Synthesis, characterization and application of trans-D-B-A-porphyrin based dyes in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.jphotochem.2011.01.003

A series of metal-free trans-D-B-A porphyrin dyes have been synthesized and characterized as photo-sensitizers for dye-sensitized solar cells (DSSCs). These dye molecules contain a porphyrin unit as a π-bridge, a substituted phenyl group as an electron do

Full text of DOI:10.1016/j.jphotochem.2011.01.003

Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 219–225
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Synthesis, characterization and application of trans-D–B–A-porphyrin based dyes
in dye-sensitized solar cells
Kalyan Kumar Pasunootia,1, Jun-Ling Songa,1, Hua Chaia, Pitchamuthu Amaladassa,
Wei-Qiao Dengb,∗, Xue-Wei Liua,∗∗
a Division of Chemistry and Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, Singapore 637371, Singapore
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of metal-free trans-D–B–A porphyrin dyes have been synthesized and characterized as photo-
sensitizers for dye-sensitized solar cells (DSSCs). These dye molecules contain a porphyrin unit as a
ꢀ-bridge, a substituted phenyl group as an electron donor and cyanoacrylic acid or carboxylic acid group
as an electron acceptor (anchoring groups). This phenomenon is more pronounced for porphyrins (Dye
4 and Dye 5) that have conjugated diphenyl aniline at the opposite side of the anchoring group. A solar
conversion efficiency of 2.38% has been achieved with a short circuit current (Jsc) of 7.63 mA/cm2, an
open circuit voltage (Voc) of 0.613 V and a fill factor (ff) of 51.0% for Dye 5 under standard global AM 1.5
solar irradiation. Results of time-dependent density functional theory (TD-DFT) calculations on these five
organic dyes support the electrochemical data.
Received 15 October 2010
Received in revised form
26 December 2010
Accepted 6 January 2011
Available online 13 January 2011
Keywords:
Dye-sensitized solar cells
Porphyrin
© 2011 Elsevier B.V. All rights reserved.
Push–pull
Charge-separation
Aggregation
1. Introduction
in solar cells [8–17]. More recently, we have reported the design and
synthesis of novel donor–acceptor aryl/hetero-arylethyne bridged
Since past few years there has been considerable attention
in photo-active molecules to convert solar energy into electrical
power. Dye-sensitized solar cells (DSSCs) have been paid much
attention due to the relatively high solar energy conversion and
potentially low cost [1–3]. Organic or inorganic dye sensitizers
have been found as one of the most important tool for DSSCs to
get high conversion efficiency. In this connection, several research
groups have been devoted to explore the efficient dyes to improve
the cell performance. Commonly, transition polypyridine ruthe-
nium complexes such as N3 [1,4], N719 [1,4,5], and Z907 [6,7]
shows the overall conversion efficiency reaching up to 11% under
100 mW/cm2 illumination of simulated solar light. In recent years,
the interests in organic dyes as substitutes of noble metal com-
plexes are increasing due to many advantages, such as diversity
of molecular structures, high molar extinction co-efficient, simple
synthesis as well as low cost and environmental issues. Metal-free
organic dyes have been developed and showed good performance
dyes for photovoltaic performances of DSSCs [18].
Among all organic dyes, porphyrins and their derivatives have
been considered as potential photosensitizers in DSSCs because
they possess an intense Soret band at 400–450 nm and moderate
Q bands at 500–650 nm [19–25]. However, porphyrin dyes showed
low efficiency compared to Ru dyes for DSSCs due to their insuffi-
cient light-harvesting properties in the visible region. To improve
the performance of porphyrin dyes, porphyrins with extended ꢀ-
conjugation and efficient anchoring groups have been designed
and this led to better photo-voltaic performance. Porphyrin based
push–pull chromophores shows large conjugation and strong elec-
tronic interaction between chromophores as previously reported
[26–28], and this type of sensitizers can facilitate the direction of
electron injection, reduce the aggregation on a TiO2 surface, tune
the level of the excited state and increase the electron coupling
between the sensitizers and the semiconductor [11]. Based on this
hypothesis, we investigate a series of unsymmetrical trans-D–B–A-
porphyrin based dye molecules (D = donor, B = bridge, A = acceptor)
with different donating group substituent on the opposite side
of the acceptor to be used for dye sensitized solar cells (DSSCs).
Here, we report the synthesis, electrochemical and photovoltaic
properties of a series of metal-free porphyrin sensitizers based on
the donor–acceptor porphyrin chromophores and their molecular
structures were shown in Fig. 1.
∗
Corresponding author.
Corresponding author. Tel.: +65 6316 8901.
∗∗
E-mail addresses: dengwq@dicp.ac.cn (W.-Q. Deng), xuewei@ntu.edu.sg
(X.-W. Liu).
1
These authors contributed equally to this work.
1010-6030/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2011.01.003

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K.K. Pasunooti et al. / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 219–225
Fig. 1. Structures of the porphyrin dyes (Dyes 1–5).
2. Experimental
placed under high vacuum. Propionic acid (0.05 M) was added
to the reaction mixture under nitrogen atmosphere at room
temperature. This reaction mixture was heated to reflux and
5-phenyldipyrromethane 1 (2.25 mmol) in propionic acid (0.25 M)
was added slowly over 20 min. The mixture was refluxed for 2 h
and then the crude product was evaporated to dryness under
vacuum. The black solid was purified by column chromatography
on silica gel to give the desired porphyrin product.
2.1. Material and instrumentation
All chemicals were obtained from commercial sources and used
without further purification. Unless otherwise noted, all reactions
were carried out in oven-dried glassware under an atmosphere of
nitrogen. Solvents and reagents were purified according to the stan-
dard procedure prior to use. Product purification by flash column
chromatography was accomplished using silica gel 60 mesh. Tech-
nical grade solvents were used for chromatography and distilled
prior to use. NMR spectra were recorded at room temperature on
a 300 MHz Bruker ACF 300, 400 MHz Bruker DPX 400 and 500 MHz
Bruker AMX 500 NMR spectrometers, respectively. The residual sol-
vent signals were taken as the reference (7.26 ppm for 1H NMR
spectroscopy). Infrared spectra were recorded on a Bio-RAD FTS
165 FT-IR Spectrometer and reported in cm−1. Samples were pre-
pared in thin film technique. HRMS (ESI) spectra were recorded on
a Finnigan/MAT LCQ quadrupole ion trap mass spectrometer, cou-
pled with the TSP4000 HPLC system and the Crystal 310 CE system.
Absorption spectrums were measured on JASCO V-670 UV–Vis-
Near IR spectrophotometer and photoluminescence analyses were
performed on a Cary Eclipses fluorescence spectrometer.
2.3.1. 5,15-Bisphenyl-10-(4-nitro)phenyl-20-
(4-methoxycarbonyl phenyl)-porphyrin (6)
Purification of the crude product by flash column chromatogra-
phy on silica gel gave purple color solid compound 6 (240 mg, 15%):
1H NMR (500 MHz, CDCl3): ı in ppm = 8.88–8.85 (m, 6H), 8.79 (d,
J = 4.6 Hz, 2H), 8.44 (dd, J = 6.5 and 1.7 Hz, 2H), 8.30 (d, J = 1.3 Hz,
2H), 8.22 (dd, J = 7.6 and 1.4 Hz, 6H), 7.81–7.73 (m, 8H), 4.11 (s, 3H),
−2.77 (s, 2H). 13C NMR (100 MHz, CDCl3): ı in ppm = 167.3, 146.9,
141.9, 134.5, 134.5, 129.6, 127.9, 127.8, 126.7, 120.6, 118.9, 52.4.
IR (CHCl3): ꢁmax = 3030, 1660, 1210, 758 cm−1. LC–MS (ESI): (M+1):
718.
2.3.2. 5,10,15-Trisphenyl-20-(4-methoxycarbonylphenyl)
porphyrin (7)
Purification of the crude product by flash column chromatogra-
phy on silica gel gave purple color solid compound 7 (400 mg, 14%):
1H NMR (500 MHz, CDCl3): ı in ppm = 8.87–8.85 (m, 6H), 8.80 (d,
J = 4.6 Hz, 2H), 8.44 (d, J = 8.1 Hz, 2H), 8.31 (d, J = 8.1 Hz, 2H), 8.22
(dd, J = 7.6 and 1.4 Hz, 6H), 7.79–7.73 (m, 9H), 4.11 (s, 3H), −2.76
(s, 2H). IR (CHCl3): ꢁmax = 3025, 2200, 1653, 760 cm−1. HRMS (ESI):
m/z: calcd for C46H33N4O2: 673.2604 [M]+; found: 673.2608.
2.2. Synthesis
2.2.1. 2,2ꢀ-(Phenylmethylene)bis(1H-pyrrole) (1) [29]
Distilled pyrrole (60.0 mL, 866.0 mmol) and benzaldehyde
(3.50 mL, 34.6 mmol) were added to a dry round-bottomed flask
and degassed with a stream of nitrogen for 5 min. TFA (0.26 mL,
3.46 mmol) was add to the reaction mixture, which stirred under
nitrogen atmosphere at room temperature for 5 min and then
quenched with 0.1 M NaOH. The reaction mixture was extracted
with ethyl acetate. The combined organic phase was washed
with brine, dried over anhydrous NaSO4 and concentrated under
reduced pressure to provide a residue. It was purified by flash col-
umn chromatography to give intermediate 1 as a pale yellow solid.
2.3.3. 5,15-Bisphenyl-10-(4-methoxy)phenyl-20-(4-methoxy
carbonylphenyl)-porphyrin (8)
Purification of the crude product by flash column chromatog-
raphy on silica gel gave purple color solid compound 8 (240 mg,
15%): 1H NMR (500 MHz, CDCl3): ı in ppm = 8.91–8.80 (m, 6H),
8.81 (d, J = 4.3 Hz, 2H), 8.45 (d, J = 7.8 Hz, 2H), 8.32 (d, J = 7.8 Hz, 2H),
8.23 (d, J = 6.7 Hz, 4H), 8.14 (d, J = 8.1 Hz, 2H), 7.79–7.74 (m, 6H),
7.29 (d, J = 8.2 Hz, 2H), 4.12 (s, 3H), 4.09 (s, 3H), −2.73 (s, 2H). 13C
NMR (100 MHz, CDCl3): ı in ppm = 167.3, 159.4, 147.1, 142.1, 135.6,
134.6, 134.5, 134.4, 129.5, 127.9, 127.7, 126.7, 120.5, 120.3, 118.3,
2.3. General experimental procedure for porphyrins 6, 7, 8 and 9
Aldehyde (1.17 mmol) and methyl 4-formylbenzoate
(1.17 equiv) were added to a dry round-bottomed flask and
112.2, 55.5, 52.4. IR (CHCl3): ꢁmax = 3030, 2350, 1650, 758 cm−1
.

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221
HRMS (ESI): m/z: calcd for C47H35N4O3: 703.2709 [M]+; found:
703.2708.
(500 MHz, CDCl3): ı in ppm = 9.04–9.03 (m, 1H), 8.92–8.86 (m, 4H),
8.84 (d, J = 4.6 Hz, 2H), 8.58 (d, J = 7.9 Hz, 2H), 8.40 (d, J = 6.1 Hz, 2H),
8.27–8.25 (m, 5H), 8.11 (d, J = 5.1 Hz, 1H), 7.83–7.79 (m, 7H), 7.49
(d, J = 8.2 Hz, 1H), 7.45 (d, J = 4.2 Hz, 5H), 7.28 (d, J = 7.5 Hz, 4H),
7.27–7.16 (m, 2H), −2.72 (s, 2H). IR (CHCl3): ꢁmax = 3015, 2400,
1670, 1560, 1220 cm−1. HRMS (ESI): m/z: calcd for C57H40N5O2:
826.3182 [M]+; found: 826.3180.
2.3.4. 5,15-Bisphenyl-10-(4-diphenylamino)phenyl-20-
(4-methoxy carbonylphenyl)-porphyrin (9)
Purification of the crude product by flash column chromatog-
raphy on silica gel gave purple color solid compound 9 (500 mg,
13%): 1H NMR (500 MHz, CDCl3): ı in ppm = 9.03 (d, J = 4.6 Hz, 1H),
8.94–8.84 (m, 4H), 8.82–8.76 (m, 2H), 8.46 (d, J = 6.4 Hz, 2H), 8.33 (d,
J = 8.1 Hz, 2H), 8.25–8.24 (m, 4H), 8.10 (d, J = 6.6 Hz, 1H), 7.83–7.75
(m, 6H), 7.70 (d, J = 7.0 Hz, 1H), 7.48 (d, J = 6.6 Hz, 1H), 7.44 (d,
J = 3.4 Hz, 5H), 7.37 (t, J = 6.3 Hz, 2H), 7.21–7.15 (m, 4H), 7.04 (d,
J = 8.6 Hz, 1H), 4.12 (s, 3H), −2.71 (s, 2H). 13C NMR (100 MHz, CDCl3):
ı in ppm = 167.4, 147.8, 147.1, 142.1, 135.6, 134.6, 129.5, 127.9,
127.8, 126.7, 124.9, 123.3, 121.2, 120.3, 118.3, 52.4. IR (CHCl3):
ꢁmax = 3020, 2405, 1550, 1215, 755 cm−1. HRMS (ESI): m/z: calcd
for C58H42N5O2: 840.3339 [M]+; found: 840.3336.
2.4.5. 5,15-Bisphenyl-10-(4-diphenylamino)phenyl-20-
(4-hydroxy methylphenyl)-porphyrin (10)
Porphyrin 9 (50 mg, 0.06 mmol) was dissolved in 2 mL of THF at
0 ◦C. After addition of LiAlH4 (6.8 mg, 0.18 mmol), the mixture was
allowed to reach room temperature and stirred for 3 h. After being
quenched by water, the mixture was stirred for further 10 min.
The solvent was removed by using a rotary evaporator, and the
residue was dissolved in dichloromethane and passed through the
filter paper. The filtrate was washed with water and dried over
Na2SO4. Evaporation of the solvent afforded purple solid, which
was purified by silica gel column chromatography to give desired
alcohol 10 (46 mg, 95%) as a purple solid. 1H NMR (500 MHz,
CDCl3): ı in ppm = 9.02 (d, J = 4.4 Hz, 2H), 8.89–8.85 (m, 6H), 8.22
(dd, J = 12.0 and 7.4 Hz, 6H) 8.09 (d, J = 8.2 Hz, 2H), 7.79–7.73 (m,
8H), 7.47 (d, J = 8.3 Hz, 2H), 7.42–7.40 (m, 8H), 7.16–7.13 (m, 2H),
5.04 (d, J = 8.1 Hz, 2H), 1.97 (t, J = 5.6 Hz, 1H) −2.71 (s, 2H). 13C
NMR (125 MHz, CDCl3): ı in ppm = 147.8, 147.5, 142.2, 135.8, 135.6,
134.7, 130.9, 129.5, 127.7, 126.7, 125.3, 124.9, 123.3, 121.2, 120.3,
2.4. General experimental procedure for porphyrin Dyes 1–4
The desired porphyrin methyl ester (0.29 mmol) was dissolved
in 50 mL of isopropanol. KOH (2.9 mmol) dissolved in water
(0.02 M) was added to this solution and the reaction mixture was
refluxed for 6 h. After cooling to room temperature, the mixture
was neutralized with 1 M HCl solution and the resulting suspen-
sion was extracted with chloroform. The organic layer was washed
with water, dried and evaporated to dryness. The crude product
was purified by column chromatography on silica gel to give the
desired porphyrin product.
120.1, 65.4. IR (CHCl3): ꢁmax = 3020, 2390, 1558, 1530, 756 cm−1
IR (CHCl3): ꢁmax = 3018, 2399, 1558, 1506, 1215, 756 cm−1
.
.
HRMS (ESI): m/z: calcd for C57H42N5O: 812.3389 [M]+; found:
812.3387.
2.4.1. 5,10-Bisphenyl-10-(4-nitro)phenyl-20-(4-carboxymethyl
phenyl)-porphyrin (Dye 1)
2.4.6. 5,15-Bisphenyl-10-(4-diphenylamino)phenyl-20-(4-formyl
phenyl)-porphyrin (11)
Purification of the crude product by flash column chromatog-
raphy on silica gel gave compound Dye 1 (185 mg, 94%): 1H NMR
(500 MHz, CDCl3): ı in ppm = 8.88–8.85 (m, 6H), 8.79 (d, J = 4.6 Hz,
2H), 8.44 (dd, J = 6.5 and 1.7 Hz, 2H), 8.30 (d, J = 1.3 Hz, 2H), 8.22
(dd, J = 7.6 and 1.4 Hz, 6H), 7.81–7.73 (m, 8H), −2.77 (s, 2H). IR
Porphyrin 10 (30 mg, 0.04 mmol) was dissolved in 2 mL of
dichloromethane and cooled to 0 ◦C. After addition of MnO2 (6.3 mg,
0.07 mmol), the mixture was allowed to reach room temperature
and stirred for 6 h. The reaction mixture was then dissolved in
excess dichloromethane and passed through the celite. The filtrate
was washed with water and dried over Na2SO4. Evaporation of the
solvent afforded purple solid, which was purified by silica gel col-
umn chromatography to give desired aldehyde 11 (27 mg, 91%) as
a purple solid. 1H NMR (400 MHz, CDCl3): ı in ppm = 10.36 (s, 1H),
9.05 (d, J = 4.7 Hz, 2H), 8.91 (d, J = 4.7 Hz, 4H), 8.80 (d, J = 4.8 Hz, 2H),
8.42 (d, J = 5.2 Hz, 2H), 8.39–8.24 (m, 6H), 8.10 (d, J = 8.3 Hz, 2H),
7.83–7.75 (m, 6H), 7.49–7.47 (m, 10H), 7.19–7.14 (m, 2H), −2.68
(s, 2H). 13C NMR (100 MHz, CDCl3): ı in ppm = 192.4, 148.7, 147.8,
147.6, 142.0, 135.6, 135.2, 134.6, 129.5, 128.0, 127.8, 126.7, 125.0,
123.4, 121.1, 120.9, 120.5, 117.8. IR (CHCl3): ꢁmax = 3020, 2390,
1550, 1516, 1215, 756 cm−1. HRMS (ESI): m/z: calcd for C57H40N5O:
810.3233 [M]+; found: 810.3234.
(CHCl3): ꢁmax = 3018, 2399, 1670, 1521, 1261, 1215, 756 cm−1
.
HRMS (ESI): m/z: calcd for C45H30N5O4: 704.7447 [M]+; found:
704.7441.
2.4.2. 5,10,15-Trisphenyl-20-(4-carboxymethylphenyl)-
porphyrin (Dye 2)
Purification of the crude product by flash column chromatog-
raphy on silica gel gave porphyrin Dye 2 (180 mg, 92%): 1H NMR
(500 MHz, CDCl3): ı in ppm = 8.87–8.81 (m, 8H), 8.51 (d, J = 7.6 Hz,
2H), 8.35 (d, J = 7.7 Hz, 2H), 8.22 (d, J = 6.2 Hz, 2H), 8.22 (d, J = 6.2 Hz,
6H), 7.77–7.76 (m, 9H), −2.77 (s, 2H). IR (CHCl3): ꢁmax = 3010, 1670,
1530, 1260, 1210 cm−1. HRMS (ESI): m/z: calcd for C45H31N4O2:
659.2447 [M]+; found: 659.2435.
2.4.3. 5,10-Bisphenyl-10-(4-methoxy)phenyl-20-
2.4.7. 5,15-Bisphenyl-10-(4-diphenylamino)phenyl-20-[4-
(4-carboxymethyl phenyl)-porphyrin (Dye 3)
(2-carboxy-2-cynovinyl)phenyl]-porphyrin (Dye 5)
Purification of the crude product by flash column chromatog-
raphy on silica gel gave porphyrin Dye 3 (190 mg, 97%): 1H NMR
(500 MHz, CDCl3): ı in ppm = 8.91–8.80 (m, 6H), 8.81 (d, J = 4.3 Hz,
2H), 8.45 (d, J = 7.8 Hz, 2H), 8.32 (d, J = 7.8 Hz, 2H), 8.23 (d, J = 6.7 Hz,
4H), 8.14 (d, J = 8.1 Hz, 2H), 7.79–7.74 (m, 6H), 7.29 (d, J = 8.2 Hz,
2H), 4.09 (s, 3H), −2.73 (s, 2H). IR (CHCl3): ꢁmax = 3018, 2399, 1653,
1215, 758 cm−1. HRMS (ESI): m/z: calcd for C46H33N4O3: 689.2553
[M]+; found: 689.2551.
2-Cynoacetic acid (25 mg, 0.3 mmol) and piperidine (60 ␮L,
0.6 mmol) were dissolved in acetonitrile (2 mL) and placed under
nitrogen atmosphere at room temperature. Porphyrin 11 in ace-
tonitrile (1 mL) was then added to the reaction mixture and heated
to reflux for 4 h. After cooling to room temperature, the mixture
was washed with 2 M aqueous HCl and extracted with chloro-
form. The organic phase was collected and dried over Na2SO4. After
removal of solvent, the crude product was purified on the silica
gel column to afford the porphyrin Dye 5 as a purple solid (45 mg,
85%). 1H NMR (400 MHz, CDCl3): ı in ppm = 10.74 (bs, 1H), 8.88
(s, 2H), 8.74–8.71 (m, 6H), 8.40–8.38 (m, 2H), 8.30–8.28 (m, 2H),
8.15–8.10 (m, 4H), 7.97 (d, J = 8.0 Hz, 2H), 7.71–7.67 (m, 6H), 7.34
(d, J = 8.3 Hz, 2H), 7.71–7.67 (m, 8H), 7.03–6.99 (m, 2H), −2.75 (s,
2.4.4. 5,15-Bisphenyl-10-(4-diphenylamino)phenyl-20-
(4-carboxy methylphenyl)-porphyrin (Dye 4)
Purification of the crude product by flash column chromatog-
raphy on silica gel gave porphyrin Dye 4 (194 mg, 98%): 1H NMR

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2H). IR (CHCl3): ꢁmax = 3010, 2350, 1520, 756 cm−1. HRMS (ESI):
m/z: calcd for C60H41N6O2: 877.3291 [M]+; found: 877.3289.
the TiO2 working electrode to form the test cell. The two electrodes
separated by 50 ␮m thermal-plastic Suryln spacers were bonding
together, and the electrolyte (EL-HPE from dyesol) was introduced
between the electrodes by capillary action.
2.5. Calculation method
2.8. Photovoltaic characterization
All calculations were performed with the Gaussian 03 program
package [30]. The geometries of molecules in vacuo were optimized
using density functional theory (DFT) with the B3LYP density func-
tional and the 6-31G* basis set. The time-dependent DFT (TD-DFT)
at the level of B3LYP/6-31G* was carried out to study the excited
state of selected molecules [31]. The 20 lowest spin-allowed singlet
transitions were involved to simulate the absorption spectra. TD-
DFT calculations were performed in tetrahydrofuran (THF) solution
(C-PCM model) with geometries obtained in vacuo. The contri-
bution of singlet excited state configurations to each electronic
transition and the simulated absorption spectra of the porphyrin
analogues and graphical molecular orbital were generated with the
Gauss View software program (version 3.09) [30].
A Sun 2000 solar simulator light source (Abet-technologies,
U.S.A.) was used to give an irradiance of 100 mW cm−2 (the equiv-
alent of one sun at air mass (AM) 1.5) at the surface of solar cells.
The current–voltage characteristics of the cell under these condi-
tions were obtained by applying external potential bias to the cell
and measuring the generated photocurrent with a Keithley model
2440 digital source meter (Keithley, U.S.A.). This process was fully
automated using lab tracer software.
2.9. Electrochemistry
Eco Chemie Autolab PGSTAT 100 with an ADC fast scan genera-
tor. Working electrodes were 1 mm diameter planar Pt disks, used
in conjunction with a Pt auxiliary electrode and an Ag/Ag+ reference
electrode was connected to the test solution via a salt bridge con-
taining 0.2 M Bu4NPF6 in THF. Accurate potentials were obtained
using ferrocene as an internal standard. All potentials reported
herein are vs. FeCp2/FeCp2+ = 0.41 V.
2.6. Electrochemistry
Electrochemistry was characterized with Eco Chemie Autolab
PGSTAT 100 with an ADC fast scan generator. Planar Pt disks
(1 mm diameter) were used as working electrodes in conjunc-
tion with a Pt auxiliary electrode. An Ag/Ag+ reference electrode
was connected to the test solution via a salt bridge containing
0.2 M Bu4NPF6 in THF. Accurate potentials were obtained using fer-
rocene as an internal standard. All potentials reported herein are
vs. FeCp2/FeCp2+ = 0.41 V.
3. Results and discussion
3.1. Synthesis of porphyrin dyes
2.7. Preparation of porphyrin sensitized TiO2 electrode
The structures of the five porphyrin dye sensitizers were pre-
sented in Fig. 1. And the synthetic pathway of porphyrin Dyes
1–5 was shown in Scheme 1. The syntheses involve two important
steps to get desired porphyrin dyes such as (1) condensation of an
appropriate aldehydes with dipyrrophenylmethane unit to afford
the porphyrins as precursors, and (2) subsequent hydrolysis of the
precursor porphyrins with KOH afforded the corresponding por-
phyrin Dyes 1–4 [29]. Porphyrins (6, 7, 8 and 9) were synthesized
by employing modified Aldler–Longo method [32]. This method
involves cross condensation of phenyl dipyrromethane (1) with
different substituted aldehydes (2–5) followed by basic hydrolysis
afforded the desired products (Dyes 1–4). Synthesis of the acrylic
acid derivative Dye-5 was achieved via LiAlH4 reduction of ester 9 to
alcohol 10 using standard procedure. MnO2 oxidation of 10 to alde-
Conducting glasses (SnO2:F, FTO, 15 ꢂ/sq, Solaronix) were
washed with 0.1 M HCl and isopropyl alcohol and then the glass
was treated with 40 mM TiCl4 aqueous solution at 70 ◦C for
40 min following a literature procedure. The 10.0 ␮m thick films of
nanocrystalline TiO2 (Ti-Nanoxide T/SP, Solaronix) were deposited
onto the side of conducting layer of FTO by doctor blading. TiO2
substrates were sintered at 500 ◦C for 30 min in air and then in
immersed 40 mM TiCl4 aqueous solution at 70 ◦C for 40 min. Then
the TiO2 plates◦were dipped into 0.5 mM dye solution in THF and
˚
left for 2 h at 50 C, and taken out and rinsed with methanol. A 400 A
Pt fabricated by e-beam evaporation on a commercial indium tin
oxide glass as the counter electrode was then clipped onto the top of
Scheme 1. Synthesis of porphyrin dyes (Dyes 1–5).

K.K. Pasunooti et al. / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 219–225
223
hyde 11, followed by subsequent condensation with cyanoacetic
acid to give the desired porphyrin product (Dye-5). These porphyrin
dyes were fully characterized by spectroscopic analyses including
NMR, FT-IR and mass spectra.
portion of the highest occupied molecular orbital (HOMO) is located
on the donor moiety and the lowest unoccupied molecular orbital
(LUMO) is located on the acceptor and the bridge moieties. The
HOMO position can be adjusted by introducing different donor
groups; and LUMO can be tuned by installing an acceptor group
on the trans position of porphyrin ring. Therefore, the band gap of
the dye molecule can be reduced through structural modification.
This allows broad absorption bands and easy intramolecular charge
separation, which eventually improve the high cell performance
of DSSCs. As shown in Fig. 4, the calculated HOMO energy level
increase in the order –NO2 < –H < –OMe < –NPh2, and same as LUMO
energy level increase in the order of –COOHꢁ –C C(CN)COOH. These
results imply the formation of a charge-separation between the
donor and acceptor during photo-induced electron transfer. In
addition, For the five dye sensitizers, the HOMO level higher than
the reduction potential of electrolyte (I−/I3−, +0.5 V vs. NHE) while
LUMO level should be more negative than the conduction band of
TiO2 (−0.5 V vs. NHE), which indicate that these molecules are suit-
able as sensitizers in DSSCs. The TD-DFT calculation results of these
Dyes 1–5 are shown in Table 2 and Table S2. The excitation energies
of them indicated that introduction of donor group such as Ph2N
group, acceptor group such as nitro-group and cyanoacetic acid into
the dye molecule can significantly increase the oscillator strength
of the Soret band and Q-band. In addition, the donor and acceptor
moieties also help to decrease the HOMO–LUMO band-gap, which
enhances the solar spectral overlap. The two dominating excitation
states, originated from HOMO → LUMO and HOMO → LUMO + 1
(Dyes 2–5) and HOMO → LUMO and HOMO − 1 → LUMO (Dye 1),
and their intensities are primarily coincident with the experimental
absorptions of these five compounds as shown in Fig. 2a.
3.2. Absorption properties in solution and on TiO2 film
The UV–vis spectra of the porphyrins (Dyes 1–5) in THF are
shown in Fig. 2 and the related absorption maxima and extinction
coefficients for the Soret- and Q-bands of the porphyrins are pro-
vided in Table S1. The bands between 400 and 650 nm are due to
ꢀ–ꢀ* absorptions of the conjugation. The dye with a cyanoacrylic
acid anchoring group (Dye 5) shows broader absorption than the
carboxylic ones (Dyes 1–4) that are red-shifted with respect to the
Soret- and Q-bands of porphyrins. The absorption spectra of films
show that there are some dyes aggregated on the surface of TiO2
nanoparticles in accordance with the reported results [9,11]. The
emission spectra of porphyrins Dyes 1–5 shown in Fig. 3, in THF.
The emission maxima (Table S1) show characteristic vibronic bands
for Dyes 1–5 at around 630 and 675 nm similar to those reported
for other porphyrins [19,26].
3.3. Photophysical properties
In this article, we performed DFT and time-depended den-
sity functional theory (TD-DFT) calculations to investigate the
intramolecular charge separation of the newly synthesized organic
dyes (Dyes 1–5). As shown in Fig. 4, in all dye molecules the major
The oxidation potential of the Dyes 1–5 was determined with
cyclic voltammetry. The measurements were performed in THF
solution with 0.2 M tetrabutyl ammonium hexafluorophosphate
(Bu4NPF6). Ferrocene was used as an internal standard at 0.63 V vs.
NHE (Figure S1). The oxidation potential of Dyes 1–5 is 1.09, 1.17,
1.16, 1.21 and 1.22 V vs. NHE respectively, which are more posi-
tive than that of I−/I3 couple (0.5 V vs. NHE) [28]. These results
−
ensure the regeneration of sensitizer radical cations by the elec-
trolyte redox couple. The excited state oxidation potential of Dyes
1–5 is −0.95, −0.87, −0.86, −0.70 and −0.69 V respectively, which
are more negative than the conduction band (CB) of TiO2 (−0.5 V
vs. NHE). Thereby, this provides enough driving force to inject elec-
trons from the excited porphyrin singlet state to the conduction
band (CB) of TiO2 (Table S3) [13,33].
Compound 1
Compound 2
Compound 3
Compound 4
Compound 5
600
650
700
750
800
Wavelength (nm)
Fig. 2. (a) UV–visible absorption spectra of Dyes 1–5 in THF solution. (b) Adsorbed
on TiO2.
Fig. 3. Emission spectra of Dyes 1–5 in THF solution.

224
K.K. Pasunooti et al. / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 219–225
Fig. 4. Molecular orbital diagrams of Dyes 1–5.
Table 1
-20
J–V characteristics of the porphyrin sensitizers and Z907 sensitized solar cell with
Compound 1
Compound 2
Compound 3
Compound 4
Compound 5
Z907
ionic liquid electrolyte under AM 1.5 irradiation condition.
Electrode
Jsc (mA/cm2)
Voc (V)
ff (%)
ꢃ (%)
-15
-10
-5
Dye 1
Dye 2
Dye 3
Dye 4
Dye 5
Z907
2.23
2.90
4.66
4.95
7.63
16.75
0.542
0.553
0.563
0.605
0.613
0.686
54.9
55.5
54.9
50.8
51.0
58.9
0.66
0.89
1.44
1.52
2.38
6.77
The performance of the dye sensitized solar cells (DSSCs) was
measured by incident photon-to-current conversion efficiency
(IPCE) and photocurrent–voltage (J–V) measurements. Fig. 5 shows
the photocurrent–voltage (J–V) curve for DSSCs based on five por-
phyrin dyes (Dyes 1–5). The short-circuit current densities (Jsc),
open-circuit voltages (Voc), fill factors (ff) and power conversion
efficiencies (ꢃ) of the cells are summarized in Table 1. The Dye 5
gave the best performance, the conversion efficiency (ꢃ) arrives
at 2.38% with Jsc 7.63 mA/cm2, Voc 0.613 V, ff 51.0%, which reaches
35% with respect to Z907-based device fabricated under the simi-
lar conditions. The highest Jsc compared with other devices in this
study is attributed to the high extinction coefficient as shown in
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Voltage (V)
Fig. 5. J–V characteristics of Dyes 1–5 and Z907.
tively suppress the dye-aggregation [34]. Otherwise, the increase
of the intramolecular charge separation maybe benefit for the
Fig. 2a. As we know, the aggregation of the dye could decrease Voc
,
increase of Voc [35]. Accordingly, the Dye 5 gave the largest Voc
.
4-methoxy-triphenylamine moiety as donating group could effec-
Therefore, the performance of these dyes is improved due to the
Table 2
Computed excitation energies (eV), oscillation strengths (f) and two highest electronic transition configurations for the optical transitions below 3.2 eV for Dye 2 in THFa.
No.
1
Transition energies, nm
575.5
Excitation energies, eVb
2.15
Oscillator strengths, f
0.0470
Main configurations H = HOMO, L = LUMO, L + 1 = LUMO + 1, etc.
H − 0 → L + 0 (+44%); H − 1 → L + 1 (+24%)
H − 0 → L + 1 (+23%); H − 1 → L + 0 (+13%)
H − 0 → L + 1 (+40%); H − 1 → L + 0 (+25%)
H − 0 → L + 0 (+22%); H − 1 → L + 1 (+12%)
H − 1 → L + 1 (+35%); H − 0 → L + 0 (+12%)
H − 1 → L + 0 (+11%)
2
3
5
6
7
540.8
408.1
402.3
371.2
368.4
2.29
3.04
3.08
3.34
3.37
0.0663
1.4710
1.6885
0.0285
0.0235
H − 1 → L + 0 (+34%); H − 0 → L + 1 (+15%)
H − 1 → L + 1 (+11%)
H − 0 → L + 2 (+41%); H − 2 → L + 0 (+36%)
H − 2 → L + 1 (+15%)
H − 0 → L + 2 (+48%); H − 2 → L + 0 (+27%)
H − 2 → L + 1 (+14%)
a
Calculated using TD-B3LYP/6-31G*//B3LYP/6-31G* and C-PCM framework. See Table S2 for more data on Dyes 1, 3–5 in supporting information.
Excitation energies with oscillation strengths having <0.01 eV are ignored.
b

K.K. Pasunooti et al. / Journal of Photochemistry and Photobiology A: Chemistry 218 (2011) 219–225
225
increase of the photocurrent and open circuit voltage. We believe
that intramolecular charge separation and broad absorption bands
with high extinction coefficient is responsible for the significant
enhancement of the conversion efficiency.
[11] Y. Wang, W.Q. Deng, X.-W. Liu, X. Wang, Int. J. Hydrogen Energy 34 (2009)
1437–1443.
[12] S. Hwang, J.H. Lee, C. Park, H. Lee, C. Kim, M.H. Lee, W. Lee, J. Park, K. Kim, N.G.
Park, Chem. Commun. 46 (2007) 4887–4889.
[13] D.P. Hagberg, T. Edvinsson, T. Marinado, G. Boschloo, A. Hagfeldt, L.C. Sun, Chem.
Commun. 21 (2006) 2245–2247.
[14] G.E. Collis, W.M. Campbell, D.L. Officer, A.K. Burrell, Org. Biomol. Chem. 3 (2005)
2075–2084.
4. Conclusion
[15] K. Hara, M. Kurashige, S. Ito, A. Shinpo, S. Suga, K. Sayama, H. Arakawa, Chem.
Commun. 2 (2003) 252–253.
[16] K. Hara, K. Sayama, Y. Ohga, A. Shinpo, S. Suga, H. Arakawa, Chem. Commun. 6
(2001) 569–570.
[17] A.K. Burrell, W.M. Campbell, D.L. Officer, S.M. Scott, K.C. Gordon, M.R. McDonald,
J. Chem. Soc. Dalton Trans. 19 (1999) 3349–3354.
[18] J.-L. Song, P. Amaladass, S.-H. Wen, K.K. Pasunooti, A. Li, Y.-L. Yu, X. Wang, W.-Q.
Deng, X.-W. Liu, New J. Chem. 35 (2011) 127–136.
[19] M. Tanaka, S. Hayashi, S. Eu, T. Umeyama, Y. Matano, H. Imahori, Chem. Com-
mun. 20 (2007) 2069–2071.
[20] M.K. Nazeeruddin, R. Humphry-Baker, D.L. Officer, W.M. Campbell, A.K. Burrell,
M. Gratzel, Langmuir 20 (2004) 6514–6517.
[21] H. Imahori, S. Hayashi, H. Hayashi, A. Oguro, S. Eu, T. Umeyama, Y. Matano, J.
Phys. Chem. C 113 (2009) 18406–18413.
[22] S. Hayashi, M. Tanaka, H. Hayashi, S. Eu, T. Umeyama, Y. Matano, Y. Araki, H.
Imahori, J. Phys. Chem. C 112 (2008) 15576–15585.
In conclusion, we have successfully designed and synthesized a
series of trans-D–B–A-porphyrin based dyes with different donor
and acceptor groups to fabricate dye-sensitized solar cells. The
results of calculation and experiments clearly demonstrate that the
band gap can be regulated by increasing the intramolecular charge-
separation and introducing different donor and acceptor. Among
the five synthesized dye molecules, porphyrin Dye-5 exhibits the
maximum power conversion efficiency (2.38%). Therefore, opti-
mization of these factors is a valid way to improve the performance
of solar cells, which provides a revelation to exploit highly efficient
porphyrin sensitizers for dye-sensitized solar cell.
[23] W.M. Campbell, K.W. Jolley, P. Wagner, K. Wagner, P.J. Walsh, K.C. Gordon, L.
Schmidt-Mende, M.K. Nazeeruddin, Q. Wang, M. Gratzel, D.L. Officer, J. Phys.
Chem. C 111 (2007) 11760–11762.
Acknowledgments
[24] E.E. Bonfantini, A.K. Burrell, W.M. Campbell, M.J. Crossley, J.J. Gosper, M.M.
Harding, D.L. Officer, D.C.W. Reid, J. Porphyrins Phthalocyanines 6 (2002)
708–719.
[25] Q. Wang, W.M. Campbell, E.E. Bonfantini, K.W. Jolley, D.L. Officer, P.J. Walsh, K.
Gordon, R. Humphrybaker, M.K. Nazeeruddin, M. Gratzel, D.L. Officer, J. Phys.
Chem. B 109 (2005) 15397–15409.
Authors would like to acknowledge the financial support from
Nanyang Technological University (RG50/08) and the Ministry of
Education (ARC24/07, no. T206B1218RS), Singapore.
Appendix A. Supplementary data
[26] N.K. Subbaiyan, L. Obraztsov, C.A. Wijesinghe, K. Tran, W. Kutner, F. D’Souza, J.
Phys. Chem. C 113 (2009) 8982–8989.
[27] N. Kristian, Y.L. Yu, P. Gunawan, R. Xu, W.-Q. Deng, X.-W. Liu, X. Wang, Elec-
trochim. Acta 54 (2009) 4916–4924.
[28] 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, C.Y. Yeh, Chem. Eur. J. 15 (2009) 1403–1412.
[29] J.K. Laha, S. Dhanalekshmi, M. Taniguchi, A. Ambroise, J.S. Lindsey, Org. Process
Res. Dev. 7 (2003) 799–812.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jphotochem.2011.01.003.
References
[30] (a) M.J. Frisch, et al., Gaussian 03, Revision C.02, Gaussian, Inc., Wallingford, CT,
2004;
[1] B. Oregan, M. Gratzel, Nature 353 (1991) 737–740.
[2] A. Mishra, M.K.R. Fischer, P. Bauerle, Angew. Chem. Int. Ed. 48 (2009)
2474–2499.
(b) R. Dennington, T. Keith, J. Millam, K. Eppinnett, W.L. Hovell, R. Gilliland,
GaussView, Semichem, Shawnee Mission, KS, 2003.
[3] W.M. Campbell, A.K. Burrell, D.L. Officer, K.W. Jolley, Coord. Chem. Rev. 248
(2004) 1363–1379.
[31] S.I. Gorelsky, B.P. Lever, J. Organomet. Chem. 635 (2001) 187–196.
[32] A.D. Adler, F.R. Longo, J.D. Finarell, J. Goldmach, J. Assour, L. Korsakof, J. Org.
Chem. 32 (1967) 476–478.
[33] D.P. Hagberg, J.H. Yum, H. Lee, F. De Angelis, T. Marinado, K.M. Karlsson, R.
Humphry-Baker, L.C. Sun, A. Hagfeldt, M. Gratzel, M.K. Nazeeruddin, J. Am.
Chem. Soc. 130 (2008) 6259–6266.
[4] H. Imahori, T. Umeyama, S. Ito, Acc. Chem. Res. 42 (2009) 1809–1818.
[5] M.K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphrybaker, E. Muller, P. Liska, N.
Vlachopoulos, M. Gratzel, J. Am. Chem. Soc. 115 (1993) 6382–6390.
[6] Q.J. Yu, S. Liu, M. Zhang, N. Cai, Y. Wang, P. Wang, J. Phys. Chem. C 113 (2009)
14559–14566.
[7] P. Wang, S.M. Zakeeruddin, J.E. Moser, M.K. Nazeeruddin, T. Sekiguchi, M.
Gratzel, Nat. Mater. 2 (2003) 402–407.
[8] H. Choi, I. Raabe, D. Kim, F. Teocoli, C. Kim, K. Song, J.H. Yum, J. Ko, M.K.
Nazeeruddin, M. Gratzel, Chem. Eur. J. 16 (2010) 1193–1201.
[9] G. Zhang, H. Bala, Y. Cheng, D. Shi, X. Lv, Q. Yu, Chem. Commun. 16 (2009)
2198–2200.
[34] (a) N. Satoh, T. Nakashima, K. Yamamoto, J. Am. Chem. Soc. 127 (2005)
13030–13038;
(b) N. Satoh, J.S. Cho, M. Higuchi, K. Yamamoto, J. Am. Chem. Soc. 125 (2003)
8104–8105;
(c) Q. Wang, S.M. Zakeeruddin, J. Cremer, P. Bauerle, R. Humphry-Baker, M.
Grätzel, J. Am. Chem. Soc. 127 (2005) 5706–5713.
[35] Y. Ooyama, Y. Harima, Eur. J. Org. Chem. (2009) 2903–2934.
[10] W. Zeng, Y. Cao, Y. Bai, Y. Wang, Y. Shi, M. Zhang, F. Wang, C. Pan, P. Wang,
Chem. Mater. 22 (2010) 1915–1925.