Azo-coupled zinc phthalocyanines: Towards broad absorption and application in dye-sensitized solar cells

Article abstract of DOI:10.1016/j.poly.2014.10.026

The synthesis of symmetrical azo-coupled zinc phthalocyanines (7, 8, 10 and 11) incorporating two spectrally complementary photogenerators (azo dyes and phthalocyanines) by NN double bonds in a single structure was described. The four sensitizers were fully characterized by Fourier Transform infrared spectroscopy, UV-Vis spectroscopy, proton nuclear magnetic resonance, mass spectrometry, elemental analyses, thermogravimetric analysis and cyclic voltammetry. All the azo-coupled phthalocyanines exhibited broad absorptions covering the range 300-800 nm. The cyclic voltammetry studies indicated that the LUMO and HOMO energy levels of four synthesized azo-coupled phthalocyanines could ensure efficient electron injection and thermodynamically favorable dye regeneration. Thermal stability studies showed that the four sensitizers were stable above 200 °C. The new compounds were tested in dye-sensitized solar cells and compound 8 showed the best photovoltaic performance, yielding 2.7% power conversion efficiency.

Full text of DOI:10.1016/j.poly.2014.10.026

Polyhedron 85 (2015) 864–873
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Azo-coupled zinc phthalocyanines: Towards broad absorption
and application in dye-sensitized solar cells
⇑
Mingliang Han, Xuejun Zhang , Xiaoxia Zhang, Chaoqiang Liao, Baiqing Zhu, Qiaoling Li
Department of Chemistry, School of Science, North University of China, Taiyuan 030051, Shanxi Province, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis of symmetrical azo-coupled zinc phthalocyanines (7, 8, 10 and 11) incorporating two spec-
trally complementary photogenerators (azo dyes and phthalocyanines) by N@N double bonds in a single
structure was described. The four sensitizers were fully characterized by Fourier Transform infrared spec-
troscopy, UV–Vis spectroscopy, proton nuclear magnetic resonance, mass spectrometry, elemental anal-
yses, thermogravimetric analysis and cyclic voltammetry. All the azo-coupled phthalocyanines exhibited
broad absorptions covering the range 300–800 nm. The cyclic voltammetry studies indicated that the
LUMO and HOMO energy levels of four synthesized azo-coupled phthalocyanines could ensure efficient
electron injection and thermodynamically favorable dye regeneration. Thermal stability studies showed
that the four sensitizers were stable above 200 °C. The new compounds were tested in dye-sensitized
solar cells and compound 8 showed the best photovoltaic performance, yielding 2.7% power conversion
efficiency.
Received 14 July 2014
Accepted 11 October 2014
Available online 1 November 2014
Keywords:
Azobenzene
Phthalocyanines
Dye-sensitized solar cells
Absorption spectra
Electrochemistry
Ó 2014 Elsevier Ltd. All rights reserved.
1
. Introduction
for the construction of molecular materials with designed elec-
tronic and optical properties [3,6]. Additionally, the facile modifi-
Dye-sensitized solar cells (DSSCs) based on dye sensitized nano-
crystalline TiO photoelectrodes have received considerable atten-
tion due to their low cost, decorative nature, ease of fabrication,
short energy pay-back time and being more environmentally
pleasant than conventional solar cells based on inorganic silicon
semiconductors [1,2]. A typical DSSC is composed of a photo-anode
cation of the chromophore skeleton of phthalocyanines provides
potential for molecular design. Based on these attributes, phthalo-
cyanines are rendered attractive for the potential application in
dye-sensitized solar cells (DSSC) [7–11].
2
Up until now, various chromophores have been introduced into
the phthalocyanine core to tune the absorption over a broad spec-
tral range and to achieve high extinction coefficients, including
coumarin derivatives [12], azo dyes [13–15], triphenylamine
groups [10], thienyls [16], multicomponent systems such as tetra-
thiafulvalenes [17], fluorenyl [18] and perylenediimide [19]. How-
ever, we found that almost all the chromophores were introduced
through oxygen bridges [12–15], thioether bonds [20] or CꢀC
bonds [7,10], and the characteristic absorptions of these chro-
mophores overlapped with the Soret-band of the phthalocyanines.
Moreover, the absorption bands of these phthalocyanines red
shifted slightly. Also, for Pc dyes, the spectral window between
400 and 600 nm still exhibited almost no absorptions [19]. In order
to overcome the problems mentioned above, azo dyes were chosen
as chromophores and these were connected onto the Pc ring
through the N@N double bond. Pcs and azo dyes show great attrac-
tiveness due to their broad availability and perfectly complemen-
tary absorption regions. Azo compounds are the oldest and
largest class of industrial synthesized organic dyes due to their
versatile application in the fields of dyes and pigments [21–23].
(
consisting of a fluorine-doped SnO
2
glass substrate, a nanocrystal-
line TiO thin film and dye sensitizers) and a Pt-doped photoinert
2
counter electrode (cathode), sandwiching a redox mediator. To
date, power conversion efficiencies of more than 9% have been
achieved for state-of-the-art DSSCs [3,4]. The performances of
DSSCs are mainly correlated with the properties of the sensitizers.
Organic sensitizers with higher LUMO energy levels, smaller
energy gaps and wide wavelength range absorption bands will be
superb [3].
Phthalocyanines (Pcs) and their metal complexes, as planar
two-dimensional aromatics, are typical NIR dyes [5]. Pcs have
strong absorptions on two distinct edges of the visible region,
namely the Q-band at around 700 nm and the Soret-band at
around 350 nm. These macrocycles, in combination with other
electro- and photoactive moieties, have been ideal building blocks
⇑
Corresponding author. Tel./fax: +86 351 3922152.
E-mail address: zhangxuejun@nuc.edu.cn (X. Zhang).
http://dx.doi.org/10.1016/j.poly.2014.10.026
277-5387/Ó 2014 Elsevier Ltd. All rights reserved.
0

M. Han et al. / Polyhedron 85 (2015) 864–873
865
Nowadays, azo dyes have been deeply investigated and applied in
high technology areas due to their idiosyncratic optical properties,
such as optical information storage [24], liquid crystalline displays
was employed, consisting of a platinum wire as the working elec-
trode, another platinum wire as the counter film electrode and a
standard calomel electrode (SCE) as the reference electrode. The
dyes were analyzed in dry THF containing tetrabutylammonium
perchlorate (0.1 M) as the supporting electrolyte.
[
25] and non-linear optics [26]. Azo dyes show intense absorptions
in the visible region (400–650 nm) and always have two
absorption bands corresponding to the long wavelength generated
⁄
by the n ?
p
transition and the short wavelength generated by
2.2. Device fabrication
⁄
the
p
?
p
transition.
Herein, we designed and synthesized four symmetrical azo-
The screen-printing method was used to prepare the TiO
toelectrodes, as reported in the literature [28,29]. The 10 wt% P25
TiO nanoparticles were well dispersed in ethanol, and then the
dispersed solution was electrosprayed through a plastic syringe,
that had a metallic microtip connected to a high-voltage power
supply, onto a washed transparent conducting glass (which had
been coated with a fluorine-doped stannic oxide layer, with a
2
pho-
coupled zinc phthalocyanines with hydroxyl and carboxyl as
anchoring groups, which incorporated two spectrally complemen-
tary photogenerators (azo dyes and Pcs) by N@N double bonds in
single structures. The new compounds showed wide and intense
absorption bands in the whole visible region and some of the
near-infrared region. To the best of our knowledge, reports on
phthalocyanine–azobenzene dyads bearing anchoring groups, such
as panchromatic and functional molecules, and their application in
dye-sensitized solar cells are still scarce. In this manuscript, the
synthesis, characterization, spectroscopic, electrochemistry, ther-
2
thickness of 4 mm and sheet resistance of 18–20
the as-prepared TiO spheres were placed between two steel plates
and laminated under a pressure of 12 MPa. The pressed TiO films
X). Subsequently,
2
2
were dried at 150 °C for 30 min and then were gradually sintered
at 500 °C for 20 min to fabricate the photoelectrode layers. The
sensitizers 7, 8, 10 and 11 were dissolved in THF at a concentration
2
mal studies and TiO photoelectrode tests of four azo-coupled zinc
phthalocyanines were reported.
ꢀ4
of 1 ꢁ 10 M. The TiO
tions and then kept at room temperature for 15 h so that the dyes
were adsorbed onto the TiO films. Then, the dye-adsorbed photo-
2
films were immersed into the dye solu-
2
. Experimental
2
electrodes were washed with THF and ethanol to remove non-
adsorbed dyes under a stream of nitrogen. The dye-sensitized elec-
trodes were assembled and sealed with the Pt-CEs using hot-melt
2
2
.1. Materials and equipment
.1.1. Materials
ionomer adhesive films (Surlyn, 25 lm thickness, DuPont 1702) to
Fe powders, 4-nitrophthalonitrile, phenol, 1-naphthalenol, 4-
acquire a gap between the two electrodes. The Pt-CE was prepared
from hexachloroplatinic acid by chemical deposition. The electro-
lyte prepared from 0.05 M iodine and 0.5 M LiI in acetonitrile
was introduced through the pre-drilled hole. The holes were then
covered with a glass cover. Photoelectrochemical data were tested
hydroxybenzoic acid, p-aminobenzoic acid, sodium hydroxide,
sodium nitrite, zinc acetate dihydrate, tetrabutylammonium per-
chlorate, vanilline and 1,8-diazabicyclo [5.4.0]-undec-7-ene (DBU)
were purchased from Aladdin. Methanol, acetone, acetic acid, chlo-
roform, n-pentanol, ethyl acetate, N,N-dimethylformamide (DMF),
dimethyl sulfoxide (DMSO) and tetrahydrofuran (THF) were pur-
chased from Damao reagent company. All reagents and solvents
were analytical grade. All solvents were dried and purified as
described by Perin and Armarego [27]. 4-Hydroxybenzoate was pre-
pared from 4-hydroxybenzoic acid according to classical proce-
dures. Column chromatography was performed on silica gel (80–
2
with a 450 W xenon light source, focused to give 1000 W/m , the
equivalent of one sun at AM 1.5, at the surface of the test cells.
In order to reduce scattered light from the edge of the glass elec-
2
trodes of the dyed TiO layer, a light-shading mask was also used
on the devices.
2
2
.3. Synthesis
1
00 mesh).
.3.1. 4-Aminophthalonitrile (1)
2.1.2. Equipment
A mixture of Fe powders (2.05 g, 33.60 mmol) and concentrated
Infrared spectra (IR) were recorded on a Shimadzu 4800S Fou-
hydrochloric acid (1 ml) in deionized water (50 ml) was heated and
stirred at 60 °C for 30 min. Then 4-nitrophthalonitrile (2.00 g,
11.56 mmol) was added portionwise over a period of 60 min. After
the addition of the 4-nitrophthalonitrile, the temperature was
raised to 75 °C and the mixture was stirred vigorously for a further
1.5 h. The reaction mixture was then filtered through a Büchner
funnel. The residues were washed with 150 ml hot methanol.
Washings were concentrated and the obtained faint yellow precip-
itate was filtered off, washed with water until the washings were
neutral. The white pure product was recovered by recrystallization
using methanol. Yield: 1.07 g (65%). M.p.: 179–181 °C. Anal. Calc.
1
rier Transform Infrared spectrophotometer using KBr pellets.
NMR spectra were obtained in DMSO-d
H
6
solvent using a Bruker
DRX 300 MHz NMR spectrometer, and chemical shifts are reported
relative to tetramethylsilane (TMS) as an internal reference. The
UVꢀVis spectra were recorded with a Techcomp 2300 spectropho-
tometer. Elemental analyses were performed on a Flash EA1112
elemental analyzer (C, H, N). MALDI-TOF mass spectrometric mea-
surements were performed on a Bruker Bifiex III MALDI-TOF. Ther-
mogravimetric measurements were carried out on
a ZCT-A
ꢀ1
instrument with a heating rate of 10 °C min under nitrogen.
The current–voltage characteristics of the cells were measured
on a computer-controlled voltage–current source meter (Advan-
test, R6243) under illumination of simulated AM1.5G solar light
from an AM1.5 solar simulator (Wacom Co., WXS-80C-3 with a
for C
8 5 3
H N : C, 67.13; H, 3.49; N, 29.37. Found: C, 67.09; H, 3.46;
ꢀ1
N, 29.28%. FT-IR (KBr tablet)
mmax, cm : 3452, 3354 (NAH), 3079
(ArAH), 2220 (C„N), 1598 (C@C), 1332 (CAN), 1638, 1494, 1256.
4
50 W xenon lamp). The incident light intensity was calibrated
2.3.2. 4-(4-Hydroxyphenylazo)phthalonitrile (2)
by using a standard mono-Si solar cell, giving the photoresponse
range of an amorphous silicon solar cell. The standard solar cell
was produced and calibrated by the Japan Quality Assurance
Organization.
Cyclic voltammetry measurements were performed on an elec-
trochemical workstation (CHI LK2005A, Tianjin Lanlike Chemistry
electronic high-tech Co., Ltd., PR China). A three electrode set-up
2.3.2.1. Diazotization. A mixture of 1 (1.43 g, 10 mmol) and concen-
trated hydrochloric acid (3 ml) in deionized water (30 ml) was stir-
red in an ice bath until blended well. Then a solution of sodium
nitrite (0.76 g, 11 mmol) was added dropwise to the slurry-like
mixture at 0–5 °C over about 20 min. The stirring was continued
for another 30 min, then the reaction mixture was filtered, and
the bright yellow solution was kept in an ice bath for the next step.

8
66
M. Han et al. / Polyhedron 85 (2015) 864–873
2
.3.2.2. Coupling. The prepared diazonium salt solution was slowly
(1H, d, ArAH), 7.64 (1H, S, ArAH), 4.29 (2H, m, Aliph. CAH), 1.30
(3H, m, Aliph. CAH).
added to a stirred solution of phenol (0.94 g, 10 mmol) as the cou-
pling component in alkali medium by adjusting the pH value to 8–
9
using a sodium hydroxide solution (30%) and the temperature
was maintained at 0–5 °C. The resulting solution was stirred for
0 min in an ice bath and then allowed to reach room temperature.
2
.3.5. 4-[(4-Hydroxy-3-methoxybenzylidene)amino]benzoic acid (5)
A mixture of vanilline (2.28 g, 15 mmol) and ethyl 4-aminoben-
3
zoic acid (2.47 g, 15 mmol) in ethanol (30 ml) was refluxed under a
nitrogen atmosphere until the starting material could no longer be
detected by TLC. After the reaction completed, the solvent was
evaporated and the yellow crude product was purified by silica
gel chromatography column with chloroform alone as the eluent.
The title compound was dried under vacuum at 60 °C overnight.
Yield: 3.41 g (76%). M.p.: 142–146 °C. Anal. Calc. for C15
C, 66.42; H, 4.79; N, 5.16. Found: C, 66.49; H, 4.72; N, 5.18%. UV–
Vis (THF) kmax, nm (log
After 1 h, the pH value of the diazo liquor was adjusted to 3–4 by
adding a diluted hydrochloric acid solution. The precipitated
jacinth solids were filtered off, washed several times with cold
water and purified by further recrystallization from acetone and
finally dried under vacuum at 60 °C overnight. Yield: 1.76 g
(
3
k
71%). M.p.: 223ꢀ225 °C. Anal. Calc. for C14
.22; N, 22.58. Found: C, 67.64; H, 3.25; N, 22.53%. UV–Vis (THF)
max, nm (log ): 378 (4.82), 489 (4.03). FT-IR (KBr tablet)
cm : 3339 (OAH), 3033 (ArAH), 2242 (C„N), 1601, 1581 (C@C),
460 (N@N), 1505, 1420, 1392, 1352, 1290, 1202, 1135. 1H NMR
DMSO-d ) d, ppm: 10.41 (1H, s, OH), 8.41 (1H, d, ArAH), 8.34
1H, s, ArAH), 7.84 (1H, d, ArAH), 7.75 (2H, d, ArAH), 6.91 (2H,
8 4 2
H N O : C, 67.74; H,
4
H13NO :
e
max
m ,
e
): 326 (4.68), 427 (3.72). FT-IR (KBr tablet)
ꢀ
1
ꢀ1
mmax, cm : 3508 (OAH), 3079 (ArAH), 2991–2862 (Aliph. CAH),
1
(
(
1
1
683 (C@O), 1593 (C@N), 1511, 1427, 1320, 1204, 1167. H NMR
) d, ppm: 12.56 (1H, s, COOH), 9.57 (1H, s, OH), 8.13
2H, d, ArAH), 7.49 (2H, d, ArAH), 8.35 (1H, s, CH@N), 7.01 (1H,
6
(
(
DMSO-d
6
d, ArAH).
d, ArAH), 6.96 (1H, s, ArAH), 6.65 (1H, d, ArAH), 3.73 (3H, s, Aliph.
CAH).
2
2
.3.3. 4-(4-Hydroxynaphthalen-1-ylazo)phthalonitrile (3)
.3.3.1. Diazotization. The same procedure and molar ratio of the
2
.3.6. 4-[2-Hydroxy-3-methoxy-4-(4-carboxy-benzylidene-
amino)phenylazo]phthalonitrile (6)
.3.6.1. Diazotization. The same procedure and molar ratio of the
reagents given in section 2.3.2.1 were followed.
reagents given in section 2.3.2.1 were followed.
2
2.3.3.2. Coupling. The prepared diazonium salt solution was slowly
added to a stirred solution of 1-naphthalenol (1.44 g, 10 mmol) in
alkali medium by adjusting the pH value to 8–9 using a sodium
hydroxide solution (30%) and the temperature was maintained at
2.3.6.2. Coupling. The prepared diazonium salt solution was slowly
added to a stirred solution of 5 (2.99 g, 10 mmol) in alkali medium
by adjusting the pH value to 8–9 using a sodium hydroxide solu-
tion (30%) and the temperature was maintained at 0–5 °C. The
resulting solution was stirred for 30 min in an ice bath and then
allowed to reach room temperature. After 30 min, the pH value
of the diazo liquor was adjusted to 3–4 by adding a diluted hydro-
chloric acid solution. The precipitated reddish brown solids were
filtered off, washed several times with cold water, purified by fur-
ther recrystallization from acetone and finally dried under vacuum
at room temperature for 2 days. Yield: 2.33 g (53%). M.p.: 161–
0
–5 °C. The resulting solution was stirred for 30 min in an ice bath
and after another 30 min the pH value of the diazo liquor was
adjusted to 3–4 by adding a diluted hydrochloric acid solution.
The precipitated red solids were filtered off, washed several times
with cold water and purified by further recrystallization from ace-
tic acid and finally dried under vacuum at room temperature for
2
C
days. Yield: 1.87 g (63%). M.p.: 292–295 °C. Anal. Calc. for
O: C, 72.48; H, 3.35; N, 18.79. Found: C, 72.41; H, 3.33;
): 308 (4.46), 453 (4.83),
18
10 4
H N
N, 18.72%. UVꢀVis (THF) kmax, nm (log
07 (3.98). FT-IR (KBr tablet) max, cm : 3259 (OAH), 3079
ArAH), 2226 (C„N), 1595 (C@C), 1482 (N@N), 1414, 1361,
e
ꢀ1
6
(
m
1
64 °C. Anal. Calc. for C23
H
15
N
5
O
4
: C, 64.94; H, 3.52; N, 16.47.
):
max, cm : 3374 (OAH),
077 (ArAH), 2925 (Aliph. CAH), 2231 (C„N), 1688 (C@O), 1599
1
Found: C, 64.87; H, 3.49; N, 16.51%. UV–Vis (THF) kmax, nm (log
e
1
310, 1256, 1183, 758. H NMR (DMSO-d
OH), 8.42 (1H, d, ArAH), 8.37 (1H, s, ArAH), 8.08 (1H, d, ArAH),
.91 (1H, d, ArAH), 7.84 (1H, d, ArAH), 7.68 (1H, d, ArAH), 7.38
1H, m, ArAH), 7.35 (1H, m, ArAH), 6.89 (1H, d, ArAH).
6
) d, ppm: 10.27 (1H, s,
ꢀ
1
3
3
(
36 (4.86), 518 (4.17). FT-IR (KBr tablet) m
7
(
C@N), 1508 (N@N), 1276, 1022 (CAOAC), 1433, 1386, 1172. 1
H
NMR (DMSO-d
6
) d, ppm: 12.85 (1H, s, COOH), 10.24 (1H, s, OH),
8
.11 (2H, d, ArAH), 7.5 (2H, d, ArAH), 8.41 (1H, s, CH@N), 8.44
2
2
.3.4. 4-(2-Hydroxy-5-ethyl-oxycarbonyl-phenylazo) phthalonitrile (4)
.3.4.1. Diazotization. The same procedure and molar ratio of the
(
1H, d, ArAH), 7.92 (1H, d, ArAH), 8.36 (1H, s, ArAH), 7.68 (1H, s,
ArAH), 7.16 (1H, s, ArAH), 3.71 (3H, s, Aliph. CAH).
reagents given in section 2.3.2.1 were followed.
2
.3.4.2. Coupling. The prepared diazonium salt solution was slowly
added to a stirred solution of ethyl 4-hydroxybenzoate (1.66 g,
0 mmol) in alkali medium by adjusting the pH value to 8–9 using
2.3.7. Tetrakis-(4-hydroxyphenylazo) phthalocyaninato zinc(II) (7)
A mixture of 2 (1.18 g, 5 mmol), DBU (4 ml, 2.6 mmol) and zinc
acetate dihydrate (0.28 g, 1.3 mmol) in n-pentanol (30 ml) was
refluxed and stirred under a nitrogen atmosphere for 24 h. There-
after, methanol (50 ml) was added to the reaction mixture, fol-
lowed by refluxing for 30 min. The resulting black suspension
was filtered off, washed with deionized water, dilute hydrochloric
acid, methanol, acetone and ethyl acetate. The black solid after fil-
tration was subjected to Soxhlet extraction using an acetone-meth-
anol (volume ratio 1:1) mixture for 24 h to afford pure 7. Yield:
1
sodium hydroxide (30%) solution and the temperature was main-
tained at 0–5 °C. The resulting solution was stirred for 30 min in
an ice bath and then allowed to reach room temperature. After 1
h, the pH value of the diazo liquor was adjusted to 3–4 by adding
diluted hydrochloric acid solution. The precipitated brown orange
solids were filtered off, washed several times with cold water and
purified by further recrystallization from acetone and finally dried
under vacuum at 60 °C overnight. Yield: 2.17 g (68%). M.p.: 183–
0.47 g (36%). M.p.: >300 °C. Anal. Calc. for C56
62.13; H, 3.02; N, 21.18. Found: C, 62.17; H, 3.05; N, 21.14%. UV–
Vis (THF) kmax, nm (log ): 376 (4.78), 649 (4.24), 718 (4.72).
MALDI-TOF MS (m/z): C56
32 16 4
H N O Zn: C,
1
85 °C. Anal. Calc. for C17
H
12
N
4
O
3
: C, 63.75; H, 3.75; N, 17.5. Found:
): 378
max, cm : 3221 (OAH),
067 (ArAH), 2984–2903 (Aliph. CAH), 2236 (C„N), 1708 (C@O),
C, 63.69; H, 3.73; N, 17.52%. UV–Vis (THF) kmax, nm (log
e
e
ꢀ1
+
(
4.80), 488 (4.01). FT-IR (KBr tablet)
m
H
32
N
16
O
4
Zn [1058.36]: M 1058.62. FT-
max, cm : 3203 (OAH), 3073 (ArAH), 1587
(C@C), 1479 (N@N), 1502, 1341, 1274, 1231, 1140, 1091, 841,
ꢀ
1
3
1
IR (KBr tablet) m
1
615, 1587 (C@C), 1446 (N@N), 1395, 1369, 1287, 1239, 1171. H
) d, ppm: 9.44 (1H, s, OH), 8.39 (1H, d, ArAH),
.36 (1H, d, ArAH), 7.89 (1H, d, ArAH), 7.87 (1H, d, ArAH), 7.73
1
NMR (DMSO-d
8
6
745. H NMR (DMSO-d
6
) d, ppm: 10.43 (4H, s, OH), 6.96–7.1 (8H,
d, ArAH), 8.93–9.06 (8H, d, ArAH), 7.85–8.34 (12H, m, Pc ring).

M. Han et al. / Polyhedron 85 (2015) 864–873
867
2
.3.8. Tetrakis-(4-hydroxynaphthalen-1-ylazo)phthalocyaninato
zinc(II) (8)
The synthesis and purification was as outlined for 7, except 3
1.48 g, 5 mmol) was employed instead of 2. The same molar ratio
of the reagents given in section 2.3.7 was followed. Yield: 0.42 g
27%). M.p.: >300 °C. Anal. Calc. for C72 Zn: C, 68.71; H,
.18; N, 17.81. Found: C, 68.68; H, 3.16; N, 17.79%. MALDI-TOF
3. Results and discussion
3.1. Synthesis and characterization
(
The synthetic routes to the azo-coupled phthalocyanines (7, 8,
10 and 11) were given in Scheme 1. The first step was the synthesis
of 4-aminophthalonitrile 1. This was reduced from 4-nitrophthalo-
nitrile using Fe powders and concentrated hydrochloric acid in
deionized water. 4-Aminophthalonitrile was used to prepare the
diazonium salt solutions to synthesize the four azo-phthalonitriles:
4-(4-hydroxyphenylazo)phthalonitrile 2, 4-(4-hydroxy-naphtha-
len-1-ylazo) phthalonitrile 3, 4-(2-hydroxyl-5-ethyl-oxycarbonyl-
phenylazo)phthalonitrile 4 and 4-[2-hydroxyl-3-methoxyl-4-(4-
carboxyl-benzylidene-amino)phenylazo]phthalonitrile 6. The cou-
pling between the arenediazonium ion and phenol derivatives
was via electrophilic aromatic substitution reactions. The arene-
diazonium ion reacted as an electrophilic reagent, but it was so mild
that only highly activated rings could be used. For this reason, the
coupling reactions were carried out in alkali medium to activate
the phenolic hydroxyl groups. The purification of the four azo-cou-
pled phthalonitriles was achieved by recrystallization. The solubil-
ity of the four azo-phthalonitriles was good in common organic
solvents, such as alcohol and ethyl acetate. The synthesis of the four
azo-coupled phthalonitrile compounds was the most important
step in these reaction sequences.
(
3
40 16 4
H N O
+
40 16 4
MS (m/z): C72H N O Zn [1258.60]: M 1258.41. UV–Vis (THF)
ꢀ1
kmax, nm (log
e): 462 (4.74), 740 (4.77). FT-IR (KBr)
m
max, cm
:
3
1
367, 3220 (OAH), 3067 (ArAH), 1598 (C@C), 1488 (N@N), 1544,
1
454, 1315, 1248, 1197, 1087, 1050, 838, 764, 747. H NMR
(
6
DMSO-d ) d, ppm: 10.20 (4H, s, OH), 8.08 (4H, d, ArAH), 7.35–
7
7
.38 (8H, m, ArAH), 7.68 (4H, d, ArAH), 7.91–8.03 (12H, m, Pc ring),
.81 (4H, d, ArAH), 6.88 (4H, d, ArAH).
2
.3.9. Tetrakis-(2-hydroxy-5-ethyl-oxycarbonyl-
phenylazo)phthalocyaninato zinc(II) (9)
The synthesis and purification was as outlined for 7, except 4
(
1.60 g, 5 mmol) was employed instead of 2. The same molar ratio
of the reagents given in section 2.3.7 was followed. Yield: 0.53 g
32%). M.p.: >300 °C. Anal. Calc. for C68 12Zn: C, 60.65; H,
.56; N, 16.64. Found: C, 65.58; H, 3.53; N, 16.59%. MALDI-TOF
(
3
48 16
H N O
+
MS (m/z): C68
H
48
N
16
O
12Zn [1346.54]: M 1346.87. UV–Vis (THF)
): 361 (4.79), 649 (4.24), 715 (4.70). FT-IR (KBr)
cm : 3361 (OAH), 2932, 2858 (Aliph. CAH), 1703 (C@O), 1598
C@C), 1494 (N@N), 1559, 1460, 1364, 1282, 1228, 1095, 838,
kmax, nm (log
e
max
m ,
ꢀ1
The template self-cyclotetramerization of the azo-coupled pht-
halonitriles 2, 3, 4 and 6 in the high-boiling solvent n-pentanol
under a nitrogen atmosphere afforded the four azo-coupled zinc
phthalocyanines 7, 8, 9 and 11, respectively. Hydrolysis of 9 with
potassium hydroxide and acidification with hydrochloric acid gave
(
7
1
6
70, 747. H NMR (DMSO-d ) (d: ppm): 9.49 (4H, s, OH), 8.62
(
(
4H, s, ArAH), 8.14 (4H, d, ArAH), 7.17 (4H, d, ArAH), 7.83–7.91
12H, m, Pc ring), 4.27 (2H, m, Aliph. CAH), 1.27 (3H, m, Aliph.
CAH).
1
0 [30]. The basicity of DBU was so sufficient that ester exchange
between ethyl ester and n-butanol inevitably occurred [9]. There-
fore, compound 9 was actually a mixture of the ethyl and n-butyl
esters. However, no further subtle purification, such as chromatog-
raphy, towards 9 was needed before further hydrolysis. The purifi-
cation of the four metal phthalocyanines was carried out by
Soxhlet extraction. All the finely washed azo-coupled phthalocya-
nines were fine black powders. Compounds 7, 8, 10 and 11 were
slightly soluble in common organic solvents, such as alcohol and
ethyl acetate, but exhibited good solubility in Py (pyridine), THF,
DMF and DMSO.
2
.3.10. Tetrakis-(2-hydroxy-5-carboxy-phenylazo)phthalocyaninato
zinc(II) (10)
A mixture of potassium hydroxide solution (10 ml, 5 mol/L),
THF (10 ml) and 9 (0.3 mmol, 0.40 g) was stirred at 75 °C for 6 h.
The black green solution was filtered and re-precipitated with
dilute hydrochloric acid. The black solid after filtration was sub-
jected to Soxhlet extraction using an acetone-methanol (volume
ratio 1:1) mixture for 24 h to afford pure 10. Yield: 0.24 g (68%).
M.p.: >300 °C. Anal. Calc. for C60
N, 18.16. Found: C, 58.31; H, 2.56; N, 18.13%. MALDI-TOF MS (m/
z): C60
nm (log
cm : 3435, 3248 (OAH), 3039 (ArAH), 1720 (C@O), 1587 (C@C),
1
NMR (DMSO-d ) d, ppm: 12.73 (4H, s, COOH), 9.48 (4H, s, OH),
8
32 16
H N O12Zn: C, 58.37; H, 2.59;
The structures of the target compounds were characterized by
elemental analysis together with spectral data (FT-IR, 1H NMR,
mass and UV–Vis). The characterization data of the new com-
pounds were consistent with the assigned formula.
+
H
32
N
16
O
12Zn [1234.40]: M 1234.69. UV–Vis (THF) kmax
,
,
e
): 362 (4.82), 647 (4.37), 716 (4.78). FT-IR (KBr) m
max
ꢀ1
1
According to FT-IR spectroscopy data, the formation of com-
494 (N@N), 1646, 1440, 1381, 1296, 1185, 1135, 846, 761.
H
pound 1 was clearly confirmed by the disappearance of the NO
2
6
ꢀ
1
bands at 1540 and 1360 cm , and the appearance of NH
2
bands
.63 (4H, s, ArAH), 8.16 (4H, d, ArAH), 7.14 (4H, d, ArAH), 7.84–
.92 (12H, m, Pc ring).
ꢀ1
at 3446 and 3357 cm . For the four azo-coupled phthalonitriles
, 3, 4 and 6, the characteristic vibrations corresponding to C„N
7
2
ꢀ1
were at 2242, 2226, 2236 and 2231 cm , respectively. The N@N
vibrations for 2–4 and 6 were observed at 1460, 1482, 1446 and
1508 cm , respectively. The IR spectra of compounds 2 and 3
2.3.11. Tetrakis-[2-hydroxy-3-methoxy-4-(4-carboxy-N-phenylmeth-
ꢀ1
animino)phenylazo]phthalocyani-nato zinc(II) (11)
ꢀ1
The synthesis and purification was as outlined for 7, except 6
showed sharp OH bands at 3339 and 3259 cm , while compounds
ꢀ1
(
2.21 g, 5 mmol) was employed instead of 2. The same molar ratio
of the reagents given in section 2.3.7 was followed. Yield: 0.54 g
24%). M.p.: >300 °C. Anal. Calc. for C92 16Zn: C, 62.53; H,
.39; N, 15.86. Found: 62.49; H, 3.36; N, 15.89%. MALDI-TOF MS
4 and 6 showed broad OH bands at 3221 and 3374 cm due to
stronger intramolecular hydrogen bonding between the OH group
and nitrogen atom in the azo groups. IR spectra of compounds 5
(
3
60 20
H N O
ꢀ1
and 6 showed aromatic CH@N peaks at 1593 and 1599 cm . A
+
ꢀ1
(m/z): C92
H
60
N
20
O
16Zn [1766.99]: M 1767.21. UV–Vis (THF) kmax
,
characteristic substituted naphthalene peak at about 758 cm
ꢀ
1
nm (log
e
): 664 (4.46), 729 (4.86). FT-IR (KBr)
mmax, cm : 3355,
for the azo-coupled phthalonitrile compound 3 was observed as
well [15].
3
1
1
220 (OAH), 2932, 2858 (Aliph. CAH), 1700 (C@O), 1598 (C@N),
488 (N@N), 1205, 1050 (CAOAC), 1649, 1559, 1542, 1457, 1140,
098, 838, 787, 747. 1H NMR (DMSO-d
) d, ppm: 12.85 (4H, s,
For the 1H NMR spectra of compounds 2–6, aromatic protons
appeared at 6.91–8.41, 6.89–8.42, 7.64–8.39, 6.65–8.13 and 7.16–
8.44 ppm, respectively. The hydroxyl protons of compounds 2–6
showed singlet peaks at 10.41, 10.27, 9.44, 9.57 and 10.24 ppm,
6
COOH), 10.24 (4H, s, OH), 8.09 (8H, d, ArAH), 7.49 (8H, d, ArAH),
8
8
.43 (4H, s, CH@N), 7.38 (4H, s, ArAH), 7.68 (4H, s, ArAH), 7.98–
.16 (12H, m, Pc ring), 3.83-3.95 (12H, s, Aliph. CAH).

8
68
M. Han et al. / Polyhedron 85 (2015) 864–873
Scheme 1. Synthetic routes for the four azo-coupled phthalonitriles (2, 3, 4 and 6) and their corresponding zinc phthalocyanines (7, 8, 9, 10 and 11).
respectively. The presence of hydrogen bonds between the OH
(C@C) as expected. The 1H NMR spectra of the four substituted
azo-phthalocyanines were almost identical with those of the start-
ing compounds 2, 3, 4 and 6 except for broadening and small shift
due to the existence of statistical isomers and possible aggregation
at the measured concentration [32]. For compounds 9 and 11, the
integral ratio of the aromatic protons to aliphatic protons was sim-
group and the nitrogen atom in azo groups led to resonances of
1
hydroxyl protons occurring in the range 9–11 ppm [13]. The
H
NMR spectra of compounds 5 and 6 showed resonances of protons
belonging to the Schiff base groups at 8.35 and 8.41 ppm. In addi-
tion, elemental analysis results of compounds 2–6 were good
agreement with the proposed structures.
ilar to the corresponding phthalonitrile compounds. For the 1
H
After cyclotetramerization, the sharp peaks for the C„N vibra-
tions completely disappeared in the FT-IR spectra [31]. The IR spec-
tra of all the azo-coupled phthalocyanines featured Pc ring
NMR spectrum of compound 10, the resonance belonging to the
carboxyl protons showed a singlet peaks at 12.73 ppm, clearly con-
firming the hydrolysis of the ester groups. The disappearance of the
resonances of the aliphatic protons of 9 also confirmed the hydro-
ꢀ1
ꢀ1
vibration peaks at around 1600 cm
(CH@N) and 1500 cm

M. Han et al. / Polyhedron 85 (2015) 864–873
869
lysis. In the mass spectra of the five azo-coupled phthalocyanines,
the molecular ion peaks, which were observed at m/z: 1058.62
+
+
+
+
[
M ] for 7, 1258.41 [M ] for 8, 1346.87 [M ] for 9, 1234.69 [M ]
+
for 10 and 1767.21 [M ] for 11, supported the desired structures.
In addition, the elemental analysis was satisfactory.
3.2. Absorption properties
The electronic absorption spectra of the phthalonitrile precur-
sors 2, 3, 4 and 6 were recorded in THF solution and the azo-cou-
pled zinc phthalocyanines 7, 8, 10 and 11 were tested both in
2
THF solution and adsorbed on anatase TiO thin films. Figs. 1–4
show the absorption spectra of the unsubstituted zinc phthalocya-
t
nine (Zn Pc), the phthalonitrile precursors 2, 3, 4 and 6 (both as ref-
t
ꢀ5
erence compounds) and the azo-coupled phthalocyanines. The UV–
Vis spectra of the azo-coupled phthalocyanines showed the charac-
teristic Q-bands. The Q-band absorptions of the azo-coupled zinc
Fig. 2. UV–Vis absorption spectra of 8, Zn Pc and 3 in THF at 1 ꢁ 10 M and 8
adsorbed on a 6 m thick TiO film.
l
2
phthalocyanines 7, 8, 10 and 11 were observed as single high
⁄
intensity bands due to
p
?
p
transitions at 718, 740, 716 and
7
29 nm, respectively. For compounds 8 and 11, the B-band, usually
located at 350 nm for the normal Pcs, were replaced by strong and
broad new absorption bands appearing at approximately 300–
6
00 nm, however for compounds 7 and 10, the B-band were clear
and located at 376 and 362 nm, respectively. The above phenom-
ena are closely related to increased expansion of the -conjugation
p
of the dyes due to the introduced azo moieties, and this could also
be used to illustrate the red shift of the Q-bands of four azo-cou-
pled phthalocyanines. The absorptions in the spectral window
between 400 and 600 nm of the four azo-coupled phthalocyanines
were remarkable, and these were caused by efficient electron
⁄
transfer from the azobenzene
p
orbital to the Pc S
1
level [30].
Additional absorptions of this kind are scarce and would make
Pcs acquire novel opto-electronic properties and collect panchro-
matic photoelectrons efficiently. It is clear that the absorption
characteristics of both azobenzene and Pc moieties are displayed
when conjugated into one molecule.
t
ꢀ5
Fig. 3. UV–Vis absorption spectra of 10, Zn Pc and 4 in THF at 1 ꢁ 10 M and 10
adsorbed on a 6 m thick TiO film.
l
2
The absorption spectra of the azo-coupled zinc phthalocyanines
2
7, 8, 10 and 11 adsorbed onto 6 lm thick TiO films were similar to
those of the solution spectra, but exhibited a small red shift and the
bands were much broader. This may be due to the presence of
2
anchoring group protons, which on adsorption on TiO release
the proton and bind to Ti4 . The Ti acts as electron withdrawing
and produces a red shift in the absorption bands [33,34]. In addi-
tion, light scattering and re-absorption from the nanoparticle sur-
+
4+
2
face of TiO may also lead to the spectra significantly broadening.
The large coverage of the solar spectrum from UV to near IR region
renders these phthalocyanines ideal light-harvesting materials in
organic solar cells.
t
ꢀ5
Fig. 4. UV–Vis absorption spectra of 11, Zn Pc and 6 in THF at 1 ꢁ 10 M and 11
adsorbed on a 6 m thick TiO film.
l
2
opt
The optical energy band gaps (Eg ) of 7, 8, 10 and 11 were esti-
mated according to the following equation [35]:
hc
¼ k
1240
opt
g
E
¼
k
max
max
where kmax is the absorption maximum.
opt
The UV–Vis data and E energies estimated from the absorp-
g
t
tion maxima of Zn Pc, 7, 8, 10 and 11 are shown in Table 1. The
decrease of the energy gaps between the HOMO and LUMO com-
pared to Zn Pc results in the red shift of the absorption bands of
the dyes.
t
t
ꢀ5
Fig. 1. UV–Vis absorption spectra of 7, Zn Pc and 2 in THF at 1 ꢁ 10 M and 7
adsorbed on a 6 m thick TiO film.
l
2

8
70
M. Han et al. / Polyhedron 85 (2015) 864–873
Table 1
The lowest unoccupied molecular orbital (LUMO) of the dye is
closely related with the electron injection efficiency of the TiO
opt
t
The UV–Vis data in THF and Eg energy values of Zn Pc and azo-coupled phthalo-
cyanines 7, 8, 10 and 11.
2
surface. Additionally, the highest occupied molecular orbital
(HOMO) of the sensitizer should lie below the energy level of the
max, nm (loge)a
opt
g
(eV)b
Comp.
k
E
ꢀ
ꢀ
B band
Q band
I /I redox couple to achieve efficient regeneration of the oxidized
3
t
dyes formed after electron injection [3]. The HOMO–LUMO levels
of the azo-coupled zinc phthalocyanines 7, 8, 10 and 11 were esti-
mated through the equation reported by de Leeuw et al. [36]:
Zn Pc
7
8
344 (4.42)
376 (4.78)
–
362 (4.82)
–
608 (4.19), 673 (4.92)
649 (4.24), 718 (4.72)
740 (4.77)
647 (4.37), 716 (4.78)
664 (4.46), 729 (4.86)
1.84
1.72
1.67
1.73
1.70
1
1
0
1
ꢀ
ꢁ
ꢁ
Oxy
E
HOMO ¼ ꢀ E
þ 4:4 eV
onset!SCE
a
The solvent is THF.
The optical energy gaps from the UV–Vis absorption spectra.
b
ꢀ
Red
E
LUMO ¼ ꢀ E
þ 4:4 eV
onset!SCE
3
.3. Electrochemical properties
Oxy
onset!SCE
Red
onset!SCE
E
and E
reduction potentials.
indicate the onset values of the oxidation and
Cyclic voltammetry experiments were carried out at room tem-
perature under nitrogen against a saturated calomel electrode
SCE) in THF containing 0.1 M tetrabutylammonium perchlorate
TBPA) on a Pt working electrode to study the electrochemical
behaviors of the four azo-coupled zinc phthalocyanines. The cyclic
voltammograms of the four compounds are illustrated in Fig. 5.
Compounds 7, 8, 10 and 11 showed similar single reversible reduc-
tion peaks attributed to the reduction of the four azobenzene moi-
eties at ꢀ0.98, ꢀ1.01, ꢀ1.05 and ꢀ1.13 V, respectively. At the same
time, the CV scans of 7, 8 and 10 showed similar quasi-reversible
oxidation waves at 0.67, 0.52 and 0.54 V, and 11 showed an irre-
versible oxidation wave at 0.55 V. This was ascribed to the oxida-
tion of the Pc rings [30].
The LUMO energy levels of compounds 7, 8, 10 and 11 were
found to be ꢀ3.0, ꢀ2.97, ꢀ2.95 and ꢀ2.94 eV, respectively. The
HOMO energy levels of compounds 7, 8, 10 and 11 were found to
be ꢀ4.75, ꢀ4.65, ꢀ4.72 and ꢀ4.73 eV, respectively. Band gap val-
ues from the electrochemically determined data were 1.75, 1.68,
(
(
1
.77 and 1.79 eV, respectively. All the data are listed in Table 2.
Compared with the energy gap calculated from the UV–Vis absorp-
tion spectra, the band gap values read from the CV graphs are lar-
ger. This is probably due to the interface dipole effect on modifying
the energy barriers between the electrodes and the organic mate-
rials [37]. However, the energy gap values read from the CV graphs
on the whole correlate well with the calculated values from the
absorption spectra.
Fig. 5. Cyclic voltammograms of azo-coupled phthalocyanines 7 (a), 8 (b), 10 (c) and 11 (d). Medium = THF containing 0.1 M TBPA. Scan rate = 100 mV sꢀ1
.

M. Han et al. / Polyhedron 85 (2015) 864–873
871
We took the previously reported zinc phthalocyanine sensitizer
TT1 [7] as a reference for assessing the LUMO levels of the sensitiz-
ers 7, 8, 10 and 11. TT1 showed 80% IPCE (incident photon-to-cur-
the N@N bond and breakage of the bonds between Zn and the Pc
ring [40,41]. It was seen that 10 and 11 had the lowest major
decomposition temperature among the four azo-coupled phthalo-
cyanines. This might be due to the existence of carboxyl groups
and Schiff base groups in the structures of 10 and 11. From the
TGA curves, we also found that the removal of carboxyl groups
and decomposition the Schiff base structures led to the weight loss
onset for 10 and 11 at 300 °C, compared to higher onset tempera-
tures for 7 and 8. Generally, the thermal data imply that 7 and 8
have better stability and also agree with the structures of the four
azo-coupled phthalocyanines.
rent conversion efficiency) at 690 nm and a record
g value of 3.5%.
The LUMO level of TT1 was ca. ꢀ3.17 eV [38], which is relatively
low, a comparative shortcoming for the further improvement of
g
values of phthalocyanine-sensitized solar cells. As observed in
Table 2, the four azo-coupled phthalocyanines have higher LUMO
energy levels than TT1, indicating that the four compounds could
be promising for providing higher electron injection efficiencies
and
redox potential of the electrolyte, we choose the reported dye N3
39] as a reference to assess the HOMO level of the four azo-cou-
g values. Also, as it was difficult to estimate the theoretical
[
3.5. Photovoltaic performances
pled phthalocyanines. N3 is a famous ruthenium-polypyridyl sen-
sitizer with a HOMO energy level of ca. ꢀ5.08 eV [38]. A sensitizer
with a HOMO level close to or higher than that of N3 might be
promising for regeneration since N3 can be regenerated well. The
HOMO levels of the four azo-coupled phthalocyanines were higher
than N3 so that the regeneration of four compounds by iodide is
thermodynamically favorable.
The performances of DSSCs are characterized primarily by the
open-circuit voltage (Voc), short-circuit photocurrent density (Jsc),
photocurrent/voltage curve (J/V curve), incident photon-to-current
conversion efficiency (IPCE), fill factor (ff) and solar energy-to-elec-
tricity conversion yield (
absorption properties of the dye, the amount of adsorbed dyes on
the TiO surface, the amount of electron injection from the photo-
excited dye to the CB of the TiO and the efficiency of the collection
g) [3]. The IPCE is directly related to the
2
2
3.4. Thermal properties
of electrons in the external circuit, and it can be represented by the
following equation:
The thermal properties of 7, 8, 10 and 11 were tested by using
TGA under a nitrogen atmosphere. For roof top applications of dye-
sensitized solar cells, the thermal stability of organic dyes
ꢀ
2
1240ðeV nmÞJphðmA cm
Þ
IPCE ð%Þ ¼
ð1Þ
ꢀ2
kðnmÞIðmW cm
Þ
2
adsorbed on nanocrystalline TiO is very essential.
The thermogravimetric curves for the four azo-coupled phthal-
ocyanines are shown in Fig. 6. From the curves, minor decomposi-
tion reactions occurred at 295, 240, 270 and 230 °C for compounds
Jph is the short-circuit photocurrent density generated by mono-
chromatic light, k and I are the wavelength and the intensity of
the monochromatic light, respectively.
7
, 8, 10 and 11, respectively, with a weight loss of 22%, 3%, 10% and
The fill factor is defined as the maximum power output of the
solar cell per unit area divided by the product of Jsc and Voc. The fill
factor can be represented by the following equation:
1
1%. This stage corresponded to the partial degradation of the aro-
matic substituents introduced onto the Pc ring. Major decomposi-
tion reactions occurred at 400, 450, 390 and 370 °C for 7, 8, 10 and
P
max
1
1 and the corresponding weight losses were 27%, 30%, 42% and
ff ¼
ð2Þ
J Voc
sc
3
3%, respectively. This stage corresponded to the degradation of
The solar energy-to-electricity conversion yield
g
is defined as
the ratio of the maximum output electrical power of the DSSC to
Table 2
the energy of the incident sunlight (I
0
). The
g
value can be calcu-
Electrochemical properties of the azo-coupled phthalocyanines 7, 8, 10 and 11.
lated from the following equation:
Comp.
E
ox/Ered (V)a
LUMO (eV)
HOMO (eV)
E
g
(eV)b
ꢀ
2
J ðmA cm ÞVocðVÞff
7
8
0.35/ꢀ1.40
0.25/ꢀ1.43
0.32/ꢀ1.45
0.33/ꢀ1.46
ꢀ3.00
ꢀ2.97
ꢀ2.95
ꢀ2.94
ꢀ4.75
ꢀ4.65
ꢀ4.72
ꢀ4.73
1.75
1.65
1.77
1.79
sc
g
ð%Þ ¼
ð3Þ
ꢀ
2
I
0
ðmW cm Þ
1
1
0
1
To judge the potential of the newly synthesized azo-coupled
zinc phthalocyanines 7, 8, 10 and 11 as sensitizers for DSSC, P25
a
Onset voltages from CV curves.
Energy gaps from the electrochemically determined values.
b
TiO
TiO
films were used to evaluate their photovoltaic performances.
electrodes (6 lm thick) were sensitized with the four azo-cou-
2
2
pled phthalocyanines. The photovoltaic characterization data for
sensitizer dyes 7, 8, 10 and 11 are summarized in Table 3. The pho-
tocurrent–voltage curves (JꢀV curves) of the four azo-coupled
2
phthalocyanines sensitized TiO cells are depicted in Fig. 7. We have
observed that the power conversion efficiencies (g) for compounds
Table 3
a
Photovoltaic characteristics of DSSCs with the azo-coupled phthalocyanine dyes 7, 8,
10 and 11 under one-sun conditions and values of the IPCE.
sc (mA cm 2
ꢀ
)
V
oc (V)
ff
g
(%)
IPCEmax (%)
Comp.
J
7
8
0
1
3.5
6.6
4.4
5.2
0.54
0.59
0.56
0.57
0.74
0.69
0.72
0.70
1.4
2.7
1.8
2.1
41
49
45
46
1
1
a
ꢀ4
The TiO
2
electrode was sensitized with 1 ꢁ 10 M sensitizer. The electrolyte
ꢀ
1
was 0.05 M iodine and 0.5 M LiI in acetonitrile. The thickness and area of TiO were
6 lm and 0.68 cm , respectively.
Fig. 6. TG curves of compounds 7, 8, 10 and 11 with a heating rate of 10 °C min
2
2
under nitrogen.

8
72
M. Han et al. / Polyhedron 85 (2015) 864–873
porated two spectrally complementary photogenerators (azo dyes
and Pcs) by N@N double bonds in single structures. Compared with
the phthalocyanines where azo moieties were introduced through
oxygen bridges, the four compounds showed impressive absorp-
tions and photocurrent responses in the visible and near-IR regions
as a result of lowering of energy gaps. Electrochemical studies were
made to investigate the HOMO and LUMO levels. The LUMO levels
of the four sensitizers were high enough to achieve efficient elec-
tron injection from the excited dyes to the conduction band of
2
TiO . In addition, the HOMO levels of the four dyes could ensure
efficient regeneration of the oxidized dyes. The four azo-coupled
phthalocyanines were tested in DSSC. The low efficiency of DSSC
using the four azo-coupled phthalocyanines is probably due to
aggregation of the azo-coupled phthalocyanine molecules on the
2
TiO surface and the symmetrical structures of the four dyes, which
are not good enough for intramolecular charge transfer and direc-
tional electron injection. Thermal stability studies showed that the
four sensitizers were stable above 200 °C.
Fig. 7. Photocurrent–voltage curves of the azo-coupled phthalocyanine dyes 7, 8,
0 and 11.
1
Acknowledgments
The authors would like to acknowledge financial support from
the National Nature Science Foundation of China (Grant number:
5
1272239) and the Shanxi Natural Science Foundation (Grant
number: 2011011022-4).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2014.10.026.
These data include MOL files and InChiKeys of the most important
compounds described in this article.
Fig. 8. IPCEs of dye-sensitized solar cells with the four different azo-coupled
phthalocyanine dyes.
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