Bisquinoxaline-fused porphyrins for dye-sensitized solar cells

Article abstract of DOI:10.1002/cssc.201100029

5,10,15,20-Tetrakis(2,4,6-trimethylphenyl)-6′-carboxylquinoxalino[2, 3-b]quinoxalino[12,13-b′]porphyrinatozinc(II) (ZnPBQ) is synthesized to evaluate the effects of π elongation of quinoxaline-fused porphyrins on the optical, electrochemical, and photovol

Full text of DOI:10.1002/cssc.201100029

DOI: 10.1002/cssc.201100029
Bisquinoxaline-Fused Porphyrins for Dye-Sensitized Solar
Cells
Hiroshi Imahori,*[a, b, c] Hiroaki Iijima,[b] Hironobu Hayashi,[b] Yuuki Toude,[b]
Tomokazu Umeyama,[b, d] Yoshihiro Matano,[b] and Seigo Ito[e]
5,10,15,20-Tetrakis(2,4,6-trimethylphenyl)-6’-
TiO2 double layers was used. The ZnPBQ cell exhibited a rela-
tively high power conversion efficiency (h) of 4.7%, which was
smaller than that of the ZnPQ cell (h=6.3%). The weaker elec-
tronic coupling between the LUMO of ZnPBQ and conduction
band (CB) of TiO2 or more tilted geometry of ZnPBQ on the
TiO2 surface may result in the low electron injection/charge
collection efficiency as well as the low incident photon-to-cur-
rent efficiency (IPCE) for the ZnPBQ cell (maximum IPCE=
56%) relative to the ZnPQ cell (maximum IPCE=75%), leading
to the lower h value of the ZnPBQ cell than that of the ZnPQ
cell. In addition, the open-circuit potential of the ZnPBQ cell
carboxylquinoxalino[2,3-b]quinoxalino[12,13-b’]porphyrinato-
zinc(II) (ZnPBQ) is synthesized to evaluate the effects of p elon-
gation of quinoxaline-fused porphyrins on the optical, electro-
chemical, and photovoltaic properties. ZnPBQ showed an in-
tensified Soret band as well as red-shifted Soret
and Q bands relative to 5,10,15,20-tetrakis(2,4,6-trimethylphe-
nyl)-6’-carboxylquinoxalino[2,3-b]porphyrinatozinc(II)
(ZnPQ),
demonstrating the improved light-harvesting property of
ZnPBQ. The optical and electrochemical HOMO–LUMO gaps
were consistent with those estimated by DFT calculations. The
photovoltaic properties were compared under optimized con-
ditions, in which a sealed device structure with TiCl4-treated,
also slightly decreased with the effect of charge recombination
ꢀ
from the electrons injected into the CB of TiO2 to I3
.
Introduction
Recently solar cells have attracted much attention because of
the potential prospect to solve energy and environmental
issues. In this regard, several forms of organic material based
solar cells have been examined as alternatives to conventional
silicon-based solar cells. One of the most promising technolo-
gies are dye-sensitized solar cells (DSSCs), which have low-cost
production as well as high power conversion efficiencies (h).[1]
Ruthenium polypyridyl complexes have been utilized as TiO2
sensitizers, which have h values of >11 %.[2] However, owing to
the limited availability and high cost of ruthenium metal,
metal-free organic chromophores[3] and inexpensive metal
complexes[4] for DSSCs are highly desirable. So far various dyes
such as coumarin,[5] indoline,[6] polyene,[7] thiophene,[8] cya-
nine,[9] hemicyanine,[10] squaraine,[11] phthalocyanine,[12] pery-
lene,[13] and tetrathiafulvalene[14] derivatives have been devel-
oped as potential sensitizers for DSSCs.
We have proposed that unsymmetrical p elongation of por-
phyrins is a promising strategy to improve the light-harvesting
property.[4a,20] Specifically, quinoxalino[2,3-b]porphyrin acid
(ZnPQ) showed broadened, redshifted, and amplified light-ab-
sorption properties than those of 5-(4-carboxyphenyl)-10,15,20-
tris(2,4,6-trimethylphenyl)porphyrinatozinc(II) (ZnP), leading to
the higher h value of 6.3% for the ZnPQ cell[4a,20g] than that of
the ZnP cell (4.4%).[20f] Therefore, quinoxaline-fused porphyrins
are highly attractive as excellent sensitizers for DSSC. Herein,
[a] Prof. H. Imahori
Institute for Integrated Cell-Material Sciences (iCeMS)
Kyoto University, Nishikyo-ku, Kyoto 615-8510 (Japan)
Fax: (+81)75-383-2571
E-mail: imahori@scl.kyoto-u.ac.jp
[b] Prof. H. Imahori, H. Iijima, H. Hayashi, Y. Toude, Prof. T. Umeyama,
Prof. Y. Matano
Porphyrins have also been used as sensitizers for DSSCs due
to their strong Soret (400–450 nm) and moderate Q bands
(550–600 nm).[15–21] Nevertheless, until recently, the cell per-
formances of porphyrin DSSCs were much lower than those of
ruthenium DSSCs as a result of the poor light-harvesting prop-
erty of typical porphyrins at 450–550 nm and >600 nm. To
overcome this drawback, the perturbation of the porphyrin p
system is an effective methodology to tailor the optical, photo-
physical, and electrochemical properties. For instance, a push–
pull porphyrin with an electron-donating diarylamino group at
the meso position and an electron-withdrawing carboxypheny-
lethynyl anchoring group at the opposite meso position led to
the remarkably improved h value of up to 11%.[19g]
Department of Molecular Engineering
Graduate School of Engineering, Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
[c] Prof. H. Imahori
Fukui Institute for Fundamental Chemistry
Kyoto University, Sakyo-ku, Kyoto 606-8103 (Japan)
[d] Prof. T. Umeyama
PRESTO, Japan Science and Technology Agency (JST)
4-1-8 Honcho, Kawaguchi, Saitama 332-0012 (Japan)
[e] S. Ito
Department of Electrical Engineering and Computer Sciences
Graduate School of Engineering, University of Hyogo
2167 Shosha, Himeji, Hyogo 671-2280 (Japan)
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100029.
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H. Imahori et al.
we report the first synthesis and the optical, electrochemical,
and photovoltaic properties of bisquinoxaline-fused porphyrin
(ZnPBQ) for DSSCs. The fundamental properties of ZnPBQ are
compared with those of ZnPQ and ZnP. We expected that
ZnPBQ would exhibit broadening and redshifting of the ab-
sorption band compared with that of ZnPQ due to further p
elongation by an additional fused quinoxaline unit relative to
ZnPQ, which would affect the cell performances. 2,4,6-Trime-
thylphenyl groups were introduced to increase solubility and
to reduce aggregation on TiO2.[21]
Figure 1. UV-visible absorption spectra of ZnPBQ (solid line), ZnPQ[20g]
(dashed line), and ZnP[20f] (dotted line) measured in dichloromethane.
bands of ZnPBQ are broadened, the absorption at 450–500 nm
is intensified relative to ZnPQ and ZnP. Additionally, the Q
bands are also redshifted in comparison with ZnPQ and ZnP.
The absorption properties of ZnPBQ are beneficial for harness-
ing solar energy in the visible region.
The steady-state fluorescence spectrum of ZnPBQ was mea-
sured in dichloromethane and compared with those of
ZnPQ[20g] and ZnP[20f] by exciting at the strongest positions of
the Soret bands (Figure 2). The wavelengths for emission
maxima are listed in Table 1. In accordance with the trend of
the longest-wavelength absorption maxima, the emission
maxima of ZnPBQ are redshifted relative to those of ZnPQ and
Results and Discussion
Synthesis
ZnP. From the intersection of the normalized absorption and
[20]
emission spectra, the zero–zero excitation energies (E0-0
)
are
The synthetic route to prepare ZnPBQ is illustrated in
Scheme 1. Synthetic precursors were prepared from
porphyrin dione 1.[20d] Condensation of 1 with 1,2-di-
aminobenzene gave quinoxalinoporphyrin 2 in 94%
yield.[20d,22] Acetoxylation of 2 occurred at the 12 posi-
tion to afford 3 in 48% yield.[23] Porphyrin acetate 3
was deprotected with potassium carbonate in metha-
nol to give 12-hydroxyl-5,10,15,20-tetrakis(2,4,6-
trimethylphenyl)quinoxalino[2,3-b]porphyrin, which
was then oxidized to the chlorin-a-dione 4 using the
Dess–Martin periodinane (DMP), giving 41% for the
two steps.[23] The condensation of porphyrin 4 with
methyl diaminobenzoate in pyridine afforded bisqui-
noxaline-fused porphyrin 5.[20d,22] Treatment of 5 with
zinc acetate, followed by hydrolysis, gave ZnPBQ.
Structures of all of the new compounds were verified
1
by H NMR and FTIR spectroscopy, and high-resolu-
tion mass spectrometry.
Optical and electrochemical properties
Figure 1 shows the UV-visible absorption spectra of
ZnPBQ, ZnPQ,[20g] and ZnP[20f] in dichloromethane.
The positions and molar absorption coefficients (e) of
Scheme 1. Synthesis of ZnPBQ. 1) 1,2-Diaminobenzene, pyridine, 1108C, 3 h, 94%;
the Soret and Q bands are listed in Table 1. The Soret
band of ZnPBQ is split into two signals, each of
which is blue- and redshifted compared with the cor-
2) silver acetate, iodine, dichloromethane, 3 h, 48%; 3) i) K2CO3, methanol, dichlorome-
thane, 3 h; ii) DMP, dichloromethane, 4 h, 41%; 4) methyl 3,4-diaminobenzoate, pyridine,
110 8C, 11 h, 93%; 5) i) Zn(OAc)2·2H2O, 1-butanol, tetrachloromethane, reflux, 2 days;
responding signals of ZnPQ. Although the Soret ii) THF, 2m NaOH (aq), reflux, 16 h, 84%.
798
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Bisquinoxaline-Fused Porphyrins
of ZnPBQ (ꢀ0.98 V vs. NHE) is
shifted in a positive direction by
0.15 V relative to ZnPQ (ꢀ1.13 V
vs. NHE).[20g] This implies that un-
symmetrical p elongation by the
additional electron-withdrawing
Table 1. Optical and electrochemical data for the porphyrins studied and driving forces for electron-transfer
processes on TiO2.
[b]
[c]
[c]
[e]
[f]
Sample
ZnPBQ
labs[a] [nm]
lem
Eox
Ered
[V]
E0-0
[eV]
Eox*[d]
[V]
DGinj
[eV]
DGreg
[eV]
(e [103 mꢀ1 cmꢀ1])
[nm]
[V]
402 (125)
465 (146)
560 (10.7)
612 (12.7)
637 (11.5)
662 (38.9)
415 (184)
458 (56.2)
578 (13.6)
622 (12.6)
422 (431)
551 (23)
667
737
0.97
ꢀ0.98
1.86
ꢀ0.89
ꢀ0.39
ꢀ0.47
quinoxaline
moiety
affects
mainly the LUMO of the porphy-
rin. The stabilization of the
LUMO relative to the HOMO
leads to a decrease in the elec-
trochemical HOMO–LUMO gaps.
The electrochemical HOMO–
LUMO gap of ZnPBQ is deter-
mined to be 1.95 eV, which is
considerably smaller than those
of ZnPQ (2.11 eV)[20g] and ZnP
(2.33 eV).[20f] The trend largely
agrees with that of the optical
HOMO–LUMO gaps (see above).
From the spectroscopic and
ZnPQ[g]
ZnP[h]
645
0.98
1.00
ꢀ1.13
ꢀ1.33
1.98
2.10
ꢀ1.00
ꢀ1.10
ꢀ0.50
ꢀ0.60
ꢀ0.48
ꢀ0.50
600
650
592 (4)
[a] Wavelengths for Soret and Q bands maxima in CH2Cl2. [b] Wavelengths for emission maxima in CH2Cl2 by ex-
citing at the Soret wavelength. [c] Ground-state redox potentials (vs. NHE). [d] Excited-state oxidation potentials
approximated from Eox and E0-0 (vs. NHE). [e] Driving forces for electron injection from the porphyrin singlet ex-
cited state (Eox*) to the conduction band (CB) of TiO2 (ꢀ0.5 V vs. NHE). [f] Driving forces for the regeneration of
porphyrin radical cation (Eox) by Iꢀ/I3ꢀ redox couple (+0.5 V vs. NHE). [g] From ref. [20g]. [h] From ref. [20f].
electrochemical measurements, driving forces for electron in-
jection from the porphyrin excited singlet state to the CB of
the TiO2 (ꢀ0.5 V vs. NHE)[20] (DGinj) and the regeneration of the
ꢀ
porphyrin radical cation by the Iꢀ/I3 redox couple (+0.5 V vs.
NHE)[20] (DGreg) for the porphyrin-sensitized solar cells are eval-
uated (Table 1). Both of the processes are sufficiently exother-
mic, ensuring efficient electron-transfer reactions.
DFT calculations
DFT calculations were employed to gain insight into the equi-
librium geometry and electronic structures for the frontier orbi-
tals of the porphyrins. The calculated structures did not show
negative frequencies, implying that the optimized geometries
were in the global energy minima.[27] It is noteworthy that
ZnPBQ possesses a planar structure, whereas ZnPQ exhibits a
saddle structure[20d] (see Figure S1 in the Supporting Informa-
tion).
Figure 2. Steady-state fluorescence spectra of ZnPBQ (solid line), ZnPQ[20g]
(dashed line), and ZnP[20f] (dotted line) measured in dichloromethane. The
spectra are normalized for comparison.
determined to be 1.86 eV for ZnPBQ, which is smaller than the
E0-0 values of 1.98 eV for ZnPQ[20g] and 2.10 eV for ZnP[20f]
(Table 1). The fluorescence lifetime (t) of ZnPBQ was obtained
in dichloromethane by a time-correlated single photon count-
ing technique.[24] The excitation wavelength was 405 nm and
the fluorescence was monitored at the strongest emission
maximum for each porphyrin. The decay curve of the fluores-
cence intensity was fitted as a single exponential to give t=
(1.00ꢁ0.01) ns for ZnPBQ, which is comparable to t=(0.99ꢁ
0.01) ns for ZnPQ,[20g] but shorter than that of t=(2.16ꢁ
0.01) ns for ZnP.[20f] Because electron injection processes from
the excited dyes to a CB of TiO2 are known to take place on a
timescale of 0.1–100 ps,[25,26] the relatively slow fluorescence
decay of ZnPBQ may have little influence on the electron injec-
Porphyrins with D4h symmetry generally reveal two energeti-
cally degenerate LUMOs (LUMO+1 and LUMO) and two nearly
degenerate HOMOs (HOMO and HOMOꢀ1). The degenerate
energy levels of LUMO and LUMO+1 are split in both ZnPBQ
and ZnPQ[20g] (Table 2). Although the degenerate energy levels
of HOMO and HOMOꢀ1 are split significantly in ZnPBQ, the
near degeneracy of HOMO and HOMOꢀ1 is retained for ZnPQ.
A variation in energy by structural modification for the LUMO
of ZnPBQ (0.21 eV vs. ZnPQ) is larger than that for the HOMO
of ZnPBQ (0.08 eV vs. ZnPQ). The HOMO–LUMO gaps of the
three porphyrins are as follows: ZnP (2.98 eV)[20f] >ZnPQ
(2.69 eV)[20g] >ZnPBQ (2.40 eV). This trend agrees well with
those of the optical and electrochemical HOMO–LUMO gaps
(see below).
tion efficiency (f ).
inj
To determine the first oxidation potential (Eox) and the first
reduction potential (Ered) of ZnPBQ in solution, differential
pulse voltammetry measurements were performed in dichloro-
methane containing 0.1m Bu4NPF6 as a supporting electrolyte
(Table 1). The Eox value of ZnPBQ (0.97 V vs. NHE) is virtually
identical to that of ZnPQ (0.98 V vs. NHE),[20g] but the Ered value
Figure 3 illustrates the electron density distributions of
ZnPBQ and ZnPQ in their respective HOMOs and LUMOs. The
electron density distribution of LUMOs around an anchoring
group is known to influence the electronic coupling between
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H. Imahori et al.
amounts of porphyrins adsorbed on the TiO2 films were deter-
mined by measuring the changes in the absorbance of the
porphyrin solutions before and after immersion of the TiO2
films. After immersion, ZnPBQ in EtOH[29] reached almost satu-
rated surface coverage (G) on the TiO2 films in 4 h, whereas
ZnPQ in MeOH showed saturation in 0.5 h (Figure 4); the trend
Table 2. Molecular orbital energy levels for porphyrins.[a]
ZnPBQ
ZnPQ
ZnP
LUMO+1
LUMO
ꢀ2.31
ꢀ2.77
ꢀ2.23
ꢀ2.56
ꢀ2.17
ꢀ2.21
(ꢀ3.46)[b]
ꢀ5.17
(ꢀ3.31)[b]
ꢀ5.25
(ꢀ3.11)[b]
ꢀ5.19
HOMO
(ꢀ5.41)[b]
ꢀ5.36
(ꢀ5.42)[b]
ꢀ5.28
(ꢀ5.44)[b]
ꢀ5.31
HOMOꢀ1
[a] Some sets of molecular orbital energy levels for porphyrins were esti-
mated by DFT calculations at the B3LYP/6-31G(d) level of theory. The en-
ergies in eV are quoted with respect to vacuum (1 hartree=27.2116 eV).
[b] The energy levels of HOMO and LUMO were determined electrochem-
ically, given that the absolute energy level of NHE was ꢀ4.44 eV under
vacuum. The data for ZnPQ and ZnP were taken from references [20g]
and [20f], respectively.
Figure 4. Plots of porphyrin surface density (G) as a function of immersion
&
*
time for ZnPBQ ( ) and ZnPQ ( ), which adsorb on the TiO2 surfaces without
the scattering layers.
was very similar to our previous report on the G value of ZnP
on TiO2.[20f,g,21b] The slower adsorption of ZnPBQ than ZnPQ is
consistent with the lower electron density of the anchoring
group in the LUMO of ZnPBQ than in ZnPQ. The saturated G
values of ZnPBQ and ZnPQ on the TiO2 films were determined
to be 1.0ꢁ10ꢀ10 and 1.1ꢁ10ꢀ10 mol cmꢀ2, respectively. By as-
suming that the porphyrin monolayers are densely packed and
orientated vertically relative to the TiO2 surface, the G values of
ZnPBQ and ZnPQ are calculated to be 1.1ꢁ10ꢀ10 molcmꢀ2. By
taking into account the good agreement between the calculat-
ed and experimental G values with the saturated adsorption
behavior of ZnPBQ and ZnPQ, densely packed monolayers of
ZnPBQ and ZnPQ are formed on the TiO2.
Photovoltaic properties of porphyrin-sensitized TiO2 cells
Figure 3. Some sets of molecular orbital diagrams for (a) ZnPBQ and
(b) ZnPQ[20g] obtained by DFT calculations at the B3LYP/6-31G(d) level of
theory.
We evaluated the cell performance of the TiO2/ZnPBQ cell in
comparison with those of the TiO2/ZnPQ[20g] and TiO2/ZnP[20f]
cells. The TiO2 electrode was modified with ZnPBQ by immers-
ing the TiO2 electrode into the porphyrin solution in ethanol
for 0.5–24 h.[29] The h value is derived from the equation: h=
JSCVOCff, in which VOC is the open-circuit potential, JSC is the
short-circuit current, and ff is the fill factor. The h value of the
TiO2/ZnPBQ cell revealed a strong dependence on the immer-
sion time (Figure 5), as observed for TiO2 cells modified with
ZnPQ[20g] and ZnP.[20f] The decrease in the h value as a function
of immersion time may be rationalized by an increase in the
porphyrin aggregation with increasing immersion time.[20,21]
The maximal h values (ZnPBQ: 3.7%; ZnPQ: 5.4%[20g]) were ob-
tained with an immersion time of 2 h for TiO2/ZnPBQ and 1 h
for the TiO2/ZnPQ cells. Note that the h value of the ZnPBQ
cell is lower than that of the ZnPQ cell. To further optimize cell
performances, ZnPBQ (0.2 mm) was sensitized onto the TiO2
surface in ethanol for 2 h, in which coadsorbent (chenodeoxy-
the excited adsorbed dye and the 3d orbital of TiO2.[28] The
electron densities of the anchoring group in the LUMO of
ZnPBQ are rather high. However, the integrated ratio of the
electron densities of the anchoring group, including the neigh-
boring quinoxaline moiety in the LUMO of ZnPBQ (18.7%), is
lower considerably than that in the LUMO of ZnPQ (32.8%).
Accordingly, the light-harvesting property of ZnPBQ is im-
proved, whereas the f value from the excited singlet state of
inj
ZnPBQ to the CB of TiO2 may be lowered compared with that
of ZnPQ (see below).
Porphyrin adsorption on TiO2
The TiO2 electrodes were immersed into 0.2 mm solutions of
porphyrin to give the porphyrin-modified TiO2 electrodes. Total
800
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Bisquinoxaline-Fused Porphyrins
Figure 5. Plots of the power conversion efficiency (h) as a function of immer-
Figure 7. Photocurrent action spectra of the TiO2/ZnPBQ+CDCA (solid line)
and TiO2/ZnPQ+CDCA[20g] (dotted line) cells.
[20g]
&
*
sion time for ZnPBQ ( ) and ZnPQ
(
) sensitized TiO2 cells. The porphyr-
ins were adsorbed on the TiO2 electrodes by immersing them into a solution
of ZnPBQ in EtOH or a solution of ZnPQ (0.2 mm) in MeOH without coad-
sorbent.
ence in the h values between the ZnPBQ- and ZnPQ-sensitized
TiO2 cells primarily results from the nonparallel values of JSC
and VOC. Each of the photocurrent action spectra (Figure 7)
follow the absorption features of the corresponding porphyrin
adsorbed on the electrodes (Figure 8), indicating that the por-
phyrin is the main source for the photocurrent generation. The
cholic acid (CDCA, 0.2 mm), diphenylphosphinic acid (DPPA,
0.05 mm), or phenylphosphonic acid (PPA, 0.05 mm)) was
added to reduce porphyrin aggregation on the TiO2 (see the
Experimental Section). Figure 6 depicts the photocurrent–volt-
age characteristics of the TiO2/ZnPBQ, TiO2/ZnPBQ+CDCA,
TiO2/ZnPBQ+DPPA, and TiO2/ZnPQ+CDCA[20g] cells under the
respective maximal h conditions. The improvement of cell per-
formance in the TiO2/ZnPBQ+CDCA cell is larger than that in
the TiO2/ZnPBQ+DPPA cell. The use of PPA as a coadsorbent
led to exclusive adsorption of PPA on TiO2, leading to little ad-
sorption of ZnPBQ.[30]
The TiO2/ZnPBQ+CDCA cell has values of h=4.7% with
JSC =10.6 mAcmꢀ2, VOC =0.66 V, and ff=0.67, which are compa-
rable to those of the TiO2/ZnP+CDCA cell (h=4.4%, JSC =
9.4 mAcmꢀ2, VOC =0.72 V, ff=0.64),[20f] but are inferior to the
TiO2/ZnPQ+CDCA cell (h=6.3%, JSC =13.2 mAcmꢀ2, VOC
=
0.71 V, ff=0.67).[20g] The h values are smaller than that of N719-
sensitized cell under our currently optimized conditions (h=
8.4%, JSC =15.7 mAcmꢀ2, VOC =0.80 V, ff=0.67).[20g] The differ-
Figure 8. Light-harvesting efficiencies of the TiO2/ZnPBQ+CDCA (solid line)
and TiO2/ZnPQ+CDCA[20g] (dotted line) electrodes. The scattering TiO2 layers
were not applied to the TiO2 electrodes to measure absorbance accurately.
IPCE is divided into three components, according to Equa-
tion (1), in which light-harvesting efficiency (LHE) is the
number of absorbed photons per number of incident photons,
f
inj
is the quantum yield for electron injection from the por-
phyrin excited state to the CB of the TiO2 electrode, and hcol is
the efficiency of charge collection:
IPCE ¼ LHE ꢂ ꢀinj ꢂ hcol
ð1Þ
The TiO2/ZnPBQ+CDCA cell discloses inferior IPCE values than
those the TiO2/ZnPQ+CDCA cell in all wavelength regions,
except the long wavelength of 670–750 nm (Figure 7). By con-
sidering the effect of the light-scattering TiO2 layer, the LHE
values of the TiO2/ZnPBQ+CDCA and the TiO2/ZnPQ+CDCA
cells are comparable, although the LHE value of the TiO2/
ZnPBQ+CDCA cell is larger at 650-750 nm than that for the
TiO2/ZnPQ+CDCA cells. Thus, we can conclude that the lower
electron injection efficiency and/or electron collection efficien-
cy of the TiO2/ZnPBQ+CDCA cell than those of the TiO2/
Figure 6. Photocurrent–voltage characteristics of the TiO2/ZnPBQ+CDCA cell
(solid line; h=4.7%, JSC =10.6 mAcmꢀ2, VOC =0.66 V, ff=0.67), the TiO2/
ZnPBQ cell (dashed line; h=3.6%, JSC =7.7 mAcmꢀ2, VOC =0.65 V, ff=0.72),
the TiO2/ZnPBQ+DPPA cell (dotted line; h=3.9%, JSC =8.8 mAcmꢀ2
OC =0.62 V, ff=0.71), and the TiO2/ZnPQ+CDCA cell[20g] (solid line with
,
V
black circles; h=6.3%, JSC =13.2 mAcmꢀ2, VOC =0.71 V, ff=0.67). Conditions:
electrolyte 0.1m LiI, 0.05m I2, 0.6m 2,3-dimethyl-1-propylimidazolium iodide,
and 0.5m 4-t-butylpyridine in CH3CN; input power: AM 1.5 under simulated
solar light (100 mWcmꢀ2).
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801

H. Imahori et al.
ZnPQ+CDCA cell are responsible for the difference in the IPCE
and resulting h values. The maximal IPCE values at Soret bands
are 56% for the TiO2/ZnPBQ+CDCA cell and 75% for the TiO2/
ZnPQ+CDCA cell.
rable to that (4.4%) of the nonfused, porphyrin-sensitized TiO2
cell, but was lower than that (6.3%) of the monoquinoxaline-
fused, porphyrin-sensitized TiO2 cell. Such fundamental infor-
mation may be useful for the molecular design of highly effi-
cient porphyrin DSSCs based on our concept: the unsymmetri-
cal p elongation of porphyrins.
The lower electron injection efficiency for the TiO2/
ZnPBQ+CDCA cell relative to that for TiO2/ZnPQ+CDCA cell
can be attributed to the weak electronic coupling between the
LUMO of ZnPBQ and the CB of the TiO2 due to the smaller
electron density of the carboxyl group in the LUMO of ZnPBQ
than that of ZnPQ (see above). Alternatively, the plausible,
more tilted geometry of ZnPBQ than ZnPQ on the TiO2 surface
may also lower the electron collection efficiency because of
fast charge recombination from the electrons in the CB of the
TiO2 to the porphyrin radical cation by the short separation dis-
tance between the porphyrin and the TiO2 through the
space.[26b] The VOC value of the TiO2/ZnPBQ+CDCA cell is slight-
ly lower than that of the TiO2/ZnPQ+CDCA cell. To elucidate
the reason for the low VOC value, we measured the current–
voltage characteristics under dark conditions (Figure 9). Clearly,
the onset for the TiO2/ZnPBQ+CDCA cell appears at a less pos-
Experimental Section
Synthesis: ZnPBQ was synthesized by following the protocol devel-
oped by Burn and co-workers[23] and by us.[20d] 2,3-Dioxo-
5,10,15,20-tetrakis(2,4,6-trimethylphenyl)chlorin (1), ZnPQ, and ZnP
were prepared according to previously reported procedures.[20d]
5,10,15,20-Tetrakis(2,4,6-trimethylphenyl)quinoxalino[2,3-b]porphy-
rin (2): 1,2-Diaminobenzene (104 mg, 0.96 mmol) and 1 (353 mg,
0.43 mmol) were charged into a two-necked flask. Dry pyridine
(40 mL) was added and the solution was stirred at 1108C for 3 h
before the solvent was evaporated. Column chromatography on
silica gel (CH2Cl2/hexane=1:1) afforded 2 as a dark violet solid
(362 mg, 0.409 mmol, 94%). M.p. >3008C; 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=8.84 (d, J=4.9 Hz, 2H; b-H), 8.74 (d, J=
4.9 Hz, 2H; b-H), 8.53 (s, 2H; b-H), 7.95 (dd, J=8.8, 4.4 Hz, 2H; Arꢀ
H), 7.79 (dd, J=8.8, 4.4 Hz, 2H; ArꢀH), 7.32 (s, 2H; Ar-H), 7.28 (s,
2H; Ar-H), 7.26 (s, 4H; ArꢀH), 2.74 (s, 6H; mesityl-H), 2.63 (s, 6H;
mesityl-H), 1.89 (s, 12H; mesityl-H), 1.75 (s, 12H; mesityl-H),
ꢀ2.36 ppm (s, 2H; NꢀH); FTIR (KBr): n˜max =2914 (br), 1611, 1474,
1377, 1212, 1168, 1112, 971, 850, 803, 764, 726 cmꢀ1; HRMS (ESI):
m/z calcd for C62H57N6: 885.4645 [M+H+]; found: 885.4603.
12-Acetoxyl-5,10,15,20-tetrakis(2,4,6-trimethylphenyl)quinoxalino-
[2,3-b]porphyrin (3): Silver acetate (142 mg, 0.851 mmol) and
iodine (150 mg, 1.18 mmol) were added to a solution of 2 (133 mg,
0.150 mmol) in dichloromethane (13 mL). The reaction mixture was
stirred at room temperature in the dark under argon for 3 h. The
reaction mixture was washed with an aqueous solution of Na2S2O3
and then water three times. The organic layer was collected, dried
over anhydrous Na2SO4, and the solvent was removed in vacuo.
Column chromatography on silica gel (CH2Cl2/hexane=1:1 to 3:1)
afforded 3 as a dark violet solid (68 mg, 0.072 mmol, 48%). M.p.
>3008C; 1H NMR (400 MHz, CDCl3, 258C, TMS): d=8.84 (d, J=
4.9 Hz, 1H; b-H), 8.82 (d, J=4.9 Hz, 1H; b-H), 8.73 (d, J=4.9 Hz,
1H; b-H), 8.64 (d, J=4.9 Hz, 1H; b-H), 8.31 (s, 1H; b-H), 7.95 (m,
2H; ArꢀH), 7.79 (d, 2H; ArꢀH), 7.31 (s, 4H; ArꢀH), 7.28 (s, 2H; Arꢀ
H), 7.26 (s, 2H; ArꢀH), 2.73 (s, 6H; mesityl-H), 2.61 (s, 3H; mesityl-
H), 2.58 (s, 3H; mesityl-H), 1.91 (s, 6H; mesityl-H), 1.87 (s, 6H; mesi-
tyl-H), 1.86 (s, 3H; mesityl-H), 1.85 (s, 6H; mesityl-H), 1.84 (s, 6H;
mesityl-H), 1.75 (s, 6H; mesityl-H), 1.74 (s, 6H; mesityl-H), ꢀ2.44 (s,
1H; N-H), ꢀ2.55 ppm(s, 1H; N-H); FTIR (KBr): n˜max =3362, 2917 (br),
2915 (br), 2856, 1775, 1572, 1437, 1342, 1262, 1188, 1106, 1028,
801, 766, 705 cmꢀ1; HRMS (ESI): m/z calcd for C62H59N6O2: 943.4700
[M+H+]; found: 943.4656.
Figure 9. Current–voltage characteristics of the TiO2/ZnPBQ+CDCA (solid
line) and the TiO2/ZnPQ+CDCA (dotted line) cells under dark conditions.
itive value than that of the TiO2/ZnPQ+CDCA cell, indicating
that the degree of charge recombination between the injected
ꢀ
electrons in the CB of TiO2 and I3 for the TiO2/ZnPBQ+CDCA
cell is higher than that for the TiO2/ZnPQ+CDCA cell. Given
that the hcol value primarily depends on the relative rates of
charge transport against charge recombination, the higher
charge recombination rate for the TiO2/ZnPBQ+CDCA cell than
the TiO2/ZnPQ+CDCA cell may also rationalize the feasible low
charge collection efficiency, resulting in the low IPCE and VOC
values.[31]
Conclusions
2,3-Dioxo-5,10,15,20-tetrakis(2,4,6-trimethylphenyl)quinoxalino-
[12,13-b’]chlorin (4): K2CO3 (210 mg, 1.52 mmol) in methanol
(10 mL) was added to a solution of 3 (217 mg, 0.230 mmol) in di-
chloromethane (10 mL) and the mixture was stirred at room tem-
perature in the dark under argon for 3 h. The reaction mixture was
washed with water three times and the organic layer was collect-
ed, dried over anhydrous Na2SO4, and the solvent was removed in
vacuo. The crude compound was directly used for the next reac-
Bisquinoxaline-fused porphyrin has been synthesized and the
photovoltaic properties were compared with monoquinoxa-
line-fused and nonfused porphyrins, for the first time. The bis-
quinoxaline-fused porphyrin showed improved light-harvesting
properties than those of the reference porphyrins. The bisqui-
noxaline-fused porphyrin-sensitized TiO2 cell exhibited a mod-
erate power conversion efficiency of 4.7%, which was compa-
802
www.chemsuschem.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 2011, 4, 797 – 805

Bisquinoxaline-Fused Porphyrins
tion due to its instability against photo-oxidation. DMP (250 mg,
0.59 mmol) was added to the solution of the crude compound in
dichloromethane (10 mL). The reaction mixture was stirred at room
temperature in the dark under argon for 4 h and then filtered
through a plug of Celite. The filtrate was collected and the solvent
was removed. Column chromatography on silica gel (CH2Cl2/
a light-scattering layer (see below) were recorded by using a
Perkin–Elmer Lambda 900 UV/VIS/NIR spectrometer. Steady-state
fluorescence spectra were acquired by using a SPEX Fluoromax-3
spectrofluorometer.
A time-correlated single-photon counting
(TCSPC) method was used for the time-resolved fluorescence
measurements on the nano- and sub-nanosecond timescale and
the time resolution was 80 ps (FWHM).[24] The excitation wave-
length was 405 nm.
hexane=2:1) afforded
4 as a dark brown solid (86 mg,
1
0.094 mmol, 41%). M.p. >3008C; H NMR (400 MHz, CDCl3, 258C,
TMS): d=8.73 (d, J=3.4 Hz, 2H; b-H), 8.52 (d, J=3.4 Hz, 2H; b-H),
7.93 (dd, J=6.3, 3.4 Hz, 2H; ArꢀH), 7.80 (dd, J=6.3, 3.4 Hz 2H; Arꢀ
H), 7.30 (s, 4H; ArꢀH), 7.25 (s, 4H; Ar-H), 2.72 (s, 6H; mesityl-H),
2.60 (s, 6H; mesityl-H), 1.83 (s, 12H; mesityl-H), 1.77 (s, 12H; mesi-
tyl-H), ꢀ1.86 ppm (s, 2H; N-H); FTIR (KBr): n˜max =3396 (br), 2965
(br), 1746, 1437, 1342, 1262, 1104, 795, 697 cmꢀ1; HRMS (FAB): m/z
calcd for C62H55N6O2: 915.4387 [M+H+]; found: 915.4379.
Electrochemistry: Electrochemical measurements were made by
using an ALS 630a electrochemical analyzer. Redox potentials of
the porphyrins were determined by differential pulse voltammetry
(DPV) in dichloromethane containing 0.1m Bu4NPF6 as a support-
ing electrolyte. A glassy carbon working electrode (3 mm diame-
ter), Ag/AgNO3 (0.01m in acetonitrile) reference electrode, and a Pt
wire counter electrode were employed. Ferrocene/ferrocenium
(+0.642 V vs. NHE) was used as an internal standard for all meas-
urements. All of the measured potentials were converted to the
NHE scale.
5,10,15,20-Tetrakis(2,4,6-trimethylphenyl)-6’-
methoxycarboxylquinoxalino[2,3-b]quinoxalino[12,13-b’]porphyrin
(5): Methyl 3,4-diaminobenzoate (26.6 mg, 0.160 mmol) and 4
(85.6 mg, 0.0935 mmol) were charged into a two-necked flask. Dry
pyridine (20 mL) was added and the reaction mixture was stirred at
110 8C for 11 h. Then, the solvent was evaporated. Column chroma-
tography on silica gel (CH2Cl2/hexane=2:1) afforded 5 as a dark
green solid (91 mg, 0.087 mmol, 93%). M.p. >3008C; 1H NMR
(400 MHz, CDCl3, 258C, TMS): d=8.89 (s, 4H; b-H), 8.62 (s, 1H; Arꢀ
H), 8.38 (d, J=7.2 Hz 1H; ArꢀH), 8.01 (d, J=7.2 Hz 1H; ArꢀH), 7.95
(m, 2H; ArꢀH), 7.85 (m, 2H; Ar-H), 7.39 (s, 2H; ArꢀH), 7.37 (s, 6H;
ArꢀH), 4.06 (s, 3H; methoxy-H), 2.77 (s, 3H; mesityl-H), 2.75 (s, 9H;
mesityl-H), 1.80 (s, 24H; mesityl-H), ꢀ2.32 ppm (s, 2H; NꢀH); FTIR
(KBr): n˜max =2921 (br), 2852 (br), 1726, 1436, 1335, 1260, 1105,
1304, 1254, 1137, 1105, 974, 849, 808, 765, 702 cmꢀ1; HRMS (FAB):
m/z calcd for C70H60N8O2: 1044.4839; found: 1044.4845 [M+].
DFT calculations: Geometry optimization and electronic structure
calculations of the porphyrins were performed by using the B3LYP
functional and 6-31G(d) basis set implemented in the Gaussian 03
program package.[32] Molecular orbitals were visualized by Molstu-
dio 3.0 software.
Preparation of porphyrin-sensitized TiO2 electrode and photovoltaic
measurements: The preparation of TiO2 electrodes and the fabrica-
tion of sealed cells for photovoltaic measurements were performed
by using the previously reported literature method.[33]
Nanocrystalline TiO2 particles (20 nm, CCIC:PST18NR, JGC-CCIC)
were used as the transparent layer of the photoanode, whereas
submicrocrystalline TiO2 particles (400 nm, CCIC:PST400C, JGC-
CCIC) were used as the light-scattering layers of the photoanode.
The working electrode was prepared by cleaning fluorine-doped
5,10,15,20-Tetrakis(2,4,6-trimethylphenyl)-6’-carboxylquinoxalino-
[2,3-b]quinoxalino[12,13-b’]porphyrinatozinc(II) (ZnPBQ): Zinc ace-
tate dihydrate (700 mg, 2.74 mmol) in 1-butanol (10 mL) was
added to a solution of 5 (91.0 mg, 0.0871 mmol) in tetrachlorome-
thane (15 mL). The reaction mixture was stirred at reflux under
argon for 2 days and filtered through a short plug of silica by
using dichloromethane as the eluent. The solvent was evaporated.
The solid was redissolved in tetrahydrofuran (20 mL) and an aque-
ous solution of NaOH (2m, 5.0 mL) was added. The mixture was
heated at reflux for 16 h. After cooling to room temperature, the
reaction mixture was treated with an aqueous solution of HCl (1m,
12 mL) and then washed with a saturated aqueous solution of
NaHCO3 and water successively. The organic layer was collected,
dried over anhydrous Na2SO4, and the solvent was removed in
vacuo. Column chromatography on silica gel (CH2Cl2/EtOAc gradi-
ent from 9:1 to 4:1) afforded ZnPBQ as a dark green solid (80 mg,
&
tin oxide (FTO) glass (Solar 4 mm thickness, 10 W/ , Nippon Sheet
Glass) with a detergent solution in an ultrasonic bath for 10 min,
rinsing with distilled water, ethanol, and air drying. The electrode
was subjected to UV/O3 irradiation for 18 min, immersed into a so-
lution of freshly prepared 40 mm aqueous TiCl4 at 708C for 30 min,
washed with distilled water, ethanol, and dried. Nanocrystalline
TiO2 paste was coated onto the FTO glass by screen printing, fol-
lowed by standing in a clean box for a few minutes, and dried at
1258C for 6 min; this was repeated to attain a final thickness of
12 mm. A layer of 4 mm submicrocrystalline TiO2 paste was deposit-
ed in the same fashion as the nanocrystalline layer. Finally, the
electrode was heated under an airflow at 3258C for 5 min, 3758C
for 5 min, 4508C for 15 min, and 5008C for 15 min. The thickness
of the films was determined by using a surface profiler (SURFCOM
130 A, ACCRETECH). The size of the TiO2 film was 0.25 cm2 (5ꢁ
5 mm). The TiO2 electrode was then subjected to immersion into a
solution of freshly prepared 40 mm aqueous TiCl4 at 708C for
30 min before rinsing with distilled water and ethanol, and being
air dried. The electrodes were sintered at 5008C for 30 min, cooled
to 708C, and immersed into the dye solution at 258C in the dark
for the prescribed times. The TiO2 electrode was immersed into a
solution of 0.20 mm porphyrin in ethanol. The TiO2 electrode modi-
fied with ZnPBQ is denoted as TiO2/ZnPBQ. To reduce the dye ag-
gregation on the TiO2, the TiO2 electrode was also immersed into a
solution of 0.20 mm porphyrin in ethanol containing CDCA
(0.20 mm), DPPA (0.05 mm), or PPA (0.05 mm). The TiO2 electrodes
coadsorbed with ZnPBQ and CDCA, DPPA, or PPA are denoted as
1
0.073 mmol, 84%). M.p. >3008C; H NMR (400 MHz, CDCl3, 258C,
TMS): d=8.91 (s, 2H; b-H), 8.90 (s, 2H; b-H), 8.84 (s, 1H; ArꢀH),
8.43 (d, J=8.4 Hz; 1H; ArꢀH), 8.08 (d, J=8.4 Hz; 1H; ArꢀH), 7.95
(dd, J=6.4, 3.4 Hz, 2H; ArꢀH), 7.85 (dd, J=6.4, 3.4 Hz, 2H; ArꢀH),
7.38 (s, 3H; ArꢀH), 7.35 (s, 3H; ArꢀH), 7.34 (s, 6H; ArꢀH), 2.81 (s,
3H; mesityl-H), 2.77 (s, 3H; mesityl-H), 2.76 (s, 6H; mesityl-H),
1.79 ppm (s, 24H; mesityl-H); FTIR (KBr): n˜max =2913 (br), 1721,
1611, 1400, 1336, 1308, 1201, 1137, 1114, 996, 954, 846, 821, 795,
759, 737, 708 cmꢀ1; HRMS (FAB): m/z calcd for C62H56N6O2Zn:
1092.3818 [M+]; found: 1092.3828.
Optical spectroscopy: UV/Vis absorption spectra of the porphyrins
in CH2Cl2 and the porphyrin monolayers on TiO2 electrodes without
ChemSusChem 2011, 4, 797 – 805
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
803

H. Imahori et al.
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spectively. The porphyrin surface coverage adsorbed on the TiO2
films (molcmꢀ2) was determined by measuring the difference in
the absorbances of the porphyrin solution before and after immer-
sion of the TiO2 films (TiO2 area of 0.25 cm2 with a thickness of
12 mm) without the scattering layer, assuming that the molar ab-
sorption coefficient in solutions is identical to that on the TiO2.
The counter electrode was prepared by drilling a small hole in FTO
&
glass (Solar 1 mm thickness, 10 W/ , Nippon Sheet Glass), rinsing
with distilled water and ethanol before treatment with 0.1m HCl/2-
propanol in an ultrasonic bath for 5 min. After heating in air at
4008C for 15 min, platinum was deposited by coating the elec-
trode with a solution of H2PtCl6 (2 mg in 1 mL ethanol) twice and
heating in air at 4008C for 15 min.
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The sandwich cell was prepared by using the dye-anchored TiO2
film as a working electrode and a counter Pt electrode, which were
assembled with a hot-melt ionomer film of Surlyn polymer gasket
(DuPont), and the superimposed electrodes were tightly held and
heated at 1308C to seal the two electrodes. The aperture of the
Surlyn frame was larger than that of the area of TiO2 film by 2 mm
and its width was 1 mm. The hole in the counter electrode was
sealed with a film of Surlyn. A hole was then made in the Surlyn
film covering the hole by a needle. A drop of an electrolyte was
put on the hole in the back of the counter electrode. It was intro-
duced into the cell by vacuum backfilling. Finally, the hole was
sealed by using Surlyn film and a cover glass (0.13–0.17 mm thick).
The edge of the FTO outside of the cell was roughened with sand-
paper. A solder was applied on each edge of the FTO electrodes.
The electrolyte solution used was 0.60m 2,3-dimethyl-1-propylimi-
dazolium iodide, 0.05m I2, 0.10m LiI, and 0.50m 4-tert-butylpyridine
in acetonitrile.
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Incident photon-to-current efficiency (IPCE) and photocurrent–volt-
age (J--V) performance were measured on an action spectrum
measurement setup (PEC-S20) and a solar simulator (PEC-L10, Pec-
cell Technologies) with
a
simulated sunlight of AM 1.5
(100 mWcmꢀ2), respectively. IPCE (%)=100ꢁ1240ꢁi/(Winl), in
which i is the photocurrent density (Acmꢀ2), Win is the incident
light intensity (Wcmꢀ2), and l is the excitation wavelength (nm).
During the photovoltaic measurements, a black plastic tape was at-
tached to the back of the TiO2 electrode, except for the TiO2 film
region, to reduce scattering light.
Acknowledgements
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This work was supported by Grant-in-Aid (no. 21350100 to H.I.)
and Strategic Japanese–Finnish Cooperative Program (JST) from
MEXT, Japan. H.I. also thanks Prof. Nikolai Tkachenko and Helge
Lemmetyinen for the fluorescence lifetime measurement of
ZnPBQ. H.H. also thanks the JSPS for a fellowship for young sci-
entists.
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Keywords: dyes/pigments · porphyrinoids · quinoxalines ·
solar cells · synthesis design
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Received: January 24, 2011
Published online on May 17, 2011
ChemSusChem 2011, 4, 797 – 805
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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