Double D-π-A dye linked by 2,2′-bipyridine dicarboxylic acid: Influence of para- and meta-substituted carboxyl anchoring group

Article abstract of DOI:10.1002/cphc.201402822

Starting from 2,2′-bipyridine dicarboxylic acid, two new (D-π-A)₂ sensitizers, including m-DA with the carboxyl anchoring group substituted meta to the donor-bridge moiety and p-DA with a para-substituted anchoring group, were synthesized in order to evaluate the impact of the position of the anchoring group on the optical, electrochemical, and photovoltaic properties of dye-sensitized solar cells. p-DA exhibits red-shifted absorption behavior compared to m-DA, owing to the more efficiently extended π-conjugation with para substitution. Both m-DA and p-DA are adsorbed on the mesoporous TiO₂ surface by using both of their carboxylic acid groups in a bianchoring mode, which is confirmed through attenuated total reflectance FTIR analysis. Red-shifted absorption of p-DA assists the achievement of a red-shifted incident photon-to-electron conversion efficiency and a higher short-circuit current density than m-DA. The photogenerated electron lifetime in TiO₂ is also found to be higher for para substituted p-DA than the meta-substituted m-DA, which results in a higher open-circuit voltage. All of the results suggest that dicarboxyl-2,2′-bipyridine can be used as an acceptor for metal-free organic sensitizers. However, the anchoring segments should be adjusted to the favorable position of the corresponding donor-bridge moieties for better conjugation.

Full text of DOI:10.1002/cphc.201402822

DOI: 10.1002/cphc.201402822
Articles
Double D–p–A Dye Linked by 2,2’-Bipyridine Dicarboxylic
Acid: Influence of para- and meta-Substituted Carboxyl
Anchoring Group
Paramaguru Ganesan,[a, b] Aravind Kumar Chandiran,[a, c] Peng Gao,*[a]
Renganathan Rajalingam,[b] Michael Grꢀtzel,[a] and Mohammad Khaja Nazeeruddin*[a]
Starting from 2,2’-bipyridine dicarboxylic acid, two new (D–p–
A)2 sensitizers, including m-DA with the carboxyl anchoring
group substituted meta to the donor-bridge moiety and p-DA
with a para-substituted anchoring group, were synthesized in
order to evaluate the impact of the position of the anchoring
group on the optical, electrochemical, and photovoltaic prop-
erties of dye-sensitized solar cells. p-DA exhibits red-shifted ab-
sorption behavior compared to m-DA, owing to the more effi-
ciently extended p-conjugation with para substitution. Both
m-DA and p-DA are adsorbed on the mesoporous TiO2 surface
by using both of their carboxylic acid groups in a bianchoring
mode, which is confirmed through attenuated total reflectance
FTIR analysis. Red-shifted absorption of p-DA assists the ach-
ievement of a red-shifted incident photon-to-electron conver-
sion efficiency and a higher short-circuit current density than
m-DA. The photogenerated electron lifetime in TiO2 is also
found to be higher for para substituted p-DA than the meta-
substituted m-DA, which results in a higher open-circuit volt-
age. All of the results suggest that dicarboxyl-2,2’-bipyridine
can be used as an acceptor for metal-free organic sensitizers.
However, the anchoring segments should be adjusted to the
favorable position of the corresponding donor-bridge moieties
for better conjugation.
1. Introduction
Dye-sensitized solar cells (DSSC), commonly known as Grꢀtzel
cells, have emerged as a promising solar energy-conversion
technology to meet the energy demand of the future.[1,2]
DSSCs have received great attention from both academia and
industry, owing to their low cost and easy fabrication methods.
Among the different components determining the per-
formance of DSSCs, sensitizers that harvest solar radiation play
a key role in achieving high efficiencies. So far, ruthenium-
based complexes and zinc-porphyrin sensitizers show promis-
ing photovoltaic power conversion efficiencies (PCE) of 11 and
13%, respectively.[3–5] Owing to the rare availability, high cost,
and low molar extinction coefficient of ruthenium-based sensi-
tizers, quests to develop organic-based sensitizers have been
receiving increasing attention in recent years. With many ap-
proaches to different organic sensitizers, the most effective
structural arrangement of an organic dye consists of donor
and acceptor groups bridged through a p spacer (D–p–A).[6]
Recently, Grꢀtzel et al. achieved a record PCE of 10.65% with
a single D–p–A organic molecule, which motivated researchers
to further develop new efficient sensitizers, sustaining impor-
tant factors such as broad absorption in the visible region,
strong binding affinity with TiO2, good electron injection, good
stability, and cost effectiveness.[7] The development of substi-
tuted multianchor dyes has been getting great attention in
recent years to enhance the binding strength and electron in-
jection efficiency.[8] The most common strategy is that a donor
and p spacer are attached to anchoring groups with two ac-
ceptors (D–p–A2),[9] and there are a few reports focused on
using the double donor–p–acceptor approach (D–p–A)2.[10] In
both of these strategies, despite the change in choices of
donor and p spacer groups, cyanoacrylic acid is the most
widely used acceptor and anchoring moiety. This is evident
from the recent review of multianchoring groups, where all
the dyes have cyanoacrylic acid as the anchoring moiety.[11]
Kloo et al. and Sun et al. observed the problem of photoisome-
rization and degradation of cyanoacrylic acid, which will affect
the dye performance in the long term.[12] When multiple cya-
noacrylic acids are employed in one molecule, the problem
will become more complicated. Therefore, we have tried to
overcome this situation by using carboxylic acids as anchoring
groups, like ruthenium dye counterparts.
[a] Dr. P. Ganesan, Dr. A. K. Chandiran, Dr. P. Gao, Prof. Dr. M. Grꢀtzel,
Prof. M. K. Nazeeruddin
Laboratory for Photonics and Interfaces
Institute of Chemical Sciences and Engineering
Ecole Polytechnique Fꢁdꢁrale de Lausanne
CH-1015 Lausanne (Switzerland)
E-mail: peng.gao@epfl.ch
mdkhaja.nazeeruddin@epfl.ch
[b] Dr. P. Ganesan, Prof. R. Rajalingam
School of Chemistry, Bharathidasan University
Tiruchirappalli 620024, Tamilnadu (India)
[c] Dr. A. K. Chandiran
Current address:
Department of Chemistry
University of California, Berkeley, CA, 94720-1460 (USA)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cphc.201402822.
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Based on the above discussion, we attempted to use two
different kinds of 2,2’-bipyridine dicarboxylic acids to develop
new types of (D-p-A)2 sensitizers. 2,2’-Bipyridine dicarboxylic
acids, which are commonly used in ruthenium dyes, were
chosen because of their flexible structural arrangement in cou-
pling two donors and the inherent two-carboxyl-acceptor part.
(Scheme 1) To the best of our knowledge, this is the first
Scheme 2. Synthetic route employed for 2,2’-bipyridine-based acceptor with
carboxyl groups at different positions.
ate precursors.[13] Hydrolysis of these precursors by using LiOH
afforded the final sensitizers m-DA and p-DA. (Scheme 3) All in-
termediates and the final compounds were characterized by
1H NMR, 13C NMR, and mass spectroscopy. The observed NMR
and mass data correlated well with the expected final com-
pounds.
Scheme 1. a) Chemical structure of commonly employed ruthenium com-
plexes containing carboxyl anchoring groups. b) Chemical structure of the
(D–p–A)2 sensitizers with carboxyl anchoring groups that attached to the
donor segment in the para or meta positions.
2.2. Optical and Electrochemical Properties
The influence of para- and meta-substituted carboxyl anchor-
ing groups on the light-harvesting properties of the sensitizers
was scrutinized by using ground-state absorption measure-
ments. It can be seen from the absorption spectra of m-DA
report in which bipyridine ac-
ceptors are used in organic sen-
sitizers. In the two sensitizers
that we designed and synthe-
sized, carboxylic acids were at-
tached to 2,2’-bipyridine with
two different situations, that is,
m-DA with the anchoring part
substituted meta to the donor
part and p-DA with a para-sub-
stituted anchoring part. The two
new sensitizers were fully char-
acterized by NMR, mass tech-
Scheme 3. Synthetic route employed for final (D–p–A)2 sensitizers m-DA and p-DA.
niques, UV/Vis absorption, pho-
toluminescence (PL), and cyclic
voltammetry (CV). The impact of the position of the anchoring
part on the optical, electrochemical and photovoltaic proper-
ties was also investigated.
and p-DA that the change in positions of the carboxyl anchor-
ing group affects the absorption spectra of the dyes. (Fig-
ure 1A) The meta-substituted ÀCOOH dye (m-DA) exhibits
a major absorption band from 370 to 525 nm with a maximum
around 471 nm. When the ÀCOOH group is present in the
para position, there is a 7 nm redshift in the maximum absorp-
tion and the absorption edge experiences a greater redshift
and appears around 555 nm. This phenomenon is attributed to
the possibility of p-conjugation shifts towards the carboxylic
group in p-DA, owing to the substitution in para position and
results in prominent charge-transfer (CT) transition. However,
owing to the meta-positioned ÀCOOH group in m-DA, the ex-
tended conjugation is not overlapped with the ÀCOOH moiety
and results in a CT band. Furthermore, the molar extinction co-
2. Results and Discussion
2.1. Synthesis and Characterization of m-DA and p-DA
The 2,2’-bipyridine-based acceptors with carboxyl groups at
different positions were synthesized, and the synthetic routes
are depicted in Scheme 2. The donor–p–spacer 1 was synthe-
sized by employing the previously reported procedure and
then converted to its stannous analogue for convenient cou-
pling with the acceptor fragments to give the methyl carboxyl-
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Figure 2. a) Cyclic voltammogram of m-DA and p-DA. b) Absorption spec-
trum of sensitizers m-DA and p-DA adsorbed on TiO2 film.
Figure 1. a) Absorption and b) emission spectra of sensitizers m-DA and p-
DA in dichloromethane (1ꢂ10À5 m).
efficient of p-DA is found to be higher than m-DA. The emis-
sion behavior of both dyes excited at their absorption maxi-
mum wavelength was recorded in dichloromethane solution
and the spectra are displayed in Figure 1B. m-DA and p-DA
have maximum emissions at 569 and 584 nm, respectively.
Similar to their absorption behavior, the maximum emission
wavelength of p-DA is red-shifted by 15 nm compared to that
of m-DA.
was estimated from the intersection of the normalized absorp-
tion and emission spectra. The LUMO levels of m-DA and p-DA
were calculated to be around À1.59 V versus NHE and À1.58 V
versus NHE, respectively, which are more negative than the
conduction-band edge of TiO2 electrode (À0.5 V vs. NHE),[15]
implying that these dyes can efficiently inject an electron from
their excited states to the CB of TiO2. It is worth noting that
there is considerable difference in the second oxidation pro-
cess of both sensitizers, which may be attributed to the varia-
tion of the positions of substituted anchoring groups. All data
are detailed in Table 1.
Energy levels of sensitizers are imperative parameters to de-
termine the ability of electron injection from the excited dye
molecules to the conduction band (CB) of TiO2 and dye regen-
eration through the redox couple. CV was carried out to mea-
sure and understand the impact of the positions of the anchor-
ing groups on the oxidation potential of the sensitizers. Fig-
ure 2a displays the CV traces of both dyes, in which two oxida-
tion peaks are observed. The first redox process (HOMO)
(0.77 V vs. NHE for m-DA and 0.74 V vs. NHE for p-DA) occurs
because of the presence of a triphenyl amine segment, as ob-
served from earlier reports, which are more positive than the
redox potential of the IÀ/I3À redox couple (ꢀ0.4 V vs. NHE),[14]
implying the thermodynamic feasibility of regeneration of the
oxidized dyes. Correspondingly, the excited-state oxidation po-
tential of the dyes (LUMO) will give information about their
electron-injection possibilities to the CB of TiO2. The LUMO
levels of the dyes were calculated from the difference between
Table 1. Photophysical and electrochemical properties of sensitizers m-
DA and p-DA.
Sensitizer Absorption Emission
lmax[a] [nm] lmax[a] [nm] [V vs. NHE] [eV]
HOMO[b]
Band gap[c] LUMO[d]
[V vs.
NHE]
m-DA
p-DA
471
478
569
584
0.77
0.74
2.36
2.32
À1.59
À1.58
[a] Absorption or emission maximum in dichloromethane solution.
[b] The HOMO was taken from the first redox potential in CV plot. Poten-
tials measured versus Fc+/Fc were converted to normal hydrogen elec-
trode (NHE) by adding +0.63 V). [c] The band gap (E0-0) was estimated
from the intersection of the normalized absorption and emission spectra.
[d] The LUMO was calculated with the expression of LUMO=HOMO -
Band Gap.
the HOMO level and the band-gap energy. The band gap (E0-0
)
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2.3. Binding Properties of m-DA and p-DA with TiO2
To analyze the binding behavior of the dyes on the TiO2 sur-
face, the absorption spectra of the two sensitizers loaded on
a TiO2 film (5 mm in thickness) were measured and the results
are displayed in Figure 2b. Both dyes have similar maximum
absorption wavelengths compared to their solution-state ab-
sorption spectra, whereas their absorption edges appeared
around 561 and 573 nm for m-DA-T and p-DA-T, respectively,
which are red-shifted by 36 and 18 nm, indicating the J-aggre-
gation of the dye molecules upon binding with the TiO2 sur-
face.[16] Attenuated total reflectance FTIR (ATR-FTIR) spectrosco-
py has been widely utilized to probe the interactions between
the carboxylic acid groups and the TiO2 surface. Figure 3
Figure 4. a) Current–voltage characteristics measured under simulated
AM 1.5 G simulated sunlight (100 mWcmÀ2) for devices employing m-DA
and p-DA. b) IPCE spectra of sensitizers m-DA and p-DA.
2.4. Photovoltaic Properties of m-DA and p-DA
DSSC devices were fabricated with these dyes to evaluate the
influence of two different types of substitution on the PCEs.
The DSSCs were all measured under simulated one-sun illumi-
nation (AM 1.5G irradiation at 100 mWcmÀ2). The photocurrent
density voltage (J–V) curves are shown in Figure 4 and the cor-
responding photovoltaic data are listed in Table 2. It is interest-
ing to look over the short-circuit current density (JSC), owing to
the distinct feature in the direction of orientation for electron
injection. m-DA with meta-substituted anchoring groups pro-
duces 7.01 mAcmÀ2, whereas the JSC of p-DA increased by
1.18 mAcmÀ2 to reach 8.19 mAcmÀ2. The observed difference
Figure 3. FTIR spectra of m-DA and p-DA in the solid state and the dyes ad-
sorbed on TiO2 films.
shows the ATR-FTIR spectra of the dyes on glasses or adsorbed
on the TiO2 surface. On a glass substrate, both m-DA and p-DA
showed a strong absorption signal around 1699 cmÀ1, which is
attributed to the C=O stretching frequency of free carboxylic
acid group (black line). Upon binding with the TiO2 surface,
this signal completely disappears, as seen in the FTIR spectra
(grey line). The complete disappearance of the free carboxylic
acid group indicates that both m-DA and p-DA are adsorbed
on the TiO2 surface by using both of their carboxylic acid
groups in a so called “bianchoring mode”. These observations
are well corroborated by the previously reported IR characteris-
tics of sensitizers with carboxylic acid anchoring groups.[17]
Table 2. Photovoltaic performance of DSSCs of m-DA and p-DA using
iodide/triiodide redox mediator.[a]
Cells
VOC
[V]
JSC
[mAcmÀ2
FF
h
]
[%]
m-DA
p-DA
0.682
0.708
7.01
8.19
0.69
0.67
3.3
3.9
[a] TiO2 film has a 8 mm scattering layer and a 5 mm transparent layer.
Electrolyte composition: 1.0m DMII (1,3-dimethylimidazolium iodide),
0.03m iodine, 0.025m NaI, 0.5 TBP, 0.1m guanidinium thiocyanide, and
acetonitrile solvent.
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in JSC values indicates a more efficient electron injection from
the LUMO of the dye molecule to the CB of TiO2 through the
para-substituted anchoring group compared with the meta-
substituted anchoring group. Similarly, the open-circuit voltage
(VOC) of p-DA was 26 mV greater than m-DA, whereas the fill
factors (FFs) of these two dyes were almost the same. Owing
to the higher JSC and VOC values with p-DA, its overall conver-
sion efficiency reached 3.9% compared to 3.3% for m-DA. Inci-
dent photon-to-current conversion efficiency (IPCE) measure-
ments can provide a clear picture of the ability of the sensitiz-
ers to convert the absorbed light energy into a current. The
photocurrent response spectra of all sensitizers were measured
with their corresponding photovoltaic devices, and the plots of
IPCE as a function of wavelength are displayed in Figure 4B. p-
DA shows slightly red-shifted IPCE compared to m-DA, which
is in line with their absorption behavior. This small red-shifted
IPCE contributes to the higher current of p-DA. Both dyes have
approximately 60% IPCE in the 400–520 nm regions.
The reason for the difference in JSC values is well supported
by their absorption and IPCE measurements. The VOC difference
is mainly governed either by the distribution of trap states
below the CB on the semiconductor or by the recombination
of the injected electrons from the TiO2 to the redox species.
Charge extraction measurements were carried out to analyze
the trap states of the dye-sensitized TiO2 surface. Figure 5a
represents the photovoltage as a function of capacitance. The
observed results suggest that both types of anchoring position
have a similar type of trap states. Hence, transient photovolt-
age decay measurements were carried out to analyze the re-
combination of electrons injected in the TiO2 through reduc-
tive electron transfer to the electrolyte, and a lower recombi-
nation process results in a longer lifetime of the electron. As
displayed in Figure 5b, at a given capacitance, the electron
lifetime of p-DA is longer than m-DA, which resulted in
a 26 mV higher VOC than p-DA. The possible reason for the dif-
ference in recombination rate is attributed to the effective con-
jugation of the para-substituted anchoring group with TiO2 in
p-DA, which injects electrons deep in the TiO2 surface for
better electron percolation. Because there is no effective conju-
gation for m-DA with TiO2, it injects only to the surface of the
semiconductor and enhances the recombination process. This
elucidation is well supported by the observation of Wiberg
et al. for the different recombination rates between a well-con-
jugated cyanoacrylic acid anchoring group and less-conjugated
rhodanine acetic acid anchoring group.[18]
Figure 5. a) Distribution of trap state and b) electron lifetimes as a function
of capacitance, as measured by using transient photocurrent and photovolt-
age decay techniques for devices made with m-DA and p-DA.
behavior was observed for the para-substituted p-DA com-
pared to m-DA. Analysis from ATR-FTIR measurements con-
firmed that both m-DA and p-DA were adsorbed on the TiO2
surface by using both of their carboxylic acid groups in a bian-
choring mode. Owing to the red-shifted absorption of p-DA, its
JSC is higher than m-DA, which is again reflected in the slightly
broadened IPCE of p-DA. As probed from transient measure-
ments, the reason for the 26 mV higher VOC of p-DA compared
to that of m-DA is the difference in their charge recombination
rate or electron lifetime. Overall the para-substituted dye ex-
hibited better photovoltaic performance than m-DA. The ob-
served results provide vital information for the future design of
double-acceptor-based sensitizers with better efficiencies.
3. Conclusions
Experimental Section
For the first time, two new (D–p–A)2 organic sensitizers (m-DA,
p-DA) employing a 2,2’-bipyridine dicarboxylic acid unit as an
acceptor and anchoring group were designed. m-DA has the
anchoring part substituted meta to the donor moiety and p-
DA has a para-substituted anchoring part. The two new dyes
were successfully synthesized and characterized by using NMR
and mass spectroscopy. The impacts of the different anchor-
ing-group substitution on optical, electrochemical, and photo-
voltaic properties were investigated. Red-shifted absorption
Synthesis of p-DA-COOMe
To a 100 mL flam- dried, N2-filled round-bottomed flask charged
with a stirrer bar, we added p-Cl-BP-COOMe (0.040 g) and 4-(4,4-di-
hexyl-6-(tributylstannyl)-4H-cyclopenta[1,2-b:5,4-b’] dithiophen-2-
yl)-N,Nbis(4-hexyloxy)phenyl)aniline 6 (485 mg). Pd(PPh3)4 (100 mg)
was then added, followed by a freeze–pump–thaw degassing pro-
cession. Anhydrous DMF (20 mL) was then added and the reaction
mixture was heated to 808C for 12 h. Dichloromethane (50 mL)
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was added to the reaction mixture, which was subsequently
washed with saturated sodium chloride aqueous solution (3ꢂ
75 mL) and then with water (3ꢂ75 mL). The organic layer was sep-
arated and dried over magnesium sulfate. After removal of the sol-
vent, the crude mixture was purified with gradient silica gel chro-
matography: 100:0 to 50:50 (hexane/DCM) to give p-DA-COOMe
(0.13 g, 36%) as a red-colored semisolid. 1H NMR (400 MHz, CDCl3):
d=8.31–8.19 (m, J=8.2 Hz, 2H), 7.72–7.63 (m, J=8.5 Hz, 2H), 7.55
(s, 2H), 7.49–7.40 (m, 4H), 7.20–6.99 (m, 10H), 6.95 (d, J=8.9 Hz,
4H), 6.89–6.84 (m, 8H), 3.97 (t, J=6.5 Hz, 8H), 3.75 (s, 6H), 1.85–
1.79 (m, 8H), 1.53–1.47 (m, 8H), 1.39 (td, J=3.7, 7.3 Hz, 20H), 1.34–
1.31 (m, 8H), 1.22–1.16 (m, 20H), 0.97–0.92 (m, 20H), 0.84 ppm (t,
J=6.8 Hz, 12H); 13C NMR (100 MHz, CDCl3): d=167.4, 160.3, 158.3,
158.1, 155.5, 154.2, 148.2, 146.6, 143.0, 141.6, 140.4, 138.5, 137.6,
134.6, 130.6, 127.1, 126.5, 125.9, 123.5, 120.7, 120.7, 118.6, 116.4,
116.2, 115.3, 68.3, 54.0, 52.3, 38.0, 31.6, 31.6, 29.7, 29.5, 29.3, 27.8,
26.9, 26.8, 25.7, 24.5, 22.6, 22.6, 19.6, 17.5, 14.1, 14.0, 13.6 ppm.
1H NMR (400 MHz, [D8]THF): d=13.31 (s, 3H), 8.88 (s, 1H), 8.33 (s,
1H), 7.83 (s, 1H), 7.46 (d, J=8.8 Hz, 3H), 7.26 (s, 1H), 7.03 (d, J=
9.0 Hz, 6H), 6.87 (dd, J=22.9, 8.8 Hz, 10H), 3.94 (t, J=6.4 Hz, 6H),
3.58 (s, 18H), 2.20 (s, 2H), 2.01 (d, J=7.8 Hz, 7H), 1.75 (d, J=
16.8 Hz, 33H), 1.55–1.45 (m, 8H), 1.39 ppm (d, J=9.0 Hz, 19H);
13C NMR (101 MHz, THF) d=160.00, 157.97, 155.77, 148.23, 146.40,
143.78, 140.39, 136.95, 134.52, 127.94, 127.09, 126.41, 125.64,
125.01, 120.39, 116.15, 115.03, 67.89, 67.39, 41.15, 38.01, 34.11,
31.60, 29.82, 29.31, 25.74, 24.59, 24.39, 24.17, 22.56, 20.55, 13.54,
13.52 ppm; HRMS (MALDI) m/z: calcd for C114H138N4O8S4: 1820.6180
[M]+; found 1819.9087; elemental analysis calcd (%): C 75.21, H
7.64; found: C 75.23, H, 7.62.
Acknowledgements
R.R. thanks the Department of Science and Technology (DST) (ref-
erence number SR/S1/PC-12/2011, DT: 20.09.2011) for the project.
G.P. thanks the Swiss Government Scholarship (Ref No:
2012.0795/India/OP.) P.G. thanks the European Community’s Sev-
Synthesis of p-DA
To a 50 mL round-bottomed flask, we added p-DA-COOMe (0.1 g)
and THF (30 mL). The mixture was cooled to 08C and 2m LiOH
(30 mL) was added slowly and the reaction mixture was stirred for
about 24 h. Dichloromethane (50 mL) was added to the reaction
mixture, which was subsequently washed with acetic acid solution
(3ꢂ75 mL) and then with water (3ꢂ75 mL). The organic layer was
separated and dried over magnesium sulfate. After removal of the
solvent, the crude mixture was purified with gradient silica gel
chromatography using DCM/methanol/acetic acid (0.8:0.1:0.1) as
a final eluent to yield p-DA (0.06 g, 36%) as s red-colored semisol-
id. m.p. 246–2488C; 1H NMR (400 MHz, CDCl3): d=8.39 (br. s., 1H),
7.71 (d, J=8.5 Hz, 2H), 7.55 (s, 2H), 7.44 (d, J=8.5 Hz, 4H), 7.28–
6.98 (m, 11H), 6.95 (d, J=8.9 Hz, 4H), 6.89–6.82 (m, 8H), 3.97 (t, J=
6.7 Hz, 8H), 2.38–2.31 (m, 4H), 1.82–1.80 (m, 4H), 1.67–1.57 (m,
8H), 1.37 (td, J=3.6, 7.4 Hz, 20H), 1.33–1.31 (m, 9H), 1.23–1.18 (m,
21H), 0.95–0.90 (m, 20H), 0.84 ppm (s, 12H); 13C NMR (101 MHz,
CDCl3d): d=179.4, 158.3, 155.6, 148.3, 140.4, 130.1, 130.0, 129.8,
127.9, 127.0, 126.6, 126.0, 120.6, 116.2, 115.3, 68.3, 54.1, 38.0, 34.0,
32.0, 31.7, 31.6, 31.6, 29.8, 29.8, 29.7, 29.7, 29.6, 29.5, 29.5, 29.4,
29.4, 29.3, 29.2, 29.1, 27.2, 27.0, 25.8, 24.7, 24.6, 22.7, 22.7, 22.6,
14.1, 14.1, 13.6 ppm; HRMS (MALDI) m/z: calcd for C114H138N4O8S4:
1818.9398 [M]+; found 1818.9828; elemental analysis calcd (%): C
75.21, H 7.64; found: C 75.26, H 7.61.
enth
Framework
Programme
(FP7/2007-2013)
ENERGY.2012.10.2.1; NANOMATCELL, grant agreement no.
308997K. M.K.N. acknowledges the World Class University pro-
gramme, Photovoltaic Materials, Department of Material Chemis-
try, Korea University, Chungnam, Korea, funded by the Ministry of
Education, Science and Technology through the National Re-
search Foundation of Korea (no. R31-2008-000-10035-0).
Keywords: 2,2’-bipyridine · bianchoring · double acceptor ·
dyes/pigments · solar cells
[1] B. O’Regan, M. Gratzel, Nature 1991, 353, 737–740.
[2] M. Grꢀtzel, Nature 2001, 414, 338–344.
[3] a) M. Grꢀtzel, J. Photochem. Photobiol. A 2004, 164, 3–14; b) M. K. Na-
zeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker, E. Mueller, P. Liska, N.
Vlachopoulos, M. Graetzel, J. Am. Chem. Soc. 1993, 115, 6382–6390;
c) M. K. Nazeeruddin, R. Splivallo, P. Liska, P. Comte, M. Gratzel, Chem.
Commun. 2003, 1456–1457; d) P. Wang, S. M. Zakeeruddin, I. Exnar, M.
Gratzel, Chem. Commun. 2002, 2972–2973; e) P. Wang, S. M. Zakeerud-
din, J. E. Moser, M. K. Nazeeruddin, T. Sekiguchi, M. Gratzel, Nat. Mater.
2003, 2, 402–407.
[4] a) A. Yella, H. W. Lee, H. N. Tsao, C. Yi, A. K. Chandiran, M. K. Nazeerud-
din, E. W. G. Diau, C. Y. Yeh, S. M. Zakeeruddin, M. Grꢀtzel, Science 2011,
334, 629–634; b) S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. F. E.
Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M. K. Nazeerud-
din, M. Grꢀtzel, Nat. Chem. 2014, 6, 242–247.
Synthesis of m-DA-COOMe
[5] a) R. Chen, X. Yang, H. Tian, L. Sun, J. Photochem. Photobiol. A 2007, 189,
295–300; b)
A similar method to that for the synthesis of p-DA-COOMe was ap-
plied to give m-DA-COOMe (0.1 g, 30%) as a red-colored solid.
1H NMR (400 MHz, CD2Cl2): d=8.76 (s, 1H), 8.03 (s, 1H), 7.58 (s,
1H), 7.51 (d, J=8.6 Hz, 3H), 7.11 (d, J=7.6 Hz, 6H), 6.90 (d, J=
8.8 Hz, 9H), 4.06 (s, 4H), 3.99 (t, J=6.5 Hz, 6H), 2.12–2.01 (m, 6H),
1.83 (dt, J=14.5, 6.6 Hz, 6H), 1.74–1.65 (m, 5H), 1.57–1.47 (m, 8H),
1.45–1.21 (m, 42H), 1.15 (s, 6H), 0.98 (t, J=7.3 Hz, 16H), 0.87 ppm
(t, J=6.8 Hz, 9H); 13C NMR (101 MHz, CD2Cl2): d=165.62, 160.08,
155.82, 155.32, 146.44, 139.14, 126.78, 125.77, 120.34, 117.55,
115.24, 68.30, 38.34, 31.65, 29.91, 29.35, 27.86, 26.86, 25.76, 22.67,
17.48, 13.87, 13.41 ppm.
R. Chen, X. Yang, H. Tian, X. Wang, A. Hagfeldt, L. Sun, Chem. Mater.
2007, 19, 4007–4015; c) Y. Hao, X. Yang, J. Cong, H. Tian, A. Hagfeldt, L.
Sun, Chem. Commun. 2009, 4031–4033.
[6] a) N. Hirata, J.-J. Lagref, E. J. Palomares, J. R. Durrant, M. K. Nazeeruddin,
M. Gratzel, D. Di Censo, Chem. Eur. J. 2004, 10, 595–602; b) J. N. Clifford,
E. Palomares, M. K. Nazeeruddin, M. Grꢀtzel, J. Nelson, X. Li, N. J. Long,
J. R. Durrant, J. Am. Chem. Soc. 2004, 126, 5225–5233; c) J.-H. Yum, P.
Chen, M. Grꢀtzel, M. K. Nazeeruddin, ChemSusChem 2008, 1, 699–707;
d) A. Kay, M. Graetzel, J. Phys. Chem. 1993, 97, 6272–6277; e) K. Hara, Y.
Dan-oh, C. Kasada, Y. Ohga, A. Shinpo, S. Suga, K. Sayama, H. Arakawa,
Langmuir 2004, 20, 4205–4210.
[7] J. Yang, P. Ganesan, J. Teuscher, T. Moehl, Y. J. Kim, C. Yi, P. Comte, K. Pei,
T. W. Holcombe, M. K. Nazeeruddin, et al., J. Am. Chem. Soc. 2014, 136,
5722–5730.
Synthesis of m-DA
[8] a) S. S. Park, Y. S. Won, Y. C. Choi, J. H. Kim, Energy Fuels 2009, 23, 3732–
3736; b) C.-H. Yang, S.-H. Liao, Y.-K. Sun, Y.-Y. Chuang, T.-L. Wang, Y.-T.
Shieh, W.-C. Lin, J. Phys. Chem. C 2010, 114, 21786–21794; c) X. Jiang,
A similar method to that for the synthesis of p-DA was applied to
give m-DA (0.12 g, 35%) as a red-colored solid. m.p. 224–2288C;
&
ChemPhysChem 0000, 00, 0 – 0
www.chemphyschem.org
6
ꢁ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

Articles
K. M. Karlsson, E. Gabrielsson, E. M. J. Johansson, M. Quintana, M. Karls-
son, L. Sun, G. Boschloo, A. Hagfeldt, Adv. Funct. Mater. 2011, 21, 2944–
2952; d) S. Ramkumar, S. Manoharan, S. Anandan, Dye. Pigment. 2012,
94, 503–511; e) M. S. Kim, H. S. Yang, D. Y. Jung, Y. S. Han, J. H. Kim, Col-
loids Surf. A 2013, 420, 22–29; f) G. Paramaguru, N. Nagarajan, R. Re-
nganathan, J. Mol. Struct. 2013, 1038, 235–241; g) Y. Ooyama, N. Yama-
guchi, I. Imae, K. Komaguchi, J. Ohshita, Y. Harima, Chem. Commun.
2013, 49, 2548–2550; h) W.-I. Hung, Y.-Y. Liao, C.-Y. Hsu, H.-H. Chou, T.-
H. Lee, W.-S. Kao, J. T. Lin, Chem. Asian J. 2014, 9, 357–366; i) W. Lee,
S. B. Yuk, J. Choi, H. J. Kim, H. W. Kim, S. H. Kim, B. Kim, M. J. Ko, J. P.
Kim, Dye. Pigment. 2014, 102, 13–21.
H. K. Kim, Dye. Pigment. 2013, 99, 599–606; c) D. Cao, J. Peng, Y. Hong,
X. Fang, L. Wang, H. Meier, Org. Lett. 2011, 13, 1610–1613; d) Y. Hong,
J.-Y. Liao, J. Fu, D.-B. Kuang, H. Meier, C.-Y. Su, D. Cao, Dye. Pigment.
2012, 94, 481–489.
[11] N. Manfredi, B. Cecconi, A. Abbotto, Eur. J. Org. Chem. 2014, 2014,
7069–7086.
[12] a) B. Zietz, E. Gabrielsson, V. Johansson, A. M. El-Zohry, L. Sun, L. Kloo,
Phys. Chem. Chem. Phys. 2014, 16, 2251–2255; b) C. Chen, X. Yang, M.
Cheng, F. Zhang, L. Sun, ChemSusChem 2013, 6, 1270–1275.
[13] P. Ganesan, A. Chandiran, P. Gao, R. Rajalingam, M. Grꢀtzel, M. K. Nazeer-
uddin, J. Phys. Chem. C 2014, 118, 16896–16903.
[9] a) H. Zhang, J. Fan, Z. Iqbal, D.-B. Kuang, L. Wang, H. Meier, D. Cao, Org.
Electron. 2013, 14, 2071–2081; b) K. S. V. Gupta, T. Suresh, S. P. Singh, A.
Islam, L. Han, M. Chandrasekharam, Org. Electron. 2014, 15, 266–275;
c) H.-C. Chu, D. Sahu, Y.-C. Hsu, H. Padhy, D. Patra, J.-T. Lin, D. Bhatta-
charya, K.-L. Lu, K.-H. Wei, H.-C. Lin, Dye. Pigment. 2012, 93, 1488–1497;
d) R. Grisorio, L. De Marco, G. Allegretta, R. Giannuzzi, G. P. Suranna, M.
Manca, P. Mastrorilli, G. Gigli, Dye. Pigment. 2013, 98, 221–231; e) G. D.
Sharma, J. A. Mikroyannidis, M. S. Roy, K. R. J. Thomas, R. J. Ball, R. Kurch-
ania, RSC Adv. 2012, 2, 11457–11464; f) V. Leandri, R. Ruffo, V. Trifiletti,
A. Abbotto, Eur. J. Org. Chem. 2013, 2013, 6793–6801; g) C. Y. Lee, C.
She, N. C. Jeong, J. T. Hupp, Chem. Commun. 2010, 46, 6090–6092; h) L.
Yu, J. Xi, H. T. Chan, T. Su, L. J. Antrobus, B. Tong, Y. Dong, W. K. Chan,
D. L. Phillips, J. Phys. Chem. C 2013, 117, 2041–2052; i) Y. S. Yang, H.
Do Kim, J.-H. Ryu, K. K. Kim, S. S. Park, K.-S. Ahn, J. H. Kim, Synth. Met.
2011, 161, 850–855.
[14] P. Gao, Y. J. Kim, J.-H. Yum, T. W. Holcombe, M. K. Nazeeruddin, M. Grat-
zel, J. Mater. Chem. A 2013, 1, 5535–5544.
[15] A. Bard, L. Faulkner, Electrochemical Methods: Fundamentals and Appli-
cations, Wiley, New York, 2001.
[16] Z.-S. Wang, K. Hara, Y. Dan-oh, C. Kasada, A. Shinpo, S. Suga, H. Araka-
wa, H. Sugihara, J. Phys. Chem. B 2005, 109, 3907–3914.
[17] A. Kira, Y. Matsubara, H. Iijima, T. Umeyama, Y. Matano, S. Ito, M. Niemi,
N. V. Tkachenko, H. Lemmetyinen, H. Imahori, J. Phys. Chem. C 2010,
114, 11293–11304.
[18] J. Wiberg, T. Marinado, D. P. Hagberg, L. Sun, A. Hagfeldt, B. Albinsson,
J. Phys. Chem. C 2009, 113, 3881–3886.
Received: November 20, 2014
[10] a) X. Ren, S. Jiang, M. Cha, G. Zhou, Z.-S. Wang, Chem. Mater. 2012, 24,
3493–3499; b) K. D. Seo, B. S. You, I. T. Choi, M. J. Ju, M. You, H. S. Kang,
Published online on && &&, 2015
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These are not the final page numbers! ÞÞ

ARTICLES
P. Ganesan, A. K. Chandiran, P. Gao,*
R. Rajalingam, M. Grꢀtzel,
M. K. Nazeeruddin*
On the bench: Two new (D–p–A)2 or-
ganic sensitizers (m-DA, p-DA) employ-
ing a 2,2’-bipyridine dicarboxylic acid
unit as an acceptor and anchoring
group are designed. The m-DA has the
anchoring part substituted meta to the
donor moiety and p-DA has a para-sub-
stituted anchoring moiety. Overall, para-
substituted p-DA exhibits better photo-
voltaic performance than meta-substi-
tuted m-DA.
&& – &&
Double D–p–A Dye Linked by 2,2’-
Bipyridine Dicarboxylic Acid: Influence
of para- and meta-Substituted
Carboxyl Anchoring Group
&
ChemPhysChem 0000, 00, 0 – 0
www.chemphyschem.org
8
ꢁ 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!