Novel planar binuclear zinc phthalocyanine sensitizer for dye-sensitized solar cells: Synthesis and spectral, electrochemical, and photovoltaic properties

Article abstract of DOI:10.1016/j.molstruc.2014.09.037

A planar binuclear zinc phthalocyanine was newly synthesized for use in dye-sensitized solar cells, based on Schiff base and asymmetric amino zinc phthalocyanine. The novel compounds were characterized using FTIR, UV-Vis, ¹H NMR, cyclic voltamm

Full text of DOI:10.1016/j.molstruc.2014.09.037

Journal of Molecular Structure 1079 (2015) 61–66
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Novel planar binuclear zinc phthalocyanine sensitizer for dye-sensitized
solar cells: Synthesis and spectral, electrochemical, and photovoltaic
properties
⇑
Baiqing Zhu, Xuejun Zhang , Mingliang Han, Pengfei Deng, Qiaoling Li
Department of Chemistry, School of Science, North University of China, Taiyuan, Shanxi 030051, China
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
ꢀ
ꢀ
ꢀ
APC and bi-NPC were designed for use
in dyes-sensitized solar cells.
Bi-NPC was synthesized based on
Schiff base.
Schiff base could form coplanar
condition between mononuclear
phthalocyanines.
ꢀ
Bi-NPC shows the higher absorbance,
the better red shift and performance.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2014
Received in revised form 1 September 2014
Accepted 11 September 2014
Available online 19 September 2014
A planar binuclear zinc phthalocyanine was newly synthesized for use in dye-sensitized solar cells, based
on Schiff base and asymmetric amino zinc phthalocyanine. The novel compounds were characterized
using FTIR, UV–Vis, 1H NMR, cyclic voltammetry and elemental analysis. From the reduction and oxida-
tion behavior, it is proved that APC and bi-NPC have negative LUMO levels and positive HOMO levels, sat-
isfying the energy gap rule, and can be employed as sensitizers for dye-sensitized solar cells (DSSCs)
applications.
Keywords:
Ó 2014 Elsevier B.V. All rights reserved.
Planar binuclear
Dye-sensitized solar cells
Schiff base
Electrochemistry
Introduction
transportation are fulfilled by the nanocrystalline metal oxide and
electrolyte. As a kind of functional material, the dye should be sol-
As one of the most promising methods for future low cost
power production from renewable energy sources, DSSCs have
the outstanding advantage of high light-to-electricity conversion
efficiencies and easy fabrication [1]. The absorption properties of
dyes dictate the light-harvesting capacity of DSSCs, since the light
absorption function is fulfilled by the dye and the electron and hole
uble in solvents that are compatible with the TiO2 and favorable for
absorption of a nonaggregated monolayer on the surface [2].
Phthalocyanine is a special dye pigment consisting of 18-
p elec-
tronics and has a similarity with biological molecules chlorophyll
and hemoglobin [3,4]. Strong Q band light absorption properties
at around 700 nm and excellent chemical stability make phthalo-
cyanine become a better choice as sensitizer using in DSSCs. In
addition, as the characteristic of color is bright and structure could
be modified, phthalocyanine is widely used in different areas, such
⇑
Corresponding author. Tel./fax: +86 3513922152.
E-mail address: zhangxuejun@nuc.edu.cn (X. Zhang).
http://dx.doi.org/10.1016/j.molstruc.2014.09.037
0022-2860/Ó 2014 Elsevier B.V. All rights reserved.

62
B. Zhu et al. / Journal of Molecular Structure 1079 (2015) 61–66
as photographic printing [5], photovoltaic cells [6–8], liquid crys-
tals [9,10], photodynamic therapy [11–14], light-emitting diodes
[15,16] and chemical sensors [17,18]. Phthalocyanines have vari-
chromic. Compared with general method of ordinary mononuclear
phthalocyanine by changing substituent to enhance conjugate sys-
tem, C@N bond of Schiff base connecting two mononuclear phthal-
ocyanines could extend conjugation between the two mononuclear
unsymmetrical phthalocyanines. To a certain extent, it integrates
and arranges the phthalocyanine molecules. B-naphthyloxy could
increase the bi-NPC solubility in DMF, on the other hand reduce
accumulation between the phthalocyanine molecules based on
the space steric hindrance. The synthesis and spectral, electrochem-
ical and photovoltaic properties of these phthalocyanines based
sensitizers are investigated.
ous central metals and extended
p-conjugation system and have
been tested several decades as sensitizers of wide-band gap oxide
semiconductors, as increasingly high incident photon-to-electric
current conversion yields are obtained [19–23]. However, poor sol-
ubility of macrocycle in organic solvent, strong tendency to aggre-
gation on the film surface, lack directionality in the excited state
and easy electron recombination between injected electron in
TiO2 conduction band and oxidized dye hinder the incident
photon-to-electric current conversion yields [2].
This paper concerns phthalocyanines in which polar were
attached to the periphery together with an additional group to
enhance solubility in organic solvents and Schiff base connect two
asymmetric mononuclear phthalocyanines. A new planar binuclear
zinc phthalocyanine (9(10), 90(100), 16(17), 160(170), 23(24),
230(240)-(b-naphthyloxy)-2(3), 20(30)-(1,4-methanamide phenyl)
zinc phthalocyanine (bi-NPC)), based on Schiff base and extended
Experimental
4-Nitro-phthalonitrile, 1,4-phthalaladehyde, b-naphthol, 1,8-
diazabicyclo[5,4,0]-undec-7-ene (DBU), 1-pentanol, N,N-Dimethyl-
formamide (DMF), tetrahydrofuran (THF), Na2Sꢁ9H2O, CHCl3, HAC,
anhydrous K2CO3, anhydrous MgSO4, anhydrous SiO2 were pur-
chased commercially. DMF, THF and CHCl3 were dried and distilled
by accustomed methods before use. All other solvents and chemi-
cals used in this work were analytical grade and used without fur-
ther purification. Column chromatography was performed on silica
gel (200–300).
p-conjugation concept connecting two mononuclear unsymmetri-
cal phthalocyanines, and asymmetric amino zinc phthalocyanine
(9(10), 16(17), 23(24)-(b-naphthyloxy)-2(3)-(amido) Zinc Phthalo-
cyanine (APC)) have been designed and synthesized for the dye-
sensitized solar cells (Fig. 1). APC has three b-naphthyloxy groups
to enhance the solubility of phthalocyanine in common organic
solvent, reduced the aggregation and turned the LUMO level of
phthalocyanine. Bi-NPC is made of two unsymmetrical amino zinc
phthalocyanines connected on Schiff base reaction. Formyl groups
of 1, 4-phthalaldehyde and the amino of asymmetric mononuclear
phthalocyanines form the Schiff base. Schiff base could provide a
coplanar condition between two mononuclear phthalocyanines
[24]. As reagents in organic synthesis and liquid crystal materials,
Schiff base compounds and their metal complexes containing imine
or methylene amine groups (ARC@NA) have important application
in medicine, catalysis, analytical chemistry, corrosion and photo
Synthesis
4-(b-Naphthyloxy) phthalonitrile (1)
Phthalonitrile derivative was prepared by similar method
reported in the literature [25,26]. 4-Nitro-phthalonitrile (1.732 g,
0.01 mol) and b-naphthol (1.442 g, 0.01 mol) were added succes-
sively with stirring to dry DMF (30 ml). After dissolution, anhy-
drous K2CO3 (2 g, 0.014 mol) was added and the reaction mixture
was stirred at 60 °C. Further anhydrous K2CO3 (1 g, 7 mmol) was
added portion-wise after 2 h. Stirring vigorously for 28 h under
nitrogen. Then the reaction mass was poured into 200 ml of cold
Fig. 1. The molecular formula of APC and bi-NPC.

B. Zhu et al. / Journal of Molecular Structure 1079 (2015) 61–66
63
water and stirred for 15 min. The precipitate was filtered, washed
several times with cold water until the filtrate became neutral, and
crystallized from EtOH–water to give the product as a pink, crystal-
line powder. Yield: 1.497 g, 55.4%, m.p. 127–128 °C. Infrared spec-
spectra of APC and bi-NPC were recorded with a Techcomp 2300
spectrophotometer. 1H NMR spectra were recorded with a Varian
Inova 400 MHz NMR system. Cyclic voltammetry measurements
were performed on an electrochemical workstation (CHILK2005A,
Tianjin Lanlike Chemistry electronic high-tech Co., Ltd., PR China).
Cyclic voltammetry experiments were performed on 1 ꢃ 10ꢂ4 mol
phthalocyanine dye solution in DMF at scan rate 100 mV/s using
tetrabutylammonium perchlorate (0.1 mol/L) as supporting elec-
trolyte. Cyclic voltammetry with a platinum wire was used as
working electrode, the platinum as the counter film electrode,
the calomel as the reference. Purge the solution with nitrogen for
1 h before the experiment.
trophotometer (IR) (KBr),
t
(cmꢂ1): 3086 (HAAr), 2236 (AC„N),
1258 (ArAOAAr), 1589, 1572 (Ar C@C). Nuclear magnetic reso-
nance (1H NMR) (DMSO-d6) d, ppm: 7.66–6.98 (m, 10H, Ar-H). Ele-
mental analysis (Anal. Calc.) for C18H10N2O: C 79.99, H 3.73, N
10.36, O 5,92; Found C 80.11, H 3.83, N 10.17, O 5.84.
9(10), 16(17), 23(24)-(b-Naphthyloxy)-2(3)-(nitryl) zinc
phthalocyanine (NPC) (2)
4-Nitro-phthalonitrile (0.173 g, 1 mmol), 4-(b-naphthyloxy)
phthalonitrile (0.811 g, 3 mmol), Zn(CH3COO)2 (0.22 g, 1 mmol)
and a catalytic amount of DBU in dry 1-pentanol (30 ml) were
heated at 160 °C with stirring under nitrogen for 24 h. After cooling
to room temperature, the reaction mixture was precipitated by
adding methanol. The product was separated by filtration as a blue
solid which was washed several times with methanol and ethanol
to remove any unreacted precursor and then dried in vacuo. The
solid material was subjected to silica gel column chromatography
and eluted with THF: benzene = 1:10 (v/v), and bluish color band
was collected. The solvent was removed under reduced pressure
to get the desired product. Yield: 0.424 g, 40.5%, m.p. > 200 °C. IR
Preparation dye-sensitized nanocrystalline TiO2 thin films
TiO2 photoelectrode (area: ca. 0.8 cm ꢃ 1.2 cm) was prepared
by a similar method reported in the literature [27,28,31]. Nano-
crystalline TiO2 films of 6 lm thickness were deposited onto
transparent conducting glass (which has been coated with a
fluorine-doped stannic oxide layer, thickness of 4 mm, sheet
resistance of 18–20
X). These films were dried at 150 °C for
20 min and then were gradually sintered at 500 °C for 20 min.
The sensitizer was dissolved in ethanol at a concentration of
1 ꢃ 10ꢂ4 mol/L. The photoelectrode was dipped into the dye
solution immediately after the high-temperature annealing and it
was still hot (80 °C) and then kept at room temperature for 24 h
so that the dye was adsorbed onto the TiO2 films. After completion
of the dye adsorption, the photoelectrode was withdrawn from
the solution and washed thoroughly with ethanol to remove
non-adsorbed dye under a stream of dry air or nitrogen.
(KBr),
t
(cmꢂ1): 3068 (HAAr), 1535, 1343 (ArANO2), 1603, 1578,
1456 (C@C), 1232, 1098, 757. 1H NMR (DMSO-d6) d, ppm: 8.64–
6.95 (m, 33H, Ar-H). Anal. Calc. for C62H33N9O5Zn: C 70.99, H
3.15, N 12.02, O 7.63; found C 70.98, H 3.14, N 12.03, O 7.59.
9(10), 16(17), 23(24)-(b-Naphthyloxy)-2(3)-(amido) Zinc
Phthalocyanine (APC) (3)
9(10), 16(17), 23(24)-(b-Naphthyloxy)-2(3)-(nitryl) Zinc Phtha-
locyanine (NPC) (0.314 g, 0.3 mmol) and Na2Sꢁ9H2O (0.216 g,
0.9 mmol) were dissolved in dry THF (30 ml). The reaction mixture
was heated at 66 °C with stirring under nitrogen for 24 h. After
cooling to room temperature, the product was separated by filtra-
tion as a blue liquid. The solvent was removed under reduced pres-
sure to get the desired product. The solid material was subjected to
Results and discussion
Design and synthesis
The synthesis of APC and bi-NPC were illustrated in the
Scheme 1. 4-(b-Naphthyloxy) phthalonitrile
through the displacement reaction of 4-nitro-phthalonitrile 1 with
b-naphthol 2 in DMF solution in a moderate yield. NPC was
prepared through 4-(b-naphthyloxy) phthalonitrile
4-nitro-phthalonitrile 1 in 1-pentanol solution in the presence of
DBU and metal salts. APC was prepared through NPC with
Na2Sꢁ9H2O 4 in THF solution. Bi-NPC was prepared through APC with
1,4-phthalaldehyde 5 in THF solution and the presence of HAC.
3 was prepared
chloroform column. Yield: 0.273 g, 89.4%, m.p. > 200 °C. IR (KBr),
m
(cmꢂ1): 3345, 1580 (ArANH2). 1H NMR (DMSO-d6) d, ppm: 7.62–
6.87 (m, 33H, ArAH), 3.43 (s, 2H, NAH). Anal. Calc. for C62H35N9O3-
Zn: C 73.08, H 3.44, N 12.38, O 4.71; found C 73.05, H 3.41, N 12.25,
O 4.69.
3
with
9(10), 90(100), 16(17), 160(170), 23(24), 230(240)-(b-Naphthyloxy)-2(3),
20(30)-(1,4-methanamide phenyl) zinc phthalocyanine (bi-NPC) (4)
9(10), 16(17), 23(24)-(b-Naphthyloxy)-2(3)-(amido) Zinc
Phthalocyanine (APC) (0.204 g, 0.2 mmol), 1,4-phthalaladehyde
(0.013 g, 0.1 mmol) and anhydrous MgSO4 (0.012 g, 0.1 mmol)
were dissolved in dry THF (30 ml). Then added a catalytic amount
of HAC and the reaction mixture was heated at 66 °C with stirring
under nitrogen for 24 h. After cooling to room temperature, the
product was separated by filtration as a blue liquid. The solvent
was removed under reduced pressure to get the desired product.
The solid material was subjected to Al2O3 gel column chromatogra-
phy and eluted with THF: ethyl acetate = 6:1 (v/v). Then the prod-
uct was washed several times with EtOH. Yield: 0.166 g, 77.8%,
UV–Vis measurements
Phthalocyanine has two characteristics of the absorption bands
in UV–Vis area: B band and Q band. The B band (Soret band) were
observed base on the transitions from the deeper
LUMO. The Q band observed was attributed to
p
–
levels to the
p
p* transitions
from the highest occupied molecular orbital (HOMO) to the lowest
unoccupied molecular orbital (LUMO) of the conjugated macrocy-
cle. Fig 2 shows the electronic absorption spectrum of APC and bi-
NPC in DMF solution. The absorption spectrum in solution shows
the characteristic absorptions of bi-NPC are 351 (B band) and 680
(Q band) nm and the characteristic absorptions of APC are 342 (B
band) and 672 (Q band) nm. Analysis of UV–Vis spectra shows that
no evidence of aggregation in solution as demonstrated by a sharp
unperturbed phthalocyanine Q band. Compared with APC in equiv-
alent environment, the bi-NPC shows the higher absorbance and
the better red shift. Owing to Schiff base group in bi-NPC integrated
the two mononuclear phthalocyanines, it makes the phthalocya-
nine possess superior arrangement. According to the Eq. (1), E0–0
m.p. > 200 °C. IR (KBr),
m
(cmꢂ1): 1638 (C@N). 1H NMR (DMSO-
d6) d, ppm: 8.02–6.94 (m, 70H, ArAH), 8.42 (s, 2H, CAH). Anal. Calc.
for C132H72N18O6Zn2: C 74.23, H 3.37, N 11.81, O 4.5; found C 74.18,
H 3.25, N 11.78, O 4.47.
Characterization methods
The Fourier transform IR (FTIR) spectra of all the samples were
measured using a Shimadzu 4800S spectrophotometer. The UV–Vis

64
B. Zhu et al. / Journal of Molecular Structure 1079 (2015) 61–66
Scheme 1. (1) K2CO3, DMF; (2) DBU, 1-pentanol, Zn(CH3COO)2; (3) THF, Na2Sꢁ9H2O; (4)THF, HAC.
Fig. 2. UV–Vis absorption spectra of bi-NPC (log
e
= 5.29) and APC (log
e
= 5.09) in
Fig. 3. Absorption spectral changes of APC (loge = 5.09) in DMF at different
DMF.
concentrations.
energy of APC and bi-NPC estimated from absorption maximum are
1.845 eV and 1.823 eV, respectively.
[29]. Aggregation is usually depicted as a coplanar association of
rings progressing from monomer to dimer and higher order com-
plexes and affects the shape of the Q band. It is dependent on
the concentration, nature of the solvent, nature of the substituents,
complex metal ions and temperature. In this study, the aggregation
behavior of APC (Fig. 3) and bi-NPC (Fig. 4) complexes were inves-
tigated in different concentration in DMF. As the concentration was
increased, the intensity of absorption of the Q band also increased
and there were no new bands due to the aggregated species [30]. It
1240
kmax
E0—0
¼
ð1Þ
kmax is the absorption maximum.
Phthalocyanines are known to form aggregation, which is due
to the strong coupling between the molecules that causes either
a red shift or a blue shift in the absorption band of the aggregate

B. Zhu et al. / Journal of Molecular Structure 1079 (2015) 61–66
65
transition energy, the energy levels of the LUMO is determined
respectively to be ꢂ0.88 V vs SCE and ꢂ0.99 V vs SCE. The LUMO
is calculated according to the following equation.
LUMO ¼ HOMO ¼ E0—0
ð2Þ
The energy level of the conduction band edge of TiO2 is ca.
ꢂ0.74 V vs SCE [33]. This makes electron injection from the excited
state of APC or bi-NPC into the conduction band of TiO2 thermody-
namically feasible. The HOMO levels of APC and bi-NPC are more
positive than the energy level of the redox couple Iꢂ/I3ꢂ (0.2 V vs
SCE) in the electrolyte, indicating more efficient phthalocyanines
regeneration by electron transfer from Iꢂ. Fig. 6 shows the sche-
matic energy diagram of APC and bi-NPC dye sensitized TiO2 elec-
trodes. APC and bi-NPC have negative LUMO levels and positive
HOMO levels, satisfying the energy gap rule. This means that the
APC and bi-NPC could be employed as sensitizers for DSSCs appli-
cations [30]. Bi-NPC has stronger electron injection than APC.
Fig. 4. Absorption spectral changes of bi-NPC (loge = 5.29) in DMF at different
concentrations.
Photovoltaic characterization
is seen that the Beer–Lambert law were obeyed for APC and bi-NPC
for concentrations ranging from 0.9 ꢃ 10ꢂ5 to 0.15 ꢃ 10ꢂ5 mol/L.
The I–V curve of dye-sensitized solar cells is measured in sun
outside the laboratory, and monochromatic incident photon-to-
electron conversion efficiency (IPCE) and total photoelectric con-
Electrochemical measurements
version efficiency (
g) are the main evaluation parameter of DSSCs
performance. They could be calculated following Eqs. (3)–(5).
The reduction and oxidation behavior of metallophthalocyanine
derivatives is due to the interaction between the phthalocyanine
ring and the central metal. First-row transition metal phthalocya-
nines differ from those of the main-group metal phthalocyanines,
due to the fact that metal ‘d’ orbitals may be positioned between
the HOMO and LUMO of the phthalocyanine ligand [32]. To get
an efficient charge separation, the HOMO and LUMO levels of
APC and bi-NPC match with the conduction-band-edge energy
level of the TiO2 and the redox potential of electrolyte for efficient
electron injection and APC and bi-NPC regeneration. Cyclic and dif-
ferential pulse voltammetric techniques in DMF solvent were used
to investigate the electrochemical behaviors of APC and bi-NPC.
Half-wave potentials (E1/2) (Eox ꢂ Ered)/2 by cyclic voltammetry or
peak potentials by differential pulse voltammetry determine the
oxidation potentials. The corresponding cyclic and differential
pulse voltammograms were presented in Fig. 5. The first oxidation
potentials (E1/2(ox)) correspond to the HOMO level of phthalocya-
nine. Quasi-reversible oxidation of APC exhibition is 0.96 V and
bi-NPC exhibits a quasi-reversible oxidation at 0.83 V. With respect
to dye-sensitization of wide-band gap semiconductors, e.g. TiO2,
the first oxidation potentials of APC and bi-NPC, and the E0–0
ꢀ
ꢁ
Iph
kPin
IPCE ¼ 1240
ð3Þ
where k is the wavelength, Iph is the photocurrent of the incident
radiation (mA/cm2) and Pin is the incident radioactive flux (W/m2).
JSC ½mA cmꢂ2ꢄVOC½Vꢄ ꢃ ff
g
½%ꢄ ¼
ꢃ 100%
ð4Þ
I0 ½WMꢂ2
ꢄ
where I0 is the photon flux (e.g. 1000 W mꢂ2 for 1.0 sun), JSC is the
short-circuit photocurrent density under irradiation, VOC is the
open-circuit voltage, and ff represents the fill factor. The fill factor
is defined by the following equation.
JPHðmaxÞVPHðmaxÞ
ff ¼
ð5Þ
JSCVOC
where JPH(max) and VPH(max) are the photocurrent and photovoltage
for maximum power output; JSC and VOC are the short-circuit photo-
current density and open-circuit photovoltage. Fig. 7 shows the I–V
curves of APC and bi-NPC. We have observed an overall efficiency
0.905% under 1 sun irradiation. Table 1 shows the photoelectric
properties of APC and bi-NPC.
Fig. 5. Cyclic and differential pulse voltammograms of APC and bi-NPC in DMF.
Fig. 6. Energy level diagram for APC and bi-NPC.

66
B. Zhu et al. / Journal of Molecular Structure 1079 (2015) 61–66
51272239) and the Natural Science Foundation of Shanxi Province,
China (Grant number: 2011011022-4).
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Table 1
The photoelectric properties of APC and bi-NPC.
DSSCs
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ff
APC
bi-NPC
2.19
2.56
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Schiff base. Compared with mononuclear phthalocyanine in fur-
thermore, the bi-NPC shows the higher absorbance and the better
red shift. Owing to chemical integration of mononuclear phthalo-
cyanines, it makes the phthalocyanine possess superior arrange-
ment. From the absorption spectrum, the reduction and
oxidation behavior, it is proved that APC and bi-NPC possess inten-
sive absorption in the far-red/near-IR region, the excited electron
can be easily injected into the TiO2 conduction and the oxidized
state of dyes can easily get reduced by taking electron form redox
electrolyte. The two sensitizers are promising in the development
of DSSCs. For the future, performance of phthalocyanine-based
TiO2 solar cells, some other substituents that are asymmetrically
should be incorporated into the phthalocyanine macrocycle.
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Acknowledgments
The authors would like to acknowledge financial support from
the National Nature Science Foundation of China (Grant number: