Broad-absorbing tetrabenzocorrolazine green photosensitizer for the enhanced conversion efficiency in dye-sensitized solar cells

Article abstract of DOI:10.1142/S108842461650098X

Novel tetrabenzocorrolazine green photosensitizer was designed and synthesized to enhance the solar energy-to-electricity conversion efficiency of dye-sensitized solar cells. This near-infrared absorbing dye exhibited superior short-circuit photocurrent density induced from the additional absorption compared to the phthalocyanine dye commonly applied as a photosensitizer covering the near-infrared region in dye-sensitized solar cells. The introduction of bulky peripheral groups and axial substituents to the novel dye led to the increase of open-circuit photovoltage. As a result, the novel tetrabenzocorrolazine dye achieved a solar energy-to-electricity conversion efficiency of 3.78% under AM 1.5G conditions which was much higher than that of the phthalocyanine reference dye.

Full text of DOI:10.1142/S108842461650098X

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2016; 20: 804–812
DOI: 10.1142/S108842461650098X
Published at http://www.worldscinet.com/jpp/
Broad-absorbing tetrabenzocorrolazine green
photosensitizer for the enhanced conversion efficiency
in dye-sensitized solar cells
Jun Choi*a, Woosung Leeb and Jae P. Kimc
a Human and Culture Convergence Technology Group, Korea Institute of Industrial Technology (KITECH),
Ansan-si, Gyeonggi-do 426-910, Republic of Korea
b ICT Textile & Apparel Group, Korea Institute of Industrial Technology (KITECH), Ansan-si, Gyeonggi-do 426-910,
Republic of Korea
c Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Republic of Korea
Received 2 April 2016
Accepted 28 April 2016
ABSTRACT: Novel tetrabenzocorrolazine green photosensitizer was designed and synthesized to
enhance the solar energy-to-electricity conversion efficiency of dye-sensitized solar cells. This near-
infrared absorbing dye exhibited superior short-circuit photocurrent density induced from the additional
absorption compared to the phthalocyanine dye commonly applied as a photosensitizer covering the
near-infrared region in dye-sensitized solar cells. The introduction of bulky peripheral groups and axial
substituents to the novel dye led to the increase of open-circuit photovoltage. As a result, the novel
tetrabenzocorrolazine dye achieved a solar energy-to-electricity conversion efficiency of 3.78% under
AM 1.5G conditions which was much higher than that of the phthalocyanine reference dye.
KEYWORDS: dye-sensitized solar cell, tetrabenzocorrolazine, ring-contraction reaction, bathochromic
shift, aggregation, phthalocyanine.
introduction of additional donors [8] or extension of
INTRODUCTION
the donor chromophore’s p-conjugation system [9] to
Dye-sensitized solar cells (DSSCs) have recently
have a broadened absorption spectrum for absorbing
attracted great attention because of their ease of
fabrication, lightweight, and cost-effectiveness compared
more light. V value can be improved by suppressing
oc
charge recombination between injected electrons on
with silicon (Si)-based photovoltaic devices [1]. Upon
the TiO2 surface and the holes in the electrolyte with
optical excitation, the dye injects an electron into the
prohibiting dye aggregation though the introduction of
conduction band of a nanocrystalline film of a wide
alkyl chains or bulky aromatic rings on the donor and/or
band gap oxide, such as titanium dioxide (TiO2), and
is subsequently regenerated back to the ground state by
the p-conjugation linker [10]. Overall, Jsc and V values
oc
can be improved by simple structural modifications of
electron donation from a redox couple present in the
the dye molecule.
electrolyte [2]. For higher efficiency, many sensitizers
Although Ru complex sensitizers (N3, N719 and
with high short-circuit photocurrent density (Jsc) values
and high open-circuit photovoltage (V ) values have been
black dye) have been reported to give high photoelectric
oc
conversion efficiencies of over 11% under AM 1.5
extensively investigated [3–7]. Jsc value can be improved
conditions [11], their use of rare and expensive metals
by the red-shift of the absorption region towards the
and the difficulty of obtaining purified dyes have limited
long wavelength end of the solar spectrum through the
their commercial applicability. On the other hand,
sensitizers without Ru such as metal-free organic dyes
or organometallic dyes are relatively inexpensive and
easily synthesized, purified, and modified. However, an
*Correspondence to: Jun Choi, email: skywork1@kitech.re.kr,
tel: +82 31-8040-6256, fax: +82 31-8040-6220
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BROAD-ABSORBING TETRABENZOCORROLAZINE GREEN PHOTOSENSITIZER
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issue with respect to these dyes is their weak absorption
Scheme 1. The detailed synthesis route of reference
dye PcS2, accomplished by the cyclization reaction
between phthalonitrile 1 and tert-butylphthalonitrile and
subsequently treated by NaOH in THF according to the
previously reported procedures [12], also appeared in
Scheme 1. The precursor of TBC2, the phthalonitrile
(2), was synthesized through a nucleophilic aromatic
substitution reaction between nitro phthalonitrile and
phenol with the bulky functional substituents, and its
coefficient of above 650 nm. Therefore, to achieve not
only the enhancement of the photovoltaic performance
but also the esthetic appreciation of the DSSCs, this near-
infrared (NIR) region absorbing green photosensitizer
should be developed. Because of their excellent light-
harvesting property with intense absorption throughout
the NIR region, various phthalocyanines (PCs) have been
designed and applied for DSSC. Recently, there was a
report of a high conversion efficiency (h = 4.6%) for
DSSCs using the unsymmetrical zinc phthalocyanine
dye having three bulky aryl groups and a carboxylic acid
group [12]. However, their lack of absorption in the visible
area (400–600 nm) and the natural tendency of forming
aggregates induced from planar molecular structure are
detrimental to solar energy-to-electricity conversion
efficiency. In order to overcome these limitations, we have
designed and developed a tetrabenzocorrolazine (TBC)
green sensitizer modified from PCs as shown in Fig. 1.
In addition to the conventional absorption range of PCs
(600–700 nm), they also showed prominent absorption
from 400 nm to 500 nm where the high solar photon
flux occurs. Such expanded light absorption led to the
higher Jsc value. In addition, their bulky peripheral groups
and the axial substituents introduced to the phosphorus
core metal prevented the formation of p–p stacks among
1
structure was confirmed by H NMR [13]. The metal-
free PC (3) was synthesized by the cyclization reaction
between the phthalonitrile 1 and twice the amount of
the phthalonitrile 2. Phenol was used as a cosolvent
instead of ethanol to prevent the transesterification. After
the cooling of the resulting slurry, the crude product
contained several kinds of metal-free PCs and this was
purified by column chromatography on silica gel using
dichloromethane as the eluent [14]. The band containing
a compound synthesized only from four phthalonitriles 2
could be separated firstly. Subsequently, the second band
containing bright green metal-free PC 3 was collected.
The TBC phosphorus compound was synthesized via a
ring-contraction reaction mediated by PBr3 in pyridine
in which a meso-nitrogen atom was extruded from an
appropriate metal-free PC precursor 3. In order to ensure
a good yield of TBC, the pyridine must be carefully
distilled over CaH2, and the phthalocyanine starting
material must be dried in a heated (50°C) vacuum
oven for several hours immediately prior to use [15].
After the ring-contraction reaction, the TBC compound
was successfully precipitated by quenching with a
MeOH/CH2Cl2 mixture to introduce the methoxy axial
substituent. Purification by silica gel chromatography
was carried out by using ethyl acetate/hexane solution
as the eluent. After the column chromatography, the
resulting compound was treated with an aqueous solution
of NaOH to hydrolyze the esteric part [12]. Purification
by silica gel chromatography was then carried out again
by using MeOH/CH2Cl2 solution as the eluent. The
TBC compound synthesized can theoretically have two
the molecules which led to the higher V value. These
oc
effects were investigated by the dye photophysical and
electrochemical properties, and the cells’ photovoltaic
performance. Electrochemical impedance spectroscopy
(EIS) was used to study their interfacial charge transfer.
RESULTS AND DISCUSSION
Synthesis
The novel TBC phosphorus compounds, TBC2, with
enhanced solubility and broadened absorption peak
for DSSC was designed and synthesized as shown in
Fig. 1. Molecular structures of PcS2, TBC2, and N719
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J. CHOI ET AL.
isomers as shown in Scheme 1, and their properties
were observed without further attempts to separate these
isomers in this work. The structure of the synthesized
TBC2 was confirmed by MALDI-TOF spectroscopy and
elemental analysis.
elsewhere, the absorption peak of the dye TBC2 had a
red-shifted Soret band and a slightly blue-shifted Q-band
compared to that of the dye PcS2 [16–18]. Therefore,
the absorption peak of the dye TBC2 became closer
to that of the panchromatic dye which is ideal for the
high photovoltaic performance of DSSC. In particular,
the conspicuous growth and bathochromic shift of
the Soret band induced from the conversion from PC
compound to TBC compound was extremely crucial for
exhibiting high solar energy-to-electricity conversion
efficiency. When it came to the aesthetic appreciation of
the DSSCs’ application, TBC dyes exhibited much more
Photophysical properties of the dyes
The absorption spectra of the synthesized dyes in
THF or CH2Cl2 solution and on TiO2 films are shown
in Figs 2(a) and 2(b), respectively. The dyes’ absorption
and emission properties are listed in Table 1. As reported
Scheme 1. Synthesis of PcS2 and TBC2
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Scheme 1. (Continued)
Table 1. Absorption and emission properties of PcS2 and TBC2
Dye
Absorption
max, M-1.cm-1
Emission
lmax
e
l
max (on TiO2)c
lmax
PcS2a
676 nm
240,530
311,582
662 nm
694 nm
TBC2b 452, 640 nm
423, 639 nm 676 nm
aAbsorptionandemissionspectraweremeasuredinTHFsolution
b
at room temperature (5 × 10-6 M). Absorption and emission
spectra were measured in CH2Cl2 solution at room temperature
(5 × 10-6 M). cAbsorption spectra were obtained through measur-
ing the dye absorbed onto TiO2 film.
vivid green colors compared with the PC dyes. Such a
bathochromic shift of the Soret band was likely to be the
result of the destabilization of the HOMO-1 (a2u) orbital
caused by the removal of a meso-nitrogen atom [15]. The
absorption maxima (lmax) of the Soret band of TBC2 was
at 452 nm, and that of the Q-band at 640 nm in CH2Cl2
solution. The molar extinction coefficient at lmax of TBC2
was 311,582 M-1.cm-1, and included in Table 1, which
was higher than that of PcS2. This was also desirable as
a photosensitizer for large photocurrent generation. On
TiO2 films, the absorption maxima (lmax) of PcS2 and
TBC2 were shown in Fig. 2(b). They exhibited blue-shifts
(14–29 nm) due to the H-aggregation of the dyes [19–21],
or deprotonation of carboxylic acid upon adsorption onto
the TiO2 surface [22]. However, these hypsochromic
shifts of the absorption spectra for the prepared dyes were
Fig. 2. UV-visible absorption spectra of (a) PcS2 in THF and
TBC2 in CH2Cl2, and (b) both on TiO2 films
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Table 2. Electrochemical properties of PcS2 and TBC2
Oxidation potential data a
Dye
b
Eox (=HOMO) (vs. NHE)
E0–0
Eox–E0–0 (=LUMO) (vs. NHE)
PcS2
0.95 eV
0.97 eV
1.81 eV
1.86 eV
-0.86 eV
-0.89 eV
TBC2
a Measured in CH2Cl2 containing 0.1 M tetrabutylammonium tetrafluoroborate
(TBABF4) as electrolyte (working electrode: glassy carbon; counter electrode: Pt;
reference electrode: Ag/Ag+; calibrated with ferrocene/ferrocenium (Fc/Fc+) as an
internal reference, and converted to NHE by addition of 630 mV [25]). b Estimated
from intersection wavelengths between absorption and emission spectra.
Fig. 3. CV and DPV (inset) curves of Fc/Fc+, PcS2, and TBC2 in CH2Cl2
smaller than those generally observed in other organic
dyes when dyes were attached to the TiO2 surface.
The HOMO level of the ring-contracted dye TBC2
was slightly more positive than that of the PcS2 while
the LUMO level of TBC2 was slightly more negative
than that of PcS2, resulting in the increase of the energy
band gap (Egap) of the HOMO-LUMO level. This may
be attributed to the blue-shifted Q-band of dye TBC2
compared with that of PcS2. Consequently, the dye TBC2
might act as a more efficient photosensitizer for DSSCs,
compared with PcS2 because its absorption spectra are in
the wavelength range where the higher solar photon flux
occurs [24].
Electrochemical properties of the dyes
The first oxidation potential (Eox), corresponding to
each dye’s HOMO level, was measured in CH2Cl2 by CV
and DPV. The excited state oxidation potential (Eox* =
Eox – E0–0), corresponding to each dye’s LUMO level, was
obtained by subtracting the zeroth–zeroth energy (E0–0),
estimated from the intersection between the normalized
absorption and emission spectra (Fig. S1) from Eox. The
electrochemical properties of the dyes and their CV with
DPV curves are shown in Table 2 and Fig. 3, respectively.
The HOMO levels of the prepared dyes ranged from 0.95
to 0.97 eV (vs. NHE) and were much more positive than
the redox potential of I-/I3- (0.4 V vs. NHE) [23]. The
LUMO levels of the dyes ranged from -0.86 to -0.89 eV
and were more negative than the conduction band energy
level of TiO2 (-0.5 V vs. NHE) [23], thus indicating
that the excited electrons of the dyes can be efficiently
injected into the conduction band of TiO2.
Photovoltaic properties of DSSCs
DSSCs were fabricated using the synthesized
dyes according to the methods described in the
experimental section, and their photovoltaic properties
were measured under standard AM 1.5G irradi-
ation conditions (100 mW.cm-2). The effects on cell
performance by the modifications of the dye structure
were assessed by measuring the conversion efficiencies.
The capabilities of the prepared dyes as possible
photosensitizers were compared with the N719 cell’s
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Table 3. Photovoltaic performance data of the cells with the prepared dyesa
Dyeb
Jsc, mA.cm-2
V , V
oc
FF, %
Area, cm2
IPCEmax, %
h, %
PcS2
TBC2
N719
3.27
7.08
0.56
0.65
0.72
0.71
0.69
0.71
0.283
0.298
0.278
21
59
72
1.30
3.78
7.09
13.34
a Measured under AM 1.5 irradiation G (100 mW.cm-1), with 0.278–0.298 cm2 working
area. b Dyes were maintained at 0.5 mM in CH2Cl2-acetonitrile (1:1) solution, with 10 mM
CDCA as co-adsorbent. Electrolyte comprised 0.7 M 1-methyl-3-propylimidazolium
iodide (MPII), 0.2 M LiI, 0.05 M I2, 0.5 M TBP in acetonitrile-valeronitrile (v/v, 85/15)
for organic dyes.
visible light to photocurrent at ca. 400–500 nm and
at ca. 600–700 nm due to the increased directional
electron-transfer property (pushing ability) of the
substituents, as well as the prevention of aggregation
[12]. The IPCE value of the cell with TBC2 reached the
highest value of 59% at 420 nm and 57% at 660 nm.
Although the dye TBC2 exhibited superior IPCE
values compared to the reference dye PcS2, its IPCE
did not exceed that of the dye N719 except the NIR
region (ca. 650–800 nm). This outcome was not very
comprehensible since this near-IR photosensitizer
had excellent absorption at ca. 400–500 nm (emax
=
311,582), as well as throughout the near-IR region
(emax = 180,924). This result could be attributed
to the poor adsorption of the dye TBC2 to the TiO2
surfaces because the steric hindrance induced by the
bulky peripheral groups and the axial substituents of
this dye inhibited close p–p aggregation between the
dye molecules, and may decrease the amount of dye
adsorbed [26].
The positive effects on the cell performance by the dye
structural modification were also appeared in J–V curves.
The ring-contracted dye TBC2 exhibited a much higher Jsc
value of 7.08 mA.cm-2 compared to the dye PcS2 owing
to the expanded light absorption in the wavelengths
where the high solar photon flux occurring as previously
mentioned. The higher molar extinction coefficient of the
dye TBC2 also affected the improvement of the Jsc value
of it. In case of open-circuit photovoltage, the dye TBC2
also exhibited higher V value of 0.65 V compared with
oc
the dye PcS2 owing to decreased molecular aggregation
by the introduction of bulky peripheral groups and the
axial substituents to the phosphorus core metal of it. The
suppressed dye aggregation prevented the recombination
between the injected electrons on TiO2 and the holes of
Fig. 4. (a) Incident photo-to-current conversion efficiency
(IPCE) spectra for DSSCs based on PcS2, TBC2, and N719
(b) Photocurrent-voltage curves for DSSCs based on PcS2,
TBC2, and N719 under illumination of AM 1.5G simulated
sunlight (100 mW.cm-2)
the electrolyte which raised the V value [10].
oc
Electrochemical impedance spectroscopy (EIS)
analysis [27] was employed to investigate the electron
recombination and interfacial charge transfer process in
DSSCs. The EIS spectra for the DSSCs based on PcS2
and TBC2 were performed under forward bias (-0.55 V)
in the dark and the Nyquist plots are shown in Fig. 5.
The major semicircles in the Nyquist plots indicate the
performance as listed in Table 3. Figures 4(a) and
4(b) show the incident photon-to-current conversion
efficiency (IPCE) spectra and photocurrent-voltage
(J–V) curves, respectively. Compared to the reference
dye PcS2, the dye TBC2 could effectively convert
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Analytical instruments and measurements
1H NMR spectra were recorded on a Bruker Avance
500 MHz (Seoul National University National Center for
inter-University Research Facilities) with the chemical
shifts against TMS. Matrix Assisted Laser Desorption/
Ionization Time Of Flight (MALDI-TOF) mass spectra
were collected on a Voyager-DE STR Biospectrometry
Workstation with a-cyano-4-hydroxy-cynamic acid
(CHCA) as the matrix. Elemental analysis was carried out
with a Flash EA 1112 CNH analyzer. UV-vis spectra and
photoluminescence spectra were recorded on a Hewlette
Packard 8452A spectrophotometer and Shimadzu
RF-5301PC spectrofluorometer, respectively. Cyclic
voltammetry (CV) and differential pulse voltammetry
(DPV) studies were performed using a three-electrode
cell with a 273A potentiostat (Princeton Applied
Research, Inc.). The measurement was carried out using
an Ag wire (Ag/Ag+), a glassy carbon and a platinum
wire as a reference, a working and a counter electrode,
respectively, in CH2Cl2 solution containing 0.1 M
tetrabutylammonium tetrafluoroborate as a supporting
electrolyte. A standard ferrocene/ferrocenium (Fc/Fc+)
redox couple was used to calibrate the oxidation peak.
DPV scan rate was 100 mV.s-1. Photocurrent–voltage
measurements were performed using a Keithley model
2400 source measure unit. Incident photon-to-current
conversion efficiency was measured as a function of
wavelength from 300 nm to 1000 nm using a specially
designed system for the DSSCs (PV Measurements, Inc.).
Electrical impedance spectra of DSSCs under dark with
-0.55 V forward bias were measured with an impedance
analyzer (Compactstat, IVIUM Tech) at frequencies of
10-1–106 Hz.
Fig. 5. Electrochemical impedance spectra (Nyquist plots) of
DSSCs based on PcS2 and TBC2 measured at -0.55 V forward
bias in the dark
charge transfer resistance at the TiO2/dye/electrolyte
interface. The radius of the major semicircle in the
Nyquist plot of TBC2 was larger than that of PcS2,
indicating that the electron recombination resistance
of TBC2 was bigger than that of PcS2 [28]. This
result coincided with the increase of V in the DSSCs
oc
based on these dyes as the suppression of electron
recombination between the injected electrons and the
electrolyte improved V .
oc
Consequently, the overall conversion efficiency value
of 3.78% achieved by the cell fabricated with TBC2
was much higher than that of the cell with the reference
dye PcS2. Without the co-adsorbent CDCA, the cell
with TBC2 showed the conversion efficiency value of
3.72%, which is not very distinctive. The co-adsorbent
CDCA did not play a crucial role since the attachment of
peripheral bulky units and methoxy axial groups already
prevented the stacking of the dye molecules. In addition
to that, the dye TBC2 was expected to be really suitable
to the co-sensitization in DSSC since there were plenty
of highly efficient sensitizers which absorbed light in the
range of 500–600 nm [2, 10, 29–31].
Fabrication of dye-sensitized solar cells
and measurements
The nanocrystalline TiO2 working electrode
comprised a TiO2 transparent layer (20 nm, synthesized)
and a TiO2 scattering layer (250 nm, G1). The Pt-coated
counter electrode was prepared by a reported procedure
[8b]. TiO2 electrodes were immersed in tert-butanol-
acetonitrile (1:1) solution containing the dyes at 0.5 mM
for 40 h at ambient temperature. They were then washed
with ethanol and dried under a stream of nitrogen. The
working and counter electrodes were sealed with Surlyn
(60 mm, Dupont) and the electrolyte was injected through
a hole in the counter electrode. The electrolyte comprised
0.7 M 1-methyl-3-propylimidazolium iodide (PMII),
0.2 M LiI (Aldrich), 0.05 M I2 (Aldrich), and 0.5 M
4-tert-butylpyridine (TBP, Aldrich) in a mixed solvent
of acetonitrile and valeronitrile (v/v, 85/18). The active
area of the dye-coated TiO2 film was 0.269–0.284 cm2
working area, measured by analyzing images from a
CCD camera (moticam 1000). TiO2 film thickness was
measured by an a-step surface profiler (KLA tencor).
EXPERIMENTAL
Materials and reagents
1,8-Diazabicyclo-7-undecene (DBU), 4-nitrophthalo-
nitrile, and 2,5-bis-(1,1-dimethylbutyl)-methoxyphenol
from TCI and 4-iodophthalonitrile, 4-(methoxy carbonyl
phenyl) boronic acid, Pd(PPh3)4, NaCO3, tert-butyl-
phthalonitrile, ZnCl2, NaOH, K2CO3, Li, NaCl, pyridine,
PBr3 and HCl from Sigma-Aldrich were purchased and
used as received without further purification. All the other
reagents and solvents were of reagent-grade quality and
obtained from commercial suppliers.
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Synthesis of dyes
triazatetrabenzcorrole having an ester in THF (9 mL).
The mixture was stirred at 70°C for 24 h. The solvent
Phthalonitrile 1 were accomplished by the Suzuki
coupling reaction between 4-iodo phthalonitrile and
4-(methoxy carbonyl phenyl) boronic acid or 3-(methoxy
carbonyl phenyl) boronic acid according to the previously
reported procedures [12].
PcS2 were accomplished by the cyclization reaction
between phthalonitrile 1 and tert-butylphthalonitrile and
subsequently treated by NaOH in THF according to the
previously reported procedures [12].
was removed in vacuo and the residue was dissolved
in aqueous solution of HCl (1.0 M, 20 mL) and 20 mL
of CH2Cl2. The resulting solution was extracted and
washed with saturated NaCl solution. After removal of
the solvent, the crude product was purified on a silica gel
column with 1/1 CH2Cl2/MeOH as the eluent to afford the
target compound as a bright green solid (0.24 g, 20.19%).
MALDI-TOF MS: m/z 1582.25 (100%, [M]+). Found C,
73.97; H, 7.49; N, 6.31. Calcd. for C98H116N7O10P: C,
74.36; H, 7.39; N, 6.19.
Preparation of 4-(2,5-Bis(1,1-dimethylbutyl)-4-
methoxyphenoxy)phthalonitrile (2). 4-Nitrophthalo-
nitrile (1 g, 5.77 mmol) and 2,5-bis-(1,1-dimethylbutyl)-
methoxyphenol (1.68 g, 5.77 mmol) were dissolved
in 30 mL of dry DMF and anhydrous K2CO3 (1.06 g,
7.66 mmol) was added in portions during 4 h. The mixture
was stirred at 80°C for 8 h under nitrogen atmosphere.After
filtering the reaction mixture, the residue was extracted
with CH2Cl2 and dried by rotary evaporation.After removal
of the solvent, the crude product was purified on a silica gel
column (CHCl3) to afford the target compound as a light
brown solid (2.13 g, 88.47%). 1H NMR (500 MHz, CDCl3,
25°C, TMS): d, ppm 7.69 (d, 1H), 7.22 (s, 1H), 7.14 (d,
1H), 6.82 (s, 1H), 6.63 (s, 1H), 3.08 (s, 3H, –O–CH3), 1.71
(m, 2H), 1.56 (m, 2H), 1.26 (d, 12H), 1.04 (m, 2H), 0.96
(m, 2H), 0.81 (t, 3H), 0.71 (t, 3H).
Preparation of unsymmetrical metal-free phthalo-
cyanine (3). Phthalonitrile 1 (0.3 g, 1.08 mmol) and
phthalonitrile 2 (0.9 g, 2.15 mmol) was dissolved in
50 mL of anhydrous xylene and 10 mL of phenol under
nitrogen atmosphere and Li (0.16 g, 22.63 mmol) was
added in the solution. The reaction mixture was stirred
at 150°C for 5 h. After cooling the solution, the resulting
slurry was extracted with 100 mL of CH2Cl2 and washed
with saturated NaCl solution. After removal of the
solvent, the crude product was purified on a silica gel
column (CH2Cl2) to afford the target compound as a
bright green solid (0.61 g, 55.60%). MALDI-TOF MS:
m/z 1520.40 (100%, [M]+). Found C, 76.37; H, 7.60; N,
7.35. Calcd. for C97H114N8O8: C, 76.65; H, 7.56; N, 7.37.
Preparation of unsymmetrical phosphorus triaza-
tetrabenzcorrole (TBC2). Phthalocyanine 3 (1.12 g,
0.74 mmol) was placed in a 100 mL flask equipped with
a condenser and gas inlet adapter and dissolved in 30 mL
of pyridine. An amount of PBr3 (9.0 g, 33.0 mmol) was
added and the resulting solution heated to 120°C and
stirred for 2 h.After 2 h, the volume of the reaction mixture
was reduced by vacuum distillation to 1–2 mL and then
treated with a 50/50 solution of CH2Cl2/MeOH for 2 h.
The resulting green solid was dissolved in CH2Cl2 and
filtered to remove excess pyridinium bromide as a white
crystalline solid. The filtrate was reduced in volume under
reduced pressure and loaded onto a silica gel column for
purification with 1/7 ethyl acetate/hexane as the eluent.
An aqueous solution of NaOH (1.0 M, 2 mL) was
added to the solution of unsymmetrical phosphorus
CONCLUSION
Novel TBC green sensitizer was designed and
synthesized to investigate the effects of an expanded
absorption range induced from the ring-contraction
reaction and an anti-aggregation by the bulky peripheral
groups and the axial substituents on the performance of
DSSCs. To the best of the authors’ knowledge, this is the
first report of applying TBC moieties exhibiting vivid
green colors as a sensitizer of DSSCs.
As we expected, the ring-contraction reaction of
phthalocyanine precursor made its Soret band largely
bathochromic-shifted and its Q-band hipsochromic-
shifted. The additional strong absorption from 400 nm
to 500 nm of tetrabenzocorrolazine dye TBC2 induced
increase of its Jsc and IPCE compared with the phthalo-
cyanine dye PcS2. In addition to that, the introduction of
bulky peripheral groups and axial substituents at the core
metal of phthalocyanine triggered decreased molecular
aggregation which led to the increased V . The radius of
oc
the semicircle in the intermediate frequency regime of the
Nyquist plot of TBC2 was bigger than that of PcS2 which
coincided with the trend of V values.
oc
As a result, the superior solar energy-to-electricity
conversion efficiency was achieved with TBC2, which
had the h value of 3.78% under AM 1.5G irradiation.
However, the fabrication of the DSSCs with these
sensitizers should be thoroughly optimized since all the
conversion efficiencies of the cells even with the dye N719
(h = 7.09%) were not high enough as reported elsewhere.
REFERENCES
1. O’Regan B and Grätzel M. Nature 1991; 353:
737–740.
2. Yella A, Lee H-W, Tsao HN, Yi C, Chandiran
AK, Nazeeruddin MdK et al. Science 2011; 334:
629–634.
3. Koumura N, Wang Z-S, Miyashita M, Uemura Y,
Sekiguchi H, CuiY et al. J. Mater. Chem. 2009; 19:
4829–4836.
4. Wang Z-S, CuiY, Dan-ohY, Kasada C, ShinpoA and
Hara K. J. Phys. Chem. C 2007; 111: 7224–7230.
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 811–812

812
J. CHOI ET AL.
5. Kim S, Lee JK, Kang SO, Ko J, Yum J-H, Fantacci
S et al. J. Am. Chem. Soc. 2006; 128: 16701–16707.
6. Ito S, Miura H, Uchida S, Takata M, Sumioka K and
Liska P. Chem. Commun. 2008; 41: 5194–5196.
7. Tang J, Wu W-J, Hua JH, Li X and Tian H. Energy
Environ. Sci. 2009; 2: 982–990.
8. (a) Tian H, Bora I, Jiang X, Gabrielsson E, Karlsson
KM, Hagfeldt A et al. J. Mater. Chem. 2011; 21:
12462–12472. (b) Park SW, Son K-I, Ko MJ, Kim K
and Park N-G. Synth. Met. 2009; 159: 2571–2577.
9. Kim SH, Kim HW, Sakong C, Namgoong J, Park
SW, Ko MJ et al. Org. Lett. 2011; 13: 5784–5787.
10. Chang YC et al. Chem. Commun. 2011; 47:
8910–8912.
11. (a) Grätzel M. J. Photochem. Photobiol. C 2003;
4: 145–153. (b) Grätzel M. J. Photochem. Photo-
biol. A 2004; 164: 3–14. (c) Park N-G and Kim K.
Physica. Status Solidi. A 2008; 205: 1895–1904.
12. Mori S, Nagata M, Nakahata Y, Yasuta K, Goto R,
Kimura M and Taya M. J. Am. Chem. Soc. 2010;
132: 4054–4055.
13. Ağritaş MS. Dyes. Pigm. 2007; 74: 490–493.
14. Cheng G, Peng X, Hao G, Kennedy VO, Ivanov IN,
KnappenbergerK,HillTJ,RodgersMAJandKennedy
ME. J. Phys. Chem. A 2003; 107: 3503–3514.
15. Fox JP and Goldberg DP. Inorg. Chem. 2003; 42:
8181–8191.
19. Chen R, Yang X, Tian H and Sun L. J. Photochem.
Photobiol. A 2007; 189: 295–300.
20. Tian H, Yang X, Chen X, Pan Y, Li L, Hagfeldt A,
et al. Chem. Commun. 2007; 36: 3741–3743.
21. Hara K, Wang Z-S, Sato T, Furube A, Katoh R,
Sugihara H et al. J. Phys. Chem. B 2005; 109:
15476–15482.
22. Lin L-Y, Tsai C-H, Wong K-T, Huang T-W, Hsieh L,
Liu S-H et al. J. Org. Chem. 2010; 75: 4778–4785.
23. Hagfeldt A and Grätzel M. Chem. Rev. 1995; 95:
49–68.
24. Sun H, Biedermann L and Bond TC. Geophys. Res.
Lett. 2007; 34: L17813–17817.
25. Hagberg DP, Edvinsson T, Marinado T, Boschloo
G, Hagfeldt A and Sun L. Chem. Commun. 2006:
2245–2247.
26. Tan L-L, Chen H-Y, Hao L-F, Shen Y, Xiao L-M,
Liu J-M et al. Phys. Chem. Chem. Phys. 2013; 15:
11909–11917.
27. (a) Bisquert J. Phys. Chem. Chem. Phys. 2003; 5:
5360–5364. (b) Wang Q, Moser J-E and Grätzel M.
J. Phys. Chem. B 2005; 109: 14945–14953.
28. (a) Kern R, Sastrawan R, Ferber L, Stangl R and
Luther J. Electrochimica. Acta. 2002; 47: 4213–4225.
(b) Kuang D, Ito S, Wenger B, Klein C, Moser JE,
Humpry-Baker R et al. J. Am. Chem. Soc. 2006; 128:
4146–4154.
16. Fujiki M, Tabei H and Isa K. J. Am. Chem. Soc.
1986; 108: 1532–1536.
17. Liu J, Zhang F, Zhao F, Tang Y, Song X and Yao G.
Photochem. Photobiol. 1995; 91: 99–104.
18. Li J, Subramanian LR and Hanack M. Eur. J. Org.
Chem. 1998; 12: 2759–2767.
29. Bessho T, Zakeeruddin SM, Yeh CY, Diau EWG
and Grätzel M. Angew. Chem. Int. Ed. 2010; 49:
6646–6649.
30. Hsieh CP et al. J. Mater. Chem. 2010; 20:
1127–1134.
31. Wu SL, et al. Energy Environ. Sci. 2010; 3: 949–955.
Copyright © 2016 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2016; 20: 812–812