Synthesis of sterically hindered phthalocyanines and their applications to dye-sensitized solar cells

Article abstract of DOI:10.1039/b803272f

Phthalocyanines with high peripheral substitutions and free from potential contamination by regioisomers have been synthesized and evaluated as photosensitizers for dye-sensitized solar cell applications. Each of the sterically hindered precursor compounds was accomplished by Suzuki-Miyaura cross-coupling reactions with the arylchloride and corresponding boronic acids. Metal free phthalocyanine-sensitized solar cells showed no photocurrent generation due to its low excited singlet state (LUMO) compared with the conduction band of TiO₂. Upon zinc metalation, the LUMO level of the phthalocyanine was pushed up, and this variation afforded an exergonic free energy change for electron injection. The zinc phthalocyanine-sensitized solar cell displayed 0.57% power conversion efficiency (η) and 4.9% maximal IPCE in the near infrared region. More importantly, the cell prepared with and without the presence of chenodeoxycholic acid revealed no difference in the power conversion efficiency. This implies that the well-known aggregation tendency of phthalocyanines that is considered to enhance the self-quenching of the phthalocyanine excited singlet state is effectively suppressed by the high degree of substitutions. The significance of the driving force for electron injection and the distance between the dye core and the TiO₂ surface is also highlighted for devising high performance phthalocyanine photosensitizers.

Full text of DOI:10.1039/b803272f

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis of sterically hindered phthalocyanines and their applications to
dye-sensitized solar cells
Seunghun Eu,a Takashi Katoh,b Tomokazu Umeyama,a Yoshihiro Matanoa and Hiroshi Imahori*a,c,d
Received 27th February 2008, Accepted 16th May 2008
First published as an Advance Article on the web 30th June 2008
DOI: 10.1039/b803272f
Phthalocyanines with high peripheral substitutions and free from potential contamination by
regioisomers have been synthesized and evaluated as photosensitizers for dye-sensitized solar cell
applications. Each of the sterically hindered precursor compounds was accomplished by
Suzuki–Miyaura cross-coupling reactions with the arylchloride and corresponding boronic acids. Metal
free phthalocyanine-sensitized solar cells showed no photocurrent generation due to its low excited
singlet state (LUMO) compared with the conduction band of TiO2. Upon zinc metalation, the LUMO
level of the phthalocyanine was pushed up, and this variation afforded an exergonic free energy change
for electron injection. The zinc phthalocyanine-sensitized solar cell displayed 0.57% power conversion
efficiency (h) and 4.9% maximal IPCE in the near infrared region. More importantly, the cell prepared
with and without the presence of chenodeoxycholic acid revealed no difference in the power conversion
efficiency. This implies that the well-known aggregation tendency of phthalocyanines that is considered
to enhance the self-quenching of the phthalocyanine excited singlet state is effectively suppressed by the
high degree of substitutions. The significance of the driving force for electron injection and the distance
between the dye core and the TiO2 surface is also highlighted for devising high performance
phthalocyanine photosensitizers.
700 nm.5 Their extreme stabilities against thermal, chemical, and
Introduction
photochemical reactions are definitively desirable features for the
Exhaustion of fossil fuels and global energy concerns have never
long-term and outdoor robustness of DSSCs. Applications of
been recognized as seriously as in recent times. In this context,
phthalocyanines for DSSCs as photosensitizers, however, have
not been successful.6 Notoriously poor solubility in common
organic solvents and a high tendency to form aggregations have
been attributed as the main reasons impeding the revelation
of their potential as use for DSSCs. Recent studies by Torres
et al. and Nazeeruddin et al. demonstrated the usefulness of
phthalocyanines for red light-harvesting provided that the degree
of aggregation is partially diminished by introducing three bulky
t-butyl groups into the macrocycle plane.7 The directionality in the
excited state of the dye was also emphasized as an important factor
for efficient light-harvesting. However, the reported compound is
a mixture of regioisomers, and this may result in the formation of
rather complex monolayers on the TiO2 surface, making it difficult
to disclose the relationship between the monolayer structure and
the photovoltaic properties. More importantly, the cell perfor-
mance is still aided with the co-adsorption of chenodeoxycholic
acid which is well-known to suppress dye aggregation on the TiO2
surface. Although the power conversion efficiency is the highest
among the reported phthalocyanine-sensitized TiO2 cells (h =
3.5%), it is much lower than those of Ru dye-based DSSCs (h = 10–
11%).2–4 Therefore, further studies are still needed to elucidate the
close relationship between molecular structure and photovoltaic
properties toward the improvement of cell performance.
research activities to acquire energy from the sun, as a clean and
inexhaustible resource, are being extensively exploited.1 After the
seminal report of O’Regan and Gra¨tzel,2 dye-sensitized solar cells
(DSSCs) with mesoporous TiO2 have been regarded as one of
the most promising candidates among a variety of regenerative
energy sources developed to date. The fundamental aspects of
the DSSCs have been well documented based on widespread
research efforts to disclose the nature of the devices including
interfacial photoinduced electron transfer, role of mesoporous
semiconductor electrode, and electrolyte.3 From the practical
and industrial point of view, however, the improvement in the
performance has been rather stagnated during the last two decades.
The main reason for this could be attributed to the limited
light-harvesting capabilities of the existing dyes, especially for
the near-infrared region.4 As such, to devise and develop novel
photosensitizer dyes that can effectively harvest red light is an
urgent task to make DSSCs practically viable.
Phathalocyanines are the proper choice for this objective due
to their strong Q band light absorption properties at around
aDepartment of Molecular Engineering, Graduate School of Engineering,
Kyoto University, Nishikyo-ku, Kyoto, 615-8510, Japan
bGoi Research Center, Chisso Petrochemical Corporation, 5-1 Goikaigan,
Ichihara, Chiba, 290-8551, Japan
Herein we report the synthesis and photovoltaic properties of
a novel highly substituted zinc phthalocyanine carboxylic acid
(ZnPc) and its metal free counterpart (H2Pc) as depicted in
Fig. 1. The compounds retain eight sterically hindered phenyl
groups where the neighboring phenyl rings are rotated about each
cInstitute for Integrated Cell-Material Sciences (iCeMS), Kyoto University,
Nishikyo-ku, Kyoto, 615-8510, Japan
dFukui Institute for Fundamental Chemistry, Kyoto University, 34-4, Takano-
Nishihiraki-cho, Sakyo-ku, Kyoto, 606-8103, Japan. E-mail: imahori@
scl.kyoto-u.ac.jp
5476 | Dalton Trans., 2008, 5476–5483
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©

Fig. 1 Structures of the phthalocyanines used in this study.
other with respect to the phthalocyanine plane to avoid steric
congestion around the ortho-protons. Moreover, the six phenyl
groups also possess bulky t-butyl moieties. Therefore, ZnPc and
H2Pc are expected to show high solubility toward common organic
solvents and a reduced tendency towards aggregation. Since the
two neighboring b positions are occupied by the same functional
groups, the target compound can be isolated free from the problem
of regioisomeric mixtures. Two carboxylic acid binding groups
could guarantee stable immobilization of the phthalocyanine
onto the TiO2 surface. Additionally, the intramolecular push–pull
character afforded by electron-donating (t-butyl) and electron-
withdrawing (carboxylic acid) groups would be anticipated to
make efficient electron transfer from the phthalocyanine excited
singlet state to the conduction band (CB) of the TiO2.
Scheme 1
bicyclo[5.4.0]undec-7-ene). Although we expected to obtain 2,3,9,
10,16,17-hexakis(4-t-butylphenyl)-23,24-bis(4-methoxycarbonyl-
phenyl)phthalocyanine, we obtained a mixture of two phthal-
ocyanine compounds. The compounds showed two molecular
ion peaks at 1630.7 and 1686.8 with an intensity ratio of
1 : 3 in the mass spectrum (MALDI-TOF). The peaks corres-
pond to 2,3,9,10,16,17-hexakis(4-t-butylphenyl)-23-(4-methoxy-
carbonylphenyl)-24-(4-pentoxycarbonylphenyl)phthalocyanine,
and 2,3,9,10,16,17-hexakis(4-t-butylphenyl)-23,24-bis(4-pentoxy-
carbonylphenyl)phthalocyanine, respectively. We believed that the
exchange reaction between the acetate and pentanoate occurred
during the cyclo-condensation reaction, as already reported.6g,h
Because both of the compounds would afford the same target
compound (i.e., H2Pc) by hydrolysis, the mixture was directly
employed for the next reaction without further purification.
The basic hydrolysis of the compounds in THF–methanol
containing aqueous potassium hydroxide solution afforded the
corresponding phthalocyanine carboxylic acid, H2Pc. To achieve
the zinc phthalocyanine carboxylic acid (ZnPc), zinc(II) was
inserted into the core of the phthalocyanine esters by treatment
with zinc acetate. The resulting compounds displayed molecular
ion peaks at 1692.5 and 1748.6 in the mass spectrum (MALDI-
TOF). The peaks correspond to 2,3,9,10,16,17-hexakis(4-
t-butylphenyl)-23-(4-methoxycarbonylphenyl)-24-(4- pentoxycar-
bonylphenyl)phthalocyanatozinc(II) and 2,3,9,10,16,17-hexakis-
(4-t-butylphenyl)-23,24-bis(4-pentoxycarbonylphenyl)phthaloc-
yanatozinc(II), respectively. The basic hydrolysis of the compounds
in THF–methanol containing aqueous potassium hydroxide
solution afforded the corresponding zinc phthalocyanine
carboxylic acid, ZnPc. Structures of all the new compounds were
verified by spectroscopic analyses including 1H NMR, 13C NMR,
FT-IR, mass spectra, and elemental analyses.
Results and discussion
Synthesis
The syntheses of the phthalocyanines used in this study were
achieved by the statistical condensation method.8 A key step in this
protocol is the preparation of adequate phthalonitrile precursors.
Synthetic routes to H2Pc and ZnPc are displayed in Scheme 1.
Precursor compound 1 was accomplished by the Suzuki–Miyaura
cross-coupling reaction between 4,5-dichlorophthalonitrile and 4-
t-butylphenylboronic acid. Suzuki–Miyaura coupling is one of the
most widely used reactions for C–C bond formation.9 However,
it is well known that the coupling reactions for substrates with
high steric hindrance or for the arylchlorides are ineffective, as
for the substrate in this study. To attain the desired compound,
we have employed the electron-rich and sterically hindered ligand,
2-(2¢,6¢-dimethoxybiphenyl)dicyclohexylphosphine (S-Phos).10
Because electron-rich ligands with high steric hindrance such
as S-Phos or P(t-Bu)3 make the palladium(0) coordinatively
unsaturated, the cross-coupling reactions even for difficult sub-
strates proceeded smoothly with moderate to good yield.11
Precursor compound 2 was also achieved through Suzuki–
Miyaura cross-coupling between 4,5-dichlorophthalonitrile and
4-(methoxycarbonylphenyl)boronic acid, but it needed longer
reaction times compared with that of 1. Owing to the presence of
the ester group, the boronic acid would be an inferior nucleophile
compared with 4-t-butylphenylboronic acid, and this could be the
reason for the longer reaction time.
After obtaining the precursor compounds, we have tried to
prepare the desired cyclotetramer by statistical condensation
of 1 and 2 in 1-pentanol in the presence of DBU (1,8-diaza-
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Optical and electrochemical properties
compounds.14 This variation results in the important consequence
of electron injection from the excited state dye (LUMO) to the
CB of the TiO2 (vide infra). On the basis of the spectroscopic and
electrochemical measurements, driving forces for electron injection
from the LUMO of the dye to the CB of the TiO2 (-0.5 V vs NHE)
UV-visible absorption spectra for H2PC and ZnPc in THF are
displayed in Fig. 2. Each of the compounds showed characteristic
optical features of zinc and metal free phthalocyanine, respectively.
The peak positions in the B and Q band regions are summarized
in Table 1. The steady state fluorescence spectra of the phthalocya-
nines were also measured in THF and the wavelengths for emission
maxima are listed in Table 1. The Stokes shifts are determined to
be 68.2 cm-1 for H2Pc and 165.6 cm-1 for ZnPc. This fairly small
value suggests that the variations in the atomic coordinates during
the electronic transitions are small for both of the compounds.
From the intersection of the normalized absorption and emission
spectra, the zero–zero excitation energies (E0–0) are determined to
be 1.73 eV for H2Pc and 1.78 eV for ZnPc in THF.12
(DGinj)15 and the regeneration of the dye radical cation by the I-/I3
-
redox couple (+0.5 V vs NHE) (DGreg)15 for the phthalocyanine-
sensitized solar cells are evaluated (Table 1). Both of the processes
for ZnPc-sensitized TiO2 cell are thermodynamically feasible,
whereas the electron injection from the excited H2Pc to the CB
of the TiO2 is thermodynamically uphill.
DFT Calculations
DFT calculations were employed to gain insight into the equilib-
rium geometry and electronic structures for the molecular orbitals
of the phthalocyanines. The calculated structures do not show
negative frequencies, implying that the optimized geometries are
in the global energy minima.16 Fig. 3 illustrates the electron
density distributions of H2Pc and ZnPc in their respective LUMOs.
Sufficient electron densities around the carboxylic acid binding
group on the LUMO of the dye are required for good electronic
coupling between the excited state of the dye and the 3d orbital
of TiO2.17 There exists little electron density distribution on
and near the carboxylic acid binding groups of the phthalo-
cyanines.
Fig. 2 UV-visible absorption spectra of ZnPc (solid) and H2Pc (dashed)
measured in 2 ¥ 10-6 M of THF solution. Optical length = 1 cm.
To determine the first oxidation potential (Eox) of the phthalo-
cyanines, differential pulse voltammetry (DPV)13 measurements
were performed in DMF containing 0.1 M tetrabutylammonium
hexafluorophosphate (TBAP) as a supporting electrolyte and the
results are summarized in Table 1. Both H2Pc and ZnPc display
one oxidation peak corresponding to the phthalocyanine radical
cation under the same sweep conditions (-0.1 V to +0.8 V vs
Ag/AgNO3). The oxidation potential of H2Pc appears at +1.27 V
(vs NHE), whereas that of ZnPc is determined to be +0.94 V
(vs NHE). This indicates that the phthalocyanine is easier to oxi-
dize upon zinc(II) metalation, as reported for other phthalocyanine
Fig. 3 LUMOs of (a) ZnPc and (b) H2Pc calculated by DFT methods
with B3LYP/3-21G(d). The protons are omitted for clarity.
Table 1 Optical and electrochemical data for the phthalocyanines and driving forces for electron transfer processes on TiO2
labs a/nm
lemb/nm
Eoxc/V
+0.94
E0–0d/eV
1.78
Eox*e/V
-0.84
DGinjf/eV
-0.34
DGregg/eV
-0.44
364.5
622.0
699
730
ZnPc
H2Pc
691.0
357.9, 416.3
619.4, 651.1
681.0, 714.5
718
747
+1.27
1.73
-0.46
+0.04
-0.77
a Wavelengths for B and Q band maxima in THF. b Wavelengths for emission maxima in THF by exciting at the B band maxima. c Ground state oxidation
potentials (vs NHE). d The zero–zero excitation energy estimated from the interaction of the normalized absorption and emission spectra. e Excited-state
oxidation potentials approximated from Eox and E0–0 (vs NHE). f Driving forces for electron injection from the phthalocyanine excited singlet state (Eox*)
to the conduction band of the TiO2 (-0.5 V vs NHE). g Driving forces for the regeneration of phthalocyanine radical cation (Eox) by I-/I3 redox couple
-
(+0.5 V vs NHE).
5478 | Dalton Trans., 2008, 5476–5483
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Photovoltaic and photoelectrochemical properties of
phthalocyanine-sensitized TiO2 cells
profiles of the h values (Fig. 5) correlate well with those of the C
values (vide supra). This is in sharp contrast with the porphyrin-
sensitized TiO2 cells in which at first the h values increase and
then decrease significantly with increasing immersion time.4b,18
The representative photocurrent–voltage characteristics of ZnPc-
sensitized TiO2 cells with an immersion time of 12 h is depicted in
Mesoporous TiO2 films (10 mm thick) were prepared from a
colloidal suspension of TiO2 nanoparticles (P25) (see Experi-
mental section). The TiO2 electrodes were immersed in THF
containing 0.05 mM phthalocyanine at room temperature to
give the phthalocyanine-modified TiO2 electrodes. Since the light-
harvesting ability and consequently the cell performance are, to a
large extent, controlled by surface coverage (C) of the dye on the
TiO2 surface, first we examined the immersing time dependency
of the C value for the phthalocyanines. Both of the dyes showed
similar and rather slow adsorption rates, and reached saturated
coverage (C) on the surface after about 10 h of immersion time
(Fig. 4). The total amount of the dyes adsorbed on the TiO2 surface
was determined by measuring the changes in the absorbance of
the dye solutions before and after immersing the TiO2 films. Dye
concentrations on the TiO2 films (0.25 cm2 of area with thickness of
10 mm) are determined to be about 1.4 ¥ 10-10 mol cm-2. Assuming
that (i) the phthalocyanine molecule is a rectangular hexahedron
and (ii) the phthalocyanine molecules are densely packed onto
the TiO2 surface with a perpendicular orientation, the occupied
area of one molecule on the TiO2 surface is calculated to be ca.
Fig. 6; h = 0.57 0.03% with JSC = 1.47 0.05 mA cm-2, VOC
=
0.54 0.02 V, and ff = 0.71 0.03.
2
2
˚
˚
103 A (4.3 ¥ 24.0 A ). Accordingly, the C value is estimated to
be 1.6 ¥ 10-10 mol cm-2, which is close to the experimental value.
The TiO2 electrodes modified with H2Pc and ZnPc are denoted as
TiO2/H2Pc and TiO2/ZnPc, respectively.
Fig. 5 Profile of the power conversion efficiency (h) of a ZnPc-sensitized
TiO2 cell depending on the immersion time of the TiO2 electrode in
ZnPc–THF solution (0.05 mM).
Fig. 6 Photocurrent–voltage characteristics of a ZnPc-sensitized TiO2
cell (h = 0.57%, Jsc = 1.47 mA cm-2, Voc = 0.54 V, ff = 0.71). Conditions:
electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propyl imidazolium
iodide, and 0.5 M 4-t-butylpyridine in CH3CN; input power: AM 1.5 under
simulated solar light (100 mW cm-2). h = Jsc ¥ Voc ¥ ff.
Fig. 4 Profile of the surface coverage (C) of ZnPc on the TiO2 electrode
depending on the immersion time of the TiO2 electrode in a THF solution
of ZnPc (0.05 mM).
To judge the potential of H2Pc and ZnPc as a photosensitizer for
DSSC, we evaluated their cell performances using P25 TiO2 films.
A 10 mm thick TiO2 electrode was modified with H2Pc (0.05 mM)
which was dissolved in THF for an immersion time of 1–72 h.
The H2Pc-sensitized TiO2 cell showed no power conversion due to
the lower energy level of the H2Pc excited singlet state (-0.46 V
vs NHE) than that of the CB of the TiO2 (-0.5 V vs NHE).15
In contrast, the h values of the ZnPc-sensitized cell gradually
increased with increasing immersion time to reach maximum h
values (hmax) of ca. 0.6% for 12 h. The power conversion efficiency
(h) is derived from the equation: h = JSC ¥ VOC ¥ ff , where JSC
is the short circuit current, VOC is the open circuit potential, and
ff is the fill factor. Further increases of the immersion time of up
to 72 h exhibited no noticeable changes in the h values. The time
For the phthalocyanine-sensitized TiO2 cell, co-adsorbates such
as chenodeoxycholic acid have been employed to reduce the
tendency of dye aggregation on the TiO2 surface.6,7 For instance,
in a previous report Sundstro¨m et al. manufactured double
enhanced performance by introducing the co-adsorbates into the
immersion bath (h = 0.29% in the absence of the co-adsorbates
and h = 0.54% in the presence of the co-adsorbates).19 To further
improve the performance of the present cell, we also introduced
chenodeoxychloic acid (2.5 mM) to the ZnPc–THF solution
(0.05 mM), and prepared the phthalocyanine-sensitized TiO2 cell.
The cell, however, exhibited no apparent difference in the h
values with the cell prepared in the absence of chenodeoxycholic
acid under the same conditions (Fig. 7); h = 0.54
0.03%,
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©

Fig. 7 Profile of the power conversion efficiency (h) of a ZnPc-sensitized
TiO2 cell depending on the immersion time of the TiO2 electrode in
ZnPc–THF solution (0.05 mM) in the presence of chenodeoxycholic acid
(2.5 mM).
Jsc = 1.44 0.06 mA cm-2, Voc = 0.54 0.03 V, and ff = 0.70 0.03
for the cell with chenodeoxycholic acid. If there exist significant
aggregations of ZnPc on the TiO2 surface, the electron injection
yield would be considerably diminished by the accelerated decay
of the ZnPc excited singlet state, leading to a fair decrease in
the cell performance, especially in the short circuit current. No
apparent difference between the two cells with and without the
presence of chenodeoxycholic acid reveals that the performance
of our present cell is not perturbed by the well-known tendency of
phthalocyanine aggregation. We understood this phenomenon to
originate from the high steric hindrance of the ZnPc. Considering
that the degree of dye aggregation on the TiO2 surface is generally
increased along with prolonged immersion, this interpretation is
consistent with the parallel correlation between the time profile of
the h values and the C values (vide supra).
To investigate the photovoltaic response of the present cell in
more detail, we measured the photocurrent action spectra of a
ZnPc-sensitized TiO2 cell under the same conditions as for the
photocurrent–voltage characteristic measurements (Fig. 8a).20 To
a large extent, the photocurrent response follows the general
trend of the absorption feature of ZnPc/TiO2 (Fig. 8b), indicating
that the phthalocyanine is the main source of the photocurrent
generation. The maximal IPCE value at the near-infrared region
is measured to be 4.9%.
Fig. 8 (a) Photocurrent action spectra of a ZnPc-sensitized TiO2 cell.
The phthalocyanine-modified TiO2 electrode was prepared under the
same conditions as for the power conversion efficiency (h) measurements.
Conditions: electrolyte 0.1 M LiI, 0.05 M I2, 0.6 M 2,3-dimethyl-1-propyl
imidazolium iodide, and 0.5 M 4-t-butylpyridine in CH3CN; input power:
AM 1.5 under simulated solar light (100 mW cm-2). (b) UV-visible
absorption spectra of TiO2/ZnPc. The thickness of the TiO2 was adjusted
to be 700–1000 nm to obtain the shape and peak position of the spectra
accurately.
10-5 to 10-3 s.23,25 Therefore, the charge collection efficiency may
not be the limiting factor for the low photocurrent generation. The
quantum yield of electron injection is controlled by the competing
processes against electron injection such as intersystem crossing,
nonradiative decay, emission, and excited-state quenching; the
most important factor is, however, the driving force for electron
injection (DGinj) from the excited state dye to the CB of the
TiO2.3c From the solution electrochemistry and spectroscopy,
the DGinj for the ZnPc-sensitized TiO2 cell is determined to be
-0.34 eV. Apparently, it indicates that electron injection is a
thermodynamically feasible process. The energy level of the CB
is, however, not laid at a fixed point but is subject to change
depending on the operating conditions of the DSSCs. Mesoporous
TiO2 films have been known to show Nernstian shifts in their
CB level depending on the degree of surface protonation.26 The
shift of the CB of about 0.3 eV related to the changes in the
electrolyte composition is also reported.22 Besides, the level of the
CB would be raised to some degree due to the electron injection
itself. The oxidation potential of the dye, in addition, is positively
shifted by the chemical adsorption on the TiO2.18c,27 Thus, the
actual driving force for the ZnPc-sensitized cell is estimated to be
Considering the full coverage of the TiO2 surface and the high
molar extinction coefficient around the 700 nm region for ZnPc,
the low IPCE value of the ZnPc-sensitized TiO2 cell cannot be
explained by light-harvesting efficiency. The remaining two factors
are the quantum yield of the electron injection from the ZnPc
excited singlet state to the CB of the TiO2 electrode, and the
efficiency of charge collection.21 The charge collection efficiency
is determined primarily by the relative rate of charge transport
and charge recombination. In DSSCs, the injected electron can
be recombined with the resulting dye cation and I-/I3 redox
-
couple before going to the outer circuit.22 The charge transport
is reported to occur on the timescale of 10-7–10-5 s, whereas the
-
recombination between the electron and I-/I3 happens on the
timescale of 10-3–1 s.23,24 Although the time scale of the charge
recombination with the resulting dye cation is known to vary
depending on the electron density on TiO2, a typical range is from
5480 | Dalton Trans., 2008, 5476–5483
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©

smaller than the above-mentioned value. In such a case, electron
injection from the excited ZnPc to the CB of the TiO2 may occur
mainly or only through surface states because there exist little
available acceptor states for efficient electron injection.28 Besides,
the electronic coupling between the LUMO of the ZnPc and the
3d orbital of the TiO2 cannot be anticipated to be large enough
due to no apparent electron density on the carboxylic acid binding
groups together with the intervening phenyl moieties between the
ZnPc core and the carboxylic acid (vide supra). Thus, both the
small driving force and the weak electronic coupling would make
ZnPc an inefficient photosensitizer for DSSCs.
recorded on a Yanagimoto micro-melting point apparatus and
were not corrected.
Electrochemical measurements were made using a BAS 50 W
electrochemical workstation. Oxidation potentials in solution were
determined by differential pulse voltammetry (DPV) with a pulse
amplitude of 50 mV in Ar saturated N, N ¢-dimethylformamide
containing 0.1 M tetrabutylammonium perchlorate (TBAP) as
supporting electrolyte. A glassy carbon working electrode (3 mm
diameter), Ag/AgNO3 reference electrode, and Pt wire counter
electrode were employed. Ferrocene/ferrocenium (+0.642 V vs
NHE) was used as an internal standard for all measurements. The
measured potentials were quoted with reference to NHE.
Certainly, the performance of our present cell is not good
and is far from our expectations. From this study, however, we
can detect invaluable clues for devising effective phthalocyanine
photosensitizers for DSSC applications.
Synthesis
4,5-Bis(4-t-butylphenyl)phthalonitrile (1). Although this com-
pound has already been reported, the previous characterization is
not complete.29 A 100 mL round-bottomed flask was charged with
4,5-dichlorophthalonitrile (1.87 g, 9.5 mmol), 4-t-butylphenyl-
boronic acid (5.0 g, 28.1 mmol), palladium(II) acetate (44 mg,
0.2 mmol), 2-(2¢,6¢-dimethoxybiphenyl)dicyclohexylphosphine
(200 mg, 0.49 mmol), K3PO4 (8.48 g, 40 mmol), and anhydrous
toluene (25 mL). The solution was stirred at 90 ◦C for 2 h. After
cooling to room temperature, the reaction mixture was washed
twice with water. The combined organic layers were washed once
with water, subsequently dried over anhydrous magnesium sulfate,
and then concentrated in vacuo. The residue was dissolved in
hexane, and the solid material was obtained by precipitation.
Dissolution in and reprecipitation with cold hexane was repeated
until no solid material appeared. Reprecipitation of the combined
crude product with ethyl acetate–hexane afforded 1 as needle-
like off-white crystals (2.44 g, 6.22 mmol, 66% yield); mp 165.2–
166.9 C; H NMR (400 MHz, CDCl3) d 7.82 (s, 2H, phenyl
H), 7.28 (d, J = 8.3 Hz, 4H, phenyl H), 7.03 (d, J = 8.3 Hz,
4H, phenyl H), 1.29 (s, 18H, t-butyl H); 13C NMR (100 MHz,
CDCl3) d 151.77, 145.81, 135.54, 134.80, 129.01, 125.46, 115.56,
113.97, 34.64, 31.20; FT-IR (KBr) nmax 2964, 2905, 2869, 2233
(CN), 1735, 1608, 1589, 1484, 1462, 1363, 1268, 1113, 1015, 915,
835, 594, 569 cm-1; HRMS (EI positive) m/z calcd 392.2252 (for
C28H28N2), found 392.2249; elemental analysis (% calcd, % found
for C28H28N2) : C (85.67, 85.37), H (7.19, 7.30), N (7.14, 6.87).
Conclusions
Sterically hindered zinc phthalocyanine carboxylic acid (ZnPc)
and its metal free counterpart (H2Pc) were synthesized and
evaluated as photosensitizers for DSSC applications. The H2Pc-
sensitized TiO2 cell showed no photocurrent response due to its
low excited singlet state compared with the CB of TiO2. The ZnPc-
sensitized TiO2 cell displayed 0.57% of power conversion efficiency
(h) and 4.9% maximal IPCE value in the near-infrared region.
Introduction of chenodeoxycholic acid revealed no noticeable
change in the cell performance, showing that the aggregation of
ZnPc is effectively suppressed by steric hindrance. The moderate
cell performance can be rationalized by the small driving force for
electron injection from the excited-state ZnPc to TiO2, and the
poor electronic coupling between the LUMO of ZnPc and the CB
of TiO2. Thus, this study affords important basic information for
devising novel phthalocyanines for DSSC applications.
◦
1
Experimental
General
All solvents and chemicals were of reagent grade quality, pur-
chased, and used without further purification unless otherwise
noted. Column chromatography and thin-layer chromatography
(TLC) were performed with UltraPure Silica Gel (230–400 mesh,
4,5-Bis(4-methoxycarbonylphenyl)phthalonitrile (2). A 100 mL
round-bottomed flask was charged with 4,5-dichloro-
phthalonitrile (1.0 g, 5.1 mmol), 4-(methoxycarbonylphenyl)-
boronic acid (2.7 g, 15.0 mmol), palladium(II) acetate (22 mg,
0.1 mmol), 2-(2¢,6¢-dimethoxybiphenyl)dicyclohexylphosphine
(100 mg, 0.25 mmol), K3PO4 (4.24 g, 20 mmol), and anhydrous
toluene (20 mL). The solution was stirred at 90 ◦C for 15 h. After
cooling to room temperature, the reaction mixture was washed
twice with water. The combined organic layers were washed once
with water, subsequently dried over anhydrous magnesium sulfate,
and then concentrated in vacuo. Reprecipitation of the crude
product from ethyl acetate twice afforded 2 as plate-like off-white
crystals (1.20 g, 2.62 mmol, 60.6% yield); mp 225.6–227.1 ◦C;
1H NMR (300 MHz, CDCl3) d 7.96 (d, J= 8.4 Hz, 4H, phenyl
H), 7.89 (s, 2H, phenyl H), 7.17 (d, J = 8.4 Hz, 4H, phenyl H),
3.92 (s, 6H, ester H); 13C NMR (75 MHz, CDCl3) d 166.20,
144.86, 141.58, 135.39, 130.50, 130.00, 129.39, 115.31, 114.97,
52.38; FT-IR (KBr) nmax 3437, 2236 (CN), 1725, 1609, 1436, 1314,
1
SiliCycle) and Silica gel 60 F254 (Merck), respectively. H NMR
spectra were measured on a JEOL EX-400 (400 MHz) or a
Varian Unity 500 (500 MHz) spectrometer. 13C NMR spectra
were measured on a JEOL EX-400 (100 MHz) spectrometer. High-
resolution mass spectra (HRMS) were recorded on a JEOL JMS-
HX 110A spectrometer (FAB) using 3-nitrobenzylic alcohol as
a matrix or a JEOL JMS-700 MStation spectrometer (EI). Ma-
trix assisted laser desorption/ionization time-of-flight (MALDI-
TOF) mass spectra were made on a BRUKER Autoflex III using
CHCA (a-cyano-4-hydroxycinnamic acid) as matrix. UV-visible
absorption spectra were measured using a Perkin-Elmer Lambda
900 UV/vis/NIR Spectrometer. Steady-state fluorescence spectra
were acquired with a SPEX Fluoromax-3 Spectrofluorometer.
Spectroscopy grade tetrahydrofuran was used for the measure-
ments of UV-visible absorption and fluorescence spectra. FT-
IR spectra were acquired using a JASCO FT/IR-470 plus or a
FT/IR-4200 spectrometer with a KBr pellet. Melting points were
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 5476–5483 | 5481
©

1282, 1188, 1115, 1105, 1018, 861, 777, 713, 523 cm-1; HRMS (EI
positive) m/z calcd 396.1110 (for C26H16N2O4), found 396.1114;
elemental analysis (% calcd, % found for C26H16N2O4) : C (72.72,
72.79), H (4.07, 4.05), O (16.14, 16.29), N (7.07, 6.87).
green solid (103 mg). The solid showed two molecular ion
peaks at 1692.5 and 1748.6 with an intensity ratio of 1 : 3 in
the mass spectrum (MALDI-TOF). The peaks correspond to
2,3,9,10,16,17-hexakis(4-t-butylphenyl)-23-(4-methoxycarbonyl-
phenyl) - 24 - (4 - pentoxycarbonylphenyl ) phthalocyanatozinc(II),
and 2,3,9,10,16,17-hexakis(4-t-butylphenyl)-23,24-bis(4-pentoxy-
carbonylphenyl)phthalocyanatozinc(II), respectively. The mixture
was directly employed for the next reaction without further
purification.
To a solution of the previously collected green solid (103 mg) in
THF–methanol (2 : 1 (v/v), 30 mL) in a 100 mL round-bottomed
flask was added a 40% aqueous KOH solution (30 mL). The
solution was stirred at reflux for 5 h, cooled to room temperature,
and the organic solvent was removed under reduced pressure.
The pH of the reaction mixture was set to 4 by adding 6 M
HCl solution, and a precipitate was formed. The precipitate was
collected and washed with copius amounts of water and dried. The
collected solid material was suspended in chloroform (100 mL),
and stirred at reflux for 1 h. Filtering, and subsequent drying
under reduced pressure of the solid afforded ZnPc as a dark green
solid (52.6 mg, 4.6% overall yield for 3 steps with reference to 2
used); mp > 300 ◦C; 1H NMR (400 MHz, THF-d8) d 10.72 (s, 1H,
carboxy H), 9.47 (s, 2H, phenyl H (23,24)), 9.40 (s, 2H, phenyl H
(9,10)), 9.39 (s, 4H, phenyl H (2,3,16,17)), 8.10 (d, 4H, J = 8.3 Hz,
carboxyphenyl H), 7.72 (d, 4H, J = 8.3 Hz, carboxyphenyl H),
7.53 (skewed d, J = 8.3 Hz, 12H, carboxyphenyl H), 7.46 (skewed
d, J = 8.3 Hz, 12H, carboxyphenyl H), 1.41 (s, 36H, t-Bu H), 1.40
(s, 18H, t-Bu H); HRMS (FAB positive) m/z calcd 1608.6846 (for
C106H96N8O4Zn), found 1608.6816 (M + H)+.
2,3,9,10,16,17-Hexakis(4-t-butylphenyl)-23,24-bis(4-carboxy-
phenyl)phthalocyanine (H2Pc). A 200 mL round-bottomed flask
was charged with 1 (4.24 g, 10.8 mmol), 2 (1.43 g, 3.60 mmol),
1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) (0.3 g, 1.97 mmol),
and 1-pentanol (60 mL). The solution was stirred at reflux
for 20 h and cooled to room temperature. The solvent was
then removed under reduced pressure. Solid material was
obtained from methanol, and subjected to silica gel column
chromatography (chloroform–hexane
=
3 : 1). The second
fraction was collected and reprecipitated from methanol. Silica
gel column chromatography (chloroform–hexane = 1 : 5) of
the previously collected material afforded a dark green solid
(1.02 g). The solid showed two molecular ion peaks at 1630.7
and 1686.8 with an intensity ratio of 1 : 3 in the mass spectrum
(MALDI-TOF). The peaks correspond to 2,3,9,10,16,17-hexakis-
(4-t-butylphenyl)-23-(4-methoxycarbonylphenyl)-24-(4-pentoxy-
carbonylphenyl)phthalocyanine, and 2,3,9,10,16,17-hexakis(4-t-
butylphenyl)-23,24-bis(4-pentoxycarbonylphenyl)phthalocyanine,
respectively. The mixture was directly employed for the next
reaction without further purification.
To a solution of the previously collected dark green solid
(1.03 g) in THF–methanol (2 : 1 (v/v), 280 mL) in a 500 mL
round-bottomed flask was added a 40% aqueous KOH solution
(40 mL). The solution was stirred at reflux for 8 h, cooled to room
temperature, and the organic solvent was removed under reduced
pressure. The pH of the reaction mixture was set to 2 by adding
HCl solution (6 M), and a precipitate was formed. The precipitate
was collected and washed with copious amounts of water and
dried. The collected solid material was suspended in chloroform
(100 mL), and stirred at reflux for 1 h. Filtering, and subsequent
drying under reduced pressure of the solid afforded H2Pc as a dark
green solid (336 mg, 6◦.0% overall yield for 2 steps with reference
to 2 used); mp > 300 C; 1H NMR (400 MHz, THF-d8) d 10.83
(s, 1H, carboxy H), 9.18 (s, 6H, phenyl H (2,3,9,10,16,17)), 9.15
(s, 2H, phenyl H (23,24)), 8.08 (d, 4H, J = 8.3 Hz, carboxyphenyl
H), 7.63 (d, 4H, J = 8.3 Hz, carboxyphenyl H), 7.49 (d, 4H, J =
8.3 Hz, carboxyphenyl H), 7.43 (s, 20H, carboxyphenyl H), 1.43
(s, 36H, t-Bu H), 1.41 (s, 18H, t-Bu H), -1.55 (br s, 2H, inner
H); FT-IR (KBr) nmax 3431, 3296, 2962, 2903, 1716, 1698, 1609,
1506, 1499, 1446, 1435, 1395, 1363, 1315, 1292, 1269, 1105, 1011,
925, 835, 764, 727, 719 cm-1; HRMS (FAB positive) m/z calcd
1547.7745 (for C106H98N8O4), found 1547.7822 (M + H)+.
Density functional theory (DFT) calculations
Geometry optimization and electronic structure calculations of
the phthalocyanines were performed using the B3LYP functional
and 3-21G(d) basis set implemented in the Gaussian 03 program
package.30 Molecular orbitals were visualized by GaussView 3.0
software.
Preparation of phthalocyanine-modified TiO2 electrode and
photovoltaic measurements
Preparation of mesoporous TiO2 films and immobilization of
the phthalocyanine on the TiO2 surface, and characterization
of the photovoltaic properties of phthalocyanine-modified TiO2
were made by following the procedures reported previously.18
Tetrahydrofuran was used as an immersion solvent in the present
experiments instead of ethanol or methanol. The TiO2 electrodes
modified with H2Pc and ZnPc are denoted as TiO2/H2Pc and
TiO2/ZnPc, respectively. The amounts of phthalocyanine ad-
sorbed on the TiO2 films were determined by measuring the
changes in the absorbance of the phthalocyanine solutions (4 mL)
before and after immersion of the TiO2 films (0.25 cm2 of projected
area). All the experimental values were given as an average from
six independent measurements.
2,3,9,10,16,17-Hexakis(4-t-butylphenyl)-23,24-bis(4-carboxy-
phenyl)phthalocyanatozinc(II) (ZnPc). To a solution of the pre-
viously collected dark green solid (500 mg, mixture of the
2,3,9,10,16,17-hexakis(4-t-butylphenyl)-23-(4-methoxycarbonyl-
phenyl)-24-(4-pentoxycarbonylphenyl)phthalocyanine and 2,3,9,
10,16,17-hexakis(4-t-butylphenyl)-23,24-bis(4-pentoxycarbonyl-
phenyl)phthalocyanine) in 1-pentanol (100 mL) in a 300 mL
round-bottomed flask was added anhydrous zinc acetate (500 mg,
3.0 mmol). The solution was stirred at 130 ◦C for 3 h under
a nitrogen atmosphere. After cooling to room temperature, the
solvent was removed under reduced pressure. Silica gel column
chromatography (chloroform) of the crude product afforded a
Acknowledgements
This work was supported by Grant-in-Aid (No. 19350068 for
H.I.) from the Ministry of Education, Culture, Sports, Science
and Technology (MEXT), Japan and NEDO. We gratefully
5482 | Dalton Trans., 2008, 5476–5483
This journal is
The Royal Society of Chemistry 2008
©

acknowledge Prof. Susumu Yoshikawa (Kyoto University) and
Prof. Shozo Yanagida and Dr Naruhiko Masaki (Osaka Univer-
sity) for the use of the equipment for photovoltaic measurements.
Computation time was provided by the Academic Center for
Computing and Media Studies, Kyoto University. S. E. thanks the
International Doctoral Program in Engineering, Graduate School
of Engineering, Kyoto University, for financial support.
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E0 = Epeak + DE/2, where Epeak is the potential of peak maximum
This journal is
The Royal Society of Chemistry 2008
Dalton Trans., 2008, 5476–5483 | 5483
©