Multi-alkylthienyl appended porphyrins for efficient dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2011.05.017

Four porphyrin dyes, incorporating multi-alkylthienyl appended porphyrins as the electron donor, the 2-cyanoacrylic acid as the electron acceptor, and different π-conjugated spacer, have been synthesized for dye-sensitized solar cells (DSSCs). All the porphyrin dyes studied in this work exhibit red-shifted and broadened electronic spectra respect to the reference P_Zn as expected. By the introduction of thienyl groups at the meso-positions, the energy level of E_ox (excited-state oxidation potentials) is significantly shifted to the positive compared with the reference P _Zn, indicating a decreased HOMO-LUMO gap. The highest power conversion efficiency of the four dyes based on DSSCs reached 5.71% under AM 1.5 G irradiation.

Full text of DOI:10.1016/j.dyepig.2011.05.017

Dyes and Pigments 91 (2011) 404e412
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Multi-alkylthienyl appended porphyrins for efficient dye-sensitized solar cells
,
,
Weiping Zhou a, Bin Zhao a b, Ping Shen a b, Shenghui Jiang a, Hongduan Huang a,
Lijun Deng a, Songting Tan a
,b,
*
a College of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan 411105, PR China
b Key Laboratory of Advanced Functional Polymeric Materials, College of Hunan Province, Xiangtan University, Xiangtan 411105, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four porphyrin dyes, incorporating multi-alkylthienyl appended porphyrins as the electron donor, the
Received 1 February 2011
Received in revised form
15 May 2011
Accepted 16 May 2011
Available online 2 June 2011
2-cyanoacrylic acid as the electron acceptor, and different
p-conjugated spacer, have been synthesized
for dye-sensitized solar cells (DSSCs). All the porphyrin dyes studied in this work exhibit red-shifted and
broadened electronic spectra respect to the reference PZn as expected. By the introduction of thienyl
groups at the meso-positions, the energy level of Eox (excited-state oxidation potentials) is significantly
shifted to the positive compared with the reference PZn, indicating a decreased HOMOeLUMO gap. The
highest power conversion efficiency of the four dyes based on DSSCs reached 5.71% under AM 1.5 G
irradiation.
Keywords:
Multi-alkylthienyl porphyrins
Thiophene derivatives
Red-shifted
Ó 2011 Elsevier Ltd. All rights reserved.
Power conversion efficiency
Dye-sensitized solar cells
Photovoltaic performance
1. Introduction
from those of ruthenium polypyridyl complexes [7]. 3) Metal
porphyrins undergo facile reduction and oxidation [8], so their
Increasing energy demand pushes human beings to find more
and more renewable energy sources. Solar energy plays a key role
as a renewable, clean and inexhaustible resource [1]. From the
breakthrough by Grätzel and co-workers [2], the research on dye-
sensitized solar cells (DSSCs) has intensified in recent years [3,4].
Nowadays, DSSCs based on the ruthenium sensitizers have ach-
optical, photophysical, and electrochemical properties can be
systematically tailored by the peripheral substitutions and/or inner
metal complexations. Porphyrins, however, generally show inferior
performance due to the limited light absorption, poor matching to
solar light distribution, and consequently possess a low value of
short circuit current. To achieve higher
h, two methods have been
ieved power conversion efficiencies (h) of 10e11% [5]. However,
studied: elongating the -system and lowering the symmetry of
p
ruthenium dyes are expensive due to the cost of ruthenium and the
typically lengthy purification steps involved in their preparation.
Alternative low cost and readily accessible ruthenium dye
replacements are under active investigation. Porphyrins are one of
the alternative dyes used as sensitizers with many advantages as
follows. 1) The porphyrins derived from chlorophylls which are the
key components of natural photosynthetic systems in green plants
[6] and absorb strongly in the range of 400e700 nm with high
molar absorbance coefficient. 2) Porphyrin dyes have been
demonstrated to possess charge-transfer kinetics indistinguishable
the macro-cycle [8], which can lead to a significantly broadened
Soret-band and a red-shifted Q-band absorptions. A DSSC device
using porphyrin sensitizers with the linker and anchor units
functionalized at the b-position is reported to show a h as high as
7.1% [9]. Diau et al. synthesized a series of dark-green porphyrin
sensitizers with phenylethylnyl as linkers and carboxyl groups as
anchors at the meso-position with the
h in the range from 2.1% to
11% in DSSCs [10].
In recent years, interest in meso-tetrathienylporphyrins is
growing due to their use as models for energy transfer reactions
[11]. In almost all porphyrin structures applied in DSSCs, the
substituents at the meso-positions are phenyl units. However, the
smaller five-membered 2-thienyl rings offer reduced steric
hindrance compared with the larger six-membered 4-phenyl rings.
* Corresponding author. College of Chemistry, Key Laboratory of Environmentally
Friendly Chemistry and Applications of Ministry of Education, Xiangtan University,
Xiangtan 411105, PR China.
In addition, the 2-thienyl group lacks one o-phenyl-H to
b-pyrrole-
E-mail address: tanst2008@163.com (S. Tan).
H interaction which allows for greater easy of rotation of the
0143-7208/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2011.05.017

W. Zhou et al. / Dyes and Pigments 91 (2011) 404e412
405
thien-2-yl porphyrins, thus allowing the thienyl groups to adopt
a coplanar arrangement to the porphyrin ring [12]. Moreover, it is
known that the attachment of thienyl units usually can induce
a bathochromic shift, an intensification of the absorptivity and an
increased lifetime of the excited-state [13]. Until now, there are few
investigations of meso-tetrathienylporphyrins applied in DSSCs. In
this study, we designed and synthesized a series of multi-thienyl
appended porphyrin dyes (Z1eZ4) with DepeA structure (see
Fig. 1) applied in DSSCs. For comparison, the structure of the
reference porphyrin dye PZn is also shown in Fig. 1. The thiophene
were measured on a PerkineElmer Lambda 25 spectrophotometer.
The PL spectra were obtained using PerkineElmer LS-50 lumines-
cence spectrophotometer. MALDIeTOF mass spectrometric
measurements were performed on Bruker Autoflex III. Electro-
chemical redox potentials were obtained by cyclic voltammetry
(CV) and differential pulse voltammetry (DPV) using a three-
electrode configuration and an electrochemistry workstation
(CHI660A, Chenhua Shanghai). The working electrode was a glassy
carbon electrode; the counter electrode was a Pt electrode, and
saturated calomel electrode (SCE) was used as a reference elec-
trode. Tetrabutylammonium perchlorate (TBAP) 0.1 M was used as
the supporting electrolyte in dry DMF. Ferrocene was added to each
sample solution at the end of the experiments, and was used as an
internal potential reference [14].
segments with varied substituents employed as bridges (
linking the porphyrin donors (D) and anchoring and acceptor
groups (A), as well as extending the -conjugated bridges. Three
p) for
p
5-alkylthien-2-yl groups are introduced to improve the electronic
spectra and to tune the redox potential. Further, the alkyl groups
can induce steric hindrance around the porphyrin core to reduce
the aggregation between the neighboring porphyrins adsorbed
onto the TiO2 surface.
2.3. General procedure for preparation of porphyrin-modified TiO2
electrode and photovoltaic measurements
The TiO2 suspension was prepared from P25 (Degussa AG,
Germany) and 1 wt% magnesium acetate solution on following
a literature procedure [15]. The suspension was deposited on
a transparent conducting glass by using a doctor blade technique.
The film was sintered at 450 ꢀC for 30 min, then treated with
40 mM TiCl4 aqueous solution at 70 ꢀC for 30 min and annealed
again at 450 ꢀC for 30 min. After the film was cooled to room
temperature, it was immersed into 5.0 ꢁ 10ꢂ4 M dye solution for
15 min in the dark. The sensitized electrode was then rinsed with
ethanol, and dried. One drop of electrolyte solution was deposited
onto the surface of the electrode and penetrated inside the
TiO2 film via capillary action. The photovoltaic measurements
were performed in a sandwich cell consisting of the porphyrin-
sensitized TiO2 electrode as the working electrode and a Pt foil as
the counter electrode. The electrolyte consists of 0.5 M LiI, 0.05 M I2,
and 0.5 M 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile.
The photocurrent-voltage (JeV) characteristics were recorded
on Keithley 2602 Source meter. Porphyrin dyes sensitized TiO2
electrodes were measured under simulated AM 1.5 irradiation
2. Experimental section
2.1. Materials and reagents
All starting materials were purchased from commercial
suppliers (Pacific ChemSource and Alfa Aesar Corp.) at analytical
grade. THF and toluene were distilled from sodiumebenzophenone
prior to use. DMF was dried and distilled under reduced pressure.
POCl3 and 1,2-dichloroethane were achieved by atmospheric
distillation. All other solvents and chemicals used in this work were
analytical grade without further purification. All chromatographic
separations were carried out on silica gel (200e300 mesh).
2.2. Analytical instruments
FT-IR spectra were measured on a PerkineElmer Spectra One
spectrophotometer. 1H and 13C NMR spectra were recorded with
a Bruker Avance 400 instrument. UVevisible spectra of the dyes
Fig. 1. Molecular structures of the porphyrin dyes Z1eZ4 and PZn
.

406
W. Zhou et al. / Dyes and Pigments 91 (2011) 404e412
Fig. 2. Synthetic routes to the porphyrin dyes. Reagents and conditions: I, POCl3, DMF, 1,2-dichloroethane; II, Mg, I2, THF, DMF; III, propionic acid; IV, Zn(OAc)2, CH3OH, CHCl3;
V, cyanoacetic acid, piperidine, CH3CN.
ꢀ
ꢁ
ꢁ
(100 mW cmꢂ2). The power conversion efficiency (
h) of the DSSCs is
calculated from short-circuit photocurrent (Jsc), the open-circuit
photovoltage (Voc), the fill factor (ff) and the intensity of the inci-
dent light (Pin) according to the following equation:
Jsc mA cmꢂ2
1240
ꢀ
IPCEð
l
Þ ¼
ꢁ
l
ðnmÞ
F
mW cmꢂ2
Where Jsc is the short-circuit photocurrent density generated by
ꢀ
ꢁ
monochromatic light,
l
is the wavelength of incident mono-
Jsc mA cmꢂ2 ꢁ VocðVÞ ꢁ ff
chromatic light, and
F is the incident light intensity.
ꢀ
ꢁ
h
¼
Pin mW cmꢂ2
2.4. Synthesis
The incident photon-to-current conversion efficiency (IPCE)
values are plotted as a function of the excited wavelength and
defined according to the following equation [16]:
5-Hexylthiophene-2-carbaldehyde (1-a), 5-methylthiophene-2-
carbaldehyde (2-a), 3,4-ethylenedioxylthienyl-2-carbaldehyde (4-a),

W. Zhou et al. / Dyes and Pigments 91 (2011) 404e412
407
were synthesized according to the methods reported in the literature
[17] with some modification. The synthetic routes to the four dyes are
shown in Fig. 2. The detailed synthetic procedures are as follows.
(0.073 g, 0.87 mmol) and piperidine (0.5 mL) were added sequen-
tially under Ar atmosphere, the mixture was heated under reflux for
16 h. After cooling to room temperature, the solution was extracted
with dichloromethane, and washed with H2O and 0.1 M HCl. The
organic layer was dried over anhydrous MgSO4. Solvent was
removed by rotary evaporation, and the residue was purified by
silica gel column chromatography with methanol/dichloromethane
(1/20) as eluent to yield a purple solid Z1 (0.11 g, 57%). FT-IR (KBr,
vmax/cmꢂ1): 2977, 2927, 2347, 2219,1651,1488,1400,1068, 989, 794,
703, 666. UVevis lmax (CHCl3)/nm: 434, 559. 1H NMR (DMSO,
2.4.1. 5-(Thien-2-yl)-10,15,20-tris(5-hexylthien-2-yl)porphyrin (1-b)
In
a 250 mL three-necked round-bottomed flask, 5-hexyl
thiophene-2-carbaldehyde (1-a) (7.2 g, 36 mmol), thiophene-2-
carbaldeyde (1.37 g, 12 mmol) were dissolved in propionic acid
(200 mL) and the solution was heated under reflux at 140 ꢀC.Pyrrole
(3.21 g, 48 mmol) was added dropwise and stirred for another 30 min.
After cooling to room temperature, the solvent was evaporated and
the crude product was separated by column chromatography
(petroleum ether/dichloromethane ¼ 3/1 as an eluent). After recrys-
tallization from CH3OH, the desired bright purple solid compound 1-b
400 MHz, d/ppm): 9.06e9.00 (m, 8 H, pyrrolic-H), 8.58 (s,1 H, vinyl-
H), 8.31 (s, 1 H, TheH), 8.11 (s, 1 H, TheH), 7.74 (d, 3 H, TheH), 7.28
(d, 3 H, TheH), 3.12 (t, 6 H, eCH2), 1.89 (t, 6 H, eCH2), 1.56e1.23 (m,
18 H, eCH2), 0.94 (m, 9 H, eCH3).13C NMR (DMSO,100 MHz,
d/ppm):
was obtained (1.2 g, 11.2%). 1H NMR (CDCl3, 400 MHz,
d/ppm):
151.1, 151.0, 150.9, 150.1, 148.2, 140.7, 133.7, 132.6, 132.0, 131.6, 124.2,
113.9, 113.4, 31.7, 31.5, 30.0, 28.7, 22.5, 14.3. MALDI-TOF MS
(C58H57N5O2S4Zn) m/z: calcd for 1047.26; found 1047.49.
9.09e9.02 (d, 8 H, pyrrolic-H), 7.91 (d, 1 H, TheH), 7.86e7.84 (d, 1 H,
TheH), 7.69 (d, 3 H, TheH), 7.50 (d, 1 H, TheH), 7.17 (d, 3 H, TheH),
3.15e3.11 (t, 6 H, eCH2), 1.99e1.92 (m, 6 H, eCH2), 1.61e1.58 (m, 6 H,
eCH2), 1.45 (m, 12 H, eCH2), 0.99e0.96 (t, 9 H, eCH3), ꢂ2.61 (s, 2 H,
2.4.5. 5-(Thien-2-yl)-10,15,20-tris(5-methylthien-2-yl)porphyrin
(2-b)
eNH). 13C NMR (CDCl3, 100 MHz,
d/ppm):148.9, 140.1, 140.0, 133.8,
133.6, 131.1, 127.8, 126.0, 123.3, 113.0, 112.9, 31.9, 31.7, 30.4, 29.0, 22.7,
14.1. MALDIeTOF MS (C54H58N4S4) m/z: calcd for 891.33; found 891.57.
The synthetic procedure for 2-b was similar to that for 1-b,
except that 2-a (6.3 g, 50 mmol) was used instead of 1-a. A purple
solid of compound 2-b was obtained (1.3 g, 11.4%). 1H NMR (CDCl3,
2.4.2. 5-(5-Formylthien-2-yl)-10,15,20-tris(5-hexylthien-2-yl)
porphyrin (1-c)
400 MHz, d/ppm): 9.10e9.02 (m, 8 H, pyrrolic-H), 7.92 (d, 1 H,
TheH), 7.86e7.85 (d, 1 H, TheH), 7.68e7.67 (d, 3 H, TheH),
7.51e7.50 (m, 1 H, TheH), 7.16 (d, 3 H, TheH), 2.82 (s, 9 H,
In a 50 mL three-necked flask, porphyrin 1-b (0.53 g, 1 mmol),
DMF (1 mL, 12 mmol), 1,2-dichloroethane (15 mL) were added
sequentially under Ar atmosphere. POCl3 (1.1 mL, 12 mmol) was
added quickly at 0 ꢀC. The mixture was heated to 80 ꢀC and stirred
for 12 h. After cooling to room temperature, poured it into sodium
acetate solution and stirred for an addition 1 h. The solution was
extracted with dichloromethane, and washed with H2O and brine.
The organic layer was dried over anhydrous MgSO4. Solvent was
removed by rotary evaporation, and the residue was purified by
silica gel column chromatography with petroleum ether/
dichloromethane (1/1) as eluent to yield 1-c as a purple-red solid
eCH3), ꢂ2.61 (s, 2 H, eNH). 13C NMR (CDCl3, 100 MHz,
d/ppm):
142.6, 133.8, 133.0, 127.8, 126.0, 124.6, 15.5. MALDIeTOF MS
(C39H28N4S4) m/z : calcd for 681.12; found 681.18.
2.4.6. 5-(5-Formylthien-2-yl)-10,15,20-tris(5-methylthien-2-yl)
porphyrin (2-c)
The synthetic procedure for 2-c was similar to that for 1-c,
except that 2-b (0.50 g, 0.74 mmol) was used instead of 1-b. A
purple solid of compound 2-c was obtained (0.21 g, 40.1%). 1H NMR
(CDCl3, 400 MHz, d/ppm): 10.24 (s, 1 H, eCHO), 9.12e8.98 (m, 8 H,
(0.22 g, 40%). 1H NMR (CDCl3, 400 MHz,
d
/ppm): 10.26 (s, 1
pyrrolic-H), 8.16 (d, 1 H, TheH), 8.00 (d, 1 H, TheH), 7.69 (d, 3 H,
TheH), 7.17 (d, 3 H, TheH), 2.83 (s, 9 H, eCH3), ꢂ2.61 (s, 2 H, eNH).
H, eCHO), 9.11e8.97 (m, 8 H, pyrrolic-H), 8.16e8.15 (d, 1 H, TheH),
8.00e7.99 (d, 1 H, TheH), 7.71e7.70 (d, 3 H, TheH), 7.18e7.17 (d,
3 H, TheH), 3.15e3.11 (t, 6 H, eCH2), 1.99e1.92 (m, 6 H, eCH2),
1.62e1.57 (m, 6 H, eCH2), 1.44(m, 12 H, eCH2), 0.99e0.96 (m, 9
H, eCH3), ꢂ2.61 (s, 2 H, eNH). 13C NMR (CDCl3, 100 MHz,
13C NMR (CDCl3, 100 MHz,
d/ppm): 183.4, 153.3, 145.6, 142.7, 140.0,
134.8, 134.0, 124.4, 113.6, 113.3, 109.7, 15.5. MALDIeTOF MS
(C40H28N4OS4) m/z : calcd for 709.15; found 709.24.
d/ppm):183.2, 153.4, 149.1, 145.7, 139.8, 134.7, 133.8, 123.3, 113.5,
2.4.7. 5-(5-Formylthien-2-yl)-10,15,20-tris(5-methylthien-2-yl)
porphyrin zine (2-d)
The synthetic procedure for 2-d was similar to that for 1-d,
except that 2-c (0.2 g, 0.28 mmol) was used instead of 1-c. A purple
solid of compound 2-d was obtained (0.21 g, 97%). 1H NMR (CDCl3,
109.7, 31.8, 31.7, 30.4, 29.0, 22.6, 14.1. MALDIeTOF MS
(C55H58N4OS4) m/z: calcd for 919.32; found 919.34.
2.4.3. 5-(5-Formylthien-2-yl)-10,15,20-tris(5-hexylthien-2-yl)
porphyrin zinc (1-d)
400 MHz, d/ppm): 10.24 (s, 1 H, eCHO), 9.12e8.99 (m, 8 H, pyrrolic-
A mixture of compound 1-c (0.55 g, 1 mmol) and Zn (OAc)2
(1.85 g, 10 mmol) in CHCl3 (150 mL) and CH3OH (10 mL) was
refluxed for 4 h. After cooling to room temperature, the mixture
was washed with H2O. The organic layer was dried over anhy-
drous MgSO4 and concentrated by rotary evaporation. A purplish-
green solid of compound 1-d was obtained (0.55 g, 95%). 1H NMR
H), 8.15 (d, 1 H, TheH), 8.01 (d, 1 H, TheH), 7.69 (d, 3 H, TheH), 7.17
(d, 3 H, TheH), 2.83 (s, 9 H, eCH3). MALDIeTOF MS (C40H26N4OS4Zn)
m/z : calcd for 771.02; found 771.09.
2.4.8. 2-Cyano-3-(5-(10,15,20-tris(5-methylthien-2-yl))
porphyrinatozinc(II)yl)thienyl acrylic acid (Z2)
(CDCl3, 400 MHz,
d
/ppm): 10.26 (s, 1 H, eCHO), 9.12e8.98 (m, 8 H,
The synthetic procedure for Z2 was similar to that for Z1, except
that 2-d (0.20 g, 0.26 mmol) was used instead of 1-d. A purple solid
of compound Z2 was obtained (0.08 g, 36.9%). FT-IR (KBr,
vmax/cmꢂ1): 2880, 2843, 2852, 2356, 2217, 1623, 1456, 1384, 996,
1071, 798, 668. UVevis lmax (CHCl3)/nm: 431, 560. 1H NMR (DMSO,
pyrrolic-H), 8.15 (d, 1 H, TheH), 7.99 (d, 1 H, TheH), 7.70 (d, 3 H,
TheH), 7.17 (d, 3 H, TheH), 3.15e3.11 (t, 6 H, eCH2), 1.99e1.92
(m, 6 H, eCH2), 1.62e1.57 (m, 6 H, eCH2), 1.44 (m, 12 H, eCH2),
0.99e0.96 (m, 9 H, eCH3). MALDIeTOF MS (C55H56N4OS4Zn) m/z:
calcd for 981.24; found 981.33.
400 MHz, d/ppm): 9.07 (m, 8 H, pyrrolic-H), 8.61 (s, 1 H, vinyl-H),
8.31 (s, 1 H, TheH), 8.12 (s, 1 H, TheH), 7.73 (s, 3 H, TheH),7.27
2.4.4. 2-Cyano-3-(5-(10,15,20-tris(5-hexylthien-2-yl))
porphyrinatozinc(II)yl)thienyl acrylic acid (Z1)
(s, 3H, TheH), 2.79 (s, 9 H, eCH3). 13C NMR (DMSO, 100 MHz,
d/ppm): 151.1, 151.0, 150.9, 150.3, 142.2, 141.0, 134.6, 133.9, 132.5,
To a solution of acetonitrile (15 mL) and toluene (5 mL),
porphyrin compound 1-d (0.180 g, 0.29 mmol), cyanoacetic acid
132.2, 131.8, 125.4, 113.4, 15.6. MALDIeTOF MS (C43H27N5O2S4Zn)
m/z : calcd for 837.15; found 837.03.

408
W. Zhou et al. / Dyes and Pigments 91 (2011) 404e412
2.4.9. 3-Hexylthiophene-2-carbaldehyde (3-a)
purple solid of compound Z3 was obtained (0.08 g, 26%). FT-IR
(KBr, vmax/cmꢂ1): 2959, 2927, 2851, 2347, 2216, 1622, 1449,
1384, 1068, 979, 793, 784. UVevis lmax (CHCl3)/nm: 432, 558. 1H
Dried magnesium pieces (0.4 g, 17 mmol) were placed into a dry
100 mL three-necked round-bottomed flask together with a few
small crystals of iodine. THF (10 mL) was added sequentially under Ar
atmosphere, and then a THF solution of 2-bromo-3-hexylthiophene
(4.41 g, 15 mmol) was dropped slowly at 60e70 ꢀC. After 6 h, the
reaction mixture was cooled down to ꢂ40 ꢀC and 6 mL DMF was
added. The reaction was allowed to progress for a further 20 h
after warming to room temperature. At last, 10 mL diluent hydro-
chloric acid was added into it. The solution was extracted
with dichloromethane, and washed with H2O. The organic layer was
dried over anhydrous MgSO4. Solvent was removed by rotary evap-
oration, and the residue was purified by silica gel column chroma-
tography with petroleum ether/dichloromethane (5/1) as eluent to
yield 3-a as a buff liquid (1.5 g, 50%). 1H NMR (CDCl3, 400 MHz,
NMR (CDCl3, 400 MHz, d/ppm): 9.25e8.93 (m, 8 H, pyrrolic-H),
8.60 (s, 1 H, vinyl-H), 8.21 (s, 1 H, TheH), 7.70e7.69 (s, 3 H,
TheH), 7.17 (s, 3 H, TheH), 2.84 (s, 9 H, eCH3), 2.52e2.49 (t, 2
H, eCH2), 1.47e1.27 (m, 4 H, eCH2), 0.93e0.80 (m, 4 H, eCH2),
0.51e0.47 (t, 3 H, eCH3). 13C NMR (DMSO, 100 MHz,
d/ppm): 151.1,
150.9, 150.8, 150.5, 145.9, 142.1, 142.1, 141.0, 140.9, 133.8, 133.2,
131.6, 132.3, 132.2, 132.1, 131.5, 130.5, 125.3, 125.0, 123.6, 113.7,
113.3, 110.0, 29.7, 29.3, 28.9, 28.3, 22.0, 20.8, 15.5, 13.9. MAL-
DIeTOF MS (C49H39N5O2S4Zn) m/z : calcd for 921.12; found
921.23.
2.4.14. 5-(3,4-Ethylenedioxyl)thienyl-10,15,20-tris(5-methylthien-
2-yl)porphyrin (4-b)
d/ppm): 10.05 (s, 1 H, eCHO), 7.66e7.64 (d, 1 H, TheH), 7.02e7.01
(d, 1 H, TheH), 2.99e2.96 (t, 2 H, eCH2), 1.71e1.64 (m, 4 H, eCH2),
The synthetic procedure for 4-b was similar to that for 1-b,
except that 4-a (2.88 g, 17 mmol) was used instead of
1.38e1.27 (m, 4 H, eCH2), 0.91e0.88 (t, 3 H, eCH3). 13C NMR (CDCl3,
100 MHz,
28.5, 22.5, 13.9.
d
/ppm): 182.1, 152.7, 137.7, 134.3, 130.6, 31.5, 31.3, 28.9,
2-thiophenealdehyde.
A
purple solid of compound 4-b was
/ppm): 9.15e9.11
obtained (1.2 g, 9.6%). 1H NMR (CDCl3, 400 MHz,
d
(m, 8 H, pyrrolic-H), 7.69e7.68 (d, 3 H, TheH), 7.17 (d, 3 H, TheH),
6.93 (s, 1 H, TheH), 4.45 (t, 2 H, eOCH2), 4.28 (t, 2 H, eOCH2), 2.84
(s, 9 H, eCH3), ꢂ2.58 (s, 2 H, eNH). 13C NMR (CDCl3, 100 MHz,
2.4.10. 5-(3-Hexylthien-2-yl)-10,15,20-tris(5-methylthien-2-yl)
porphyrin (3-b)
The synthetic procedure for 3-b was similar to that for 2-b, except
that 3-a (1.88 g,17 mmol) was used instead of 2-thiophenealdehyde.
A purple solid of compound 3-b was obtained (1.0 g, 7%). 1H NMR
d/ppm): 142.5, 140.5, 140.3, 133.8, 133.7, 131.0, 129.0, 128.2, 125.3,
124.5, 117.0, 113.2, 112.7, 108.7, 101.3, 64.8, 15.5. MALDI-TOF MS
(C41H30N4O2S4) m/z: calcd for 739.12; found 739.19.
(CDCl3, 400 MHz, d/ppm): 9.12e8.90 (m, 8 H, pyrrolic-H), 7.75e7.74
(t,1 H, TheH), 7.69 (d, 3 H, TheH), 7.39e7.37 (t,1 H, TheH), 7.17 (s, 3 H,
TheH), 2.84e2.75 (s, 9 H, eCH3), 2.49e2.45(t, 2 H, eCH2), 1.45
(t, 2 H, eCH2),1.28 (t, 2 H, eCH2), 0.94e0.81 (m, 4 H, eCH2), 0.54e0.51
2.4.15. 5-[5-(3,4-Ethylenedioxyl-5-formyl)thienyl-10,15,20-tris(5-
methylthien-2-yl) porphyrin (4-c)
The synthetic procedure for 4-c was similar to that for 1-c,
except that 4-b (0.60 g, 0.81 mmol) was used instead of 2-b. A
purple solid of compound 4-c was obtained (0.30 g, 48.3%). 1H
(t, 3 H, eCH3), ꢂ2.60 (s, 2 H, eNH).13C NMR (CDCl3,100 MHz,
d/ppm):
145.8, 142.5, 140.2, 133.8, 133.7, 127.2, 125.6, 124.5, 113.0, 112.7, 111.1,
31.2, 30.3, 29.3, 28.7, 22.7, 22.2, 14.1. MALDI-TOF MS (C45H40N4S4)
m/z: calcd for 765.21; found 765.30.
NMR (CDCl3, 400 MHz, d/ppm): 10.30 (s, 1 H, eCHO), 9.16e9.05 (m,
8 H, pyrrolic-H), 7.70 (d, 3 H, TheH), 7.17 (d, 3 H, TheH), 4.60 (t, 2
H, eOCH2), 4.33 (t, 2 H, eOCH2), 2.84 (s, 9 H, eCH3), ꢂ2.60 (s, 2
2.4.11. 5-(3-Hexyl-5-formylthien-2-yl)-10,15,20-tris (5-
methylthien-2-yl)porphyrin (3-c)
The synthetic procedure for 3-c was similar to that for 1-c, except
that 3-b (0.5 g, 0.58 mmol) was used instead of 2-b. A purple solid of
compound 3-c was obtained (0.21 g, 40.7%). 1H NMR (CDCl3,
H, eNH). 13C NMR (CDCl3, 100 MHz,
d/ppm): 180.0, 147.2, 142.7,
140.1, 140.0, 133.9, 128.5, 124.6, 119.6, 114.0, 113.2, 106.2, 65.5, 64.6,
15.5. MALDI-TOF MS (C42H30N4O3S4) m/z: calcd for 767.12; found
767.26.
400 MHz,
d
/ppm): 10.21 (s, 1 H, eCHO), 9.11e8.80 (m, 8 H, pyrrolic-
2.4.16. 5-[5-(3,4-Ethylenedioxyl-5-formyl)thienyl-10,15,20-tris(5-
methylthien-2-yl) porphyrin zine (4-d)
The synthetic procedure for 4-d was similar to that for 1-d,
except that 4-c (0.3 g, 0.39 mmol) was used instead of 1-c. A
purple solid of compound 4-d was obtained (0.31 g, 95%). 1H
H), 8.06 (d, 1 H, TheH), 7.68 (d, 3 H, TheH), 7.16 (d, 3 H, TheH), 2.82
(s, 9 H, eCH3) 2.45e2.42 (t, 2 H, eCH2),1.45e1.42 (m, 2 H, eCH2),1.25
(m, 2 H, eCH2), 0.92e0.80 (m, 4 H, eCH2), 0.52e0.49 (m, 3
H, eCH3), ꢂ2.61 (s, 2 H, eNH). 13C NMR (CDCl3, 100 MHz,
d/ppm):
183.2,148.0,147.5,143.3,142.7,140.2,140.0,136.0,133.9,124.6,113.8,
113.2, 108.8, 31.1, 30.0, 29.7, 28.6, 22.2, 21.5, 15.5, 13.7. MALDIeTOF
MS (C46H40N4OS4) m/z: calcd for 793.20; found 793.28.
NMR (CDCl3, 400 MHz, d/ppm): 10.31 (s, 1 H, eCHO), 9.15e9.02
(m, 8 H, pyrrolic-H), 7.71 (d, 3 H, TheH), 7.17 (d, 3 H, TheH), 4.60
(t, 2 H, eOCH2), 4.33 (t, 2 H, eOCH2), 2.84 (s, 9 H, eCH3). MAL-
DIeTOF MS (C42H28N4O3S4Zn) m/z: calcd for 829.05; found
829.11.
2.4.12. 5-(3-Hexyl-5-formylthien-2-yl)-10,15,20-tris(5-
methylthien-2-yl)porphyrin zine (3-d)
The synthetic procedure for 3-d was similar to that for 1-d,
except that 3-c (0.21 g, 0.24 mmol) was used instead of 1-c. A
purple solid of compound 3-d was obtained (0.20 g, 90%). 1H NMR
2.4.17. 2-Cyano-3-(3,4-ethylenedioxyl-5-(10,15,20-tris(5-
methylthien-2-yl)) porphyrinatozinc(II)yl)thienyl acrylic acid (Z4)
The synthetic procedure for Z4 was similar to that for Z1,
except that 4-d (0.20 g, 0.24 mmol) was used instead of 1-d. A
purple solid of compound Z4 was obtained (0.12 g, 56.1%). FT-IR
(KBr, vmax/cmꢂ1): 2966, 2843, 2364, 2206, 1611, 1579, 1441, 1383,
1319, 1261, 1015, 1071, 799, 712. UVevis lmax (CHCl3)/nm: 435,
(CDCl3, 400 MHz, d/ppm): 10.21 (s, 1 H, eCHO), 9.11e8.83 (m, 8 H,
pyrrolic-H), 8.07 (d, 1 H, TheH), 7.67 (d, 3 H, TheH), 7.16 (d, 3 H,
TheH), 2.82(s, 9 H, eCH3) 2.45e2.42(t, 2 H, eCH2), 1.45e1.42 (m,
2 H, eCH2), 1.25 (m, 2 H, eCH2), 0.92e0.80 (m, 4 H, eCH2),
0.52e0.49 (m, 3 H, eCH3). MALDIeTOF MS (C46H38N4OS4Zn) m/z:
calcd for 855.15; found 855.21.
562. 1H NMR (DMSO, 400 MHz,
d/ppm): 9.16e9.06 (m, 8 H,
pyrrolic-H), 8.51 (s, 1 H, vinyl-H), 7.72 (d, 3 H, TheH), 7.27 (d, 3 H,
TheH), 4.65 (t, 2 H, eOCH2). 4.37 (t, 2 H, eOCH2), 2.78 (s, 9
2.4.13. 2-Cyano-3-(4-hexyl-5-(10,15,20-tris(5-methylthien-2-yl))
porphyrinatozinc(II)yl)thienyl acrylic acid (Z3)
The synthetic procedure for Z3 was similar to that for Z1,
except that 3-d (0.20 g, 0.21 mmol) was used instead of 1-d. A
H, eCH3). 13C NMR (DMSO, 100 MHz,
d/ppm): 151.1, 150.9, 150.8,
150.4, 142.2, 141.0, 133.9, 132.3, 132.1, 131.9, 125.4, 113.3, 22.9, 15.5.
MALDIeTOF MS (C45H29N5O4S4Zn) m/z: calcd for 895.04; found
895.12.

W. Zhou et al. / Dyes and Pigments 91 (2011) 404e412
409
Fig. 3. Normalized absorption spectra of the porphyrin dyes in CHCl3. The inset is an
expansion of the Q-band region.
Fig. 4. UVevisible absorption spectra of the porphyrin dyes adsorbed on TiO2 films.
3. Results and discussion
are collected in Table 1. Owing to their similar structures, all the
porphyrin dyes exhibit the strong absorption peaks located in the
range of 400e440 nm (Soret-band) and weak Q-bands around
3.1. Synthesis
560 nm which are attributed to
p
e
p
* electron transition. The
The detailed synthetic routes of four porphyrin dyes as well as
the intermediates are shown in Fig. 2. Functionalized porphyrins as
donors in this study were synthesized by the AdlereLongo method
[12]. Taking Z1 as an example, the dye was synthesized as the
following steps. First, 5-hexylthiophene-2-aldehyde 1-a, was
obtained from 2-hexylthiophene by Vilsmeier formylation with
POCl3 and DMF in 1,2-dichloroethane. Then compound 1-a was
reacted with 2-thiophenealdehyde and pyrrole by the AdlereLongo
method to obtain crude compound. After reduced pressure distil-
lation, recrystallization and purification twice by column chroma-
tography on silica gel, the key intermediate 1-b was achieved.
Subsequently, compound 1-b was reacted with POCl3 and DMF in
1,2-dichloroethane via Vilsmeier formylation to convert to 1-c.
Finally, compound 1-c was treated with Zn(OAc)2 to obtain 1-d and
then reacted with cyanoacetic acid by Knoevenagel reaction to give
the target dye Z1. Z2, Z3 and Z4 were synthesized by the similar
procedure. It is noted that compound 3-a was synthesized by
Grignard reaction with 2-bromo-3-hexylthiophene as the starting
material. The structures of the four dyes were verified by FT-IR, 1H
NMR, 13C NMR and MALDIeTOF mass spectra.
values of the molar extinction coefficient (
3
) at the maximum
absorption wavelength (lmax) for the Z-series of dyes are in the
range of 0.86e2.59 ꢁ 105 dm3 molꢂ1 cmꢂ1. As expected, the
UVevisible absorption bands of the four dyes are sensitive to
substituents on the periphery of the porphyrin ring respectively. All
of the Soret-bands of Z1e4 are red-shifted and broadened signifi-
cantly relative to the reference porphyrin PZn [18], and the Q-bands
are red-shifted and broadened evidently in the expansion. The
maximum absorption wavelength (lmax) for these dyes are 434,
431, 433 and 435 nm, respectively. And all the Soret-bands of the
four dyes are 9e13 nm red-shifted than the porphyrins which
contain alkylphenyl units at the meso-position [18,19]. The increase
in red-shift of the absorption band is possibly due to the electronic
effect of the thienyl groups [11]. This result conforms that the alkyl-
thienyls appending on porphyrin cycles have positive effect on
improving the UVevisible absorption of the porphyrin dyes. The
molar extinction coefficient (3) of Z4 is smaller than three dyes
(Z1e3) indicating the inferior ability of light harvesting. We also
3.2. Optical properties
The UVevisible absorption spectra of Z1eZ4 and PZn [18] in the
CHCl3 solution are displayed in Fig. 3, and the corresponding data
Table 1
UVevisible, PL spectral data of the porphyrin dyes.
Dye Soret/Q-band(s)a
Soret/Q-band Gc/10ꢂ8
lem (nm)d lint (nm)e
a
(3
, 105 Mꢂ1 cmꢂ1
)
(f)b
mol cmꢂ2
Z1
Z2
Z3
Z4
434 (2.57), 560 (0.23) 451/560
431 (1.93), 560 (0.17) 452/561
433 (1.63), 558 (0.13) 450/559
435 (0.86), 559 (0.07) 452/573
2.90
4.84
3.83
4.27
e
695
692
656
641
593
590
580
593
448
PZn 420 (2.21), 547 (0.07)
e
598, 645
a
Absorption spectra was measured in CHCl3 solution.
Absorption spectra was obtained on TiO2 films.
Amount of the dyes adsorbed on TiO2 films.
b
c
d
Wavelengths for emission spectra in CHCl3 solution by exciting at Soret
wavelength.
e
Measured by the intercept of the normalized absorption and emission spectra.
Fig. 5. PL spectra of the porphyrin dyes in CHCl3.

410
W. Zhou et al. / Dyes and Pigments 91 (2011) 404e412
Fig. 7. IPCE plots for the DSSCs based on the porphyrin dyes.
Fig. 6. Cyclic voltammograms of the porphyrin dyes.
cyclic voltammetry (Fig. 6). All the data are summarized in Table 2,
which are consistent with previous reports [11]. The reduction
potentials of the four dyes (Z1e4) are ꢂ1.104, ꢂ1.216, ꢂ1.176
and ꢂ1.124 eV, respectively, which are positive than PZn
(ꢂ1.290 eV). Eox is the first oxidation potentials (vs. NHE), and E0e0
is determined from the intercept of the normalized absorption and
emission spectra. Then we consider the potential levels of
measured the adsorption of the four porphyrin dyes on TiO2 films
on quartz plate (Fig. 4) to obtain red-shifted and broadened spectra
in comparison with those of the solution spectra, which should
result from the formation of the J-type aggregate of porphyrins on
the TiO2 surface [14].
The emission spectra of these porphyrin dyes are presented in
Fig. 5 and fluorescence data are also listed in Table 1. The maximum
fluorescence emission wavelength (lem) of Z1 and Z2 are 695 and
692 nm, respectively, which are red-shifted relative to Z3 (656 nm)
and Z4 (641 nm). The observed red-shifts of lem should be attrib-
uted to the increase of the effective conjugation in Z1 and Z2
molecules compared to Z3 and Z4. This can be explained by the fact
that there are much bigger dihedral angles between the prophyrin
(EoxeE0e0
) corresponding to the LUMO (lowest unoccupied
molecular orbital) level of the dyes. By the introduction of thie-
nylgroups at the meso-positions of porphyrin dyes, the energy level
of LUMO is significantly shifted to the positive compared with the
reference PZn [18] and the HOMO energy levels are negatively
shifted, indicating
a decreased HOMOeLUMO energy gap.
Furthermore, for dyes Z1eZ4 with the introduction of electron-
donating groups into the porphyrin ring, a decrease of the
HOMOeLUMO energy gaps relative to PZn is observed, which is
consistent with the red-shift in the absorption and emission
spectra. As shown in Table 2, the sufficiently low HOMO energy
level of the dye will ensure that there is enough driving force for the
dye regeneration reaction to compete efficiently with the recapture
of the injected electrons by the dye cation radical. While the LUMO
energy level is more negative than the conduction band (CB) of TiO2
(ꢂ0.5 V vs. NHE) [21,22], indicating sufficient driving force for
electron transfer from the excited dye molecules to the conduction
band of TiO2 electrode.
core and the
p
-bridge due to the substituents for Z3 and Z4, which
-A system thus resulting in
lower the -conjugation [20] of the D-
p
p
the ineffective intramolecular electron transfer.
3.3. Electrochemical properties
The electrochemical studies are performed on the four dyes to
elucidate the effect of thienyl groups on the energy level of the
porphyrin macro-cycle. The reduction potentials were determined
by differential pulse voltammetry (DPV) and the oxidation poten-
tials (Eox) corresponding to the HOMO (highest occupied molecular
orbital) energy levels of the porphyrin dyes were determined via
Table 2
Electrochemical data for the porphyrin dyes and driving forces for electron transfer
processes on the TiO2.
d (V)
D
Ginje(eV)
D
Gregf(eV)
a
b
Dye
E0ꢂ0 (eV)
Eox (V)
Eredc (V)
Eox
*
Z1
Z2
Z3
Z4
PZn
2.091
2.102
2.126
2.091
2.77
1.220
1.207
1.161
1.167
1.230
ꢂ1.104
ꢂ1.216
ꢂ1.176
ꢂ1.124
ꢂ1.290
ꢂ0.871
ꢂ0.895
ꢂ0.977
ꢂ0.924
ꢂ1.540
ꢂ0.371
ꢂ0.395
ꢂ0.477
ꢂ0.424
ꢂ1.040
ꢂ0.720
ꢂ0.707
ꢂ0.661
ꢂ0.667
ꢂ0.730
a
Determined from the intercept of the normalized absorption and emission
spectra.
b
c
First oxidation potentials (vs. NHE).
First reduction potentials (vs. NHE).
Excited-state oxidation potentials approximated from Eox and E0e0 (vs. NHE).
Driving forces for electron injection from the porphyrin excited singlet state
d
e
(Eox*) to the conduction band of TiO2 (ꢂ0.5 V vs. NHE).
f
ꢂ
Driving forces for regeneration of the porphyrin radical cation by Iꢂ/I3 redox
couple (þ0.5 V vs. NHE).
Fig. 8. Currentevoltage characteristics of the porphyrin dyes.

W. Zhou et al. / Dyes and Pigments 91 (2011) 404e412
411
Table 3
more solar light, we should further design and synthesis new
porphyrin dyes which have broadened spectra between the Soret-
bands, the Q-bands and/or intensify the Q-bands as much as they
can.
Photovoltaic performances of DSSCs based on the porphyrin dyes.
Dye
Jsc (mA/cm2)
Voc (V)
ff
h (%)
Z1
Z2
Z3
Z4
12.13 ꢃ 0.10
14.18 ꢃ 0.13
11.37 ꢃ 0.18
11.56 ꢃ 0.18
0.56 ꢃ 0.01
0.59 ꢃ 0.01
0.57 ꢃ 0.01
0.56 ꢃ 0.01
0.65 ꢃ 0.01
0.68 ꢃ 0.01
0.70 ꢃ 0.01
0.70 ꢃ 0.02
4.44 ꢃ 0.10
5.71 ꢃ 0.05
4.54 ꢃ 0.05
4.54 ꢃ 0.10
Acknowledgments
This work was supported by the National Natural Science
Foundation of China (50973092, 51003089), Specialized Research
Fund for the Doctoral Program of Higher Education of China
(20094301120005, 20104301110003), and Natural Science Foun-
dation (09JJ3020, 09JJ4005) of Hunan Province of China.
3.4. Photovoltaic properties of porphyrin-sensitized TiO2 cells
The incident photo-to-current conversion efficiencies (IPCE)
plots for the DSSCs based on the four porphyrin dyes are shown in
Fig. 7. The four dyes all respond in the broad rang of 350e800 nm
and show the maximum IPCE values around 450 nm, which is in
accord with the spectra of the dyes in solid films. Among the four
dyes, Z2 exhibits the highest IPCE (w78%) due to its largest
adsorbed amounts of the dye on the TiO2 film and relatively better
ability of light harvesting, which implies this sensitizer would show
a relatively large photocurrent in DSSCs. On the other hand, Z4
shows inferior IPCE in the range of 550e650 nm relative to other
References
[1] Robertson N. Optimizing dyes for dye-sensitized solar cells. Angewandte
Chemie International Edition 2006;45:2338e45.
[2] O’Regan B, Grätzel M. A low-cost high-efficiency solar cell based on dye-
sensitized colloidal TiO2 film. Nature 1991;353:737e9.
[3] Mishra A, Fischer MKR, Bäerle P. Metal-free organic dyes for dye-sensitized
solar cells: from structure: property relationships to design rules. Ange-
wandte Chemie International Edition 2009;48:2474e99.
three dyes which mainly resulted from the smaller
worse ability of light harvesting.
G value and
[4] Imahori H, Umeyama T, Ito S. Large
p-aromatic molecules as potential
sensitizers for highly efficient dye-sensitized solar cells. Accounts of Chemical
Research 2009;42(11):1809e18.
[5] (a) Nazeeruddin MK, Angelis DF, Fantacci S, Selloni A, Viscardi G, Liska P, et al.
Combined experimental and DFT-TDDFT computational study of
photoelectrochemical cell ruthenium sensitizers. Journal of the American
Chemical Society 2005;127:16835e47;
The photovoltaic characteristics of DSSCs based on the four dyes
are shown in Fig. 8 and the device performance parameters are
listed in Table 3. The open-circuit photovoltage (Voc) values are in
the order of Z2 > Z3 > Z1 z Z4, and the short-circuit photocurrent
density (Jsc) are Z2 > Z1 > Z3 z Z4. The measured results of Voc
values are consistent with the discussion of their electrochemical
properties. To investigate the absorptive abilities and explain the Jsc
of the Z-series of dyes the thin films were evaluated for the total
(b) Wang P, Klein C, Moser JE, Baker RH, Ha NLC, Charvet R, et al. Amphiphilic
ruthenium sensitizer with 4,40-diphosphonic acid-2,20-bipyridine as
anchoring ligand for nanocrystalline dye sensitized solar cells. Journal of
Physical Chemistry B 2004;108:17553e9.
[6] Calogero G, Marco GD, Caramori S, Cazzanti S, Argazzi R, Bignozzi CA. Natural
dye senstizers for photoelectrochemical cells. Energy
& Environmental
amount of the porphyrin adsorbed. The adsorbed amounts (G) of
Science 2009;2:1162e72.
the four dyes are exhibited in Table 1. The adsorbed amounts of four
[7] Eu S, Hayashi S, Umeyama T, Matano Y, Araki Y, Imahori H. Quinoxaline-fused
porphyrins for dye-sensitized solar cells. Journal of Physical Chemistry C
2008;112:4396e405.
[8] Liao MS, Scheiner S. Electronic structure and bonding in metal porphyrins,
metal¼Fe, Co, Ni, Cu, Zn. Journal of Chemical Physics 2002;117:205e19.
[9] Campbell WM, Jolley KW, Wagner P, Wagner K, Walsh PJ, Gordon KC, et al.
Highly efficient porphyrin sensitizers for dye-sensitized solar cells. Journal of
Physical Chemistry C 2007;111:11760e2.
[10] (a) Hsieh CP, Lu HP, Chiu CL, Lee CW, Chuang SH, Mai CL, et al. Synthesis and
characterization of porphyrin sensitizers with various electron-donating
substituents for highly efficient dye-sensitized solar cells. Journal of Mate-
rials Chemistry 2010;20:1127e34;
dyes are 2.90 ꢁ 10ꢂ8, 4.84 ꢁ 10ꢂ8, 3.83 ꢁ 10ꢂ8, and 4.27 ꢁ 10ꢂ8
mol/cm2 for Z1, Z2, Z3, and Z4, respectively. The
G is in the order of
Z2 > Z4 > Z3 > Z1. Because
G is related to the molecular size and
the thickness of dye aggregates [23], which correlates directly to
the absorptive abilities of the four dyes and may influence the
photovoltaic performance of solar cells. For Z1, the n-hexyl
substituents around the porphyrin core sterically hinder adsorp-
tion. In detail, Z3 features a long alkyl chain on the thiophene
bridge between the porphyrin core and the anchor group resulted
in a much bigger dihedral angle of the porphyrin to reduce the
coplanarity of the donor and the acceptor unit. And in the unit of
Z4, the EDOT (3,4-ethylenedioxythienyl) segment is larger than the
thienyl segment leading to a more twisted nonplanar geometry.
Finally, due to the smallest molecular size, Z2 is prone to be the
(b) Lu HP, Tsai CY, Yen WN, Hsieh CP, Lee CW, Yeh CY, et al. Control of dye
aggregation and electron injection for highly efficient porphyrin sensitizers
adsorbed on semiconductor films with varying ratios of coadsorbate. Journal
of Physical Chemistry C 2009;113:20990e7;
(c) Bessho T, Zakeeruddin SM, Yeh CY, Diau EWG, Grätzel M. Highly efficient
mesoscopic dye-sensitized solar cells based on donoreacceptor-substituted
porphyrins. Angewandte Chemie International Edition 2010;49:6646e9;
(d) Imahori H, Matsubara Y, Iijima H, Umeyama T, Matano Y, Ito S, et al. Effects
of meso-diarylamino group of porphyrins as sensitizers in dye-Sensitized solar
cells on optical, electrochemical, and photovoltaicproperties. Journal of
Physical Chemistry C 2010;114:10656e65.
largest
G
and result in the highest Jsc, which is in agreement with
(4.54%)
the IPCE. The DSSCs based on Z3 and Z4 show the same
h
because of their almost identical Voc and Jsc values. Summarily, Z2
obtains the best IPCE, the hightest Voc value (0.59 V) and Jsc value
[11] Bhyrappa P, Bhavana P. Meso-tetrathienylporphyrins: electrochemical and
axial ligation properties. Chemical Physical Letters 2001;349:399e404.
[12] Boylea NM, Rochfordb J, Prycea MT. Thienyl-appended porphyrins: synthesis,
photophysical and electrochemical properties, and their applications. Coor-
dination Chemistry Reviews 2010;254:77e102.
[13] Ziessel R, Bäuerle P, Ammann M, Barbieri A, Barigelletti F. Exciton-like energy
collection in an oligothiophene wire end-capped by Ru- and Os-polypyridine
chromophores. Chemical Communication 2005;6:802e5.
(14.18 mA/cm2), and the best
h of 5.71%.
4. Conclusions
In conclusion, four porphyrin dyes (Z1eZ4) with thienyl
appended porphyrins as electron donors, thiophene derivatives as
linkers and 2-cyanoacrylic acids as anchor groups were designed,
[14] Li G, Jiang K, Bao P, Li Y, Li S, Yang L. Molecular design of triarylamine-based
organic dyes for efficient dye-sensitized solar cells. New Journal of Chemistry
2009;33:868e76.
[15] Kalyanasundaram K, Grätzel M. Applications of functionalized transition
metal complexes in photonic and optoelectronic devices. Coordination
Chemistry Reviews 1998;77:347e414.
synthesized and characterized for DSSCs to obtain the highest
h up
to 5.71%. The appended alkylthienyl units and various thiophene
linkers significantly influence the photophysical, electrochemical
and photovoltaic properties of the four dyes. With the introduction
of these alkylthienyl units around the porphyrin cycles, the
absorption spectra were red-shifted and broadened and the
HOMOeLUMO energy gaps were decreased as expected. To utilize
[16] Kamat PV, Haria M, Hotchandani S. C60 cluster as an electron shuttle in
a
Ru(II)-polypyridyl sensitizer-based photochemical solar cell. Journal of
Physical Chemistry B 2004;108:5166e70.
[17] Koumura N, Wang ZS, Mori S, Miyashita M, Suzuki E, Hara K. Alkyl-func-
tionalized organic dyes for efficient molecular photovoltaics. Journal of the
American Chemical Society 2006;128:14256e7.

412
W. Zhou et al. / Dyes and Pigments 91 (2011) 404e412
[18] Xiang N, Huang X, Feng X, Liu Y, Zhao B, Deng L, et al. The structural modi-
fication of thiophene-linked porphyrin sensitizers for dye-sensitized solar
cells. Dyes and Pigments 2011;88:75e83.
[19] Liu Y, Xiang N, Feng X, Shen P, Zhou W, Weng C, et al. Thiophene-linked
porphyrin derivatives for dye-sensitized solar cells. Chemical Communication
2009;18:2499e501.
[20] Umeyama T, Takamatsu T, Tezuka N, Matano Y, Araki Y, Wada T, et al. Synthesis
andphotophysicaland photovoltaicpropertiesof porphyrinefuran andthiophene
alternating copolymers. Journal of Physical Chemistry C 2009;113:10798e806.
[21] Hagfeldt A, Grätzel M. Light-induced redox reactions in nanocrystalline
systems. Chemical Reviews 1995;95(1):49e68.
[22] Hara K, Sato T, Katoh R, Furube A, Ohga Y, Shinpo A, et al. Molecular design of
coumarin dyes for efficient dye-sensitized solar cells. Journal of Physical
Chemistry B 2003;107(2):597e606.
[23] Wang ZS, Koumura N, Cui Y, Takahashi M, Sekiguchi H, Mori A, et al. Hex-
ylthiophene-functionalized carbazole dyes for efficient molecular photovol-
taics: tuning of solar-cell performance by structural modification. Chemistry
of Materials 2008;20:3993e4003.