Light-induced electron transfer on a trichromophore dye modified ITO electrode

Article abstract of DOI:10.1021/jp983463o

A novel trichromophore amphiphilic dye N-methyl-2,4,6-tri-(4-dioctadecylamino)stilbazolium iodide (P₃n) has been synthesized and successfully transferred on semiconducting transparent indium-tin oxide (ITO) electrodes by a Langmuir-Blodgett (LB

Full text of DOI:10.1021/jp983463o

J. Phys. Chem. B 1999, 103, 4377-4381
4377
Light-Induced Electron Transfer on a Trichromophore Dye Modified ITO Electrode
Deng-Guo Wu, Chun-Hui Huang,* Liang-Bing Gan, Wen Zhang, and Jie Zheng
State Key Laboratory of Rare Earth Materials Chemistry and Applications, Peking UniVersity,
Beijing 100871, China
Hong Xia Luo and Nan Qiang Li
Department of Chemistry, Peking UniVersity, Beijing 100871, China
ReceiVed: August 24, 1998; In Final Form: February 11, 1999
3
A novel trichromophore amphiphilic dye N-methyl-2,4,6-tri-(4-dioctadecylamino)stilbazolium iodide (P n)
has been synthesized and successfully transferred on semiconducting transparent indium-tin oxide (ITO)
electrodes by a Langmuir-Blodgett (LB) technique. Photocurrent generation was studied in a conventional
photoelectrochemical cell. An action spectrum of the photocurrent generation is coincident with the absorption
spectrum of the LB film-modified electrode, indicating that the dye aggregate in the LB film is responsible
for the photocurrent. Some factors that may affect the observed photocurrent, such as the presence of O
2
, the
2
+
concentrations of methyl viologen (MV ) and hydroquinone (H
2
Q), pH, and the bias voltage have been
investigated. Models for the mechanism of light-induced electron transfer under different conditions are
proposed.
Introduction
CHART 1
The study of photochemical and photophysical phenomena
in organized assemblies is an area of considerable activity.
Chlorophyll, porphyrin, and phthalocyanine pigments having a
large π-conjugation effect in the molecules are classified as
1
excellent photosynthesis materials in nature. One found that if
reaction of 2,4,6-trimethylpridine (5 mL) and iodic methyl (2.4
mL) for 12 h at room temperature, and the product was purified
by recrystallization in absolute ethanol. N-Methyl-2,4,6-tri-(4-
dioctadecylamine)stilbazolium iodide (P3n) was synthesized by
condensing N-methyl-2,4,6-trimethylpyridine iodide (53 mg)
with 4-dioctadecylaminobenzaldehyde (410 mg) in refluxing
complex solvents of absolute ethanol and chloroform for 3
weeks; hexahydropyridine was used as catalyst. The product
was purified by column chromatography on silica gel with
chloroform and methanol (20:1) as eluent, and the yield is ca.
an electron donor and (or) acceptor is attached to those
sensitizers, the quantum efficiencies can be efficiently en-
2
hanced. Fujihira and co-workers have studied the relationship
between the sensitizers and push-pull electron radicals by a
Langmuir-Blodgett (LB) technique, and they found that only
if the pushing radical-sensitizer-pulling radical is attached in a
line can the quantum efficiencies of photoelectric conversion
3
be the best. Our group has systematically studied the photo-
electric conversion properties of some dye molecules with
second-order nonlinear optical character and has found that the
higher the molecular hyperpolarizability, the better the molecular
photoelectric conversion efficiency in the system of stilbazolium
2
5%. The elemental analysis is as follows. Found for P3n: C,
7
9.44; H, 11.38; N, 2.61. Calcd for C138H245N4I: C, 79.44; H,
1
4
-6
11.79; N, 2.69. H NMR (CDCl3) δ: 0.86 (t, 18H, 6CH3), 1.26
and its congeners. Recently, our work also found that if two
chromophores are connected through a suitable alkyl chain, it
can obtain more than 2 times the photocurrent compared with
(
)
m, 180H, 90CH2), 1.57 (m, 12H, 6CH2), 3.25 (t, 12H, 6CH2N-
+
, 4.04 (s, 3H, N -CH3), 6.46-6.52 (m, 3H, CHd), 6.55 (d,
7
6H, 3 phenyl), 6.92 (d, 3H, 3CHd), 7.52 (d, 6H, phenyl), 7.65
the single chromophore. As a part of our systematic study, we
(
s, 2H, pyridinyl). Mp, 199.9 °C. Methyl viologen diiodide
designed a novel dye molecule P3n (Chart 1) in which there
are three electron donors through three bridges connected by
one acceptor. By a LB technique, we successfully transferred
the molecule onto the indium-tin oxide (ITO) substrate.
In this paper, we report an investigation of the photocurrent
generation from a trichromophore stilbazolium dye LB film
modified ITO electrode. Mechanistic models for photocurrent
generation under different conditions are proposed.
2
+
(MV ) was synthesized by reaction of 4,4′-dipyridyl with
1
methyl iodide. Its identity was confirmed by H NMR analysis.
The electrolyte for the electrochemical experiment was KCl
(AR). Hydroquinone (H2Q) (AR) was recrystallized from water
before use. The spreading solvent used for film deposition was
chloroform. The water subphase was in-house deionized water
purified by an Easy pure RF compact ultrapure system (Barn-
stead Co.) (R ≈ 18 MΩ).
Apparatus. C, H, N analysis of the compounds were
Experiment Section
1
performed by using a Carlo Erba 1106 elemental analyzer. H
Materials. The 4-dioctadecylaminobenzaldehyde was syn-
thesized according to the previous method reported. The
N-methyl-2,4,6-trimethylpyridine iodide was synthesized by
NMR spectra were obtained by using a Brucker ARX 400.
Electronic spectra were measured with a Shimadzu model 3100
UV-vis-NIR spectrophotometer. LB film modified ITO
8
1
0.1021/jp983463o CCC: $18.00 © 1999 American Chemical Society
Published on Web 04/28/1999

4378 J. Phys. Chem. B, Vol. 103, No. 21, 1999
Wu et al.
Figure 1. Surface pressure-area isotherm of P
interface (20 ( 1 °C).
3
n at the air/water
Figure 2. (a) UV-vis absorption spectra of the P
photocurrent action spectrum of the P
irradiation with 464 nm light in 0.5 mol L electrolyte solution with
zero bias voltage. The intensities of different wavelengths are all
normalized.
3
n LB film; (b)
n modified ITO electrode, upon
3
-
1
electrodes were fabricated by using a model 622 NIMA
Langmuir-Blodgett trough. The light source of the photoelec-
trochemical studied was a 500 W Xe arc lamp. The light beam
was passed through a group of filters (300-800 nm, Toshiba
Co., Japan, and Schott Co.) in order to get given band-pass of
light. The light intensity at each wavelength was calibrated by
an energy and power meter (Scientech).
2
limiting area per molecule is ca. 154 Å , extrapolated from the
surface pressure versus the molecular area isotherm under 30
-
1
mN m . The value is about three times larger than that for
compound (C16H33)2N-C6H4-CHdCH-C5H4N-CH3I, ca. 53
2
9
Å , indicating that there are indeed three chromophores in the
molecule and that the three chromophore stilbazolin radicals
may be arranged on the substrate with a tilt angle. The second-
harmonic generation (SHG) experiment shows that there is a
tilt angle of 44° of the chromophores, relative to the normal of
LB Film Preparation. Solution of the P3n in chloroform was
spread dropwise onto the clean water subphase by syringe at a
subphase temperature of 20 ( 1 °C. The chloroform was
allowed to evaporate for 15 min, and the floating film was then
compressed at a rate of 40 cm2 min-1. The substrate for
1
0
the substrate, which is similar to that obtained from films of
monolayer deposition was transparent electrodes of indium-
9
,11
the hemicyanine halides.
tin oxide (ITO) coated borosilicate glass with a sheet resistance
-
2
of 50 Ω cm . The ITO plates were cleaned by detergent and
chloroform and then rinsed with pure water. To ensure the
formation of the hydrophilic surface, the plate was immersed
for 2 days in a saturated sodium methanol solution and then
thoroughly rinsed with pure water under ultrasonication several
times.
The electronic absorption spectra of the P3n LB film is shown
in Figure 2a. The λmax occurs at ca. 500 nm, which is
significantly blue-shifted from the chloroform solution (λmax )
12
520 nm), suggesting the presence of H-aggregates in the film.
Photocurrent Generation from Dye-ITO Electrode. A
steady cathodic photocurrent ranging from 1300 to 1700 nA
-
2
For deposition of the monolayer, the ITO slide was first
immersed in the subphase, when the monolayer was formed
3
cm (Table 1, condition 1) was obtained from the dye P n
-
1
monolayer (transferred under 30 mN m surface pressure)
modified electrode when it was illuminated by white light with
-1
and compressed to 30 (or 10) mN m . The slide was then raised
-
1
-2
at a rate of 4 mm min . Only monolayer films having transfer
ratios of 1.0 ( 0.1 were used in all experiments.
110 mW cm light intensity and without any bias voltage in
-1
0.5 mol L KCl electrolyte solution. The photoelectric response
was very stable when switching on and off many times. The
action spectrum of the cathodic photocurrents for the dye
Photoelectrochemical and Electrochemical Measurements.
A conventional glass three-electrode cell (30 mL capacity)
2
having a flat window with a 0.38 cm effective light irradiation
-1
monolayer transferred under 30 mN m surface pressure is also
area for illumination was used. The counter electrode was a
polished Pt wire, and the reference was a saturated calomel
electrode (SCE). All photoelectrochemical data were recorded
by a model CH 600 voltammetric analyzer controlled by a
computer. Cyclic voltammetry (CV) experiments were per-
formed on an EG&G PAR 273 potentiostat/galvanostat with
EG&G PAR 270 electrochemical software. The supporting
electrolyte was an aqueous solution of 0.5 mol L KCl. Unless
specified, oxygen was removed from the solutions by bubbling
N2 before every measurement.
shown in Figure 2b. The spectrum responses coincide with the
absorption spectrum of the dye films, suggesting that the
aggregate of the dye in the LB film is responsible for the
-2
photocurrent generation. About 117 nA cm photocurrent can
-1
be obtained for the dye P3n transferred under 30 mN m surface
pressure irradiated by a 464 nm light with an intensity of 1.14
16
-2 -1
-1
×
10 photons cm
s
in 0.5 mol L KCl electrolyte solution
-1
with zero bias voltage. The quantum yield æ is about 0.41%
for the monolayer dye P3n modified electrode (Table 1, condition
2
) (the absorbance ratio of the film is about 1.6% for the dye
under an incident light of 464 nm).
Results and Discussion
Table 1 shows that the photocurrent generated from the LB
-
1
Characterization of LB Film and Dye-ITO Electrode.
Surface pressure measurements on monolayers were used to
determine the average values of the area per molecule occupied
by the dye at the air/water interface. Figure 1 shows a typical
surface pressure-area π-A isotherm for the dye P3n. The
film deposited under 10 mN m is lower than that from the
-1
LB film deposited under 30 mN m . About 16% of the limiting
area per molecule is reduced when the surface pressure is
-
1
increased from 10 to 30 mN m , while 70% photocurrent can
be increased under irradiation of light of 464 nm. It may be

Light-Induced Electron Transfer
J. Phys. Chem. B, Vol. 103, No. 21, 1999 4379
TABLE 1: P
3
n Photocurrent Generation I and Quantum Yield æ under Different Conditionsa
condition 1
condition 2
condition 3
condition 4
condition 5
condition 6
condition 7
deposition
pressure
mN m
I
I
æ
I
æ
I
æ
I
æ
I
æ
I
æ
-
1
-2
-2
-2
-2
-2
-2
-2
nA cm
nA cm
%
nA cm
%
nA cm
%
nA cm
%
nA cm
%
nA cm
%
3
1
0
0
1336-1705
700-970
117
69.2
0.41
0.24
202.8
109.1
0.69
0.37
345.7
172.4
1.2
0.59
368.9
184.4
1.3
0.63
475.9
234.2
1.63
0.80
600.3
243.2
2.1
0.83
a
-2
All the photocurrent data (except condition 1) were obtained under 464 nm irradiation monochromatized from white light of 110 mW cm
-
2
through a band-pass filter (KL45+GG420). Condition 1: irradiation under a white light of 110 mW cm . Condition 2: irradiation under
-
1
2+
monochromatized light at 464 nm. Condition 3: under -100 mV bias voltage. Condition 4: under 5 mmol L MV . Condition 5: under 5 mmol
-
1
2+
-1
2+
L
5
MV and -100 mV bias voltage. Condition 6: under pH ca. 2 and 5 mmol L MV and oxygen-saturated. Condition 7: under pH ca. 2 and
1
-
2+
mmol L MV , oxygen-saturated, and -100 mV bias voltage.
n monolayer transferred under 30 mN m-
1
1
Figure 4. CV of the P
surface pressure on ITO substrate as working electrode in 0.5 mol L
3
-
KCl aqueous solution. Curves 1-3 is the number of sweeps, and the
-
1
inset shows CV in pH ca. 2 solution. Sweep rate ) 100 mV s
3
Figure 3. UV-vis absorption spectra of the P n multilayer films.
Number of layers: (a) 1, (b) 3, (c) 5, (d) 7. Inset shows a linear
relationship between the absorbance at 500 nm and the number of
-
1
monolayers transferred under 30 mN m surface pressure.
due to the difference of magnitude of the active molecules per
area in the LB films.
A linear relationship between the absorbance at 500 nm and
the number of monolayers transferred was observed (Figure 3),
indicating that there are no interactions between the layers that
significantly affect the extinction coefficient. However, the
photocurrent first increases as the the number of layers increases,
and after three layers it levels off and then gradually decreases.
The reason is that the film resistance increases as the number
of layers is increased, since there are some long hydrocarbon
chains in the P3n molecule, indicating the presence of a
competitive process between the number of active molecules
and the film resistance.
Figure 5. Photocurrent vs bias voltage for LB film of dye P n on ITO
3
-
1
Electrochemical Properties of the Dye. To estimate the
redox potentials of the excited-state dye and to discuss the
mechanism of photocurrent generation, CV studies were carried
out. Typical voltammograms of the P3n films transferred under
electrode in 0.5 mol L KCl aqueous solution upon irradiation with a
-
2
1
10 mW cm white light.
4). The potential is -7 mV vs SCE. The positive shift of the
reduction peak indicates that the proton is involved in the redox
reaction and that the reduction reaction takes place more easily
-
1
3
0 mN m are shown in Figure 4. Monolayer dye-ITO was
-
1
used as a working electrode in 0.5 mol L KCl aqueous
solution. The reduction and oxidation potentials of the couples
1
4
in an acidic rather than in a neutral medium. Note that those
CV data refer to the dye in the LB film on an ITO semiconduct-
ing electrode, not to the dye in solution.
-
-
dye/dye and dye /dye are -730 and -384 mV vs SCE,
respectively, in a neutral medium. The difference between the
anodic and cathodic peak potentials (∆Ep ) 350 mV) are larger
than the ideal value (60 mV), indicating slow kinetics of the
couple.13 Figure 4 shows that the cathodic current is ca. 370
µA, while its anodic current is only 25 µA, indicating that there
is an irreversible chemical reaction on the electrode. Experi-
mental results show that the cathodic current is decreased
successively as the number of sweeps increases, while the anodic
current does not, which agrees with the fact that the redox wave
is irreversible. A weak reduction peak, ca. 45 µA, was obtained
when the pH of the electrolyte solution was adjusted to ca. pH
Bias Voltage Dependence of Electron Injection. To un-
derstand the effect of bias voltage on electron injection, the
relationship between bias voltage and photocurrent generation
is investigated. The photocurrent is increased as the applied
negative bias voltage is increased (Figure 5). About 203 nA
-
2
cm photocurrent can be obtained at -100 mV, and under
irradiation of light of 464 nm, the quantum yield reaches ca.
0.69% (Table 1, condition 3). This can be readily attributed to
an acceleration in the rate of charge separation, resulting in an
increasing proportion of the electron-hole pair separation
occurring within the LB film. This increase in magnitude could
2
with dilute HCl solution in an acidic medium (inset of Figure

4380 J. Phys. Chem. B, Vol. 103, No. 21, 1999
Wu et al.
2+
Figure 6. Dependence of the photocurrent on the MV concentration
-
2
-1
upon irradiation with a 110 mW cm white light in 0.5 mol L KCl
aqueous solution.
Figure 7. Effect of pH on the photocurrent in Britton-Kobinson buffer
solution (b) under zero bias voltage and (2) under -100 mV in 0.5
-1
-2
mol L KCl electrolyte solution upon irradiation with a 110 mW cm
white light.
be attributable, at least in part, to an increase of the yield for
electron injection from the ITO conduction band to the HOMO
of the dye at negative bias.
electron acceptor and thus assists generation of a cathodic
photocurrent, so the protons play twofold roles in the system.
Under favorable conditions, e.g., pH ca. 2, in the presence of
Effect of Electron Donors and Acceptors. Figure 6 shows
the dependence of the photocurrent on MV2 concentration. It
+
2
+
-1
2+
is seen that the photocurrent increases with increasing MV
O and 5 mmol L MV , with -100 mV bias voltage, a
2
2+
-2
concentration, then it levels off when the MV concentration
photocurrent of ca. 600 nA cm was obtained under 464 nm
-
1
is larger than ca. 8 mmol L . If the electrolyte solution was
saturated by bubbling oxygen, the photocurrent would be
noticeably increased by ca. 50%, indicating that O2 is a favorable
factor in the electron-transfer process because it acts as an
electron acceptor through the formation of a superoxide anion
radical.14 If H2Q was added to the electrolyte solution, the
photocurrent would quickly decrease and become an anodic
irradiation; the quantum yield is 2.1% (Table 1, condition 7),
which is the biggest quantum yield in all stilbazolin pigments
6
so far. The open-circuit voltage was 0.675 V for the P n
3
monolayer film.
Mechanism of Photocurrent Generation from the Dye-
ITO Electrode. For the mechanism of light sensitization for
some semiconductors, both energy and electron transfer were
assumed to be possible. Van der Auweraer et al. reported that
light sensitization of anodic and cathodic photocurrents belonged
-
1
current, though the H2Q concentration is 0.1 mmol L ,
indicating that the presence an electron donor is unfavorable to
production of a cathodic photocurrent in the system. According
to the Franck-Condon principle, the only possibility under these
circumstances is electron transfer between the ITO electrode
and dye* (excited state) in the LB film. Electrons flow from
the ITO semiconductor to the HOMO of the dye in the LB films,
and the excited state loses electrons to the electrolyte, the dye*
acting as an electron transporter. Whatever factors cause this
to occur, as long as they can improve the electron transporting
17
to the mechanism of energy transfer. To elucidate the
mechanism for the cathodic and anodic photocurrent in this
system, the energies of the relevant electronic states must be
estimated. The electron affinity the conduction band (E ) and
c
valence band (E ) edges of the ITO electrode surface are
v
18
estimated to be ca. -4.5 and -8.3 eV, respectively. The lowest
levels available to accept an electron and the first excited singlet
state for the dye P n film transferred under 30 mN m are
-
1
3
2+
ability, they may enhance the photocurrent. MV , for example,
can accept electrons from the excited state, and it increases the
cathodic photocurrent. H2Q, which donates electrons to the
HOMO of the dye, prohibits the flow of electrons from ITO to
the HOMO of the dye, thus resulting in a reduced photocurrent
and even reversing the electron flow.
assumed to be -4.01 eV (-0.73 V vs SCE) and -6.49 eV on
an absolute scale, respectively, with reference to a reduction
potential of -0.73 V (vs SCE) and band gap of 2.48 eV (500
nm). The reduction potential of the dye P n in an acidic medium
3
is -4.733 eV (-0.007V vs SCE), the reduction potential of
2
+
19
MV is -4.51 eV (-0.23V vs SCE), and the oxidation
19
Effect of pH Value. The effect of pH value on the
photocurrent generation was investigated in a Britton-Kobinson
potential of H2Q is -4.61 eV (-0.13 V vs SCE), respectively,
on an absolute scale.
-
1
buffer solution containing 0.5 mol L KCl. Figure 7 shows
that the photocurrent decreases rapidly with increasing pH value.
To understand the bias voltage dependence of pH value, the
photocurrent was also determined under -100 mV. The change
tendency of the photocurrent under the different pH is coincident
with that under zero bias voltage, except the photocurrent is
larger than that at the corresponding pH value of zero bias
voltage, indicating that an acidic medium is favorable for the
production of a cathodic photocurrent while the alkaline medium
is not. This may result from the acidic medium being able to
decrease the flat band potential of the electrode.16 On the other
hand, the nitrogen of aminobenzenes in the title compound P3n
The direction of photocurrent generation are shown in Scheme
1. It can be seen that the direction of the photocurrent depends
not only on the dye sensitized by light but also on the nature of
the redox couple in the aqueous phase surrounding the electrode.
In the presence of redox couples favoring electron donation the
photocurrent exhibited anodic photocurrent, while with electron
acceptors in the aqueous phase it exhibited cathodic photocur-
rent.
The cathodic photocurrent probably involves an electron
transfer from the excited dye aggregate to the electron acceptor
with a subsequent electron transfer from the conduction band
of the ITO electrode to the hole residing in the dye aggregate1
9
+
2+
can combine with H ion in an acidic system; it may accept
in the presence of some electron acceptors, such as O2, MV ,
+
2+
3+
protons to became HP3n , H2P3n or H3P3n , which is an
and the acidic medium in the monolayer film system.

Light-Induced Electron Transfer
J. Phys. Chem. B, Vol. 103, No. 21, 1999 4381
SCHEME 1: Schematic Diagram Showing Electron
Transfer Processes of (a) Cathodic Photocurrent and (b)
Anodic Photocurrenta
Acknowledgment. The project is financially supported by
a climbing program and NNSFC (No. 29471005, 29671001)
References and Notes
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(
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monolayer film. The quantum yield is 2.1% under favorable
conditions.
(
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