Performance of four artificial chlorin-type sensitizers with different stereostructures in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.dyepig.2013.01.013

Four artificial chlorin-type sensitizers have been prepared and their photovoltaic performance in dye-sensitized solar cells has been evaluated. It was found that the photovoltaic performance increased with growing absorption intensity of the Q band in a TiO₂ film. This result indicates the contribution of intrinsic enhanced absorption properties of chlorins in the Q band regions on the improvement of the overall photovoltaic performance in DSSCs. On the other hand, their capability of solar energy conversion also exhibits close relationship with the geometry of the four sensitizers, in which the orientation of a sterically demanding 2,6-dichlorophenyl group towards either the 5- or 15-position (the anchor group is at the 20-position) gives higher solar energy-to-electricity conversion efficiency.

Full text of DOI:10.1016/j.dyepig.2013.01.013

Dyes and Pigments 98 (2013) 181e189
Contents lists available at SciVerse ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Performance of four artificial chlorin-type sensitizers with different
stereostructures in dye-sensitized solar cells
Xiujun Liu a, Chengjie Li a, Xiao Peng a, Yongzhu Zhou a, Zhe Zeng a, Yuanchao Li a,
,
Tianyi Zhang a, Bao Zhang a, Yi Dong b, Dongming Sun c, Ping Cheng c, Yaqing Feng a
*
a School of Chemical Engineering and Technology, Tianjin University, No. 92 Weijin LU, Nankai QU, Tianjin 300072, PR China
b Physical and Theoretical Chemistry, University of Saarland, Saarbrücken 66123, Germany
c School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Four artificial chlorin-type sensitizers have been prepared and their photovoltaic performance in dye-
sensitized solar cells has been evaluated. It was found that the photovoltaic performance increased
with growing absorption intensity of the Q band in a TiO2 film. This result indicates the contribution of
intrinsic enhanced absorption properties of chlorins in the Q band regions on the improvement of the
overall photovoltaic performance in DSSCs. On the other hand, their capability of solar energy conversion
also exhibits close relationship with the geometry of the four sensitizers, in which the orientation of
a sterically demanding 2,6-dichlorophenyl group towards either the 5- or 15-position (the anchor group
is at the 20-position) gives higher solar energy-to-electricity conversion efficiency.
Received 21 November 2012
Received in revised form
14 January 2013
Accepted 15 January 2013
Available online 30 January 2013
Keywords:
Porphyrin
Chlorin
Ó 2013 Elsevier Ltd. All rights reserved.
1,3-Dipolar cycloaddition reaction
Dye-sensitized solar cell
Absorption spectra in TiO2 films
Photovoltaic performance
1. Introduction
by the role of chlorophylls (17,18-dihydroporphyrin) and bacterio-
chlorophylls (7,8,17,18-tetrahydroporphyrin) [35e37] in the natural
Due to their high solar energy-to-electricity conversion effi-
ciencies, easy fabrication, and low production costs [1], dye sensi-
tized solar cells (DSSCs) have attracted much attention as a new
generation of devices to utilize sunlight. Many kinds of sensitizers
have been developed, such as triarylamine [2e8], coumarin [9e11],
Ru-polypyridyl complexes [12e17], porphyrin [18e27], and other
organic sensitizers [28e33]. Among them, porphyrins show special
properties with high performance in DSSCs because of their strong
absorption and emission in the visible region, tunable redox po-
tentials and structural flexibility (four meso-positions and eight
photosynthesis as the main pigments to utilize solar energy over
a large window of the solar radiation spectrum, dihydroporphyrins
and tetrahydroporphyrins have attracted considerable interest
recently [38e43] in terms of powerful solar energy harvesting in the
area above 600 nm. In this field, many remarkable contributions
have been made by Wang and his co-workers who used chlorophyll
derivatives as sensitizers in DSSCs [40e42]. To artificially prepare
dihydroporphyrins or tetrahydroporphyrins, numerous methods
have been reported, e.g. DielseAlder reaction [44], reduction by
diimide [45], oxidation by OsO4 [46] and 1,3-dipolar cycloaddition
reaction [47e54] on the peripheral double bonds of pyrrolic rings. In
connection with our previous study on 1,3-dipolar cycloaddition
reaction between A4-porphyrin and nitrile oxides [48], we plan to
furnish the chlorin-type framework by this reaction between an
A3B-porphyrin and a nitrile oxide, and use the artificial chlorins in
DSSCs.
b
-positions) [34]. So far the best-performing porphyrin dyes have
been reported with conversion efficiency around 12% in DSSCs [18].
However, most of the highly efficient porphyrin sensitizers in DSSCs
lack the light-harvesting ability in the spectrum ranging from
600 nm to 800 nm. Therefore, it is absolutely necessary to extend the
absorption region of the sensitizers to efficiently utilize light energy
of more than 600 nm for the future development of DSSCs. Inspired
In this paper, we synthesized four chlorin-type sensitizers with
strong absorption at long wavelength, and studied their spectral,
electro-chemical, and photovoltaic properties, meanwhile we also
investigated the influencing factors of the photovoltaic perfor-
mance of the chlorin-type sensitizers in DSSCs.
* Corresponding author. Tel.: þ86 22 27891958; fax: þ86 22 27401795.
E-mail address: yqfeng@tju.edu.cn (Y. Feng).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.01.013

182
X. Liu et al. / Dyes and Pigments 98 (2013) 181e189
2. Experimental section
522 (14.7), 551 (16.7), 592 (8.3), 646 (21). IR(KBr, cmꢀ1): 3023.69,
2951.19, 2921.48,1723.45,1609.50,1568.29,1517.07,1431.52,1277.77,
1108.20, 790.35, 718.45.
2.1. Materials and analytical measurements
Cyc-2: 9.2% yield, 1H-NMR (400 MHz, CDCl3):
d
ꢀ1.98 (s, 1H,
Dry toluene was prepared by distillation under nitrogen in the
presence of sodium and benzophenone ketyl, and dichloromethane
(CH2Cl2) was distilled before being used for chromatography. Col-
umn chromatography (CC): silica gel (300e400 mesh or silica H).
Nuclear magnetic resonance (1H-, 1H-Roesy) spectra: Varian
pyrrole-NH), ꢀ1.92 (s, 1H, pyrrole-NH), 2.53 (s, 3H, eCH3), 2.66 (s,
3H, eCH3), 2.67 (s, 3H, eCH3), 4.09 (s, 3H, eCOOCH3), 6.80 (d,
J ¼ 7.7 Hz, 1H, Ph-H), 6.86 (d, J ¼ 7.8 Hz, 1H, Cl2Ph-m-H), 6.91 (d,
J ¼ 7.7 Hz, 1H, Ph-H), 7.11 (d, J ¼ 7.8 Hz, 1H, Cl2Ph-m-H), 7.16 (t,
J ¼ 7.8 Hz, 1H, Cl2Ph-p-H), 7.19 (d, J ¼ 10.1 Hz, 1H, H-8), 7.40 (d,
J ¼ 7.9 Hz,1H, Ph-H), 7.42 (d, J ¼ 7.9 Hz,1H, Ph-H), 7.48 (d, J ¼ 7.9 Hz,
1H, Ph-H), 7.54e7.57 (m, 2H, Ph-H), 7.65 (d, J ¼ 7.9 Hz,1H, Ph-H), 7.85
(d, J ¼ 7.9 Hz, 1H, Ph-H), 7.88 (d, J ¼ 10.1 Hz, 1H, H-7), 7.93 (d,
J ¼ 7.9 Hz, 1H, Ph-H), 8.16e8.20 (m, 3H, 2 Ph-H þ 1 pyrrole-H), 8.27
(d, J ¼ 8.0 Hz, 1H, Ph-H), 8.40e8.44(m, 4H, 2 Ph-H þ 2 pyrrole-H),
8.48 (d, J ¼ 7.9 Hz, 1H, Ph-H), 8.53 (d, J ¼ 4.5 Hz, 1H, pyrrole-H),
8.60 (d, J ¼ 4.5 Hz, 2H, pyrrole-H). Assignments aided by 1H ROESY
spectra. HRMS (ESI) (C56H41Cl2N5O3: exact mass ¼ 901.2586): calcd
for [M þ H]þ: 902.2659, found: 902.2667. UVeVis (CH2Cl2) lmax (nm)
(ε (103 Mꢀ1 cmꢀ1)): 418 (191.3), 522 (15.7), 549 (17.3), 594 (8.7), 646
(21.3). IR(KBr, cmꢀ1): 3349.19, 3023.22, 2949.06, 2918.64, 1721.43,
1606.17, 1567.84, 1515.69, 1432.53, 1279.26, 1108.63, 792.10, 716.37.
500 or Bruker AV400; chemical shifts (
d
) in ppm, with
d
(CHCl3) ¼
7.26 ppm,
d
(DMSO) ¼ 2.50 ppm for 1H NMR. HR-ESI-MS: Bruker
microTof-QII, positive ion mode and Bruker Autoflex Tof/Tof III. The
X-ray dates: a Rigaku CCD diffractometer using Mo K
a radi-
ꢀ
ation ¼ 0.71073A, at 113 (2) K. The UVeVis spectra: Shimadzu UV-
1800 in 10 mm quartz cell Spectrometer using EtOH as solvent.
The Fluorescence spectra: a Varian Cary Eclipse Spectrometer,
emission wavelengths
l in nm and the fluorescence lifetime:
HORIBA JY Fluorolog, single electron photoelectric counter (argon
saturated EtOH, excitation at 366 nm, detection at the maxima of
flourescence). Cyclic voltammetry measurements: CHI 660D (the
working electrode and the counter electrode are Pt wires, and
a Ag/Agþ is used as the reference electrode. Fc/Fcþ redox couple is
employed for calibration. The electrolyte is n-Bu4NClO4 (TBAP, 0.1 M)
in dry CH2Cl2).
Cyc-3: 8.9% yield, 1H-NMR (400 MHz, CDCl3):
d
ꢀ1.86 (s, 1H,
pyrrole-NH), ꢀ1.81 (s,1H, pyrrole-NH), 2.66e2.68 (m, 9H, eCH3), 4.08
(s, 3H, eCOOCH3), 6.89 (d, J ¼ 8.0 Hz, 1H, Cl2Ph-m-H), 7.07 (d,
J ¼ 8.0 Hz, 1H, Cl2Ph-m-H), 7.11 (d, J ¼ 10.2 Hz, 1H, H-18), 7.12 (d,
J ¼ 7.5 Hz, 1H, Ph-H), 7.20 (t, J ¼ 8.0 Hz, 1H, Cl2Ph-p-H), 7.42 (d,
J ¼ 7.6 Hz, 1H, Ph-H), 7.47 (d, J ¼ 7.9 Hz, 1H, Ph-H), 7.51e7.57 (m, 4H,
Ph-H), 7.65 (d, J ¼ 7.8 Hz,1H, Ph-H), 7.71 (d, J ¼ 7.5 Hz,1H, Ph-H), 7.85
(s,1H, Ph-H), 7.89 (d, J ¼ 10.2 Hz, 1H, H-17), 7.97 (s 1H, Ph-H), 8.07 (d,
J ¼ 4.4 Hz, 2H,1 Ph-H þ 1 pyrrole-H), 8.15 (d, J ¼ 7.6 Hz,1H, Ph-H), 8.18
(d, J ¼ 7.9 Hz, 1H, Ph-H), 8.27 (d, J ¼ 7.6 Hz, 1H, Ph-H), 8.40
(d, J ¼ 4.5 Hz, 1H, pyrrole-H), 8.48 (d, J ¼ 7.8 Hz, 1H, Ph-H), 8.51 (s,
2H, pyrrole-H), 8.59 (d, J ¼ 4.4 Hz, 1H, pyrrole-H), 8.69 (d,
J ¼ 4.5 Hz, 1H, pyrrole-H). Assignments aided by 1H ROESY spectra.
HRMS (ESI) (C56H41Cl2N5O3: exact mass ¼ 901.2586): calcd for
[M þ H]þ: 902.2659, found: 902.2638. UVeVis (CH2Cl2) lmax (nm)
(ε (103 Mꢀ1 cmꢀ1)): 416 (217.3), 521 (16.3), 550 (20), 593 (9.7),
645(25.3). IR(KBr, cmꢀ1): 3349.20, 3020.67, 2947.46, 2917.91,1722.33,
1604.38, 1567.30, 1514.18, 1433.11, 1276.12, 1106.74, 792.50, 718.04.
2.2. Synthesis of porphyrins
2.2.1. Synthesis of 5-((4-methoxycarbonyl)phenyl)-10,15,20-
triphenylporphyrin (P0)
Compound P0 bearing an ester group was prepared according to
the reported methods [55]. The yield of P0 is 15%, 1H-NMR
(400 M Hz, CDCl3):
d
ꢀ2.76 (s, 2H, pyrrole-NH), 2.71(s, 9H, eCH3),
4.12 (s, 3H, eCOOCH3), 7.56 (d, J ¼ 7.5 Hz, 6H, Ph-H), 8.10 (d,
J ¼ 7.5 Hz, 6H, Ph-H), 8.31 (d, J ¼ 7.8 Hz, 2H, Ph-H), 8.44 (d,
J ¼ 7.8 Hz, 2H, Ph-H), 8.78 (d, J ¼ 4.4 Hz, 2H, pyrrole-H), 8.88(s, 6H,
pyrrole-H). HRMS (ESI) (C49H38N4O2: exact mass ¼ 714.2995): calcd
for [M þ H]þ: 715.3068, found: 715.3069. UVeVis (CH2Cl2) lmax
(nm) (ε (103 Mꢀ1 cmꢀ1)): 420 (433.3), 517 (16.3), 552 (8.7), 592 (5),
648 (5.3). IR(KBr, cmꢀ1):3021.87, 2920.34, 1726.64, 1609.22,
1560.19, 1507.16, 1436.77, 1275.84, 1107.53, 796.79, 732.84.
Cyc-4: 9.5% yield, 1H-NMR (400 MHz, CDCl3):
d
ꢀ1.90 (s, 1H,
pyrrole-NH), ꢀ1.81 (s, 1H, pyrrole-NH), 2.53 (s, 3H, eCH3), 2.67 (s,
6H, eCH3), 4.08 (s, 3H, eCOOCH3), 6.80 (d, J ¼ 7.2 Hz, 1H, Ph-H),
6.85 (d, J ¼ 8.0 Hz, 1H, Cl2Ph-m-H), 6.89 (d, J ¼ 7.2 Hz, 1H, Ph-H),
7.11 (d, J ¼ 8.0 Hz, 1H, Cl2Ph-m-H), 7.15 (t, J ¼ 8.0 Hz, 1H, Cl2Ph-p-
H), 7.18 (d, J ¼ 10.2 Hz, 1H, H-17), 7.40 (d, J ¼ 7.5 Hz, 1H, Ph-H), 7.47
(d, J ¼ 7.7 Hz, 1H, Ph-H), 7.49e7.57 (m, 3H, Ph-H), 7.79 (d, J ¼ 8.3 Hz,
1H, Ph-H), 7.82 (d, J ¼ 10.2 Hz, 1H, H-18), 7.86 (d, J ¼ 7.3 Hz, 1H, Ph-
H), 7.90e8.00 (m, 2H, Ph-H), 8.06 (s, 1H, Ph-H), 8.18 (d, J ¼ 4.6 Hz,
2H, 1 Ph-H þ 1 pyrrole-H), 8.30 (d, J ¼ 4.5 Hz, 1H, pyrrole-H), 8.31
(d, J ¼ 8.3 Hz, 1H, Ph-H), 8.48e8.55 (m, 3H, 1 Ph-H þ 2 pyrrole-H),
8.59 (d, J ¼ 4.6 Hz, 1H, pyrrole-H), 8.65e8.75 (m, 2H, 1 Ph-H þ 1
pyrrole-H). Assignments aided by 1H ROESY spectra. HRMS (ESI)
(C56H41Cl2N5O3: exact mass ¼ 901.2586): calcd for [M þ H]þ:
902.2659, found: 902.2658. UVeVis (CH2Cl2) lmax (nm)
(ε (103 Mꢀ1 cmꢀ1)): 417 (170.3), 522 (13), 550 (16), 594 (8.3),
645(20.3). IR(KBr, cmꢀ1): 3350.82, 3021.35, 2922.32, 2855.82,
1723.69, 1605.01, 1568.30, 1515.15, 1433.99, 1277.28, 1108.59,
791.63, 725.82.
2.2.2. General procedure for the 1,3-dipolar cycloadditions
The general procedure of the reaction is described as follows: A
toluene (30 mL) solution of porphyrins P0 (0.1 g, 0.14 mmol) and
nitrile oxides (0.7 mmol) was refluxed for 6 h under N2. Further
portion of nitrile oxides (0.7 mmol) was added and the reflux
prolonged for an extra period of 6 h. The solvent was then evapo-
rated in vacuo and the residue was purified by column chroma-
tography on silica gel (silica H) to give the cycloadducts
Cyc-1: 10% yield, 1H-NMR (500 MHz, CDCl3):
d
ꢀ2.01 (s, 1H,
pyrrole-NH), ꢀ1.94 (s, 1H, pyrrole-NH), 2.53 (s, 3H, eCH3), 2.66
(s, 3H, eCH3), 2.68 (s, 3H, eCH3), 4.09 (s, 3H, eCOOCH3), 6.81
(d, J ¼ 7.8 Hz, 1H, Ph-H), 6.86 (d, J ¼ 7.8 Hz, 1H, Cl2Ph-m-H), 6.91
(d, J ¼ 7.8 Hz, 1H, Ph-H), 7.11 (d, J ¼ 7.8 Hz, 1H, Cl2Ph-m-H), 7.17
(t, J ¼ 7.8 Hz, 1H, Cl2Ph-p-H), 7.20 (d, J ¼ 10.1 Hz, 1H, H-7), 7.40 (d,
J ¼ 7.8 Hz,1H, Ph-H), 7.42 (d, J ¼ 7.9 Hz,1H, Ph-H), 7.53 (s, 2H, Ph-H),
7.57 (d, J ¼ 7.9 Hz, 1H, Ph-H), 7.65 (d, J ¼ 7.7 Hz, 1H, Ph-H), 7.89 (d,
J ¼ 10.1 Hz, 1H, H-8), 7.94 (d, J ¼ 7.8 Hz, 1H, Ph-H), 7.97 (s, 1H, Ph-H),
8.08 (d, J ¼ 6.5 Hz, 2H, Ph-H), 8.22 (d, J ¼ 4.5 Hz, 1H, pyrrole-H), 8.35
(d, J ¼ 7.5 Hz,1H, Ph-H), 8.37e8.46(m, 4H,1 Ph-H þ 3 pyrrole-H), 8.49
(d, J ¼ 7.7 Hz, 1H, Ph-H), 8.51 (d, J ¼ 4.5 Hz, 1H, pyrrole-H), 8.55 (d,
J ¼ 4.5 Hz, 1H, pyrrole-H), 8.71 (d, J ¼ 4.5 Hz, 1H, pyrrole-H). As-
signments aided by 1H ROESY spectra. HRMS (ESI) (C56H41Cl2N5O3:
exact mass ¼ 901.2586): calcd for [M þ H]þ: 902.2659, found:
902.2658. UVeVis (CH2Cl2) lmax (nm) (ε (103 Mꢀ1 cmꢀ1)): 419 (192),
Cyc-5: 9.8% yield, 1H-NMR (400 MHz, CDCl3):
d
ꢀ2.01 (s, 1H,
pyrrole-NH), ꢀ1.91 (s, 1H, pyrrole-NH), 2.51 (s, 6H, eCH3), 2.64 (s,
3H, eCH3), 4.07 (s, 3H, eCOOCH3), 6.83 (d, J ¼ 7.6 Hz, 2H, Ph-H),
6.86e6.93 (m, 4H, 2 Cl2Ph-m-H þ 2 Ph-H), 7.01 (dd, J ¼ 2.1 Hz,
10.0 Hz, 2H, H-8, H-17), 7.11 (d, J ¼ 7.8 Hz, 2H, Cl2Ph-m-H), 7.18 (t,
J ¼ 7.8 Hz, 2H, Cl2Ph-p-H), 7.36 (d, J ¼ 7.6 Hz, 2H, Ph-H), 7.44
(d, J ¼ 7.8 Hz, 1H, Ph-H), 7.60 (d, J ¼ 7.5 Hz, 1H, Ph-H), 7.68

X. Liu et al. / Dyes and Pigments 98 (2013) 181e189
183
(d, J ¼ 10.0 Hz, 1H, H-7), 7.70 (d, J ¼ 7.8 Hz, 1H, Ph-H), 7.74
(d, J ¼ 10.0 Hz, 1H, H-18), 7.80e7.88 (m, 4H, 2 Ph-H þ 2 pyrrole-H),
7.93 (d, J ¼ 7.8 Hz, 1H, Ph-H), 8.11 (d, J ¼ 4.4 Hz, 1H, pyrrole-H), 8.23
(d, J ¼ 4.4 Hz, 1H, pyrrole-H), 8.28 (d, J ¼ 7.5 Hz, 1H, Ph-H), 8.34
(d, J ¼ 7.8 Hz, 1H, Ph-H), 8.48 (q, J ¼ 8.4 Hz, 2H, Ph-H). Assign-
ments aided by 1H ROESY spectra. HRMS (ESI) (C63H44Cl4N6O4:
exact mass ¼ 1088.2178): calcd for [M þ H]þ: 1089.2251, found:
1089.2247. Crystallographic data for Cyc-5: C63H44Cl4N6O4,
Ph-H), 7.77 (d, J ¼ 10.6 Hz, 1H, H-17), 7.82 (d, J ¼ 7.2 Hz, 1H, Ph-H),
7.86 (d, J ¼ 4.5 Hz, 1H, pyrrole-H), 7.99 (d, J ¼ 7.2 Hz, 1H, Ph-H), 8.05
(d, J ¼ 7.8 Hz, 1H, Ph-H), 8.09 (d, J ¼ 4.5 Hz, 1H, pyrrole-H), 8.11 (d,
J ¼ 8.2 Hz,1H, Ph-H), 8.18 (d, J ¼ 7.8 Hz,1H, Ph-H), 8.24 (d, J ¼ 7.8 Hz,
1H, Ph-H), 8.29 (q, J ¼ 4.5 Hz, 2H, pyrrole-H), 8.34 (d, J ¼ 4.5 Hz, 1H,
pyrrole-H), 8.43 (d, J ¼ 4.5 Hz, 1H, pyrrole-H). MALDI Tof: m/z calcd
for C55H37Cl2N5O3Zn 949.16; found 949.32 [M]þ. UVeVis (CH2Cl2)
lmax (nm) (ε (103 Mꢀ1 cmꢀ1)): 418(243.3), 585(11.7), 612(28).
IR(KBr, cmꢀ1):3417.00, 3021.48, 2919.45, 1733.55, 1606.64, 1572.43,
1506.29, 1429.79, 1341.49, 1079.50, 795.42, 728.86.
Mw ¼ 1090.87, monoclinic, space group Cc, a ¼ 12.465(10),
ꢀ
b ¼ 32.62(3), c ¼ 15.008(15) A,
a
¼ 90,
b
¼ 99.42(5),
g
¼ 90 ,
3
ꢀ
3
V ¼ 6020(9) A , Dc ¼ 1.212 g/cm , Z ¼ 4, R1 ¼ 0.1272(4641),
wR2 ¼ 0.3649(10162). UVeVis (CH2Cl2) lmax (nm) (ε (103 Mꢀ1
cmꢀ1)): 389 (192.7), 538 (40.3), 644 (5.7), 704(68.7). IR(KBr,
cmꢀ1):3428.07, 3020.42, 2950.71, 2917.84, 2861.26, 1724.88,
1605.32, 1571.26, 1505.81, 1428.22, 1272.59, 1071.03, 795.37, 729.33.
Dye-4: 6.3 mg, 66%, 1H-NMR (500 MHz, DMSO):
d 2.45 (s, 3H, e
CH3), 2.60 (s, 3H, eCH3), 2.61 (s, 3H, eCH3), 6.70 (d, J ¼ 7.4 Hz, 1H,
Ph-H), 6.80 (d, J ¼ 7.4 Hz, 1H, Ph-H), 7.14 (d, J ¼ 10.7 Hz, 1H, H-17),
7.16 (d, J ¼ 7.9 Hz, 1H, Ph-H), 7.31e7.37 (m, 2H, Cl2Ph-m-H), 7.41 (t,
J ¼ 8.1 Hz, 1H, Cl2Ph-p-H), 7.48 (d, J ¼ 7.7 Hz, 1H, Ph-H), 7.52 (d,
J ¼ 7.5 Hz,1H, Ph-H), 7.55 (d, J ¼ 7.6 Hz, 2H, Ph-H), 7.75 (d, J ¼ 7.5 Hz,
1H, Ph-H), 7.78 (d, J ¼ 10.7 Hz, 1H, H-18), 7.82 (d, J ¼ 6.7 Hz, 2H, Ph-
H), 7.90 (d, J ¼ 4.5 Hz, 1H, pyrrole-H), 7.96e8.02 (m, 2H, Ph-H),
8.03e8.08 (m, 2H, 1 pyrrole-H þ 1 Ph-H), 8.20 (d, J ¼ 8.0 Hz, 1H,
Ph-H), 8.30 (q, J ¼ 4.5 Hz, 2H, pyrrole-H), 8.34 (d, J ¼ 4.5 Hz, 1H,
pyrrole-H), 8.35e8.41 (m, 2H, Ph-H), 8.44 (d, J ¼ 4.5 Hz,1H, pyrrole-
H), 13.15 (s, br. H, eCOOH). MALDI Tof: m/z calcd for
C55H37Cl2N5O3Zn 949.16; found 949.42 [M]þ. UVeVis (CH2Cl2) lmax
(nm) (ε (103 Mꢀ1 cmꢀ1)): 418(239), 585(13), 612(29.7). IR(KBr,
cmꢀ1): 3428.07, 3020.42, 2917.84, 1724.88, 1605.32, 1571.26,
1505.81, 1428.82, 1340.23, 1071.03, 795.37, 729.33.
2.2.3. Synthesis of chlorin-type dye sensitizers
A mixed solution of free-base monoadducts (0.01 mmol) in
CHCl3 (18 mL) and Zn(OAc)2,2H2O (4.39 mg, 0.02 mmol) in MeOH
were refluxed for 30 min, and the reaction mixture was filtered
through a short silica pad to give the zinc monoadducts. Then, the
zinc monoadducts was dissolved in THF/EtOH/H2O (8 mL/8 mL/
2 mL), and KOH (30 equiv) was added. The reaction was refluxed for
30 min, subsequently, the reaction mixture was acidified by diluted
HCl (5%) to adjust pH <7. Next, the mixture was extracted by CH2Cl2
to collect the organic layer which was washed with water, dried
and concentrated. Chromatography (CH2Cl2: MeOH ¼ 50:1) affor-
ded chlorin-type sensitizers.
2.3. Absorption spectra on TiO2 films
Dye-1: 6.8 mg, 72%, 1H-NMR (500 MHz, DMSO):
d 2.45 (s, 3H, e
CH3), 2.60 (s, 3H, eCH3), 2.62 (s, 3H, eCH3), 6.69 (d, J ¼ 7.6 Hz, 1H,
Ph-H), 6.84 (d, J ¼ 7.6 Hz, 1H, Ph-H), 7.17 (d, J ¼ 10.6 Hz, 1H, H-7),
7.17 (d, J ¼ 7.5 Hz, 1H, Ph-H), 7.34 (t, J ¼ 8.0 Hz, 2H, Cl2Ph-m-H), 7.41
(t, J ¼ 8.0 Hz,1H, Cl2Ph-p-H), 7.43 (d, J ¼ 7.7 Hz,1H, Ph-H), 7.50e7.62
Absorption spectra of the sensitizers deposited on TiO2 films
were measured with a Shimadzu UV-1800 spectrometer. The TiO2
films with a typical thickness of 5 mm were dipped into 0.3 mM
ethanol solution of chlorin dyes for 15 min and then the dye-
adsorbed films are rinsed with ethanol three times and dried in
air before measurement of the absorption spectra.
(m, 4H, Ph-H), 7.76 (d, J ¼ 10.6 Hz, 1H, H-8), 7.83 (d, J ¼ 7.0 Hz, 1H,
̊
Ph-H), 7.94 (d, J ¼ 4.5 Hz, 1H, pyrrole-H), 7.99 (d, J ¼ 6.5 Hz, 3H, Ph-
H), 8.11 (d, J ¼ 4.5 Hz, 1H, pyrrole-H), 8.19 (d, J ¼ 7.7 Hz, 1H, Ph-H),
8.23 (d, J ¼ 7.7 Hz, 1H, Ph-H), 8.26 (d, J ¼ 4.6 Hz, 1H, pyrrole-H),
8.27e8.35 (m, 4H, 2 Ph-H þ 2 pyrrole-H), 8.45 (d, J ¼ 4.6 Hz, 1H,
pyrrole-H), 13.16 (s, br. H, eCOOH). MALDI Tof: m/z calcd for
C55H37Cl2N5O3Zn 949.16; found 949.30 [M]þ. UVeVis (CH2Cl2) lmax
(nm) (ε (103 Mꢀ1 cmꢀ1)): 420(240.7), 586(9.7), 614(27.7). IR(KBr,
cmꢀ1): 3435.66, 3020.08, 2919.82, 1689.01, 1607.60, 1574.84,
1507.60, 1428.22, 1341.06, 1108.87, 791.47, 717.66.
2.4. Measurement of surface coverage (
G)
TiO2 films 0.16 cm2 in size, w16
mm in thickness were dipped
into an ethanolic solution containing each dye sensitizer for 2 h and
then washed with ethanol to remove free dye sensitizers on the
surface. The adsorbed dye sensitizers were estimated when dis-
solved in 3 mL of 0.1 M EtONa solution. The absorption spectra of
the EtONa solution of each sensitizer were measured to obtain the
Dye-2: 6.5 mg, 68%, 1H-NMR (500 MHz, DMSO):
d 2.45 (s, 3H, e
CH3), 2.60 (s, 3H, eCH3), 2.60 (s, 3H, eCH3), 6.70 (d, J ¼ 7.8 Hz, 1H,
Ph-H), 6.84 (d, J ¼ 7.8 Hz, 1H, Ph-H), 7.17 (d, J ¼ 10.7 Hz, 1H, H-8),
7.18 (d, J ¼ 8.0 Hz, 1H, Ph-H), 7.34 (t, J ¼ 7.9 Hz, 2H, Cl2Ph-m-H), 7.42
(t, J ¼ 7.9 Hz, 1H, Cl2Ph-p-H), 7.43 (d, J ¼ 7.8 Hz, 1H, Ph-H), 7.48 (d,
J ¼ 7.3 Hz, 1H, Ph-H), 7.54e7.62 (m, 3H, Ph-H), 7.76 (d, J ¼ 10.7 Hz,
2H, H-7 þ 1 Ph-H), 7.92 (d, J ¼ 4.5 Hz, 1H, pyrrole-H), 7.99 (d,
J ¼ 7.9 Hz,1H, Ph-H), 8.06 (d, J ¼ 7.2 Hz, 2H, Ph-H), 8.12 (d, J ¼ 4.5 Hz,
1H, pyrrole-H), 8.19 (d, J ¼ 7.9 Hz, 1H, Ph-H), 8.22 (d, J ¼ 7.7 Hz, 1H,
Ph-H), 8.27 (d, J ¼ 4.5 Hz, 2H, 1 pyrrole-H þ 1 Ph-H), 8.31 (d,
J ¼ 4.5 Hz, 2H,1 pyrrole-H þ 1 Ph-H), 8.36 (d, J ¼ 4.5 Hz,1H, pyrrole-
H), 8.41 (d, J ¼ 4.5 Hz,1H, pyrrole-H), 13.17 (s, br. H, eCOOH). MALDI
Tof: m/z calcd for C55H37Cl2N5O3Zn 949.16; found 949.20 [M]þ. UVe
Vis (CH2Cl2) lmax (nm) (ε (103 Mꢀ1 cmꢀ1)): 420(241.3), 586(12.3),
613(28.7). IR(KBr, cmꢀ1):3433.08, 3020.69, 2920.01, 1731.28,
1607.65, 1574,69, 1507.97, 1429.48, 1341.51, 1109.03, 793.04, 719.40.
surface coverage (G) according to the standard method [56].
2.5. Fabrication of DSSCs and photovoltaic measurements
The optically transparent electrode (OTE) with 0.16 cm2 working
area contains 20 nm TiO2 nanoparticles with thickness of 12
and 400 nm TiO2 nanoparticles (diffusive layer) with thickness of
m was prepared by screen print for light-harvesting. The elec-
mm
4
m
trolyte consisted of 0.06 M LiI, 0.03 M I2, 0.6 M 1,2-dimethyl-3-
propylimidazolium iodide (DMPII), and 0.5 M 4-tert-butylpyr-
idine (TBP) in a mixture of acetonitrile and valeronitrile (volume
ratio, 85: 15) [57]. The porphyrin dyes sensitized TiO2 electrodes
were measured under simulated AM 1.5 irradiation (100 mW/cm2)
and the photocurrentevoltage (JeV) characteristics were recorded
on Keithley 2400 Source meter (solar AAA simulator, oriel China,
calibrated with a standard crystalline silicon solar). The power
Dye-3: 6.6 mg, 70%, 1H-NMR (500 MHz, DMSO):
d 2.59 (s, 6H, e
CH3), 2.61 (s, 3H, eCH3), 7.03 (d, J ¼ 7.8 Hz, 1H, Ph-H), 7.11 (d,
J ¼ 10.6 Hz, 1H, H-18), 7.16 (d, J ¼ 8.2 Hz, 1H, Cl2Ph-m-H), 7.19 (d,
J ¼ 8.2 Hz, 1H, Cl2Ph-m-H), 7.42 (t, J ¼ 8.2 Hz, 1H, Cl2Ph-p-H), 7.43
(d, J ¼ 7.5 Hz, 1H, Ph-H), 7.47 (d, J ¼ 7.8 Hz, 1H, Ph-H), 7.51 (d,
J ¼ 7.6 Hz, 2H, Ph-H), 7.53e7.61 (m, 4H, Ph-H), 7.75 (d, J ¼ 7.6 Hz,1H,
conversion efficiency (h) of the DSSCs was calculated from short-
circuit photocurrent (Jsc), the open-circuit photovoltage (Voc), the
fill factor (FF) and the intensity of the incident light (Pin) according
to the following equation:

184
X. Liu et al. / Dyes and Pigments 98 (2013) 181e189
À
Á
Jsc mA=cm2 ꢁ VocðVÞ ꢁ FF
À
Á
h
¼
Pin mW=cm2
The action spectra of monochromatic incident photon-to-
current conversion efficiency (IPCE) for the solar cells were per-
formed by using a commercial setup (QTest Station 2000 IPCE
Measurement System, CROWNTECH, USA).
2.6. Theoretical calculations
Full ground geometry optimization of dyes was carried out by
using DFT calculation with B3LYP exchange-correlations functional
and the 6-31G(d, p) basis set and with CPCM solvent model (EtOH)
[43,58e62]. The TD-DFT calculations with the BP 86 exchange-
correlations functional and the 6-31G(d, p) basis set were based
on the optimized structure with solvation effect described by CPCM
(EtOH) for the set of sensitizers [63e68].
Fig. 1. 1H-chemical shift data with 2D ROESY correlations for compound Cyc-1.
3. Results and discussions
1,3-dipolar cycloaddition reaction affording both the monoadducts
and the bisadducts. The chlorin-type sensitizers were then
obtained after the metallization of the monoadducts and following
hydrolysis of the COOMe group. In detail, the solution of P0 and
an excess of nitrile oxides in toluene was heated to reflux under N2
3.1. Synthesis and characterization
The synthetic route to artificial chlorin-type sensitizers is
depicted in Scheme 1. Here, P0 [55] was treated with nitrile oxide
[47,48] so that the reaction substances underwent the typical
Scheme 1. The synthetic route to the chlorin-type sensitizers.

X. Liu et al. / Dyes and Pigments 98 (2013) 181e189
185
Fig. 2. The crystal structure of the bisadduct Cyc-5 (left: front view, right: side view).
until TLC revealed no further progress of the reaction. Subse-
quently, metallization of the free base monoadducts with
Zn(OAc)2,2H2O, followed by alkaline hydrolysis of the COOMe
group, gave the chlorin-type sensitizers. As P0 was a A3B-type
porphyrin, there should be a certain selectivity to form mono-
adducts in the cycloaddition reaction. However, four monoadducts
with similar yields were isolated and demonstrated the equal
reactivity of the peripheral double bonds in pyrrolic rings of this
kind of A3B-porphyrin. In addition, o-dichlorobenzene was also
employed as an alternative to toluene as the solvent in the 1,3-
dipolar cycloaddition reaction, but the yield decreased sharply
due to the elevated reaction temperature.
rings, the signals of the protons in four meso-benzene rings were
obtained. The protons of the saturated pyrrolic ring at 7.20 ppm and
7.89 ppm were coupling with the aromatic protons at 7.94 ppm (of
the benzene ring 1) and 7.57 ppm (of the benzene ring 2). This
result indicates the occurrence of the cycloaddition reaction on the
pyrrolic ring between two 4-methylphenyl groups. In addition,
another signal of aromatic proton in benzene ring 1 (6.91 ppm)
correlated to the proton with the signal at 8.22 ppm (J ¼ 4.5 Hz)
which should be assigned to the unsaturated pyrrolic proton. Fol-
lowing another coupling of the signal with that of the unsaturated
pyrrolic ring proton at 8.22 ppm, the other pyrrolic proton signal at
8.51 ppm (J ¼ 4.5 Hz) was picked up. As the signals at 8.43 ppm
which were assigned to the aromatic proton in 4-CH3COOPh
group correlated to pyrrolic proton signal at 8.51 ppm, the four
meso-substituted aromatic rings and cycloaddition reaction
site were identified besides the spatial orientation of the 2,6-
dichlorophenyl group. On the basis of the coupling between the
signal at 2.53 ppm from benzene ring 1 and signal at 7.11 ppm (of
the aromatic proton in 2,6-dichlorophenyl group), the orientation
of the 2,6-dichlorophenyl group was confirmed to point towards
benzene ring 1 which was next to the 4-carboxymethylphenyl
group. So the exact structure of this monoadduct was established.
By the same way, the structures of the other three monoadducts
were established.
The respective structures of the four isomeric monoadducts
were deduced from one- and two-dimensional 1H-NMR spectro-
scopy. In the 1H-NMR spectra of monoadducts (Figure S8eS11 in
Supplementary Materials), the split of the signals corresponding to
b
-protons and phenyl protons were detected at low field. These
changes demonstrate the lower symmetry of the porphyrin
framework after the cycloaddition reaction. Meanwhile, two sets
of doublets (J ¼ 9.8e10.7 Hz) were observed at the similar chemical
shift values, suggesting their assignment as that of saturated pyr-
rolic protons (H-7 and H-8 or H-17 and H-18). The identification of
the cycloaddition site was achieved by tracing the HeH correlations
of the monoadducts. Here, we took the analysis of Cyc-1 as an
example (Fig.1 and Figure S1 in Supplementary Materials). First, we
marked the meso-benzene rings with CH3 as 1, 2, 3 according to the
chemical shift value of the CH3 group at 2.53 ppm, 2.66 ppm,
2.68 ppm, respectively, and the meso-benzene ring with COOCH3 as
4. On the basis of the coupling between the protons in the CH3 or
COOCH3 group and the protons in meso-benzene rings themselves,
eight aromatic protons belonging to different benzene rings were
found, e.g. the coupling partner of CH3 (2.53 ppm) at 6.81 ppm and
7.40 ppm could be assigned to the aromatic protons in benzene ring
1. Then, because of the correlation between the known eight aro-
matic protons and the unknown eight aromatic protons in benzene
In addition, four bisadducts were also separated in the cyclo-
addition. One of the bisadducts Cyc-5 has been characterized by
X-ray analysis (Fig. 2). In its structure, the two 2,6-dichlorophenyl
groups were added to the opposite pyrrolic ring in the same ori-
entation. The structures of the other three bisadducts have not yet
been fully elucidated.
3.2. Optical and electrochemical properties
The molecular structures of the four chlorin sensitizers Dye-1,
Dye-2, Dye-3 and Dye-4 were exhibited in Fig. 3. And their UVeVis
Fig. 3. The molecular structures of four chlorin sensitizers Dye-1, Dye-2, Dye-3 and Dye-4.

186
X. Liu et al. / Dyes and Pigments 98 (2013) 181e189
Fig. 4. The UVeVis spectra of P0, Dye-1, Dye-2, Dye-3 and Dye-4 in EtOH (3 ꢁ 10ꢀ6 mol/L) (left) and the UVeVis spectra of Dye-1, Dye-2, Dye-3 and Dye-4 on TiO2 films (5
mm)
(right).
spectra in EtOH are shown in Fig. 4 (left). The absorption maxima
and extinction coefficients for the Soret- and Q-bands of all prod-
ucts are listed in Table 1. Compared with the spectrum of P0, the
spectra of four chlorin-type sensitizers show a distinct red-shift and
higher molar extinction coefficients in the 600 nme620 nm spec-
tral ranges due to the partial saturated pyrrolic ring resulting from
the cycloaddition reaction [47,69], which is accompanied by lower
molar extinction coefficients of their Soret bands. In addition, the
Soret bands of Dye-3 and Dye-4 show a w2 nm blue shift compared
with that of the Dye-1 and Dye-2. This phenomena indicates
greater distortion of the porphyrin framework when the dichlor-
ophenyl is introduced to the pyrrolic ring IV (Scheme 1).
vibronic bands for Dye-1, Dye-2, Dye-3 and Dye-4 at around 620
and 670 nm. The strongest fluorescence quenching was observed in
the spectra of Dye-1 due to the possible electron transfer from
porphyrin core to the COOH group. Meanwhile, the time-resolved
fluorescence spectroscopy was used to detect the fluorescence
lifetime of the chlorin-type sensitizers. All profiles decay as a single
exponential with a time constant s in nanoseconds (Table 1) which
is several orders of magnitude greater than that of the electron
injection (in picoseconds) into the conduction band of the TiO2 [24].
Thus, the excited electron of the four dyes could have enough time
to transfer into the semiconductor under illumination.
Cyclic voltammetry was employed to determine the redox po-
tentials of the chlorins (see Figure S5 in Supplementary Materials).
All chlorin-type sensitizers show reversible oxidation waves and
quasi-reversible reduction waves. The first oxidation potentials vs
Fc/Fcþ are listed in Table 2. Their first oxidation potentials corre-
sponding to the HOMO energy of the dye are all lower than that of
the Iꢀ/Iꢀ3 couple [71], while the LUMO energy are all higher than
that of the conduction band of the TiO2 semi-conductor. This result
ensures the generation of current in the presence of sunlight.
Additional one-electron oxidations are present at a potential of
about 0.4 V, corresponding to the formation of the dication from
the radical cation. There are also reversible one-electron reductions
at around ꢀ1.4 V due to the reduction of the porphyrin ring and
another irreversible one-electron reduction at about ꢀ1.8 V as
a result of the reduction of the porphyrin anion.
The light-harvesting capability of the dyes is a significant
parameter which determines the amount of solar energy that can
be captured by DSSCs and consequently affects the photocurrent
generated. Fig. 4 (right) exhibits the UVeVis spectra of the four
chlorin-type sensitizers anchored on the TiO2 films (5 mm). Com-
pared to the sensitizers in solution, the anchorage onto TiO2 films
shifts the absorption peaks to a longer wavelength region together
with a substantially broadened bandwidth. These changes may
attribute to the J-type aggregation of the dyes on the TiO2 surface
[70]. Furthermore, the absorption intensity of the Q bands of the
sensitizers behaves similar to that of Soret bands and are in the
order Dye-1 > Dye-4 > Dye-2 > Dye-3. Here, we should emphasize
that the Q band absorption intensity of Dye-1 is even stronger than
that of the Soret band. The enhancement of the Q bands absorption
after being deposited on TiO2 films proposes the dominant role of Q
bands absorption in light-harvesting and light-converting for this
kind of sensitizers.
The steady-state fluorescence spectra of the four chlorin-type
sensitizers, exhibited in Fig. 5, was obtained in EtOH with excita-
tion wavelength at 420 nm. The spectra show characteristic
Table 1
Absorption, fluorescence and fluorescence lifetime for porphyrins dyes.
lmaxa, nm (ε, 103 Mꢀ1,cmꢀ1
)
Emission lmax
sc (ns)
b
(nm)
P0
420(433.3), 517(16.3), 552(8.7),
592(5.0), 648(5.3)
Dye-1
Dye-2
Dye-3
Dye-4
420(240.7), 586(9.7), 614(27.7)
420(240.7), 586(12.3), 613(28.7)
418(243.3), 585(11.7), 612(28)
418(239), 585(13), 612(29.7)
617, 670
617, 669
616, 671
616, 670
1.85
1.64
1.72
1.69
a
Absorption data were obtained in EtOH solution at 298 K.
Emission spectra were obtained at 298 K in EtOH by exciting at 420 nm.
Fluorescence lifetime was measured in argon saturated EtOH by exciting at
b
c
Fig. 5. The emission spectra of Dye-1, Dye-2, Dye-3 and Dye-4 in EtOH (3 ꢁ 10ꢀ6 mol/L,
366 nm.
excitation at 420 nm).

X. Liu et al. / Dyes and Pigments 98 (2013) 181e189
187
Table 2
3.4. Photovoltaic properties of porphyrin-sensitized DSSCs
Electrochemical data for chlorin-type sensitizers.
Eoxa (V)
HOMOb (eV)
Eg (eV)
LUMOd (eV)
c
Excitation of the porphyrin molecules with visible light will lead
to the electronically excited state that undergoes electron-transfer
quenching as a consequence of electrons being injected into the
conduction band of the semiconductor. The oxidized porphyrin is
subsequently reduced back to the ground state (S) by the electron
donor (Iꢀ) in the electrolyte. The electrons in the conduction band
are collected and subsequently pass through the external circuit
to arrive at the counter electrode. The photocurrentephotovoltage
(IeV) and the incident photon-to-current conversion efficiency
(IPCE) curves of the DSSCs based on the four isomeric chlorin-type
sensitizers have been measured (Fig. 7 and Table 3), in which the
Zn-3 [73] (see Supplementary Materials, Figure S3) was used as
reference in the DSSCs. The same photovoltaic performance order,
Dye-1>Dye-4>Dye-2>Dye-3, were observed in the IeV curves and
the IPCE profiles. This order is consistent with that of the Q band
absorption properties of the chlorin-type sensitizers in TiO2 films.
Thus, this result demonstrates the significance of the absorption in
long-wavelength on the overall solar energy conversion capability.
Dye-1
Dye-2
Dye-3
Dye-4
0.203
0.198
0.207
0.202
ꢀ5.003
ꢀ4.998
ꢀ5.007
ꢀ5.002
2.01
2.00
2.01
2.01
ꢀ2.993
ꢀ2.998
ꢀ2.997
ꢀ2.992
First oxidation potentials vs ferrocene/ferrocenium (Fc/Fcþ) couple.
a
b
c
þ
HOMO ¼ ꢀ4:8 ev ꢀ ðEox1 ꢀ EFc=Fc Þ:
Eg is obtained from the intersecting point between the UVeVis spectra and the
emission spectra.
d
LUMO ¼ HOMO þ Eg.
In addition, through measuring surface coverage (G) of each sen-
sitizer (Dye-2>Dye-1>Dye-4>Dye-3), we have found that the
amount of dyes adsorbed onto the TiO2 film was not proportional to
the photovoltaic performance. Dye-2 attains the maximum
G value,
however, its solar energy conversion capability in DSSCs locates in
the middle among the four dyes. For Dye-2 with the dichlorophenyl
group oriented to the 10-position (opposite to COOH group), this
structure is prone to aggregation which could lead to the suggested
exciton annihilation [43,74] when anchored onto a TiO2 film and
Fig. 6. Energy levels (eV) of HOMO, LUMO, LUMO þ 1, and LUMO þ 2 molecular or-
consequently leads to a decrease of conversion efficiency (
h) in
bitals for the four chlorin-type sensitizers based on DFT calculations.
spite of the maximum . On the other hand, Dye-3 with the
G
dichlorophenyl group oriented towards the 20-position (COOH
group) makes itself difficult to anchor onto TiO2 film, and affords
3.3. DFT calculation
the minimum
mance. By contrast, Dye-1 and Dye-4 with the dichlorophenyl
group oriented to the 5- or 15-position give the middle value. In
their condition, the dichlorophenyl group doesn’t block anchorage
onto TiO2 films and can also reduce the aggregation of sensitizers.
Therefore, Dye-1 and Dye-4 give better photovoltaic performance
among the four chlorin-type sensitizers. So, the photovoltaic per-
formance was also closely bound with the stereo-structures of the
G value as well as the poorest photovoltaic perfor-
In order to further understand the electronic properties asso-
ciated with the geometry of the four chlorin-type sensitizers with
2,6-dichlorophenyl group, ab initio quantum mechanical calcula-
tions were performed with the TD-DFT variant hybrid density
functional theory (BP 86) in conjunction with the 6-31G(d,p) basis
set as implemented in the Gaussian 09 program package [72]. The
distribution of the electron cloud at the molecular orbitals HOMO,
LUMO, LUMOþ1 and LUMOþ2 are shown in Figure S6 (in
Supplementary Materials). Fig. 6 displays the molecular orbital
energy levels of the four chlorin-type sensitizers which are com-
pared with the conduction band of TiO2 and Iꢀ/I3ꢀ based on DFT
calculations. The reasonable energy match between them suggests
that a favourable current could be produced in these porphyrin-
sensitized solar cells under sunlight.
G
four sensitizers. In brief, we have realized that the
h exhibited close
relationship with the Q band absorption in TiO2 films and the
“spatial orientation” of the dichlorophenyl group. When the sen-
sitizer has a stronger Q band absorption intensity after connecting
with TiO2 films and the steric hindrance group orients to the meso-
substituent being perpendicular to the meso-position of the anchor
group, it will give better performance in the DSSCs. In our study,
Fig. 7. The IPCE profiles (left) and the IeV curves of the chlorin-type sensitizers (right).

188
X. Liu et al. / Dyes and Pigments 98 (2013) 181e189
[5] Cheng X, Sun S, Liang M, Shi Y, Sun Z, Xue S. Organic dyes incorporating the
Table 3
cyclopentadithiophene moiety for efficient dye-sensitized solar cells. Dyes
Pigments 2012;92(3):1292e9.
[6] Wu W, Yang J, Hua J, Tang J, Zhang L, Lomg Y, et al. Efficient and stable dye-
sensitized solar cells based on phenothiazine sensitizers with thiophene units.
J Mater Chem 2012;20(9):1772e9.
[7] Ying W, Guo F, Li J, Zhang Q, Wu W, Tian H, et al. Series of new D-A-pi-A
organic broadly absorbing sensitizers containing isoindigo unit for highly
efficient dye-sensitized solar cells. ACS Appl Mater Interfaces 2012;4(8):
4215e24.
The constants of the DSSCs based on the chlorin-type sensitizers.
Dyes
G
, mol/cm2
Jsc, mA/cm2
Voc, V
FF
h, %
Zn-3
7.78
8.55
7.21
4.35
8.37
630
610
600
590
600
72.94
69.96
67.41
72.06
71.38
3.57
3.65
2.92
1.85
3.60
Dye-1
Dye-2
Dye-3
Dye-4
8.55 ꢁ 10ꢀ8
1.17 ꢁ 10ꢀ7
5.39 ꢁ 10ꢀ8
7.85 ꢁ 10ꢀ8
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carbazole antennas for efficient and stable dye-sensitized solar cells. Energy
Environ Sci 2010;3(11):1736e45.
Dye-1 gave the highest efficiency of 3.65% with a short circuit
photocurrent density of 8.55 mA/cm2, an open-circuit voltage of
610 mV, and a fill factor of 0.70 under the standard illumination test
conditions.
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sensitized solar cells. New J Chem 2003;27:783e5.
4. Conclusions
[11] Seo KD, Choi IT, Park YG, Kang S, Lee JY, Kim HK. Novel D-A-
p-A coumarin
dyes containing low band-gap chromophores for dye-sensitised solar cells.
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In summary, the artificial chlorin-type sensitizers with high
absorption above 600 nm have been implemented in DSSCs. We
find that the photovoltaic performance of the four sensitizers
closely relates with the Q band absorption after being deposited on
TiO2 film and the stereo-structures of the sensitizers. The increas-
ing Q band absorption will improve the total conversion efficiency
from solar energy to electricity. On the other hand, the steric hin-
drance group being perpendicular to the anchor group will give
a positive effect on the capacity of converting solar energy for this
kind of chlorin-type sensitizers, while the steric hindrance of the
anchor group will have a negative effect. The results offer a new
proposed strategy to the design of chlorin-type sensitizer for better
utilization of solar energy in future. However, 4-carboxyphenyl
group and the porphyrin framework are not coplanar due to the
steric hindrance in space, in which the electron transfer would be
blocked. Thus introduction of the alkyne or alkene or a thiophene
ring [24e27] between them could be a reasonable way to enhance
the electron transfer from excited porphyrin under light. In addi-
tion, inspired by Wang’s work [70], addition of some coadsorbent,
e.g. CDCA or other organic dyes, could be another potential method
to improve the photovoltaic performance of chlorin-type sensi-
tizers. Further work on these is in progress.
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Evaluation of a ruthenium oxyquinolate architecture for dye-sensitized solar
cells. Inorg Chem 2011;51(1):1e3.
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Acknowledgements
We thank Prof. Dr. Song Xue for the measurement of IPCE.
This work is supported by National Natural Science Foundation of
China (No. 21076147), Natural Science Foundation of Tianjin (No.
10JCZDJC23700), National International S&T Cooperation Founda-
tion of China (No. 2012DFG41980) and Independent Innovation
Foundation of Tianjin University (2010).
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
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