Modulation of energy levels by donor groups: An effective approach for optimizing the efficiency of zinc-porphyrin based solar cells

Article abstract of DOI:10.1039/c2jm16804a

In the design of efficient sensitizers for dye-sensitized solar cells (DSSCs), it is vital to modulate the HOMO and LUMO orbitals by introducing suitable donor and acceptor groups. In this respect, triphenylamine has been extensively used as the donor gro

Full text of DOI:10.1039/c2jm16804a

View Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2012, 22, 7434
www.rsc.org/materials
PAPER
Modulation of energy levels by donor groups: an effective approach for
optimizing the efficiency of zinc-porphyrin based solar cells†
*
Bo Liu, Weihong Zhu, Yueqiang Wang, Wenjun Wu, Xin Li, Baoqin Chen, Yi-Tao Long and Yongshu Xie
*
Received 23rd December 2011, Accepted 13th February 2012
DOI: 10.1039/c2jm16804a
In the design of efficient sensitizers for dye-sensitized solar cells (DSSCs), it is vital to modulate the
HOMO and LUMO orbitals by introducing suitable donor and acceptor groups. In this respect,
triphenylamine has been extensively used as the donor group. Porphyrin has strong absorption in the
UV-Vis range, which makes it a promising dye candidate. In addition, a porphyrin molecule contains
eight b positions and four meso-positions, which may be conveniently utilized for introducing multiple
donor and acceptor moieties. In this work, various numbers of triphenylamine and trimethoxyphenyl
groups are introduced to the porphyrin meso-positions as electron donors, with the aim to
systematically modulate the energy levels and investigate the effect on the efficiency of the DSSCs. The
HOMO–LUMO energy gaps remain almost constant irrespective of the donor groups, thus, the
variation of the donors leads to a contradictory effect on the processes of electron injection and dye
regeneration. In the case of the zinc porphyrin with two triphenylamine units and one
trimethoxyphenyl group as the electron donors (M3T2P), moderate and well-matched HOMO and
LUMO energy levels are observed, thus affording high efficiencies for both the electron injection and
the dye regeneration processes. The overall conversion efficiencies (h) of the DSSCs based on these dyes
lie in the range of 2.70–5.45%, with the optimized performance achieved by M3T2P, which can be
rationalized by the efficient electron injection and dye regeneration processes, and the long electron
lifetime, as demonstrated by the electrochemical impedance spectroscopy measurements. These results
provide further insights into the strategy for elevating the efficiencies of DSSCs, especially those based
on porphyrins, simply by modulating the electron donor groups.
organic dyes for practical use because of their low cost, high
molar absorption coefficients, and facile modification of the
molecular structures. Highly efficient organic dyes are typically
composed of a donor–p-conjugated unit–acceptor (D–p–A)
framework with a well defined architecture.3 Thus, organic dyes
based on coumarin, triphenylamine (TPA), thiophene, polyene,
cyanine, hemicyanine, squaraine and phthalocyanine moieties
have been explored as potential sensitizers for DSSCs.4–10 In this
respect, we have developed various efficient dyes incorporating
indoline, isophorone and benzothiadiazole units.11 In the
photosynthetic cores of bacteria and plants, solar energy is
collected by chromophores based on porphyrin, and the captured
radiant energy is converted efficiently to chemical energy.
Inspired by nature, porphyrins have attracted increasing interest
as DSSC sensitizers. They have rigid macrocyclic structures,
exhibiting strong absorption in the visible region12 and electron
transfer kinetics indistinguishable from those of the efficient
ruthenium polypyridyl complexes. More importantly, because of
their multiple reaction sites, including four meso and eight
b-positions, their optical, electrochemical, and photophysical
properties can be easily modulated by the peripheral substitu-
tions and inner metal complexations.13,14 Thus, numerous
Introduction
The utilization of inexhaustible solar energy is a key to meeting
a rapidly increasing global demand for clean and renewable
energy sources. In this respect, dye-sensitized solar cells (DSSCs)
are promising devices as an alternative to the traditional silicon
based solar cells. The most developed DSSC dyes are based on
ruthenium polypyridyl complexes which show a power conver-
sion efficiency up to 11%.1 The advantages of such ruthenium
complexes include broad absorption in the near-UV and visible
regions and appropriate excited-state oxidation potentials for
electron injection into the conduction band of the commonly
used semiconductor TiO2.2 However, the cost and environmental
concern of ruthenium complexes limit their wide applications
and encourage the exploration of cheaper and safer dyes. Thus,
much effort has been devoted to the development of new efficient
Key Laboratory for Advanced Materials and Institute of Fine Chemicals,
East China University of Science and Technology, Shanghai, P. R.
China. E-mail: yshxie@ecust.edu.cn; Fax: +(86) 21-6425-2758; Tel:
+(86) 21-6425-0772
† Electronic Supplementary Information (ESI) available: HRMS, NMR
spectra and optimized geometry. See DOI: 10.1039c2jm16804a
7434 | J. Mater. Chem., 2012, 22, 7434–7444
This journal is ª The Royal Society of Chemistry 2012

View Online
porphyrins have been synthesized and used as sensitizers for
DSSCs,15–17 with the highest conversion efficiency of 11.9%,17
which has even surpassed the records for the DSSCs based on
ruthenium sensitizers. Nevertheless, high performance porphyrin
sensitizers are still relatively scarce.
In the design of dye molecules, the highest occupied molecular
orbital (HOMO), the lowest unoccupied molecular orbital
(LUMO), and the HOMO–LUMO energy gap are among the
most important parameters dominating the dye performance in
DSSCs. Generally, the HOMO and LUMO orbitals of solar cell
sensitizers should match the corresponding energy levels of the
electrolyte and the conduction band (CB) of the TiO2 electrode,
thus ensuring efficient dye regeneration and electron injection
processes. In addition, the HOMO–LUMO band gap should also
correspond to a suitable absorption wavelength for harvesting
sunlight. In the case of porphyrins, the electronic structure is
influenced considerably by the electron donors introduced into
the meso-positions.15 In principle, the tuning of the HOMO and
LUMO levels could be achieved through the choice of suitable
donor groups.18 Such tuning effects on the photovoltaic perfor-
mance of porphyrin based DSSCs have not been fully investi-
gated. For the purpose of investigating the effects of the meso-
donor groups on the cell performance, here we designed and
synthesized a series of novel porphyrin sensitizers T2P, T3P,
MT2P, MT3P and M3T2P (Scheme 1). In each of the dyes,
a 4-ethynylbenzoic acid group was attached to one of the
meso-positions as the electron-withdrawing and anchoring
moiety, and various numbers of triphenylamine and
trimethoxyphenyl groups were introduced to the rest of the meso-
positions as the electron donors. Interestingly, a systematic
tuning of the HOMO–LUMO levels could be effectively ach-
ieved. M3T2P shows the best photovoltaic performance with the
overall conversion efficiency (h) of 5.45%, associated with rela-
tively high Jsc and Voc values resulting from the suitable HOMO
and LUMO levels in combination with the long electron lifetime.
Scheme 2 The synthetic routes for the dyes. Reaction conditions: (i)
TFA, CH2Cl2; (ii) NBS, CH2Cl2; (iii) methyl 4-ethynylbenzoate, AsPh3,
Pd2(dba)3, THF, Et3N; (iv) LiOH$H2O, THF, H2O; (v)
Zn(CH3COO)2$2H2O, CH2Cl2, CH3OH.
Tetrabutylammonium hexafluorophosphate (TBAPF6) was
vacuum-dried for 48 h. The transparent FTO conducting glass
(fluorine-doped SnO2, transmission >90% in the visible range,
sheet resistance 15 U squareꢀ1) and the TiO2 paste was purchased
from Geao Science and Educational Co. Ltd. The FTO con-
ducting glass was washed with a detergent solution, deionized
water, acetone, and ethanol successively under ultrasonication for
20 min before use.
Experimental section
Materials and reagents
Equipment and apparatus
All reagents and solvents were obtained from commercial sources
and used without further purification unless otherwise noted.
ꢀ
THF was dried over 4 A molecular sieves, and distilled under
1H NMR spectra were obtained using a Bruker AM 400 spec-
trometer. HRMS were performed using a Waters LCT Premier
XE spectrometer. UV-Vis absorption spectra were recorded on
nitrogen from sodium benzophenone prior to use.
Scheme 1 Molecular structures of the porphyrin sensitizers.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7434–7444 | 7435

View Online
cooling and then dipping into a 0.2 mM porphyrin/ethanol solu-
tion for 12 h at room temperature. The counter electrode was
prepared by sputtering with three drops of 0.02 M H2PtCl6 in 2-
propanol followed by annealing treatment at 400 ꢁC for 15 min. A
hole (0.8 mm diameter) was drilled in the counter electrode. The
perforated sheet was cleaned by ultrasound in an ethanol bath for
10 min. For the assembly of DSSCs, the dye-covered TiO2 elec-
trode and Pt counter electrode were assembled into a sandwich
type cell and sealed with a hot-melt gasket of 25 mm thickness
made of ionomer Surlyn 1702 (DuPont). A drop of electrolyte
solution was put in the hole in the back of the counter electrode,
and introduced into the cell via vacuum backfilling. Finally, the
hole was sealed using a UV-melt gum and a cover glass. The
electrolyte solution contains 0.1 M LiI, 0.05 M I2, 0.6 M 1-butyl-3-
methylimidazolium iodide (BMII), and 0.5 M 4-tert-butylpyr-
idine (TBP) in a mixture of acetonitrile and valeronitrile (volume
ratio 1 : 1).
Fig. 1 (a) Absorption spectra and (b) emission spectra of the porphyrins
measured in THF at 298 K.
a Varian Cary 100 spectrophotometer and fluorescence spectra
were recorded on a Varian Cary Eclipse fluorescence spectro-
photometer. The cyclic voltammograms of the dyes were
obtained in acetonitrile with a Versastat II electrochemical
workstation (Princeton Applied Research) using 0.1 M TBAPF6
(Aldrich) as the supporting electrolyte, the sensitizer attached to
a nanocrystalline TiO2 film deposited on the conducting FTO
glass as the working electrode, a platinum wire as the counter
electrode, and a regular calomel electrode in saturated KCl
Synthesis of the dyes
5,15-Bis(4-(diphenylamino)phenyl)porphyrin (1a). To a solution
of 4-(diphenylamino)benzaldehyde (0.94 g, 3.42 mmol) and
dipyrromethane (0.5 g, 3.42 mmol) in CH2Cl2 (800 mL) was
added trifluoroacetic acid (0.25 mL). The solution was stirred at
room temperature for 3 h. Then, DDQ (2,3-dichloro-5,6-dicya-
nobenzoquinone) (1.2 g, 5.29 mmol) was added and stirred at
room temperature for another 1 h. After the reaction was
quenched by the addition of 6 mL triethylamine, the solvent was
removed in vacuo. The residue was purified by column chroma-
tography (silica gel) using CH2Cl2 as the eluent. Recrystallization
from CH2Cl2/CH3OH gave 1a (650 mg, 46%). 1H NMR
(400 MHz, CDCl3, ppm): d ꢀ3.02 (s, 2H, NH), 7.14–7.19 (m, 4H,
phenyl), 7.44 (d, J ¼ 4.4 Hz, 16H, phenyl), 7.51 (d, J ¼ 8.4 Hz,
4H, phenyl), 8.13 (d, J ¼ 8.4 Hz, 4H, phenyl), 9.22 (d, J ¼ 4.4 Hz,
4H, pyrrolic), 9.42 (d, J ¼ 4.4 Hz, 4H, pyrrolic), 10.31 (s, 2H,
meso). HRMS (ESI, m/z): [M + H]+ calcd for C56H41N6,
797.3393; found, 797.3391. IR (KBr pellet, cmꢀ1): 3418(br),
3025(w), 2921(w), 1637(m), 1617(m), 1592(s), 1494(s), 1399(m),
1384(m), 1277(s), 994(m), 956(m), 794(m), 694(s), 621(m).
5,15-Bis(N,N-bis(4-methoxyphenyl)aminophenyl)porphyrin
(1b). The synthesis was performed following the procedure
similar to that for 1a starting from 4-[bis(4-methoxyphenyl)
amino]benzaldehyde (1.73 g, 5.18 mmol) and dipyrromethane
solution as the reference electrode. The scan rate was 100 mV sꢀ1
.
Photovoltaic measurements employed an AM 1.5 solar simu-
lator equipped with a 300 W xenon lamp (model no. 91160,
Oriel). The power of the simulated light was calibrated to
100 mW cmꢀ2 using a Newport Oriel PV reference cell system
(model 91150 V). I–V curves were obtained by applying an
external bias to the cell and measuring the generated photocur-
rent with a model 2400 source meter (Keithley Instruments, Inc.
USA). The voltage step and delay time of the photocurrent were
10 mV and 40 ms, respectively. Action spectra of the incident
monochromatic photon-to-electron conversion efficiency (IPCE)
for the solar cells were obtained with a Newport-74125 system
(Newport Instruments). The intensity of monochromatic light
was measured with a Si detector (Newport-71640). The electro-
chemical impedance spectroscopy (EIS) measurements of all the
DSSCs were performed using a Zahner IM6e Impedance
Analyzer (ZAHNER-Elektrik GmbH & CoKG, Kronach,
Germany). The frequency range is 0.1 Hz–100 kHz. The
magnitude of the alternative signal is 10 mV.
Fabrication of the solar cells
1
(0.76 g, 5.18 mmol). Yield: 400 mg, 17%. H NMR (400 MHz,
The preparation of TiO2 electrodes and the fabrication of the
sealed cells for photovoltaic measurements were performed
CDCl3, ppm): d ꢀ2.99 (s, 2H, NH), 3.87 (s, 12H, MeO), 6.99 (d,
J ¼ 8.8 Hz, 8H, phenyl), 7.33–7.39 (m, 12H, phenyl), 8.06 (d, J ¼
8.0 Hz, 4H, phenyl), 9.21 (d, J ¼ 4.4 Hz, 4H, pyrrolic), 9.39 (d,
J ¼ 4.4 Hz, 4H, pyrrolic), 10.27 (s, 2H, meso). HRMS (ESI, m/z):
[M + H]+ calcd for C60H49N6O4, 917.3815; found, 917.3815. IR
(KBr pellet, cmꢀ1): 3414(br), 2921(m), 2851(w), 1795(w),
1635(m), 1617(s), 1503(vs), 1402(m), 1384(s), 1241(s), 1036(m),
988(m), 830(w), 795(w), 623(w).
€
following the procedures previously reported by Gratzel and co-
workers.19 A screen-printed double layer of TiO2 particles was
used as the photoelectrode. A 10 mm thick film of 13 nm sized TiO2
particles was first printed on the FTO conducting glass, which was
kept in a clean box for 5 min, and then dried at 125 ꢁC over 6 min,
and further coated by a 4 mm thick second layer of 400 nm light-
scattering anatase particles. Finally, the electrodes coated with the
TiO2 pastes were gradually sintered in a muffle furnace at 275 ꢁC
for 5 min, at 325 ꢁC for 5 min, at 375 ꢁC for 5 min, at 450 ꢁC for 15
min and at 500 ꢁC for 15 min, respectively. The size of the TiO2
film was 0.28 cm2. These films were immersed into a 40 mM
aqueous TiCl4 solution at 70 ꢁC for 30 min, washed with water and
ethanol, and then heated again at 450 ꢁC for 30 min followed by
5,10,15-Tris(4-(diphenylamino)phenyl)porphyrin (1c). The
synthesis was performed following the procedure for 1a starting
from 4-(diphenylamino)benzaldehyde (283 mg, 1.04 mmol), 5-(4-
(diphenylamino)phenyl)dipyrromethane (202 mg, 0.52 mmol),
1
and dipyrromethane (76 mg, 0.52 mmol). Yield: 20 mg, 4%. H
NMR (400 MHz, CDCl3, ppm): d ꢀ2.91 (s, 2H, NH), 7.13–7.17
(m, 6H, phenyl), 7.40–7.49 (m, 30H, phenyl), 8.09 (t, J ¼ 9.0 Hz,
7436 | J. Mater. Chem., 2012, 22, 7434–7444
This journal is ª The Royal Society of Chemistry 2012

View Online
6H, phenyl), 9.04 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 9.06 (d, J ¼ 4.4
Hz, 2H, pyrrolic), 9.15 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.34 (d, J ¼
4.8 Hz, 2H, pyrrolic), 10.18 (s, 1H, meso). HRMS (ESI, m/z): [M
+ H]+ calcd for C74H54N7, 1040.4441; found, 1040.4440. IR (KBr
pellet, cmꢀ1): 3417(br), 3029(w), 2921(w), 1637(m), 1617(m),
1591(vs), 1492(vs), 1403(m), 1313(m), 1279(s), 1175(w), 1154(w),
994(m), 797(m), 751(m), 695(s), 622(m), 515(m), 465(w).
1384(m), 1327(m), 1313(m), 1280(s), 977(w), 962(m), 790(m),
749(m), 736(m), 695(s), 516(m), 473(w).
5,15-Bis(N,N-bis(4-methoxyphenyl)aminophenyl)-10-bromo-
porphyrin (2b). Yield: 42%. 1H NMR (400 MHz, CDCl3, ppm):
d ꢀ2.90 (s, 2H, NH), 3.87 (s, 12H, MeO), 6.99 (d, 8H, J ¼ 9.2 Hz,
phenyl), 7.31 (d, J ¼ 8.4 Hz, 4H, phenyl), 7.36–7.40 (m, 8H,
phenyl), 7.98 (d, J ¼ 8.8 Hz, 4H, phenyl), 9.08 (d, J ¼ 4.8 Hz, 2H,
pyrrolic), 9.09 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.27 (d, J ¼ 4.4 Hz,
2H, pyrrolic), 9.73 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 10.13 (s, 1H,
meso). HRMS (ESI, m/z): [M + H]+ calcd for C60H48BrN6O4,
995.2920; found, 995.2913. IR (KBr pellet, cmꢀ1): 3448(br),
2924(m), 2853(w), 1828(w), 1599(m), 1503(vs), 1384(m),
1277(m), 1241(s), 1178(w), 1036(m), 827(w), 789(w), 528(w).
5,10,15-Tris(4-(diphenylamino)phenyl)-20-bromoporphyrin
(2c). Yield: 95%. 1H NMR (400 MHz, CDCl3, ppm): d ꢀ2.67 (s,
2H, NH), 7.13–7.18 (m, 6H, phenyl), 7.40–7.47 (m, 30H, phenyl),
8.04 (d, J ¼ 8.4 Hz, 6H, phenyl), 8.94 (d, J ¼ 4.8 Hz, 2H,
pyrrolic), 8.96 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.03 (d, J ¼ 4.4, 2H,
pyrrolic), 9.67 (d, J ¼ 4.8, 2H, pyrrolic). HRMS (ESI, m/z): [M +
H]+ calcd for C74H53BrN7, 1118.3546; found, 1118.3551. IR
(KBr pellet, cm-1): 3448(br), 3029(w), 2921(w), 2848(w), 1591(s),
1492(vs), 1384(m), 1313(m), 1278(s), 1105(vs), 965(s), 798(m),
752(m), 695(s), 511(m), 468(m).
5,10,15-Tris(N,N-bis(4-methoxyphenyl)aminophenyl)porphyrin
(1d). The synthesis was performed following the procedure similar
to that for 1a starting from 4-(diphenylamino)benzaldehyde (3.17
g, 9.50 mmol), 5-(N,N-bis(4-methoxyphenyl)aminophenyl)dipyr-
romethane (2.14 g, 4.75 mmol), and dipyrromethane (0.69 g, 4.75
1
mmol). Yield: 360 mg, 6%. H NMR (400 MHz, CDCl3, ppm):
d ꢀ2.87 (s, 2H, NH), 3.85 (s, 6H, MeO), 3.86 (s, 12H, MeO),
6.96–6.70 (m, 12H, phenyl), 7.28–7.38 (m, 18H, phenyl), 7.99–
8.04 (m, 6H, phenyl), 9.01 (d, J ¼ 4.8, 2H, pyrrolic), 9.03 (d, J ¼
4.4, 2H, pyrrolic), 9.14 (d, J ¼ 4.4, 2H, pyrrolic), 9.31 (d, J ¼ 4.8,
2H, pyrrolic), 10.14 (s, 1H, meso). HRMS (ESI, m/z): [M + H]+
calcd for C80H66N7O6, 1220.5075; found, 1220.5078. IR (KBr
pellet, cmꢀ1): 3419(br), 3035(w), 2929(w), 2832(w), 1600(s),
1503(vs), 1463(s), 1403(m), 1384(m), 1317(s), 1240(vs), 1189(m),
1105(m), 1035(s), 964(m), 828(m), 798(m), 600(w), 581(w),
529(w).
5,15-Bis(4-(diphenylamino)phenyl)-10-(3,4,5-trimethoxyphenyl)
porphyrin (1e). The synthesis was performed following the
procedure similar to that for 1a except that the reaction mixture
was refluxed for 48 h in a mixed solvent of CH2Cl2 (1600 mL) and
MeOH (160 mL). Thus, the reaction of 4-(diphenylamino)benz-
aldehyde (1.89 g, 6.9 mmol), 5-(3,4,5-trimethoxyphenyl)dipyrro-
methane (1.08 g, 3.45 mmol), and dipyrromethane (0.51 g, 3.45
5,10,15-Tris(N,N-bis(4-methoxyphenyl)aminophenyl)-20-bro-
moporphyrin (2d). Yield: 87%. 1H NMR (400 MHz, CDCl3,
ppm): d ꢀ2.63 (s, 2H, NH), 3.85 (s, 6H, MeO), 3.86 (s, 12H,
MeO), 6.95–7.00 (m, 12H, phenyl), 7.27–7.31 (m, 6H, phenyl),
7.34–7.38 (m, 12H, phenyl), 7.96 (d, J ¼ 8.4 Hz, 6H, phenyl), 8.92
(d, J ¼ 4.8 Hz, 2H, pyrrolic), 8.94 (d, J ¼ 4.8 Hz, 2H, pyrrolic),
9.02 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 9.64 (d, J ¼ 4.8 Hz, 2H,
pyrrolic). HRMS (ESI, m/z): [M + H]+ calcd for C80H65BrN7O6,
1298.4180; found, 1298.4178. IR (KBr pellet, cmꢀ1): 3442(br),
2949(w), 2832(w), 1600(s), 1503(vs), 1465(s), 1440(w), 1384(w),
1317(m), 1240(s), 1178(w), 1105(w), 1035(m), 965(m), 828(m),
799(m), 582(w).
1
mmol) afforded 1e. Yield: 160 mg, 4.6%. H NMR (400 MHz,
CDCl3, ppm): d ꢀ2.93 (s, 2H, NH), 3.96 (s, 6H, MeO), 4.19 (s,
3H, MeO), 7.13–7.17 (m, 4H, phenyl), 7.42 (d, J ¼ 4.0 Hz, 16H,
phenyl), 7.48 (t, J ¼ 4.2 Hz, 6H, phenyl), 8.09 (d, J ¼ 8.4 Hz, 4H,
phenyl), 9.00 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 9.04 (d, J ¼ 4.8 Hz,
2H, pyrrolic), 9.15 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 9.34 (d, J ¼ 4.8
Hz, 2H, pyrrolic), 10.19 (s, 1H, meso). HRMS (ESI, m/z): [M +
H]+ calcd for C65H51N6O3, 963.4023; found, 963.4025. IR (KBr
pellet, cmꢀ1): 3416(br), 3027(w), 2935(w), 1637(m), 1617(s),
1591(s), 1492(s), 1406(m), 1279(m), 1127(m), 972(w), 798(w),
695(m), 622(m).
5,15-Bis(4-(diphenylamino)phenyl)-10-(3,4,5-trimethoxyphenyl)-
20-bromoporphyrin (2e). Yield: 90%. 1H NMR (400 MHz, CDCl3,
ppm): d ꢀ2.70 (s, 2H, NH), 3.96 (s, 6H, MeO), 4.18 (s, 3H, MeO),
7.14–7.18 (m, 4H, phenyl), 7.41–7.48 (m, 22H, phenyl), 8.02–8.05
(d, J ¼ 8.8 Hz, 4H, phenyl), 8.92 (d, J ¼ 4.8 Hz, 2H, pyrrolic),
8.94 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.04 (d, J ¼ 4.4 Hz, 2H,
pyrrolic), 9.69 (d, J ¼ 4.8 Hz, 2H, pyrrolic). HRMS (ESI, m/z):
[M + H]+ calcd for C65H50BrN6O3, 1041.3128; found, 1041.3127.
IR (KBr pellet, cmꢀ1): 3453(br), 3027(w), 2935(w), 1636(m),
1591(s), 1492(vs), 1405(w), 1313(w), 1279(s), 1236(w), 1127(s),
1002(w), 973(w), 799(m), 753(w), 696(m), 622(w).
General procedure for bromination
To a solution of porphyrin (0.2 mmol) in CH2Cl2 (200 mL) was
added NBS (0.2 mmol) at room temperature. After 1 h, the
reaction was quenched with acetone (5 mL). The solvent was
removed under reduced pressure. The product was isolated by
silica gel columns and recrystallized from CH2Cl2/CH3OH.
5,15-Bis(4-(diphenylamino)phenyl)-10-bromoporphyrin (2a).
General procedure for the Sonogashira coupling reactions
Bromoporphyrin (0.08 mmol), methyl 4-ethynylbenzoate
(0.16 mmol), Pd2(dba)3 (0.04 mmol), AsPh3 (0.16 mmol) were
placed in a Schlenk flask, which was flushed with nitrogen and
then charged with dry THF (25 mL) and Et3N (5 mL). The
mixture was stirred at 60 ꢁC for 3 h. Then, the solvent was
removed under reduced pressure, and the residue was dissolved
in CH2Cl2 and washed with water, dried over anhydrous sodium
sulfate, and evaporated. The residue was purified on a silica gel
column. Recrystallization from CH2Cl2/CH3OH gave the target
compound.
1
Yield: 45%. H NMR (400 MHz, CDCl3, ppm): d ꢀ2.92 (s, 2H,
NH), 7.15–7.19 (m, 4H, phenyl), 7.42–7.49 (m, 20H, phenyl),
8.05 (d, J ¼ 8.4 Hz, 4H, phenyl), 9.08 (d, J ¼ 4.8 Hz, 2H,
pyrrolic), 9.09 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.29 (d, J ¼ 4.4 Hz,
2H, pyrrolic), 9.76 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 10.15 (s, 1H,
meso). HRMS (ESI, m/z): [M + H]+ calcd for C56H40BrN6,
875.2498; found, 875.2504. IR (KBr pellet, cmꢀ1): 3415(br),
2917(w), 2851(w), 1640(m), 1619(m), 1591(vs), 1492(vs), 1402(w),
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7434–7444 | 7437

View Online
5,15-Bis(4-(diphenylamino)phenyl)-10-(4-methoxycarbonylph-
1
enylethynyl)porphyrin (3a). Yield: 50%. H NMR (400 MHz,
4.8 Hz, 2H, pyrrolic), 9.76 (d, J ¼ 4.8 Hz, 2H, pyrrolic). HRMS
(ESI, m/z): [M + H]+ calcd for C75H57N6O5, 1121.4390; found,
1121.4391. IR (KBr pellet, cmꢀ1): 3420(br), 3027(w), 2935(w),
2187(w), 1719(m), 1636(m), 1617(m), 1592(s), 1490(s), 1401(m),
1384(m), 1274(s), 1127(m), 974(m), 798(m), 695(m), 622(w).
CDCl3, ppm): d ꢀ2.51 (s, 2H, NH), 4.01 (s, 3H, COOMe), 7.14–
7.19 (m, 4H, phenyl), 7.42–7.46 (m, 16H, phenyl), 7.47–7.51 (m,
4H, phenyl), 8.07 (t, J ¼ 4.0 Hz, 6H, phenyl), 8.22 (d, J ¼ 8.4 Hz,
2H, phenyl), 9.06 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.10 (d, J ¼ 4.8
Hz, 2H, pyrrolic), 9.26 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 9.79 (d, J ¼
4.4 Hz, 2H, pyrrolic), 10.13 (s, 1H, meso). HRMS (ESI, m/z): [M
+ H]+ calcd for C66H47N6O2, 955.3760; found, 955.3766. IR
(KBr pellet, cmꢀ1): 3448(br), 2921(w), 2847(w), 2187(w),
1720(m), 1637(m), 1592(s), 1491(s), 1402(m), 1384(m), 1313(w),
1275(s), 1106(w), 994(m), 973(w), 796(w), 694(s), 518(m).
5,15-Bis(N,N-bis(4-methoxyphenyl)aminophenyl)-10-(4-meth-
oxycarbonylphenylethynyl)porphyrin (3b). Yield: 55%. 1H NMR
(400 MHz, CDCl3, ppm): d ꢀ2.45 (s, 2H, NH), 3.87 (s, 12H,
OMe), 4.01 (s, 3H, COOMe), 6.99 (d, J ¼ 8.8 Hz, 8H, phenyl),
7.33 (d, J ¼ 8.4 Hz, 4H, phenyl), 7.37–7.39 (m, 8H, phenyl), 8.01
(d, J ¼ 8.4 Hz, 4H, phenyl), 8.07 (d, J ¼ 8.4 Hz, 2H, phenyl), 8.23
(d, J ¼ 8.0 Hz, 2H, phenyl), 9.07 (d, J ¼ 4.4 Hz, 2H, pyrrolic),
9.11 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 9.25 (d, J ¼ 4.8 Hz, 2H,
pyrrolic), 9.78 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 10.12 (s, 1H, meso).
HRMS (ESI, m/z): [M + H]+ calcd for C70H55N6O6, 1075.4183;
found, 1075.4180. IR (KBr pellet, cmꢀ1): 3447(br), 2928(w),
2826(w), 2187(w), 1797(w), 1719(m), 1602(s), 1504(vs), 1459(w),
1438(w), 1317(w), 1275(s), 1241(s), 1176(m), 1106(m), 1035(m),
994(m), 821(w), 788(w), 486(w).
General procedure for hydrolysis of carboxylic ester group
A mixture of the porphyrin with a carboxylate group (0.03
mmol) and LiOH$H2O (0.3 mmol) in THF (30 mL) and H2O (4
mL) was refluxed for 12 h under nitrogen. Then, the solvent was
removed in vacuo. The residue was dissolved in THF and the
precipitate filtered. The filtrate was concentrated in vacuo to
afford the crude product, which was purified on a silica gel
column, followed by recrystallization from CH2Cl2/CH3OH.
5,15-Bis(4-(diphenylamino)phenyl)-10-(4-carboxyphenylethynyl)
porphyrin (4a). Yield: 82%. 1H NMR (400 MHz, DMSO-
d6:CDCl3 ¼ 3:1, ppm): d ꢀ2.60 (s, 2H, NH), 7.08–7.31 (m, 6H,
phenyl), 7.31–7.52 (m, 22H, phenyl), 8.12 (d, J ¼ 8.4 Hz, 4H,
phenyl), 9.01–9.22 (m, 4H, pyrrolic), 9.53 (br s, 2H, pyrrolic), 9.90
(br s, 2H, pyrrolic), 10.47 (s, 1H, meso), 13.10 (br s, 1H, COOH).
HRMS (ESI, m/z): [M + H]+ calcd for C65H45N6O2, 941.3604;
found, 941.3613. IR (KBr pellet, cmꢀ1): 3433(br), 3120(m),
3021(m), 2923(w), 2187(m), 1797(w), 1731(m), 1688(m), 1591(s),
1556(w), 1493(s), 1403(m), 1384(s), 1313(m), 1278(s), 1173(m),
973(m), 794(m), 751(w), 695(m).
5,10,15-Tris(4-(diphenylamino)phenyl)-20-(4-methoxycarbo-
nylphenylethynyl)porphyrin (3c). Yield: 55%. 1H NMR (400
MHz, CDCl3, ppm): d ꢀ2.20 (s, 2H, NH), 4.01 (s, 3H, COOMe),
7.14–7.18 (m, 6H, phenyl), 7.41–7.48 (m, 30H, phenyl), 8.03–8.09
(m, 8H, phenyl), 8.24 (d, J ¼ 8.4, 2H, phenyl), 8.91 (d, J ¼ 4.4 Hz,
2H, pyrrolic), 8.93 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 9.06 (d, J ¼ 4.8,
2H, pyrrolic), 9.73 (d, J ¼ 4.4, 2H, pyrrolic). HRMS (ESI, m/z):
[M + H]+ calcd for C84H60N7O2, 1198.4808; found, 1198.4805.
IR (KBr pellet, cmꢀ1): 3420(br), 3031(w), 2922(w), 2851(w),
2192(w), 1718(m), 1591(s), 1493(s), 1400(m), 1384(m), 1313(m),
1275(s), 1175(w), 1154(w), 1107(w), 972(m), 798(m), 753(w),
622(w).
5,15-Bis(N,N-bis(4-methoxyphenyl)aminophenyl)-10-(4-car-
1
boxylphenylethynyl)porphyrin (4b). Yield: 81%. H NMR (400
MHz, DMSO-d6, ppm): d ꢀ2.63 (s, 2H, NH), 3.82 (s, 12H,
OMe), 7.08 (d, J ¼ 9.2 Hz, 8H, phenyl), 7.21 (d, J ¼ 8.4 Hz, 4H,
phenyl), 7.39 (d, J ¼ 8.8 Hz, 8H, phenyl), 8.04 (d, J ¼ 8.0 Hz, 4H,
phenyl), 8.21 (d, J ¼ 8.0 Hz, 2H, phenyl), 8.29 (d, J ¼ 8.4 Hz, 2H,
phenyl), 9.03 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.09 (d, J ¼ 4.8 Hz,
2H, pyrrolic), 9.54 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 9.87 (d, J ¼ 4.4
Hz, 2H, pyrrolic), 10.48 (s, 1H, meso), 13.26 (br s, 1H, COOH).
HRMS (ESI, m/z): [M + H]+ calcd for C69H53N6O6, 1061.4027;
found, 1061.4036. IR (KBr pellet, cmꢀ1): 3443(br), 2967(w),
2830(w), 2191(w), 1801(w), 1685(m), 1603(s), 1504(vs), 1403(m),
1317(m), 1284(m), 1241(s), 1177(w), 1035(m), 828(w), 797(m).
586(w), 528(w).
5,10,15-Tris(N,N-bis(4-methoxyphenyl)aminophenyl)-20-(4-
1
methoxycarbonylphenylethynyl)porphyrin (3d). Yield: 51%. H
NMR (400 MHz, CDCl3, ppm): d ꢀ2.16 (s, 2H, NH), 3.86 (s, 6H,
MeO), 3.87 (s, 12H, MeO), 4.01 (s, 3H, COOMe), 6.96–7.01(m,
12H, phenyl), 7.27–7.32 (m, 6H, phenyl), 7.33–7.39 (m, 12H,
phenyl), 7.98 (t, J ¼ 8.6 Hz, 6H, phenyl), 8.08 (d, J ¼ 8.4 Hz, 2H,
phenyl), 8.24 (d, J ¼ 8.0 Hz, 2H, phenyl), 8.89 (d, J ¼ 4.8 Hz, 2H,
pyrrolic), 8.91 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.05 (d, J ¼ 4.4 Hz,
2H, pyrrolic), 9.70 (d, J ¼ 4.8 Hz, 2H, pyrrolic). HRMS (ESI,
m/z): [M + H]+ calcd for C90H72N7O8, 1378.5442; found,
1378.5442. IR (KBr pellet, cmꢀ1): 3441(br), 2995(w), 2928(w),
2832(w), 2188(w), 1720(m), 1601(s), 1503(vs), 1465(s), 1439(m),
1317(m), 1274(s), 1240(s), 1176(m), 1105(m), 1035(m), 971(w),
828(m), 732(w).
5,10,15-Tris(4-(diphenylamino)phenyl)-20-(4-carboxylpheny-
lethynyl)porphyrin (4c). Yield: 71%. 1H NMR (400 MHz,
DMSO-d6, ppm): d ꢀ2.47 (s, 2H, NH), 7.20–7.63 (m, 36H,
phenyl), 8.04 (s, 6H, phenyl), 8.21 (d, J ¼ 9.2, 4H, phenyl), 8.82–
8.92 (m, 4H, pyrrolic), 8.97 (br s, 2H, pyrrolic), 9.72 (br s, 2H,
pyrrolic), 13.18 (br s, 1H, COOH). HRMS (ESI, m/z): [M + H]+
calcd for C83H58N7O2, 1184.4652; found, 1184.4651. IR (KBr
pellet, cmꢀ1): 3421(br), 2922(s), 2851(m), 2191(w), 1683(m),
1591(s), 1492(s), 1400(m), 1384(m), 1313(m), 1277(s), 1175(w),
1154(w), 1101(w), 972(m), 940(w), 797(m), 752(w), 696(m),
623(w), 516(w).
5,10,15-Tris(N,N-bis(4-methoxyphenyl)aminophenyl)-20-(4-
carboxylphenylethynyl)porphyrin (4d). Yield: 76%. 1H NMR
(400 MHz, DMSO-d6, ppm): d ꢀ2.55 (s, 2H, NH), 3.77 (s, 6H,
MeO), 3.78 (s, 12H, MeO), 6.94–6.96 (m, 12H, phenyl), 7.09–7.15
(m, 6H, phenyl), 7.30–7.34 (m, 12H, phenyl), 7.86–7.92 (m, 6H,
phenyl), 8.14–8.24 (m, 4H, phenyl), 8.72–8.83 (m, 4H, pyrrolic),
8.87 (br s, 2H, pyrrolic), 9.59 (br s, 2H, pyrrolic), 13.23 (br s, 1H,
COOH). HRMS (ESI, m/z): [M + H]+ calcd for C89H70N7O8,
5,15-Bis(4-(diphenylamino)phenyl)-10-(3,4,5-trimethoxyphe-
nyl)-20-(4-methoxycarbonylphenylethynyl)porphyrin
(3e).
Yield: 59%. 1H NMR (400 MHz, CDCl3, ppm): d ꢀ2.25 (s, 2H,
NH), 3.97 (s, 6H, MeO), 4.02 (s, 3H, MeO), 4.18 (s, 3H, MeO),
7.14–7.19 (m, 4H, phenyl), 7.42–7.49 (m, 22H, phenyl), 8.04–8.11
(m, 6H, phenyl), 8.26 (d, J ¼ 8.0 Hz, 2H, phenyl), 8.89 (d, J ¼ 4.8
Hz, 2H, pyrrolic), 8.91 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 9.07 (d, J ¼
7438 | J. Mater. Chem., 2012, 22, 7434–7444
This journal is ª The Royal Society of Chemistry 2012

View Online
1364.5286; found, 1364.5287. IR (KBr pellet, cmꢀ1): 3430(br),
3125(m), 2927(s), 2676(m), 2603(m), 2494(m), 2187(w), 1602(m),
1504(vs), 1472(m), 1384(s), 1318(w), 1241(s), 1170(s), 1036(m),
978(m), 828(w), 802(w), 529(w).
2494(m), 2187(w), 1592(m), 1475(s), 1443(m), 1397(s), 1279(w),
1171(m), 1036(s), 993(m), 850(w), 806(w), 695(w), 462(w).
Zinc(II) 5,10,15-tris(N,N-bis(4-methoxyphenyl)aminophenyl)-
20-(4-carboxylphenylethynyl)porphyrin (MT3P). Yield: 90%. 1H
NMR (400 MHz, DMSO-d6, ppm): d 3.79 (s, 18H, MeO), 7.05 (d,
J ¼ 6.8 Hz, 12H, phenyl), 7.13–7.18 (m, 6H, phenyl), 7.36 (d, J ¼
6.4 Hz, 12H, phenyl), 7.96 (t, J ¼ 8.2 Hz, 6H, phenyl), 8.18 (s,
4H, phenyl), 8.81–8.92 (m, 4H, pyrrolic), 8.99 (br s, 2H, pyrrolic),
9.76 (br s, 2H, pyrrolic). HRMS (ESI, m/z): [M + H]+ calcd for
C89H68N7O8Zn, 1426.4421; found, 1426.4429. IR (KBr pellet,
cmꢀ1): 3440(br), 2975(m), 2935(s), 2738(m), 2676(s), 2492(m),
2185(w), 1602(m), 1504(vs), 1474(w), 1384(s), 1241(s), 1170(m),
1035(m), 993(m), 828(w), 806(w).
5,15-Bis(4-(diphenylamino)phenyl)-10-(3,4,5-trimethoxyph-
enyl)-20-(4-carboxylphenylethynyl)porphyrin (4e). Yield:
80%. 1H NMR (400 MHz, CDCl3, ppm): d ꢀ2.24 (s, 2H, NH),
3.97 (s, 6H, MeO), 4.19 (s, 3H, MeO), 7.14–7.18 (m, 4H, phenyl),
7.42–7.49 (m, 22H, phenyl), 8.05–8.12 (m, 6H, phenyl), 8.31 (d,
J ¼ 7.6 Hz, 2H, phenyl), 8.86–8.95 (m, 4H, pyrrolic), 9.08 (d, J ¼
4.4 Hz, 2H, pyrrolic), 9.76 (d, J ¼ 4.4 Hz, 2H, pyrrolic). HRMS
(ESI, m/z): [M + H]+ calcd for C74H55N6O5, 1107.4234; found,
1107.4238. IR (KBr pellet, cmꢀ1): 3438(br), 2920(m), 2850(w),
2182(w), 1797(w), 1635(w), 1591(s), 1492(s), 1403(m), 1384(m),
1314(m), 1278(m), 1127(s), 974(m), 796(w), 695(m), 519(w).
Zinc(II)
5,15-bis(4-(diphenylamino)phenyl)-10-(3,4,5-trime-
thoxyphenyl)-20-(4-carboxylphenylethynyl)porphyrin (M3T2P).
Yield: 85%. 1H NMR (400 MHz, DMSO-d6, ppm): d 3.91 (s, 6H,
MeO), 3.99 (s, 3H, MeO), 7.18 (t, J ¼ 7.2 Hz, 4H, phenyl), 7.38
(d, J ¼ 7.6 Hz, 12H, phenyl), 7.45–7.50 (m, 10H, phenyl), 8.05–
8.18 (m, 8H, phenyl), 8.85 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 8.89 (d,
J ¼ 4.4 Hz, 2H, pyrrolic), 9.00 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.81
(br s, 2H, pyrrolic). HRMS (ESI, m/z): [M + H]+ calcd for
C74H53N6O5Zn, 1169.3369; found, 1169.3370. IR (KBr pellet,
cmꢀ1): 3438(br), 2959(m), 2924(s), 2852(m), 2187(w), 1797(w),
1592(s), 1491(vs), 1406(s), 1384(s), 1314(w), 1262(s), 1126(m),
1101(m), 999(m), 796(m), 746(w), 696(m), 519(w), 472(w).
General procedure for zinc coordination
To a solution of the porphyrin (0.021 mmol) in CH2Cl2 (50 mL)
was added a CH3OH (5 mL) solution of Zn(OAc)2$2H2O (1.05
mmol). After the mixture was stirred at room temperature
overnight, the solvent was removed under reduced pressure, and
the residue was purified on a silica gel column, followed by
recrystallization from CH2Cl2/CH3OH.
Zinc(II)
5,15-bis(4-(diphenylamino)phenyl)-10-(4-carboxy-
lphenylethynyl)porphyrin (T2P). Yield: 91%. 1H NMR (400
MHz, DMSO-d6, ppm): d 7.17–7.22 (m, 4H, phenyl), 7.39–7.44
(m, 12H, phenyl), 7.47–7.52 (m, 8H, phenyl), 8.10 (d, J ¼ 8.4 Hz,
4H, phenyl), 8.21 (s, 4H, phenyl), 8.99 (d, J ¼ 4.8 Hz, 2H,
pyrrolic), 9.06 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.46 (d, J ¼ 4.4 Hz,
2H, pyrrolic), 9.83 (d, J ¼ 4.8 Hz, 2H, pyrrolic), 10.31 (s, 1H,
meso). HRMS (ESI, m/z): [M + H]+ calcd for C65H43N6O2Zn,
1003.2739; found, 1003.2738. IR (KBr pellet, cmꢀ1): 3423(br),
2921(m), 2851(m), 2187(w), 1797(w), 1685(m), 1637(m), 1592(s),
1491(s), 1406(m), 1384(m), 1314(w), 1278(m), 1107(m), 998(m),
872(w), 791(w), 696(m), 473(m).
Results and discussion
Synthesis
In general, highly efficient DSSC dyes have a push–pull structure.
Triarylamine (TPA) moieties have been widely used as electron
donors due to their strong electron-donating ability.20 Alkoxyl
groups are weaker electron donors than TPA moieties and stronger
than alkyl groups.21 Accordingly, for the purpose of gradually
increasing the electron-donating ability, various numbers of TPA
and methoxyl groups were combined and introduced to afford the
target porphyrin dyes (Scheme 2). The meso-substituted porphyrin
intermediates 1a–1d were prepared by TFA-promoted condensa-
tion of corresponding dipyrromethanes and aldehydes in CH2Cl2.
For the synthesis of 1e, methanol was added to suppress the
scrambling process, but the reaction at room temperature afforded
no product. Thus, the reaction mixture was refluxed for 48 h, and
1e was obtained in 4.6% yield.
Zinc(II) 5,15-bis(N,N-bis(4-methoxyphenyl)aminophenyl)-10-
(4-carboxylphenylethynyl)porphyrin (MT2P). Yield: 90%. 1H
NMR (400 MHz, DMSO-d6, ppm): d 3.80 (s, 12H, MeO), 7.06 (d,
J ¼ 8.8 Hz, 8H, phenyl), 7.20 (d, J ¼ 8.0 Hz, 4H, phenyl), 7.37 (d,
J ¼ 8.4 Hz, 8H, phenyl), 8.00 (d, J ¼ 7.6 Hz, 4H, phenyl), 8.12–
8.19 (m, 4H, phenyl), 8.97 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.05 (d,
J ¼ 4.4 Hz, 2H, pyrrolic), 9.43 (d, J ¼ 4.4 Hz, 2H, pyrrolic), 9.81
(d, J ¼ 4.4 Hz, 2H, pyrrolic), 10.27 (s, 1H, meso). HRMS (ESI,
m/z): [M + H]+ calcd for C69H51N6O6Zn, 1123.3162; found,
1123.3158. IR (KBr pellet, cmꢀ1): 3472(br), 3414(vs), 3235(m),
2920(m), 2847(w), 2187(w), 1793(w), 1637(m), 1617(s), 1503(vs),
1384(s), 1317(w), 1240(s), 1176(m), 997(m), 830(w), 788(w),
624(m).
Ethyne bridges have been shown to allow efficient conjugation
and strong electronic interaction between chromophores, and to
provide a well-defined and rigid structural arrangement.22 In
view of this, 4-ethynylbenzoic acid is selected as the electron-
acceptor and the anchoring group, which might facilitate elec-
tron transfer from the excited dye to the TiO2 surface and would
lead to an improved energy conversion efficiency. The ethynyl
group was attached to the meso-position via Sonogashira
coupling reactions in satisfactory yields. The NMR and HRMS
spectra of the intermediates and the target porphyrins are shown
in the ESI† (Figs. S1–S50).
Zinc(II) 5,10,15-tris(4-(diphenylamino)phenyl)-20-(4-carboxy-
lphenylethynyl)porphyrin (T3P). Yield: 86%. 1H NMR
(400 MHz, DMSO-d6: CDCl3 ¼ 5: 1, ppm): d 7.14–7.22 (m, 6H,
phenyl), 7.36–7.48 (m, 30H, phenyl), 8.04–8.15 (m, 8H, phenyl),
8.26 (d, J ¼ 7.2, 2H, phenyl), 8.90 (d, J ¼ 4.4 Hz, 2H, pyrrolic),
8.92 (d, J ¼ 4.8 Hz, 2H, pyrrolic),9.04 (d, J ¼ 4.8, 2H, pyrrolic),
9.79 (d, J ¼ 4.4 Hz, 2H, pyrrolic). HRMS (ESI, m/z): [M + H]+
calcd for C83H56N7O2Zn, 1246.3787; found, 1246.3802. IR (KBr
pellet, cmꢀ1): 3430(br), 2938(m), 2738(m), 2676(s), 2603(m),
Absorption properties in solutions and on TiO2 films
The UV-visible absorption spectra of the porphyrins in THF are
shown Fig. 1a. All the porphyrin dyes exhibit the typical features
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7434–7444 | 7439

View Online
Table 1 Absorption and emission data for the porphyrin dyes in THF
Electrochemical properties
Porphyrins Absorption lmax/nm (3/103 Mꢀ1 cmꢀ1
)
Emission lmax/nma
For a typical DSSC sensitizer, suitable HOMO and LUMO
orbitals are required to enable efficient dye regeneration and
electron injection processes. To evaluate this possibility, the
cyclic voltammograms (Fig. 3) were recorded in acetonitrile, and
the data are summarized in Table 2. The HOMO energy levels lie
in the range between 0.75 and 1.10 V with the sequence of T2P z
T3P < M3T2P < MT2P z MT3P. It is interesting that the
variation in the number of the triphenylamine donor groups
caused very little changes in the HOMO energy levels. Thus, T2P
and T3P have almost equal values. On the other hand, the
introduction of electron-donating methoxy groups to the tri-
phenylamine rings has pronounced effects. Thus, MT2P and
MT3P have significantly higher HOMO energies than those of
T2P and T3P. In the molecule of M3T2P, a moderate electron
donor, a trimethoxyphenyl unit is directly introduced onto the
meso position, which results in a moderate HOMO energy level.24
The LUMO energy levels for these compounds have a similar
variation trend. These results clearly indicate that the energy
levels could be conveniently tuned by the variation of the donor
groups. The LUMO levels (Fig. 4) of the porphyrins are more
negative than the conduction edge (–0.50 V vs. NHE) of TiO2,
which implies that the electron injection from the excited states of
the dyes to the conduction band (CB) of TiO2 is possible. On the
other hand, the HOMO levels are more positive than the
T2P
T3P
MT2P
MT3P
M3T2P
443 (129), 573 (7.0), 621 (10.4)
449 (180), 581 (8.8), 632 (20.2)
440 (83.2), 574 (6.0), 623 (8.8)
448 (83.0), 581 (5.0), 636 (12.2)
447 (94.8), 578 (4.1), 629 (8.7)
635
651
642
660
645
a
Excitation wavelengths/nm: T2P, 443; T3P, 449; MT2P, 440; MT3P,
448; M3T2P, 447.
of a porphyrin ring, with an intense Soret band in the range 400–
500 nm and less intense Q bands in a range from 550 to 750 nm.
As expected, the absorption bands are sensitive to the substitu-
ents. With increasing number of electron donors, the Soret and Q
bands are broadened and red-shifted. Thus, T3P and MT3P
exhibit slightly broader absorption bands at longer wavelengths,
compared with other porphyrins. For the fluorescence spectra in
THF (Fig. 1b), the trend for the variation of the emission
wavelength (Table 1) is similar to that of the absorption bands.12c
Fig. 2 shows the absorption spectra of the porphyrins in THF
and on 5 mm thick TiO2 films. All of the porphyrins show
comparable absorptions on the TiO2 films. Generally, when
sensitizers are anchored onto a nanocrystalline TiO2 surface, the
deprotonation and H-aggregates will result in a blue-shift, while
J-aggregates usually lead to a red-shift. Compared with the
solution spectra, the Soret and Q bands of the porphyrin/TiO2
films were only slightly shifted but considerably broadened
relative to the solution spectra. In addition, the presence of the
blue shoulders of the Soret bands was observed. These obser-
vations can be related to H-type aggregation of the porphyrins
on the TiO2 surfaces.23 The broadened absorption bands and the
red-shifted absorption onset of the thin-film samples afford
increased light-harvesting and better utilization of the long-
wavelength light.
Fig. 3 Cyclic voltammetry plots of the dyes adsorbed to a nanocrystal-
line TiO2 film deposited on conducting FTO glass.
Table 2 Electrochemical properties of the porphyrin dyes
Porphyrins HOMOa/V (vs. NHE) E0–0b/V LUMOc/V (vs. NHE)
T2P
T3P
MT2P
MT3P
M3T2P
1.10
1.09
0.76
0.75
0.99
1.84
1.79
1.83
1.78
1.82
ꢀ0.74
ꢀ0.70
ꢀ1.07
ꢀ1.03
ꢀ0.83
a
HOMO levels were measured in acetonitrile with 0.1
M
tetrabutylammonium hexafluorophosphate (TBAPF6) as electrolyte
(working electrode: FTO/TiO2/dye; reference electrode: SCE; calibrated
with ferrocene/ferrocenium (Fc/Fc+) as an external reference. Counter
b
electrode: Pt). E0–0 was estimated from the absorption threshold of
dyes adsorbed on the TiO2 film. The LUMO was calculated using the
c
Fig. 2 Normalized UV-visible spectra of the porphyrins in THF (red
curves) and on TiO2 films (5 mm) in air (black curves).
equation LUMO ¼ HOMO ꢀ E0–0
.
7440 | J. Mater. Chem., 2012, 22, 7434–7444
This journal is ª The Royal Society of Chemistry 2012

View Online
and matched HOMO and LUMO energy levels. In such a case,
both the electron injection and the dye regeneration processes
will be efficient, thus enabling an optimized cell performance.
This agrees well with the cell performance measurements
(vide infra).
DFT calculations
To gain further insight into the electronic properties of the
porphyrins, DFT (density functional theory) calculations were
performed. Geometry optimization was carried out at the
B3LYP/6-31G* level.25 Fig. 5 shows the corresponding molec-
ular orbitals. The HOMO orbitals of the porphyrin dyes are
delocalized over the donors and the porphyrin macrocycles while
the LUMO orbitals are delocalized over the porphyrin macro-
cycles, the ethynylphenylene linkers and the carboxyl anchoring
groups. Accordingly, the excitation of the electron from the
HOMO to the LUMO orbital can result in the electron redis-
tribution from the donor unit to the anchoring moiety, thus
realizing electron injection to the conduction band of TiO2.
Fig. 4 Schematic energy-level diagram of the porphyrins. LUMO is
estimated by subtracting E0–0 from HOMO.
ꢀ
oxidation potential for the Iꢀ/I3 redox couple (+0.40 V vs.
NHE), indicating that the oxidized dyes generated by the elec-
tron injection process could thermodynamically accept electrons
from Iꢀ ions to regenerate the dyes. Thus, these porphyrins may
serve as sensitizers for DSSCs.
It is noteworthy that for all the porphyrin dyes, the HOMO–
LUMO energy gaps remain almost constant with the values
varying in a small range of 1.78–1.84 V, despite the fact that the
numbers and types of electron donors are varied to a large extent.
This observation is in sharp contrast to the effective modulation
of the HOMO–LUMO energy gaps by the modification of the p-
conjugation systems.16a Due to the relatively constant HOMO–
LUMO energy gaps for these porphyrin sensitizers, a higher
HOMO energy level is associated with a higher LUMO energy
level. Such a case is favorable for the electron injection process,
but unfavorable for the dye regeneration process. On the other
hand, a lower HOMO energy level associated with a lower
LUMO energy level will be unfavorable for the electron injection
process, but favorable for the dye regeneration process. Hence,
the variation of the donor groups will have contradictory effects
on the processes of electron injection and dye regeneration. Thus,
it may be anticipated that for these porphyrin sensitizers with
similar structures and comparable HOMO–LUMO gaps, a suit-
able combination of the donor moieties may result in moderate
Photovoltaic performance of DSSCs
To check the possibility of utilizing these porphyrins as DSSC
sensitizers, the porphyrins were loaded onto TiO2 nano-
particulate films to serve as working electrodes for the fabrica-
tion of DSSC devices. The incident photon-to-current
conversion efficiency (IPCE) measurements (Fig. 6) reflect the
efficiency of photoelectric conversion at various wavelengths.
Because of the strong absorption bands with high molar
absorption coefficients, these porphyrin dyes have moderate to
high IPCE values in a large wavelength range of 400–700 nm,
indicative of the possibility to manufacture DSSCs with high
efficiencies. The corresponding current–voltage curves are shown
in Fig. 7, and the photovoltaic parameters are summarized in
Table 3. The overall power conversion efficiencies (h) lie in the
range of 2.70–5.45%, exhibiting an order of MT3P < MT2P <
T3P < T2P < M3T2P, which has the same sequence as that for
the IPCE curves. Thus, the cell based on the sensitization of
M3T2P exhibits the highest IPCE curve, which corresponds to
Fig. 5 Frontier molecular orbital profiles of the porphyrin dyes calculated by DFT.
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7434–7444 | 7441

View Online
Table 3 Photovoltaic parameters of porphyrin-based dye-sensitized
solar cells under AM1.5 illumination (power 100 mW cmꢀ2) with an
active area of 0.28 cm2
Porphyrins
Jsc/mA cmꢀ2
Voc/mV
ff
h (%)
T2P
T3P
MT2P
MT3P
M3T2P
13.04
12.38
10.19
7.86
624
573
560
533
665
0.54
0.58
0.62
0.64
0.53
4.36
4.10
3.53
2.70
5.45
15.36
the interface. In the Bode plots (Fig. 8b), the peaks at higher
frequencies correspond to the charge transfer at the counter
electrode, and the peaks at lower frequencies are related to the
charge recombination rate, and its reciprocal is regarded as the
electron lifetime.26 The peak maximum is shifted to lower
frequency from MT3P to M3T2P in the same sequence as that
observed for the IPCE curves. The electron lifetime values
derived from the curve fitting for the cells based on MT3P,
MT2P, T3P, T2P, and M3T2P are 23.9, 30.9, 40.4, 53.7, and
99.7 ms, respectively. Thus, M3T2P has the longest electron
lifetime, indicative of a more effective suppression of the back
reaction between the injected electrons and the electrolyte,
resulting in the improvement of the Voc due to reduced charge
recombination rate.27 The combination of the highest Voc and Jsc
values (vide supra) affords the best photovoltaic performance of
the solar cell based on M3T2P.
Fig. 6 IPCE spectra of DSSCs based on the porphyrin dyes.
the highest Jsc. For M3T2P, a moderate electron donating tri-
methoxyphenyl group and two triphenylamine groups were
introduced onto the meso-positions to afford moderate HOMO
and LUMO energy levels among the porphyrin dyes (vide supra).
The resulting HOMO level is more positive than those of MT2P
and MT3P so that the dye regeneration efficiency for the DSSCs
based on M3T2P may be higher than those for MT2P and
MT3P. On the other hand, the LUMO level of M3T2P is more
negative than those of T2P and T3P, so that the electron injec-
tion efficiency is higher compared with T2P and T3P. Hence, the
DSSCs based on M3T2P exhibited the highest IPCE curve,
showing the highest h value of 5.45%.
Electrochemical impedance spectroscopy
To further clarify the interrelation between the dyes and the
photoelectric properties of the DSSCs, electrochemical imped-
ance spectral (EIS) analyses were performed in the dark. The EIS
analysis provides information about interfacial charge transfer
processes. Fig. 8a shows the typical Nyquist plots of both elec-
trodes at an applied bias of ꢀ0.55 V. In the Nyquist plots, the
high-frequency semicircle on the left indicates the Pt/redox
(Iꢀ/I3ꢀ) interface charge transfer and the middle-frequency
semicircle on the right corresponds to the electron transport
process at the TiO2/dye/electrolyte interface. The radii of the
semicircles lie in the order of MT3P < MT2P < T3P < T2P <
M3T2P, indicating the sequence of charge transfer resistance at
Fig.
8 Electrochemical impedance spectra of DSSCs based on
Fig. 7 Current–voltage characteristics of the DSSCs based on the
porphyrin dyes measured at ꢀ0.55 V bias in the dark. (a) Nyquist plots;
porphyrin dyes.
(b) Bode phase plots.
7442 | J. Mater. Chem., 2012, 22, 7434–7444
This journal is ª The Royal Society of Chemistry 2012

View Online
Y. Cui, K. Hara, Y. Dan-ho, C. Kasada and A. Shinpo, Adv.
Mater., 2007, 19, 1138; (e) Z. S. Wang, Y. Cui, Y. Dan-oh,
C. Kasada, A. Shinpo and K. Hara, J. Phys. Chem. C, 2008, 112,
17011.
Conclusions
In summary, we have synthesized novel porphyrin-based DSSC
dyes with various numbers and types of donor groups attached at
the meso-positions of porphyrins. The influences of the electron
donors on spectral, electrochemical, and photovoltaic properties
of the porphyrin sensitizers were investigated. The tuning of
HOMO–LUMO energy levels and the corresponding optical and
electrochemical properties were effectively achieved by the vari-
ation of donor groups. Thus, the DSSC devices based on the
M3T2P sensitizer have achieved a power conversion efficiency of
5.45%, with Jsc, Voc and ff values of 15.36 mA cmꢀ2, 665 mV, and
0.53, respectively. Its best performance may be ascribed to
a balance of efficient dye regeneration and electron injection
processes resulting from the suitable arrangements of the HOMO
and LUMO energy levels, in combination with the longest elec-
tron lifetime. These results demonstrate that the introduction of
appropriate electron donors might significantly enhance the
photoelectrical performance of the DSSCs by modulating
the HOMO and LUMO energy levels. It is noteworthy that the
variation of the position and number of the methoxyl groups can
be easily realized, and it is very effective for modulating the
energy levels and DSSC efficiencies. Furthermore, other alkoxyl
or phenoxyl substituents can be conveniently utilized instead of
the methoxyl groups, which leaves space for further optimization
of the dye structures and cell performance. The results are
beneficial for further design of efficient DSSC sensitizers by the
variation of the donor groups.
5 (a) C. Teng, X. C. Yang, C. Yang, H. N. Tian, S. F. Li, X. N. Wang,
A. Hagfeldt and L. C. Sun, J. Phys. Chem. C, 2010, 114, 11305; (b)
H. N. Tian, I. Bora, X. Jiang, E. Gabrielsson, K. M. Karlsson,
A. Hagfeldt and L. C. Sun, J. Mater. Chem., 2011, 21, 12462; (c)
Y. Numata, I. Ashraful, Y. Shirai and L. Y. Han, Chem. Commun.,
2011, 47, 6159; (d) S. F. Zhang, X. D. Yang, K. Zhang, H. Chen,
M. Yanagidaa and L. Y. Han, Phys. Chem. Chem. Phys., 2011,
19310, DOI: 10.1039/c1cp22832c; (e) W. J. Wu, X. D. Xu,
H. B. Yang, J. L. Hua, X. Y. Zhang, L. Zhang, Y. T. Long and
H. Tian, J. Mater. Chem., 2011, 21, 10666; (f) K. T. Kamtekar,
C. S. Wang, S. Bettington, A. S. Batsanov, I. F. Perepichka,
M. R. Bryce, J. H. Ahn, M. Rabinal and M. C. Petty, J. Mater.
Chem., 2006, 16, 3823; (g) A. L. Kanibolotsky, I. F. Perepichka and
P. J. Skabara, Chem. Soc. Rev., 2010, 39, 2695.
6 (a) R. Z. Li, J. Y. Liu, N. Cai, M. Zhang and P. Wang, J. Phys. Chem.
B, 2010, 114, 4461; (b) D. F. Zhou, N. Cai, H. J. Long, M. Zhang,
Y. H. Wang and P. Wang, J. Phys. Chem. C, 2011, 115, 3163; (c)
N. Cai, S. J. Moon, L. Cevey-Ha, T. Moehl, R. Humphry-Baker,
€
P. Wang, S. M. Zakeeruddin and M. Gratzel, Nano Lett., 2011, 11,
1452; (d) S. Kim, H. Choi, D. Kim, K. Song, S. O. Kang and
J. Ko, Tetrahedron, 2007, 63, 9206; (e) S. Kim, H. Choi, C. Baik,
K. Song, S. O. Kang and J. Ko, Tetrahedron, 2007, 63, 11436; (f)
I. Jung, J. K. Lee, K. H. Song, K. Song, S. O. Kang and J. Ko,
J. Org. Chem., 2007, 72, 3652.
7 (a) Q. H. Yao, L. Shan, F. Y. Li, D. D. Yin and C. H. Huang, New J.
Chem., 2003, 27, 1277; (b) Y. S. Chen, C. Li, Z. H. Zeng, W. B. Wang,
X. S. Wang and B. W. Zhang, J. Mater. Chem., 2005, 15, 1654; (c)
T. Kitamura, M. Ikeda, K. Shigaki, T. Inoue, N. A. Anderson,
X. Ai, T. Lian and S. Yanagida, Chem. Mater., 2004, 16, 1806; (d)
K. Hara, T. Sato, R. Katoh, A. Furabe, T. Yoshihara, M. Murai,
M. Kurashige, S. Ito, A. Shinpo, S. Suga and H. Arakawa, Adv.
Funct. Mater., 2005, 15, 246; (e) K. Hara, M. Kurashige, S. Ito,
A. Shinpo, S. Suga, K. Sayama and H. Arakawa, Chem. Commun.,
2003, 252.
8 (a) M. Velusamy, K. R. J. Thomas, J. T. Lin, Y. Hsu and K. Ho, Org.
Lett., 2005, 7, 1899; (b) D. P. Hagberg, T. Edvinsson, T. Marinado,
G. Boschloo, A. Hagfeldt and L. C. Sun, Chem. Commun., 2006,
2245; (c) M. Liang, W. Xu, F. Cai, P. Chen, B. Peng, J. Chen and
Z. Li, J. Phys. Chem. C, 2007, 111, 4465.
9 (a) A. Ehret, L. Stuhl and M. T. Spitler, J. Phys. Chem. B, 2001, 105,
9960; (b) S. Ushiroda, N. Ruzycki, Y. Lu, M. T. Spitler and
B. A. Parkinson, J. Am. Chem. Soc., 2005, 127, 5158; (c) W. J. Wu,
J. L. Hua, Y. H. Jin, W. H. Zhan and H. Tian, Photochem.
Photobiol. Sci., 2008, 7, 63; (d) Q. H. Yao, L. Shan, F. Y. Li,
D. D. Yin and C. H. Huang, New J. Chem., 2003, 27, 1277; (e)
Y. S. Chen, C. Li, Z. H. Zeng, W. B. Wang, X. S. Wang and
B. W. Zhang, J. Mater. Chem., 2005, 15, 1654.
Acknowledgements
This work was financially supported by NSFC/China, the
Program for Professor of Special Appointment (Eastern Scholar)
at Shanghai Institutions of Higher Learning, Program for New
Century Excellent Talents in University (NCET), Innovation
Program of Shanghai Municipal Education Commission, the
Fundamental Research Funds for the Central Universities
(WK1013002),
and
SRFDP
(200802510011
and
20100074110015).
10 (a) J. Warnan, F. Buchet, Y. Pellegrin, E. Blart and F. Odobel, Org.
Notes and references
ꢁ
Lett., 2011, 13, 3944; (b) M. Garcıa-Iglesias, J. J. Cid, J. H. Yum,
A. Forneli, P. Vazquez, M. K. Nazeeruddin, E. Palomares,
ꢁ
€
1 (a) M. Gratzel, Acc. Chem. Res., 2009, 42, 1788; (b) Y. Cao, Y. Bai,
Q. Yu, Y. Cheng, S. Liu, D. Shi, F. Gao and P. Wang, J. Phys.
Chem. C, 2009, 113, 6290; (c) A. Hagfeldt, G. Boschloo, L. C. Sun,
L. Kloo and H. Pettersson, Chem. Rev., 2010, 110, 6595.
€
M. Gratzel and T. Torres, Energy Environ. Sci., 2011, 4, 189.
11 (a) B. Liu, W. H. Zhu, Q. Zhang, W. J. Wu, M. Xu, Z. J. Ning,
Y. S. Xie and H. Tian, Chem. Commun., 2009, 1766; (b) W. Q. Li,
Y. Z. Wu, X. Li, Y. S. Xie and W. H. Zhu, Energy Environ. Sci.,
2011, 4, 1830; (c) W. H. Zhu, Y. Z. Wu, S. T. Wang, W. Q. Li,
X. Li, J. Chen, Z. S. Wang and H. Tian, Adv. Funct. Mater., 2011,
21, 756.
12 (a) H. Imahori, T. Umeyama and S. Ito, Acc. Chem. Res., 2009, 42,
1809; (b) M. G. Walter, A. B. Rudine and C. C. Wamser,
J. Porphyrins Phthalocyanines, 2010, 14, 759; (c) M. Gouterman,
J. Mol. Spectrosc., 1961, 6, 138.
13 (a) C. W. Lee, H. P. Lu, C. M. Lan, Y. L. Huang, Y. R. Liang,
W. N. Yen, Y. C. Liu, Y. S. Lin, E. W. G. Diau and C. Y. Yeh,
Chem.–Eur. J., 2009, 15, 1403; (b) C. L. Mai, W. K. Huang,
H. P. Lu, C. W. Lee, C. L. Chiu, Y. R. Liang, E. W. G. Diau and
C. Y. Yeh, Chem. Commun., 2010, 46, 809.
14 (a) C. Y. Lee, C. X. She, N. C. Jeong and J. T. Hupp, Chem.
Commun., 2010, 46, 6090; (b) H. Imahori, Y. Matsubara, H. Iijima,
T. Umeyama, Y. Matano, S. Ito, M. Niemi, N. V. Tkachenko and
H. Lemmetyinen, J. Phys. Chem. C, 2010, 114, 10656; (c)
M. V. Martınez-Dıaz, G. de la Torre and T. Torres, Chem.
€
2 K. Kalayansundaram and M. Gratzel, Coord. Chem. Rev., 1998, 177,
347.
3 (a) A. Mishra, M. K. R. Fischer and P. Bauerle, Angew. Chem., Int.
€
Ed., 2009, 48, 2474; (b) D. P. Hagberg, X. Jiang, E. Gabrielsson,
M. Linder, T. Marinado, T. Brinck, A. Hagfeldt and L. C. Sun,
J. Mater. Chem., 2009, 19, 7232; (c) W. J. Wu, J. B. Yang,
J. L. Hua, J. Tang, L. Zhang, Y. T. Long and H. Tian, J. Mater.
Chem., 2010, 20, 1772; (d) D. Wang, S. C. Hou, H. W. Wu,
C. Zhang, Z. Z. Chu and D. C. Zou, J. Mater. Chem., 2011, 21,
6383; (e) X. Cai, S. C. Hou, H. W. Wu, Z. B. Lv, Y. P. Fu,
D. Wang, C. Zhang, H. Kafafy, Z. Z. Chu and D. C. Zou, Phys.
Chem. Chem. Phys., 2012, 14, 125.
4 (a) K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga and
H. Arakawa, Chem. Commun., 2001, 569; (b) K. Hara,
M. Kurashige, Y. Danoh, C. Kasada, A. Shinpo, S. Suga,
K. Sayama and H. Arakawa, New J. Chem., 2003, 27, 783; (c)
Z. S. Wang, Y. Cui, Y. Dan-oh, C. Kasada, A. Shinpo and
K. Hara, J. Phys. Chem. C, 2007, 111, 7224; (d) Z. S. Wang,
This journal is ª The Royal Society of Chemistry 2012
J. Mater. Chem., 2012, 22, 7434–7444 | 7443

View Online
Commun., 2010, 46, 7090; (d) C. J. Jiao, N. N. Zu, K. W. Huang,
P. Wang and J. S. Wu, Org. Lett., 2011, 13, 3652; (e) Y. Z. Liu,
H. Lin, J. T. Dy, K. Tamaki, J. Nakazaki, D. Nakayama,
S. Uchida, T. Kuboa and H. Segawa, Chem. Commun., 2011, 47,
4010; (f) J. K. Park, J. P. Chen, H. R. Lee, S. W. Park,
H. Shinokubo, A. Osuka and D. Kim, J. Phys. Chem. C, 2009, 113,
21956; (g) R. A. Jensen, H. V. Ryswyk, C. She, J. M. Szarko,
L. X. Chen and J. T. Hupp, Langmuir, 2010, 26, 1401.
22 C. W. Chang, L. Y. Luo, C. K. Chou, C. F. Lo, C. Y. Lin, C. S. Hung,
Y. P. Lee and E. W. G. Diau, J. Phys. Chem. C, 2009, 113, 11524.
23 (a) C. Y. Lin, Y. C. Wang, S. J. Hsu, C. F. Lo and E. W. G. Diau,
J. Phys. Chem. C, 2010, 114, 687; (b) C. Y. Lin, C. F. Lo, L. Luo,
H. P. Lu, C. S. Hung and E. W. G. Diau, J. Phys. Chem. C, 2009,
113, 755; (c) C. F. Lo, L. Luo, E. W. G. Diau, I. J. Chang and
C. Y. Lin, Chem. Commun., 2006, 1430.
24 C. F. Lo, S. J. Hsu, C. L. Wang, Y. H. Cheng, H. P. Lu, E. W. G. Diau
and C. Y. Lin, J. Phys. Chem. C, 2010, 114, 12018.
15 (a) C. P. Hsieh, H. P. Lu, C. L. Chiu, C. W. Lee, S. H. Chuang,
C. L. Mai, W. N. Yen, S. J. Hsu, E. W. G. Diau and C. Y. Yeh,
J. Mater. Chem., 2010, 20, 1127; (b) T. Bessho, S. M. Zakeeruddin,
25 (a) Gaussian 03, Revision D.01, M. J. Frisch, G. W. Trucks,
H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman,
J. A. MontgomeryJr., T. Vreven, K. N. Kudin, J. C. Burant,
J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci,
M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji,
M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa,
M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene,
X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken,
C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev,
A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala,
K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg,
V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain,
O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford,
J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz,
I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill,
B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople,
Gaussian, Inc., Wallingford CT, 2004; (b) A. D. Becke, J. Chem.
Phys., 1993, 98, 5648; (c) P. J. Stephens, F. J. Devlin,
C. F. Chabalowski and M. J. Frisch, J. Phys. Chem., 1994, 98,
11623; (d) P. C. Hariharan and J. A. Pople, Theor. Chim. Acta,
1973, 28, 213.
€
C. Y. Yeh, E. W. G. Diau and M. Gratzel, Angew. Chem., Int. Ed.,
2010, 49, 6646.
16 (a) C. L. Wang, Y. C. Chang, C. M. Lan, C. F. Lo, E. W. G. Diau and
C. Y. Lin, Energy Environ. Sci., 2011, 4, 1788; (b) Y. C. Chang,
C. L. Wang, T. Y. Pan, S. H. Hong, C. M. Lan, H. H. Kuo,
C. F. Lo, H. Y. Hsu, C. Y. Lin and E. W. G. Diau, Chem.
Commun., 2011, 47, 8910.
17 A. Yella, H. W. Lee, H. N. Tsao, C. Y. Yi, A. K. Chandiran,
M. K. Nazeeruddin, E. W. G. Diau, C. Y. Yeh, S. M Zakeeruddin
€
and M. Gratzel, Science, 2011, 334, 629.
18 (a) J. Tang, J. L. Hua, W. J. Wu, J. Li, Z. G. Jin, Y. T. Long and
H. Tian, Energy Environ. Sci., 2010, 3, 1736; (b) S. L. Wu,
H. P. Lu, H. T. Yu, S. H. Chuang, C. L. Chiu, C. W. Lee,
E. W. G. Diau and C. Y. Yeh, Energy Environ. Sci., 2010, 3,
949.
€
19 S. Ito, T. N. Murakami, P. Comte, P. Liska, C. Gratzel,
M. K. Nazeeruddin and M. Gratzel, Thin Solid Films, 2008, 516, 4613.
€
20 (a) Z. J. Ning and H. Tian, Chem. Commun., 2009, 5483; (b)
W. H. Liu, I. C. Wu, C. H. Lai, C. H. Lai, Y. T. Li, C. L. Chen,
Y. Y. Hsub, P. T. Chou and Y. Chi, Chem. Commun., 2008, 5152;
(c) B. S. Chen, D. Y. Chen, C. L. Chen, C. W. Hsu, H. C. Hsu,
K. L. Wu, S. H. Liu, P. T. Chou and Y. Chi, J. Mater. Chem.,
2011, 21, 1937; (d) B. S. Chen, Y. J. Chen and P. T. Chou,
J. Mater. Chem., 2011, 21, 4090.
26 (a) Y. D. Lin and T. J. Chow, J. Mater. Chem., 2011, 21, 14907; (b)
S. Q. Fan, C. Kim, B. Fang, K. X. Liao, G. J. Yang, C. J. Li,
J. J. Kim and J. Ko, J. Phys. Chem. C, 2011, 115, 7747.
21 H. Im, S. Kim, C. Park, S. H. Jang, C. J. Kim, K. Kim, N. G. Park
and C. Kim, Chem. Commun., 2010, 46, 1335.
27 Y. P. Hong, J. Y. Liao, D. Cao, X. F. Zang, D. B. Kuang,
L. Y. Wang, H. Meier and C. Y. Su, J. Org. Chem., 2011, 76, 8015.
7444 | J. Mater. Chem., 2012, 22, 7434–7444
This journal is ª The Royal Society of Chemistry 2012