Molecular engineering of organic sensitizers for dye-sensitized solar cell applications

Article abstract of DOI:10.1021/ja800066y

Novel unsymmetrical organic sensitizers comprising donor, electron-conducting, and anchoring groups were engineered at a molecular level and synthesized for sensitization of mesoscopic titanium dioxide injection solar cells. The unsymmetrical organic sens

Full text of DOI:10.1021/ja800066y

Published on Web 04/18/2008
Molecular Engineering of Organic Sensitizers for
Dye-Sensitized Solar Cell Applications
Daniel P. Hagberg,† Jun-Ho Yum,§ HyoJoong Lee,§ Filippo De Angelis,|
Tannia Marinado,‡ Karl Martin Karlsson,† Robin Humphry-Baker,§ Licheng Sun,*,†
Anders Hagfeldt,*,‡ Michael Grätzel,§ and Md. K. Nazeeruddin*,§
Organic Chemistry and Physical Chemistry, Center of Molecular DeVices, Royal Institute of
Technology, Teknikringen 30, 10044 Stockholm, Sweden, Laboratory for Photonics and
Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Swiss
Federal Institute of Technology, CH-1015 Lausanne, Switzerland, and Istituto CNR di Scienze e
Tecnologie Molecolari (ISTM), c/o Dipartimento di Chimica, UniVersità di Perugia, Via Elce di
Sotto 8, I-06123, Perugia, Italy
Received January 4, 2008; E-mail: lichengs@kth.se; hagfeldt@kth.se; mdkhaja.nazeeruddin@epfl.ch
Abstract: Novel unsymmetrical organic sensitizers comprising donor, electron-conducting, and anchoring
groups were engineered at a molecular level and synthesized for sensitization of mesoscopic titanium
dioxide injection solar cells. The unsymmetrical organic sensitizers 3-(5-(4-(diphenylamino)styryl)thiophen-
2-yl)-2-cyanoacrylic acid (D5), 3-(5-bis(4-(diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D7), 5-(4-
(bis(4-methoxyphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D9), and 3-(5-bis(4,4′-dimethoxy-
diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D11) anchored onto TiO2 and were tested in dye-
sensitized solar cell with a volatile electrolyte. The monochromatic incident photon-to-current conversion
efficiency of these sensitizers is above 80%, and D11-sensitized solar cells yield a short-circuit photocurrent
density of 13.90 ( 0.2 mA/cm2, an open-circuit voltage of 740 ( 10 mV, and a fill factor of 0.70 ( 0.02,
corresponding to an overall conversion efficiency of 7.20% under standard AM 1.5 sun light. Detailed
investigations of these sensitizers reveal that the long electron lifetime is responsible for differences in
observed open-circuit potential of the cell. As an alternative to liquid electrolyte cells, a solid-state organic
hole transporter is used in combination with the D9 sensitizer, which exhibited an efficiency of 3.25%.
Density functional theory/time-dependent density functional theory calculations have been employed to
gain insight into the electronic structure and excited states of the investigated species.
Introduction
conducting substrate, an anchored molecular sensitizer, and a
redox electrolyte. Ruthenium sensitizers have shown very
Dye-sensitized solar cells (DSCs) have attracted significant
attention as low-cost alternatives to conventional semiconductor
photovoltaic devices.1–10 These cells are composed of a wide
band gap TiO2 semiconductor deposited on a transparent
impressive solar-to-electric power conversion efficiencies, reach-
ing 11% at standard AM 1.5 sun light.11,12 Several groups,
including ours, have developed metal-free organic sensitizers
and obtained efficiencies in the range of 4-8%.13–16 The critical
factors that influence sensitization are the excited-state redox
potential, which should match the energy of the conduction band
edge of the oxide, light excitation associated with vectorial
electron flow from the light-harvesting moiety of the dye toward
the surface of the semiconductor surface, conjugation across
the donor and anchoring groups, and electronic coupling
† Organic Chemistry, Royal Institute of Technology.
‡ Physical Chemistry, Royal Institute of Technology.
§ Swiss Federal Institute of Technology.
| CNR-ISTM, Perugia.
(1) Nazeeruddin, M. K. Coord. Chem. ReV. 2004, 248, 1161.
(2) Asbury, J. B.; Ellingson, R. J.; Gosh, H. N.; Ferrere, S.; Notzig, A. J.;
Lian, T. J. Phys. Chem. B 1999, 103, 3110–3119.
(3) Park, N.-G.; Kang, M. G.; Kim, K. M.; Ryu, K. S.; Chang, S. H.;
Kim, D.-K.; Van de Lagemaat, J.; Benkstein, K. D.; Frank, A. J.
Langmuir 2004, 20, 4246–4253.
(11) Nazeeruddin, M. K.; De Angelis, F.; Fantacci, S.; Selloni, A.; Viscardi,
G.; Liska, P.; Ito, S.; Bessho, T.; Grätzel, M. J. Am. Chem. Soc. 2005,
127, 16835–16847.
(4) Heimer, T. A.; Heilweil, E. J.; Bignozzi, C. A.; Meyer, G. J. J. Phys.
Chem. A 2000, 104, 4256–4262.
(5) Saito, Y.; Fukuri, N.; Senadeera, R.; Kitamura, T.; Wada, Y.; Yanagida,
S. Electrochem. Commun. 2004, 6, 71–74.
(12) M.Grätzel, J. Photochem. Photobiol. A 2004, 164, 3–14.
(13) Hara, K.; Sato, T.; Katoh, R.; Furube, A.; Ohga, Y.; Shinpo, A.; Suga,
S.; Sayama, K.; Sugihara, H.; Arakawa, H. J. Phys. Chem. B 2003,
107, 597–606.
(6) Qiu, F. L.; Fisher, A. C.; Walker, A. B.; Peter, L. M. Electrochem.
Commun. 2003, 5, 711–716.
(7) Kamat, P. V.; Haria, M.; Hotchandani, S. J. Phys. Chem. B 2004,
108, 5166–5170.
(14) Horiuchi, T.; Miura, H.; Sumioka, K.; Uchida, S. J. Am. Chem. Soc.
2004, 126, 12218–12219.
(8) Bisquert, J.; Cahen, D.; Hodes, G.; Ruehle, S.; Zaban, A. J. Phys.
Chem. B 2004, 108, 8106–8118.
(15) Kim, S.; Lee, J. K.; Kang, S. O.; Ko, J.; Yum, J.-H.; Fantacci, S.; De
Angelis, F.; DiCenso, Md., D.; Nazeeruddin, K.; Grätzel, M. J. Am.
Chem. Soc. 2006, 128, 16701–16707.
(9) Figgemeier, E.; Hagfeldt, A. Int. J. Photoenergy 2004, 6, 127–140.
(10) Furube, A.; Katoh, R.; Yoshihara, T.; Hara, K.; Murata, S.; Arakawa,
H.; Tachiya, M. J. Phys. Chem. B 2004, 108, 12588–12592.
(16) Hagberg, D. P.; Edvinsson, T.; Marinado, T.; Boschloo, G.; Hagfeldt,
A.; Sun, L. Chem. Commun. 2006, 2245–2247.
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2008 American Chemical Society
J. AM. CHEM. SOC. 2008, 130, 6259–6266 6259

A R T I C L E S
Hagberg et al.
Figure 1. Molecular structures of D5, D7, D9, and D11.
between the lowest unoccupied molecular orbital (LUMO) of
the dye and the TiO2 conduction band. The major factors for
low conversion efficiency of many organic dyes in the DSC
are the formation of dye aggregates on the semiconductor
surface and recombination of conduction-band electrons with
triiodide.17 Therefore, to obtain optimal performance, aggrega-
tion of organic dyes and recombination need to be avoided
through appropriate structural modification.18,19 On the basis
of this strategy, we have designed and synthesized a series of
organic sensitizers, 3-(5-(4-(diphenylamino)styryl)thiophen-2-
yl)-2-cyanoacrylic acid (D5),16 3-(5-bis(4-(diphenylami-
no)styryl)thiophen-2-yl)-2-cyanoacrylic acid (D7), 5-(4-(bis(4-
methoxyphenylamino)styryl)thiophen-2-yl)-2- cyanoacrylic acid
(D9), and 3-(5-bis(4,4′-dimethoxydiphenylamino)styryl)thiophen-
2-yl)-2-cyanoacrylic acid (D11). In this paper, we report detailed
investigation of photovoltaic performance of the D5, D7, D9,
and D11 sensitizers in liquid- and solid-state-based electrolytes.
Results and Discussion
Figure 1 shows the molecular structure of the D5, D7, D9,
and D11 organic sensitizers, which were synthesized according
to well-known reactions resulting in moderate yields. Aniline
was functionalized by an Ullman-type coupling, and the
thiophene moiety was coupled to the donor by Wittig reaction
(D5 and D9) or Horner-Wadsworth-Emmons reaction (D7 and
D11).20,21 Formylation of the thiophene moiety followed by
condensation of the aldehyde by the Knoevenagel protocol in
the presence of piperidine yielded the final sensitizers (see
Experimental Section for detailed information).
Figure 2. Normalized absorption (solid lines, top) and emission (dashed
lines, bottom) spectra of D5 (gray lines), D7 (black lines), D9 (red lines),
and D11 (blue lines) measured in ethanol. The emission spectra were
obtained by using the same solution by exciting at absorption maximum at
298 K, and the low-energy shoulder is an instrument artifact.
Figure 2 shows the UV/vis spectra of the novel organic
sensitizers, D5, D7, D9, and D11, measured in ethanol solution.
(17) Liu, D.; Fessenden, R. W.; Hug, G. L.; Kamat, P. V. J. Phys. Chem.
B 1997, 101, 2583–2590.
(18) Burfeindt, B.; Hannappel, T.; Storck, W.; Willig, F. J. Phys. Chem.
1996, 100, 16463–16465.
The absorption spectra of the D5 dye displays a strong visible
band at 441 nm (ε ) 33 000 M-1 cm-1) due to the π-π*
transitions of the conjugated molecule. The D7 sensitizer that
contains two diphenylaniline units also exhibits similar spectral
properties (absorption maxima at 441 nm, ε ) 31 000 M-1
cm-1). However, the D9 (462 nm, ε ) 33 000 M-1 cm-1) and
(19) Sayama, K.; Tsukagoshi, S.; Hara, K.; Ohga, Y.; Shinpou, A.; Abe,
Y.; Suga, S.; Arakawa, H. J. Phys. Chem. B 2002, 106, 1363–1371.
(20) Wadsworth, W. S., Jr.; Emmons, W. D. J. Am. Chem. Soc. 1961, 83,
1733–1738.
(21) Hu, Z. Y.; Fort, A.; Barzoukas, M.; Jen, A. K. Y.; Barlow, S.; Marder,
S. R. J. Phys. Chem. B 2004, 108, 8626–8630.
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6260 J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008

Molecular Engineering of Organic Sensitizers for Solar Cells
A R T I C L E S
Table 1. Experimental Spectral and Electrochemical Properties of
calculations on the three organic sensitizers by using the
Gaussian03 program package.22 In particular, we used B3LYP
as exchange-correlation functional23 and 6-31g* as basis set.24
Solvation effects were included by means of the Polarizable
Continuum model.24 The geometrical structures of the dyes have
been optimized in vacuo, and time-dependent (TD) DFT excited-
state calculations were performed in water solution. Solvation
effects were included by means of the Polarizable Continuum
model.25
the Dyes
a
a
b
c
d
Absmax
[nm]
ε
Emmax
[nm]
E(S+/S)
E(0-0)
E(S+/S*)
[V] vs NHE [V] (Abs/Em) [V] vs NHE
dye
[M-1cm-1
]
D5
D7
D9
D11
441
441
462
458
33 000
31 000
33 000
38 000
621
595
632
620
1.08
1.07
0.91
0.92
2.37
2.42
2.32
2.33
-1.29
-1.35
-1.41
-1.41
a Absorption and emission spectra of dyes in ethanol solution. b The
ground-state oxidation potential of the dyes was measured with DPV
under the following conditions: Pt working electrode and Pt counter
electrode; electrolyte, 0.1 M tetrabutylammonium hexafluorophosphate,
TBA(PF6), in acetonitrile. Potentials measured vs Fc+/Fc were converted
to normal hydrogen electrode (NHE) by addition of +0.63 V. c 0-0
transition energy, E(0-0), estimated from the intercept of the normalized
absorption and emission spectra in ethanol. d Estimated LUMO energies,
E(LUMO), vs NHE from the estimated highest occupied molecular orbital
(HOMO) energies obtained from the ground-state oxidation potential by
We report in Figure 3 the calculated molecular orbital energy
diagram for D5, D9, and D11, along with isodensity plots of
the HOMO and LUMO for the D9 and D11 systems (the HOMO
and LUMO for D5, not shown, have localization similar to that
in D9).16 The D9 HOMO is a π orbital delocalized throughout
the entire molecule, whereas in D11, the almost degenerate
HOMO and HOMO-1 (not shown) couple, although having a
similar character, is mainly localized on the two methoxy-
substituted diphenylamino moieties and on the double C-C
bond, with little contributions from the thiophenes and cy-
anoacrylic acid groups. The LUMO is, on the other hand, very
similar in D5, D9, and D11, being in all cases a single π* orbital
delocalized across the thiophenes and cyanoacrylic acid groups,
with sizable contributions from the latter. Given LUMO
composition, its energy is less sensitive than that of the HOMO
to methoxy substituents. The cyanoacrylic acid contribution to
the LUMO, which represents the final state in the charge-transfer
transitions characterizing the new dyes, see below, is relevant
to ensure a high electronic coupling between the dye excited
state and the TiO2 conduction band, which in turn, translates
into high photocurrent densities in DSC devices. The electronic
structures of the novel organic sensitizers are quite similar to
those of the recently reported 3-{5-[N,N-bis(9,9-dimethylfluo-
rene-2-yl)phenyl]-thiophen-2-yl}-2-cyano-acrylic acid (JK-1)
adding the 0-0 transition energy, E(0-0)
.
D11 (458 nm, ε ) 38 000 M-1 cm-1), which contain methoxy
groups, show red-shifted absorption maxima compared to the
D5 and D7 sensitizers (see Table 1). The absorption spectra of
D5, D7, D9, and D11 adsorbed onto 3.5 µm thick TiO2
electrodes are similar to the corresponding solution spectra but
exhibit a small red shift due to the interaction of the anchoring
groups with the surface titanium ions.
The DPV of D5 sensitizer measured in acetonitrile containing
0.1 M TBA(PF6) with 0.1 V scan rate shows a symmetrical
peak at 1.07 V vs NHE because of the oxidation of the
4-(diphenylamino) group. It is interesting to note that in D9
sensitizer, the oxidation potential (0.91 V vs NHE) shifted
cathodically by 0.11 V, compared to that in the D5 sensitizer,
demonstrating the extent of destabilization of HOMO caused
by dimethoxy groups. The difference in the oxidation potential
between D7 and D11 sensitizers is consistent with that of D5
and D9 sensitizers. The destabilization of the HOMO of the
D9 and D11 sensitizers caused by methoxy substitution is
reflected in absorption spectra of these sensitizers.
and
3-{5′-[N,N-bis(9,9-dimethylfluorene-2-yl)phenyl]-2,2′-
bisthiophen-5-yl}-2-cyano-acrylic acid (JK-2) dyes,15 with the
HOMO (and HOMO-1 for D11) and LUMO being the only
molecular orbitals present in the energy range exceeding 1 eV.
For D5, D9, and D11, we calculate the lowest singlet-singlet
excitation energy at 2.03, 1.88, and 1.73 eV, respectively. For
D5 and D9, the lowest transition corresponds to a single HOMO
f LUMO excitation and is followed in D9 by a HOMO-1 f
LUMO transition at 2.83 eV. For D11, the lowest excitation is
followed at 1.89 eV by an almost degenerate transition; both
excited states are originated by HOMO f LUMO and HOMO-1
f LUMO contributions, showing therefore a strong charge-
transfer character. The two almost degenerate lowest excitations
of D11 are followed, at higher energy, by a HOMO-2 f LUMO
transition at 2.82 eV, therefore with similar energy and character
as in D9.
The excited-state oxidation potential of the sensitizer plays
an important role in the electron-injection process. By neglecting
any entropy change during light absorption, the value can be
derived from the ground-state oxidation couple and the 0-0
excitation energy, E(0-0), according to eq 1.
+
+
(
)
(
)
E S ⁄S* ) E S ⁄S - E
(1)
(
0-0
)
From the absorption/emission spectra, E(0-0) energies of 2.37,
2.42, 2.32, and 2.33 eV were extracted for D5, D7, D9, and
D11, respectively. The excited-state oxidation potential of these
sensitizers is more negative (-1.29 V vs NHE) than the
equivalent potential for N719 sensitizer (- 0.98 V vs NHE)
and the TiO2 conduction band.13 Therefore, these types of
sensitizers have sufficient driving force for electron injection
to TiO2 and become very attractive for other semiconductor
materials having conduction bands more negative than the
conduction bands of TiO2. An increased gap between the
conduction band and the redox couple yields higher open-circuit
voltage, enhancing the cell efficiency considerably. On the other
hand, the oxidation potential of the sensitizers is more positive
It is worth comparing our calculated electronic structure to
the available electrochemical and spectroscopic data obtained
in the context of the present investigation. The electrochemical
oxidation potential data are consistent with the trends in the
HOMOs and LUMOs energies calculated for D5, D9, and D11,
showing a slight increase (by 160 mV) in the oxidation potential
when going from D5 to D9 and no appreciable variations of
(22) Frisch, M. J. et al. Gaussian 03, revision B.05; Gaussian, Inc.:
Wallingford, CT, 2004.
-
than the I-/I3 redox couple (∼0.4 V vs NHE), ensuring that
there is enough driving force for the dye regeneration reaction
to compete efficiently with the recapture of the injected electrons
by the dye cation radical.
(23) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(24) Ditchfield, R.; Herhe, W. J.; Pople, J. A. J. Chem. Phys. 1971, 54,
724–728.
(25) (a) Barone, V.; Cossi, M. J. Phys Chem. A 1998, 102, 1995–2001.
(b) Cossi, M.; Rega, N.; Scalmani, G.; Barone, V. J. Comput. Chem.
2003, 24, 669–681.
To gain insight into the geometrical and electronic structure
of the new dyes, we performed density functional theory (DFT)
9
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A R T I C L E S
Hagberg et al.
Figure 3. Molecular orbital energy diagram and isodensity surface plots of the HOMO and LUMO of D9 and D11.
this parameter when going from D9 to D11. The decreased
oxidation potential measured for D9 compared to that of D5 is
directly related to the calculated HOMO destabilization (0.18
eV), related to the presence of the electron-donating methoxy-
substituents. The lowest TDDFT excitation energies, although
qualitatively reproducing the similar spectroscopic behavior of
D9 and D11 with absorption maxima at ca. 2.7 eV, considerably
underestimate both the absorption maxima and the estimated
E(0-0) transitions, which are determined at ca. 2.3 eV for both
dyes. This underestimate of the lowest excitation energies, which
shows up also in terms of HOMO-LUMO gaps, see Figure 3,
was already found in similar charge-transfer JK-1 and JK-2
dyes15 and can be related to a deficiency of DFT to deal with
extensively delocalized states, such as those we are facing in
the present cases.
The D5, D7, D9, and D11 sensitizers have been used to
manufacture solar cell devices to explore current-voltage
characteristics by using 7 (transparent) + 5 (scattering) µm TiO2
layers. Figure 4 shows the incident monochromatic photon-to-
current conversion efficiency (IPCE) obtained with a sandwich
cell by using 0.6 M N-methyl-N-butyl imidazolium iodide, 0.04
M iodine, 0.025 M LiI, 0.05 M guanidinium thiocyanate, and
0.28 M tert-butylpyridine in 15/85 (v/v) mixture of valeronitrile
and acetonitrile as redox electrolyte. The IPCE data of all
sensitizers plotted as a function of excitation wavelength exhibit
a high plateau at 85%. D7 showed IPCE to be blue-shifted
around 10 nm, which is coincident with the result of absorption
spectra, as mentioned above. The IPCE spectra of the D9 and
D11 sensitizers that contain methoxy groups exhibit 30 nm red
shift compared to the D5 and D7 sensitizers, which is consistent
with the absorption spectra. Under standard global AM 1.5 solar
conditions, the D5-sensitized cell gave a short-circuit photo-
current density (Jsc) of 12.0 ( 0.20 mA/cm2, an open-circuit
voltage (Voc) of 688 ( 10 mV, and a fill factor (ff) of 0.72 (
0.02, corresponding to an overall conversion efficiency η,
derived from the equation η ) JscVocff/light intensity, of 5.94%
(see Table 2). The D7-sensitized cell gave Jsc of 11.00 ( 0.20
mA/cm2, Voc of 695 ( 10 mV, and ff of 0.71 ( 0.02,
corresponding to an overall conversion efficiency η of 5.43%.
Figure 4. IPCE (top) and current-voltage characteristics (bottom) obtained
with a nanocrystalline TiO2 film supported on FTO conducting glass and
derivatized with monolayer of D5 (gray line), D7 (black line), D9 (red line),
and D11 (blue line).
The reason for the low efficiency of D7-sensitized cell compared
to the D5 cell is the lower photocurrent due to the narrow IPCE
spectra of D7 compared those of D5. In contrast, D9- and D11-
sensitized cells exhibited higher photovoltaic performance when
compared to D5-sensitized cells because of enhanced spectral
9
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Molecular Engineering of Organic Sensitizers for Solar Cells
A R T I C L E S
Table 2. Current-Voltage Characteristics Obtained with a TiO2
Film (7 + 5 µm) on FTO Conducting Glass and Derivatized with
Monolayer of D5, D7, D9, and D11
dye
Jsc (mA/cm2)
Voc (mV)
ff
η (%)
D5
D7
D9
D11
12.00
11.00
14.00
13.50
688
695
694
744
0.72
0.71
0.71
0.70
5.94
5.43
6.90
7.03
Table 3. Current-Voltage Characteristics Obtained with
D11-Sensitized Solar Cells for Various Thicknesses of the
Nanocrystalline TiO2 Films
thickness (µm)
Jsc (mA/cm2)
Voc (mV)
ff
η (%)
2.5 + 5
5 + 5
12.30
12.90
13.50
13.90
765
753
744
744
0.70
0.70
0.70
0.70
6.59
6.80
7.03
7.23
7 + 5
10 + 5
response. The D9-sensitized cell yielded Jsc of 14.00 ( 0.20
mA/cm2, Voc of 694 ( 10 mV, and ff of 0.71 ( 0.02,
corresponding to an overall conversion efficiency η of 6.90%.
And the D11-sensitized cell (having four methoxy groups)
yielded Jsc of 13.90 ( 0.20 mA/cm2, Voc of 744 ( 10 mV, and
ff of 0.70 ( 0.02, corresponding to an overall conversion
efficiency η of 7.23%. The higher efficiency of the D9 and D11
sensitizers compared to the D5 and D7 sensitizers demonstrates
the beneficial influence of alkoxy units on photovoltage and
photocurrent, caused by the enhanced red response. The
increased photovoltage of D11-sensitized solar cell (∼50mV,
see Table 2) compared to that of the D5-sensitized solar cell is
mainly due to the increased lifetime of the conduction-band
electrons; a detailed discussion about this effect will be done
in the section on electron kinetics.
Figure 5. Electron lifetime (top) and capacitance (bottom) obtained with
a 7 µm transparent nanocrystalline TiO2 film supported onto a conducting
glass sheet and derivatized with a monolayer of D5 (gray), D7 (black), D9
(red), and D11 (blue).
In order to see the impact of the high molar extinction
coefficient of these sensitizers on photovoltaic properties, we
have fabricated solar cells having TiO2 films of various
thicknesses and using D11 sensitizer. The photocurrent increased
with the thickness of TiO2 nanocrystalline layer; on the other
hand, the photovoltage decreased with increasing thickness (see
Table 3). The increased surface area due to higher thickness of
TiO2 film allows for better light harvesting, which however
enhances the possibility for injected electrons to recombine with
the oxidized redox species triiodide. The devices using a 2.5
µm thin TiO2 layer yielded remarkably high photocurrent (12.3
mA/cm2) and 79% IPCE, which we have attributed to the high
molar extinction coefficient of the dye.
The Voc of a DSC is determined by the difference between
the quasi-Fermi level (nEF) in the TiO2 under illumination and
the Fermi level of the electrolyte (redox potential, EF,redox). The
increase in Voc could be caused by two different mechanisms:
one is a retardation of the recombination between injected
electrons and oxidized species in the electrolyte, and another is
a band edge movement with respect to the redox potential.26,27
The electron lifetime in a DSC can be estimated from the
photovoltage response to a small perturbing light pulse.27 The
apparent photovoltage decay at the open-circuit condition can
be obtained from fitting the following exponential equation:
electron lifetime (τ) for the above-discussed DSCs based on 7
µm transparent TiO2 films as a function of Voc (nEF - EF,redox),
resulting from various bias-light intensities. The apparent
electron lifetimes (τ) in D7- and D11-sensitized solar cell are
∼2.0 and ∼7.5 times longer than that of the solar cell sensitized
with the D5, which shows the effect of disubstituted donor
moieties on the electron lifetime. The disubstituted donor
moieties in D7 and D11 prevent the triiodide in the electrolyte
from recombining with injected electrons in the TiO2 condution
band, leading to increased open-circuit potentials when com-
pared to the D5 sensitizer. The long apparent electron lifetimes
(τ) in D9- and D11-sensitized solar cell when compared to that
of D5-sensitized solar cell are plausibly caused by the effect of
methoxy moieties. D11-sensitized solar cell consequently
showed the longest electron lifetime, which is presumably
caused by the synergetic effect of disubstituted donor moieties
and methoxy moieties. Figure 5 (bottom) shows the chemical
capacitance as a function of Voc at different light intensities.
The chemical capacitance follows a characteristic exponential
rise with increasing forward bias. The low chemical capacitance
at fixed Voc might be due to either a negative shift of the
conduction band or a longer lifetime of electrons. The electron
lifetimes of D7-, D9-, and D11-sensitized solar cells were longer
than that of D5, as mentioned above, and thus contributed to
lowering the chemical capacitance. Consequently, the D11-
∆Vphoto(t) ) ∆Vphoto(0) exp(-t ⁄ τ)
(2)
where t is the time at the end of a small light pulse, and τ is the
apparent electron lifetime. The apparent electron lifetime can
be obtained from fitting the photovoltage decay with an
exponential function. Figure 5 shows a plot for the change in
(26) Zhang, Z.; Evans, N.; Zakeeruddin, S. M.; Humphry-Baker, R.; Grätzel,
M. J. Phys. Chem. C 2007, 111, 398.
(27) Schlichthörl, G.; Huang, S. Y.; Sprague, J.; Frank, A. J. J. Phys. Chem.
B 1997, 101, 8141–8155.
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A R T I C L E S
Hagberg et al.
Table 4. Current-Voltage Characteristics Obtained with D5-, D7-,
D9-, and D11-Sensitized Solar Cells as Solid-State Cellsa
dye
Jsc (mA/cm2)
Voc (mV)
ff
η (%)
D5
D7
D9
D11
6.31
5.02
7.72
5.85
865
785
756
811
0.57
0.71
0.56
0.63
3.11
2.79
3.25
3.01
a The solid-state cell configuration was used to measure this
spectrum. The 1.7 µm TiO2 film composed of 20 nm anatase particles
was used for derivation of sensitizers, and the Spiro was incorporated as
a hole conductor under evaporated gold cathode.
sensitized solar cell gives the highest Voc because of both
disubstituted donor and methoxy groups.
These organic dyes were evaluated as sensitizers for the solid-
state dye-sensitized solar cells in which spiro-MeOTAD hole
conductor is used as a redox couple.28,29 Under standard global
AM 1.5 solar conditions, all the D-series dye-sensitized cells
showed only 50% of their liquid-electrolyte counterparts in
overall efficiency. D9 gave the best performance, Jsc of 7.72
mA/cm2, Voc of 756 mV, and ff of 0.56, leading to an overall
conversion efficiency η of 3.25% (see Table 4). The D9-
sensitized cell demonstrated an almost linear behavior under
different light intensities, ranging from 9.2 to 99% sun with a
slightly higher efficiency (∼3.5%) at low intensity (Figure 6,
top panel). The IPCE of the D9-sensitized device exhibited
relatively high values of 40-50% in the range of 420-530 nm
(Figure 6b), which are higher than that of the standard N719-
based cell.29 Considering the film thickness (1.7 µm) of TiO2
used, the results from D-series sensitizers look promising in
the application of solid-state cells.
Figure 6. IPCE (top) and current-voltage characteristics (bottom) obtained
with solid-state solar cells derivatized with D9.
We note that in a solid-state device, the dyes with one
diphenylaniline moiety (D5 and D9) always gave a higher
efficiency than the dyes that contain two diphenylaniline groups
(D7 and D11). Also, the dyes that contain methoxy-donor groups
(D9 and D11) showed a better photovoltaic performance
compared to the corresponding sensitizers (D5 and D7) that
contain no methoxy group. Those observations in solid-state
cells are somewhat different from the results in liquid cells,
which means that other factors, such as hole-transfer yield and
pore penetration by intimate contacts between dye and hole
conductor, should also be considered in the performance of
solid-state cells, as pointed out recently.28c The solid-state solar
cell with the D9 sensitizer exhibited best performance, which
can be ranked to the second place next to indoline dye (D102),
which showed an efficiency of 4.1% (Jsc of 7.7 mA/cm2, Voc of
866 mV, and ff of 0.61).29 It is remarkable to note that the D9
and thermal stability, and the efficiency remained 70% of the
initial value after 1000 h. However, under similar conditions,
the D11-sensitized solar cells interestingly showed excellent
photochemical and thermal stability, and the efficiency remained
90% of the initial value after 1000 h.
Conclusion
In summary, we have developed a series of organic sensitiz-
ers, which demonstrate the effect of substituted donor groups
on controlling optical and photovoltaic properties. The electron
kinetics data of the DSCs, with the D5, D7, D9, and D11 organic
dyes, together with DFT/TDDFT calculations, unravel the
substitution effect, which prevents triiodide present in the
electrolyte from recombining with injected electrons in the TiO2,
leading to increased open-circuit potentials. For thin-film solid-
state solar cells based on these organic dyes, D9 showed an
excellent performance. Structural modification of D9 to increase
the molar extinction coefficient is anticipated to give even better
performance.
dye has a lower extinction coefficient (ꢀ ) 33 000 M-1 cm-1
)
compared to that of the D102 sensitizer (ꢀ ) 55 800 M-1 cm-1
at 491 nm); yet, the photocurrent of both sensitizers is ∼7.7
mA/cm2. These data indicate that the D9 sensitizer has a better
absorbed photon-to-current conversion efficiency than D102 for
the same device structure of solid-state cell.
Experimental Section
Materials and Measurements. The UV/visible and emission
spectra of the dyes in solution were recorded on a Cary Varian
spectrophotometer and a HR-2000 Ocean Optics fiber optics
spectrophotometer, respectively.
The D5-, D7-, and D9-sensitized solar cells were subjected
to accelerated testing in a solar simulator at 100 mW cm-2
intensity at 60 °C. The cells showed reasonable photochemical
Differential pulse voltammetry was performed with a CH
Instruments 660 potentiostat by using 0.1 M TBA(PF6) in aceto-
nitrile (Fluka, >99.9%) as supporting electrolyte. A Pt working
electrode, a Pt counter electrode, and an Ag/Ag+ reference electrode
were used. The system was internally calibrated with ferrocene/
ferrocenium (Fc/Fc+).
(28) (a) Snaith, H. J.; Schmidt-Mende, L.; Grätzel, M. Phys. ReV. B 2006,
74, 045306. (b) Snaith, H. J.; Grätzel, M. Appl. Phys. Lett. 2006, 89,
262114. (c) Kroeze, J. E.; Hirata, N.; Schmidt-Mende, L.; Orizu, C.;
Ogier, S. D.; Carr, K.; Grätzel, M.; Durrant, J. R. AdV. Func. Mater.
2006, 16, 1832–1838.
(29) Schmidt-Mende, L.; Bach, U.; Humphry-Baker, R.; Horiuchi, T.;
Miura, H.; Ito, S.; Uchida, S.; Grätzel, M. AdV. Mater. 2005, 17, 813–
815.
For photovoltaic measurements of the DSCs, the irradiation
source was a 450 W xenon light source (Osram XBO 450, U.S.A.)
9
6264 J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008

Molecular Engineering of Organic Sensitizers for Solar Cells
A R T I C L E S
using a Tempax 113 solar filter. The output power of the AM 1.5
solar simulator was calibrated by using a reference Si photodiode
equipped with a colored matched IR-cutoff filter (KG-3, Schott) in
order to reduce the mismatch in the region of 350-750 nm between
the simulated light and AM 1.5 to less than 2%.30 The measurement
delay of photo I-V characteristics of DSCs was fixed to 40 ms.
The measurement of IPCE was plotted as a function of excitation
wavelength by using the incident light from a 300 W xenon lamp
(ILC Technology, U.S.A.), which was focused through a Gemini-
180 double monochromator (Jobin Yvon Ltd., U.K.). The photo-
electrochemical measurements were carried out by using a Xe lamp
source designed to give 100 mW/cm2 at AM 1.5 G sun light and
a voltage source meter to sweep the voltage across the irradiated
cell and, at the same time, measure the generated current. The IPCE
is determined by wavelength scanning the light source on the cell
and measuring the short-circuit current as a function of wavelength.
Photo-generated transients were observed by using an exciting pulse
generated by green-light-emitting (535 nm) diodes with a white-
light bias. From the current decay, the photo-generated charge in
the cell is measured. The corresponding voltage decay gives the
electron lifetime.
Dye-Sensitized Solar Cells. The photo-anodes composed of
nanocrystalline TiO2 were prepared by using a previously reported
procedure.30,31 FTO glass plates (Nippon Sheet Glass, Solar 4 mm
thickness) were used for transparent conducting electrodes. After
TiCl4 treatment, a paste composed of 20 nm anatase TiO2 particles
for the transparent nanocrystalline layer was coated on the FTO
glass plates by screen printing. This coating-drying procedure was
repeated to increase the thickness to the required value. After drying
the nanocrystalline TiO2 layer, a paste for the scattering layer
containing 400 nm sized anatase particles (CCIC, HPW-400) was
deposited by two screen printings. The resulting layer was composed
of various thicknesses of transparent layer and 5 µm of scattering
layer, the thicknesses being measured by using Alpha-step 200
surface profilometer (Tencor Instruments, San Jose, CA). The TiO2
electrodes were gradually heated under an air flow, and then, the
TiO2 electrodes were treated by TiCl4 and sintered at 500 °C for
30 min. The TiO2 electrodes were immersed into the D5, D7, D9,
and D11 solutions (0.3 mM in ethanol containing 10 mM 3a,7a-
dihydroxy-5b-cholic acid (Cheno)) and kept at room temperature
for 4 h. Counter electrodes were prepared by coating with a drop
of H2PtCl6 solution (2 mg of Pt in 1 mL of ethanol) on a FTO
plate (TEC 15/2.2 mm thickness, Libbey-Owens-Ford Industries)
and heating at 400 °C for 15 min. The dye-adsorbed TiO2 electrode
and Pt counter electrode were assembled into a sealed sandwich-
type cell by heating with a hot-melt ionomer film (Surlyn 1702, 25
µm thickness, Du-Pont) as a spacer between the electrodes. An
electrolyte solution (electrolyte of 0.6 M N-methyl-N-butyl imidi-
azolium iodide, 0.04 M iodine, 0.025 M LiI, 0.05 M guanidinium
thiocyanate, and 0.28 M tert-butylpyridine in 15/85 (v/v) mixture
of valeronitrile and acetonitrile) was used as redox couple. Finally,
the hole was sealed by using additional Bynel and a cover glass
(0.1 mm thickness). An antireflection and UV cut-off film (λ <
380 nm, ARKTOP, Asahi Glass) was attached to the DSC surface.
In order to reduce scattered light from the edge of the glass
electrodes of the dyed TiO2 layer, light shading masks were used
on the DSCs, so that the active area of DSCs was fixed to 0.2 cm2.
For solid-state solar cells, ∼100 nm compact layer of TiO2 was
deposited by spray pyrolysis after cleaning FTO glass substrates
(15Ω/, Pilkington) with Helmanex (a detergent), acetone, and
ethanol. A nanoporous layer (∼1.7 µm) of 20 nm TiO2 was prepared
by doctor-blading technique, followed by heating slowly to 500 °C;
then, the preparation was baked at this temperature for 30 min under
an oxygen flow. After cooling, the as-prepared films were treated
with 0.02 M TiCl4 aqueous solution overnight, then washed with a
copious amount of deionized water, and dried. Prior to the dye
uptake, the substrates were annealed at 500 °C for 30 min under
oxygen flow and then cooled to ∼80 °C before being put into the
dye solutions for 6 h. After soaking in the dye solution, the
substrates were rinsed in acetonitrile, and the hole-conducting matrix
was spin-coated on their upper surface by following our typical
procedures.28 The hole-transporting material used was 2,2′,7,7,′-
tetrakis(N,N-dimethoxyphenyl-amine)-9,9′-spirobifluorene (spiro-
MeOTAD), which was dissolved in chlorobenzene. tert-Butyl
pyridine and Li[CF3SO2]2N were added as additives. The device
fabrication was finished by evaporating a 50 nm gold electrode on
the top.
Synthesis of Sensitizers. All reactions were carried out under
nitrogen atmosphere with the use of standard inert atmosphere and
Schlenk techniques. Dichloromethane (DCM), dimethylformamide
(DMF), hexane, and tetrahydrofuran (THF) were dried by passing
1
through a solvent column composed of activated alumina. H and
13C NMR spectra were recorded on Bruker 500 and 400 MHz
instruments by using the residual signals δ 7.26 and 77.0 ppm from
CDCl3, 2.50 and 39.4 ppm from d6-DMSO, and 2.05, 29.84, and
206.26 ppm from d6-acetone as internal references for 1H and 13C,
respectively. HR-MS was performed by using a Q-Tof Micro
(Micromass Inc., Manchester, England) mass spectrometer equipped
with Z-spray ionization source.
3-(5-(4-(Diphenylamino)styryl)thiophen-2-yl)-2-cyanoacrylic
Acid (D5). The synthetic procedure and characterizations were
16
presented in an earlier publication.
3-(5-Bis(4-(diphenylamino)styryl)thiophen-2-yl). To a stirred
solution of 4,4′-bis(N,N-diphenylamino)benzophenone32 (1.84 g,
3.65 mmol) and DMF (50 mL), t-BuOK (1.02 g, 9.13 mmol) was
added, and the mixture was cooled to 0 °C over 30 min. Diethyl
thiophenephosphonate (851 mg, 3.64 mmol) in DMF (10 mL) was
added dropwise, and the solution was kept at 0 °C for 1 h. The
reaction mixture was allowed to warm to ambient temperature and
was stirred under nitrogen atmosphere for 24 h. Ether/water
extraction, followed by solvent evaporation and column chroma-
tography over silica gel (20% DCM in petroleum ether) yielded a
1
yellow oil (1.3 g, 61%). H NMR (500 MHz, d6-acetone): δ 7.40
(s, 1H), 7.32 (m, 10H), 7.25 (d, J ) 5.1 Hz, 1H), 7.17 (m, 8H),
7.07 (m, 9H), 6.97 (d, J ) 8.8 Hz, 2H), 6.93 (d, J ) 3.6 Hz, 1H).
13C NMR (125 MHz, d6-acetone): 149.7, 149.6, 149.5, 149.1, 143.5,
140.6, 137.6, 135.3, 133.1, 131.3, 130.9, 129.3, 128.2, 127.9, 126.3,
126.3, 126.2, 125.1, 124.9, 124.7, 121.7. MS (APCI-ES) m/z: 597.3
[M + H]+.
3-(5-Bis(4-(diphenylamino)styryl)thiophen-2-yl)-2-carb-
aldehyde. 3-(5-bis(4-(diphenylamino)styryl)thiophen-2-yl) (1.3 g,
2.23mmol) was dissolved in DMF and cooled to -78 °C. The
mixture was kept under nitrogen atmosphere. To the mixture, POCl3
(0.34 g, 2.23 mmol) was added dropwise. The reaction mixture
was allowed to warm to 0 °C, whereupon it turned red. The mixture
was heated to 50 °C for 3 h. The reaction mixture was cooled to
ambient temperature and quenched by aq HCl (0.1 M, 100 mL).
The solution was washed with brine/ether. The organic phase was
dried with NaSO4. The solvent removal, followed by column
chromatography (10% petroleum ether in DCM), yielded the
product as an orange solid (0.86 g, 63%), mp 234.0-234.8. 1H
NMR (500 MHz, d6-acetone): δ 9.85 (s, 1H), 7.75 (d, J ) 3.9 Hz,
1H), 7.51 (s, 1H), 7.41 (d, J ) 8.9 Hz, 2H), 7.34 (m, 8H), 7.27 (d,
J ) 4.0 Hz, 1H), 7.22 (m, 6H), 7.17 (d, J ) 8.6 Hz, 2H), 7.09 (m,
8H), 6.99 (d, J ) 8.8 Hz, 2H). 13C NMR (125 MHz, d6-acetone):
184.8, 152.7, 150.6, 149.7, 149.2, 145.9, 144.9, 138.0, 132.8, 131.9,
131.4, 129.8, 127.3, 126.7, 126.2, 125.5, 124.9, 124.0, 120.9. MS
(APCI-ES) m/z: 625.3 [M + H]+.
(30) Wang, P.; Zakeeruddin, S. M.; Comte, P.; Charvet, R.; Humphry-
Baker, R.; Grätzel, M. J. Phys. Chem. B 2003, 107, 14336–14341.
(31) Nazeeruddin, M. K.; Péchy, P.; Renouard, T.; Zakeeruddin, S. M.;
Humphry-Baker, R.; Comte, P.; Liska, P.; Le, C.; Costa, E.; Shklover,
V.; Spiccia, L.; Deacon, G. B.; Bignozzi, C. A.; Grätzel, M. J. Am.
Chem. Soc. 2001, 123, 1613–1624.
3-(5-Bis(4-(diphenylamino)styryl)thiophen-2-yl)-2-cyano-
acrylic Acid (D7). A 30 mL acetonitrile solution of 3-(5-bis(4-
(diphenylamino)styryl)thiophen-2-yl)-2-carbaldehyde (404 mg, 0.07
(32) Plater, M. J.; Jackson, T. Tetrahedron 2003, 59, 4673–4685.
9
J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008 6265

A R T I C L E S
Hagberg et al.
mmol), cyanoacetic acid (56 mg, 0.07 mmol), and piperidine (1.7
mg, 0.02 mmol) was refluxed for 4 h under nitrogen atmosphere.
Purification by extraction (petroleum ether and aq HCl (0.1 M))
and filtration of the formed solid yielded the product as a dark red
solid, D7 (254 mg, 56%), mp 214.5-215.5 °C. 1H NMR (500 MHz,
d6-DMSO): δ 8.29 (s, 1H), 7.80 (d, J ) 4.1 Hz, 1H), 7.49 (s, 1H),
7.30 (m, 11H), 7.19 (d, J ) 7.6 Hz, 4H), 7.06 (m, 12H), 6.90 (d,
J ) 8.8 Hz, 2H). 13C NMR (125 MHz, d6-DMSO): 163.8, 151.2,
147.9, 147.5, 146.9, 146.5, 146.3, 143.8, 139.3, 135.0, 133.9, 130.6,
130.5, 129.6, 129.5, 128.1, 124.9, 124.5, 123.6, 123.5, 122.5, 121.7,
118.7, 116.4, 96.8. HR-MS (TOF MS ESI) m/z: 714.2186 [M +
Na+]. Calcd for C23H17NNaOS (M + Na+): 714.2191.
Bis-(4-methoxyphenyl)-phenyl-amine. A mixture of aniline
(1.02 g, 10.4 mmol), copper powder (4.55 g, 72 mmol), K2CO3
(18.56 g, 134 mmol), 18-crown-6 (100 mg), and iodoanisol (29.43
g, 126 mmol) was heated to reflux at 188 °C under nitrogen
atmosphere with stirring for 30 h. The reaction mixture was allowed
to cool to ambient temperature and was dissolved in DCM. The
solution was washed with distilled water, and the organic phase
was dried with NaSO4 and concentrated by evaporation. Column
chromatography (10% DCM in petroleum ether) yielded the product
as a light yellow solid (2.19 g, 68%). 1H NMR (500 MHz, CDCl3):
δ 7.20-7.15 (m, 2H), 7.06 (d, J ) 8.91 Hz, 4H), 6.95 (d, J ) 7.82
Hz, 2H), 6.87 (t, J ) 7.28, 7.28 Hz, 1H), 6.83 (d, J ) 8.98 Hz,
4H), 3.80 (s, 6H). 13C NMR (125 MHz, d6-acetone): 155.7, 148.8,
141.2, 128.9, 126.4, 120.9, 120.6, 114.6, 55.5. MS (APCI-ES) m/z:
306.1 [M + H]+.
5-(4-(Bis(4-methoxyphenylamino)styryl)thiophen-2-yl)-2-
carbaldehyde.33 This compound was synthesized from bis-(4-
methoxyphenyl)-phenyl-amine following the reported procedure.33
5-(4-(Bis(4-methoxyphenylamino)styryl)thiophen-2-yl)-2-
cyanoacrylic Acid (D9). A 30 mL acetonitrile solution of 5-(4-
(bis(4-methoxyphenylamino)styryl)thiophen-2-yl)-2-carbaldehyde
(360 mg, 0.08 mmol), cyanoacetic acid (102 mg, 0.16 mmol), and
piperidine (1.7 mg, 0.02 mmol) was refluxed for 4 h under nitrogen
atmosphere. Purification by extraction (petroleum ether and aq HCl
(0.1 M)) and filtration of the formed solid yielded the product as a
dark red solid, D9 (283 mg, 68%), mp 223.5-224.4 °C. 1H NMR
(500 MHz, d6-DMSO): δ 8.42 (s, 1H), 7.91 (d, J ) 4.10 Hz, 1H),
7.47 (d, J ) 8.86 Hz, 1H), 7.37-7.29 (m, 2H), 7.16 (d, J ) 16.04
Hz, 1H), 7.11-7.05 (m, 4H), 6.97-6.91 (m, 4H), 6.70 (d, J )
8.85 Hz, 2H), 3.75 (s, 6H). 13C NMR (125 MHz, d6-DMSO): 163.7,
156.2, 153.1, 149.0, 146.3, 141.4, 139.2, 133.0, 132.8, 128.2, 127.2,
126.9, 126.2, 117.9, 117.6, 116.6, 114.9, 96.7, 55.2. HR-MS (TOF
MS ESI) m/z: 531.1349 [M + Na+]. Calcd for C23H17NNaOS (M
+ Na+): 531.1355.
4,4′-Bis(N,N-(4,4′-dimethoxydiphenylamino))benzophenone.
A mixture of 4,4′-bis(amino)benzophenone (1.16 g, 5.54 mmol),
copper powder (2 g, 32 mmol), K2CO3 (9.06 g, 65 mmol), 18-
crown-6 (100 mg), and iodoanisol (20.5 g, 109 mmol) was heated
to reflux at 188 °C under nitrogen atmosphere with stirring for 60 h.
The reaction mixture was allowed to cool to ambient temperature
and was dissolved in DCM. The solution was washed with distilled
water, and the organic phase was dried with NaSO4 and concen-
trated by evaporation. Column chromatography (10% DCM in
petroleum ether) yielded the product as a yellow solid (1.95 g, 56%).
1H NMR (500 MHz, d6-acetone): δ 7.60 (d, J ) 8.9 Hz, 4H), 7.08
(d, J ) 9.0 Hz, 8H), 6.97 (d, J ) 9.0 Hz, 8H), 6.78 (d, J ) 8.9 Hz,
4H). 13C NMR (125 MHz, d6-acetone): 194.0, 159.2, 154.1, 141.2,
133.1, 130.8, 129.9, 118.1, 116.893, 56.8. MS (APCI-ES) m/z: 637.3
[M + H]+.
mmol) in THF (10 mL) was added dropwise, and the solution was
kept at 0 °C for 1 h. The reaction mixture was refluxed and stirred
under nitrogen atmosphere for 12 h. The reaction was quenched
with EtOH. Ether/water extraction, followed by solvent evaporation
and column chromatography over silica gel (20% DCM in
1
petroleum ether), yielded a yellow oil (0.242 g, 89%). H NMR
(500 MHz, d6-acetone): δ 7.28 (s, 1H), 7.20 (d, J ) 8.9 Hz, 2H),
7.18 (d, J ) 5.2 Hz, 1H), 7.11 (d, J ) 9.0 Hz, 4H), 7.04 (d, J )
9.0 Hz, 4H), 6.99 (m, 5H), 6.90 (m, 9H), 6.77 (d, J ) 8.8 Hz, 2H),
3.78 (s, 12H). 13C NMR (125 MHz, d6-acetone): 166.9, 166.7,
159.3, 158.7, 152.5, 151.3, 151.0, 149.8, 144.2, 141.7, 141.4, 138.9,
137.8, 137.3, 137.1, 136.4, 136.2, 131.3, 129.9, 129.1, 125.2, 65.3.
MS (APCI-ES) m/z: 717.4 [M + H]+.
3-(5-Bis(4,4′-dimethoxydiphenylamino)styryl)thiophen-2-
yl)-2-carbaldehyde.
3-(5-Bis(4,4′-dimethoxydiphenylami-
no)styryl)thiophen-2-yl) (0.242 g, 0.338 mmol) was dissolved in
DMF and cooled to -78 °C. The mixture was evacuated and kept
under nitrogen atmosphere. To the mixture, POCl3 (0.34 g, 0.419
mmol) was added dropwise. The reaction mixture was allowed to
warm to 0 °C, whereupon it turned red. The mixture was heated to
50 °C for 3 h. The reaction mixture was cooled to ambient
temperature and quenched by aq HCl (0.1 M, 100 mL). The solution
was washed with brine/ether. The organic phase was dried with
NaSO4, and solvent removal followed by column chromatography
(10% petroleum ether in DCM) yielded the product as a dark orange
1
oil (166 mg, 59%). H NMR (500 MHz, d6-acetone): δ 9.82 (s,
1H), 7.71 (d, J ) 3.9 Hz, 1H), 7.39 (s, 1H), 7.28 (d, J ) 9.0 Hz,
2H), 7.20 (d, J ) 3.9 Hz, 1H), 7.15 (d, J ) 9.0 Hz, 4H), 7.08 (d,
J ) 8.9 Hz, 4H), 7.04 (m, 4H), 6.93 (m, 9H), 6.79 (d, J ) 8.9 Hz,
2H), 3.79 (s, 12H). 13C NMR (125 MHz, d6-acetone): 184.7, 158.6,
158.1, 153.3, 151.5, 151.1, 146.7, 144.4, 142.8, 142.0, 138.1, 134.0,
132.4, 131.4, 129.8, 129.1, 128.4, 123.9, 120.5, 119.6, 116.7, 56.7.
MS (APCI-ES) m/z: 744.4 [M + H]+.
3-(5-Bis(4,4′-dimethoxydiphenylamino)styryl)thiophen-2-
yl)-2-cyanoacrylic acid (D11). A 30 mL acetonitrile solution of
3-(5-bis(4,4′-dimethoxydiphenylamino)styryl)thiophen-2-yl)-2-car-
baldehyde (20 mg, 0.027 mmol), cyanoacetic acid (5 mg, 0.059
mmol), and piperidine (0.85 mg, 0.01 mmol) was refluxed for 4 h
under nitrogen atmosphere. Purification by extraction (petroleum
ether and aq HCl (0.1 M)) and filtration of the formed solid yielded
the product as dark red solid, D11 (19 mg, 87%), mp 220.5-
221.5 °C. 1H NMR (500 MHz, d6-DMSO): δ 8.28 (s, 1H), 7.76 (d,
J ) 4.15 Hz, 1H), 7.39 (s, 1H), 7.23 (d, J ) 4.18 Hz, 1H),
7.22-7.17 (m, 6H), 7.07-7.03 (m, 1H), 6.98-6.90 (m, 10H), 6.81
(d, J ) 8.65 Hz, 1H), 6.69 (d, J ) 8.94 Hz, 1H), 3.74 (s, 12H).
13C NMR (125 MHz, d6-DMSO): 156.0, 155.9, 149.0, 148.6, 139.7,
139.4, 134.7, 131.9, 130.1, 129.9, 128.2, 127.9, 127.4, 126.9, 118.8,
118.0, 117.5, 114.9, 114.8, 55.1, 55.1. HR-MS (TOF MS ESI) m/z:
834.2608 [M + Na+]. Calcd for C23H17NNaOS (M + Na+):
834.2608.
Acknowledgment. We acknowledge the Swedish Reasearch
Council, Swedish Energy Agency, and the Knut and Alice
Wallenberg Foundation for financial support and the Korea
Foundation for International Cooperation of Science & Technol-
ogy through the Global Research Laboratory (GRL) Program
funded by the Ministry of Science and Technology. F.D.A.
thanks MIUR (FIRB 2003: Molecular compounds and hybrid
nanostructured materials with resonant and nonresonant optical
properties for photonic devices) and CNR (PROMO 2006) for
financial support. We also thank Mr. Lien-Hoa Tran (Stockholm
University) for the HR-MS measurements.
3-(5-Bis(4,4′-dimethoxydiphenylamino)styryl)thiophen-2-yl). To
a stirred solution of 4,4′-bis(N,N-(4,4′-dimethoxydiphenylami-
no))benzophenone (0.24 g, 0.378 mmol) and THF (50 mL), NaH
(20.5 mg, 0.529 mmol) was added, and the mixture was cooled to
0 °C over 30 min. Diethyl thiophenephosphonate (106 mg, 0.378
Supporting Information Available: Complete ref 22. This
material is available free of charge via the Internet at http://
pubs.acs.org.
(33) Spraul, B. K.; Suresh, S.; Sassa, T.; Herranz, M. A.; Echegoyen, L.;
Wada, T.; Perahia, D., Jr Tetrahedron Lett. 2004, 45, 3253–3256.
JA800066Y
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6266 J. AM. CHEM. SOC. VOL. 130, NO. 19, 2008