2-Ethynyl-6-methylthieno[3,2-b]thiophene as an efficient π spacer for porphyrin-based dyes

Article abstract of DOI:10.1016/j.dyepig.2015.06.024

In this study, three donor-π-acceptor (D-π-A) type unsymmetrical porphyrin-based dyes have been designed and synthesized for dye-sensitized solar cells (DSSCs). The photophysical, electrochemical and photovoltaic properties of the dyes were successfully a

Full text of DOI:10.1016/j.dyepig.2015.06.024

Accepted Manuscript
2-Ethynyl-6-methylthieno[3,2-b]thiophene as an efficient π spacer for porphyrin-based
dyes
Huifang Zhao, Jun Long, Xi Luo, Bin Zhao, Songting Tan
PII:
S0143-7208(15)00242-9
10.1016/j.dyepig.2015.06.024
DYPI 4826
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 13 May 2015
Revised Date: 12 June 2015
Accepted Date: 15 June 2015
Please cite this article as: Zhao H, Long J, Luo X, Zhao B, Tan S, 2-Ethynyl-6-
methylthieno[3,2-b]thiophene as an efficient π spacer for porphyrin-based dyes, Dyes and Pigments
(2015), doi: 10.1016/j.dyepig.2015.06.024.
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ACCEPTED MANUSCRIPT
2
-Ethynyl-6-methylthieno[3,2-b]thiophene as an efficient π spacer for
porphyrin-based dyes
a
a
a
a,b
a,b*
Huifang Zhao , Jun Long , Xi Luo , Bin Zhao , Songting Tan
a
College of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications
of Ministry of Education, Xiangtan University, Xiangtan 411105, P. R. China
b
Key Laboratory of Advanced Functional Polymeric Materials of College of Hunan Province,
Xiangtan University, Xiangtan, 411105, P. R. China
*
Corresponding author: tanst2008@163.com (S. Tan)
ABSTRACT: In this study, three donor-π-acceptor (D-π-A) type unsymmetrical
porphyrin-based dyes have been designed and synthesized for dye-sensitized solar
cells (DSSCs). The photophysical, electrochemical and photovoltaic properties of the
dyes were successfully adjusted by using functionalized arylamines (triphenylamine
or N, N-dimethyl-4-(acetylene)-aniline) as donor, porphyrin derivatives as conjugated
π bridge, ethynyl-thiophene or ethynyl-thiophthene as π spacer and carboxylic acid as
anchor group. Compared with the dye containing ethynyl-thiophene as π spacer, the
dyes containing 2-ethynyl-6-methylthieno[3,2-b]thiophene as π spacer between the
porphyrin macrocycle and the carboxylic acid anchoring group show broadened
absorption spectra on TiO films and red-shifted absorption spectra in solution. The
2
dye based on triphenylamine and 2-ethynyl-6-methylthieno[3,2-b]thiophene exhibited
a power conversion efficiency (PCE) of 4.53% under simulated air mass 1.5 global
sunlight.
These
results
indicate
that
the
introduction
of
1

ACCEPTED MANUSCRIPT
2
-ethynyl-6-methylthieno[3,2-b]thiophene in the meso-positions of porphyrin dyes is
an effective approach to improve the photovoltaic performance.
Keywords: Conjugated π bridge; Porphyrin-based dyes; Power conversion efficiency;
Dye-sensitized solar cells; Synthesis
1
. Introduction
As the fossil energy gradually reduces and the demand of energy increases in the
world, the exploration of new energy has became a hot scientific research. Solar
energy, as a kind of widely distributed, ample and eco-friendly energy, has caused the
world’s research. Dye-sensitized solar cells (DSSCs) were one of the most effective
renewable photovoltaic devices because they were found to display low cost, ease of
fabrication, short energy pay back time, low sensitivity to temperature changes and
environmental friendliness compared with the conventional inorganic solar cells [1].
The sensitizer was one of the key components for achieving the device with high
efficiency and durability. Up to now, a recorded power conversion efficiency (PCE)
above 13.0% has been reported for DSSCs [2, 3]. The ruthenium polypyridyl complex
as sensitizers in the DSSCs was the first one reached PCE of 10% [4]. However,
ruthenium was a noble and expensive metal, which may limit the large-scale
application as sensitizers. For this reason, porphyrin and its derivatives with large
π-conjugated systems have been received considerable attention as sensitizers for
DSSC applications [5, 6].
The structure of porphyrin exhibits four meso-positions and eight β-positions,
which can serve to functionalization. β-Linked porphyrin derivatives were reported to
2

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show promising device performances with a benchmark efficiency of 7.1% in 2007
[
7]. Meso-linked porphyrin derivative sensitizers with a push-pull framework
appeared in 2009 [8]. With a structural design involving long alkoxyl chains to
envelop the porphyrin core to suppress the dye aggregation for a push-pull zinc
porphyrin, the DSSC device achieved a record PCE of 13.0% in 2014 [2]. Recently,
unsymmetrical porphyrin sensitizers [9-11] with unique directionality have been
proven to be high-performance sensitizers. In the past decades, various functional
groups and their derivatives [12-14] have been employed at meso-positions of
porphyrin macrocycles to develop an effective part of the donor-π-acceptor (D-π-A)
structure.
According to previous reports, the PCE of porphyrin sensitizers could be
significantly improved by grafting the molecule with properly chosen donor, π spacer
and acceptor/anchoring groups. Combination of a wide range of donor moieties, such
as functionalized arylamines, polyaromatic or heterocyclic donors, with ethynyl
benzoic acid anchoring group at the meso-position of the porphyrin has revealed a
significantly improved cell performance [9-12]. Thiophene derivatives have been
widely utilized in metal-free organic sensitizers [15-17] and porphyrin-based
sensitizers [18, 19] as an effective π bridge in DSSCs. Compared with thiophene,
thiophthene [20] exhibits larger π delocalization, so it was widely used as π bridge for
metal-free organic sensitizers. However, it rarely used as π bridge in porphyrin
sensitizers. It was revealed that, in porphyrin dyes, the elongation of π conjugation or
loss of molecular structure symmetry could cause a split in π and π* energy levels and
3

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a decrease in the energy gap between the highest occupied molecular orbital (HOMO)
and the lowest unoccupied molecular orbital (LUMO), thus resulted in broadening
and red-shifting of the absorption band together with an increasing intensity of the Q
band relative to that of the Soret band [21]. In this article, in contrast to many
available porphyrin sensitizers which have been reported with ethynyl benzoic acid
anchoring group [22, 23], we attempted to introduce either ethynyl-thiophene or
2
-ethynyl-6-methylthieno[3,2-b]thiophene as π spacer between the porphyrin
macrocycle and the carboxylic acid anchoring group and studied its influence on solar
cell performance.
Herein, we reported three new unsymmetrical zinc-porphyrin dyes, as shown in
Fig. 1, appended with ethynyl-thiophene or ethynyl-thiophthene π spacer between
porphyrin and anchoring group to facilitate the shift of the absorption towards the red
region in solution. We predicted that increasing the conjugated degrees of π-bridge
moieties could improve the light-harvesting capability and reduce the intermolecular
aggregation [24]. Thus, a higher short circuit current density (J ) and a higher PCE
sc
could be achieved. The photophysical, electrochemical and photovoltaic properties of
the porphyrin dyes with different donors and π spacers were fully investigated.
2
. Experimental section
2
.1 Materials and reagents
All starting materials were purchased from commercial suppliers at analytical
grade. Tetrahydrofuran (THF) was distilled from sodium-benzophenone prior to use.
N,N-Dimethylformamide (DMF) was dried and distilled under reduced pressure.
4

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Phosphorus oxychloride (POCl ) and 1,2-dichloroethane were achieved by
3
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 (200-300 mesh). Compound 8, 9, 13 [3, 25, 26] and
compound 14 were synthesized according to the methods reported in the literatures
[
27].
2
.2 Instruments and characterizations
1
H NMR spectra was measured on a Bruker Avance 400 instrument, using CDCl and
3
deuterated dimethyl sulfoxidi (DMSO-d6) as solvents and tetramethylsilane (TMS;
δ=0 ppm) as internal standard. MALDI-TOF mass spectrometric measurements were
performed on Bruker Autoflex III instrument. GC-MS were measured on an
Agilengt-7890A-5975C instrument. UV-Visible spectra of the dyes were measured on
a Perkin-Elmer Lamada 25 spectrometer. The total amounts of the three dyes
adsorbed on the TiO films were determined by measuring the absorbance of the
2
sensitizers, which were dissolved from the dye-coated TiO films into DMF/H O =
2
2
−
1
4
:1 with 0.1 mol L NaOH. The PL spectra were obtained using Perkin-Elmer LS-50
luminescence spectrometer. Electrochemical impedances were measured on an
electrochemistry workstation in the dark. Electrochemical redox potentials were
obtained by cyclic voltammetry, which measured by a three-electrode configuration
-1
and an electrochemistry workstation at a scanning rate of 50 mV s . The silver
electrode (SCE), platinum disk electrode and Pt electrode were used as reference
electrode, working electrode and the counter electrode, respectively.
5

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Tetrabutylammonium perchlorate (TBAP) 0.1 M was used as supporting electrolyte in
nitrogen-purged anhydrous CHCl . Ferrocene/ferrocenium redox couple was used for
3
potential calibration.
2
.3 General procedure for preparation of porphyrin-modified TiO electrode and
2
photovoltaic measurements
Fluorine-doped SnO conducting glass (FTO) were cleaned by ultrasonication in
2
water, acetone, ethanol, isopropanol, respectively, and subjected to an O treatment
3
o
for 10min. The substrates were treated in a 40 mM TiCl solution at 70 C for 30 min,
4
o
then rinsed with deionized water and ethanol, sintered at 450 C for 30 min. The TiO
2
suspension was composed of P25 (Degussa AG, Germany) [28], acetylacetone, 1 wt%
magnesium acetate, and magnetic stirring for 24 h. Then the paste was deposited onto
the substrates by the method of blade coating to obtain a TiO film of 10-15 ꢀm
2
o
thickness. The TiO film was gradually heated to 450 C, baked at this temperature for
2
3
0 min. The 200 nm particles sized TiO colloid was coated on the electrode by the
2
same method, resulting in a TiO light-scattering layer of 4-6 ꢀm thickness. The
2
o
double-layer TiO were sintered at 450 C for 30 min. After the substrate was cooled
2
o
to the room temperature, then treated with TiCl solution at 70 C for 30 min and
4
o
sintered at 450 C for 30 min.
After the film was cooled to room temperature, it was immersed in optimized
solvent system of THF and ethanol (volume ratio 1:4) with 0.25 mM porphyrin dyes
and 10 mM Chenodeoxycholic acid (CDCA) for 3 h in the dark. The sensitized
electrode was then rinsed with ethanol and dried. A drop of electrolyte was deposited
6

ACCEPTED MANUSCRIPT
onto the surface of the electrode and a Pt foil counter electrode was clipped onto the
top of the TiO electrode to assemble the dye sensitized solar cells for photovoltaic
2
performance measurements. The electrolyte consisted of 0.5 M LiI, 0.05 M I , and 0.5
2
M 4-tert-butylpyridine (TBP) in 3-methoxypropionitrile and the efficient irradiated
2
area of the cell was 0.196 cm . The photocurrent-voltage (J–V) characteristics were
recorded on Keithley 2602 Source meter. Dye sensitized TiO electrodes were
2
measured under simulated air mass 1.5 global sunlight. The power conversion
efficiency (η) of the DSSC was calculated from short-circuit photocurrent (J ), the
sc
open-circuit photovoltage (V ), the fill factor (FF) and the intensity of the incident
oc
light (P ) according to the following equation:
in
The measurement of monochromatic incident photo-to-current conversion
efficiency (IPCE) for the solar cell was also detected with a Zolix DCS300PA data
acquisition system.
2
.4 Synthesis
N-(4-(di(1H-pyrrol-2-yl)methyl)phenyl)-4-methyl-N-p-tolylbenzenamine (2)
Compound 1 (3.01 g, 10 mmol) and pyrrol (17.40 mL, 0.25 mmol) were added to
a dry round-bottomed flask and degassed with a stream of argon for 30 min. Indium
trichloride (0.22 g, 1 mmol) was added, and the solution was stirred under argon at
room temperature for 30 min and then quenched with 0.1 M aqueous NaOH. The
organic phase was washed with water and dried over anhydrous MgSO . Then the
4
solvent was removed under vacuum to afford an orange crude product, and then it was
7

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purified by column chromatography using a hexane/ethyl acetate mixture (5/1) as
1
eluent. A brownish yellow solid was obtained. Yield 2.51 g (59.9%). H NMR (CDCl ,
3
400 MHz, δ/ppm): 7.95 (s, 2H), 7.05–7.04 (d, J=2 Hz, 6H), 6.98–6.94 (d, J=8 Hz, 6H),
6.70 (s, 2H), 6.16 (s, 2H), 5.94 (s, 2H), 5.40 (s, 1H), 2.30 (s, 6H).
1-(3-Bromothiophen-2-yl)ethanone (3)
To a stirred solution of 3-bromothiophene (10 g, 62.00 mmol) in refined DMF
(200 mL), acetyl chloride (5.80 g, 74.40 mmol) was added, and then slowly added
o
AlCl (16.35 g, 124 mmol). The solution was stirred at 0 C for 3 h and then quenched
3
with water. The organic phase was washed with 0.1 M aqueous NaOH and dried over
anhydrous MgSO . The solvent was removed under reduced pressure to give the
4
product without any purification.
3
-Methylthieno[3,2-b]thiophene-2-carboxylate (4)
To a degassed solution of compound 3 (5 g, 24.38 mmol) and K CO (6.77 g,
2
3
4
8.76 mmol) in refined DMF (100 mL), ethyl-2-mercaptoacetate (3.52 gꢀ29.26
mmol)was added. After the solution was stirred at room temperature under argon for
4 h, NaOH (178 mg, 4.46 mmol) in ethanol (5 mL) was added and the mixture was
2
stirred for an additional 24 h, and then quenched with water. The organic phase was
washed with water and dried over anhydrous MgSO . The solvent was removed under
4
reduced pressure. The residue was purified by column chromatography using
hexane/ethyl acetate=5/1 to as eluent. A white solid was obtained. Yield 2.12 g
1
(
41.0%). H NMR (400 MHz, CDCl , δ/ppm): 7.56–7.55 (d, J=2 Hz, 1H), 7.31–7.30
3
(d, J=2 Hz, 1H), 4.39–4.36 (m, 3H), 2.69 (s, 3H).
8

ACCEPTED MANUSCRIPT
5
-Bromo-3-methylthieno[3,2-b]thiophene-2-carboxylate (5)
To a stirred solution of compound 4 (2.12 g, 10 mmol) in DMF (50 mL), a
solution of N-bromosuccinimide (NBS) (2.15 g, 12 mmol) in DMF (20 mL) was
o
o
slowly added at 0 C, and then the mixture was warmed up to 60 C for 24 h. After the
reaction was quenched with acetone (5 mL), the mixture was poured into water and
extracted with dichloromethane (DCM). The solvent was removed under reduced
pressure. The residue was purified by column chromatography using hexane/ethyl
1
acetate = 5/1 to as eluent. A white crystal was obtained. Yield 2.05 g (70.9%). H
NMR (400 MHz, CDCl , δ/ppm): 7.30 (s, 1H), 4.39–4.34 (m, 3H), 2.69 (s, 3H).
3
3
-Methyl-5-(2-(trimethylsilyl)ethynyl)thieno[3,2-b]thiophene-2-carboxylate (6)
A mixture of compound 5 (2.05 g, 7.09 mmol), trimethylsilylacetylene (3 mL, 21
mmol ), Pd(PPh ) (32 mg, 46.60 ꢀmol), CuI (13 mg, 69.90 ꢀmol), THF (30 mL) and
3
4
Et N (5 mL) was gently refluxed for 12 h under argon. The reaction liquid was poured
3
into water and extracted with DCM. The solvent was removed under reduced pressure.
The residue was purified by column chromatography using hexane/ethyl acetate = 5/1
1
to as eluent to give the product. Yield 1.42 g (65.1%) as a light yellow solid. H NMR
(
400 MHz, CDCl , δ/ppm): 7.38 (s, 1H), 4.42–4.37 (m, 3H), 2.68 (s, 3H), 0.30 (s,
3
9
H).
5
-Ethynyl-3-methylthieno[3,2-b]thiophene-2-carboxylic acid (7)
2
M aqueous NaOH (3.75 mL, 7.50 mmol) was added into a solution of
compound 6 (1.16 g, 3.75 mmol) in MeOH (10 mL) at 0 °C. The reaction mixture was
stirred for 5 h at room temperature. The mixture was neutralized and concentrated in
9

ACCEPTED MANUSCRIPT
vacuo. The mixture was poured into water and extracted with ethyl acetate. The
organic layer was washed with brine, and dried over anhydrous MgSO . Filtration and
4
1
concentration under vacuum gave 7 as a brown solid. Yield 0.81 g (97.6 %). H NMR
(
400 MHz, CDCl , δ/ppm): 7.38 (s, 1H), 3.87 (s, 1H), 2.68 (s, 3H).
3
5
-(4-(Di-p-tolylamino)phenyl)-10,20-bis(2,6-dioctoxyphenyl)porphyrin (10)
To a degassed solution of dipyrromethane (0.40 g, 2.74 mmol), compound 2
(1.15 g, 2.74 mmol) and compound 8 (2.00 g, 5.50 mmol) in DCM (700 mL),
trifluoroacetic acid (0.27 mL, 3.73 mmol) was added. After the solution was stirred at
o
2
5 C under argon for 4 h, 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) (2 g,
.27 mmol) was added and the mixture was then stirred for additional 1 h. Then, the
8
mixture was basified with Et N (5 mL). The solvent was removed under reduced
3
pressure and the residue was purified by column chromatography using DCM/hexane
1
=
1/2 as eluent to give a red solid. Yield 0.68 (18.5%). H NMR (400 MHz, CDCl ,
3
δ/ppm): 10.13 (s, 1H), 9.77–9.69 (m, 4H), 9.12–9.04 (m, 4H), 8.97 (s, 2H), 8.90 (s,
2
3
0
H), 8.83 (s, 2H), 7.98–7.96 (m, 4H), 7.88 (s, 2H), 7.21 (s, 2H), 6.88 (s, 4H),
.92–3.87 (m, 8H), 2.32–2.26 (s, 6H), 0.95–0.88 (m, 8H), 0.87–0.82 (m, 8H),
.68–0.61 (m, 8H), 0.59–0.54 (m, 28H), 0.49–0.42 (m, 8H), –3.02 (s, 2H).
MALDI-TOF MS (C H N O ) m/z: calcd for 1246.808; found 1246.905.
8
4
103
5
4
5
-Bromo-15-(4-(di-p-tolylamino)phenyl)-10,20-bis(2,6-dioctoxyphenyl)porphyrin (11)
To a stirred solution of porphyrin 10 (0.68 g, 0.55 mmol) in DCM (350 mL), was
slowly added a solution of NBS (97 mg, 0.55 mmol) in DCM (40 mL) in a period of 6
h at 0 °C under argon. After the reaction was quenched with acetone (10 mL), the
1
0

ACCEPTED MANUSCRIPT
solvent was removed under reduced pressure. The residue was purified by column
chromatography (silica gel) using DCM/hexane=1/2 as eluent. Recrystallization from
1
MeOH/DCM gave the product as a purple powder. Yield 0.53 g (72.6%). H NMR
(
400 MHz, CDCl , δ/ppm): 9.77–9.69 (m, 4H), 9.12–9.04 (m, 4H), 8.97 (s, 2H), 8.90
3
(s, 2H), 8.83 (s, 2H), 7.98–7.96 (m, 4H), 7.88 (s, 2H), 7.21 (s, 2H), 6.88 (s, 4H),
3
0
.92–3.87 (m, 8H), 2.32–2.26 (s, 6H), 0.95–0.88 (m, 8H), 0.87–0.82 (m, 8H),
.68–0.61 (m, 8H), 0.59–0.54 (m, 28H), 0.49–0.42 (m, 8H), –2.98 (s, 2H).
MALDI-TOF MS (C H N O Br) m/z: calcd for 1326.717; found 1326.530.
8
4
102
5
4
[
5-Bromo-15-(4-(di-p-tolylamino)phenyl)-10,20-bis(2,6-dioctoxyphenyl)porphyrinato]
zinc(II) (12)
A suspension of compound 11 (0.53 g, 0.40 mmol) and Zn(OAc) •2H O (1.04 g,
2
2
4
.40 mmol) in a mixture of DCM (60 mL) and methanol (40 mL) were stirred at 23
ο
C for 3 h. The reaction was quenched with water (100 mL), and the mixture was
extracted with DCM (2 × 100 mL). The combined extracts were washed with water
and dried over anhydrous MgSO . The solvent was removed under reduced pressure
4
1
to give the product. Yield 0.52 g (98.1%). H NMR (400 MHz, CDCl , δ/ppm):
3
9
.77–9.69 (m, 4H), 9.12–9.04 (m, 4H), 8.97 (s, 2H), 8.90 (s, 2H), 8.83 (s, 2H),
.98–7.96 (m, 4H), 7.88 (s, 2H), 7.21 (s, 2H), 6.88 (s, 4H), 3.92–3.87 (m, 8H),
.32–2.26 (s, 6H), 0.95–0.88 (m, 8H), 0.87–0.82 (m, 8H), 0.68–0.61 (m, 8H),
.59–0.54 (m, 28H), 0.49–0.42 (m, 8H). MALDI-TOF MS (C H N O BrZn) m/z:
7
2
0
84
100
5
4
calcd for 1387.600; found 1387.706.
[
5,15-bis(2,6-dioctoxyphenyl)-10-(4-(di-p-tolylamino)phenyl)-20-(5-carboxythiophen
11

ACCEPTED MANUSCRIPT
e-2-yl-ethynyl)porphyrinato] zinc(II) (SF)
Compound SF was prepared under modified conditions of literature procedure.
Pd(PPh ) (12 mg, 0.01 mmol) and CuI (2 mg, 0.01 mmol) was added into a solution
3
4
of compound 12 (139 mg, 0.10 mmol) and compound 13 (40 mg, 0.30 mmol) in fresh
distilled THF (35.0 mL) and anhydrous Et N (6 mL) under argon. The reaction was
3
o
stirred at 45 C for 14 h. Then, the reaction was quenched with water (100 mL), and
the mixture was extracted with DCM (2 × 50 mL). The combined extracts were
washed with water and dried over anhydrous MgSO . The solvent was removed under
4
reduced pressure to give the product. The crude product was purified using silica gel
column chromatography (eluent: DCM/CH OH = 25:1) to give a green solid. Yield 95
3
1
mg (65.2%). H NMR (400 MHz, DMSO, δ/ppm): 9.56–9.49 (m, 4H), 9.13–9.04 (m,
4H), 8.83 (s, 2H), 8.78–8.77 (m, 3H), 8.67–8.60 (m, 2H), 7.96–7.92 (m, 5H), 7.69 (s,
2H), 7.21 (s, 2H), 6.89 (s, 4H), 3.92–3.87 (m, 8H), 2.48 (s, 6H), 0.95–0.88 (m, 8H),
0.87–0.82 (m, 8H), 0.68–0.61 (m, 8H), 0.59–0.54 (m, 28H), 0.49–0.42 (m, 8H). FT-IR
-1
(
vmax/cm ): 2177 (v ). MALDI-TOF MS (C H N O SZ ) m/z: calcd for 1458.699;
C≡C 91 103 5 6 n
found 1458.706.
[
5,15-bis(2,6-dioctoxyphenyl)-10-(4-(di-p-tolylamino)phenyl)-20-(5-carboxy-6-methyl
thieno[3,2-b]thiophene-2-yl-ethynyl)porphyrinato] zinc(II) (BSF)
The synthetic procedure for BSF was similar to that for SF, except that
compound 7 (66 mg, 0.3 mmol) was used instead of compound 13. The crude product
was purified using silica gel column chromatography (eluent: DCM/CH OH = 25:1)
3
1
to give a green solid. Yield 89 mg (58.2%). H NMR (400 MHz, DMSO, δ/ppm):
1
2

ACCEPTED MANUSCRIPT
9
2
3
0
.57–9.49 (m, 4H), 9.13–9.04 (m, 4H), 8.83 (s, 2H), 8.78–8.77 (m, 2H), 8.67–8.60 (m,
H), 8.05 (s, 1H), 7.96–7.92 (m, 4H), 7.69 (s, 2H), 7.21 (s, 2H), 6.89 (s, 4H),
.92–3.87 (m, 8H), 2.48 (s, 6H), 2.33 (s, 3H), 0.95–0.88 (m, 8H), 0.87–0.82 (m, 8H),
-1
.68–0.61 (m, 8H), 0.59–0.54 (m, 28H), 0.49–0.42 (m, 8H). FT-IR (vmax/cm ): 2177
vC≡C). MALDI-TOF MS (C H N O S Z ) m/z: calcd for 1528.687; found
(
94
105
5
6 2 n
1
528.984.
[
5,15-bis(2,6-dioctoxyphenyl)-10-((4-N,N-dimethylamino-phenyl)ethynyl)-20-(5-carb
oxy-6-methylthieno[3,2-b]thiophene-2-yl-ethynyl)porphyrinato] zinc(II) (NSF)
The synthetic procedure for NSF was similar to that for SF, except that
compound 14 (138 mg, 0.11 mmol) and compound 7 (132 mg, 0.60 mmol) were used
instead of compound 12 and compound 13, respectively. The crude product was
purified using silica gel column chromatography (eluent: DCM/CH OH = 25:1) to
3
1
give a green solid. Yield 94 mg (61.0%). H NMR (400 MHz, DMSO, δ/ppm): 9.57 (s,
2
2
2
2
H), 9.29 (s, 2H), 8.85–8.74 (m, 4H), 8.05 (s, 1H), 7.90–7.89 (d, J=2 Hz, 2H), 7.69 (s,
H), 7.47–7.46 (d, J=2 Hz, 2H), 6.99 (s, 4H), 3.92–3.87 (m, 8H), 2.60–2.55 (s, 6H),
.16 (s, 3H), 0.95–0.88 (m, 8H), 0.87–0.82 (m, 8H), 0.68–0.61 (m, 8H), 0.59–0.54 (m,
-1
8H), 0.49–0.42 (m, 8H). FT-IR (vmax/cm ): 2177 (vC≡C). MALDI-TOF MS
C H N O S Z ) m/z: calcd for 1399.620; found 1399.721.
(
8
4
97
5
6 2 n
3
. Results and discussion
3
.1 Synthesis
The detailed synthetic routes of three porphyrin dyes as well as the intermediates
were shown in Scheme 1. Firstly, the compound 2 was synthesized from
1
3

ACCEPTED MANUSCRIPT
4
-(dip-tolylamino) benzaldehyde in pyrrol using InCl as catalyst. Then compound 2
3
was reacted with compound 8 and 9 to obtain the key intermediate 10. Subsequently,
compound 10 was reacted with NBS in DCM to convert to 11 Finally, compound 11
was treated with Zn(OAc) •H O to obtain the key intermediate 12. Compound 3 was
.
2
2
synthesized from 3-bromine thiophene by Vilsmeier formylation reaction. Compound
was prepared from 3 with ethyl-2-mercaptoacetate by free radical reaction. Then, by
4
means of Sonogashira reaction, 5 was reacted with trimethylsilylacetylene to afford 6.
Finally, 7 was obtained from ordinary hydrolysis reaction. Compound 8, 9, 13 and 14
were synthesized according to the methods reported in the literatures. The target
molecules were produced with the Sonogashira cross-coupling method [25].
3
.2 Photophysical properties
The photophysical properties of SF, BSF and NSF were investigated by UV-Vis
–
5
absorption spectroscopy in diluted (10 M) chloroform solution. The spectra and data
are shown in Fig. 2 and Table 1, respectively. The three dyes showed typical
absorption bands associated with porphyrins, namely, an intense Soret band at around
4
50 nm and a series of lower-intensity Q bands at longer wavelength (550−650 nm).
Among all the three dyes, NSF displayed a relatively weaker Soret band absorption,
indicating the inferior ability of light harvesting. The values of the molar extinction
coefficient (ε) at the maximum absorption wavelength (λmax) were 2.12, 1.42, 0.86 ×
5
–1
–1
1
0 M cm for SF, BSF, NSF dyes, respectively. As expected, all of the Soret-bands
and Q bands of BSF and NSF were obvious red-shifted significantly relative to SF. In
CHCl solution, the maximum absorption wavelength (λmax) for these dyes were 450,
3
1
4

ACCEPTED MANUSCRIPT
4
57, 458 nm, respectively. The Soret-bands of BSF and NSF which used
-ethynyl-6-methylthieno[3,2-b]thiophene as π spacer were 7–8 nm red-shifted than
2
SF. The Q bands of BSF and NSF were 4 nm and 28 nm red-shifted relative to SF,
respectively. The red-shift of BSF and NSF, which might be due to the electronic
effect of the π spacer and the Q band red-shift of NSF in solution is more obvious
than BSF, which may be largely affected by the dimethyl aniline electron donor group.
This result conformed that the ethynyl-thiophthene groups linking at the
meso-position of porphyrin had positive effect on improving the UV-Vis absorption
of the porphyrin dyes.
We also measured the adsorption of those three porphyrin dyes on TiO films
2
(Fig. 3). The corresponding data were also summarized in Table 1. The corresponding
absorption peaks in Soret-bands were 452, 440 and 477 nm for SF, BSF and NSF,
respectively. The absorption peak of the dye BSF was blue-shifted on the TiO surface
2
in comparison with those in solutions, due to the formation of H-aggregation.
However, the absorption peaks of the dyes SF and NSF were red-shifted compared to
those in solution, which should result from the formation of the J-aggregation of
porphyrins on the TiO surface [29, 30]. In addition, the absorption spectra of these
2
three dyes on the TiO film were broadened relative to the solution spectra.
2
The emission spectra of the three porphyrin dyes are presented in Fig. 4 and
fluorescence data are also listed in Table 1. The maximum fluorescence emission
wavelength (λ ) of BSF was 640 nm, which was a little red-shifted to SF (636 nm).
em
The observed red-shift of λem should be attributed to the increase of the effective
1
5

ACCEPTED MANUSCRIPT
conjugation in BSF molecules compared to SF. But the λ of NSF (623 nm) displays
em
an antipodally blue-shifted relative to SF due to different donor units.
3
.3 Electrochemical properties
To evaluate the thermodynamic possibility of electron transfer from the excited
dye molecule to the conductive band of TiO , cyclic voltammetry (CV) was
2
performed in CHCl with 0.1 M tetrabutylammonium perchlorate (TBAP) as a
3
supporting electrolyte (Fig.5). The first oxidation potential (E ) versus normal
ox
hydrogen electrode (NHE) E , which corresponds to the highest occupied molecular
ox
orbital (HOMO versus NHE), was calibrated by addition of 0.63 V to the potential
+
(
versus silver electrode) versus Fc/Fc by CV. The reduction potential versus NHE
(Ered), which corresponds to the lowest unoccupied molecular orbital (LUMO versus
NHE), could be calculated by E -E . And the zerothezeroth energies (E ) were
ox 0-0
0-0
determined by the intersection of the normalized UV-Vis absorption spectrum and
steady-state fluorescence emission spectrum. All the data are summarized in Table 2.
The Eox energies of SF, BSF and NSF were found to locate at 0.83, 0.79 and
0
.64 V, respectively. Compared with SF, the E s of BSF and NSF were shifted to
ox
negative, which could be attributed to the expansion of their π-conjugated units [31].
The reduction potentials of the three dyes (SF, BSF and NSF) were –1.23, –1.25 and
–
1.43, respectively. It was evident that the HOMO levels of dyes BSF and NSF were
more negative than SF, which resulted in the decrease of their energy gaps between
the HOMO and LUMO. At the same time, the decrease of their energy gaps showed a
positive effect on the DSSCs performance due to the narrower gap between the
1
6

ACCEPTED MANUSCRIPT
HOMO level and the redox potential of iodine/iodide leading to less waste of energy.
It was evident that the LUMO values were more negative than the conduction band of
TiO (about –0.5 V versus NHE), which the electrons could be efficiently injected into
2
the conduction band of TiO from the excited dyes. The Egap between the conduction
2
band and the E (versus NHE) of SF, BSF and NSF were 0.73, 0.75 and 0.93 eV.
red
Theoretically, an energy gap of 0.2 eV was necessary for efficient electron injection
[
32]. The above experimental results indicated that the three dyes could be used as the
qualified sensitizers for efficient DSSCs.
3
.4 Photovoltaic properties of porphyrin-sensitized TiO cells
2
The incident photo-to-current conversion efficiencies (IPCE) plots for the
DSSCs based on the three porphyrin dyes are shown in Fig. 6. All the three dyes
responded in the broad range of 300–750 nm and showed the maximum IPCE values
around 460 nm, which was in accord with the spectra of the dyes on TiO . It was
2
noticed that the solar cell sensitized by dye SF presented an IPCE value of 55% at 466
nm. Dyes BSF and NSF got higher IPCE values of 71% at 467 nm and 82% at 470
nm, respectively. Among the three dyes, SF showed inferior IPCE relative to other
two dyes which mainly resulted from its worse light harvesting ability in long
wavelength region in solution.
The photocurrent density-voltage curves (J–V) of DSSCs based on SF, BSF and
NSF are shown in Fig. 7 and the detailed photovoltaic parameters are summarized in
–2
Table 3. The SF-based cell gave a short circuit current density (J ) of 6.09 mA cm ,
sc
an open circuit voltage (V ) of 0.52 V, and a fill factor (FF) of 0.67, which yielded an
oc
1
7

ACCEPTED MANUSCRIPT
overall power conversion efficiency (PCE) of 2.10%. And the cell based on NSF
–
2
showed a J of 9.41 mA cm , a V of 0.62 V, and a FF of 0.68, leading to a PCE of
sc
oc
3
.98%. Under the same conditions, the BSF-based cell showed the highest PCE
–
2
among the three dyes of 4.53%, with a highest J of 10.66 mA cm . The V values
sc
oc
were in the order of NSF ≈ BSF > SF, the lowest V of SF which might due to the
oc
higher charge recombination rate in TiO /dye/electrolyte interface. The measured Jsc
2
–2
–2
increased in the order of SF (6.09 mA cm ) < NSF (9.41 mA cm ) < BSF (10.66
–2
mA cm ), consistent with the increased tendency of IPCE. To investigate the
absorptive abilities and explain the J of the three dyes, the thin films were evaluated
sc
for the total amount of the porphyrin adsorbed. The adsorbed amounts (Γ) of the three
dyes on TiO film are exhibited in Table 1. The amount of the three dyes (SF, BSF
2
–
9
–9
–8
–
2
and NSF) absorbed on TiO film were 7.72 × 10 , 8.19×10 , 1.58×10 mol cm
,
2
respectively. These data suggested that SF had the worst light-harvesting ability on
TiO film thus caused the low J [33]. Although NSF exhibited the highest Γ, its
2
sc
lowest ε brought about the lower J than BSF. Finally, the maximum conversion
sc
efficiency has been achieved for the cell sensitized with BSF.
3
.5 Electrochemical impedance spectroscopy studies
To particularly elucidate correlation of Voc with the three dyes the
electrochemical impedance spectroscopy (EIS) was measured in the dark [34]. The
Nyquist plots of SF, BSF and NSF based DSSCs are shown in Fig. 8. Two
semicircles were observed in the Nyquist plots. The small and large semicircles,
respectively located in the high and middle frequency regions, were assigned to the
1
8

ACCEPTED MANUSCRIPT
charge transfer at Pt/electrolyte and TiO /dye/electrolyte interface [35, 36],
2
respectively. The charge recombination resistance (R ) on the TiO surface, which
rec
2
was related to the charge recombination rate between injected electron and electron
−
acceptor ( 3
I
) in the electrolyte, was estimated by the large semicircle [37]. A large
Rrec means a small charge recombination rate and vice versa. The Rrec values for SF,
BSF and NSF were estimated to be 23 ꢁ, 37 ꢁ and 46 ꢁ, respectively. The result was
consistent with the changing tendencies of V values in the cells of the three dyes
oc
mentioned above.
DSSCs based on BSF and NSF produced higher V values than SF (0.52 V <
oc
0
.61 Vꢁ0.62 V), which could also be explained by the electron lifetime. Fig. 9
showed the Bode plot for DSSCs based on SF, BSF and NSF. The peaks located at
the high-frequency (right) and middle-frequency (left) respectively correspond to the
small semicircle (left) and large semicircle (right) in the Nyquist plots (Fig. 9). The
reciprocal of the peak frequency for the middle-frequency peak was regarded as the
electron lifetime since it represented the charge transfer process at the
TiO /dye/electrolyte interface. Therefore we could speculate that the electronic
2
lifetime of the DSSC based on BSF and NSF was longer than that of DSSC based on
SF, thereby explaining the tendencies of V for the three dyes.
oc
4
. Conclusions
In summary, three new donor-π-acceptor type unsymmetrical porphyrins (SF,
BSF and NSF) were designed and synthesized. The photophysical, electrochemical
and photovoltaic properties of the porphyrin dyes were greatly influenced by the
1
9

ACCEPTED MANUSCRIPT
donor units and π spacers. It showed that the absorption spectra of both BSF and NSF
dyes which used 2-ethynyl-6-methylthieno[3,2-b]thiophene as π spacer were
red-shifted in solution and broadened on TiO films, respectively, and the amount of
2
the two dyes absorbed on TiO film significantly increased. Under simulated air mass
2
1
.5 global sunlight, BSF-based DSSC exhibited the highest η of 4.53% among the
three dyes.
Acknowledgements
This work was supported by the National Nature Science Foundation of China
(
No. 51173154), and the Nature Science Foundation of Hunan Province of China
13JJ2025, 2015JJ2141).
(
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2
6

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Table 1 Maximum absorption and emission data of SF, BSF and NSF dyes.
λ /nm
λmax/nm
Γ/
λem
λint
abs
dyes
5
-1
-1 a
b
-1
c
d
e
(
ε, 10 M cm )
(on TiO2 )
(mol cm )
(nm)
(nm)
–
9
9
8
SF
450(2.12), 621(0.23)
457(1.42), 625(0.19)
458(0.86), 649(0.20)
452
440
477
7.72 × 10
8.19 × 10
1.58 × 10
636
640
623
602
607
599
–
–
BSF
NSF
a
b
Absorption spectra was measured in CHCl solution. Absorption spectra was obtained on TiO
3
2
c
d
films. Amount of the dyes adsorbed on TiO films. Wavelengths for emission spectra in CHCl
2
3
e
solution by exciting at Soret wavelength. Measured by the intercept of the normalized absorption
and emission spectra.
Table 2 Electrochemical data of SF, BSF and NSF dyes
d
E
red/V/ vs
a
b
c
e
Dye
λ /nm
E
0-0/eV
E /V vs NHE
E gap/V
int
ox
NHE
SF
602
607
599
2.06
2.04
2.07
0.83
0.79
0.64
–
–
–
1.23
1.25
1.43
0.73
0.75
0.93
BSF
NSF
a,b
c
Measured by the intercept of the normalized absorption and emission spectra. First oxidation
d
potentials (vs. NHE). Excited-state oxidation potentials approximated from Eox and E0-0 (vs.
e
NHE). Driving forces for electron injection from the porphyrin excited singlet state(E ) to the
ox
conduction band of TiO (–0.5 V vs. NHE).
2
Table 3 Photovoltaic performances of DSSCs based on SF, BSF and NSF dyes.
–
2
Dye J (mA cm )
Voc (V)
FF
η (%)
sc
SF
6.09
10.66
9.41
0.52
0.61
0.62
0.67
0.69
0.68
2.10
4.53
3.98
BSF
NSF
2
7

ACCEPTED MANUSCRIPT
Captions for Figures and Scheme
Fig. 1 Molecular structures of SF, BSF and NSF dyes.
Scheme. 1 Synthetic routes of SF, BSF and NSF dyes.
-5
Fig. 2 Absorption spectra of SF, BSF and NSF dyes in CHCl solutions (10 M).
3
Fig. 3 Normalized absorption spectra of SF, BSF and NSF dyes on TiO thin-films.
2
Fig. 4 Normalized emission spectra for SF, BSF and NSF dyes in solution.
Fig. 5 Cyclic Voltammogram of SF, BSF and BSF dyes.
Fig. 6 IPCE action spectrum for the DSSC based on the SF, BSF and NSF
dye-sensitized TiO films.
2
Fig. 7 J
Fig. 8 EIS Nyquist plots for DSSCs based on the three dyes measured in the dark
under 0.70 V bias.
Fig. 9 EIS Bode plots for DSSCs based on the three dyes measured in the dark
under 0.70 V bias.
–V curves for DSSCs with SF, BSF and NSF dyes.
–
–
2
8

ACCEPTED MANUSCRIPT
Fig. 1 Molecular structures of SF, BSF and NSF dyes.
2
9