3,4-Ethylenedioxythiophene (EDOT) and 3,4-ethylenedithiathiophene (EDTT) as terminal blocks for oligothiophene dyes for DSSCs

Article abstract of DOI:10.1016/j.tetlet.2016.09.053

Thiophene pentamers with a cyanoacrylic acid anchoring group at one end and 3,4-ethylenedioxythiophene (EDOT) or 3,4-ethylenedithiathiophene (EDTT) units at the other end have been synthesized. Preliminary evaluation in DSSCs with a iodide/triiodide liquid electrolyte shows that in spite of the very close electronic properties of the two dyes, replacing EDOT by EDTT leads to a noticeable improvement of conversion efficiency under simulated solar light.

Full text of DOI:10.1016/j.tetlet.2016.09.053

Accepted Manuscript
3,4-ethylenedioxythiophene (EDOT) and 3,4-ethylenedithiathiophene) (EDTT)
as terminal blocks for oligothiophene dyes for DSSCs
Dora Demeter, Fabio Melchiorre, Paolo Biagini, Siriporn Jungsuttiwong,
Riccardo Po, Jean Roncali
PII:
S0040-4039(16)31211-4
DOI:
http://dx.doi.org/10.1016/j.tetlet.2016.09.053
TETL 48123
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
16 July 2016
9 September 2016
14 September 2016
Please cite this article as: Demeter, D., Melchiorre, F., Biagini, P., Jungsuttiwong, S., Po, R., Roncali, J., 3,4-
ethylenedioxythiophene (EDOT) and 3,4-ethylenedithiathiophene) (EDTT) as terminal blocks for oligothiophene
dyes for DSSCs, Tetrahedron Letters (2016), doi: http://dx.doi.org/10.1016/j.tetlet.2016.09.053
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3,4-ethylenedioxythiophene
(EDOT)
and
3,4-
ethylenedithiathiophene) (EDTT) as terminal blocks
for oligothiophene dyes for DSSCs
Dora Demeter,a) Fabio Melchiorre, b) Paolo Biagini,*b) Siriporn Jungsuttiwong, c)
Riccardo Po b) and Jean Roncali* a)
a Group Linear Conjugated Systems, CNRS Moltech-Anjou, University of Angers, 2Bd Lavoisier 49045 Angers,
France. (jeanroncali@gmail.com)
bCentro ricerche per le energie non convenzionali, Istituto ENI Donegani, ENI S.p.A.,via G. Fauser 4, 28100
Novara, Italy. (paolo.biagini@eni.com)
cCenter for organic electronics and alternative energies, Department of Chemistry, University of Ubon
Ratchathani, Ubon Ratchathani, 34190,Thailand
Abstract: Thiophene pentamers with a cyanoacrylic acid anchoring group at one end and 3,4-
ethylenedioxythiophene (EDOT) or 3,4-ethylenedithiathiophene (EDTT) units at the other
end have been synthesized. Preliminary evaluation in DSSCs with a iodide/triiodide liquid
electrolyte shows that in spite of the very close electronic properties of the two dyes,
replacing EDOT by EDTT leads to a noticeable improvement of conversion efficiency under
simulated solar light.
Introduction
Dye-sensitized solar cells (DSSCs) based on titanium dioxide have been a focus of
sustained interest for more than two decades.1-3 For many years ruthenium complexes have
represented the major class of sensitizers for DSSCs. While these dyes have permitted to
reach conversion efficiencies of ~12%,4 problems of cost, purification and environmental
impact have triggered the development of purely organic dyes.5-7 Most of these metal-free
sensitizers are built according to a D-π-A model in which a bulky electron-donor block such
as e.g. triarylamine or carbazole is connected to an electron-acceptor unit (A) through a π-
conjugating spacer. In many cases A is a cyanoacrylic acid group which acts at the same time
as electron-withdrawing unit and anchoring group on the TiO2 surface.5-7 During the past
decade many D-π-A dyes have been synthesized and PCE exceeding 10.0% have been
reported.8 The optimization of the structure of the dyes has led to analyze various structural
factors such as the size of the donor group,9 the nature of the -conjugating spacer, the
introduction of auxiliary acceptor group,5-7 or the hydrophilic/lipophilic balance of the dye.10
As for organic electronics in general, the development of active materials of simple structure
1

by efficient, clean and scalable synthesis represents a major issue for the a possible industrial
future.11-14
In this context, chromophores based on linearly -conjugated oligothiophenes have
been described by several groups.15-19 Thus, we have shown that octyl-substituted thiophene
tetramer, pentamer and heptamer with a cyanoacrylic acid end-group can reach PCE
approaching that obtained with the reference ruthenium dye N719 under the same
conditions.16,17
In our continuing interest for structure-properties relationships in oligothiophene
sensitizers16,17,19,20 we have undertaken a systematic analysis of the effects of insertion of 3,4-
ethylenedioxythiophene (EDOT) and 3,4-ethylenedithiathiophene (EDTT) on the electronic
properties of the dyes and on the performances of the resulting DSSCs.20 In this context, we
report here preliminary results on two quinquethiophene-chromophores in which the terminal
thiophene ring of a reference dye 5T17 has been replaced by (EDOT) (O4T) and (EDTT)
(S4T), (Scheme 1). In both cases an hexyl chain was attached at the terminal ring in order to
prevent possible reaction of the highly reactive free -position of the cation radical of these
two systems. As in the case of “classical” D-π-A dyes, the redox gradient resulting from the
introduction of a terminal donor unit is expected to contribute to localize the positive charge
of the oxidized state on the terminal thiophene ring and thus to limit intramolecular
recombination while facilitating hole-transfer to the redox shuttle of the electrolyte.
Scheme 1. Chemical structure of the target chromophores.
2

Results and discussion
The synthesis of O4T and S4T is depicted in Scheme 2. EDTT (2),21 2-
tributylstannyl-5-hexyl-3,4-ethylenedioxythiophene
(1b),22
3,3’’-dioctyl-2,2’:5’,2’’-
terthiophene (5)17 were prepared according to the already reported procedures. Treatment of
EDTT (2) with butyllithium and bromohexane in THF gave the corresponding 2-hexyl
derivatives 2a in 35 % yield. Compound 2a was converted into the corresponding Stille
reagents 2b by reaction with butyllithium and tributyltinchloride. Stille coupling of 1b and 2b
with 2-bromothiophene afforded the bithienyls 3a and 4a which were then again converted
into the corresponding stannylated derivatives 3b and 4b. Vilsmeier-Hack formylation of
4,4”-dioctylterthienyl 5 gave aldehyde 6a in 89 % yield. Bromination of 6a by NBS afforded
the bromoaldehyde 6b in 89% yield. Coupling of 6b with the Stille reagents 3b and 4b in the
presence of a palladium catalyst gave aldehydes 7 and 8 in 72 and 50 % yield respectively.
Finally a Knoevenagel condensation of aldehydes 7 and 8 with cyanoacrylic acid afforded the
target compounds O4T and S4T in 70 and 75 % yields respectively.
3

Scheme 2. Synthesis of O4T and S4T. a) n-BuLi, bromohexane, THF, -78°C; b) n-BuLi, THF, -78°C, Bu3SnCl;
c) Pd(PPh3)4, toluene reflux 12 h; d) POCl3/DMF, dichloroethane, 0°C; e) NBS,CHCl3/AcOH ; f) cyanoacetic
acid, AcONH4/AcOH, reflux 12h.
Fig. 1 shows the UV-Vis absorption spectra of O4T and S4T in dichloromethane
(DCM). The spectrum of O4T exhibits a first transition with a maximum at 414 nm
(max1) assigned to a * transition followed by a more intense band with max2 at 497
nm corresponding to an internal charge transfer. Replacement of the terminal 2-hexyl-
EDOT by 2-hexyl-EDTT leads to a further small red shift of max1 to 418 nm but to a 12
nm hypsochromic shift of max2 with a decrease of the relative intensity of the ICT
transition. As we have shown in many examples, the introduction of EDOT in a conjugated
oligothiophene chain leads to a planarization and rigidification of the systems through
intramolecular non covalent sulfur-oxygen interactions.22-25 These self-rigidification
effects are illustrated by the emergence of a vibrational fine structure in the absorption
spectrum as observed in Fig. 1 for O4T.
Although these results indicate that O4T has a smaller HOMO-LUMO gap than S4T, it is
well known that EDTT is a stronger donor than EDOT with an oxidation potential 0.18 V
lower than that of EDOT (1.32 vs 1.50 V).21 This apparent discrepancy could be due to
the generation of a dihedral angle between the plane of the conjugated system and the
EDTT end-group due to steric interactions between EDTT and the adjacent thiophene.26
0.5
0.4
0.3
0.2
0.1
0.0
300
400
500
600
700
800
Wavelength (nm)
Fig. 1. UV-Vis absorption in spectra in DCM. Blue: O4T, red: S4T.
Cyclic voltammetry was performed in DCM in the presence of tetrabutylammonium
hexafluorophosphate as the electrolyte. The CV of OT4 presents two successive reversible
oxidation processes corresponding to the formation of the cation radical and di-cation with
4

anodic peak potentials Epa1 and Epa2 at 0.71 and 1.03 V (Fig.2). Replacement of EDOT by
EDTT leads to a positive shift of Epa1 to 0.78V, which is consistent with the shorter effective
conjugation length indicated by UV-Vis absorption results. Comparison of the data in Table 1
to those of quinquethiophene 5T shows that the EDOT and EDTT end-groups lead to a
negative shift of Epa1 corresponding to an increase of the HOMO level by 0.19 and 0.12 eV
respectively.
The energy level of the HOMO and LUMO have been calculated by B3LYP/6-
31G(d,p) in CH2Cl2 solvent and the obtained results are in satisfying agreement with
experimental results with in particular a decrease of the HOMO level when replacing EDOT
by EDTT (Table 1 and Fig. 3).
5 A
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0.0
0.2 0.4
0.6 0.8
1.0 1.2
E (V vs SCE)
E (V vs SCE)
Fig.2. Cyclic voltammograms recorded in in 0.10 M Bu4NPF6/DCM, scan rate 100 mV s-1, Pt
electrodes. Blue O4T, Red: S4T.
Table 1. Data of UV-Vis absorption spectroscopy (in DCM) and cyclic voltammetry in 0.10 M Bu4NPF6/DCM,
scan rate 100 mV s-1, Pt electrodes, for the target compounds.
c
d
e
e
Eb
(V) (eV)
EHOMO
(eV)
ELUMO
(eV)
EHOMO
(eV)
ELUMO
(eV)
log
Epa1
Epr
(V)

max1,max2,
Dye
(nm)
M-1cm-1)
5Ta
OT4
ST4
2.59
0.90
-5.86
-5.67
-5.74
-3.41
-3.18
-3.18
-4.97
-4.87
-4.95
-2.86
-2.84
-2.85
372, 479
414, 497
418, 485
4.54
4.56
-1.06
-1.08
2.49
0.71
4.54
0.78 2.56 -1.10
a
c
from ref. 17; b based on max. using E0ox with an offset of 4.99 eV for SCE vs the vacuum level. d determined
by EHOMO-E. e calculated by B3LYP/6-31G(d,p) in CH2Cl2 solvent.
5

Fig. 3. HOMO (Left) and LUMO (right) distribution of the dyes calculated by B3LYP/6-31G(d,p) in
CH2Cl2 solvent. Top : O4T, bottom: S4T.
O4T and S4T have been evaluated as sensitizers in DSSC with a liquid electrolytes
according to already reported procedures.17 The absorption spectra of O4T and S4T adsorbed
on mesoporous TiO2 under the conditions used for cells fabrication are shown in Fig. 4. The
small differences observed in the absorption maxima and band intensities agree well with the
optical data of the two dyes.
Fig. 4. UV-Vis absorption spectra of O4T (blue) and S4T (red) adsorbed on mesoporous TiO2
.
Fig. 5 shows the current density vs voltage curves of the cells under AM 1.5 simulated
solar illumination with a power light intensity of 100 mW cm-2. The cells based on O4T
deliver a short-circuit current-density (Jsc) of 10.7 mA cm-2 and an open-circuit voltage (Voc)
of 0.63 V which combined with a fill factor (FF) of 0.75 leads to a PCE of 5.06 %.
6

Fig. 5. Top: Current-density vs voltage curves for DSSCs based on O4T (blue) and S4T (red) under AM 1.5
simulation solar illumination with a power light intensity of 100 mW cm-2. Bottom: external quantum efficiency
of DSSCs based on O4T (dashed line) and S4T (solid line).
These values are lower than those obtained with the octyl-substituted pentathiophene (5T)
(12.6 mA cm-2 and 6.30 %).17 However direct comparison is difficult and care should be
observed when comparing O4T and S4T with 5 as the different placement of the octyl chains
and the absence of end-capping hexyl chain in 5T can strongly affect the efficiency of the
dyes.17 On the other hand, comparison of the J/V curves for O4T and S4T shows that
replacement of EDOT by EDTT leads to an increase of Jsc from 10.7 to 14.4 mA cm-2 with a
slight increase of Voc to 0.64 V leading to a PCE of 6.73 %. The external quantum efficiency
spectra in Fig. 5 agree well with the results obtained under simulated solar illumination and
show a broad maximum of ~ 60% in the 440-480 nm region for O4T raising to 75 % for
S4T. These results clearly show that S4T leads to significantly higher efficiency than O4T.
While a definitive explanation of this effect requires further work, this result can be
tentatively attributed to a reduced charge-recombination due the synergistic effects of the
stronger electron donor properties of EDTT and of the dihedral twist angle between EDTT
and the rest of the conjugated chain. These combined effect could contribute to localize the
7

positive charge of the oxidized dye at the end of the chain thus limiting charge recombination
while favoring hole transfer to the redox shuttle of the electrolyte.
Table 2. Photovoltaic characteristics of the DSSC based on O4T and S4T under simulation solar illumination
(average values of three cells with deviation of less than 5%).
Jsc
Voc
(V)
PCE
(%)
Compd
FF
mA cm-2
OT4
ST4
10.7
14.4
0.63
0.64
0.75
0.73
5.06
6.73
Conclusion
Optical, electrochemical and theoretical results show that in spite of a stronger
intrinsic donor properties, the EDTT end-group leads to a shorter effective conjugation than
EDOT due possibly to steric hindrance to planarity. Nevertheless, replacement of EDOT by
EDTT leads to significant increase of conversion efficiency of the corresponding DSSCs.
Although a detailed understanding of the origin of this phenomenon clearly requires further
work, these first results can already have interesting implications for the structural
optimization of metal-free sensitizers for DSSCs. On the other hand, they also underline the
need of further investigations of structure-properties relationships in this class of compounds.
Work in this direction is now underway and will be reported in future publications.
References
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2013160201A1, (priority April 24th 2012), assigned to ENI SpA and CNRS.
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Mater. Chem. C 2015, 3, 7756-7761.
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9

Experimental Part
General
13
NMR spectra were recorded with a Bruker AVANCE III 300 (1H, 300 MHz and C, 75 MHz).
Chemical shifts are given in ppm relative to TMS. UV-Vis spectra were recorded with a Perkin Elmer
Lambda 19 or 950 spectrometer. Melting points are uncorrected. Matrix Assisted Laser
Desorption/Ionization was performed on MALDI-TOF MS BIFLEX III Bruker Daltonics spectrometer
using dithranol as matrix.
Cyclic voltammetry was performed in dichloromethane solutions purchased from SDS (HPLC
grade). Tetrabutylammonium hexafluorophosphate (0.10 M as supporting electrolyte) was purchased
from Acros and used without purification. Solutions were deaerated by nitrogen bubbling prior to each
experiment. Experiments were carried out in a one-compartment cell equipped with platinum
electrodes and saturated calomel reference electrode (SCE) with a Biologic SP-150 potentiostat with
positive feedback compensation.
Synthesis
EDTT (2),26 2-tributylstannyl-5-hexyl-3,4-ethylenedioxythiophene (1b)22 and 3,3’’-
dioctyl-2,2’:5’,2’’-terthiophene (5)17 were prepared according to the already reported
procedures.
6a. POCl3 (0.5 mL, 5.3 mmol) and 3mL of dry DMF are added to a solution of
dioctylterthienyl 5 (1.67 g, 3.53 mmol) in 1,2-dichloroethane (50 mL) at 0 °C under Ar. After
24 h stirring at 70 °C the solution is cooled to room temp, diluted with DCM and stirred with
a solution of NaCOOCH3 for 2 h. The organic layer is then washed with water and brine and
dried over MgSO4. After solvent removal the residue is chromatographed on silica gel 1:1
1
(eluent DCM/petroleum ether (1:1) to afford the desired compound (1.57 g, 89%). H NMR
(300 MHz, CDCl3: 9.83 (s, 1H), 7.59 (s, 1H), 7.27 (dd, 2H), 7.10 (d, 1H), 6.96 (d, 1H), 2.85-
2.75 (m, 4H), 1.17 -1.54 (m, 4H), 1.14-1.26 (m, 20H), 0.88-0.85 (m, 6H).
6b. NBS (0.61 g, 3.44 mmol) is added in small portions to a solution of aldehyde 6b
(1.57 g, 3.13 mmol) in 100 mL of 1:1 chloroform/acetic acid at 0 °C. After 3 h stirring at
room temp the mixture is poured into water and extracted with DCM. The organic layer is
washed with water, aqueous sodium bicarbonate, brine and again with water, and dried over
Na2SO4. After solvent removal chromatography on silica gel with 1:1 DCM/petroleum ether
1
as eluent gives (1.6 g, 89%) of a slightly yellow oil. H NMR (300 MHz, CDCl3: 9.83 (s,
1H), 7.59 (s, 1H), 7.23 (d, 1H), 7.05 (d, 1H), 6.91 (s, 1H), 2.81 (t, 2H), 2.71 (t, 2H), 1.73 -
1.56 (m, 4H), 1.14-1.26 (m, 20H), 0.89-0.85 (m, 6H).
10

3a. A mixture of 2-bromothiophene (0.75 g, 4.5 mmol), Stille reagent 1b (2.91 g, 5.63
mmol) and Pd(PPh3)4 (0.26 g, 5mol %) in 50 mL of anhydrous toluene is refluxed for 12 h.
After concentration, the residue is taken in DCM and the organic phase is washed twice with
an aqueous solution of NaHCO3 then with water. After drying over MgSO4 and solvent
removal, the residue is chromatographed on silica gel eluting with 1:3 DCM/petroleum ether
1
(1: 3) to give (0.78 g, 60 %) of yellowish oil . H NMR (300 MHz, CDCl3): 7.16 (dd, 1H),
7.14 (dd, 1H), 7.00 (dd, 1H), 4.32-4.29 (m, 2H), 4.24-4.20 (m, 2H), 2.63 (t, 2H), 1.65-1.55
(m, 2H), 1.39-1.27 (m, 6), 0.92-0.86 (m, 3H). 13C NMR (75 MHz, CDCl3): 137.4, 137.2,
135.1, 126.9, 122.9, 121.9, 116.1, 107.8, 65.0, 64.5, 31.5, 30.3, 28.7, 25.6, 22.5, 14.1.
2a. n-BuLi (1.6 M in hexane, 9.65 mmol, 6.03 mL) is added dropwise at -78 °C to a
solution of EDTT (1.6 g, 9.19 mmol) in dry THF under Ar atmosphere. The mixture is stirred
for 1 h at -78°C and 1-bromohexane (12 mmol, 1.72 ml) is added dropwise. After 30 min
stirring the mixture is allowed to warm slowly to room temperature and stirred for 12 h. After
dilution with diethyl ether, the organic phase is washed successively with a saturated NH4Cl
and water. After drying over MgSO4 and evaporation of the solvent, the residue is
chromatographed on silica gel eluting with 1:2 DCM petroleum ether to afford (0.8 g, 35%) of
1
a colorless oil. H NMR (300 MHz, CDCl3): 6.78 (s, 1H), 3.19 (s, 4H), 2.68 (t, 2H), 1.64-
13
1.59 (m, 2H), 1.33-1.29 (m, 6), 0.86 (t, 3H) C NMR (75 MHz, CDCl3): 137.1, 124.8, 120.6,
114.6, 31.5, 30.2, 28.8, 28.2, 27.8, 27.7, 22.5, 14.0. MS MALDI: 258.1. HRMS: calculated
258.0571; found 258.0574.
2b. n-BuLi (2.5 M in hexane, 3.11 mmol, 1.24 mL) is added dropwise at -20 °C to a
solution of 2a (0.73 g, 2.82 mmol) in dry THF under Ar atmosphere. The mixture is stirred for
1 h at -78°C. Tributyltin chloride (0.97 ml, 3.38 mmol) is added dropwise at the same
temperature. The mixture is stirred at -20°C for 30 min and then allowed to warm slowly to
room temperature. After dilution with diethyl ether, a saturated solution of NaF is added and
the mixture is stirred for 1 h. The precipitate is filtered off using celite and the filtrate is
washed with a saturated solution of NaHCO3 and water. After drying over MgSO4, the solvent
1
is evaporated and the product is used in the next step without further purification. H NMR
(300 MHz, CDCl3): 3.19-3.16 (m, 4H), 2.70 (t, 2H), 1.64-1.50 (m, 8H), 1.39-1.26 (m, 12),
1.17-1.12 (m, 6H), 0.94-0.86 (m, 12H).
4a. A mixture of 2-bromohiophene (0.46 g, 2.81 mmol), Stille reagent 2b (1.54 g,
2.81 mmol) and Pd(PPh3)4 (0.32 g, 10 mol %) in 50 mL of anhydrous toluene is refluxed for
12 h. After concentration, the residue is taken in DCM and the organic phase is washed twice
with an aqueous solution of NaHCO3 then with water. After drying over MgSO4 and solvent
11

removal, the residue is chromatographed on silica gel eluting with 1:3 DCM petroleum ether
1
to afford 0.43 g, (46 %)of a yellowish oil. H NMR (300 MHz, CDCl3): 7.28 (d, 1H), 7.23 (d,
1H), 7.06 (dd, 1H), 3.27-3.19 (m, 4H), 2.70 (t, 2H), 1.69-1.59 (m, 2H), 1.41-1.28 (m, 6), 0.92-
13
0.85 (m, 3H). C NMR (75 MHz, CDCl3): 135.9, 135.4, 127.3, 125.3, 124.9, 122.1, 121.9,
31.5, 30.2, 28.8, 28.4, 28.2, 27.6, 22.5, 14.0. MS MALDI: 340.1. HRMS: calculated
340.0448; found 340.0445.
(4b) n-BuLi (2.5 M in hexane, 1.51 mmol, 0.6 mL) is added dropwise at -78 °C to a
solution containing of 3a (0.43 g, 1.26 mmol) in dry THF. The mixture is stirred for 1 h at at -
78 °C. Tributyltin chloride (0.43 ml, 1.51 mmol) is then added dropwise at -40°C. The
mixture is stirred at -40°C for 30 min and allowed to warm slowly to room temp. After
dilution with diethyl ether, a saturated solution of NaF is added and the mixture is stirred for 1
h. The precipitate is filtered off using celite and the filtrate is washed with a saturated solution
of NaHCO3 then with water. After drying over MgSO4, the solvent is evaporated and the
1
product is used in the next step without further purification. H NMR (300 MHz, CDCl3):
7.35 (d, 2H), 7.10 (d, 2H), 3.24-3.22 (m, 4H), 2.72 (t, 2H), 1.64-1.51 (m, 8H), 1.41-1.25 (m,
12), 1.37-1.08 (m, 6H), 0.93-0.86 (m, 12H)
7. A mixture of aldehyde 6b (0.74 g, 1.34 mmol), Stille reagent 3b (0.66 g, 1.12
mmol) and Pd(PPh3)4 (77 mg, 5%) in 50 mL of anhydrous toluene is refluxed for 12 h. After
concentration, the residue is taken in DCM and the organic phase is washed twice with an
aqueous solution of NaHCO3 then with water. After drying over MgSO4 and solvent removal,
the residue is chromatographed on silica gel eluting with DCM to afford a red-orange solid
(0.65 g, 72 %). M.p.: 110-115 °C.
8. Prepared using the procedure already described for 7 from aldehyde 6b (0.21 g, 0.38
mmol), Stille reagent 24 (0.3 g, 0.47 mmol) and Pd(PPh3)4 (44 mg, 10%) in 50 mL of
1
anhydrous toluene to afford a red-orange oil (150 mg, 50 %). H NMR (300 MHz, CDCl3):
9.83 (s, 1H), 7.59 (s, 1H), 7.26 (d, 1H), 7.15 (d, 1H), 7.12 (d, 1H), 7.11 (d, 1H), 7.03 (s, 1H),
3.31-3.27 (m, 2H), 3.25-4.21 (m, 2H), 2.85-2.69 (m, 6H), 1.71-1.63 (m, 6H), 1.39-1.28 (m,
26), 0.89-0.85 (m, 9H). 13C NMR (75 MHz, CDCl3): 182.4, 141.1, 141.0, 140.2, 140.1, 139.0,
138.1, 136.1, 135.9, 135.5, 134.7, 134.3, 128.7, 127.8, 126.5, 125.9, 125.7, 125.3, 123.9,
122.4, 122.3, 31.8, 31.5, 30.4, 30.2, 29.6, 29.4, 29.3, 29.2, 28.8, 28.5, 28.2, 27.5, 22.6, 22.5,
14.0. MS MALDI: 838.3. HRMS: calculated 838.2533; found 838.2510 (2.6 ppm)
OT4. Aldehyde 7 (0.2 g, 0.25 mmol) is mixed with cyanoacetic acid (43 mg, 0.50
mmol), ammonium acetate (99 mg, 1.24 mmol) in glacial acetic acid (30 mL) and the solution
is refluxed for 12 h. After slowly cooling to room temp, a dark precipitate is formed, filtered
12

and washed with a diluted solution of sodium hydroxide and then with water to afford a black
1
solid (150 mg, 70%). M.p.: 240-245 °C. H NMR (300 MHz, CDCl3): 8.20 (s, 1H), 7.60 (s,
1H), 7.30 (d, 1H), 7.10 (d, 1H), 7.04 (d, 1H), 7.01 (d, 1H), 6.97 (s, 1H), 4.34-4.32 (m, 2H),
4.24-4.21 (m, 2H), 2.84-2.73 (m, 4H), 2.64 (t, 2H), 1.72-1.58 (m, 6H), 1.45-1.36 (m, 26),
0.91-0.83 (m, 9H). 13C NMR (75 MHz, CDCl3 not every carbon is visible): 141.8, 141.2,
140.5, 139.2, 137.9, 137.6, 137.5, 136.2, 134.6, 134.0, 133.6, 132.4, 128.4, 128.1, 126.0,
125.8, 123.9, 122.3, 116.8, 107.8, 65.1, 64.4, 31.9, 31.8, 31.5, 30.3, 30.1, 29.7, 29.6, 29.5,
29.4, 29.2, 28.8, 25.7, 22.6, 22.5, 14.1 MS MALDI: 873.1 HRMS: calcd 873.30476; found
873.30721.
ST4. Prepared using the procedure already described for OT4 from aldehyde 8 (0.1 g,
0.12 mmol), cyanoacetic acid (20 mg, 0.24 mmol), ammonium acetate (47 mg, 0.6 mmol) in
30 mL of glacial acetic acid (30 mL) to afford a black solid (75 mg, 75%). M.p.116-118°C.
1H NMR (300 MHz, THF-d8): 8.28 (s, 1H), 7.74 (s, 1H), 7.38 (d, 1H), 7.22 (d, 1H), 7.17 (m,
3H), 3.26-3.28 (m, 2H), 3.24-3.22 (m, 2H), 2.90-2.82 (m, 4H), 2.72 (t, 2H), 2.02-2.04 (m,
6H), 1.26-1.29 (m, 26), 0.82-0.88 (m, 9H). 13C NMR (75 MHz, THF-d8): 204.3, 164.0, 145.7,
141.9, 141.5, 140.9, 138.7, 136.4, 136.3, 135.7, 134.9, 134.3, 132.3, 132.2, 129.4, 129.1,
129.0, 128.9, 127.3, 126.9, 126.2, 125.6, 124.6, 123.7, 123.6, 116.4, 100.2, 32.6, 32.3, 31.1,
30.9, 30.4, 30.3, 30.2, 30.1, 29.8, 29.5, 28.9, 28.6, 27.8, 25.6, 23.4, 23.2, 14.2. MS MALDI:
905.3. HRMS: calcd 905.2591; found 905.2590.
13

Highlights



Synthesis of metal-free sensitizers for dye-sensitized solar cells
Analysis of the effect of the structure of the terminal thiophenic ring
Replacement of EDOT by EDTT leads to a noticeable improvement of conversion
efficiency
14

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3,4-ethylenedioxythiophene (EDOT) and
3,4-ethylenedithiathiophene) (EDTT) as terminal blocks
for oligothiophene dyes for DSSCs
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Dora Demeter, Fabio Melchiorre, Paolo Biagini,* Siriporn Jungsuttiwong, Riccardo Po and Jean Roncali*
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