Symmetric and unsymmetric donor functionalization. comparing structural and spectral benefits of chromophores for dye-sensitized solar cells

Article abstract of DOI:10.1039/b911397p

A series of organic chromophores have been synthesized in order to investigate the benefits of structural versus spectral properties as well as the absorption properties and solar cell performance with unsymmetrically introduced substituents in the chromophore. Exceptionally high open-circuit voltage, V_oc, was found for the symmetrical structurally benefited dye, which also gave the best overall solar cell performance.

Full text of DOI:10.1039/b911397p

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Symmetric and unsymmetric donor functionalization. comparing structural
and spectral benefits of chromophores for dye-sensitized solar cells†
Daniel P. Hagberg,a Xiao Jiang,b Erik Gabrielsson,a Mats Linder,c Tannia Marinado,b Tore Brinck,c
a
Anders Hagfeldtbd and Licheng Sun
*
Received 10th June 2009, Accepted 31st July 2009
First published as an Advance Article on the web 24th August 2009
DOI: 10.1039/b911397p
A series of organic chromophores have been synthesized in order to investigate the benefits of structural
versus spectral properties as well as the absorption properties and solar cell performance with
unsymmetrically introduced substituents in the chromophore. Exceptionally high open-circuit voltage,
Voc, was found for the symmetrical structurally benefited dye, which also gave the best overall solar
cell performance.
LUMO excitation, while the panchromatic absorbance of
Introduction
common Ru-dyes is due to numerous weakly allowed transitions.
The demand for alternative power sources has drawn attention
The HOMO-1 energy of organic dyes is generally too low to
to a variety of light-harvesting devices. Among these, dye-
make any significant contribution except in the near-UV region.
sensitized solar cells (DSCs)1 have attracted a large number of
Moreover, it has proven difficult to absorb the near-IR photons
research groups in recent decades. So far, the conventional
ruthenium-based sensitizers such as N3/N7192,3 and black dye4
by extending the p-conjugation of the molecule, without nega-
tively affecting the stability and overall efficiency.23 Given these
have reached the promising solar-energy-to-electricity conver-
limitations, it remains a challenge for organic sensitizers to
sion efficiencies of 11% under AM 1.5 irradiation. In compar-
compete with the high incident photon-to-current conversion
ison, organic sensitizers have reached efficiencies of 6–9.8%.5–19
efficiency (IPCE) of e.g. N719.2
However, organic chromophores for DSCs exhibiting high molar
In recent years a number of chromophores with insulating
extinction coefficients, can be prepared and easily modified and
alkyl groups, both on the linker and the donor, have been pub-
are environmentally friendly. The high extinction coefficients are
lished.11,19 It has been shown that the efficiencies of the sensitizers
needed in solid-state devices where limitation of the photovoltaic
are strongly dependent on the position of these insulating groups.
performance by low hole mobility and insufficient pore filling
Hara and co-workers have elegantly elucidated the influence of
requires thin TiO2 films.20 Organic chromophores based on tri-
the position/number of alkyl chains on the open-circuit voltage,
Voc, and hence the efficiency of the sensitizers.6 Moreover, it is
phenylamine (TPA) donors have proved potent sensitizers in
DSCs over the past few years.10,11,17–19,21,22 Many research groups
well-established that electron-donating groups at the para-posi-
focus on broadening the absorption spectra of the chromophores
tion of the TPA phenyls, e.g. amine or alkoxy groups, red-shift
in order to increase their efficiency. The spectral red-shift of
the absorption spectrum and increase efficiency, where the degree
organic sensitizers is frequently associated with parallel limita-
of red-shift is dependent on the electron-donating strength of the
tions; firstly, expanding the absorption by increasing the conju-
substituents.11,17 Tian et al. reported a series of chromophores
gation can result in sensitization problems often referred to as
aggregation,23–25 secondly, decreasing the HOMO–LUMO gap
with aryl amines at the para-position of the TPA moiety and they
indeed shifted the spectra bathochromically.14 One drawback
can result in driving force limitations for the injection and
with this strategy however, is the risk of raising the HOMO
regeneration.26 One principal difference between organic and Ru-
potential too high, hence not matching the redox potential of the
based chromophores is that the main absorption band in the
electrolyte.
organic dye arises mainly due to a single, strong HOMO /
We have designed and synthesized a series of chromophores
based on the TPA donor group (Fig. 1). The TPA group has been
substituted in the para-position with additional phenyl groups,
aOrganic Chemistry, Center of Molecular Devices, Department of
Chemistry, KTH Chemical Science and Engineering, 10044 Stockholm,
Sweden
bInorganic Chemistry, Center of Molecular Devices, Department of
Chemistry, KTH Chemical Science and Engineering, 10044 Stockholm,
Sweden
cPhysical Chemistry, Center of Molecular Devices, Department of
Chemistry, KTH Chemical Science and Engineering, 10044 Stockholm,
Sweden
dDepartment of Physical and Analytical Chemistry, Uppsala University,
Box 259, 75105 Uppsala, Sweden
† Electronic supplementary information (ESI) available: H NMR and
1
13C NMR spectra for the new compounds described in this paper,
complete ref. 29 and molecular coordinates. See DOI: 10.1039/b911397p
Fig. 1 Structures of the chromophores.
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functionalized with p-dimethylamine (D29) and/or p,o-butoxy
groups (D35, D37). This series can be used to study the impact of
structural versus spectral benefits on the overall solar cell effi-
ciency. The chromophore D29, with dimethylaniline substituents
can be regarded as a chromophore with spectral benefits, due to
the strong electron-donating properties of the amine groups.
The chromophore D35, on the other hand, receives structural
benefits from the large steric bulk introduced by the four butoxy
groups, but it is not red-shifted as much as D29, due to lower
electron-donating strength. The chromophore D37 is a hybrid of
the two parent chromophores D29 and D35, and is included since
it encompasses a combination of properties that might be
optimal for solar cell performance. To the best of our knowledge,
this is the first time such an unsymmetric organic chromophore
has been presented, however, unsymmetric squaraine dyes have
been investigated.27 Moreover, breaking the symmetry of the
molecule gives rise to orbital rearrangements. As has been
observed in a different class of dyes, two explicit acceptor groups
give two near-degenerate LUMOs.28
absorption band is predicted to be broadened at the blue end,
because of a larger contribution from the HOMO-1 / LUMO
transition, which is essentially forbidden in D29 and D35. This
difference is mainly due to a better overlap between HOMO-1
and LUMO in D37 (Fig. 2). The calculations predict a systematic
red-shift, when increasing the number of dimethyl amines in the
functional groups.
Results and discussion
The straightforward synthesis of the chromophores, illustrated in
Scheme 1, is strongly based on the Suzuki coupling reaction of
commercially available materials.31 Suzuki coupling was carried
out with unprotected 5-formyl-2-thiophene-boronic acid and
4-(diphenylamino)-bromobenzene under microwave irradiation
to directly yield the aldehyde in 1, necessary in the final reactions.
Bromination of the para-positions of the triphenylamine moiety
was carried out with NBS to yield 2. Additional Suzuki coupling
reactions under microwave irradiation introduced the function-
alized phenyl groups. The final reaction of the three dyes was the
condensation of the respective aldehydes with cyanoacetic acid
by the Knoevenagel reaction in presence of piperidine.
Prior to the synthesis of the three novel organic chromophores
(Scheme 1), their electronic and spectral properties were evalu-
ated using hybrid-DFT computational methods. Gas-phase
optimized geometries and absorption spectra were obtained at
the B3LYP/6-31G(d) level, employing the Gaussian03 program
package.29 Time-dependent DFT was used to obtain the vertical
transitions. Although it is well-known that hybrid-DFT gravely
underestimates the charge-transfer excitations,30 present in all
organic sensitizers, B3LYP does successfully reproduce the
general characteristics of the spectra, making it the method of
choice in this study. Fig. 3 displays the simulated absorption
spectra together with the relative energy levels of the three dyes.
The calculations indicate that the unsymmetric dye D37 has more
evenly spaced occupied frontier orbitals, having contributions
from both the D29 and D35 electronic structures. The main
Table 1 presents the experimental spectral and electrochemical
properties of the dyes in ethanol solution and adsorbed onto
TiO2, respectively. The predicted red-shift in the absorption
maxima, due to the increasing electron-donating strength of the
moieties, was observed. As seen in Fig. 4, the difference is small,
and the theoretical overestimation of absorption wavelength is
evident. In the experimental spectrum for D37, a slight broad-
ening at the blue end of the visible band is observed. We believe
that this is due to the second transition discussed above. The
appearance of only one band can be explained by solvation
broadening and the fact that the two vertical transitions are
closer in reality than in the theoretical spectrum, thus embedded
Scheme 1 Synthesis of the chromophores.
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Fig. 2 Isodensity plots of the new organic chromophores, showing HOMO-1, HOMO and LUMO. Good charge separation is obtained by transferring
an electron from an occupied orbital to the LUMO, facilitating electron injection. D37 is the only of the three dyes where the HOMO-1 orbital extends
over the linker region, which explains the relatively strong second transition. Note that the butoxy groups have been truncated in the calculations, in
order to save computational time. This does not significantly affect the electronic or spectral properties.
in the same band. The estimated LUMO energy levels, calculated
from E(HOMOox) ꢀ E0–0, seem to be sufficiently more negative
than the TiO2 conduction-band edge for all three dyes, reported
in Table 1.
However, the LUMO energy level of D37 is surprisingly
negative, and not at all in the same region as the two parent
chromophores, D29 and D35, which was predicted. The mono-
chromatic IPCE spectra are presented in Fig. 5, and show that
the highest IPCE is obtained from the D35 solar cell. The IPCE
of D37 is a combination of the two parent chromophores, but not
quite as red as the D29 nor as high as the D35. A comparison of
the photovoltaic performances of solar cells based on the
different dyes D29, D35 and D37 under AM 1.5 illumination is
presented in Table 2. The efficiencies were 4.83, 6.00 and 5.24%
for D29, D35 and D37, respectively.
The D35 dye showed remarkably high Voc, 0.75 V, which we
Fig. 3 Calculated gas-phase absorption spectra of the senzitisers. Each
believe is due to an insulating effect of the bulky butoxy chains.17
spectrum was generated by employing an arbitrary Gaussian broadening
of 3000 cmꢀ1, and the calculated transitions are shown as vertical lines.
The right panel shows relative energy levels.
This is also supported by our preliminary electron-lifetime
studies, which are presented in Fig. 6. Comparing the electron
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Table 1 Experimental data for the spectral and electrochemical properties of the dyes
Dye
Absmax/nma
3/Mꢀ1cmꢀ1
Emmax/nm
E(HOMOox)/Vb vs. NHE
E0–0/eVc (Abs/Em)
E(LUMO)/Vd vs. NHE
D29
D35
D37
482i, 456ii
445i, 444ii
459i, 446ii
70 200
70 100
37 200
612
597
576
0.84
1.04
0.81
2.31
2.41
2.41
ꢀ1.47
ꢀ1.37
ꢀ1.6
a
b
Absorption of the dyes in EtOH solutioni and adsorbed onto TiO2
.
following conditions: Pt working electrode and Pt counter electrode; electrolyte, 0.05 M tetrabutylammonium hexafluorophosphate, TBA(PF6), in
The ground-state oxidation potential of the dyes was measured under the
ii
c
DMF. Potentials measured vs. Fc+/Fc were converted to normal hydrogen electrode (NHE) by addition of +0.63 V. 0–0 transition energy,
estimated from the intercept of the normalized absorption and emission spectra in ethanol. Estimated LUMO energies vs. NHE from the
d
estimated highest-occupied molecular orbital (HOMO) energies obtained from the ground-state oxidation potential, E(HOMOox), by subtracting the
0–0 transition energy, E0–0.
Table 2 Current and voltage characteristics of DSCsa
Dye
Voc/V
Jsc/mA cmꢀ2
ff
h (%)
D29b
D35c
D37b
0.67
0.75
0.71
12.00
12.96
12.50
0.60
0.61
0.59
4.83
6.00
5.24
a
Photovoltaic performance under AM 1.5 irradiation of DSCs based on
D29, D35, and D37 dyes, respectively, based on 0.6 M
tetrabutylammonium iodide (TBAI), 0.1 M LiI, 0.05 M I2, 0.5 M 4-
tert-butylpyridine (4-TBP), 0.05 M guanidinium thiocyanate (GuSCN)
b
in acetonitrile. Dye bath: acetonitrile solution (2 ꢁ 10ꢀ4 M) with
c
addition of CDCA (6 ꢁ 10ꢀ3 M). Dye bath: ethanol solution (2 ꢁ
10ꢀ4 M) with addition of CDCA (6 ꢁ 10ꢀ3 M).
Fig. 4 Normalized absorption spectra of D29 (red), D35 (green), and
D37 (black line) measured in ethanol.
Fig. 6 Electron lifetime as a function of extracted charge under open-
circuit conditions for DSCs based on D29, D35, and D37, respectively.
Fig. 5 Spectra of incident photon-to-current conversion efficiency
(IPCE) for DSCs based on D29, D35, and D37 dyes.
photovoltaic performance. We are currently working on the
synthesis of spectrally and structurally benefited chromophores.
lifetimes as a function of Voc, DSCs based on D35 showed
significantly longer electron lifetimes compared to D37 and D29.
This provides support for the four insulating butoxy chains
introduced in D35 preventing electrons in TiO2 from recombin-
ing with redox species yielding a high Voc. Despite the red-shift in
the absorption spectrum and hence a more optimal HOMO and
Conclusions
In conclusion, this series of chromophores showing high effi-
ciencies in DSCs were used in a comparative study of structural
versus spectral benefits. The TPA donor units were functional-
ized with electron-donating groups and bulky substituents,
respectively. Neither the red-shift of the symmetrical dye D29 nor
the expanded spectral response from the unsymmetrical dye D37
can compete with the structural benefits of the symmetrical dye
D35. In the present study, solar cells based on D35 yielded the
ꢀ
LUMO energy-level composition with regards to the TiO2–Iꢀ/I3
system, the D29 dye showed a comparatively low performance
due to the lack of insulating bulky substituents. The predicted
broader spectral response of D37 was not decisive for the
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highest efficiency, thereby showing that structural properties are
of central importance to solar cell efficiency.
Electrochemical measurements
Electrochemical experiments were performed with a CH Instru-
ments electrochemical workstation (model 660A) using
a conventional three-electrode electrochemical cell. The sup-
porting electrolyte consisted of 0.05 M TBAPF6 in DMF and dye
concentrations of 1 ꢁ 10ꢀ3 M. A glass carbon disk was used as
the working electrode, a platinum wire served as a counter
electrode, a Ag/Ag+ electrode was utilized as a reference elec-
trode, and the scan rate was 100 mV sꢀ1. All potentials were
calibrated vs. a normal hydrogen electrode (NHE) by the addi-
tion of ferrocene as an internal standard taking Eꢂ (Fc/Fc+) ¼
630 mV vs. NHE.
Experimental section
General procedure for preparation of solar cells
Fluorine-doped tin oxide (FTO) glass plates (Pilkington-TEC8)
were cleaned using (in order) detergent solution, water and
ethanol using an ultrasonic bath overnight. The conducting glass
substrates were immersed in 40 mM aqueous TiCl4 solution at
70 ꢂC for 30 min and washed with water and ethanol. The screen-
printing procedure was repeated (layers of ꢃ3 mm) with TiO2
paste to obtain a transparent nanocrystalline film of thickness
around 10 mm. The preparation of TiO2 paste (ꢃ25 nm colloidal
particles) is described elsewhere.32 A scattering layer (ꢃ3 mm,
PST-400C, JGC Catalysts and Chemicals LTD) was deposited
and a final thickness of 13.0 ꢄ 0.2 mm was attained. The TiO2
electrodes were gradually heated in an oven (Nabertherm
Controller P320) in an air atmosphere. The temperature gradient
program had four levels at 180 ꢂC (10 min), 320 ꢂC (10 min),
Electron-lifetime measurements
Electron lifetimes in the complete dye sensitized solar cell device
were measured in a system using a red-light-emitting diode
(Luxeon Star 1 W, lmax ¼ 640 nm) as light source. Voltage traces
were recorded using a 16-bit resolution data acquisition board
(DAQ National Instruments) in combination with a current
amplifier (Stanford Research Systems SR570) and a custom-
made system using electromagnetic relay switches. Electron
lifetimes were determined by monitoring the transient voltage
responses after a small light intensity modulation (square-wave
modulation, <10% intensity of 0.5 Hz), and the step response was
recorded by the DAQ board. The voltage response was well fitted
to first-order decay, and time constants were thus obtained.
ꢂ
ꢂ
390 C (10 min) and 500 C (60 min). After sintering, the elec-
trodes once again passed, as described above, a post-TiCl4
treatment. A second and final sintering, at 500 ꢂC forꢂ30 min, was
performed. When the temperature decreased to 70 C after the
sintering, the electrodes were immersed into 0.2 mM dye solu-
tions and kept for 16–17 h in darkness at room temperature. The
solvent was ethanol (99.5%) for D35, acetonitrile (99.8%) for D29
and D37, respectively. After the adsorption of the dyes, the
electrode was rinsed with the same solvent. The electrodes were
assembled with a platinized counter electrode using a hot-melt
Surlyn film. The redox electrolyte consisted of 0.1 M LiI (99.9%),
0.6 M TBAI (98%), 0.05 M I2 (99.9%), 0.5 M 4-TBP (99%), and
0.05 M GuSCN in acetonitrile (99.8%) was introduced through
a hole drilled in the back of the counter electrode. Finally, the
hole was also sealed with Surlyn film.
General synthetic procedure
1H and 13C NMR spectra were recorded on Bruker 500 and 400
MHz instruments by using the residual signals d ¼ 7.26 ppm and
77.0 ppm from CDCl3, d ¼ 2.50 and 39.4 ppm from [D6]-DMSO
and d ¼ 2.05, 29.84, and 206.26 ppm from [D6]-acetone, as
1
internal references for H and 13C respectively. HRMS was per-
formed using a Q-Tof Micro (Micromass Inc., Manchester,
England) mass spectrometer equipped with a Z-spray ionization
source.
Photocurrent density–voltage (J–V) measurements.
The current–voltage characteristics and incident photon-to-
current conversion efficiencies (IPCEs) of the prepared solar
cells were studied. Current–voltage measurements were carried
out with a Newport Oriel 300 W Solar Simulator (Model 91160)
5-(4-(Diphenylamino)phenyl)thiophene-2-carbaldehyde (1). The
synthetic procedure and characterizations were presented in an
earlier publication.23
with AM 1.5
G filter. The incident light intensity was
5-(4-(Bis(4-bromophenyl)amino)phenyl)thiophene-2-carbalde-
1000 W mꢀ2 calibrated with a standard Si solar cell. For the
J–V curves, the solar cells were evaluated by using a slightly
larger black mask (working area 0.25 cm2, aperture area of
0.49 cm2) on the cell surface to avoid diffusive light. IPCE
measurements were carried out with a Xenon arc lamp (300 W),
a 1/8 m monochromator, a source/meter, and a power meter
with a 818-UV detector head.
hyde (2). 1 (400 mg, 1.13 mmol) was dissolved in THF (50 mL)
ꢂ
and the solution was cooled to 0 C. NBS (411 mg, 2.31 mmol)
was added in one portion and the mixture was stirred for 1 h at
0 ꢂC. The mixture was allowed to warm to ambient temperature
and stirring was continued for 1.5 h. The reaction was quenched
by addition of water (50 mL), extracted with diethyl ether
(50 mL), washed with water (3 ꢁ 50 mL) and purified by filtra-
tion through a plug of silica gel with DCM. Solvent removal by
rotary evaporation yielded 2 as a yellow solid (577 mg, 100%). 1H
NMR (500 MHz, CDCl3) d ppm 9.86 (s, 1H), 7.71 (d, J ¼ 3.9 Hz,
1H), 7.54 (d, J ¼ 8.7 Hz, 2H), 7.39 (d, J ¼ 8.8 Hz, 4H), 7.32 (d, J
¼ 4.0 Hz, 1H), 7.06 (d, J ¼ 8.7 Hz, 2H), 6.99 (d, J ¼ 5.8 Hz, 4H);
13C NMR (126 MHz, CDCl3) d ppm 182.6, 153.9, 148.0, 145.7,
141.7, 137.6, 132.6, 127.5, 127.4, 126.2, 123.27, 123.2, 116.7.
Photophysical measurements
All UV–vis spectra of dye solutions (1 ꢁ 10ꢀ5 M in ethanol) were
recorded on a Lambda 750 spectrophotometer using a 1 cm
cuvette. All fluorescence spectra were recorded on a Cary Eclipse
fluorescence spectrophotometer.
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5-(4-(Bis(40-(dimethylamino)biphenyl-4-yl)amino)phenyl)thio-
phene-2-carbaldehyde (5a). 2 (200 mg, 390 mmol), 4-(dimethyla-
mino)phenylboronic acid (142 mg, 857 mmol), K2CO3 (269 mg,
1.95 mmol) and PdCl2(dppf) (31.8 mg, 39 mmol) was added to
a mixture of DCM and methanol (3:2, ꢂ5 mL). The mixture was
heated by microwave irradiation to 70 C for 20 min. The reac-
tion was quenched by the addition of water (20 mL) and
extracted with DCM (2 ꢁ 30 mL). The combined extracts were
dried over anhydrous MgSO4 and filtered. Solvent removal by
rotary evaporation followed by column chromatography over
silica gel with DCM yielded 5a as an orange solid (165 mg, 71%).
1H NMR (500 MHz, CDCl3) d ppm 9.86 (s, 1H), 7.71 (d, J ¼ 4.0
Hz, 1H), 7.54 (d, J ¼ 8.7 Hz, 2H), 7.50 (d, J ¼ 8.6 Hz, 8H), 7.31
(d, J ¼ 4.0 Hz, 1H), 7.20 (d, J ¼ 8.5 Hz, 4H), 7.14 (d, J ¼ 8.7 Hz,
2H), 6.81 (d, J ¼ 8.8 Hz, 4H), 3.00 (s, 12H); 13C NMR (126 MHz,
CDCl3) d ppm 182.5, 154.7, 149.8, 149.1, 144.9, 141.1, 137.7,
136.7, 128.4, 127.3, 127.2, 127.0, 125.7, 125.4, 122.7, 122.0, 112.7,
40.5; HRMS (TOF-MS-ESI) m/z: 593.2474 [M+]; calcd for
C39H35N3OS [M+]: 593.2495.
CDCl3) d ppm 9.86 (s, 1H), 7.71 (d, J ¼ 4.0 Hz, 1H), 7.54 (d, J ¼
8.8 Hz, 2H), 7.48 (d, J ¼ 8.6 Hz, 4H), 7.31 (d, J ¼ 4.0 Hz, 1H),
7.28 ꢀ 7.25 (m, 2H), 7.21 ꢀ 7.15 (m, 6H), 6.57 ꢀ 6.53 (m, 4H),
4.02 ꢀ 3.95 (m, 8H), 1.81 ꢀ 1.72 (m, 8H), 1.55 ꢀ 1.49 (m, 4H),
1.45 ꢀ 1.41 (m, 4H), 1.00 (t, J ¼ 7.4, 7.4 Hz, 6H), 0.94 (t, J ¼ 7.4,
7.4 Hz, 6H); 13C NMR (126 MHz, CDCl3) d ppm 182.5, 159.6,
156.9, 154.8, 149.2, 145.0, 141.1, 137.7, 133.9, 130.8, 130.3, 127.1,
125.8, 124.4, 122.8, 122.7, 122.3, 105.3, 100.4, 68.1, 67.7, 31.3,
31.1, 19.3, 19.2, 13.8, 13.8; HRMS (TOF-MS-ESI) m/z: 795.3959
[M+]; calcd for C51H57NO5S [M+]: 795.3952 and 5c (22 mg, 22%)
as an orange solid; 1H NMR (500 MHz, [D6]-acetone) d ppm 9.88
(s, 1H), 7.90 (d, J ¼ 4.0 Hz, 1H), 7.67 (d, J ¼ 8.7 Hz, 2H), 7.57 (d,
J ¼ 8.6 Hz, 2H), 7.52 (m, 5H), 7.25 (d, J ¼ 8.4 Hz, 1H), 7.16 (m,
4H), 7.09 (d, J ¼ 8.7 Hz, 2H), 6.80 (d, J ¼ 8.8 Hz, 2H), 6.61 (d,
J ¼ 2.3 Hz, 1H), 6.57 (dd, J ¼ 8.4, 2.3 Hz, 1H), 4.00 (m, 4H), 2.95
(s, 6H), 1.71 (m, 4H), 1.45 (m, 4H), 0.95 (t, J ¼ 7.4, 7.4 Hz, 3H),
0.89 (t, J ¼ 7.4, 7.4 Hz, 3H); 13C NMR (126 MHz, [D6]-acetone)
d ppm 182.6, 159.7, 156.9, 154.8, 149.8, 149.2, 145.0, 141.1, 137.7,
136.8, 134.0, 130.8, 130.4, 128.5, 127.3, 127.2, 127.1, 125.8, 125.5,
124.4, 122.8, 122.7, 122.2, 112.8, 105.3, 100.5, 68.2, 67.8, 40.6,
31.4, 31.2, 19.4, 19.29, 13.9, 13.8; HRMS (TOF-MS-ESI) m/z:
694.3223 [M+]; calcd for C45H46N2O3S [M+]: 694.3224.
(E)-3-(5-(4-(Bis(40-(dimethylamino)biphenyl-4-yl)amino)phenyl)-
thiophen-2-yl)-2-cyanoacrylic acid (D29). A 50 mL acetonitrile
solution of 5a (110 mg, 185 mmol) and cyanoacetic acid (18.9 mg,
222 mmol) was refluxed in the presence of piperidine (40 mg,
476 mmol) for 4 h. The solvent was removed by rotary evapora-
tion. The product was purified by chromatography over silica gel
(short column) using a DCM to DCM–methanol (9 : 1) gradient
and washed with a 1 M HCl solution upon which the product
precipitated. The precipitate was dissolved in methanol which was
removed by rotary evaporation to yield D29 (87 mg, 71%) as
(E)-3-(5-(4-(Bis(20,40-dibutoxybiphenyl-4-yl)amino)phenyl)thio-
phen-2-yl)-2-cyanoacrylic acid (D35). A 10 mL acetonitrile solu-
tion of 5b (75 mg, 94 mmol) and cyanoacetic acid (9.62 mg,
113 mmol) was refluxed in the presence of piperidine (27 mg,
317 mmol) for 24 h. Additional cyanoacetic acid (10 mg,
117 mmol) and piperidine (27 mg, 317 mmol) was added and reflux
ꢂ
continued for 3 h. The temperature was reduced to 50 C and
1
a dark-red solid. H NMR (500 MHz, [D6]-DMSO) d ppm 8.47
stirring continued for 40 h. The solvent was removed by rotary
evaporation and the product purified by chromatography over
silica gel (short column) using a DCM to DCM–methanol (9 : 1)
gradient. The solvent was removed by rotary evaporation. The
product was dissolved in DCM (20 mL) and washed with 1 M
HCl (2 ꢁ 20 mL) and the solvent was removed by rotary evap-
oration to finally yield D35 (70 mg, 86%) as a red solid. 1H NMR
(500 MHz, [D6]-DMSO) d ppm 8.45 (s, 1H), 7.98 (d, J ¼ 4.1 Hz,
1H), 7.69 (d, J ¼ 8.7 Hz, 2H), 7.64 (d, J ¼ 4.0 Hz, 1H), 7.47 (d,
J ¼ 8.5 Hz, 4H), 7.23 (d, J ¼ 8.4 Hz, 2H), 7.11 (d, J ¼ 8.5 Hz,
4H), 7.05 (d, J ¼ 8.7 Hz, 2H), 6.62 (d, J ¼ 1.9 Hz, 2H), 6.58 (dd,
J ¼ 8.5, 2.0 Hz, 2H), 4.02 ꢀ 3.95 (m, 8H), 1.73 ꢀ 1.60 (m, 8H),
1.45 (dd, J ¼ 14.9, 7.4 Hz, 4H), 1.39 ꢀ 1.34 (m, 4H), 0.94 (t, J ¼
7.4, 7.4 Hz, 6H), 0.87 (t, J ¼ 7.4, 7.4 Hz, 6H); 13C NMR (126
MHz, [D6]-DMSO) d ppm 163.6, 159.2, 156.4, 148.5, 144.2,
133.7, 133.4, 131.4, 130.4, 130.1, 128.5, 127.3, 125.2, 124.1, 123.8,
121.7, 121.5, 116.6, 105.8, 100.1, 67.5, 67.1, 30.7, 30.5, 18.7, 18.6,
13.6, 13.5; HRMS (TOF-MS-ESI) m/z: 861.3956 [Mꢀ]; calcd for
C54H57N2O6S [Mꢀ]: 861.3943.
(s, 1H), 8.00 (m, 1H), 7.73 (m, 6H), 7.66 (m, 5H), 7.50 (s, 4H), 7.18
(d, J ¼ 8.3 Hz, 4H), 7.10 (d, J ¼ 8.4 Hz, 2H), 3.08 (s, 12H); 13
C
NMR (126 MHz, [D6]-DMSO) d ppm 163.7, 153.0, 148.1, 146.6,
145.4, 141.7, 134.7, 134.6, 133.6, 127.6, 127.5, 127.4, 127.4, 126.0,
125.0, 124.2, 122.6, 118.3, 116.5, 97.3, 43.5; HRMS (TOF-MS-
ESI) m/z: 660.2562 [M+]; calcd for C42H36N4O2S [M+]: 660.2553.
5-(4-(Bis(20,40-dibutoxybiphenyl-4-yl)amino)phenyl)thiophene-2-
carbaldehyde (5b), 5-(4-((20,40-dibutoxybiphenyl-4-yl)(40-(dimethy-
lamino)biphenyl-4-yl)amino)phenyl)thiophene-2-carbaldehyde (5c).
2 (74 mg, 144 mmol), 2,4-dibutoxyphenylboronic acid (81 mg,
303 mmol), K2CO3 (100 mg, 721 mmol) and PdCl2(dppf) was
added to a mixture of DCM and methanol (3 : 2, 3 mL). The
mixture was heated by microwave irradiation to 60 ꢂC for 30 min.
The reaction was quenched by addition of 10 mL water and
extracted with DCM (2 ꢁ 20 mL). Solvent removal by rotary
evaporation followed by column chromatography over silica gel
with ethyl acetate–pentane (1 : 5) yielded a crude intermediate
(88 mg). The crude intermediate was added to a mixture of
toluene and methanol (3:2, 3 mL) together with 4-(dimethyla-
mino)phenylboronic acid (56.6 mg, 342 mmol), K2CO3 (118 mg,
855 mmol) and PdCl2(dppf) (14 mg, 17ꢂmmol). The mixture was
heated by microwave irradiation to 70 C for 20 min. The reac-
tion was quenched by addition of water (20 mL) and extracted
with DCM (2 ꢁ 20 mL). Solvent removal by rotary evaporation
followed by column chromatography over silica gel using
a DCM–pentane 1 : 1 to DCM–methanol 9 : 1 gradient and
(E)-2-Cyano-3-(5-(4-((20,40-dibutoxybiphenyl-4-yl)(40-(dimethyla-
mino)biphenyl-4-yl)amino)phenyl)thiophen-2-yl)acrylic acid (D37).
A 10 mL acetonitrile solution of 5c (23 mg, 33 mmol) and cyano-
acetic acid (3.38 mg, 40 mmol) was refluxed in the presence of
piperidine (27 mg, 317 mmol) for 4 h. The solvent was removed by
rotary evaporation. The product was purified by chromatography
over silica gel (short column) using a DCM to DCM–methanol
(9 : 1) gradient. The solvent was removed by rotary evaporation,
the product dissolved in DCM (20 mL) and washed with 1 M HCl
1
yielded 5b (35 mg, 30%) as a yellow solid; H NMR (500 MHz,
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 7232–7238 | 7237

View Article Online
10 K. R. J. Thomas, Y.-C. Hsu, J. T. Lin, K.-M. Lee, K.-C. Ho, C.-
H. Lai, Y.-M. Cheng and P.-T. Chou, Chem. Mater., 2008, 20,
1830.
11 D. P. Hagberg, J.-H. Yum, H. Lee, F. De Angelis, T. Marinado,
K. M. Karlsson, R. Humphry-Baker, L. Sun, A. Hagfeldt,
€
M. Gratzel and Md. K. Nazeeruddin, J. Am. Chem. Soc., 2008, 130,
(2 ꢁ 20 mL). Removal of the solvent with rotary evaporation
yielded D37 (17 mg, 67%) as a dark red solid. 1H NMR (500 MHz,
[D6]-DMSO) d ppm 8.47 (s, 1H), 8.00 (d, J ¼ 4.2 Hz, 1H), 7.71
(d, J ¼ 8.7 Hz, 2H), 7.68 ꢀ 7.59 (m, 5H), 7.48 (d, J ¼ 8.6 Hz, 2H),
7.24 (d, J ¼ 8.4 Hz, 1H), 7.15 (t, J ¼ 8.7, 8.5 Hz, 5H), 7.07 (d, J ¼ 8.7
Hz, 2H), 6.63 (d, J ¼ 2.2 Hz, 1H), 6.58 (dd, J ¼ 8.5, 2.2 Hz, 1H), 4.03
ꢀ 3.95 (m, 4H), 3.02 (s, 6H), 1.75 ꢀ 1.61 (m, 4H), 1.50 ꢀ 1.32 (m,
6259.
12 S. Ito, H. Miura, S. Uchida, M. Takata, K. Sumioka, P. Liska,
ꢀ
€
P. Comte, P. Pechy and M. Gratzel, Chem. Commun., 2008, 5194.
13 H. Qin, S. Wenger, M. Xu, X. Jing, P. Wang, S. M. Zakeeruddin and
4H), 0.95 (t, J ¼ 7.4, 7.4 Hz, 3H), 0.88 (t, J ¼ 7.4, 7.4 Hz, 3H); 13
C
€
M. Gratzel, J. Am. Chem. Soc., 2008, 130, 9202.
NMR (126 MHz, [D6]-DMSO) d ppm 163.7, 159.3, 156.4, 153.2,
148.4, 146.6, 144.2, 141.7, 133.9, 133.4, 130.5, 130.2, 127.4, 127.0,
125.3, 125.1, 124.2, 123.9, 121.8, 121.6, 116.5, 105.9, 100.2, 97.1,
67.6, 67.1, 30.7, 30.6, 18.7, 18.7, 13.6, 13.6; HRMS (TOF-MS-ESI)
m/z: 761.3289 [M+]; calcd for C48H47N3O4S [M+]: 761.3282.
14 Z. Ning, Q. Zhang, W. Wu, H. Pei, B. Liu and H. Tian, J. Org. Chem.,
2008, 73, 3791.
15 D. Kuang, S. Uchida, R. Humphry-Baker, S. M. Zakeeruddin and
€
M. Gratzel, Angew. Chem., Int. Ed., 2008, 47, 1923.
16 H. Choi, C. Biak, S. O. Kang, J. Ko, M.-S. Kang,
€
Md. K. Nazeeruddin and M. Gratzel, Angew. Chem.,Int. Ed., 2008,
47, 327.
Acknowledgements
17 J.-H. Yum, D. P. Hagberg, S.-J. Moon, K. M. Karlsson,
T. Marinado, L. Sun, A. Hagfeldt, Md. K. Nazeeruddin and
We acknowledge the Swedish Research Council, K & A
Wallenberg Foundation, the Swedish Energy Agency for finan-
cial support of this work.
€
M. Gratzel, Angew. Chem., Int. Ed., 2009, 48, 1576.
18 J. T. Lin, P.-C. Chen, Y.-S. Yen, Y.-C. Hsu, H.-H. Chou and
M.-C. P. Yeh, Org. Lett., 2009, 11, 97.
19 G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu and P. Wang,
Chem. Commun., 2009, 2198.
20 L. Schmidt-Mende, U. Bach, R. Humphry-Baker, T. Horiuchi,
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