Synthesis, characterization and electrochemistry of novel unsymmetrical zinc phthalocyanines sensitizer with extended conjugation

Article abstract of DOI:10.1016/j.molstruc.2012.04.046

Synthesis and structural characterization of peripherally polar substituent zinc phthalocyanine, based on extended π-conjugation concept, were carried out for efficient far-red/near-IR performance in nanocrystalline TiO₂ films. The new compound

Full text of DOI:10.1016/j.molstruc.2012.04.046

Journal of Molecular Structure 1022 (2012) 153–158
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Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstruc
Synthesis, characterization and electrochemistry of novel unsymmetrical zinc
phthalocyanines sensitizer with extended conjugation
⇑
Xuejun Zhang , Lijun Mao, Dan Zhang, Lei Zhang
Department of Chemistry, College of Science, North University of China, Taiyuan, Shanxi 030051, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Synthesis and structural characterization of peripherally polar substituent zinc phthalocyanine, based on
extended p-conjugation concept, were carried out for efficient far-red/near-IR performance in nanocrys-
talline TiO2 films. The new compounds were characterized using FTIR, UV–Vis, fluorescence spectrosco-
pies, 1H NMR, cyclic voltammetry, differential pulse voltammetry and elemental analysis. From the
reduction and oxidation behavior, it is proved that PPC and QPC have negative LUMO levels and positive
HOMO levels, satisfying the energy gap rule, PPC and QPC can be employed as sensitizers for DSSC
applications.
Received 2 January 2012
Received in revised form 10 April 2012
Accepted 17 April 2012
Available online 9 May 2012
Keywords:
Phthalocyanine
Unsymmetrical
Dye-sensitized solar cells
Electrochemistry
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
sensitizers employed so far in such cells are bis(tetrabutylammo-
nium)-cis-di(thiocyanato)-N,N0-bis(4-carboxylato-40-carboxylic
The conversion of sunlight to electricity using dye-sensitized
solar cells (DSSCs) has been researched as one of the most promis-
ing methods for future low cost power production from renewable
energy sources [1,2]. In DSSC the light absorption function is ful-
filled by the dye and the electron and hole transporting are fulfilled
by the nanocrystalline metal oxide and electrolyte. Therefore the
absorption properties of the dye dictate the light-harvesting capac-
ity of the cell. As for the sensitizing dye, there are several theoret-
ical requirements that it has to comply with. First, anchoring
groups such as carboxylates, sulfonates or phosphonates which
are capable of covalently bonding to AOH groups on the TiO2 sur-
face. Second, the lowest unoccupied molecular orbital (LUMO) of
the dye should have a higher energy than the conduction band
edge of the TiO2 and have a good orbital overlap to facilitate elec-
tron injection. Third, ideally, the dye should have intensive absorp-
tion in the whole solar spectrum. Finally, mainly practical,
requirement is that the dye should be soluble in solvents that are
compatible with the TiO2 and favorable for adsorption of a nonag-
gregated monolayer of the dye on the nanoparticle surface [3,4].
acid-2,20-bipyridine)ruthenium(II) (the N719 dye), and trithiocy-
anato 4,40400-tricaboxy-2,20:60,200-terpyridine ruthenium(II) (the
black dye), produced solar-energy-to-electricity conversion effi-
ciencies of up to 11% under AM 1.5 irradiation [7,8]. In spite of this,
the main drawback of ruthenium-based sensitizer is their lack of
absorption in the red region of the visible spectrum and their high
cost. Phthalocyanines are the proper choice for this objective due
to their strong Q band light absorption properties at around
700 nm and good chemical stability [9–11].
In the past, phthalocyanines have been repeatedly tested as
photosensitizers for DSSC [12–16]. However, poor incident pho-
ton-to-electric current conversion yields were obtained. This may
be due to the poor solubility of the macrocycle in organic solvent,
a strong tendency to aggregation on the film surface, lack of direc-
tionality in the excited state and easy electron recombination be-
tween injected electron in TiO2 conduction band and oxidized
dye [4]. For this reason, one of the important aims of research on
the chemistry of phthalocyanines is to enhance their solubility in
common organic solvents. Also, the presence of suitable substitu-
ents is known to offer a useful way of regulating the wavelength
of the Q band [17–19].
Phthalocyanines are macrocyclic complexes whose
p systems,
localized over an arrangement of alternated carbon and nitrogen
atoms, provide their unique physical properties, electrochemical
and photochemical [5,6]. The most successful charge-transfer
This paper concerns phthalocyanines in which polar groups were
attached to the periphery together with an additional group to
enhance solubility in the organic solvents. We have designed
two new unsymmetrical zinc phthalocyanines, 9,16,23-tri-tert-
butoxy-2-(4-carboxy-phenoxy) zinc phthalocyanine (PPC) and
9,16,23-tri(4-carboxy-phenoxy)-2-(2-methyl-8-oxy-quinoline)
zinc phthalocyanine (QPC). PPC has three tert-butoxy groups, which
⇑
Corresponding author. Address: North University of China, Xueyuan Road No. 3,
Taiyuan, Shanxi 030051, China. Tel./fax: +86 351 3922152.
E-mail address: zhangxuejun@nuc.edu.cn (X. Zhang).
0022-2860/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molstruc.2012.04.046

154
X. Zhang et al. / Journal of Molecular Structure 1022 (2012) 153–158
act as electron releasing, enhance the solubility of phthalocyanine in
common organic solvents, reduce the aggregation and tune the
LUMOlevelof phthalocyanine. Thesubstituents ofcarboxy-phenoxy
DMF (10 ml). After dissolution, anhydrous K2CO3 (1.0 g, 7 mmol)
was added and the reaction mixture was stirred at 45 °C. Further
K2CO3 (0.5 g, 3.5 mmol) was added portion-wise after 2 h. Stirring
vigorously for 28 h under nitrogen. Then the reaction mass was
poured into 150 ml of cold water and stirred for 15 min. The pre-
cipitate was filtered, washed several times with cold water until
the filtrate became neutral, and crystallized from EtOH–water to
give the product as a brown, crystalline powder. Yield: 0.67 g,
extended
p-conjugation that increase the absorption bands of
phthalocyanine macrocycle. The presence of carboxylic acid act as
electron acceptors for the study of photoinduced electron transfer
processes as well as grafting onto nanocrystalline TiO2. Also, 2-
methyl-8-oxy-quinoline extended p-conjugation that shift the low
energy absorption of the phthalocyanines to longer wavelengths.
The effects of the substituents are discussed. The solution electro-
chemistry of the complexes is also investigated.
46.8%, m.p. 165–166 °C. IR (KBr), t
(cmꢀ1): 3101, 3074, 3038
(HAAr), 2259 (AC„N), 1587, (Ar C@C), 1254 (ArAOAAr). 1H
NMR (DMSO-d6) d, ppm: 7.19–8.35 (m, 8H, ArAH), 2.48 (s, 3H,
ACH3). Anal. Calc. for C18H11N3O (285.30 g/mol): C, 75.78; H,
3.89; N, 14.73; O, 5.61 Found: C, 75.82; H, 3.93; N, 14.71.
2. Experimental
4-Nitro-phthalonitrile, 4-hydroxybenzoic acid, tert-butyl alco-
hol, 2-methy-8-quino-linol, 1,8-diazabicyclo [5.4.0]-undec-7-ene
(DBU), 1-pentanol, anhydrous K2CO3, N,N-Dimethylformamide
(DMF), Dimethyl sulfoxide (DMSO), CHCl3, anhydrous AlCl3 were
purchased commercially. DMF 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 (80–100).
2.1.4. 9,16,23-Tri-tert-butoxy-2-(4-carboxy-phenoxy) zinc
phthalocyanine (PPC)
4-(4-Carboxy-phenoxy) phthalonitrile (0.132 g, 0.5 mmol),
4-(tert-butoxy) phthalonitrile (0.3 g, 1.5 mmol), Zn(CH3COO)2
(0.11, 0.5 mmol) and a catalytic amount of DBU in dry 1-pentanol
(10 ml) was heated at 160 °C with stirring under nitrogen for 24 h.
After cooling to room temperature, the reaction mixture was pre-
cipitated by adding methanol. The product was separated by filtra-
tion as a green 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 col-
umn chromatography and eluted with DMF: CH3OH = 1:2 (v/v),
and the second greenish color band was the desired phthalocya-
nine. The solvent was removed under reduced pressure to get the
2.1. Synthesis
2.1.1. 4-Tert-butoxy phthalonitrile (1)
Phthalonitrile derivatives were prepared by a similar method
reported in the literature [20–22]. 4-Nitro-phthalonitrile (1.73 g,
0.01 mol) and tert-butyl alcohol (0.74 g, 0.01 mol) were added suc-
cessively with stirring to dry DMF (15 ml). After dissolution, anhy-
drous K2CO3 (2.0 g, 0.014 mol) was added and the reaction mixture
was stirred at 60 °C. Further K2CO3 (1 g, 0.007 mol) was added por-
tion-wise after 2 h. Stirring vigorously for 28 h under nitrogen.
Then the reaction mass was poured into 200 ml of cold water
and stirred for 15 min. The precipitate was filtered, washed several
times with cold water until the filtrate became neutral, and crystal-
lized from ethanol (EtOH)–water to give the product as a yellow,
crystalline powder. Yield: 0.87 g, 47.3%, melting point (m.p.)
desired product. Yield: 0.18 g, 38.7%, m.p. >200 °C. IR (KBr),
t
(cmꢀ1): 3110 (AOAH), 2928, 2860, 1718 (AC@O), 1653, 1396,
1558, 1466 (C@C), 1230, 1092, 745. 1H NMR (DMSO-d6) d, ppm:
7.11–7.90 (m, 16H, ArAH), 2.48 (s, 27H, AC(CH3)3). Ultraviolet vis-
ible spectrophotometer (UV–Vis), in DMF (kmax, loge) 677 (4.45),
343 (4.24). Anal. Calc. for C51H44N8O6Zn (928.27 g/mol): C, 65.93;
H, 4.78; N, 12.07; O, 10.34; Zn, 6.89 Found: C, 65.76; H, 4.96; N,
12.12.
2.1.5. 9,16,23-Tri(4-carboxy-phenoxy)-2-(2-methyl-8-oxy-quinoline)
zinc phthalocyanine (QPC)
272–273 °C. Infrared spectrophotometer (IR) (KBr),
t
(cmꢀ1):
4-(2-methyl-8-oxy-quinoline) phthalonitrile (0.143 g, 0.5 mmol),
4-(4-carboxy-phenoxy) phthalonitrile (0.396 g, 1.5 mmol), Zn(CH3
COO)2 (0.11, 0.5 mmol) and a catalytic amount of DBU in dry
1-pentanol (10 ml) was heated at 160 °C with stirring under nitrogen
for 20 h. After cooling to room temperature, the reaction mixture
was precipitated by adding methanol. The product was separated
by filtration as a green 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 DMF: CH3OH = 1:1 (v/v), and the
second greenish color band was the desired phthalocyanine. The
solvent was removed under reduced pressure to get the desired
3074 (HAAr), 2233 (AC„N), 1585, 1569 (Ar C@C), 1254
(ArAOAAr). Nuclear magnetic resonance (1H NMR) (DMSO-d6) d,
ppm: 7.56–8.07 (m, 3H, ArAH), 2.48 (s, 9H, AC(CH3)3). Elemental
analysis (Anal. Calc.) for C12H12N2O (200.24 g/mol): C, 71.98; H,
6.04; N, 13.99; O, 7.99 Found: C, 71.86; H, 6.06; N, 13.87.
2.1.2. 4-(4-Carboxy-phenoxy) phthalonitrile (2)
4-Nitro-phthalonitrile (0.866 g, 5 mmol) and 4-hydroxybenzoic
acid (0.69 g, 5 mmol) were added successively with stirring to dry
DMF (10 ml). After dissolution, anhydrous K2CO3 (1.0 g, 7 mmol)
was added and the reaction mixture was stirred at 60 °C. Further
K2CO3 (0.5 g, 3.5 mmol) was added portion-wise after 2 h. After
stirring vigorously the reaction mixture for 24 h under nitrogen,
the undissolved salt was removed by filtration. The solution was
concentrated under vacuum till a precipitate was obtained. Then
the precipitate was crystallized from EtOH–water to give the prod-
uct as a mahogany, crystalline powder. Yield: 0.81 g, 61.3%, m.p.
product. Yield: 0.21 g, 36.7%, m.p. >200 °C. IR (KBr),
t
(cmꢀ1): 3420
(AOAH), 2361, 2345, 1655 (AC@O), 1599, 1396, 1560, 1466 (C@C),
1234, 1094, 748. 1H NMR (DMSO-d6) d, ppm: 7.23–7.93 (m, 16H,
ArAH), 2.48 (s, 3H, ACH3). UV–Vis, in DMF (kmax, log
e) 678 (4.90),
356 (4.57). Anal. Calc. for 63H35N9O10Zn (1143.41 g/mol): C,
C
66.25; H, 3.09; N, 11.04; O, 14.02; Zn, 5.60 Found: C, 66.19; H,
3.23; N, 14.12.
252–255 °C. IR (KBr),
t
(cmꢀ1): 3398 (AOAH), 3099 (HAAr), 2230
(AC„N), 1663 (AC@O) 1591, (Ar C@C), 1256 (ArAOAAr). 1H
NMR (DMSO-d6) d, ppm: 7.03–8.04 (m, 7H, ArAH). Anal. Calc. for
2.2. Characterization methods
C15H8N2O3 (264.24 g/mol): C, 71.98; H, 3.05; N, 10.60; O, 18.16
Found: C, 72.04; H, 3.01; N, 10.53.
The UV–Vis spectra were recorded with a Techcomp 2300 spec-
trophotometer. The Fourier transform IR (FTIR) spectra of all the
samples were measured using a Shimadzu 4800S spectrophotom-
eter. Steady state fluorescence spectra were recorded using a
Spex model Fluoromax-3 spectrofluorometer for solutions having
2.1.3. 4-(2-Methyl-8-oxy-quinoline) phthalonitrile (3)
4-Nitro-phthalonitrile (0.866 g, 5 mmol) and 2-methy-8-quino-
linol (0.80 g, 5 mmol) were added successively with stirring to dry

X. Zhang et al. / Journal of Molecular Structure 1022 (2012) 153–158
155
CN
O N
CN
2
COOH
N
CH
3
C(CH
)
3OH
3
OH
a
OH
CH
N
CN
CN
3
O
HOOC
CN
CN
O
(H3C)3C
NC
NC
O
(3)
(2)
(1)
X
2
b
b
N
N
N
N
N
X2
Zn
N
X1
N
N
X2
X =
2
HOOC
O
QPC: X1=
X2=-O-C(CH )
X1=
PPC:
HOOC
O
3 3
N
CH3
O
Scheme 1. (a) K2CO3, DMF and (b) DBU, 1-pentanol, Zn(CH3COO)2.
(b)
(a)
(b)
(a)
300
400
500
600
700
800
900
1000
300
400
500
600
700
800
900
1000
Wavelength (nm)
Wavelength (nm)
Fig. 2. UV–Vis absorption spectra of QPC (a) in DMF and (b) adsorbed onto a 6 lm
thick TiO2 films.
Fig. 1. UV–Vis absorption spectra of PPC (a) in DMF and (b) adsorbed onto a 6 lm
thick TiO2 films.
A platinum wire was used as working electrode, the other platinum
as the counter film electrode, the calomel (SCE) as the reference.
optical density at the wavelength of excitation (kex) ꢁ 0.11. 1H
NMR spectra were obtained at 400 MHz using a Varian Inova
400 MHz NMR system. The chemical shifts are relative to tetra-
methylsilane (TMS). Elemental analyses were performed on a Flash
EA1112 Elemental Analyzer.
2.3. Preparation dye-sensitized nanocrystalline TiO2 thin films
Cyclic and differential pulse voltammetry measurements were
performed on an electrochemical workstation (CHI LK2005A, Tianjin
Lanlike Chemistry electronic high-tech Co., Ltd., PR China). Cyclic
voltammetry experiments were performed on 1 ꢂ 10ꢀ4 mol/L
phthalocyanine dye solution in DMF at scan rate 100 mV/s using tet-
rabutylammonium perchlorate (0.1 mol/L) as supporting electrolyte.
TiO2 photoelectrode (area: ca. 0.8 cm ꢂ 1.2 cm) was prepared
by a similar method reported in the literature [23–25]. Nanocrys-
talline TiO2 films of 6 lm thickness were deposited onto transpar-
ent 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

156
X. Zhang et al. / Journal of Molecular Structure 1022 (2012) 153–158
1.6
1.2
0.8
0.4
0.0
A
E
550
600
650
700
750
800
850
Wavelength (nm)
300
400
500
600
700
800
Wavelength (nm)
Fig. 5. Fluorescence spectra of PPC ( ) in DMF and ( ) adsorbed onto a 6 lm
thick TiO2 films.
Fig. 3. Absorption spectral changes for PPC in DMF at different concentrations: (A)
5 ꢂ 10ꢀ5, (B) 2 ꢂ 10ꢀ5, (C) 1 ꢂ 10ꢀ5, (D) 5 ꢂ 10ꢀ6, (E) 2 ꢂ 10ꢀ6 mol/L.
3.2. UV–Vis and electrochemical measurements
were gradually sintered at 500 °C for 20 min. The sensitizer was
dissolved in ethanol at a concentration of 1 ꢂ 10ꢀ4 mol/L. The pho-
toelectrode 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.
Figs. 1 and 2 show absorption spectrum of PPC and QPC in DMF
solution and compared to that of the phthalocyanine adsorbed
onto 6 lm thick nanocrystalline TiO2 films. The absorption spec-
trum in solution showed characteristic absorptions between 678
and 612 nm in the Q band region. The Q band observed was attrib-
uted to p–
pꢃ transitions from the highest occupied molecular orbi-
tal (HOMO) to the LUMO of the conjugated macrocycle. The other
bands (Soret band) in the ultraviolet (UV) region at 343–356 nm
were observed due to the transitions from the deeper
the LUMO. The absorption spectrum of the phthalocyanine ad-
sorbed onto 6 m thick TiO2 electrode exhibit an obvious red shift.
p levels to
3. Results and discussion
l
The interaction between the carboxylate group and the surface Ti4+
ions may lead to increased delocalization of the pꢃ orbital of the
conjugated framework. The energy of the pꢃ level is decreased by
this delocalization, which explains the red shift for the absorption
threshold [26,27]. PPC and QPC adsorbed onto nanocrystalline TiO2
films not only broaden the range of nanocrystalline TiO2 films
spectral response, but also conducive to inject electrons into the
conduction band of TiO2.
3.1. Design and synthesis
The synthesis of PPC and QPC were illustrated in Scheme 1. The
phthalonitrile derivatives 1, 2 and 3 were obtained through dis-
placement reaction of 4-Nitro- phthalonitrile with tert-butyl alco-
hol, 4-hydroxybenzoic acid, 2-methy-8-quinolinol, respectively.
PPC and QPC were prepared through 4-tert-butoxy phthalonitrile
1 with 4-(4-carboxy-phenoxy) phthalonitrile 2, and 4-(4-car-
boxy-phenoxy) phthalonitrile 2 with 4-(2-methyl-8-oxy-quino-
line) phthalonitrile 3, in 1-pentanol solution in the presence of
DBU and metal salts.
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
2.0
A
1.6
E
1.2
0.8
0.4
0.0
300
400
500
600
700
800
550
600
650
700
750
800
850
Wavelength (nm)
Wavelength (nm)
Fig. 4. Absorption spectral changes for QPC in DMF at different concentrations: (A)
5 ꢂ 10ꢀ5, (B) 2 ꢂ 10ꢀ5, (C) 1 ꢂ 10ꢀ5, (D) 5 ꢂ 10ꢀ6, (E) 2 ꢂ 10ꢀ6 mol/L.
Fig. 6. Fluorescence spectra of QPC ( ) in DMF and ( ) adsorbed onto a 6 lm
thick TiO2 films.

X. Zhang et al. / Journal of Molecular Structure 1022 (2012) 153–158
157
Table 1
UV–Vis data for PPC and QPC in DMF.
1 µA
Compound
Absorption kmax, nm (log
e
, molꢀ1 cmꢀ1
)
E0–0 (eV)
PPC
QPC
343 (4.24) 615 (3.84) 677 (4.45)
356 (4.57) 612 (4.24) 678 (4.90)
1.83
1.83
1 µA
2.0
0.0
0.4
0.8
1.2
1.6
Potential (V vs NHE)
Fig. 8. Cyclic ( ) and differential pulse voltammograms ( ) of QPC in DMF.
-1.5
LUMO
2.0
0.0
0.4
0.8
1.2
1.6
Injection
-1.0
Potential (V vs NHE)
QPC (- 0.98 V)
TiO2 (- 0.74V)
PPC (- 0.88 V)
Fig. 7. Cyclic ( ) and differential pulse voltammograms ( ) of PPC in DMF.
-0.5
[26]. 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,
complexed metal ions and temperature. In this study, the aggrega-
tion behavior of PPC (Fig. 3) and QPC (Fig. 4) complexes were inves-
tigated in different concentration in DMF. 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 aggre-
gated species [28,29]. It is seen that the Beer-Lambert law was
obeyed for PPC and QPC for concentrations ranging from
5 ꢂ 10ꢀ5 to 2 ꢂ 10ꢀ6 mol/L.
0
-
-
I /I (0.2 V)
3
0.5
hv
QPC (0.85 V)
PPC (0.95 V)
1.0
1.5
HOMO
Fig. 9. Energy level diagram for PPC and QPC.
The emission spectra of PPC and QPC, shown in Figs. 5 and 6,
were obtained at room temperature in DMF solution. The emission
maxima were observed at 694 and 762 nm, 699 and 771 nm,
respectively. Singlet state (E0–0) energy of PPC and QPC estimated
from excitation and emission spectra are summarized in Table 1.
The emission intensity was quenched when the two sensitizers ad-
first oxidation potentials (E1/2(ox)) correspond to the HOMO level
of phthalocyanines. The PPC exhibits a quasi-reversible oxidation
at 0.95 V, and QPC at 0.85 V. With respect to dye-sensitization of
wide-band gap semiconductors, e.g. TiO2, the first oxidation poten-
tials of PPC and QPC and the E0–0 transition energy, the energy lev-
els of the LUMO were deduced to be ꢀ0.86 V vs SCE and ꢀ0.95 V vs
SCE. The LUMO is calculated according to the following equation.
sorbed respectively onto 6 lm thick TiO2 layer as a consequence of
electron injection from excited singlet state of phthalocyanine
macrocycle into the conduction of TiO2.
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 [30]. To get
an efficient charge separation, the HOMO and LUMO levels of
PPC and QPC must match with the conduction-band-edge energy
level of the TiO2 and the redox potential of electrolyte for efficient
electron injection and PPC and QPC regeneration. The electrochem-
ical behavior of PPC and QPC were investigated using cyclic and
differential pulse voltammetric techniques in DMF solvent. The
oxidation potentials were determined from half-wave potentials
(E1/2) (Eox ꢀ Ered)/2 by cyclic voltammetry or peak potentials by dif-
ferential pulse voltammetry. The corresponding cyclic and differ-
ential pulse voltammograms were presented in Figs. 7 and 8. The
LUMO ¼ HOMO ꢀ E0—0
ð1Þ
The energy level of the conduction band edge of TiO2 is ca.
ꢀ0.74 V vs SCE [31]. This makes electron injection from the excited
state of PPC or QPC into the conduction band of TiO2 thermody-
namically feasible. The HOMO levels of PPC and QPꢀC are more po-
sitive than the energy level of the redox couple Iꢀ/I3 (0.2 V vs SCE)
in the electrolyte, indicating more efficient phthalocyanines regen-
eration by electron transfer from Iꢀ.
Fig. 9 shows the schematic energy diagram of PPC and QPC dye-
sensitized TiO2 electrodes. PPC and QPC have negative LUMO levels
and positive HOMO levels, satisfying the energy gap rule. This
means that, the PPC and QPC can be employed as sensitizers for
DSSC applications [32]. QPC has stronger electron injection than
PPC.

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X. Zhang et al. / Journal of Molecular Structure 1022 (2012) 153–158
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In conclusion, we have designed two new unsymmetrical zinc
phthalocyanines, PPC and QPC, based on extended
p-conjugation
concept. From the absorption spectrum, the reduction and oxida-
tion behavior, it is proved that PPC and QPC possess intensive
absorption in the far-red/near-IR region, the excited electron can
be easily injected into the TiO2 conduction and the oxidized state
of dye can easily get reduced by taking electron form redox electro-
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The authors would like to acknowledge financial support from
the National Nature Science Foundation of China (Grant Number:
20871108) and the Natural Science Foundation of Shanxi Province,
China (Grant Number: 2011011022-4).
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