, Z = 2, m(Mo–Ka) = 0.707 mm21
,
9829 reflections measured, 6238 unique (Rint = 0.0507) which were used in
all calculations. R1 (I > 2sI) = 0.0655 and the final wR2 (F2) was 0.1219 for
all data. CCDC 188996. See http://www.rsc.org/suppdata/cc/b2/b206243g/
for crystallographic data in CIF or other electronic format.
The cyclic voltammogram of poly(3) on glassy carbon
electrode in monomer free solution is also shown in Fig. 2. Two
reversible oxidaton waves are observed at +0.66 and +0.97 V
and are attributed to the electroactivity of the dithiolene unit and
polythiophene chain, respectively. The single peak at +0.04 V is
also related to the oxidation of the metal dithiolene unit in the
polymer; compared to the redox behaviour of the monomer, this
process has lost reversibility and has been shifted to a more
positive potential by 130 mV. Two irreversible reduction peaks
are seen at 21.04 and 21.26 V. The electrochemical bandgap of
the polymer is estimated to be ca. 1.4 eV (difference between
the onset for the oxidation of the polythiophene chain and the
onset of the reduction peak at 21.04 V). Conductivity
measurements have been performed by impedance spectros-
copy. The conductivities for the series of metallopolymers are
ca. 1026 to 1025 S cm21 in the range 0 to +1 V, similar to those
reported for analogous bis(dithiolene) polymers.3
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Housecroft, E. Mukhtar and E. Schofield, Chem. Commun., 2002, 284.
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7 Charge Percolation in Electroactive Polymers, in Electroactive Polymer
Electrochemistry, Part 1 Fundamentals, ed. M. E. G. Lyons, 1994,
Plenum Press, New York.
The electronic absorption spectrum of the polymer grown on
ITO glass, along with spectra of 3 at 20.40 and +0.25 V in
MeCN, are shown in Fig. 4. A fresh solution of the monomer
gives an identical spectrum to that obtained from a solution of
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