Fluorene functionalised sexithiophenes - Utilising intramolecular charge transfer to extend the photocurrent spectrum in organic solar cells

Article abstract of DOI:10.1039/b609858d

A new series of oligothiophenes bearing electron deficient fluorene units have been prepared and characterised. The materials are functionalised by C ₈/C₁₁ alkyl chains or triethylene glycol side groups, yet the higher oligomers remain poorly soluble. The absorption characteristics of a sexithiophene analogue (compound 3) have been studied by UV-vis and photoinduced absorption spectroscopy. Photovoltaic cells have been fabricated from blends of 3 and fullerene derivative [6,6]-phenyl-C₆₁ butyric acid methyl ester (PCBM). The photocurrent spectrum of the device matches the absorption spectrum of the sexithiophene system which incorporates an intramolecular charge transfer band arising from the 1,3-dithiole-fluorene units. A modest power conversion efficiency of 0.1% was achieved. The Royal Society of Chemistry.

Full text of DOI:10.1039/b609858d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Fluorene functionalised sexithiophenes—utilising intramolecular charge
transfer to extend the photocurrent spectrum in organic solar cells{
Peter J. Skabara,*a Rory Berridge,b Igor M. Serebryakov,{b Alexander L. Kanibolotsky,a
Lyudmila Kanibolotskaya,a Sergey Gordeyev,a Igor F. Perepichka,§*c N. Serdar Sariciftci*d and
Christoph Winderd
Received 11th July 2006, Accepted 28th November 2006
First published as an Advance Article on the web 14th December 2006
DOI: 10.1039/b609858d
A new series of oligothiophenes bearing electron deficient fluorene units have been prepared and
characterised. The materials are functionalised by C8/C11 alkyl chains or triethylene glycol side
groups, yet the higher oligomers remain poorly soluble. The absorption characteristics of a
sexithiophene analogue (compound 3) have been studied by UV-vis and photoinduced absorption
spectroscopy. Photovoltaic cells have been fabricated from blends of 3 and fullerene derivative
[6,6]-phenyl-C61 butyric acid methyl ester (PCBM). The photocurrent spectrum of the device
matches the absorption spectrum of the sexithiophene system which incorporates an
intramolecular charge transfer band arising from the 1,3-dithiole-fluorene units. A modest power
conversion efficiency of 0.1% was achieved.
substituents and ICT energies. The same relationship was
Introduction
identified for the values of the first two reduction processes of
Polythiophenes and well-defined oligothiophenes have been
the compounds, which are features of the fluorene units.
investigated as organic semiconductor materials for applica-
Electropolymerisation of a trinitro analogue of the terthio-
phene series gave polymer 1.16 Photoinduced IR spectroscopy
tion in various electronic devices, such as organic photovoltaic
cells (OPVs),1–4 light emitting diodes and electrochromics,5–8
and organic field effect transistors.9–12 In OPVs, p-electron
of this polymer showed evidence of a long lived charge
separated state, indicating that the material could be a good
rich conjugated systems are either blended or tethered to
candidate for OPVs. However, even though the repeat unit is
electron acceptor molecules and, almost exclusively, the latter
furnished with a triethylene glycol chain, this functionality is
are derivatives of the fullerene C60.13 Photoexcitation of the
not sufficient to impart good solubility to the polymer, which
organic material(s) generates a charge separated state which,
under constrained device parameters, allows the generation of
a potential difference across the OPV device.
is an obvious problem towards processing the material. To this
end, we decided to turn our attention towards the synthesis of
oligomeric analogues, which are soluble in common organic
solvents and can be processed by standard and cheap solution
based techniques. Herein, we report the synthesis and
characterisation of compounds 2 and 3 and the application
of the latter in an OPV device.
We have previously reported the synthesis of a series of
thiophene14 and terthiophene15,16 monomers bearing substi-
tuted fluorene units. The substituents vary in number and in
their electron withdrawing strength (nitro and ester function-
alities were applied). Intramolecular charge transfer (ICT)
between fluorene and 1,3-dithiole units is a common feature
in these materials and we found a Hammett-type dependence
between the sum of the nucleophilic constants of the
Synthesis
In the search for more soluble materials, the most obvious
option was to increase the number of long-chain substituents
in the fluorene moiety whilst retaining strong acceptor ability.
In line with this, we chose to investigate comparatively easily
accessible fluorene derivatives, namely diesters of fluorene-2,7-
dicarboxylic acid and 2,7-dialkylsulfonyl fluorenes. Each of
the series was prepared in two modifications, either with a long
(C11 or C8) n-alkyl substituent or with a triethylene glycol
monomethyl ether chain which should favour solubility of the
compounds in correspondingly low- and high-polar solvents.
aWestCHEM, Department of Pure and Applied Chemistry, University of
Strathclyde, 295, Cathedral Street, Glasgow, UK G1 1XL.
E-mail: peter.skabara@strath.ac.uk; Fax: +44 141 548 4822;
Tel: +44 141 548 4648
bSchool of Chemistry, University of Manchester, Oxford Road,
Manchester, UK M13 9PL
cL. M. Litvinenko Institute of Physical Organic and Coal Chemistry,
National Academy of Sciences of Ukraine, Donetsk 83114, Ukraine
dLinz Institute for Organic Solar Cells (LIOS), Physical Chemistry,
Johannes Kepler University Linz, Linz A-4040, Austria.
E-mail: Serdar.Sariciftci@jk.uni-linz.ac.at
{ Electronic supplementary information (ESI) available: experimental
and analytical data for all new compounds, details of CV and SEC
experiments. See DOI: 10.1039/b609858d
{ Present Address: Department of Organic Chemistry, Donetsk
National University, Donetsk 83055, Ukraine.
Fluorene-2,7-dicarboxylic acid and its diesters
Apart from the known procedures of direct carboxylation of
fluorene,17,18 for which the isomeric purity of products had not
been assessed, a promising option was the conversion of 2,7-
diacetylfluorene 4, which, in our experience, could be easily
§ Currently at: Department of Chemistry, Durham University,
Durham, UK DH1 3LE. E-mail: i.f.perepichka@durham.ac.uk.
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instead of sodium hydroxide under the same conditions
resulted in unchanged diacetylfluorene.
Another way of using the haloformic reaction was to
prepare 2,7-bis(tribromoacetyl)fluorene in a separate stage
before its cleavage. However, the procedure described for
a,a,a-tribromoacetophenone25 yielded only contaminated
2,7-bis(dibromoacetyl)fluorene 5 which could also be obtained
in quantitative yield by bromination of 2,7-diacetylfluorene 4
with bromine in hot acetic acid (Scheme 1). This product,
however, appeared much more reactive towards further
bromination in aqueous dioxane–sodium carbonate solutions
which, accompanied by fast destruction of the tribromo
derivative,26 allowed the isolation of the target fluorene-2,7-
dicarboxylic acid 6 in almost quantitative yield.
Diesters of fluorene-2,7-dicarboxylic acid (7a,b) were
synthesised by an analogous procedure to that described16
for 2,5,7-trinitrofluorene-4-carboxylic acid (triethylene glycol
monomethyl ether) ester. However, nitration of these com-
pounds produced target products (8a,b) contaminated with,
most probably, 4,6-isomers (13–30% depending on conditions,
NMR evidence), and, in the case of diundecyl ester, the
monosubstituted compound which was poorly soluble in the
nitrating mixture. Although all admixtures could be removed
to a desired extent (less than 3%) by repetitive recrystallisation,
this rendered only moderate yields.
Nitration of free fluorene-2,7-dicarboxylic acid led to even
more complex mixtures of products, from which the target
compound could be isolated only in low yield (12%). Our
attempt to obtain chloroanhydride from this product led to
its decomposition, as was found for 2,5,7-trinitrofluorene-4-
carboxylic acid.16
Fluorene-2,7-disulfones
The most straightforward route to the target compounds
(Scheme 2) included the reduction of fluorene-2,7-disulfonyl
chloride 9 into the corresponding dimercaptan, followed by its
subsequent alkylation to give 10a,b and oxidation of the result-
ing sulfides (11a,b). However, the only described27 synthetic
procedure for fluorene-2,7-dimercaptan produced, as it had
been found, only a low yield of target compound along
with insoluble solid, most probably polydisulfide. The same
product was isolated from an attempt to obtain a protected
derivative, as was described for 2-S-acetylthiofluorene.28
Though the polydisulfide could be converted into mercaptan
sodium salt29 with subsequent alkylation, this procedure was
found to be inferior to direct in situ alkylation of fluorene-2,7-
dimercaptan after reduction of disulfonyl chloride 9 with
lithium aluminium hydride (Scheme 2).
obtained and purified. Its oxidation furnished fluorenone-2,7-
dicarboxylic acid19 in high yield. However, in our hands,
subsequent reduction of this intermediate with hydrazine
hydrate in ethylene glycol20 failed to yield the target product.
Possibly, the use of a higher boiling solvent such as diethylene
glycol21 could afford the desired result but we did not explore
this route any further.
The conversion of acetylfluorenes into the corresponding
carboxylic acids, without simultneous oxidation of the
methylene group, could be achieved in two ways. Coupling
of 2,7-diacetylfluorene 4 with diethyl oxalate, followed by
cleavage of the resulting bis-diketoester with lead tetraacetate,
is known to give only moderate yield (33%) of fluorene-2,7-
Oxidation of the sulfides (10a,b) to the corresponding
sulfones (11a,b) with hydrogen peroxide in boiling acetic acid
proceeded smoothly within five minutes, whereas refluxing
over 2 hours, as was suggested for fluorenyl-2-methylsulfide,30
gave a product heavily contaminated with the corresponding
fluorenone derivative (MS evidence) and other unspecified
products.
dicarboxylic acid.22
A number of substituted fluorene-
carboxylic acids was obtained by haloformic cleavage of
acetylfluorenes in excellent yields;23,24 however, applied to
2,7-diacetylfluorene 4, this procedure yielded fluorenone-2,7-
dicarboxylic acid. Use of a milder base (sodium carbonate)
Nitration of sulfones 11a,b to afford compounds 12a,b
was regioselective, due to the greater orientating power of
the sulfonyl substituents compared to the carboxyl groups.
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Scheme 1
Scheme 2
However, the stronger electron withdrawing ability of the
sulfonyl groups required more vigorous nitration conditions,
in which the alkyl chains underwent attack by nitric acid to a
substantial extent (NMR evidence). The resulting contami-
nants were removed by recrystallisation.
pyridine. Due to the poor solubility of fluorene-2,7-
dicarboxylic acid diundecyl ester 8a in DMF, the coupling
was carried out in a DMF–chloroform mixture.
Chemical coupling of terthiophenes
Coupling of the fluorene units 8a,b and 12a,b with
terthiophene 13 (via the salt 14) was achieved as described16
in overall satisfactory yields. The coupling of disulfonyl
derivatives 12a,b proceeded smoothly in DMF, whereas the
less reactive dicarbonyl compounds 8a,b required addition of
As we have demonstrated previously,15,16 terthiophene analo-
gues of 15 and 16 can be polymerised electrochemically. This
procedure is analogous to coupling using a chemical oxidant.
In this part of the work, we chose to focus on the two most
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wave indicates that both fluorene moieties are reduced at the
same potential resulting in bis(radical anion) 32( 2) species, i.e.
?
there is no electronic communication between the two fluorene
units in compound 3, as expected from its structure. Conse-
quently, the second reduction wave represents the formation of
bis(dianion) 342. The difference of DEred1–2 y 0.35 V between
the first and second reduction processes is typical for fluorene
acceptors (y0.3–0.5 V,19,31 although fluorene acceptors with
strong ICT show decreased DEred1–2 of y0.2 V14,32).
Scheme 3
soluble terthiophene derivatives, compounds 15a and 16b.
Ferric chloride was added to solutions of the terthiophenes
in nitrobenzene. At higher temperatures (up to 60 uC) and
prolonged reaction times, we obtained polymeric material that
was insoluble (after evaporation of the solvent) (Scheme 3). By
performing the same reactions at room temperature for 3 hours
we obtained mainly oligomeric material, but the isolated
materials were adequately soluble in chlorinated solvents.
MALDI-TOF mass spectrometry showed that the products
were obtained as mixtures of oligomers (6T, 9T, 12T) and the
main fraction from the reaction of 15a (sexithiophene 3,
Scheme 3) was isolated by column chromatography to give a
clean spectrum (see Fig. S1 in ESI{). For reasons of solubility
and monodispersity, absorption and electrochemical studies,
and device fabrication were performed only on compound 3.
Oxidation of compound 3 to the radical cation (as followed
from the current ipaox1, which is ca. twice as low as ipc
red1
)
ox1
occurs as an electrochemically irreversible process with Epa
y +0.8 V vs. Ag/Ag+ (Fig. 1) Obviously, Epaox1 corresponds to
oxidation of the sexithiophene chain to produce a polaron
state and this process is followed by further oxidation
(presumably to a bipolaron state, which is complicated by
the simultaneous oxidation of dithiole rings—in the case of
9-dithiolylidene-nitrofluorene acceptors the substrates are
irreversibly oxidised above +1 V14,16,32). Cycling the potentials
between 0 V and +1.2 V (as well as +1.4 V) did not show
anodic electropolymerisation (or dimerisation) of 3.
Spectroelectrochemical (SEC) experiments in benzonitrile
solution were performed to establish the spectroscopic
signatures of species generated in CV. In the neutral state,
compound 3 has lmax = 439 nm with two shoulders at 380 and
520 nm (in benzonitrile). On reduction of the fluorene units in
3 to radical anions (simultaneously on both fluorenes), the
intensities of the peaks at 439 and 520 nm are increased and
their maxima are slightly blue shifted (Fig. 2a). In addition,
new broad absorption peaks appear in the long wavelength
visible region (600–750 nm; isosbestic points at 530 and
580 nm) together with very low-intensity NIR absorption
above 900 nm (Fig. 2a,c). Further reduction of the fluorene
moieties to dianions results in a decrease of intensity for the
peaks at y440–500 nm and in the NIR region (.900 nm),
whereas the long wavelength visible absorption is broadened to
y600–850 nm (Fig. 2a–c). Repeating the SEC experiment
with y5 times higher concentration of 3 allowed the clear
observation of NIR absorption of the fluorene radical anions
on reduction of 3 (broad band at y900–1300 nm peaked at
Electrochemistry and spectroelectrochemistry
Electrochemical studies of compound 3 were performed in
benzonitrile solution using tetrabutylammonium hexafluoro-
phosphate (Bu4NPF6) as supporting electrolyte and Ag/Ag+ as
reference electrode (0.01 M AgNO3 in acetonitrile; potential
+0.187 V vs. Fc/Fc+ couple). Cyclic voltammetry experiments
demonstrated electroactivity of 3 in both reduction and
oxidation processes (Fig. 1). Reduction of compound 3 shows
typical behaviour observed for electron acceptors of the
fluorene series,14,19,31,32 i.e. two consecutive single electron
reduction waves resulting in a radical anion and dianion (on
each fluorene moiety), respectively. In the case of 3, the first
red1
reduction at E1/2
= 20.96 V is almost a reversible process
(DEpc-pared1 = 82 mV), whereas the second reduction is electro-
red2
red2
chemically irreversible (Epc
= 21.35 V; DEpc-pa
#
200 mV; ipc & ipa). Good reversibility of the first reduction
lmax = 1035 nm and low intense shoulder at y1450 nm), which
disappears on further reduction to the dianion species (Fig. 3).
Such NIR absorption is characteristic for radical anions of
fluorene electron acceptors and it was previously observed
for electrochemically generated radical anions of polynitro-
fluorene derivatives, charge transfer complexes of fluorene
acceptors with tetrathiafulvalene family donors, and ICT
complexation in fluorene–s-tetrathiafulvalene diads.33,34
Oxidation of compound 3 in the SEC experiment led to the
observation of the radical cation at ca. +1.2 V (vs. Ag wire) by
a slight decrease of intensity of the peak at 439 nm and an
increase in the intensity of the shoulder at 520 nm (isosbestic
+
?
point at y485 nm) (Fig. 4a). The radical cation 3 is also
characterised by two new absorption bands: a long wavelength
vis-NIR band at y650–900 nm and a broad low energy
NIR absorbtion at .1200 nm (peaked at lmax = 1530 nm)
(Fig. 4a,c). Further increase of the potential results in a
decrease of NIR absorption at 1530 nm, indicating further
Fig. 1 Cyclic voltammetry of compound 3 in benzonitrile, 0.1 N
Bu4NPF6; scan rate 100 mV s21 (solid line: second cycle for reduction/
oxidation; dashed line: oxidation up to +1.15 V).
+
?
oxidative transformations of the 3 species (more detailed
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Fig. 3 3D representation of SEC experiment of 3 in benzonitrile,
0.1 M Bu4NPF6 at high concentration in potential range of 0 to
21.55 V (1 mm quartz cell, transmission mode, Pt grid working
electrode, potentials are vs. Ag wire); NIR region has been shown.
opt
the bare sexithiophene a-6T (Eg = 2.1 eV from the onset)35
and is complicated by the intramolecular charge transfer band
of the donor–acceptor units (the ICT band for the terthio-
phene 15a is clearly distinguishable and has a maximum at
CV
532 nm). The electrochemical energy gap (Eg
= 1.5 eV,
estimated from onsets of reduction and oxidation processes
in the CV experiment, see Fig. 1) of compound 3 is signifi-
cantly lower than the optical value, showing the diad character
of the molecule (longest wavelength in 3 is due to ICT from the
dithiole donor ring on to the acceptor fluorene moiety,14,19,32
whereas the lowest oxidation potential arises from the
sexithiophene conjugated chain). The photoluminescence
shows a weak single peak around 620 nm in toluene solution,
which is bathochromically shifted to 700 nm in the solid state
(Fig. 5).
Photoinduced absorption (PIA) was measured on a home
made setup in a liquid N2 bath cryostat. The sample was
excited by a modulated 476 nm line of an Ar+ laser. Changes
in the transmission dT of a tungsten lamp were detected with a
Si/InGaAsSb sandwich detector and measured with a lock-in
amplifier. dT was normalized to the white light transmission T,
giving the PIA = 2dT/T.
The PIA spectrum of 3, Fig. 6, shows a broad peak between
0.9 and 1.9 eV with a plateau between 1.2 and 1.5 eV and a
shoulder at 1.0 eV. Additionally, an emerging PIA feature for
small probe energies is observed. Fig. 7 shows the expansion of
the PIA for the NIR-MIR with a PIA peak around 0.4 eV and
infrared active vibration (IRAV) bands in the MIR. The IRAV
bands are a clear indication of charged species. The PIA at
0.4 eV might be assigned to the cation absorption of 3.
The modulation frequency dependences at various probe
energies are shown in Fig. 8. The PIA at 1.1 eV and 1.38 eV
shows different modulation frequency dependences. Therefore,
it is concluded that the broad absorption in the VIS-NIR
originates from more than one photoexcited species. The
shoulder at 1.1 eV shows the weakest dependence on the
modulation frequency, indicating the shortest lifetimes. Fitting
of the in-phase component (X) results in a lifetime of t =
0.07 ms and a distribution factor of a = 0.40. The measured
Fig. 2 Spectroelectrochemistry of 3 in benzonitrile, 0.1 M Bu4NPF6
(1 mm quartz cell, transmission mode, Pt grid working electrode,
?
potentials are vs. Ag wire): a) reduction
3
A 32( 2)
;
b) three
dimensional representation of the SEC experiment in the potential
range of 0 to 21.45 V; c) 3D representation of SEC experiment,
magnified 600–1000 nm region.
studies are hindered by the irreversibility of the process as
followed from CV experiments).
Absorption and photoinduced absorption
spectroscopy
The optical absorption of 3, see Fig. 5, shows an onset around
opt
650 nm (Eg = 1.9 eV) in toluene solution as well as in the
solid state. This is slightly lower than the optical energy gap of
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Fig. 5 Absorption (thick line) and photoluminescence spectra
(dashed line) of 3 in a toluene solution (upper part) and as a solid
film (lower part). Photoluminescence is excited at 500 nm for the
solution and 476 nm for the thin film.
Fig. 6 PIA spectra of 3 thin film (full line, left axis) and a 3 : PCBM
1 : 1 blend film (open squares, right axis), spectra recorded at 80 K,
excitation at 476 nm with 40 mW.
Fig. 4 SEC of 3 in benzonitrile, 0.1 M Bu4NPF6 (1 mm quartz cell,
transmission mode, Pt grid working electrode, potentials are vs. Ag
wire): a) oxidation 3 A 3 +; b) 3D representation of the SEC
experiment in potential range of +0.8 to +2.0 V, visible region; c) 3D
?
representation of SEC experiment, NIR region.
Fig. 7 PIA spectrum in the MIR-NIR infrared spectrum of a 3 film
drop cast on KBr. Spectra are recorded at 80 K, 30 mW illumination
at 476 nm. The PIA is obtained by subtracting the spectrum under
illumination from the spectrum in the dark.
range for the out-of-phase signal (Y) as well as the
dependences at 0.62 and at 1.1 eV could not be fitted by any
modulation frequency model. Fig. 9 shows the excitation
intensity dependence at various probe energies. The signals are
fitted by a power law and scale with k # 1. Such a dependence
indicates first order kinetic recombination behaviour. This
strongly indicates intramolecular photoexcited species.
butyric acid methyl ester (PCBM) as a strong electron acceptor
red1
was investigated (E1/2
= 20.69 V vs. Ag wire).36 The PIA
spectrum shows peaks at 1.47 eV with a shoulder around
In order to find out more information concerning the cation
species of 3, a blend with fullerene derivative [6,6]-phenyl-C61
1.7 eV and at 0.7 eV (cf. with SEC experiments on generation
of 3 +, Fig. 4). All peaks in the blend show quite similar
?
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Fig. 8 Modulation frequency dependence of the PIA of 3 at 1.38 eV,
1.1 eV and 0.62 eV. No satisfactory fits could be obtained for these
frequency dependences. For the 1.1 eV PIA, the in-phase component
(squares) is fitted, giving a lifetime of 0.07 ms and a distribution
factor of 0.4.
Fig. 10 Modulation frequency dependence of the PIA of 3 : PCBM
1 : 1 blend at 1.71 eV, 1.46 eV and 0.69 eV.
dependences on the modulation frequency at all investigated
probe energies (see Fig. 10). None of the peaks could be fitted
by a modulation dependence model. The excitation power
dependence, Fig. 11, varies in a small range between k = 0.55
and k =0.65, fitted by a power law. These results indicate
bimolecular recombination as the dominant recombination
mechanism. The recombination behaviour of the photoexcita-
tions is quite different in the fullerene blend than for pristine 3,
leading us to conclude that different photophysical processes
are involved.
For 6T cation, absorption peaks at 1.55 and 0.8 eV are
reported.37–40 These energies are quite comparable with the
PIA of the 3 : PCBM blends. Although the two systems, 6T
and 3, are not directly comparable, it might be concluded that
the positive charge is primarily located on the sexithiophene
backbone. Furthermore, the PIA around 1.1 eV in 3 is
quenched in the blend with PCBM. The lifetime of this feature
Fig. 11 Excitation power dependence of the PIA of 3 : PCBM 1 : 1 at
1.71.eV, 1.46 eV and 0.69 eV. Measurements were performed at 80 K
with 37 Hz modulation. Fits are provided by a power law (thin line).
is found to be rather short and different to the positive charge.
We assume therefore that this might be correlated to a triplet
state or to a charge transfer state.
Photovoltaic devices
Photovoltaic devices were prepared for 3 : PCBM blends in a
sandwich type geometry. ITO coated with PEDOT (Baytron
PH) served as the positive contact, evaporated LiF/Al contact
as the negative contact. The I–V characteristics and photo-
current spectrum are shown in Fig. 12. The device charac-
teristics are mainly dominated by a low shunt resistance.
Under illumination, the photoeffect is observed. The photo-
current spectrum matches the absorption spectrum well and
shows its onset around 650 nm, coinciding with that of 3.
Devices show a photoeffect with a power conversion efficiency
ge = 0.1%. The device performance is mainly limited by the
low fill factor. The open circuit voltage of 0.64 V is com-
parable with other polythiophene fullerene bulk heterojunction
devices.
Fig. 9 Excitation power dependence of the PIA of 3 at 1.38 eV,
1.1 eV and 0.62 eV. Fits (thin line) are done by a power law. For high
excitation intensities, saturation is observed. These points are not
included into the fitting procedure.
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Conclusions
New routes towards fluorene functionalised oligothiophenes
have been investigated. Within this theme, a new sexithiophene
material with pendant 1,3-dithiol-2-ylidene-fluorene units has
been prepared, characterised and used in a bulk heterojunction
solar cell device. Whilst the power conversion efficiency is
modest, the photocurrent spectrum indicates that the ICT
process is also involved in the conversion of absorbed light to
current. This sets a precedent for incorporating pendant
donor–acceptor moieties into conjugated macromolecules for
the purpose of improving light harvesting efficiency.
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Acknowledgements
AK thanks the EPRSC (GR/T28379) and RB The Leverhulme
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