Novel oxindole based sensitizers: Synthesis and application in dye-sensitized solar cells

Article abstract of DOI:10.1021/ol401527a

Two novel oxindole sensitizers have been synthesized for dye-sensitized solar cell applications. These new dyes can provide an additional pathway to inject electrons into the photoanode through the partial chelation of their amide carbonyl groups to the TiO₂ surface. Incorporation of an electron deficient pyridine in the acceptor of the TI125 dye was found to enhance the photovoltage and conversion efficiency of the cell.

Full text of DOI:10.1021/ol401527a

ORGANIC
LETTERS
XXXX
Vol. XX, No. XX
000–000
Novel Oxindole Based Sensitizers:
Synthesis and Application
in Dye-Sensitized Solar Cells
Yogesh S. Tingare,† Ming-Tai Shen,† Chaochin Su,*,‡ Shih-Yu Ho,‡ Sheng-Han Tsai,‡
Bo-Ren Chen,† and Wen-Ren Li*,†
Department of Chemistry, National Central University, Chung-Li, Taiwan 32001 and
Institute of Organic and Polymeric Materials, National Taipei University of Technology,
Taipei, Taiwan 10608
ch01@ncu.edu.tw; f10913@ntut.edu.tw
Received May 30, 2013
ABSTRACT
Two novel oxindole sensitizers have been synthesized for dye-sensitized solar cell applications. These new dyes can provide an additional pathway
to inject electrons into the photoanode through the partial chelation of their amide carbonyl groups to the TiO2 surface. Incorporation of an electron
deficient pyridine in the acceptor of the TI125 dye was found to enhance the photovoltage and conversion efficiency of the cell.
Energy generation from renewable energy sources has
become an important scientific and technological chal-
lenge of the 21st century. Clean and abundant solar power
is widely recognized as a future energy resource. Accord-
ingly, varieties of light-harvesting devices were developed
and the cost-effective dye-sensitized solar cells (DSSCs)
have gained particular attention.1 In DSSCs, the dye is
considered the crucial component and research has been
focused on optimizing and developing new sensitizers.1c,2
In view of limited ruthenium resources, organic sensitizers
have attracted considerable interest for their low material
cost, ease of modification, and high molar extinction
coefficients.2,3 Generally, organic dyes are configured with
donorꢀπ-bridgeꢀacceptor (DꢀπꢀA) structures for en-
abling photoinduced charge separation. Various functional
groups such as triphenylamine, coumarin, tetrahydroquino-
line, cyanine, merocyanine, indoline, phenothiazine, and
phenoxazine have been successfully utilized for constructing
efficient DꢀπꢀA sensitizers for DSSC.2,4 Recently, new
organic sensitizers representing the ‘‘DꢀAꢀπꢀA’’ config-
uration were designed to optimize the energy levels of metal-
free sensitizers.2a Several kinds of electron-withdrawing moi-
eties such as benzothiadiazole,5 benzotriazole,6 quinoxaline,7
diketopyrrolopyrrole,8 and phthalimide9 were successfully
† National Central University.
‡ National Taipei University of Technology.
(3) Yen, Y.-S.; Chou, H.-H.; Chen, Y.-C.; Hsu, C.-Y.; Lin, J.-T.
J. Mater. Chem. 2012, 22, 8734.
€
(1) (a) Gratzel, M. J. Photochem. Photobiol. A 2004, 164, 3.
€
(b) O’Regan, B.; Gratzel, M. Nature 1991, 353, 737. (c) Robertson, N.
Angew. Chem., Int. Ed. 2006, 45, 2338.
(4) (a) Li, C.; Wonneberger, H. Adv. Mater. 2012, 24, 613. (b) Ning,
Z.; Fu, Y.; Tian, H. Energy Environ. Sci. 2010, 3, 1170. (c) Ning, Z.; Tian,
H. Chem. Commun. 2009, 5483.
(5) (a) Zhu, W.; Wu, Y.; Wang, S.; Li, W.; Li, X.; Chen, J.; Wang,
Z.-S.; Tian, H. Adv. Funct. Mater. 2011, 21, 756. (b) Velusamy, M.; Justin
Thomas, K. R.; Lin, J. T.; Hsu, Y.-C.; Ho, K.-C. Org. Lett. 2005, 7, 1899.
(2) (a) Wu, Y.; Zhu, W. Chem. Soc. Rev. 2013, 42, 2039. (b) Yin, J.-F.;
Velayudham, M.; Bhattacharya, D.; Lin, H.-C.; Lu, K.-L. Coord. Chem.
Rev. 2012, 256, 3008. (c) Hagfeldt, A.; Boschloo, G.; Sun, L.; Kloo, L.;
Pettersson, H. Chem. Rev. 2010, 110, 6595. (d) Mishra, A.; Fischer,
€
M. K. R.; Bauerle, P. Angew. Chem., Int. Ed. 2009, 48, 2474.
r
10.1021/ol401527a
XXXX American Chemical Society

Scheme 1. Synthesis of TI124 and TI125
Figure 1. Chemical structures of oxindole based dyes.
incorporated as the additional acceptors in the DꢀAꢀπꢀA
sensitizers. Such inclusion of electron-deficient groups
was found to improve the photostability of dyes and
adjust their energy levels.5a However, the concern hin-
dering the present DꢀAꢀπꢀA dyes is their relatively
low photovoltages.2a Owing to the easy molecular tai-
loring of organic dyes, enhancement in photovoltage
could be achieved by assisting fast electron injection
through fine-tuning or changing the inserted additional
acceptors in the DꢀAꢀπꢀA dyes.
amide group is located near the anchoring moiety which can
be benefited to accelerate the electron transfer to the TiO2;11
and (3) the inclusion of a nitrogen atom in the heterocyclic
aryl acceptor of the TI125 sensitizer can provide a better
driving force for electron injection (than the TI124 dye) and
thus achieve higher efficiency.12
Scheme 1 depicts the synthesis of two newly designed sen-
sitizers TI124 and TI125. Our synthesis started with the
selective bromination of isatin (1),13 followed by a Wolffꢀ
Herein, we report novel organic sensitizers TI124 and
TI125 (Figure 1) representing the first case of incorpora-
tion of oxindoles as the electron-deficient chromophores in
sensitizers. The oxindoles are natural indole alkaloids and
have abundant use within the pharmaceutical industry.10
However, their applications as sensitizers have not been
explored by other groups. Considering its bicyclic struc-
ture consisting of a six-membered benzene ring fused to a
five-membered amide containing ring, the oxindole could
be a promising candidate in constructing either a DꢀAꢀA
dye or a DꢀAꢀπꢀA sensitizer. In general, the oxindole
based dyes bear beneficial structural characteristics: (1) a
possible chelation of the oxindole to the titanium ions on the
TiO2 surface which can help to increase the electron injection
yield into the photoanode and might suppress the charge
recombination reaction;11 (2) the electron-withdrawing
Kishner reduction to give 5-bromo-2-oxindole (3).14
A
Knoevenagel reaction15 was then used to condense com-
pound 3 with 5-[4-(diphenylamino) phenyl]thiophene-2-
carbaldehyde16 to yield compound 4 as a single Z-isomer.17
One of the reasons for the predominate formation of the
Z-isomer is the electrostatic interaction between the carbo-
nyl oxygen of the amide and the partial positively charged
sulfur of the thiophene.15 Ester 5a and 5b were syn-
thesized by Ullmann-type amination coupling reactions of
compound 4 with 4-bromobenzoic acid methyl ester and
6-bromo-pyridine-3-carboxylic acid methyl ester, respec-
tively. Finally, basic hydrolysis of ester 5a and 5b offered
sensitizers TI124 and TI125, respectively. Detailed synthetic
procedures and characteristic data for all the compounds are
provided in the Supporting Information (SI).
(6) (a) Mao, J.; Guo, F.; Ying, W.; Wu, W.; Li, J.; Hua, J. Chem.;
Asian J. 2012, 7, 982. (b) Cui, Y.; Wu, Y.; Lu, X.; Zhang, X.; Zhou, G.;
Miapeh, F. B.; Zhu, W.; Wang, Z.-S. Chem. Mater. 2011, 23, 4394.
(7) (a) Shi, J.; Chen, J.; Chai, Z.; Wang, H.; Tang, R.; Fan, K.; Wu,
M.; Han, H.; Qin, J.; Peng, T.; Li, Q.; Li, Z. J. Mater. Chem. 2012, 22,
18830. (b) Pei, K.; Wu, Y.; Wu, W.; Zhang, Q.; Chen, B.; Tian, H.; Zhu,
W. Chem.;Eur. J. 2012, 18, 8190. (c) Lu, X.; Feng, Q.; Lan, T.; Zhou,
G.; Wang, Z.-S. Chem. Mater. 2012, 24, 3179. (d) Chang, D.-W.; Lee,
H.-J.; Kim, J.-H.; Park, S.-Y.; Park, S.-M.; Dai, L.; Baek, J.-B. Org.
Lett. 2011, 13, 3880.
The absorption spectra of TI124 and TI125 were inves-
tigated using UVꢀvis absorption spectroscopy (Figure 2).
€
(12) Delcamp, J. H.; Yella, A.; Nazeeruddin, M. K.; Gratzel, M.
Chem. Commun. 2012, 48, 2295.
(13) Chaubey, A. K.; Pandeya, S. N. Int. J. Res. Ayurveda Pharm.
2011, 2, 1763.
(14) Mologni, L.; Rostagno, R.; Brussolo, S.; Knowles, P. P.; Kjaer,
S.; Murray-Rust, J.; Rosso, E.; Zambon, A.; Scapozza, L.; McDonald,
N. Q.; Lucchini, V.; Gambacorti-Passerini, C. Bioorg. Med. Chem. 2010,
18, 1482.
(8) (a) Qu, S.; Wang, B.; Guo, F.; Li, J.; Wu, W.; Kong, C.; Long, Y.;
Hua, J. Dyes Pigm. 2012, 92, 1384. (b) Qu, S.; Wu, W.; Hua, J.; Kong, C.;
Long, Y.; Tian, H. J. Phys. Chem. C 2009, 114, 1343.
(9) (a) Li, W.; Wu, Y.; Zhang, Q.; Tian, H.; Zhu, W. ACS Appl.
Mater. Interfaces 2012, 4, 1822. (b) Feng, Q.; Lu, X.; Zhou, G.; Wang,
Z.-S. Phys. Chem. Chem. Phys. 2012, 14, 7993.
(10) (a) Silva, J. F. M.; Garden, S. J.; Pinto, A. C. J. Braz. Chem. Soc.
2001, 12, 273. (b) Sun, L.; Tran, N.; Tang, F.; App, H.; Hirth, P.;
McMahon, G.; Tang, C. J. Med. Chem. 1998, 41, 2588.
(15) (a) Sun, L.; Tran, N.; Tang, F.; App, H.; Hirth, P.; McMahon,
G.; Tang, C. J. Med. Chem. 1998, 41, 2588. (b) Rumer, J. W.; Dai, S.-Y.;
Levick, M.; Biniek, L.; Procter, D. J.; McCulloch, I. J. Polym. Sci., Part
A: Polym. Chem. 2013, 51, 1285.
(16) Liu, W.-H.; Wu, I. C.; Lai, C.-H.; Lai, C.-H.; Chou, P.-T.; Li,
Y.-T.; Chen, C.-L.; Hsu, Y.-Y.; Chi, Y. Chem. Commun. 2008, 5152.
(17) Ankati, H.; Akubathini, S. K.; Kamila, S.; Mukherjee, C.;
D’Mello, S. R.; Biehl, E. R. Open Org. Chem. J. 2009, 3, 1.
(11) Mao, J.; He, N.; Ning, Z.; Zhang, Q.; Guo, F.; Chen, L.; Wu, W.;
Hua, J.; Tian, H. Angew. Chem. 2012, 124, 10011.
B
Org. Lett., Vol. XX, No. XX, XXXX

Table 1. Electrochemical Properties of TI124 and TI125 Dyes
b
c
d
d
EOX
EHOMO
ELUMO
E0ꢀ0
dyes
λ
max/nma
(V Vs Fc/Fc )
(eV)
(eV)
(eV)
þ
TI124
TI125
488
491
0.30
0.51
5.10
5.31
3.02
3.23
2.08
2.08
a Measured in CH2Cl2. b The Ag/AgNO3 reference electrode was cali-
brated with a ferrocene/ferrocinium (Fc/Fcþ) redox couple. c EHOMO = Eox
ꢀ EFc/Fcþ þ 4.8 eV. d ELUMO = EHOMO ꢀ E0ꢀ0. The band gap, E0ꢀ0, was
estimated from the onset of the absorption spectrum measured in CH2Cl2.
Figure 2. Absorption spectra of TI124 and TI125 dyes in
CH2Cl2 (a) and on films (b).
The absorption spectra of TI124 and TI125 dyes
(Figure 2a) in CH2Cl2 showed strong absorption bands
at 488 and 491 nm, respectively. The absorption maximum
(λmax) of TI125 was slightly more red-shifted (ca. 3 nm)
than that of TI124. Surprisingly, the molar extinc-
tion coefficient (ε) for TI124 (ε = 3.82 ꢁ 104 Mꢀ1 cmꢀ1
)
was a little higher when compared to TI125 (ε = 3.45 ꢁ
104 Mꢀ1 cmꢀ1). The UVꢀvis absorption spectra of TI124
and TI125 dyes anchored onto the TiO2 films (Figure 2b)
were broadened and blue-shifted when compared to sensi-
tizers in CH2Cl2. To the best of our knowledge, organic
dyes featured blue shifts in the absorption peaks when
attached onto the TiO2 surface, and such a phenomenon
was attributed to the formation of aggregation of sensiti-
zers or deprotonation of carboxylic acids.11,18 In contrast
to the solution, the sensitizer TI125 on film showed higher
absorption than the dye TI124.
The highest occupied molecular orbital (HOMO) and
the lowest unoccupied molecular orbital (LUMO) energy
levels play a crucial role in affecting the dye-regeneration
and electron-transfer processes in DSSCs. An efficient
sensitizer should possess a sufficiently low HOMO energy
level for efficient regeneration of the oxidized dye by the
electrolyte, whereas the LUMO energy level should be
sufficiently high for electron injection into the TiO2 elec-
trode. We estimated the HOMO and LUMO energy levels
of TI124 and TI125 from their UVꢀvis absorption spectra
and their cyclic voltammograms to ensure that they were
suitable for injecting electrons and dye regeneration after
the electron donation.19 Table 1 reveals that the HOMO
energylevelsofTI124 and TI125 aresufficientlylowerthan
Figure 3. Schematic diagrams of the frontier molecular orbitals
of TI124 and TI125 calculated at the B3LYP/6-31g (d,p) level of
theory.
These results revealed that the additional nitrogen atom
incorporated in the aryl acceptor of the dye TI125 might
assist the dye in achieving higher efficiency. To gain more
insight into the structural and electronic features of oxindole
based dyes, molecular orbital calculations were performed.
The HOMO orbitals of TI124 and TI125 dyes were localized
on the electron-rich donor groups (Figure 3). It is noteworthy
that the LUMO levels of TI124 and TI125 dyes are confined
to the amide portion of the oxindoles and do not feature
contributions from the carboxylic groups, while the electron
densities of LUMOþ1 were populated on the anchoring
carboxylic acids. These calculations indicate that the oxi-
ndole dyes in this study are poised for effective electron
injection through the anchoring carboxylate moieties as well
as the amide groups of the oxindoles.
ꢀ
that of the Iꢀ/I3 (ꢀ4.8 eV) redox potential, and their
LUMOs are adequate for efficient electron injection. The
HOMO and LUMO energy levels of TI125 were shifted by
0.21 eV relative to that of the TI124 dye. Compared to the
dye TI124, the sensitizer TI125 with stabilized energy levels
can more effectively inject excited electrons into the TiO2
band and can also be reduced rapidly by the electrolyte.
(18) Tian, H.; Yang, X.; Chen, R.; Pan, Y.; Li, L.; Hagfeldt, A.; Sun,
L. Chem. Commun. 2007, 3741.
(19) (a) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan,
G. C. Adv. Mater. 2011, 23, 2367. (b) Chang, W.-C.; Chen, H.-S.; Li,
T.-Y.; Hsu, N.-M.; Tingare, Y. S.; Li, C.-Y.; Liu, Y.-C.; Su, C.; Li,
W.-R. Angew. Chem., Int. Ed. 2010, 49, 8161.
The interaction of sensitizers with the TiO2 surface and
the charge injection are crucial factors affecting the DSSCs
Org. Lett., Vol. XX, No. XX, XXXX
C

Figure 4. FTIR spectra of the dye powders and dyes adsorbed on
films (a) TI124 (blue) and (b) TI125 (red).
Figure 5. (a) Current versus potential plots. (b) Inset: Mono-
chromatic IPCE spectra. (c) EIS spectra measured in dark of
DSSCs based on TI124 and TI125 dyes.
performance. Prior to making the solar devices, we exam-
ined the adsorption pattern of anchored dyes on films
to understand the electron transfer mechanism involved
in oxindole based sensitizers. FTIR spectra of TI124 and
TI125 powders and on films were measured and shown in
Figure 4. The bands observed at 1702 and 1708 cmꢀ1 in the
FTIR spectra of TI124 and TI125 powders, respectively,
can be assigned to amide carbonyl groups of the oxindoles.
When the dye was adsorbed on the TiO2 surface, the peak
for CdO stretching diminished which indicated the partial
chelation of the amide carbonyl group of the oxindole to the
titanium ions on the TiO2.11 The oxindole based sensitizers
might help to improve the photoinduced electron injection
yield and gain good photocurrent conversion efficiency.
Figure 5 presents the photocurrent densityꢀvoltage
curves and the incident photon to current conversion
efficiency (IPCE) spectra of DSSCs assembled with multi-
ple layered TiO2 films with TI124 and TI125 sensitizers.
Table 2 summarizesthe corresponding short-circuit photo-
current densities (Jsc), open-circuit voltages (Voc), fill
factors (FFs), and solar-to-electricity conversion efficien-
cies (η). The TI124-sensitized DSSC exhibited an overall η
= 4.10% with cell parameters of Jsc = 8.33 mA cmꢀ2, Voc
= 680 mV, and FF = 0.724. Furthermore, the TI125
sensitizer with the electron deficient heterocyclic nitrogen
containing acceptor improved the Voc by ca. 50 mV. The
DSSC based on the TI125 sensitizer achieved Jsc = 10.48
mA cmꢀ2, Voc = 730 mV, and FF = 0.707, corresponding
to an overall η = 5.41%. The enhanced η was due to its
increase in the Voc which could be attributed to the 0.21 eV
lower HOMO energy level of the TI125 sensitizer, com-
pared to that of the TI124 dye, which provides a better
driving force for dye regeneration. Furthermore, electro-
chemical impedance spectroscopy (EIS) in the dark
(Figure 5c) as well as under illumination (Figure S2
and Table S1 in SI) was performed to analyze the origin
of improvement in Voc. The EIS results revealed that
the sensitizer TI125 possessed a higher lifetime and can
more effectively suppress the electron recombination than
the TI124 dye. The photocurrent densityꢀvoltage curves
obtained are in good accord with the corresponding IPCE
measurements. The IPCE curve for the DSSC anchored
Table 2. Photovoltaic Parameters of Devices Incorporating
TI125 and TI124 Sensitizer
sensitizer
JSC (mA cmꢀ2
)
VOC (mV)
FF
η (%)
TI124
TI125
8.33
680
730
0.724
0.707
4.10
5.41
10.48
with the TI124 dye exhibited common broad characteristics
that covered the visible spectrum in the range of 400 to
650 nm and reached the maximum IPCE value of 42% at
542 nm (Figure 5b). The TI125 sensitizer with the pyridine
containing acceptor displayed a red-shifted range and in-
creasing IPCE maximum (55% at 542 nm). These results
indicate that minor structural modification in the oxindole
based sensitizer can effectively alter the physical parameters
and the overall photoelectric conversion efficiency of the
solar cell.
In conclusion, we have demonstrated the first case of
successful usage of oxindoles for constructing sensitizers
for DSSC applications. These newly developed dyes ensured
an effective electron transfer and suppression of a charge
recombination reaction due to the partial chelation of their
amide carbonyl groups to the TiO2 surface. The inclusion of
electron deficient pyridine in the acceptor of the TI125 dye
exhibited enhancement in Voc and resulted in higher overall
conversion efficiency than the TI124 sensitizer. Our finding
offers an alternative approach for future rational dye design
and will help to foster interest in the exploration of oxindole
based sensitizers for application in DSSCs.
Acknowledgment. We thank the National Science Coun-
cil, ROC, for financial support (Grants NSC 100-2113M-
008-004-MY3 and NSC 98-2113M-027-003-MY3).
Supporting Information Available. Details of experi-
mental procedures, synthesis and characterization of
compounds. This material is available free of charge via
the Internet at http://pubs.acs.org.
The authors declare no competing financial interest.
D
Org. Lett., Vol. XX, No. XX, XXXX