Synthesis and properties of end-capped sexithiophenes incorporating the ethylene dithiothiophene unit

Article abstract of DOI:10.1039/b415610b

Two new end-capped sexithiophene derivatives (10a and 10b) have been synthesised and characterised by absorption spectroscopy, cyclic voltammetry and UV-vis spectroelectrochemistry. Both oligomers undergo two reversible, sequential oxidation processes (10a: E₁^1/2 = +0.61 V, E₂^1/2 = +0.97 V; 10b: E₁^1/2 = +0.70 V, E₂^1/2 = +0.91 V vs. Ag/AgCl) and the electronic band gaps of the materials are ca. 2.2 eV (solid state). Sexithiophene 10a has been incorporated as an electron donor material, with C₆₀ as acceptor, in a bilayer photovoltaic device. The power conversion efficiency is a modest 0.12%, whilst the maximum IPCE at 350 nm gave an external quantum efficiency of ca. 12%. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b415610b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and properties of end-capped sexithiophenes incorporating the
ethylene dithiothiophene unit
a
a
b
b
Christopher R. Mason, Peter J. Skabara,* Domenico Cupertino, John Schofield, Farideh Meghdadi,
c
c
Berndt Ebner and N. Serdar Sariciftci
c
Received 8th October 2004, Accepted 21st December 2004
First published as an Advance Article on the web 2nd February 2005
DOI: 10.1039/b415610b
Two new end-capped sexithiophene derivatives (10a and 10b) have been synthesised
and characterised by absorption spectroscopy, cyclic voltammetry and UV-vis
spectroelectrochemistry. Both oligomers undergo two reversible, sequential oxidation processes
K
1
K
K
1
K
2
(10a: E
5 +0.61 V, E
2
5 +0.97 V; 10b: E
5 +0.70 V, E
5 +0.91 V vs. Ag/AgCl) and the
electronic band gaps of the materials are ca. 2.2 eV (solid state). Sexithiophene 10a has been
incorporated as an electron donor material, with C60 as acceptor, in a bilayer photovoltaic device.
The power conversion efficiency is a modest 0.12%, whilst the maximum IPCE at 350 nm gave an
external quantum efficiency of ca. 12%.
Introduction
the structure of the oligomer and morphology of the bulk
material.
Polythiophene and its many variants have been studied
extensively for their use in numerous applications, such as
1
electroluminescence, photovoltaics, field effect transistors,
2
3
4
5
electrochromics and sensors. The electronic properties of
these materials rely on the effective conjugation length
within the polymers and thereby depend heavily on the
degree of planarity in the main chain and the number of
undesirable a–b9 links between repeat units. Well-defined
oligothiophenes are also of high interest as electronic mate-
6
rials and offer several advantages over analogous polymeric
materials: (i) monodispersity, (ii) a precise structure with
no isomeric impurities, (iii) high chemical stability, (iv)
good solubility, (v) direct processability from solution or
by vacuum deposition techniques. Moreover, these attributes
allow a direct and unequivocal correlation between struc-
ture and electronic properties and there is wide coverage
in the literature focussing on morphological, electronic
and spectroelectrochemical aspects of neutral and charged
Results and discussion
7
oligothiophenes.
Synthesis
Recently, we have reported the synthesis, electropolymerisa-
tion and characterisation of some 39,49-bis(alkylsulfanyl)-
8
terthiophenes (A–D). Our main interest in this work was the
The preparation of alkylterthiophenes 9a and 9b (Scheme 1)
followed a similar strategy used to prepare terthiophenes
8
,9
significance of the ethylenedithiothiophene (EDTT) unit,
which is a direct and underexplored analogue of the well-
known EDOT unit. Indeed, poly(EDOT) and poly(EDTT)
possess very different electronic characteristics, yet the latter
has a greater potential for interchain interactions due to the
increased number of sulfur atoms in the chain. From this
viewpoint, the role of EDTT in polythiophene materials is
highly interesting in terms of electronic and charge transport
properties, as well as self-assembly and film morphology. To
this end, we have synthesised the end-capped sexithiophenes
A–D. In this case, vinylene trithiocarbonate 3 was reacted
with one equivalent of LDA to obtain the organolithio inter-
mediate, which was subsequently reacted with 5-methyl-
thiophene-2-carbaldehyde and 2 to give the corresponding
alcohols 4a and 4b as racemic mixtures (91 and 87%, respec-
tively). The aldehyde 2 was prepared simply from 2-hexyl-
thiophene 1 by lithiation with n-butyllithium followed by
addition of DMF. Further treatment of alcohols 4a,b with two
equivalents of LDA and 2-thiophenecarbaldehyde afforded
compounds 5a and 5b. 4,5-Bis(2-arylhydroxymethyl)-1,3-
dithiole-2-thiones are unstable and undergo various rearrange-
1
0a and 10b in order to gain a closer understanding of the
1
0
electronic contribution of the EDTT unit and its effect upon
ment pathways, therefore diols 5a and 5b were reacted
almost immediately with MnO2 to give the stable diketone
products 6a and 6b (75% overall yield from 4a,b in both cases).
*peter.skabara@man.ac.uk
1
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Fig. 1 Cyclic voltammograms of compounds 9a (—), 10a (---) and
…
1
0b ( ) in CH
2
Cl
2
vs. Ag/AgCl reference electrode, Au disk working
electrode, TBAPF
6
as the supporting electrolyte (0.1 M), substrate
21
2
concentration 10 M, scan rate 100 mV s
3
.
irreversible oxidation wave at +1.00 V (Fig. 1) corresponding
+
?
to the redox process 9a A 9a . A small reduction peak is
observed at +0.60 V, which is probably due to the emergence
of a dimerised product (10a) at the working electrode. In
contrast, the sexithiophene derivatives 10a and 10b display
two sequential quasi-reversible oxidation processes leading
K
to radical cation and dication species, respectively (10a: E
K
1
5
+
0.61 V, DEa2c 5 100 mV; E
2
5 +0.97 V, DEa2c 5 50 mV;
K
1
K
2
1
0b: E
5 +0.70 V, DEa2c 5 50 mV; E
5 +0.91 V, DEa2c 5
60 mV). A film of the methyl substituted sexithiophene was
1
deposited by spin coating onto ITO glass and the solid-state
voltammogram of 10a is depicted in Fig. 2. The reversibility of
the redox processes is diminished due to the self-assembly and
charge trapping in the solid state. The first oxidation peak is
shifted to a more positive value by 0.24 V to +0.90 V, whilst
the corresponding reduction process remains unchanged at
Scheme 1 Reagents and conditions: (i) n-BuLi, THF, 0 uC, 15 min,
then DMF; (ii) LDA, THF, 278 uC, 20 min, then 2 or 5-methyl-
thiophene-2-carbaldehyde; (iii) LDA, THF, 278 uC, 20 min, then
+0.56 V. The second oxidation wave is less affected in the solid
state (E
K
2
5 +0.95 V) but the difference between the oxidation
and reduction peaks is increased to 170 mV. There was no
2
-thiophenecarbaldehyde; (iv) MnO
AcOH (3:1, v/v); (vi) P 10, NaHCO
NaOEt, THF, then BrCH CH Br; (viii) FeCl
2
, CH
, 1,4-dioxane, 90 uC, 3 h; (vii)
, CHCl
2 2 2 2 2
Cl ; (v) Hg(OAc) , CH Cl /
4
S
3
2
2
3
3
.
The thiocarbonyl derivatives were converted to their oxone
analogues by treatment with mercuric acetate (7a, 85%; 7b,
8
2%), before these compounds were cyclised to the corres-
ponding thiophenes using phosphorus pentasulfide (8a, 69%;
b, 75%). Alkylation of the dithiole-one species was accom-
8
plished by the action of sodium ethoxide, releasing the
dithiolate intermediate, followed by the addition of 1,2-
dibromoethane (9a, 76%; 9b, 67%). Finally, the sexithiophene
oligomers 10a and 10b were prepared by reacting the
terthiophenes 9a,b with ferric chloride in chloroform and the
materials were isolated in 50 and 40% yields, respectively.
Electrochemistry
The redox properties of the sexithiophene derivatives have
been studied in solution and as thin films on ITO glass.
The cyclic voltammogram of compound 9a shows a single
Fig. 2 Cyclic voltammogram of compound 10a as a spin-coated film
on ITO glass, under identical conditions to those presented in Fig. 1.
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evidence of coupling reactions during repetitive cycling
experiments, indicating that the alkyl groups are successful
in inhibiting any polymerisation processes. In contrast to the
solid state CV of dimethyl sexithiophene (DMSxT, which is
analogous to 10a but without the ethylene dithio substituents),
(lmax 457 nm) in 10a due to the elongation of the conjugated
chain. In comparison, the absorption maximum for DMSxT in
1
solution is 438 nm and unsubstituted sexithiophene (a-6T)
2
1
3
has a value of 432 nm. The difference in values of lmax
between DMSxT and 10a is due to the electronic contribution
of the alkyl sulfanyl groups and this effect is also observed for
1
0a shows good reversibility, although the oxidation peaks of
1
1
14
DMSxT are very similar (+0.91 and +1.05 V, vs. Ag/AgCl).
tris(butylsulfanyl)sexithiophene 11 (lmax 449 nm, CHCl3).
In the negative scan, we observed a quasi-reversible reduction
wave at 21.74 V (DEc2a 5 100 mV). The difference between
the onset of the first oxidation and reduction waves gives an
estimate of the electrochemical band gap of the material and
this was found to be 2.20 eV.
Absorption studies/spectroelectrochemistry
In contrast to the electrochemical behaviour of the two
sexithiophene derivatives, the absorption spectra of the mate-
rials in solution are identical. Fig. 3 shows the spectra for
compounds 9a and 10a in solution (dichloromethane) and for
1
0a also in the solid state. The absorption maximum for the
terthiophene unit appears at 373 nm, which is slightly higher
8
than that for compound C (368 nm) and in the range
expected for an additional alkyl group appended to the parent
terthiophene. This peak is shifted bathochromically by 84 nm
The optical band gap of the material can be deduced from
the longest wavelength absorption edge. The absorption peak
for 10a is particularly sharp for a conjugated system and the
band edge (536 nm) equates to a value of E
the solid state, the absorption maximum is further shifted to
77 nm (E 5 2.15 eV), as a function of interchain interactions
g
5 2.3 eV. In
4
g
1
5
in the solid. The optical band gaps are in excellent agreement
with the electrochemical measurement for E . In the solid-state
g
spectrum of 10b (Fig. 3(b)), the absorption peak shows greater
complexity than that of 10a (maxima at 481, 510 and 551 nm),
indicative of some higher ordering due to the hexyl chains. The
spectrum is essentially unchanged after annealing at 100 uC
for 1 h. The shoulder of the longest wavelength absorption
peak is extremely broad and covers several hundred nm,
making it difficult to assign the optical band gap for 10b in the
solid state. The same behaviour is observed for poly(C) and
8
poly(D).
Spectroelectrochemical (SEC) measurements in acetonitrile
were conducted for both sexithiophene derivatives as thin films
drop cast on ITO glass (Fig. 4). Here it is worth noting that the
drop-cast films gave absorption spectra more reminiscent of
the solution state experiments rather than the spin-coated
films. Above +0.85 V two additional peaks emerge at 651 and
.1100 nm for compound 10a and these continue to grow up to
+
1.40 V, which is just beyond the second oxidation process for
the material. Since the oxidation of 10a surpasses the dication
threshold, these peaks should be characteristic of the evolution
of a bipolaronic species. The oxidation peaks could be
sufficiently close together (+0.90 and +1.03 V) to bypass the
experimental detection of the intermediate polaron (10a ).
However, in the spectroelectrochemical characterisation of
DMSxT, peaks arising at 685 (p-dimer association) and 860 nm
+
?
?
+ 11
(
radical cation) were assigned to the signature of DMSxT .
…
Fig. 3 Absorption spectra of (a) compounds 9a ( ) and 10a (—) in
Similar conclusions have been made from experiments on
1
6
dichloromethane solution and 10a (---) as a film on ITO glass, and (b)
compound 10b as a film on ITO glass.
methoxyoligothiophene analogues.
For compound 10b,
similar absorption peaks are observed for the bipolaron at
1
448 | J. Mater. Chem., 2005, 15, 1446–1453
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Fig. 5 Photoluminescence spectra in blends of 10a and C60 (—) and
1
0a film (—). The electron transfer to C60 quenches the photolumines-
…
cence of 10a. Inset: Absorption (—) and photoluminescence (
spectra of pristine 10a.
)
Fig. 6 Photoinduced absorption spectra of mixed layer 10a:C60 (—)
…
Fig. 4 UV-vis spectroelectrochemical measurements of (a) 10a and
b) 10b on ITO glass.
and pure 10a ( ) solid films. Excitation at 476 nm, T 5 18 K.
(
Fig. 6 displays the photoinduced absorption (PIA) spectrum
6
31 and .1100 nm, although above +1.25 V we observed a
of pristine 10a and a co-evaporated film of 10a:C . The
6
0
rapid decrease in peak intensity due to the dissolution of the
material into the solvent. Compared to simple oligothiophenes
such as DMSxT and 11, the absorption bands for oligomers
photoinduced absorption spectrum for the pure oligomer
exhibits two peaks; these features can be assigned to a charged
species (polaronic state) at 0.7 and 1.5 eV. The high energy
band at 1.5 eV possibly has an additional contribution
from triplet–triplet absorption shifting the spectrum to higher
energies. The PIA signals of the co-evaporated film exhibit
two peaks at 0.73 and 1.4 eV. These peaks can be attributed
to the photoexcited positive polaron species without the
1
0a and 10b at significantly longer wavelengths (860 vs.
1100 nm, corresponding to the polaronic species) indicate
.
a more delocalised charged state and significant electronic
contribution from the ethylenedithio units.
Photophysical studies and photovoltaic properties
additional feature of a triplet. The PIA band centred at
2
1
.12 eV corresponds to the absorption of the C60 radical
2
Photoinduced charge transfer between conjugated polymers/
oligomers and fullerene is of considerable interest for photo-
2,23
anion
due to intermolecular charge separation, which is
2
5
1
7–24
necessary for solar energy conversion.
voltaic energy conversion.
Photoinduced absorption
The current–voltage characteristics of a bilayer heterostruc-
ture photovoltaic device of ITO|PEDOT:PSS|10a (40 nm)|C60
(60 nm)|Al are displayed in Fig. 7. Diode behaviour with
rectification of about 10 is observed. Under white light
studies have been carried out on photovoltaic devices based
on 10a as a donor, with C60 as the acceptor. Fig. 5 shows
the PL spectrum of pristine 10a with a maximum at 650 nm
(
1.9 eV) using an excitation wavelength of 476 nm. It is seen
2
2
that the relatively strong PL of 10a is significantly quenched by
blending of 10a with C60. This quenching can be related to the
illumination at 100 mW cm , through the ITO electrode, an
open circuit voltage (Voc) of 0.27 V and a short circuit current
2
2
photoinduced charge transfer from 10a excited state to C60
.
(Isc) of 1.14 mA cm have been measured. Using these data, a
filling factor (FF) of 0.4 and a power conversion efficiency of
0.12% were calculated. Fig. 8 shows the photocurrent spectrum
Fig. 5 (inset) shows the PL and absorption spectra of 10a with
the onset of absorption at 570 nm.
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EA1108 elemental analyser. Absorption spectra were mea-
sured on a Unicam UV 300 spectrophotometer.
All CV and spectroelectrochemical measurements were
performed on a CH Instruments 660A Electrochemical
2 2
Workstation with iR compensation, using anhydrous CH Cl
or acetonitrile as the solvent, aqueous Ag/AgCl as the
reference electrode and platinum wire and gold disk (or
ITO glass for SEC) as the counter and working electrodes,
respectively. All solutions were degassed (Ar) and, where
2
3
relevant, contained the substrate in concentrations ca. 10 M,
together with n-Bu NPF (0.1 M) as the supporting electrolyte.
Under these conditions, the redox potential for the FcH/FcH
couple was +0.48 V (CH Cl , vs. Ag/AgCl).
4
6
+
2
2
The photoluminescence (PL) and photoinduced absorption
+
(
PIA) spectra were measured using an excitation Ar laser
Fig. 7 Logarithmic and linear (inset) representations of the current
density–voltage characteristics of a 10a|C60 cell. The empty circles
at 476 nm. The changes in sample transmission DT resulting
from the mechanically chopped laser pump beam were detected
with a Si/InGaAsSb detector and using a lock-in-amplifier
2
2
represent data obtained under 100 mW cm white light illumination
while the full circles represent data measured in the dark.
(
typical chopping frequency was 73 Hz). DT were corrected for
the sample luminescence and normalised by the sample trans-
mission T to obtain the PIA (DT/T) spectrum. PIA and PL
measurements of co-evaporated films of 10a and C60 were
carried out at 18 K. Photovoltaic cells were produced as a
bilayer heterostructure of 10a/C60 using thermal evaporation.
This active bilayer was deposited on PEDOT:PSS coated ITO
substrates. Aluminium (Al) was used as the top electrode. The
thickness of the active layer was varied between 40 and 150 nm.
Current–voltage characteristics of photovoltaic cells were mea-
2
2
sured in the dark and under illumination with 100 mW cm
1.5 AM) from a solar simulator. Incident photon to collected
(
electron (IPCE) was measured with monochromatic light from
a Xe lamp and detected by a lock-in-amplifier.
5
-Hexylthiophene-2-carbaldehyde (2)
Fig. 8 IPCE characteristics of the bilayer device using
ITO|PEDOT|10a (40 nm)|C60 (60 nm)|Al (—) is compared with the
…
To a solution of 2-hexylthiophene 1 (6.50 ml, 36.1 mmol) in
dry tetrahydrofuran (80 ml), under dry nitrogen at 0 uC, was
added butyllithium (2.5 M in hexanes, 18.0 ml, 45.1 mmol).
The mixture was allowed to stir for 15 min at which time
dimethylformamide (4.21 ml, 54.1 mmol) was added. The
mixture was allowed to stir for 30 min whilst warming to
room temperature. Ammonium chloride solution (1 M, 100 ml)
was added and the product extracted into dichloromethane
absorption spectra of 10a (---) and C60
( ).
of the photovoltaic cell and compares the absorption spectra
of pristine 10a and C60 films. The spectrum shows the
maximum incident photon to collected electron (IPCE) at
3
50 nm with an external quantum efficiency about 12%, which
2
6,27
is comparable to other oligomer/C60 blends.
In order to
maximise the photocurrent, further optimisation of the device
structure will be necessary.
(3 6 75 ml) and dried (MgSO ). The solvents were removed
4
under reduced pressure and the product isolated by column
chromatography (silica, petroleum ether (bp 40–60 uC)–ethyl
acetate, 9:1 v/v) to afford the product as a yellow/orange oil
Experimental
1
6.43 g, 90.8%), H NMR (CDCl
(
3
) d/ppm: 9.85 (1H, s), 7.65
General
(1H, d, J 5 3.85), 6.94 (1H, d, J 5 3.7), 2.90 (2H, t, J 5 7.1),
1
1
.75 (2H, m), 1.35 (6H, m), 0.95 (3H, t, J 5 6.9); MS (EI) m/z:
+
Melting points were taken using a Stuart Scientific SMP1
13
96 (62%, M ), 167 (15%), 125 (100%), 97 (43%).
1
Melting Point apparatus and are uncorrected. H and
C
NMR spectra were recorded on a Varian Unity Innova
instrument at 300 and 75 MHz, respectively; chemical shifts
are given in ppm; all J values are in Hz. IR spectra for the
characterisation of the compounds were recorded on a ATI
Mattson Genesis Series FTIR spectrometer. Mass spectra (EI)
and high resolution mass spectra were recorded on Micromass
Trio 2000 and Kratos Concept spectrometers, respectively.
Elemental analyses were obtained on a Carlo Erba Instruments
4
-[Hydroxyl(5-methyl-2-thiophen-2-yl)ethyl][1,3]dithiole-2-
thione (4a)
A solution of 1,3-dithiole-2-thione 3 (4.78 g, 35.7 mmol)
in dry tetrahydrofuran (80 ml) under dry nitrogen was
cooled to 278 uC. Lithium diisopropylamide mono(tetrahy-
drofuran) (1.5 M in cyclohexanes, 21.9 ml, 32.8 mmol) was
added and the mixture allowed to stir for 20 min. A solution of
1
450 | J. Mater. Chem., 2005, 15, 1446–1453
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5
-methylthiophene-2-carbaldehyde (4.2 ml, 39.3 mmol) was
diisopropylamide mono(tetrahydrofuran) (1.5 M in cyclo-
hexanes, 38.1 ml, 57.2 mmol) and thiophene-2-carbaldehyde
(2.67 ml, 28.6 mmol). The product was obtained as a red oil
(y10 g).
added and the mixture stirred for a further 10 min. The
reaction was left to stir for 1.5 h whilst warming to room
temperature. Saturated sodium hydrogencarbonate (150 ml)
was added and the organic phase was removed. The aqueous
phase was washed with dichloromethane (3 6 50 ml) and the
4-(5-Methyl-2-thenoyl)-5-(2-thenoyl)[1,3]dithiol-2-one (6a)
combined organic extracts dried (MgSO ). After evaporation
4
To a solution of the diol 5a (y11 g) in dichloromethane
of the solvent under vacuum, the product was isolated by
column chromatography (silica, petroleum ether (bp 40–
(
250 ml), manganese dioxide (10 6 excess w/w, y110 g) was
added portionwise and the mixture allowed to stir for
approximately 2 min. The mixture was filtered through a
silica plug (eluted with dichloromethane) and the solvent
removed under reduced pressure. The product was isolated
by recrystallisation (dichloromethane–petroleum ether (bp 40–
6
0 uC)–dichloromethane, 1:1 v/v, with gradual change to
dichloromethane), to afford the product as a red oil (8.42 g,
1
0.8%), H NMR (CDCl
9
3
) d/ppm: 6.95 (1H, s), 6.90 (1H, d,
J 5 3.4), 6.67 (1H, d, J 5 3.5), 5.90 (1H, s), 2.90–3.05 (1H, br
1
s), 2.51 (3H, s); C NMR (CDCl
3
3
) d/ppm: 213.8, 150.0, 142.2,
6
0 uC)) to afford the product as a yellow solid (8.99 g, 75.4%
1
41.3, 126.5, 125.4, 124.3, 124.2, 68.9, 15.8; MS (CI) m/z: 261
+
from 4a), mp 126–128 uC (Found: C, 46.11; H, 2.05; S, 43.78%.
21
M + H, 100%) 243 (4%); FT-IR (KBr): n/cm : 3395, 3055,
(
1
Calc. for C14
CDCl ) d/ppm: 7.77 (2H, m), 7.59 (1H, d, J 5 3.95), 7.15 (1H,
t, J 5 4.0), 6.84 (1H, d, J 5 3.9) 2.56 (s, 3H); C NMR
CDCl ) d/ppm: 207.8, 176.2, 175.4, 154.8, 144.9, 144.3, 142.5,
8 2 5
H O S : C, 45.63; H, 2.19; S, 43.50%); H NMR
2
9 8 4
913, 1482, 1060, 803; HRMS (EI) m/z: calc. for C H OS
(
3
2
59.9458, found 259.9455.
1
3
(
3
4
-[(5-Hexyl-2-thiophen-2-yl)hydroxymethyl][1,3]dithiole-2-
1
40.3, 137.5, 136.7, 135.7, 128.9, 127.9, 16.6; MS (CI) m/z: 369
+
thione (4b)
21
:
(M + H, 100%), 295 (8%), 265 (17%); FT-IR (KBr): n/cm
3090, 1637, 1522, 1444, 1411, 1351, 1274, 1070.
Compound 4b was prepared as described for 4a using 1,3-
dithiole-2-thione 3 (4.00 g, 29.9 mmol), lithium diisopropyl-
amide mono(tetrahydrofuran) (1.5 M in cyclohexanes, 21.9 ml,
4-(5-Hexyl-2-thenoyl)-5-(2-thenoyl)[1,3]dithiol-2-one (6b)
3
2.8 mmol) and 5-hexylthiophene-2-carbaldehyde 2 (6.43 g,
The diketone 6b was prepared as described for 6a from the diol
b (y10 g) in dichloromethane (250 ml) and manganese
3
2.8 mmol) in tetrahydrofuran. The product was obtained as a
5
1
red oil (8.58 g, 87.1%), H NMR (CDCl ) d/ppm: 6.96 (1H, s),
3
dioxide (10 6 excess w/w, y100 g). The mixture was filtered
through a silica plug (eluted with dichloromethane) and the
solvent removed under reduced pressure to afford the product
6
.94 (1H, d, J 5 3.7), 6.70 (1H, d, J 5 3.6), 5.90 (1H, s), 2.94
(
(
1H, s), 2.81 (2H, t, J 5 7.65), 1.70 (2H, m), 1.36 (6H, m), 0.94
1
3
3H, t, J 5 6.7 and 6.6); C NMR (CDCl
3
) d/ppm: 213.4,
1
as an orange–red oil (8.50 g, 74.6% from 4b), H NMR
1
2
49.6, 148.0, 140.6, 125.9, 123.8, 123.7, 68.7, 31.4, 30.2, 28.7,
+
(
CDCl ) d/ppm: 7.73 (2H, m), 7.58 (1H, d, J 5 3.95), 7.14 (1H,
3
2.5, 14.0; MS (CI) m/z: 331 (M + H, 100%), 313 (8%); FT-IR
t, J 5 3.95 and 4.0), 6.83 (1H, d, J 5 3.95), 2.83 (2H, t,
1
21
(
KBr) n/cm : 3399, 2953, 2926, 2853, 1463, 1063, 1033, 803;
3
J 5 7.55), 1.68 (2H, m), 1.32 (6H, m), 0.93 (3H, t, J 5 6.6);
C
HRMS (EI) m/z: calc. for C14 330.0240, found 330.0240.
H18OS
4
NMR (CDCl ) d/ppm: 207.8, 176.3, 175.5, 161.0, 145.1, 144.4,
3
1
2
3
42.6, 140.0, 137.4, 136.5, 135.7, 128.9, 126.7, 31.7, 31.4, 31.1,
+
4
2
-[Hydroxy(5-methylthiophen-2-yl)methyl]-5-(hydroxythiophen-
-ylmethyl[1,3]dithiole-2-thione (5a)
8.8, 22.8, 14.3; MS (CI) m/z: 439 (M + H, 10%), 365 (13%),
35 (23%); FT-IR (KBr): n/cm 3089, 2926, 2854, 1631, 1523,
2
1
A solution of the alcohol 4a (8.42 g, 32.4 mmol) in dry
1444, 1409, 1353, 1272, 1069; HRMS (EI) m/z: calc. for
tetrahydrofuran (120 ml) under dry nitrogen was cooled to
C H O S 437.9910, found 437.9906.
1
9
18
2 5
2
78 uC. Lithium diisopropylamide mono(tetrahydrofuran)
1.5 M in cyclohexanes, 47.5 ml, 71.3 mmol) was added and the
mixture allowed to stir for 20 min. Thiophene-2-carbaldehyde
3.33 ml, 33.6 mmol) was added and the mixture stirred for
0 min. The reaction was stirred for a further 1.5 h whilst
(
4-(5-Methyl-2-thenoyl)-5-(2-thenoyl)[1,3]dithiol-2-one (7a)
To a solution of the diketone 6a (8.99 g, 24.4 mmol) in
dichloromethane–glacial acetic acid (3:1 v/v, 400 ml) was
added mercuric acetate (10.90 g, 34.2 mmol). The mixture was
stirred at room temperature for 16 h and filtered through a
silica plug (eluting with dichloromethane). The organic extract
was washed with water (2 6 100 ml), saturated sodium
(
1
warming to room temperature. Saturated sodium hydrogen-
carbonate (200 ml) was added and the organic phase was
removed. The aqueous phase was washed with dichloro-
methane (3 6 50 ml) and the combined organic extracts dried
4
hydrogencarbonate (2 6 100 ml) and dried (MgSO ). After
(
MgSO
and the product was isolated by column chromatography
silica, dichloromethane then ethyl acetate), to afford the
product as a red oil (y11 g).
4
). The solvent was removed under reduced pressure
evaporation of the solvent, the product was isolated by recry-
stallisation (dichloromethane–petroleum ether (bp 40–60 uC)
to afford the product as an off white–yellow solid (7.30 g,
(
8
4.9%), mp 174–177 uC (Found: C, 47.55; H, 2.24; S, 36.05%.
1
Calc. for C14
CDCl ) d/ppm: 7.76 (1H, d, J 5 3.8), 7.73 (1H, d, J 5 4.0),
.55 (1H, d, J 5 3.95), 7.14 (1H, t, J 5 3.9), 6.77 (1H, d,
8 3 4
H O S : C, 47.71; H, 2.29; S, 36.38%); H NMR
4
2
-[(5-Hexylthiophen-2-yl)hydroxymethyl]-5-(hydroxythiophen-
-ylmethyl[1,3]dithiole-2-thione (5b)
(
3
7
1
3
The product was obtained using the same conditions as
J 5 3.95), 2.54 (3H, s); C NMR (CDCl ) d/ppm: 187.3, 177.8,
3
those for 5a from alcohol 4b (8.58 g, 26.0 mmol), lithium
177.0, 154.4, 142.7, 140.6, 137.2, 136.6, 136.1, 135.6, 135.5,
This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 1446–1453 | 1451

View Article Online
+
1
28.8, 127.7, 16.5; MS (CI) m/z: 353 (M + H, 100%), 295
15%), 265 (21%); FT-IR (KBr) n/cm : 3092, 1670, 1633,
5-(5-Methylthiophen-2-yl)-7-thiophen-2-yl-2,3-dihydrothieno-
2
1
(
[3,4-b][1,4]dithiine [9a]
1
529, 1441, 1410, 1351, 1278.
To a solution of the terthiophene 8a (2.6 g, 7.38 mmol) in dry
tetrahydrofuran (80 ml) under dry nitrogen was added sodium
ethoxide (0.2 M solution in ethanol, 81.3 ml, 16.3 mmol). The
reaction was allowed to stir for 30 min at which time 1,2-
dibromoethane (0.64 ml, 7.38 mmol) was added and left to
stir at room temperature for 16 h. The reaction mixture was
filtered through a silica plug and the solvent removed under
reduced pressure. The product was isolated by column
chromatography (silica, petroleum ether (bp 40–60 uC)–
dichloromethane, 4:1 v/v) to afford the product as a yellow
solid (1.97 g, 75.8%), mp 102–104 uC (Found: C, 51.00; H,
4
-(5-Methyl-2-thenoyl)-5-(2-thenoyl)[1,3]dithiol-2-one (7b)
The tricarbonyl 7b was prepared as described for 7a from
the diketone 6b (8.50 g, 19.4 mmol) in dichloromethane–
glacial acetic acid (3:1 v/v, 360 ml) and mercuric acetate
(
8.66 g, 27.2 mmol). The product was obtained as an orange–
1
red oil (6.74 g, 82.3%), H NMR (CDCl ) d/ppm: 7.72 (2H, m),
3
7
.55 (1H, d, J 5 3.8), 7.1 (1H, t, J 5 3.85), 6.82 (1H, d, J 5 4.0)
.82 (2H, t, J 5 7.5 and 7.6), 1.66 (2H, m) 1.33 (6H, m), 0.92
2
1
3
(
3H, t, J 5 6.65); C NMR (CDCl
3
) d/ppm: 187.3, 177.9,
77.1, 160.6, 142.8, 140.3, 137.1, 136.5, 135.6, 135.5, 129.2,
28.8, 126.6, 31.7, 31.4, 31.1, 28.8, 22.8, 14.3; MS (CI) m/z: 423
1
1
3.43; S, 45.01%. Calc. for C15
H
12
S
5
: C, 51.10; H, 3.43;
1
S, 45.47%); H NMR (CDCl
(2H, m), 6.77 (1H, d, J 5 3.7), 3.29 (4H, s), 2.53 (3H, s);
NMR (CDCl ) d/ppm: 141.0, 135.1, 132.5, 128.2, 127.7, 127.1,
126.6, 126.3, 126.0, 125.9, 123.7, 123.1, 28.5, 15.6; MS (EI)
3
) d/ppm: 7.34 (2H, m), 7.11
+
21
:
13
C
(
M + H, 100%), 367 (9%), 335 (21%); FT-IR (KBr) n/cm
3
092, 2927, 2855, 1696, 1633, 1528, 1444, 1409, 1353 1273;
3
HRMS (EI) m/z: calc. for C H O S 442.0139, found
1
9
18
3 4
+
21
4
42.0135.
m/z: 352 (M , 49%), 260 (12%); FT-IR (KBr) n/cm : 3073,
912, 1490, 1271.
2
4
-(5-Methyl-2-thiophen-2-yl)-6-thiophen-2-yl-thieno[3,4-
d][1,3]dithiol-2-one (8a)
5-(5-Hexylthiophen-2-yl)-7-thiophen-2-yl-2,3-dihydrothieno[3,4-
b][1,4]dithiine (9b)
A mixture of the tricarbonyl 7a (4.00 g, 11.4 mmol), sodium
hydrogencarbonate (4.8 g, 57.1 mmol) and phosphorus
pentasulfide (25.3 g, 56.8 mmol) in 1,4-dioxane (200 ml) was
stirred whilst the temperature was raised to 90 uC and
maintained for 3 h. The mixture was cooled, water (200 ml)
The terthiophene 9b was prepared as described for 9a from
the terthiophene 8b (5.02 g, 11.9 mmol) in tetrahydrofuran
(100 ml), sodium ethoxide (0.2 M solution in ethanol, 130.8 ml,
26.2 mmol) and 1,2-dibromoethane (1.03 ml, 11.9 mmol). The
product was isolated by column chromatography (silica,
petroleum ether (bp 40–60 uC)–dichloromethane, 3:1 v/v) to
2 2
was added (CAUTION! H S and CO evolution) and the
suspension was allowed to stir overnight. The crude product
was filtered, washed with boiling water and dried in vacuo. The
crude product was redissolved in chloroform, dried (MgSO4),
and stirred with decolourising charcoal for 30 min. The
mixture was filtered through a silica plug (eluted with chloro-
form), the solvent reduced in volume and the product
precipitated with petroleum ether (bp 40–60 uC). The product
was isolated by filtration as an orange solid (2.76 g, 69.0%),
1
afford the product as a green–orange oil (3.34 g, 66.5%), H
NMR (CDCl ) d/ppm: 7.35 (2H, m), 7.17 (1H, d, J 5 3.7) 7.10
3
(1H, t, J 5 4.4), 6.79 (1H, d, J 5 3.5), 3.28 (4H, s), 2.86 (2H, t,
1
J 5 7.55), 1.74 (2H, m), 1.38 (6H, m), 0.95 (3H, t, J 5 6.6);
3
C
NMR (CDCl ) d/ppm: 147.2, 135.1, 132.3, 128.3, 127.7, 127.1,
3
126.6, 126.3, 125.9, 124.8, 123.7, 122.9, 31.8, 30.4, 29.1, 28.5,
+
22.8, 14.4; MS (EI) m/z: 422 (M , 65%), 351 (72%) 323 (19%);
2
1
mp 138–140 uC (Found: C, 48.04; H, 2.04; S, 45.75%. Calc.
FT-IR (KBr) n/cm : 3065, 2924, 2852, 1277; HRMS (EI) m/z:
calc. for C20 422.0325, found 422.0325.
1
: C, 47.70; H, 2.29; S, 45.43%); H NMR
for C14
H
8
OS
5
22 5
H S
(
CDCl ) d/ppm: 7.39 (1H, d, J 5 5.1) 7.24 (1H, d, J 5 3.7),
3
7
2
.13 (1H, t, J 5 3.7), 7.05 (1H, d, J 5 3.7), 6.78 (1H, d, J 5 3.7),
+
5,59-Dimethyl-39,39-,49,49--ethylenedithio-
.56 (3H, s); MS (CI) m/z: 353 (M + H, 100%), 324 (8%); FT-
2,29:59,20:50,2-:5-,29-:59-,20--sexithiophene (10a)
2
1
IR (KBr) n/cm : 2957, 2914, 1707, 1644, 1501.
To a solution of the terthiophene 9a (1.45 g, 4.12 mmol) in
anhydrous chloroform (60 ml) under dry nitrogen was added
anhydrous iron(III) chloride (0.87 g, 5.36 mmol). The reaction
was allowed to stir for 1 h at which time sodium cyanide was
added (0.1 M solution in water, 100 ml) and the temperature
was raised to 60 uC and maintained at this temperature for 3 h.
The aqueous phase was removed (CAUTION! cyanide, excess
aqueous iron(II) sulfate added to all aqueous washes) and the
organic phase was washed with water (4 6 100 ml) and dried
4
-(5-Hexyl-2-thiophen-2-yl)-6-thiophen-2-yl-thieno[3,4-
d][1,3]dithiol-2-one (8b)
The terthiophene 8b was prepared as described for 8a from the
tricarbonyl 7b (6.74 g, 15.9 mmol) in 1,4-dioxane (200 ml),
phosphorus pentasulfide (35.5 g, 79.9 mmol) and sodium
hydrogencarbonate (6.79 g, 80.8 mmol). The product was
obtained as an orange solid (5.02 g, 74.5%), mp 109–111 uC
(
Found: C, 53.99; H, 4.15; S, 37.58%. Calc. for C H OS : C,
1
(MgSO ). The solvent was reduced in volume and the crude
4
9
18
5
1
3.99; H, 4.29; S, 37.93%); H NMR (CDCl
5
3
) d/ppm: 7.29 (1H,
product precipitated with petroleum ether (bp 40–60 uC). The
crude product was washed (using Soxhlet apparatus) with
methanol and then extracted with chloroform. Once more, the
solvent was reduced in volume and the product precipitated
with petroleum ether (bp 40–60 uC)) to afford the product as
a red–deep red solid (0.72 g, 49.8%), mp 223 uC (by DSC)
d, J 5 5.0), 7.13 (1H, d, J 5 3.4), 7.03 (1H, t, J 5 4.4), 6.95
(
(
(
1H, d, J 5 3.6), 6.69 (1H, d, J 5 3.4) 2.76 (2H, t, J 5 7.6) 1.64
2H, m), 1.28 (6H, m), 0.85 (3H, t, J 5 6.3); MS (CI) m/z: 423
+
21
M + H, 100%), 391 (32%); FT-IR (KBr) n/cm : 2923, 2851,
1
717, 1651, 1503.
1
452 | J. Mater. Chem., 2005, 15, 1446–1453
This journal is ß The Royal Society of Chemistry 2005

View Article Online
E. Ort ´ı , P. M. Viruela and R. Pou-Am e´ rigo, J. Mater. Chem., 2000,
10, 2458; J. Roncali, J. Mater. Chem., 1999, 9, 1875.
T. Otsubo, Y. Aso and K. Takimiya, J. Mater. Chem., 2002, 12,
(
Found: C, 51.16; H, 2.78; S, 45.49%. Calc. for C30
H
22
S
10: C,
) d/ppm: 7.25
2H, d, J 5 3.8), 7.19 (2H, d, J 5 3.7), 7.14 (2H, d, J 5 3.5),
.77 (2H, d, J 5 3.65), 3.33 (8H, m) 2.52 (6H, s); MS (MALDI
1
5
1.25; H, 3.15; S, 45.60%); H NMR (CDCl
3
6
(
2
565.
6
7 A. Bongini, G. Barbarella, M. Zambianchi, V. Hern a´ ndez and
J. T. L o´ pez Navarette, Synth. Met., 2000, 108, 27; K. Tanaka,
Y. Matsuura, Y. Oshima, T. Yamabe and S. Hotta, Synth. Met.,
+
21
TOF) m/z: 702 (M , 100%); FT-IR (KBr) n/cm : 3054, 2913,
2
872, 1489, 1275, 790.
1
994, 66, 295; V. Hern a´ ndez, J. Casado, F. K. Ram ´ı rez, G. Zotti,
S. Hotta and J. T. L o´ pez Navarette, Synth. Met., 1996, 76, 277;
V. Hern a´ ndez, J. Casado, F. J. Ram ´ı rez, L. J. Alemany, S. Hotta
and J. T. L o´ pez Navarette, J. Phys. Chem., 1996, 100, 289;
V. Hern a´ ndez, J. Casado, F. J. Ram ´ı rez, G. Zotti, S. Hotta and
J. T. L o´ pez Navarette, J. Chem. Phys., 1996, 104, 9271; Ll. Fajar ´ı ,
E. Brillas, C. Alem a´ n and L. Juli a´ , J. Org. Chem., 1998, 63, 5324;
D. Fichou, J. Mater. Chem., 2000, 10, 571; J. Casado, H. E. Katz,
V. Hern a´ ndez and J. T. L o´ pez Navarette, J. Phys. Chem. B, 2002,
106, 2488; S. Ponomarenko and S. Kirchmeyer, J. Mater. Chem.,
5
2
,59-Dihexyl-39,39-,49,49--ethylenedithio-
,29:59,20:50,2-:5-,29-:59-,20--sexithiophene (10b)
The sexithiophene 10b was prepared as described for 10a from
the terthiophene 9b (1.77 g, 4.19 mmol) in chloroform (30 ml)
and anhydrous iron(III) chloride (0.75 g, 4.61 mmol). The
crude product was precipitated from solution with petroleum
ether (bp 40–60 uC) and washed (using Soxhlet extraction)
with methanol and extracted with chloroform. Once again, the
solvent was reduced in volume and the product precipitated
with petroleum ether (bp 40–60 uC) to afford the product as
a red–deep red solid (0.71 g, 40.2%), mp 177 uC (by DSC)
2
003, 13, 197; J. Casado, M. C. Ruiz Delgado, Y. Shirota,
V. Hern a´ ndez and J. T. L o´ pez Navarette, J. Phys. Chem. B, 2003,
07, 2637.
1
8 C. Pozo-Gonzalo, T. Khan, J. J. W. McDouall, P. J. Skabara,
D. M. Roberts, M. E. Light, S. J. Coles, M. B. Hursthouse,
H. Neugebauer, A. Cravino and N. S. Sariciftci, J. Mater. Chem.,
2002, 12, 500.
P. J. Skabara, I. M. Serebryakov, D. M. Roberts, I. F. Perepichka,
S. J. Coles and M. B. Hursthouse, J. Org. Chem., 1999, 64, 6418.
0 (a) I. M. Serebryakov, P. J. Skabara and I. F. Perepichka, J. Chem.
Soc., Perkin Trans. 2, 1999, 1405; (b) T. Khan, P. J. Skabara,
S. J. Coles and M. B. Hursthouse, Chem. Commun., 2001, 369.
1 J. Casado, M. Bengoechea, J. T. L o´ pez Navarrete and T. F. Otero,
Synth. Met., 1998, 95, 93.
2 S. Hotta and K. Waragai, J. Phys. Chem., 1993, 97, 7427.
3 C. Van Pham, A. Burkhardt, R. Shabana, D. D. Cunningham,
H. B. Mark, Jr. and H. Zimmer, Phosphorus, Sulfur, Silicon Relat.
Elem., 1989, 46, 153.
(
Found: C, 56.68; H, 5.08; S, 37.60%. Calc. for C30
22
H S
10: C,
) d/ppm: 7.27 (2H,
d, J 5 3.95), 7.20 (2H, d, J 5 3.8), 7.18 (2H, d, J 5 3.8), 6.80
2H, d, J 5 3.7), 3.35 (8H, m), 2.86 (4H, t, J 5 7.35), 1.75 (4H,
m), 1.37 (12H, m), 0.94 (6H, t, J 5 6.85); MS (MALDI TOF)
9
1
5
6.97; H, 5.02; S, 38.01%); H NMR (CDCl
3
1
(
1
+
21
m/z: 842 (M , 100%); FT-IR (KBr) n/cm : 3056, 2921, 2851,
274, 1459, 785.
1
1
1
Acknowledgements
1
4 F. Goldoni, D. Iarossi, A. Mucci and L. Schenetti, Chem.
Commun., 1997, 2175.
5 J. Roncali, Chem. Rev., 1997, 97, 173.
We thank Christoph Winder for his technical support and
fruitful discussions.
1
1
6 Y. Yu, E. Gunic, B. Zinger and L. L. Miller, J. Am. Chem. Soc.,
1
996, 118, 1013.
7 N. S. Sariciftci, L. Smilowitz, A. J. Heeger and F. Wudl, Science,
992, 258, 1474.
a
a
Christopher R. Mason, Peter J. Skabara,* Domenico Cupertino,
b
1
b
John Schofield, Farideh Meghdadi, Berndt Ebner and
c
c
1
c
N. Serdar Sariciftci
1
1
8 C. Y. Yang and A. J. Heeger, Synth. Met., 1996, 104, 4267.
9 C. J. Barabec, H. Johannson, F. Padinger, H. Neugebauer,
J. C. Hummelen and N. S. Sariciftci, Sol. Energy Mater. Sol.
Cells, 2000, 61, 19.
0 N. S. Sariciftci, D. Braun, C. Zhang, V. I. Srdanov, A. J. Heeger,
G. Stucky and F. Wudl, Appl. Phys. Lett., 1993, 62, 585.
1 C. Winder, C. Lungschmied, G. Matt, N. S. Sariciftci,
A. F. Nogueira, I. Montanari, J. R. Durrant, C. Arndt,
U. Zokhavets and G. Gobsch, Synth. Met., 2003, 139, 577.
2 S. Luzzati, M. Panigoni and M. Catellani, Synth. Met., 2001, 116,
171.
23 R. A. Janssen, M. P. T. Christiaans, R. Pakbaz, D. Moses,
J. C. Hummelen and N. S. Sariciftci, J. Chem. Phys., 1995, 102,
2628.
24 J. Poplawsky, E. Ehrenfreund, J. Cornil, J. L. Bredas, R. Pugh,
M. Ibrahim and J. Frank, Synth. Met., 1995, 69, 401.
25 M. A. Loi, P. Denk, H. Hoppe, H. Neugebauer, C. Winder,
D. Meissner, C. Brabec, N. S. Sariciftci, A. Gouloumis, P. Vazquez
and T. Torres, J. Mater. Chem., 2003, 13, 700.
a
Department of Chemistry, University of Manchester, Oxford Road,
Manchester, UK M13 9PL. E-mail: peter.skabara@man.ac.uk
Avecia Ltd, Hexagon House, Blackley, P. O. Box 42, Manchester, UK
M9 8ZS
Linz Institute for Organic Solar Cells (LIOS), Physical Chemistry,
Johannes Kepler University Linz, Linz A-4040, Austria
b
2
c
2
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This journal is ß The Royal Society of Chemistry 2005
J. Mater. Chem., 2005, 15, 1446–1453 | 1453