Meso-Schiff-base substituted porphyrin dimer dyes for dye-sensitized solar cells: Synthesis, electrochemical, and photovoltaic properties

Article abstract of DOI:10.1007/s11164-013-1525-1

Two novel large conjugated system meso-Schiff-base substituted porphyrin dimers, which contained the thienyl or p-methylphenyl functional groups, have been designed and synthesized for the efficient green dyes for dye-sensitized solar cells (DSSCs) applic

Full text of DOI:10.1007/s11164-013-1525-1

Res Chem Intermed
DOI 10.1007/s11164-013-1525-1
Meso-Schiff-base substituted porphyrin dimer dyes
for dye-sensitized solar cells: synthesis, electrochemical,
and photovoltaic properties
•
Xuejun Zhang Ying Zhu Xiaobo Wu
•
•
•
Huipeng He Gaolei Wang Qiaoling Li
•
Received: 21 September 2013 / Accepted: 22 December 2013
Ó Springer Science+Business Media Dordrecht 2014
Abstract Two novel large conjugated system meso-Schiff-base substituted por-
phyrin dimers, which contained the thienyl or p-methylphenyl functional groups, have
been designed and synthesized for the efficient green dyes for dye-sensitized solar
cells (DSSCs) applications. The synthesis and characterization of the large conjugated
system based on porphyrin dimer derivatives have been investigated through their
spectral, electrochemical, and photovoltaic performances by DSSCs study. Due to the
increase of the conjugated system, the two porphyrin dimers have some large red-shift
at the maximum absorption, high fluorescence quantum yield. However, p-(10,15,20-
triphenyl-5-porphyrinyl) benzylidene aminophenyl-10,15,20-(2-thienyl)-porphyrin,
based on DSSCs reaching 2.86 % under AM 1.5 G irradiation, showed slightly better
comprehensive properties than the p-(10,15,20-triphenyl-5-porphyrinyl) benzylidene
aminophenyl-10,15, 20-(p-methylphenyl)-porphyrin.
Keywords Porphyrin dimer ꢀ Schiff base ꢀ Synthesis ꢀ Electrochemistry ꢀ
DSSCs
Introduction
Dye-sensitized solar cells (DSSCs) have attracted considerable attention because of
their ease of fabrication and cost-effectiveness compared with silicon (Si)-based
photovoltaic devices [1]. In recent years, efficiencies in the conversion of light to
electric power of up to 11 % have been obtained in nanocrystalline TiO2 solar cells
sensitized with polypyridyl ruthenium complexes [2–4]. However, the ruthenium
dyes face the problems of costs and environmental issues, which will limit their
X. Zhang (&) ꢀ Y. Zhu ꢀ X. Wu ꢀ H. He ꢀ G. Wang ꢀ Q. Li
Department of Chemistry, College of Science, North University of China,
Taiyuan 030051, Shanxi, China
e-mail: zhangxuejun@nuc.edu.cn
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X. Zhang et al.
large-scale application to DSSCs and encourage exploration of cheaper and safer
sensitizers [5–9]. Therefore, organic dyes, such as coumarin, indoline, oligoene,
thiophene, triarylamine, perylene, cyanine, and hemicyanine derivatives [10–14],
have drawn great interest. Among the alternative organic dyes, porphyrins having
strong absorption and emission in the visible region as well as tunable redox
potentials are promising candidates for light harvesting dyes for DSSCs. So far, the
¨
Gratzel group has reported porphyrin-sensitized solar cells with cobalt(II/III)-based
redox electrolyte, whose power conversion efficiency exceeds 12 % [15, 16].
Compared to porphyrin monomers, porphyrin dimers have similar structures to
the reaction center P680 and P700 structures of the bacterium Rhodopseudomonas,
which can be attributed to the rapid energy and electron-transfer owing to the direct
linking and short center-to-center distance between porphyrin subunits [17, 18]. The
strong excitonic interaction between two porphyrin rings imparted predominant
spectroscopic, photovoltaic, and nonlinear optical properties to the molecules. Over
the past two decades, great efforts have been devoted to synthesizing such
interesting compounds. A large variety of porphyrin dimers have been synthesized,
which contained different types of functional groups with –C=C–C=C– [19], –O–
[20], –OCOC– [21], –C6H4– [22], and –(CH2)n– [23] as a bridge to substitute the
two porphyrins at the meso- or beta-positions and had different central metal ions, to
develop other usage as special functional materials [24–26]. And some of them have
been characterized and tested in DSSCs. The C=N unit is a polar double bond,
which could not only increase the conjugation degree of the molecules but also
effectively promote the transmission of the excited electron to prevent the electronic
quenching and improve the photoelectric conversion efficiency. Monomer Schiff
bases have been synthesized and regarded as dye sensitizers in solar cells, and
showed a good performance [27–29]. Therefore, meso-Schiff-base substituted
porphyrin dimers, which have large conjugation, easy modification with 6 meso and
16 beta positions, two metal coordination centers and the polar double bond of the
–C=N– unit, could exhibit excellent characteristics.
To investigate the performance of the porphyrin dimers with the –C=N– group as
a bridge and to get better photosensitive quality, we have designed and synthesized
two porphyrin derivatives containing one –C=N– group as a bridge and two
different types of functional groups with thienyl and p-methylphenyl substituted at
the meso-position of the two porphyrin rings, respectively (Fig. 1). The new
compounds are expected to have broader and intenser absorptions than the
corresponding porphyrin monomer. Here, the synthesis and the spectral, electro-
chemical, and photovoltaic properties of these porphyrin dimers based on sensitizers
are reported. For comparison, the cell performance of the device using p-(10,15,20-
triphenyl-5-porphyrinyl) benzylidene aminophenyl-10,15,20-(2-thienyl)-porphyrin
(TPBATP) as a sensitizer slightly outperforms the porphyrin of p-(10,15,20-
triphenyl-5-porphyrin-yl) benzylidene aminophenyl-10,15,20-(p-methylphenyl)-
porphyrin (TPBAMP) dye. Therefore, TPBATP might be a promising green dye
for future colorful DSSCs application. The aim of this paper is to describe the
synthesis of such TPBAMP or TPBATP with other groups and –C=N– linker
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Meso-Schiff-base substituted porphyrin dimer dyes
Fig. 1 Molecular structure of TPBAMP and TPBATP
between the two porphyrins parts, as well as evaluate their electrochemistry
compared to the free monomer currently used for DSSCs.
Experimental
Analytical measurements
1H NMR spectra were recorded with a Varian Inova 400 MHz NMR system.
MALDI-TOF mass spectrometric measurements were performed on Bruker Bifiex
III MALDI-TOF. UV–Vis spectra of the dyes were measured on a Techcomp 2300
spectrophotometer. The photoluminescence (PL) spectra were obtained using
Hitachi F-2500 luminescence spectrometer. Cyclic and differential pulse voltam-
metry measurements were performed on an electrochemical workstation
(CHILK2005A; Tianjin Lanlike Chemistry Electronic High-tech, China).
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X. Zhang et al.
Preparation of materials
All starting materials, such as pyrrole, benzaldehyde, p-methyl benzaldehyde,
2-thiophenecarboxaldehyde, p-nitrobenzaldehyde, propionic acid, stannous chloride,
paraformaldehyde, zinc chloride, hexamethylenetetramine, glacial acetic acid,
chloroform, nitrobenzene, absolute methanol, N,N-dimethylformamide, and hydro-
chloric acid, were purchased from Aladdin. Pyrrole was dried and distilled before use.
All other solvents and chemicals used in this work were of analytical grade and used
without further purification. Column chromatography was performed with the
indicated solvents using E. Merck silica gel 60 (particle size 0.035–0.070 mm).
Macherey–Nagel precoated plates (SIL G-200, 2 mm) were used for preparative thin-
layer chromatography. Yields refer to chromatographically and spectroscopically
pure compounds.
Synthesis
5-(p-Nitrophenyl)-10,15,20-(p-methylphenyl) porphyrin 1
In a three-necked flask, 4-methylbenzaldehyde (2.88 g, 24 mmol) and 4-nitro-
benzaldehyde (1.208 g, 8 mmol) were dissolved in 150 ml propionic acid. The
solution was slowly heated to 140 °C, and then freshly distilled pyrrole (2.15 g,
32 mmol) was dissolved in 10 ml of propionic acid, added dropwise to the reaction
mixture over a period of 10 min, and stirred for another 30 min. After cooling to room
temperature, half of the solvent was evaporated and 150 ml CH3OH was added. The
mixture was cooled overnight and filtrated. The crude product was purified by column
chromatography (eluent: dichloromethane:petroleum ether = 5:1, v/v). A dark
1
purple solid of compound 1 was obtained (0.84 g, 15 %). H NMR (400 MHz,
CDCl3, ppm): 8.88 (m, 8H, b-pyrrole), 8.11–8.13 (d, 8H, J = 7.2 Hz, o-Ph), 7.57–
7.59 (d, 8H, J = 8.4 Hz, m-Ph), 2.73 (s, 9H, Me-H), -2.74 (s, 2H, NH). IR (KBr
pellet, cm-1): 3,349, 966 (tN–H, pyrrole), 3,057 (tC–H, pyrrole), 1,506, 1,471, 1,219
(tCN, pyrrole), 1,509 (tC–C, pyrrole), 3,024, 1,153, 1,182 (tC–H, phenyl), 1,596 (tC=C
,
phenyl), 1,607 (tC–C, phenyl), 2915 (tC–H, methyl), 1,566 (tNO ). MALDI-TOF MS
calcd. For C47H23N5O2 689.72; found 689.74. Elem. Anal. For C47H23N5O2 calcd.: C,
81.85; H, 3.36; N, 10.15; O, 4.64 Found: C, 81.84; H, 3.36; N, 10.16; O, 4.63.
2
5-(p-Aminophenyl)-10,15,20-(p-methylphenyl) porphyrin 2
In a three-necked flask, 5-(p-nitrophenyl)-10,15,20-(p-methylphenyl) porphyrin
(2 g, 2.8 mmol) was dissolved in 60 ml concentrated hydrochloric acid and then
excess SnCl2ꢀ2H2O (2 g, 8.8 mmol) was added, the mixed solution was stirred at
room temperature for 2 h, heated to 70 °C, and after continuing for 0.5 h, was
cooled and the isolated dark green 5-(p-aminophenyl)-10,15,20-(p-methylphenyl)
porphyrin hydrochloride solid was dispersed in 200 ml deionized water. The pH of
the solution was adjusted to pH 8–9, and filtered. The resulting solid was dissolved
in chloroform and filtered, then recrystallized from chloroform–methanol to get a
purple solid. The solid was dried under vacuum at 60 °C. Bright purple crystals of
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Meso-Schiff-base substituted porphyrin dimer dyes
compound 2 were obtained by silica column chromatography (eluent: dichloro-
1
methane) (yield: 85.1 %). H NMR (400 MHz, CDCl3, ppm): 8.89 (m, 8H, b-
pyrrole), 8.11–8.13 (d, 8H, J = 7.2 Hz, o-Ph), 7.57–7.59 (d, 8H, J = 8.0 Hz, m-
Ph), 4.01 (s, 2H, NH2), 2.75(s, 9H, Me-H), -2.74(s, 2H, NH). IR (KBr pellet,
cm-1): 3,345, 965 (tN–H, pyrrole), 3,055 (tC–H, pyrrole), 1,503, 1,470, 1,215 (tC–N
pyrrole), 1,507 (tC–C, pyrrole), 3,022, 1,152, 1,180 (tC–H, phenyl), 1,595 (tC=C
,
,
phenyl), 1,606 (tC–C, phenyl), 2,915 (tC–H, methyl), 3,448, 1,613 (tN–H). MALDI-
TOF MS calcd. For C47H25N5 659.73; found: 659.72. Elem. Anal. For C47H25N5
calcd.: C, 81.56; H, 3.82; N, 10.62. Found: C, 81.57; H, 3.82; N, 10.61.
5-(p-Nitrophenyl)-10,15,20-(2-thienyl) porphyrin 3
Compound 3 was synthesized by the same method as compound 1. 1H NMR
(400 MHz, CDCl3, ppm): 8.71 (m, 8H, b-pyrrole), 7.48–7,62 (m, 3H, thienyl), 7.92–
7.93 (m, 6H, thienyl), 8.20–8.23 (d, 2H, J = 7.2 Hz, o-Ph), 7.74–7.78 (d, 2H,
J = 8.2 Hz, m-Ph), -2.75 (s, 2H, NH). IR (KBr pellet, cm-1): 3,334, 961 (tN–H
,
pyrrole), 3,068 (tC–H, pyrrole), 3,093 (tC–H, thienyl), 3,069, 1,154, (tC–H, phenyl),
1,569 (tC=C, phenyl), 1,560 (tNO ). MALDI-TOF MS calcd. For C38H17N5O2S3
671.76; found: 671.74. Elem. Anal. For C38H17N5O2S3 calcd.: C, 67.94; H, 2.55; N,
10.43; S, 14.32; O, 4.76; Found: C, 67.95; H, 2.54; N, 10.45; S, 14.31; O, 4.75.
2
5-(p-Aminophenyl)-10,15,20-(2-thienyl) porphyrin 4
Compound 4 was synthesized by the same method as compound 2. 1H NMR
(400 MHz, CDCl3, ppm): 8.73 (m, 8H, b-pyrrole), 7.49–7,64 (m, 3H, thienyl),
7.93–7.94 (m, 6H, thienyl), 8.20–8.23 (d, 2H, J = 7.2 Hz, o-Ph), 7.74–7.78 (d, 2H,
J = 8.0 Hz, m-Ph), 4.05 (s, 2H, NH2), -2.75 (s, 2H, NH). IR (KBr pellet, cm-1):
3,330, 960 (tN–H, pyrrole), 3,065 (tC–H, pyrrole), 3,090 (tC–H, thienyl), 3,069, 1,154
(tC–H, phenyl), 1,565 (tC=C, phenyl), 3,440, 1,612 (tN–H). MALDI-TOF MS calcd.
For C38H19N5S3 641.78; found: 641.75. Elem. Anal. For C38H19N5S3 calcd.: C,
71.12; H, 2.98; N, 10.91;S, 14.99; Found: C, 71.10; H, 2.99; N, 10.93; S, 14.98.
Tetraphenylporphyrin 5
Benzaldehyde(3.4 g, 32 mmol)was dissolved in 150 ml propionic acid.The solution
was slowly heated 140 °C, and then freshly distilled pyrrole (2.15 g, 32 mmol) was
dissolved in 10 ml propionic acid, added dropwise to the reaction solution over a
period of 10 min, and stirred for a further 30 min. After cooling to room
temperature, half of the solvent was evaporated and 150 ml CH3OH was added. The
mixture was cooled overnight and filtered. The crude product was purified by
column chromatography (eluent: dichloromethane) (yield: 25 %). 1H NMR
(400 MHz, CDCl3, ppm): 8.84 (d, 8H, J = 4.8 Hz, b-pyrrole), 8.26–8.30 (m, 8H,
o-Ph), 7.82–7.68 (m, 12H, m,p-Ph), -2.73 (s, 2H, NH). IR (KBr pellet, cm-1):
3,318, 965 (tN–H, pyrrole), 3,050 (tC–H, pyrrole), 1,602 (tC=C, phenyl) 3081, 1,130
(tC–H, phenyl). MALDI-TOF MS calcd. for C44H30N4 614.74; found: 614.72. Elem.
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X. Zhang et al.
Anal. For C44H30N4 calcd.: C, 85.97; H, 4.92; N, 9.11. Found: C, 85.96; H, 4.93; N,
9.11.
5-(4-Chloromethylphenyl)-10,15,20-phenyl porphyrin 6
Paraformaldehyde (2.4 g, 62.8 mmol), sodium chloride (0.3 g, 5.1 mmol), anhy-
drous zinc chloride (0.3 g, 2.1 mmol) and dichloromethane (30 ml) were added to a
250-ml three-necked flask and cooled to -10 °C in an ice salt bath. Then, 4.5 ml
(68.19 mmol) chlorosulfonic acid was added and stirred until the solid dissolved.
Tetraphenylporphyrin (0.3 g, 0.48 mmol) was dissolved in 5 ml dry chloroform,
added dropwise to the reaction solution in 5 min, continued for 12 min, then poured
into 30 ml ice-water to terminate the reaction. It was then layered, extracted with
chloroform, washed with brine until neutral, and dried with anhydrous sodium
sulfate. The crude product was purified by column chromatography (eluent:
chloroform:petroleum ether = 5:2, v/v) (yield: 33 %). 1H NMR (400 MHz, CDCl3,
ppm): dH, ppm –2.77 (s, 2H, NH), 4.93 (s, 2H, CH2Cl), 7.72–7.77 (m, 11H, m,p-
Ph), 8.19–8.22 (m, 8H, o-Ph), 8.84 (m, 8H, b-pyrrole). IR (KBr pellet, cm-1): 3,320,
963 (tN–H, pyrrole), 3,055 (tC–H, pyrrole), 3,060, 1,480 (tC–H, phenyl), 1,590 (tC=C
,
phenyl), 756 (tC–Cl, CH2Cl). MALDI-TOF MS calcd. for C45H31ClN4 663.21;
found: 663.18. Elem. Anal. For C45H31ClN4 calcd.: C, 81.50; H, 4.71; N, 8.45; Cl,
5.34; Found: C, 81.43; H, 4.70; N, 8.53; Cl, 5.35.
5-(4-Formylphenyl)-10,15,20-phenyl porphyrin 7
In a 50-ml round-bottomed flask, 5-(4-chloromethyl phenyl)-10,15,20-phenyl
porphyrin (0.6 g, 0.9 mmol), hexamethylenetetramine (1.26 g, 9.0 mmol), and
35 ml chloroform was added and stirred for 15 h at 45 °C. The solvent was rotating
steamed out, and 30 ml 60 % acetic acid was added. The mixture was heated to
reflux, continuing to respond for a further 10 h. It was then cooled, diluted with
more water to precipitate the porphyrin solid, filtered, washed with water and then
the porphyrin solid was dissolved in chloroform, washed with water until neutral,
and dried over using anhydrous sodium sulfate. Purple crystals were obtained by
column chromatography (eluent: chloroform:petroleum ether = 3:1, v/v) (yield:
1
84 %). H NMR (400 MHz, CDCl3, ppm): -2.77 (s, 2H, NH), 7.72–7.77 (m, 9H,
m,p-Ph), 8.20–8.25 (m, 8H, o-Ph), 8.88 (s, 8H, b-Ph), 10.34 (s, 1H, CHO). IR (KBr
pellet, cm-1): 3,320, 965 (tN–H, pyrrole), 3,055 (tC–H, pyrrole), 3,062, 1,481 (tC–H
,
phenyl), 1,595 (tC=C, phenyl), 1,720 (tC=O, CHO). MALDI-TOF MS calcd. for
C45H30N4O 642.75; Found: 642.72. Elem. Anal. For C45H30N4O calcd.: C, 84.09;
H, 4.70; N, 8.72; O, 2.49; Found: C, 84.08; H, 4.71; N, 8.73; O, 2.49.
p-(10,15,20-Triphenyl-5-porphyrinyl) benzylidene aminophenyl-10,15,20-(p-
methylphenyl) porphyrin (TPBAMP)
5-(P-Aminophenyl)-10,15,20-(p-methylphenyl) porphyrin (0.34 g, 0.5 mmol), 5-(4-
formylphenyl)-10,15,20-phenyl porphyrin (0.64 g, 1.0 mmol), and 5 ml acetic acid
were added in anhydrous ethanol, and heated to reflux for 2 h. Purple crystals were
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Meso-Schiff-base substituted porphyrin dimer dyes
precipitated, and immediately filtered, and the filtered cake was washed repeatedly
with hot ethanol until the filtrate was colorless. The filtered cake was collected.
TPBAMP was obtained by column chromatography (eluent: benzene:ethyl
1
acetate = 5:1, v/v) (yield: 60 %). H NMR (400 MHz, CDCl3, ppm): 8.95 (m,
16H, b-pyrrole), 8.70 (s, 1H, Schiff base) 8.20–8.33 (m, 16H, o-Ph), 7.80–7.86 (m,
9H, m-Ph), 2.82 (s, 9H, Me-H), -2.66(s, 4H, NH). IR (KBr pellet, cm-1): 3,329,
962 (tN–H, pyrrole), 3053 (tC–H, pyrrole), 1,558, 1,471, 1,198 (tC–N, pyrrole), 1,509
(tC–C, pyrrole), 2,990, 1,078, 1,176 (tC–H, phenyl), 1,686 (tC=C, phenyl), 1,686
(tC–C, phenyl), 2,908 (tC–H, methyl), 1,655 (tC=N, Schiff base). MALDI-TOF MS
calcd. for C92H53N9 1284.47; Found: 1284.45. Elem. Anal. For C92H53N9 calcd.: C,
86.03; H, 4.16; N, 9.81. Found: C, 86.02; H, 4.15; N, 9.82.
p-(10,15,20-Triphenyl-5-porphyrinyl) benzylidene aminophenyl-10,15,20-(2-
thienyl) porphyrin (TPBATP)
1
TPBATP was synthesized by the same method as TPBAMP. H NMR (400 MHz,
CDCl3, ppm): 8.84 (m, 16H, b-pyrrole), 8.72 (s, 1H, Schiff base)7.49–7.64 (m, 3H,
thienyl), 8.07–8.09 (m, 6H, thienyl), 8.21–8.24 (m, 10H, o-Ph), 7.75–7.78 (m, 13H,
m,p-Ph), -2.78 (s, 4H, NH). IR (KBr pellet, cm-1): 3,325, 966 (tN–H, pyrrole),
3,060 (tC–H, pyrrole), 1,550, 1,460 (tC–N, pyrrole), 1,508 (tC–C, pyrrole), 2,990,
1,514 (tC–H, phenyl), 1,570 (tC=C, phenyl), 1,685 (tC–C, phenyl), 3,185 (tC–H
,
thieny), 1,657 (tC=N, Schiff base). MALDI-TOF MS calcd. for C83H47N9S3
1266.52; Found: 1266.51. Elem. Anal. For C83H47N9S3 calcd.: C, 78.71; H, 3.74; N,
9.95; S, 7.60. Found: 78.70; H, 3.76; N, 9.92; S, 7.62.
Results and discussion
Synthesis and chemical characterization
The two novel porphyrin dimer dyes were synthesized according to several classical
reactions. The synthetic strategy is shown in Scheme 1. Compounds 1, 3, and 5
were synthesized by adapting the classical Adler–Longo procedure [30], compounds
2 and 4 were obtained by reduction of the nitro group using SnCl2 in concentrated
hydrochloric acid. 5-(4-Chloromethylphenyl)-10,15,20-phenyl porphyrin 6 was
synthesized according to the literature [31], and then compound 6 was converted
into 5-(4-formylphenyl)-10,15,20-phenyl porphyrin 7 by the Sommelet reaction
[32]. Finally, compounds 2 and 4 reacted with 5-(4-formylphenyl)-10,15,20-phenyl
porphyrin 7 by the Bonus-elimination reaction [33] to give TPBAMP and TPBATP,
respectively.
Optical and electrochemical properties
The UV–Vis absorption spectra of TPBAMP and TPBATP in DMF solution are
shown in Fig. 2a, and the peak positions and molar absorption coefficients (e) of
Soret and Q bands of porphyrin dyes are listed in Table 1. The UV–Vis absorption
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X. Zhang et al.
Scheme 1 Synthesis of TPBAMP and TPBATP
Table 1 UV and PL spectral data of porphyrin dyes
Dyes
(kmax/nm)a (e/10-5mol/l-1cm-1
)
kbem (nm)
kcin (nm)
Ud
TPPH2
404 (2.447), 512 (0.504), 546
(0.229), 590 (0.155), 646 (0.194)
651.0, 714.0
582.0
570.0
578.5
0.11
TPBAMP
TPBATP
a
414 (2.283), 512 (0.152), 546 (0.075),
590 (0.049), 644 (0.044)
652.0, 716.5
654.5, 720.0
0.14
0.17
417 (2.481), 516 (0.250), 553
(0.117), 599 (0.066), 652 (0.072)
Absorption spectra measured in DMF solution(1 9 10-5 M)
b
c
d
Wavelengths for emission spectra in DMF solution by exciting at Soret wavelength
Measured by the intercept of the normalized absorption and emission spectra
Fluorescence quantum yield
spectra of TPBAMP and TPBATP exhibit a strong Soret and four moderate
Q-bands, respectively. The Soret of TPBAMP and TPBATP at 414 and 417 nm are
obviously red-shifted about 10 nm compared to TPPH2 at 404, which indicates that
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Meso-Schiff-base substituted porphyrin dimer dyes
the conjugated degree of the porphyrin dimer is much larger than that of the
porphyrin monomer. However, as a result of expansion of the p-system by
introducing thiophene units, the Soret and Q-bands of TPBATP are both broadened
and slightly red-shifted compared with TPBAMP. The molar extinction coefficients
(e) of the Soret bands are 2.283 9 105 mol l-1 cm-1 for TPBAMP and
2.481 9 105 mol l-1 cm-1 for TPBATP, respectively, both of which are much
higher than that of Ru(dcbpy)2(NCS)2 (N3 dye: ca.1.6 9 105 mol l-1 cm-1) [34].
As seen from Fig. 2b, the absorption spectra of the two porphyrin dyes adsorbed
onto TiO2 films are similar to those of the corresponding solution spectra, but
obviously red-shifted and broadening due to the formation of the J-type aggregates
of porphyrins on the TiO2 surface. Additionally, the absorption band of TPBATP is
broader and slightly red-shifted in comparison to those of TPBAMP, which should
be attributed to a more extensive conjugation of molecules containing the thienyl.
This indicates that TPBATP has an enhanced ability to harvest solar light.
The steady-state fluorescence spectra of TPBAMP and TPBATP were measured
in DMF by exciting at the peak position of the Soret band (Fig. 3) and the
wavelengths for emission maxima are listed in Table 1. The emission spectra of
TPBAMP and TPBATP display two emission maxima which are the strong peaks
at 652.0 and 654.5 nm and the weak peaks at 716.5, 720.0 nm, corresponding to
Q(0, 0) and Q(0, 1) transition, respectively. The fluorescence of the Q-band is
attributed to the transition from the first excited singlet state S1 to the ground state
S0, S1 ? S0. The quantum yields U of the (S1 ? S0) fluorescence are 0.14
(TPBAMP) and 0.17 (TPBATP), while the Soret fluorescence (S2 ? S0) is not
observed at room temperature or at the experimental excited wavelengths of 414
and 417 nm.
To determine the fluorescence quantum yield, fluorescence intensity takes the
yield of S1 fluorescence of TPPH2 in DMF (Us = 0.11, k = 404 nm) as a standard.
Quantum yield (U) was calculated by the following equation:
Ux ¼ UsðFx=FsÞðAs=AxÞ
where Ux, Fx and Ax denote the quantum yield, integrated intensity, and absorbance
at excited wavelength of complexes to be tested, respectively, while Us, Fs, and As
are the quantum yield, integrated intensity, and absorbance at excited wavelength of
standard sample, respectively. The results indicate that the porphyrin dimer dyes
with –C=N– as the bridge are more likely to launch fluorescence than the porphyrin
monomer which is due to the increase of the conjugated system. Fluorescence
quantum yield (U) of TPBATP is higher than that of TPBAMP, beyond that the
thiophene ring is a better electron donor than the benzene ring.
The electrochemical properties of these porphyrins were investigated with cyclic
voltammetry, using tetrabutylammonium perchlorate as the supporting electrolyte
(0.1 M) in DMF. Figure 4 shows examples of cyclic voltammograms. The
porphyrin dimers show a reversible oxidation peak and a quasi-reversible reduction
peak. For TPBATP, the first oxidation potential at 0.980 V, is shifted slightly, by
40 mV, to the negative relative to those of TPBAMP at 1.020 V. The oxidation
potential is the first abstraction of an electron of the porphyrin ring from the highest
occupied molecular orbital (HOMO). So this shift may be attributed to thiophene
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X. Zhang et al.
Fig. 2 UV–Vis absorption spectra of TPBAMP and TPBATP in DMF solution (a) and TiO2 films (b)
Table 2 Electrochemical data for porphyrin dyes and driving forces for electron-transfer processes on
the TiO2
Dyes
Ea0–0 (eV)
Ebox (V)
Ecred (V)
Ead (V)
DGeinj (eV)
DGfreg (eV)
ox
TPBAMP
TPBATP
a
1.925
1.901
1.020
0.980
-1.580
-1.456
-0.905
-0.921
-0.165
-0.181
-0.820
-0.780
Determined from the intercept of the normalized absorption and emission spectra
b
c
d
e
First oxidation potentials (vs. SCE)
First reduction potentials (vs. SCE)
Excited-state oxidation potentials approximated from Eox and E0–0 (vs. SCE)
Driving forces for electron injection from the porphyrin excited singlet state Eaox to the conduction band
of TiO2 (-0.74 V vs. SCE)
Driving forces for regeneration of the porphyrin radical cation by I-/I-3 redox couple (?0.2 V vs. SCE)
f
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Meso-Schiff-base substituted porphyrin dimer dyes
Fig. 3 Normalized PL spectra of TPBAMP and TPBATP in DMF solution (1 9 10-5 M)
Fig. 4 Cyclic voltammograms and differential pulse voltammerty for TPBAMP and TPBATP in DMF
solution containing 0.1 M TBAP as supporting electrolyte, measured at a scan rate of 100 Mv s-1
rings which have denser electron cloud than benzene rings. The redox potentials of
the porphyrin dyes were obtained by cyclic voltammetry and differential pulse
voltammetry (Fig. 4), and the corresponding data are summarized in Table 2. The
E0–0 was calculated according to the following equation:
1240
E0ꢁ0
¼
;
kmax is the absorption maximum:
kmax
The zero–zero excitation energies (E0–0) of TPBAMP and TPBATP are,
respectively, 1.925 and 1.901 eV. The ground state potentials of TPBAMP and
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X. Zhang et al.
Table 3 Photovoltaic performance of DSSCs for the two porphyrin dyes
Dyes
Jsc (mA/cm2)
Voc (V)
ff
g (%)
TPBAMP
TPBATP
6.20
7.53
0.61
0.63
0.64
0.60
2.44
2.86
TPBATP (1.020 and 0.980 V) are both higher than the redox potential (0.2 V vs.
SCE) of the iodide/triiodide couple. This could lead to fast dye regeneration,
avoiding the geminate charge recombination between oxidized dye molecules and
photoinjected electrons in the nanocrystalline titania film. As shown in Table 2, the
excited-state oxidation potentials of two porphyrin dyes are more negative than the
equivalent potential of the TiO2 conduction band (CB) edge (-0.74 V vs. SCE),
which suggests that the LUMO levels of porphyrin dimer should all be capable of
injecting electrons into the conducting bands of TiO2. Meanwhile, the driving forces
for electron injection from the excited porphyrin singlet state to the CB of TiO2
(-0.74 V vs. SCE) (DGing), and the regeneration of porphyrin radical cation by I-/
I-3 couple (0.2 V vs. SCE)(DGreg) for the porphyrin-sensitized TiO2 cells, were
determined and the corresponding data are listed in Table 2. The energy-level
diagram of porphyrins dyes, TiO2 and I-/I3 are shown clearly in Fig. 5. These
-
results show that the excitation state energy of dye TPBATP best matches with the
energy level of nanosized titania, which could be used asa dye sensitizer in solar
cells. Both of the processes are thermodynamically feasible.
Cell fabrication and photovoltaic properties of porphyrin-sensitized TiO2 solar
cells
The porphyrins were sensitized onto TiO2 nanoparticulate film to serve as working
electrodes in DSSC devices. The TiO2 nanoparticles (20 nm), prepared with a sol–
gel method [35–39] were screen-printed onto the F-doped SnO2 (FTO,30 X cm-2
;
Sinonar, Taiwan) glass substrate [40]. Crystallization of TiO2 film (thickness
10–15 lm and active area 0.16 cm-2) was performed by annealing in two stages:
heating at 450 °C for 5 min followed by heating at 500 °C for 30 min. The TiO2
film was then immersed in an aqueous solution of TiCl4 (50 mM, 70 °C, 30 min)
followed by the same two-stage thermal treatment for final annealing of the
electrode. The electrode was then immersed in a porphyrin/ethanol solution
(0.2 mM, 25 °C, 2 h) containing chenodeoxycholic acid (CDCA; 0.2 mM unless
otherwise specified) for dye loading onto the TiO2 film. Counter electrodes were
prepared by coating with a drop of H2PtCl6 solution (2 mg of Pt in 1 ml of ethanol)
on an FTO plate. The dye-adsorbed TiO2 electrode and Pt-counter electrode were
assembled into a sealed sandwich-type cell. A drop of the electrolyte, which was
composed of 0.6 M 1,2-dimetyl-3-propyl imidazolium iodide, 0.05 M iodine, 0.1 M
LiI, and 0.5 M tert-butylpyridine in acetonitrile, was then introduced into the space
between the two electrodes, so completing the fabrication of these DSSC devices.
Figure 6 shows the J–V curves of DSSCs based on TPBAMP and TPBATP.
Photoelectrochemical data were measured using a 450-W xenon light source that
123

Meso-Schiff-base substituted porphyrin dimer dyes
Fig. 5 Overview of the evaluated energy levels of TPBAMP and TPBATP dyes determined by cyclic
voltammetry
was focused to give 100 W cm2 [41], the equivalent of one sun at Air Mass (AM)
1.5, at the surface of the test cell. The solar-to-electricity conversion efficiency (g)
of the DSSCs was calculated according to the following equation:
Pmp JmpVmp JscðmAcmꢁ2ÞVocðVÞff
g ¼
¼
¼
:
Pin
Pin
PinðmWcmꢁ2
Þ
where Pmp is the maximum output power (=JmpVmp) and Pin is the input radiation
power (for one solar radiation Pin = 100 mWcm-2); the fill factor (ff) is defined.
JmpVmp
ff ¼
:
JscVoc
Jsc and Voc is the short-circuit photocurrent density and the open-circuit
photovoltage, Jmp and Vmp are the photocurrent and photovoltage, respectively, for
maximum power output. The detailed parameters (Jsc, Voc, ff, and g) of porphyrin
dimer dyes are summarized in Table 3. Under standard global AM 1.5 solar
conditions, the TPBATP-sensitized cell gave a Jsc of 7.53 mA/cm2, Voc of 0.63 V,
and ff of 0.60, corresponding to an overall conversion efficiency of 2.86 %, being
slightly better comprehensive properties than the TPBAMP-sensitized cell (Jsc of
6.20 mA/cm2, Voc of 0.61 V, ff of 0.64 and g % of 2.44), from which we can see
that the two porphyrin dyes display rather similar Voc and ff values, while the Jsc and
g% of the TPBAMP-sensitized TiO2 cell are low than those of the TPBATP-
sensitized cells. The different g values of the two porphyrin-sensitized primarily
result from the difference of Jsc, which could be explained by the introduction of a
thiophene group, because a thiophene group in the TPBATP dyes has higher molar
extinction coefficients and a stronger donating effect than the benzene ring in the
TPBAMP dyes which results in a high electron injection rate and a fast dye-
regeneration in order to avoid the geminated charge recombination between
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X. Zhang et al.
Fig. 6 Photocurrent density–voltage characteristics for DSSCs based on the two dyes
oxidized sensitizers and photoinjected electrons in the nanocrystalline TiO2 film,
thus enhancing the TPBATP-sensitized cell performance.
Conclusions
The novel porphyrin dimers dyes (–C=N– as a bridge) for DSSCs applications were
successfully synthesized and obtained materials with interesting optical and
electrochemical properties. Under standard global AM 1.5 solar conditions, the
TPBATP sensitizer cell gave a Jsc of 7.53 mA/cm2, Voc of 0.63 V, and ff of 0.60,
corresponding to an overall conversion efficiency of 2.86 %, being slightly better
comprehensive properties than the TPBAMP-sensitized cell. By introducing a
thiophene group to the porphyrin ring, the novel porphyrin dimer dye has a stronger
donating effect, which results in a high electron injection rate and a fast dye-
regeneration in order to avoid the geminate charge recombination between the
oxidized sensitizers and the photoinjected electrons in the nanocrystalline TiO2 film,
thus enhancing the TPBATP-sensitized cell performance. However, the g values of
the two dyes are still low, which might be the result fromoflow solubility in the
iodide/triiodide-based redox electrolytes, molecular aggregation, and small
adsorbed amounts on the TiO2 films. Further improvement of the power conversion
efficiency of porphyrin derivative dyes may be by introduction of long-chain
alkyloxy, carboxyl groups to dide/triiodide–based redox electrolytesporphyrin ring
of dimers, metal coordination and optimization of the solar cells fabricated
conditions.
Acknowledgments The authors would like to acknowledge financial support from the National Nature
Science Foundation of China (Grant number: 51272239) and the Natural Science Foundation of Shanxi
Province, China (Grant number: 2011011022-4).
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Meso-Schiff-base substituted porphyrin dimer dyes
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