Hydrogen bonding effects on the surface structure and photoelectrochemical properties of nanostructured SnO2 electrodes modified with porphyrin and fullerene composites

Article abstract of DOI:10.1021/jp0537409

Hydrogen bonding effects on surface structure, photophysical properties, and photoelectrochemistry have been examined in a mixed film of porphyrin and fullerene composites with and without hydrogen bonding on indium tin oxide and nanostructured SnO₂ electrodes. The nanostructured SnO₂ electrodes modified with the mixed films of porphyrin and fullerene composites with hydrogen bonding exhibited efficient photocurrent generation compared to the reference systems without hydrogen bonding. Atomic force microscopy, infrared reflection absorption, and ultraviolet-visible absorption spectroscopies and time-resolved fluorescence lifetime and transient absorption spectroscopic measurements disclosed the relationship between the surface structure and photophysical and photoelectrochemical properties relating to the formation of hydrogen bonding between the porphyrins and/or the C₆₀ moieties in the films on the electrode surface. These results show that hydrogen bonding is a highly promising methodology for the fabrication of donor and acceptor composites on nanostructured semiconducting electrodes, which exhibit high photoelectrochemical properties.

Full text of DOI:10.1021/jp0537409

J. Phys. Chem. B 2005, 109, 18465-18474
18465
Hydrogen Bonding Effects on the Surface Structure and Photoelectrochemical Properties of
Nanostructured SnO2 Electrodes Modified with Porphyrin and Fullerene Composites
Hiroshi Imahori,*,†,‡,§ Jia-Cheng Liu,† Hiroki Hotta,‡ Aiko Kira,† Tomokazu Umeyama,†
Yoshihiro Matano,† Guifeng Li,| Shen Ye,*,| Marja Isosomppi, Nikolai V. Tkachenko,*, and
Helge Lemmetyinen
Department of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Nishikyo-ku,
Kyoto 615-8510, Japan, PRESTO, Japan Science and Technology Agency (JST), Japan, Fukui Institute for
Fundamental Chemistry, Kyoto UniVersity, 34-4, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan,
Catalysis Research Center, Hokkaido UniVersity, Sapporo 001-0021, Japan, and Institute of Materials
Chemistry, Tampere UniVersity of Technology, P. O. Box 541, FIN-33101 Tampere, Finland
ReceiVed: July 8, 2005
Hydrogen bonding effects on surface structure, photophysical properties, and photoelectrochemistry have
been examined in a mixed film of porphyrin and fullerene composites with and without hydrogen bonding on
indium tin oxide and nanostructured SnO2 electrodes. The nanostructured SnO2 electrodes modified with the
mixed films of porphyrin and fullerene composites with hydrogen bonding exhibited efficient photocurrent
generation compared to the reference systems without hydrogen bonding. Atomic force microscopy, infrared
reflection absorption, and ultraviolet-visible absorption spectroscopies and time-resolved fluorescence lifetime
and transient absorption spectroscopic measurements disclosed the relationship between the surface structure
and photophysical and photoelectrochemical properties relating to the formation of hydrogen bonding between
the porphyrins and/or the C60 moieties in the films on the electrode surface. These results show that hydrogen
bonding is a highly promising methodology for the fabrication of donor and acceptor composites on
nanostructured semiconducting electrodes, which exhibit high photoelectrochemical properties.
Introduction
carriers are generated and simultaneously separated across the
dye-semiconductor or donor-acceptor heterointerface. Typi-
cally, bulk heterojunction solar cells possess an interpenetrating
network of donor (D) and acceptor (A) molecules in the blend
film, revealing photoinduced charge separation and the transport
of created charges to the electrodes. Thus, design and construc-
tion of electron- and hole-transporting, nanostructured highways
in the D-A interpenetrating network are pivotal for enhancing
both charge separation efficiency and charge carrier mobility.
Supramolecular assembly of D-A molecules is a potential
approach to create such a bulk heterojunction layer with a phase-
separated, interpenetrating network involving nanostructured
electron and hole highways. However, there have so far been
few examples where a noncovalent bonding strategy has been
employed in a mixed film of donor and acceptor to improve
photocurrent generation efficiency in photoelectrochemical
devices.15a,19,20
Noncovalent bonding such as hydrogen bonding, coordination
bonding, electrostatic interaction, van der Waals interaction, and
π-π interaction plays an important role in biological activities
in nature. For instance, photosynthesis employs such weak
interaction to arrange donor-acceptor (D-A) molecules in a
protein matrix precisely, exhibiting a vectorial cascade of energy
and electron-transfer processes.1 Stimulated by this vision, the
utilization of weak interaction has recently merited increasing
attention as a sophisticated methodology to assemble supra-
molecular photosynthetic architectures.2 A variety of nonco-
valently bonded D-A systems have been constructed in fluid
media to mimic photoinduced energy and electron-transfer
processes in photosynthesis.3-11
Along this line, achieving efficient solar energy conversion
at low cost is one of the most important technological challenges
for the near future. During the past decade, organic solar cells
have been discussed as a promising alternative to inorganic
semiconductors for renewable energy production. Specifically,
considerable attention has been drawn toward developing the
dye-sensitized12 and bulk heterojunction13-18 solar cells. The
distinguishing characteristic of these cells is that excitations are
generated upon light absorption, and within 100 fs charge
Here, we report the first mixed films of porphyrin and
fullerene with hydrogen bonding on tin oxide (SnO2) as well
as indium tin oxide (ITO) electrodes to reveal efficient photo-
current generation. The compounds used in this study are shown
in Figure 1. A combination of porphyrin as a donor as well as
a sensitizer with fullerene as an acceptor was chosen, because
such a D-A system is known to exhibit a long-lived charge-
separated state with a high quantum yield via photoinduced
electron transfer.21,22 Given the fact that porphyrins tend to make
a complex with fullerenes in a solid state due to the π-π
interaction23-25 and the small reorganization energy of the
porphyrin and fullerene system,21,22 we can expect fast photo-
induced charge separation and slow charge recombination in
the mixed film. More importantly, the resulting electron and
* To whom correspondence should be addressed. E-mail: (H.I.)
imahori@scl.kyoto-u.ac.jp; (S.Y.) ye@cat.hokudai.ac.jp; (N.V.T.)
nikolai.tkachenko@tut.fi.
† Kyoto University.
‡ PRESTO, JST.
§ Fukui Institute.
| Hokkaido University.
Tampere University of Technology.
10.1021/jp0537409 CCC: $30.25 © 2005 American Chemical Society
Published on Web 09/09/2005

18466 J. Phys. Chem. B, Vol. 109, No. 39, 2005
Imahori et al.
Synthesis and Characterization. Compound 1. To a solution
of 3,5-dihexadecyloxybenzyl alcohol27 (5.8 g, 9.85 mmol) in
dried dichloromethane (90 mL) was added a suspension of PCC
(5.5 g) in dried dichloromethane (20 mL) with stirring for 3.5
h at room temperature. The resulting mixture was filtered, and
the residue was washed several times with dichloromethane.
The filtrate was collected, and the solvent was removed under
reduced pressure. Flash column chromatography on silica gel
(hexane/ethyl acetate ) 96:4) afforded 1 as a white solid (4.4
g, 7.5 mmol, 76%). Mp 54-55 °C; IR (KBr) 3100, 2953 2917,
2851, 1710, 1683, 1596, 1472, 1392, 1325, 1319, 1180, 1059,
943, 848, 834, 734, 717, 675 cm-1; 1H NMR (400 MHz, CDCl3)
δ 0.88 (t, J ) 6.8 Hz, 6H), 1.26 (m, 52H), 1.79 (m, 4H), 3.98
(t, J ) 6.4 Hz, 4H), 6.70 (t, J ) 2.0 Hz, 1H), 6.98 (d, J ) 2.0
Hz, 2H), 9.89 (s, 1H); MALDI-TOFMS (positive mode) m/z
588 (M + H+); Anal. Calcd for C39H70O3: C, 79.80; H, 12.02.
Found: C, 79.59; H, 12.05.
Figure 1. Porphyrin and fullerene derivatives used in this study.
Compound 2. A solution of 5-(4-methoxycarbonylphenyl)-
dipyrromethane28 (1.40 g, 5 mmol) and 1 (2.93 g, 5 mmol) in
chloroform (500 mL) with stirring at room temperature under
nitrogen was treated with boron trifluoride diethyl etherate (0.64
mL, 5.06 mmol).28 After 2 h, p-chloranil (1.86 g, 7.6 mmol)
was added, and the reaction mixture was stirred overnight.
Triethylamine (2.1 mL, 15.2 mmol) was added, and the resulting
mixture was concentrated. Subsequent flash column chroma-
tography on silica gel (hexane/toluene ) 3:1f1:3) yielded 2
as a reddish purple solid (0.41 g, 0.24 mmol, 9.7%). Mp 127-
128 °C; IR (KBr) 3315, 2921, 2852, 1725, 1590, 1471, 1430,
1394, 1354, 1311, 1276, 1256, 1155, 1109, 1100, 1054, 1022,
hole pair must be separated through a tailored network of the
acceptors and the donors, respectively, to suppress charge
recombination. Such a electron- or hole-transporting highway
would be constructed with the help of hydrogen bonding. To
evaluate the hydrogen-bonding effect on the photoelectrochemi-
cal properties of the D-A systems, carboxyl and alkoxycarbonyl
groups were introduced to porphyrin and C60 to yield zinc-
porphyrin (ZnP) acid and ester as a donor (denoted as ZnP-
acid and ZnP-ester) and C60 acid and C60 ester as an acceptor
(denoted as C60-acid and C60-ester), respectively (Figure 1).
1
974, 927, 848, 799, 778, 732, 720, 633 cm-1; H NMR (400
Experimental Section
MHz, CDCl3) δ 2.83 (br s, 2H), 0.85 (t, J ) 6.8 Hz, 12H),
1.22 (m, 104 H), 1.85 (m, 8H), 4.11 (t, J ) 6.4 Hz, 8H), 4.12
(s, 6H), 6.88 (t, J ) 2.0 Hz, 2H), 7.36 (d, J ) 2.0 Hz, 4H),
8.30 (d, J ) 8.4 Hz, 4H), 8.44 (d, J ) 8.4 Hz, 4 H), 8.77 (d, J
) 4.8 Hz, â-H, 4H), 8.98 (d, J ) 4.8 Hz, â-H, 4H); MALDI-
TOFMS (positive mode) m/z 1693 (M + H+).
General Methods. Melting points were recorded on a
Yanagimoto micromelting point apparatus and not corrected.
1H NMR spectra were measured on a JEOL EX-400 spectrom-
eter. Matrix-assisted laser desorption/ionization (MALDI) time-
of-flight mass spectra (TOF) were measured on a Kratos
compact MALDI I (Shimadzu). UVvisible absorption spectra
were obtained on a Perkin-Elmer Lambda 900UV/vis/near-IR
spectrometer.
Compound 3. A mixture of 2 (0.17 g, 0.1 mmol) in 100 mL
of tetrahydrofuran (THF)-ethanol (1:1) and potassium hydrox-
ide (0.513 g) in water (5 mL) was refluxed for 8 h. After cooling
and the removal of solvent, the residue was diluted with water
(100 mL) and desired porphyrin dipotassium salt was filtered.
Acidification (pH 2) of an aqueous suspension of the porphyrin
dipotassium salt with concentrated hydrochloric acid and
subsequent filtration gave 3 as a reddish purple power (0.16 g,
0.1 mmol, 96%). Mp >300 °C; IR (KBr) 3318, 3090, 2921,
2852, 2533, 1690, 1599, 1589, 1541 1467, 1430, 1383, 1354,
1310, 1292, 1250, 1162 1129, 1057, 1020, 974, 928, 849, 830,
Infrared reflection absorption (IRRA) spectra were recorded
by a Bio-Rad FTS 575C FT-IR spectrometer equipped with a
liquid-N2-cooled MCT detector and a Harrick Reflector grazing
angle reflectance unit.26 The samples for IRRA observation were
prepared by direct deposition of these functional molecules on
a slide glass (2 × 2 cm2) covered by a 200 nm thick gold film
for IRRA observation, by spin-coating method. The IRRA
spectrum of a bare gold film was used as the reference. The
1
799, 778, 724, 711, 699 cm-1; H NMR (400 MHz, CDCl3/
spectral resolution used was 4 cm-1
.
DMSO-d6 ) 3:2) δ 2.91 (br s, 2H), 0.83 (t, J ) 6.0 Hz, 12H),
1.18 (m, 104H), 1.81 (m, 8H), 4.10 (t, J ) 6.0 Hz, 8H), 6.86 (t,
J ) 2.0 Hz, 2H), 7.31 (d, J ) 2.0 Hz, 4H), 8.20 (d, J ) 8.0 Hz,
4 H), 8.41(d, J ) 8.0 Hz, 4 H), 8.83 (d, J ) 4.8 Hz, â-H, 4H),
8.95 (d, J ) 4.8 Hz, â-H, 4H); MALDI-TOFMS (positive mode)
m/z 1665 (M + H+).
ZnP-Acid. To the solution of 3 (0.30 g, 0.18 mmol) in 300
mL of THF-ethanol (1:1) was added a solution of zinc acetate
dihydrate (0.20 g, 0.91 mmol) in methanol (4 mL). The reaction
mixture was refluxed overnight. After cooling and following
removal of solvent, the residue was treated with 150 mL of 1
M HCl aqueous solution and 500 mL of CHCl3 with stirring
overnight. The CHCl3 layer was collected, washed with water
several times, dried over anhydrous Na2SO4, and concentrated
in vacuo. The resulting solid was purified by chromatography
on silica gel eluting with CH2Cl2/THF (95:5) to yield ZnP-acid
Atomic force microscopy (AFM) measurements were carried
out using a Digital Nanoscope III in the tapping mode. Steady-
state fluorescence spectra were measured on a Fluorolog 3
spectrofluorimeter (ISA Inc.) equipped with a cooled IR-
sensitive photomultiplier (R2658). All solvents and chemicals
were of reagent grade quality and were purchased commercially
and used without further purification unless otherwise noted.
Tetrabutylammonium hexafluorophosphate used as a supporting
electrolyte for the electrochemical measurements was obtained
from Fluka and recrystallized from methanol. Thin-layer chro-
matography and flash column chromatography were performed
with Alt. 5554 DCAlufolien Kieselgel 60 F254 (Merck) and silica
gel 60N (Kanto Chemicals), respectively. ITO electrodes (190-
200 nm ITO on transparent glass slides) were commercially
available from Tokyo Sanyo Sinku. The roughness factor (R )
1.3) was estimated by AFM measurement with tapping mode.

Modified Nanostructured SnO2 Electrodes
J. Phys. Chem. B, Vol. 109, No. 39, 2005 18467
SCHEME 1
as a purple red solid (0.30 g, 0.17 mmol, 96%). Mp 281-282
°C; IR (KBr) 3096, 2923, 2852, 2667, 2542, 1691 1602, 1466,
1457, 1430, 1419, 1387, 1350, 1312, 1287, 1205, 1166, 1127,
1071, 1057, 998, 946, 796, 719, 698 cm-1; 1H NMR (400 MHz,
CDCl3) δ 0.85 (t, J ) 6.8 Hz, 12H), 1.22 (m, 104H), 1.86 (m,
8H), 4.22 (t, J ) 6.0 Hz, 8H), 6.90 (t, J ) 2.0 Hz, 2H), 7.38 (d,
J ) 2.0 Hz, 4H), 8.34 (d, J ) 8.4 Hz, 4H), 8.50 (d, J ) 7.6 Hz,
4 H), 8.89 (d, J ) 4.8 Hz, â-H, 4H), 9.11 (d, J ) 4.8 Hz, â-H,
4H); MALDI-TOFMS (positive mode) m/z 1727 (M + H+).
ZnP-Ester. To the solution of 2 (0.10 g, 0.06 mmol) in 100
mL of CHCl3 was added a solution of zinc acetate dihydrate
(0.10 g, 0.46 mmol) in methanol (2 mL). The mixture was
refluxed for 3 h. After cooling, the resulting solution was washed
with saturated sodium bicarbonate aqueous solution and water
successively and dried over Na2SO4, and then the solvent was
evaporated. The resulting solid was purified by chromatography
on silica gel eluting with CH2Cl2 to yield ZnP-ester as a red
purple solid (0.09 g, 0.05 mmol, 90%). Mp 86-87 °C; IR (KBr)
3440, 2921, 2852, 1724, 1696, 1591, 1470, 1433, 1394, 1348,
and/or SnO2/ITO substrates, while that of IRRA absorption was
performed on the gold substrate.
Photoelectrochemical Measurements. Photoelectrochemical
measurements were performed in a one-compartment Pyrex UV
cell (5 mL) with nitrogen-saturated electrolyte solution contain-
ing 0.5 M LiI and 0.01 M I2 in acetonitrile. The samples were
excited with monochromatic light using a 500 W Xe arc lamp
(Ritsu UXL-500D-0) coupled with a monochromator (Ritsu MC-
10N). The illuminated area on the modified surface was 0.20
cm2. The light intensity was modulated with neutral density
filters (HOYA). The casting film was immersed into the
electrolyte solution containing 0.5 M LiI and 0.01 M I2 in
acetonitrile as a working electrode. The photocurrent was
measured in a three-electrode arrangement, a modified SnO2/
ITO working electrode, a platinum wire counter electrode (the
-
distance between the electrodes is 0.3 mm), and an I-/I3
reference electrode using an ALS 630a electrochemical analyzer.
The light intensity was monitored by an optical power meter
(Anritsu ML9002A) and corrected. The I-/I3 reference elec-
-
1
1273, 1154, 1111, 1099, 1053, 998, 945, 797, 719 cm-1; H
trode was made from platinum wire covered with glass ruggin
capillary filled with the acetonitrile solution containing 0.5 M
LiI and 0.01 M I2. The potential measured was converted to
the saturated calomel electrode (SCE) scale by adding +0.05
V. The stability of the reference electrode potential was
confirmed under the experimental conditions.
NMR (400 MHz, CDCl3) δ 0.85 (t, J ) 6.8 Hz, 12H), 1.21 (m,
104H), 1.84 (m, 8H), 4.09 (t, J ) 6.4 Hz, 8H), 4.10 (s, 6H),
6.86 (t, J ) 2.0 Hz, 2H), 7.36 (d, J ) 2.0 Hz, 4H), 8.29 (d, J
) 8.4 Hz, 4H), 8.42 (d, J ) 8.0 Hz, 4H), 8.87 (d, J ) 4.8 Hz,
â-H, 4H), 9.08 (d, J ) 4.8 Hz, â-H, 4H); MALDI-TOFMS
(positive mode) m/z 1754 (M + H+).
Spectral Measurements. A pump-probe method was used
to measure transient absorption spectra in sub-picosecond-
nanosecond time domain. The measurements were carried out
using the instrument described previously.29 The transient spectra
were recorded by a CCD detector coupled with a monochro-
mator in the visible and near-infrared ranges. The second
harmonic (420 nm) of a Ti:sapphire laser was used for the
excitation. In addition, the samples were excited at 555 nm using
an optical parametric amplifier (CDP 2017, CDP Inc., Russia)
after multipass femtosecond amplifier and mixing base harmonic
with an idle beam of the parametric amplifier. A typical time
resolution of the instrument was 200-300 fs (fwhm). Emission
decays were measured using a time-correlated single-photon
counting (TCSPC) technique. The excitation wavelength was
590 nm, and the time resolution was 40 ps.
Preparation of Films. Porous nanostructured SnO2 films
were fabricated by repeated spin coating of 10% colloidal
solution (pH 10, Chemat Technology) onto optically transparent
ITO electrode, followed by drying at 473 K for 10 min.20
Finally, the dried films were annealed at 673 K for 1 h. The
nanostructured SnO2 film electrode is referred to as SnO2/ITO.
The thickness of the SnO2 layer was determined by using a
surface roughness/profile measuring instrument (SURFCOM
130A, ACCRETECH). The thicknesses of the SnO2 layer on
the ITO electrode prior to chemical modification were 0.4 and
1.1 µm in this study, where organic layers were further deposited
by spin-coating and dipping methods, respectively. A known
amount of ZnP, C60, or the mixed solution in THF was spin-
coated onto ITO or SnO2/ITO or gold electrodes (spin-coating
method). Modification of ITO or SnO2/ITO electrode was also
carried out by immersing the SnO2/ITO electrode into the THF
solution containing the compounds, followed by washing the
substrate with THF repeatedly (dipping method). The charac-
terization of photochemical behavior was carried out on the ITO
Results and Discussion
Synthesis. Synthetic strategy of porphyrin derivatives with
carboxyl groups is shown in Scheme 1. 3,5-Dihexadecyloxy-

18468 J. Phys. Chem. B, Vol. 109, No. 39, 2005
Imahori et al.
Figure 2. UV-visible absorption spectra of a mixed film of ZnP-
acid and C60-acid ((ZnP-acid + C60-acid)/SnO2/ITO, solid line), a film
of ZnP-acid (ZnP-acid/SnO2/ITO, dotted line), an equimolar THF
solution of ZnP-acid and C60-acid (ZnP-acid + C60-acid, dashed line),
and action spectrum of (ZnP-acid + C60-acid)/SnO2/ITO system (solid
line with open circles): input power, 85 µW cm-2 (λex ) 435 nm);
applied potential, 0.15 V vs SCE; 0.5 M LiI and 0.01 M I2 in
acetonitrile. The spectra were normalized at the Soret band for
comparison. Expanded spectra in the Soret band region are shown as
an inset. The modified SnO2/ITO electrodes were prepared by immers-
ing SnO2/ITO electrodes into an equimolar THF solution (1.5 mM) of
ZnP-acid and C60-acid.
Figure 3. IRRA spectra in the CdO stretching region (1650-1850
cm-1) for the thin films of porphyrin and/or fullerene spin-coated on
gold surface as (a) ZnP-ester, (b) ZnP-acid, (c) C60-ester, (d) C60-acid,
(e) ZnP-acid + C60-acid, (f) ZnP-acid + C60-ester, (g) ZnP-ester +
C60-acid, and (h) ZnP-ester + C60-ester. IR intensities for mixed films
were normalized to the same concentration as those for single-
component films.
between the porphyrin and the C60.20,25 Similar spectral behavior
was observed for the modified SnO2/ITO electrodes prepared
by the spin-coating method.
benzyl alcohol was oxidized using PCC to yield benzaldehyde
1.27 Porphyrin 2 was synthesized by the condensation of 5-(4-
methoxycarbonylphenyl)dipyrromethane with 1 in the presence
of BF3‚OEt2.28 Porphyrin diester 2 was hydrolyzed to give
porphyrin dicarboxylic acid 3. ZnP-acid was obtained by
treatment of 3 with Zn(OAc)2. ZnP-ester was prepared from 2
in the same manner. C60-ester30 and C60-acid30 were synthesized
by following previously reported procedures.
Characterization and Surface Structure of SnO2/ITO and
ITO Electrodes Modified with Porphyrin and Fullerene
Composites. We have successfully observed enhanced photo-
current generation in ITO electrode modified with the mixed
film of porphyrin and hydrogen-bonded C60 derivative relative
to the reference system without hydrogen bonding.19b To
improve the photovoltaic properties of the previous system,
hydrogen-bonding strategy has been also applied to the por-
phyrin moiety in addition to the C60 moiety (Figure 1). More
importantly, ITO electrode is replaced by SnO2/ITO electrode
to suppress charge recombination. Once the separated electron
is injected into the conduction band (CB) of SnO2, there would
be less chance to recombine the electron in the conduction band
and the hole in porphyrin radical cation or electron carrier in
the electrolyte solution, leading to the remarkable improvement
of photocurrent generation efficiency.
Figure 3 shows IRRA spectra in the CdO stretching region
(1650-1850 cm-1) for the thin films of porphyrin and/or
fullerene spin-coated on gold surface as (a) ZnP-ester, (b) ZnP-
acid, (c) C60-ester, (d) C60-acid, (e) ZnP-acid + C60-acid, (f)
ZnP-acid + C60-ester, (g) ZnP-ester + C60-acid, and (h) ZnP-
ester + C60-ester. Detailed peak information is summarized in
Table 1. As shown in Figure 3a, ZnP-ester gives an IR peak at
1725 cm-1 which can be assigned to the free CdO stretching
mode of the ester moiety. In contrast with ZnP-ester, ZnP-acid
shows an IR absorption peak from CdO stretching at lower
wavenumber (1688 cm-1, Figure 3b). Normally, it is known
that the peak from CdO stretching mode shifts to lower
frequency region when the hydrogen bonding is formed between
carbonyl group and a hydrogen-bonding donor.31-35 Thus, the
red shift in peak position of the CdO stretching can be attributed
to the formation of intermolecular hydrogen bonding between
ZnP-acid molecules, possibly dimer or some higher aggregates.
Furthermore, C60-ester shows an IR peak at 1748 cm-1 with
a shoulder at 1729 cm-1 (Figure 3c), while C60-acid gives a
broad IR peak around 1720 cm-1 (Figure 3d), suggesting the
formation of hydrogen bonding between C60-acid molecules.
The reason for the relatively broad shape of the IRRA spectrum
of C60-acid is not clear at this stage and may be related to the
un-uniform aggregates of C60-acid on the substrate.
SnO2/ITO electrodes were immersed into an equimolar THF
solution (1.5 mM) of porphyrin (ZnP-acid, ZnP-ester) and/or
fullerene (C60-acid, C60-ester) to give modified SnO2/ITO
electrodes. Figure 2 shows absorption spectra of a mixed film
of ZnP-acid and C60-acid (denoted as (ZnP-acid + C60-acid)/
SnO2/ITO), a film of ZnP-acid (denoted as ZnP-acid/SnO2/ITO),
and the equimolar THF solution of ZnP-acid and C60-acid
(denoted as ZnP-acid + C60-acid). The Soret bands of (ZnP-
acid + C60-acid)/SnO2/ITO and ZnP-acid/SnO2/ITO are broad-
ened and red-shifted relative to that of ZnP-acid + C60-acid in
THF due to the interaction between the porphyrins. In addition,
the broad long wavelength absorption of (ZnP-acid + C60-acid)/
SnO2/ITO in the 600-700 nm regions is diagnostic of the
charge-transfer absorption band due to the π-complex formed
On the other hand, useful structural information about the
hydrogen-bonding interaction between different types of mol-
ecules can be obtained from the IRRA spectra of mixed films
of porphyrin and fullerene composites. As shown in Figure 3e,f,
an IR peak at 1688 cm-1 was clearly observed for ZnP-acid
mixed with C60-acid or C60-ester. It is unlikely that the hydrogen-
bonding interaction between ZnP-acid and C60-acid or C60-ester
is strong since these IR spectra look like a superimposition of
IR spectra of two single respective components. For example,
the CdO stretching mode of C60-ester in the equimolar mixture
film with ZnP-acid shows nearly the same feature as that of a
single C60-ester component and obvious changes in peak position
and intensity were not observed (Figure 3f), indicating that
interaction between ZnP-acid and C60-ester species is very

Modified Nanostructured SnO2 Electrodes
J. Phys. Chem. B, Vol. 109, No. 39, 2005 18469
TABLE 1: IRRA Frequencies, IPCE Values, and Fluorescence Lifetimes
IPCE (dipping
IPCE (spin-coating
method) valueb/%
fluorescence
lifetimec/ps
system
IRRA freq/cm-1
method) valuea/%
ZnP-acid + C60-acid
ZnP-acid + C60-ester
ZnP-ester + C60-acid
ZnP-ester + C60-ester
ZnP-acid
ZnP-ester
C60-acid
1688, 1720
1689, 1749
1694, 1727
1726, 1749
1688
1725
1720
36
28
15
7
26
6
18
8.2
7.1
2.6
3.5
1.2
e
d
d
40
62
d
380
e
e
C60-ester
1748, 1729
e
e
e
a Input power, 85 µW cm-2 (λex) 435 nm); absorbance, 1.00 ( 0.05 at excitation wavelength; thickness of SnO2 layer, 1.1 µm; applied potential,
0.15 V vs SCE. b Input power, 92 µW cm-2 (λex ) 435 nm); absorbance, 0.25 ( 0.03 at excitation wavelength; thickness of SnO2 layer, 0.4 µm;
applied potential, 0.15 V vs SCE. c The samples were prepared by dipping method. The fluorescence decays were measured by time-correlated
single photon counting method (λex ) 590 nm); the monitoring wavelength was at 660 nm. d The decay was too fast to give the fluorescence
lifetime accurately. e Not measured.
Figure 4. AFM images of (a) ZnP-acid/SnO2/ITO, (b) ZnP-ester/SnO2/ITO, (c) C60-acid/SnO2/ITO, and (d) C60-ester/SnO2/ITO electrodes.
limited. At the same time, we still expect that a small part of
ZnP-acid molecules may form hydrogen bonding with C60-acid
molecules, although the hydrogen bonding interaction between
the same types of molecules (i.e., ZnP-acid and ZnP-acid, C60-
acid and C60-acid) seemed to be stronger. It is noteworthy that
an equimolar mixture of ZnP-ester and C60-acid shows slightly
different behavior (Figure 1g), where the peak position of
CdO stretching from C60-acid in the mixture film was narrower
and was shifted to 1694 cm-1 in comparison with that of the
single C60-acid component, suggesting that the hydrogen bond-
ing in C60-acid was intensified in the mixture with ZnP-ester
(vide infra). As expected, no change was observed in the mixture
of ZnP-ester molecules and C60-ester molecules (Figure 3h) since
there is no hydrogen-bonding donor.
As demonstrated above, the formation of hydrogen bonding
was confirmed in the mixed film of ZnP-acid + C60-acid, ZnP-
acid + C60-ester as well as ZnP-ester + C60-acid but not in
ZnP-ester + C60-ester. The hydrogen-bonding interaction be-
tween the ZnP-acid molecules or C60-acid molecules seems to
be more important than that between different types of
molecules.
(Figures 4 and 5). The images of ZnP-acid/SnO2/ITO (Figure
4a) and ZnP-ester/SnO2/ITO (Figure 4b) are similar to that of
bare SnO2/ITO, which exhibit arrays of spherical particles with
20 nm size. In contrast, the images of C60-acid/SnO2/ITO (Figure
4c) and C60-ester/SnO2/ITO (Figure 4d) show arrays of spherical
particles with 30-40 nm size, which is larger than that of ZnP-
acid/SnO2/ITO and ZnP-ester/SnO2/ITO. These results together
with the IRRA spectra suggest that the C60 molecules are more
aggregative than the porphyrins (vide supra). The AFM images
of the porphyrin and C60 mixed system are quite different from
those of either the porphyrin or the C60 system. The AFM images
of (ZnP-acid + C60-acid)/SnO2/ITO (Figure 5a), (ZnP-acid +
C60-ester)/SnO2/ITO (Figure 5b), and (ZnP-ester + C60-acid)/
SnO2/ITO (Figure 5c) are much smoother than those of (ZnP-
ester + C60-ester)/SnO2/ ITO (Figure 5d) as well as of ZnP-
acid/SnO2/ITO, ZnP-ester/SnO2/ITO, C60-acid/ SnO2/ITO, and
C60-ester/SnO2/ITO (see Supporting Information, S1). A similar
trend was observed for the AFM images on ITO and mica
surfaces. These results demonstrate that both the intermolecular
interaction between the porphyrin and the C60 and the hydrogen
bonding from either the porphyrin or the C60 moieties are
required for the smooth morphology on the surface in which
the porphyrin and the C60 molecules would be self-assembled
AFM measurements were performed for the SnO2/ITO
electrodes modified with the compounds by spin-coating method

18470 J. Phys. Chem. B, Vol. 109, No. 39, 2005
Imahori et al.
Figure 5. AFM images of (a) (ZnP-acid + C60-acid)/SnO2/ITO, (b) (ZnP-acid + C60-ester)/SnO2/ITO, (c) (ZnP-ester + C60-acid)/SnO2/ITO, and
(d) (ZnP-ester + C60-ester)/SnO2/ITO ([ZnP]/[C60] ) 1:1) electrodes.
off. The intensity of the photocurrent was maintained almost
constant during the irradiation at least 1 h. The anodic
photocurrent increases with increasing positive bias to the ITO
electrode from -0.05 to +0.4 V vs SCE, as shown in Figure 6.
The agreement of the action spectrum with the absorption
spectrum of (ZnP-acid + C60-acid)/SnO2/ITO, which is largely
similar to that of ZnP-acid/SnO2/ITO (Figure 2), demonstrates
that the direction of the electron flow is from the electrolyte to
the ITO via the excited singlet state of the porphyrin moiety
(vide infra). Similar photoelectrochemical behavior was ob-
served for (ZnP-acid + C60-ester)/SnO2/ITO, (ZnP-ester + C60-
acid)/SnO2/ITO, (ZnP-ester + C60-ester)/SnO2/ITO, ZnP-acid/
SnO2/ITO, and ZnP-ester/SnO2/ITO systems.
The IPCE (incident photon-to-photocurrent efficiency) values
were compared for (ZnP-acid + C60-acid)/SnO2/ITO, (ZnP-acid
+ C60-ester)/SnO2/ITO, (ZnP-ester + C60-acid)/SnO2/ITO, (ZnP-
ester + C60-ester)/SnO2/ITO, ZnP-acid/SnO2/ ITO, and ZnP-
Figure 6. Photocurrent vs applied potential curves of (ZnP-acid +
C60-acid)/SnO2/ITO system. The dark currents are shown as a dashed
line with open circles. Experimental conditions were as follows:
excitation power, 85 µW cm-2 (λex ) 435 nm); absorbance, 1.00 at
excitation wavelength; thickness of SnO2 layer, 1.1 µm; 0.5 M LiI and
0.01 M I2 in acetonitrile. The photoelectrochemical response at applied
potential of 0.15 V vs SCE is shown as an inset.
ester/SnO2/ITO systems under the same conditions (applied
potential, 0.15 V vs SCE; λex ) 435 nm). The excitation
wavelength at the Soret band guarantees the selective excitation
of the porphyrin rather than that of C60 moiety. The IPCE values
obtained by the dipping method are in the order of (ZnP-acid
+ C60-acid)/SnO2/ITO (36%) > (ZnP-acid + C60-ester)/SnO2/
ITO (28%) > ZnP-acid/SnO2/ITO (26%) > (ZnP-ester + C60-
acid)/SnO2/ITO (15%) > (ZnP-ester + C60-ester)/SnO2/ITO
(7%) > ZnP-ester/SnO2/ITO (6%) systems (Table 1). The IPCE
values are larger by 1-2 orders of magnitudes than those of a
similar porphyrin-hydrogen-bonded C60 system on ITO elec-
trode.19b The IPCE values from ZnP-acid systems are much
larger than those from ZnP-ester systems. This difference may
result from direct adsorption of ZnP-acid onto the SnO2 surface,
where an electron is directly injected into the conduction band
of the SnO2 surface from the excited singlet state of the
porphyrin to generate photocurrent.36,37 In both the porphyrin
carboxylic acid system and the porphyrin ester system, IPCE
values increase significantly when C60 acid rather than C60 ester
is employed. This suggests that not only the direct electron
injection takes place but also a competitive electron transfer
with a phase-separated, interpenetrating network involving
hydrogen-bonded electron and/or hole-transporting nanostruc-
tures (vide infra).
Photoelectrochemical Properties of SnO2/ITO and ITO
Electrodes Modified with Porphyrin and Fullerene Com-
posites. Photoelectrochemical measurements were performed
in a nitrogen-saturated acetonitrile solution containing 0.5 M
LiI and 0.01 M I2 using the modified SnO2/ITO as the working
-
electrode, a platinum counter electrode, and a I-/I3 reference
electrode. For instance, a stable anodic photocurrent from the
electrolyte to (ZnP-acid + C60-acid)/SnO2/ITO electrode, pre-
pared by the dipping method, appeared immediately upon
irradiation of the modified SnO2/ITO electrode (Figure 6). The
photocurrent fell down instantly when the illumination was cut

Modified Nanostructured SnO2 Electrodes
J. Phys. Chem. B, Vol. 109, No. 39, 2005 18471
occurs from the porphyrin excited singlet state to C60-acid,
followed by the electron injection from the reduced C60 to the
conduction band of the SnO2 surface (vide infra). Besides,
electron and hole relay may occur in the arrays of hydrogen-
bonded C60-acid and ZnP-acid, respectively, as indicated from
the dominant hydrogen-bonding interaction between the identical
ZnP-acid molecules or C60-acid molecules rather than between
different types of molecules (vide supra). It is noteworthy that
the IPCE value is improved in the mixed system with hydrogen
bonding as compared to the reference system. Although the
largest IPCE value (36%) is achieved in the (ZnP-acid + C60-
acid)/SnO2/ITO system,38,39 the hydrogen-bonding effect on
photocurrent generation is not prominent in comparison with
(ZnP-acid + C60-ester)/SnO2/ITO and ZnP-acid/SnO2/ITO
systems. It may be originated from the dipping method in which
ZnP-acid and C60-acid tend to adsorb onto the SnO2 surface
rather than hydrogen bonding to each other. To differentiate
the two competitive processes, the fabrication of these molecules
on the SnO2/ITO electrodes were carried out by using spin-
coating method, which reduces the possibility of adsorbing ZnP-
acid and C60-acid onto the SnO2/ITO electrodes significantly.
Although the IPCE values obtained by the spin-coating method
are smaller than those obtained by the dipping method, the
former trend for the IPCE values is unambiguous: (ZnP-acid
+ C60-acid)/SnO2/ITO (18%) > (ZnP-acid + C60-ester)/SnO2/
ITO (8.2%) > (ZnP-ester + C60-acid)/SnO2/ITO (7.1%) > ZnP-
acid/SnO2/ITO (3.5%) > (ZnP-ester + C60-ester)/SnO2/ITO
(2.6%) > ZnP-ester/SnO2/ITO (1.2%) systems. The trend for
the IPCE values is in good agreement with the results on the
IRRA spectra and the AFM measurements. For instance, the
IPCE value (18%) of the (ZnP-acid + C60-acid)/SnO2/ITO
system is much larger than those of (ZnP-acid + C60-ester)/
SnO2/ITO (8.2%) and ZnP-acid/SnO2/ITO (3.5%) systems
(Table 1). Furthermore, a prominent effect for the photocurrent
generation was also observed for (ZnP-ester + C60-acid)/SnO2/
ITO (7.1%) in comparison with those of (ZnP-ester + C60-ester)/
SnO2/ITO (2.6%) and ZnP-ester/SnO2/ITO (1.2%) systems. The
increase may be attributed to the enhancement of hydrogen
bonding between C60-acid molecules in the mixed film, as shown
in Figure 3g. These results demonstrate that photocurrent
generation is much enhanced in a hydrogen-bonded porphyrin-
fullerene system compared with the reference system without
hydrogen bonding.
Figure 7. Steady-state fluorescence spectra of (a) ZnP-acid/SnO2/ITO
(dotted line), (ZnP-acid + C60-ester)/SnO2/ITO (solid line, [ZnP]/[C60]
) 1:1), and (ZnP-acid + C60-acid)/SnO2/ITO (dashed line and solid
line with white circles (×10); [ZnP]/[C60] ) 1:1) electrodes. (b) ZnP-
ester/SnO2/ITO (dotted line (×10-2)), (ZnP-ester + C60-ester)/SnO2/
ITO (solid line, [ZnP]/[C60] ) 1:1), and (ZnP-ester + C60-acid)/SnO2/
ITO (dashed line, [ZnP]/[C60] ) 1:1) electrodes. The excitation
wavelength was 430 nm.
and the C60 moieties,40 which agrees with the results on the
IRRA spectrum (Figure 3g). On the other hand, (ZnP-ester +
C60-ester)/SnO2/ITO exhibit CT emission (800 nm),25 in addition
to the weak emission from the porphyrin. In the case of (ZnP-
ester + C60-ester)/SnO2/ITO, ZnP-ester and C60-ester molecules
are susceptible to make close contact due to the π-π interaction
because of the absence of hydrogen bonding. Accordingly, most
of the ZnP-ester and C60-ester make the supramolecular complex
to exhibit CT emission, whereas the minor, uncomplex ZnP-
ester and C60-ester reveals the quenching of fluorescence from
the porphyrin moiety via photoinduced ET (vide infra).
Photoinduced electron-transfer process was confirmed by the
comparison of picosecond fluorescence lifetime measurements
for (ZnP-acid + C60-acid)/SnO2/ITO, (ZnP-acid + C60-ester)/
SnO2/ITO, (ZnP-ester + C60-acid)/SnO2/ITO, (ZnP-ester + C60-
ester)/SnO2/ITO, ZnP-acid/SnO2/ITO, and ZnP-ester/SnO2/ITO
systems prepared by the dipping method. The fluorescence
lifetimes on the SnO2/ITO surfaces were measured by a time-
correlated single-photon counting technique at emission wave-
length of 660 nm due to the porphyrin moiety with excitation
at 590 nm. The decay curves of the fluorescence intensity were
fitted by three exponentials (Figure 8). However, in all cases,
the fastest component was the dominating one (>80%). The
Photophysical Studies. At first, steady-state fluorescence
spectra (λex ) 430 nm) were measured for the better under-
standing of the photodynamical behavior of the porphyrin-
fullerene system on the SnO2 electrodes prepared by the dipping
method (Figure 7). Fluorescence from the porphyrin is strongly
quenched in (ZnP-acid + C60-acid)/SnO2/ITO and (ZnP-acid
+ C60-ester)/SnO2/ITO relative to ZnP-acid/SnO2/ITO. In the
ZnP-acid system, no emission from the C60 moiety (720 nm) is
seen except the case of (ZnP-acid + C60-acid)/SnO2/ITO40
(Figure 7a). The intensity of the fluorescence due to the
porphyrin moiety in the ZnP-acid system is in the order of (ZnP-
acid + C60-acid)/SnO2/ITO < (ZnP-acid + C60-ester)/SnO2/
ITO < ZnP-acid/SnO2/ITO, which is consistent with that of the
IPCE values. Similar porphyrin fluorescence quenching was
observed for (ZnP-ester + C60-acid)/SnO2/ITO and (ZnP-ester
+ C60-ester)/SnO2/ITO in comparison with ZnP-ester/SnO2/ITO
(Figure 7b). These results suggest the occurrence of photoin-
duced electron transfer or partial charge transfer (i.e., exciplex
formation) from the porphyrin excited singlet state to the C60
moiety (vide infra).29 It is noteworthy that (ZnP-ester + C60-
acid)/SnO2/ITO reveals the emission from both the porphyrin

18472 J. Phys. Chem. B, Vol. 109, No. 39, 2005
Imahori et al.
Figure 9. Transient absorption spectrum of (ZnP-ester + C60-ester)/
Figure 8. Fluorescence decay curves of (a) ZnP-ester/SnO2/ITO
(square), (b) (ZnP-ester + C60-ester)/SnO2/ITO (upper triangle, [ZnP]/
[C60] ) 1:1), and (c) (ZnP-ester + C60-acid)/SnO2/ITO (downward
triangle, [ZnP]/[C60] ) 1:1) electrodes observed at 660 nm by the single
photon counting method. The excitation wavelength is 590 nm.
SnO2/ITO at a time delay of 10 ps excited at 555 nm.
SCHEME 2: Photocurrent Generation Diagram for the
(ZnP-acid + C60-acid)/SnO2/ITO System
lifetimes of the fastest component are summarized in Table 1.
The fluorescence lifetimes of (ZnP-acid + C60-acid)/SnO2/ITO,
(ZnP-acid + C60-ester)/SnO2/ITO, and ZnP-acid/SnO2/ITO are
much shorter than the time resolution of the present system (<40
ps).41 This is in accordance with the stepwise electron transfer
from the porphyrin excited singlet state to the C60, followed by
electron injection to the CB of the SnO2 electrode, in addition
to the fast, direct electron injection of the porphyrin singlet
excited state to the CB of the SnO2 electrode through the
chemical adsorption of carboxylic acid group onto the SnO2
surface.36,37 The fluorescence lifetimes of (ZnP-ester + C60-
acid)/SnO2/ITO (40 ps) and (ZnP-ester + C60-ester)/SnO2/ITO
(62 ps) are shorter than that of ZnP-ester/ITO systems (380 ps),
where direct electron injection process from the porphyrin
excited singlet state to the CB of the SnO2 electrode is
suppressed due to the absence of chemical adsorption of the
porphyrin onto the SnO2 surface. These results support the
occurrence of photoinduced electron transfer or partial charge
transfer from the porphyrin singlet excited state to the C60 moiety
rather than self-quenching of the porphyrin excited state due to
the aggregation to yield the charge-separated state, which
eventually leads to the photocurrent generation.
The formation of the charge-separated state in the mixed films
was detected using the femtosecond pump-probe method for
(ZnP-ester + C60-ester)/SnO2/ITO, ZnP-acid/SnO2/ITO, and
ZnP-ester/SnO2/ITO systems with the excitation wavelength of
555 nm. The transient absorption spectrum of (ZnP-ester + C60-
ester)/SnO2/ITO, prepared by dipping method, reveals the
characteristic bands around 650 nm due to zinc porphyrin radical
cation25 and 1000 nm due to C60 radical anion25 (Figure 9),
which thus can be assigned to the charge-separated state or to
the exciplex state.21,25,29 With the time resolution of the
instrument used an instant formation of the bands was observed,
which indicates that the primary exciplex formation in the
sample can be as fast as 1013 s-1. This is consistent with the
observation of CT emission due to the π-complexation between
the porphyrin and the C60 in the steady-state fluorescence
spectrum of (ZnP-ester + C60-ester)/SnO2/ITO. The decay of
the transient absorption was a complex process with the fastest
relaxation rate of 1011 s-1, which was followed by a slower
relaxation with the rate constant even smaller than 109 s-1. This
can be understood in terms of different types of structures
formed and a variety of charge-separation pathways realized in
the system. In contrast, the transient absorption spectra of ZnP-
acid/SnO2/ITO and ZnP-ester/SnO2/ITO exhibit the character-
istic bleaching around 590 nm due to the porphyrin excited
singlet state.21,25,29,42
Based on the energetics of the photoactive and radical ion
species involved in (ZnP-aicd + C60-acid)/SnO2/ITO system,
the mechanism of anodic photocurrent generation is proposed
1
(Scheme 2). First, an electron transfer takes place from ZnP*
(ZnP-acid, 0.97 V; ZnP-ester, 0.96 V (vs NHE)) to the
conduction band of SnO2 (0 V vs NHE)20 or C60 (C60-acid,
-0.43 V; C60-ester, 0.32 V (vs NHE)),20 yielding the porphyrin
radical cation (ZnP•+) and the electron in the conduction band
or ZnP•+ and C60 radical anion (C60•-).43 In the latter case, the
reduced C60•- gives an electron to the conduction band of SnO2
(0 V vs SCE)20 to yield the same state. On the other hand, I-
(0.5 V vs NHE)20 donates an electron to ZnP•+ (ZnP-acid, 1.08
V; ZnP-ester, 1.09 V (vs NHE))20 to generate anodic photo-
current. The hydrogen bonding in the porphyrin or the C60
molecules or both may be responsible for the large IPCE values
in comparison with the reference system without the hydrogen
bonding. Such hydrogen bonding would facilitate the initial
charge separation between the porphyrin and the C60 and the
separated electron and hole relay through the hydrogen-bonded
arrays which suppress the undesirable charge recombination. It
is well-known that the use of acid functionalized dyes helps in

Modified Nanostructured SnO2 Electrodes
J. Phys. Chem. B, Vol. 109, No. 39, 2005 18473
adsorbing the dye to the metal oxide.12 The advantage of this
approach compared to such conventional dye-sensitized solar
cells is the acid groups can contribute both binding to SnO2
and binding the molecules to one another. Although the present
system is rather complex relative to the dye-sensitized solar cells,
such binding properties would be modulated by varying the
chemical modification conditions (i.e., spin-coating, dipping,
and electrophoretic deposition methods) to optimize the photo-
electrochemical properties.
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Conclusion
Hydrogen bonding effects on photocurrent generation have
been examined in the mixed films of porphyrin and/or fullerene
with and without hydrogen bonding on nanostructured SnO2
electrodes. The nanostructured SnO2 electrodes modified with
the mixed films of porphyrin and fullerene with hydrogen
bonding exhibit efficient photocurrent generation (up to IPCE
value of 36%) as compared to the reference systems without
hydrogen bonding. Atomic force microscopy, infrared reflection
absorption and ultravioletvisible absorption spectroscopies, and
time-resolved fluorescence lifetime and transient absorption
measurements support the significant contribution of hydrogen-
bonding interaction between the porphyrins or the C60 moieties
or the both in the films on the electrode surface for the
photocurrent generation efficiency. These results show that
hydrogen bonding is a highly promising methodology for the
construction of molecular photoelectrochemical devices.
Acknowledgment. This work was supported by the Integra-
tive Industry-Academia Partnership (IIAP) including Kyoto
University, Nippon Telegraph and Telephone Corp., Pioneer
Corp., Hitachi, Ltd., Mitsubishi Chemical Corp., and Rohm Co.,
Ltd. H.I. also thanks Grant-in-Aid from MEXT, Japan (No.
16310073 and 21st Century COE on Kyoto University Alliance
for Chemistry) for financial support. S.Y. acknowledges support
from Nippon Sheet Glass Foundation for Materials Science and
Engineering. M.I., N.V.T., and H.L. thank the Academy of
Finland for the financial support.
Supporting Information Available: Section profiles of
AFM images (S1). This material is available free of charge via
the Internet at http://pubs.acs.org.
References and Notes
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C60-acid on the IPCE values under the same experimental conditions. The
largest IPCE value (36%) was obtained when a mixed solution with the
relative ratio of [ZnP-acid]:[C60-acid] ) 1:1 was employed for the
preparation of the modified SnO2 electrode.
(39) Thickness of the SnO2 layer (0.1-1.1 µm) on the ITO electrode
was also varied to optimize the IPCE value. The largest IPCE value (36%)
was achieved when the thickness of the SnO2 layer was 1.1 µm. Further
increase of the thickness of the SnO2 layer resulted in the formation of
extremely rough SnO2 layer.
(40) The weak emission from the C60 moiety (C60-acid) may be attributed
to the direct excitation of the C60 moiety in which some part of the identical
C60-acid molecules form hydrogen-bonded aggregates without making a
supramolecular complex with the zinc porphyrin, as suggested from the
IRRA spectra.
(41) We also tried to measure the fluorescence lifetimes on SnO2
electrodes using an up-conversion method (time resolution, ∼100 fs), but
it was not possible because the excitation light destroys the sample at the
excitation spot very quickly.
(42) A reliable conclusion could not be drawn from the transient
absorption spectra of (ZnP-acid + C60-acid)/SnO2/ITO, (ZnP-acid + C60-
ester)/SnO2/ITO, and (ZnP-ester + C60-acid)/SnO2/ITO systems because
of the weak signals and the degradation of the samples under laser
irradiation.
(43) The first reduction and oxidation potentials were determined by
using cyclic voltammetric measurements in THF containing 0.1 M Bu4-
NPF6. The energy level of the porphyrin excited singlet state was estimated
to be 2.05 eV in THF from the absorption and emission spectra.
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