Photoinduced electron transfer at a gold electrode modified with a self-assembled monolayer of fullerene

Article abstract of DOI:10.1039/a809918i

A stable anodic photocurrent was observed in the presence of an electron sacrificer when a gold electrode modified with a self-assembled monolayer of C₆₀ was illuminated with monochromic light, indicating the generation of a vectorial electron

Full text of DOI:10.1039/a809918i

Photoinduced electron transfer at a gold electrode modified with a
self-assembled monolayer of fullerene
Hiroshi Imahori,*a Takayuki Azuma,a Shinichiro Ozawa,a Hiroko Yamada,a Kiminori Ushida,b Anawat
Ajavakom,a Hiroyuki Noriedaa and Yoshiteru Sakata*a
a The Institute of Scientific and Industrial Research, Osaka University, Mihoga-oka, Ibaraki, Osaka 567-0047, Japan.
E-mail: imahori@sanken.osaka-u.ac.jp;sakata@sanken.osaka-u.ac.jp
b The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
Received (in Cambridge, UK) 21st December 1998, Accepted 22nd February 1999
A stable anodic photocurrent was observed in the presence of
NO2
NO2
O
NHCO-(CH2)10-Br
CHO
an electron sacrificer when a gold electrode modified with a
self-assembled monolayer of C60 was illuminated with
monochromic light, indicating the generation of a vectorial
electron flow from the electron donor to the gold electrode
via the excited states of C60.
CHO
ii–iv
i
O
2
3
4
v, vi
The unique three-dimensional structure of C60 has prompted
many researchers to apply it to materials science.1 In this
context, fullerene thin films formed using a variety of methods,
including Langmuir–Blodgett (LB) techniques and evaporation,
have displayed interesting physical and chemical properties.2
Self-assembled monolayers (SAMs) are useful for constructing
highly ordered, two- and three-dimensional structures on
substrates.3 Several groups have already reported the prepara-
tion, electrochemistry and surface properties of C60 SAMs.4–9
However, to the best of our knowledge, there have been no
reports of the photoelectrochemical properties of SAMs of
fullerenes. Here we report the preparation and photoelec-
trochemistry of a SAM of C60-tethered alkanethiol 1 on a gold
electrode (Fig. 1).
The synthetic route to C60-linked polyalkanethiol 1 is shown
in Scheme 1. It is well established that alkanethiols with a
polymethylene chain form densely packed monolayers on gold
surfaces. It is reported that the presence of an amido group in a
chain enhances the stability of monolayers due to inter-
molecular hydrogen bonding.10 Thus, it is expected that the
attachment of a long alkyl chain to C60 with an amide linkage
would allow us to produce a well-ordered C60 SAM on a gold
electrode. Aldehyde 5 was prepared via six steps from
2-nitrobenzaldehyde 2. Compound 1 was synthesized by the
condensation of N-methylglycine, C60 and 5.11 The structures of
1 and related compounds were confirmed by spectroscopic
methods including 1H NMR and mass spectra.† Cyclic
voltammetry of 1 in CH2Cl2 containing 0.1 m Bu4NPF6 as
electrolyte exhibited reversible waves due to the first and the
second reductions of C60 (20.59, 21.00 V vs. Ag/AgCl), as
shown in Fig. 2.
NHCO-(CH2)10-SH
CHO
vii
1
5
Scheme 1 Reagents and conditions: i, neopentyl glycol, TsOH, 91%; ii, H2,
Pd/C, 72%; iii, 11-bromoundecanoic acid, N-methylmorpholine, 2-chloro-
4,6-dimethoxy-1,3,5-triazine, 99%; iv, H2SO4, TFA, 99%; v, potassium
thioacetate, 80%; vi, KOH, 96%; vii, N-methylglycine, C60, 13%.
soaking, the electrode was washed well with CHCl3 and dried
with a stream of argon. Cyclic voltammetry of 1/Au in CH2Cl2
containing 0.1 m Bu4NPF6 with a sweep rate of 100 mV s21
showed a wave at E1/2 = 20.63 V (vs. Ag/AgCl) and a second
smaller wave at E1/2 = 21.07 V, which correspond to the first
and second reduction/oxidation peaks of C60, respectively (Fig.
2). The values of the peak splittings (DEpeak = 210 mV) are
large, compared with the ideal value in solution, indicating the
slow kinetics between the C60 and the gold electrode due to
structural constraints. Integration of the area under the curve
observed for the first reduction due to the C60 corresponds to a
surface coverage of G = 1.4 3 10210 mol cm22 (120 Å2
molecule21),‡ which agrees well with the value (1.4 3 10210
mol cm22) in similar C60 SAM systems.8,9 The value is
somewhat larger than the values of the hexagonal (78 Å2
A monolayer of 1 was formed by the spontaneous bonding of
1 onto Au(111)/Cr/Si(100) substrates (hereafter, 1/Au, where /
represents an interface). The bonding was carried out from a 20
mm CHCl3 solution for 20 h to complete the formation. After
(CH2)10
S
CONH
CH3
N
Fig. 2 Cyclic voltammograms of (a) 1 in CH2Cl2 (dotted line) and (b) 1/Au
in CH2Cl2: sweep rate 100 mV s21, electrode area 0.48 cm2, initial potential
0 V.
Au (111)
Fig. 1 A self-assembled monolayer of 1/Au (111).
Chem. Commun., 1999, 557–558
557

quenched by AsA (20.19 V).15 The resulting C60 anion radical
would give an electron to the gold electrode, resulting in the
recovery of the initial state. Overall, electron flow occurs from
AsA to the gold electrode via C60.
In conclusion, a photoelectrochemical cell with a gold
electrode modified with a SAM of C60 has been constructed for
the first time. The high quantum yield implies that a combina-
tion of C60 and SAMs is promising for applications in materials
science. Our results will provide the basic information for the
development of photovoltaic devices and sensors.
This work was supported by a Grant-in-Aid for COE
Research and Scientific Research on the Priority Area of
Electrochemistry of Ordered Interfaces and Creation of Deloc-
alized Electronic Systems from Ministry of Education, Science,
Sports and Culture, Japan. Y. S. thanks the Mitsubishi
Foundation for financial support.
Fig. 3 Action spectrum of (a) C60 SAM cell and (b) absorption spectrum of
1 in CHCl3 (8.88 mm).
Notes and references
molecule21) or the simple square (98 Å2 molecule21) packing
in similar C60 LB films.12 This may be related to steric
hindrance around the phenyl group on the pyrrolidine ring
attached to C60.
† Selected data for 1: dH(270 MHz, CDCl3) 11.84 (br s, 1H), 8.64 (d, J 8,
1H), 7.48 (d, J 8, 1H), 7.36 (t, J 8, 1H), 7.08 (t, J 8, 1H), 5.10 (s, 1H), 5.09
(d, J 10, 1H), 4.29 (d, J 10, 1H), 2.95 (s, 3H), 2.59 (q, J 8, 2H), 2.49 (t, J 8,
2H), 1.8–0.8 (m, 17H); m/z (FAB-MS) 1070 (M + H+).
Photoelectrochemical measurements were carried out for
1/Au in an argon-saturated 0.1 m Na2SO4 solution containing 50
mm ascorbic acid (AsA) as an electron sacrificer using a
modified gold electrode as the working electrode, a platinum
counter electrode, and a Ag/AgCl reference electrode (hereaf-
ter, Au/1/AsA/Pt, where / represents an interface). A stable
anodic photocurrent flowed immediately after the gold elec-
trode was irradiated and fell instantly when the illumination was
terminated. The photoelectrochemical response was repeated
for tens of times without any signs of attenuation when the light
was switched on and off. An increment of the anodic
photocurrent with an increase of positive bias to the gold
electrode (2400 mV to +200 mV) in the system demonstrated
that the direction of the photocurrent takes place from the
cathode to the anode through the electrolyte. The intensity of the
photocurrent for the Au/1/AsA/Pt cell is an order of magnitude
larger than those of the Au/1/Pt cell or the bare Au/AsA/Pt cell,
indicating the involvement of AsA and C60 for the generation of
the photocurrent. The agreement of the action spectrum with the
absorption of 1 in CHCl3 from 350–500 nm (Fig. 3) shows that
C60 is the photoactive species. Under excitation with l = 403 ±
6.9 nm light of 6.6 mW cm22 and 0.1 V bias voltage, we
obtained a photocurrent density of 290 nA cm22. Assuming that
the absorption coefficient of 1 on the gold surface is the same as
that in CHCl3, absorbance of 1/Au at 403 nm is calculated to be
7.81 3 1024. Given the absorbance for 1/Au, we can estimate
that the quantum yield of the Au/1/AsA /Pt cell is 7.5%.§ The
value is at least one order of magnitude larger than those in
similar photoelectrochemical cells of porphyrin SAMs13 and
comparable to those (1.2–8.2%) in similar C60 LB cells.14 These
results indicate that C60 is an excellent electron mediator as well
as a good electron acceptor.15 Based on these data together with
previous results, we can propose the photocurrent generation
mechanism. It is plausible that the excited singlet state (1.11 V
vs. Ag/AgCl) and/or the triplet state (0.82 V) of the C60 are
‡ The roughness factor (1.1) was estimated by iodine chemisorption on the
Au(111) surface.
§ Absorption spectra for 1/Au could not be obtained in reflection or
transmission mode because of the low absorption coefficient of C60 as well
as the low value of the surface coverage.
1 M. S. Dresselhaus, G. Dresselhaus and P. C. Eklund, Science of
Fullerenes and Carbon Nanotubes, Academic Press, San Diego,
1996.
2 C. A. Mirkin and W. B. Caldwell, Tetrahedron, 1996, 52, 5113.
3 A. Ulman, Introduction to Ultrathin Organic Films, Academic Press,
San Diego, 1991.
4 J. A. Chupa, S. Xu, R. F. Fischetti, R. M. Strongin, J. P. McCauley, Jr.,
A. B. Smith, III, J. K. Blasie, L. J. Peticolas and J. C. Bean, J. Am. Chem.
Soc., 1993, 115, 4383.
5 K. Chen, W. B. Caldwell and C. A. Mirkin, J. Am. Chem. Soc., 1993,
115, 1193.
6 W. B. Caldwell, K. Chen, C. A. Mirkin and S. J. Babinec, Langmuir,
1993, 9, 1945.
7 V. V. Tsukruk, L. M. Lander and W. J. Brittain, Langmuir, 1994, 10,
996.
8 X. Shi, W. B. Caldwell, K. Chen and C. A. Mirkin, J. Am. Chem. Soc.,
1994, 116, 11 598.
9 F. Arias, L. A. Godínez, S. R. Wilson, A. E. Kaifer and L. Echegoyen,
J. Am. Chem. Soc., 1996, 118, 6086.
10 R. C. Sabapathy, S. Bhattacharyya, M. C. Leavy, W. E. Cleland, Jr. and
C. L. Hussey, Langmuir, 1998, 14, 124.
11 M. Maggini, G. Scorrano and M. Prato, J. Am. Chem. Soc. 1993, 115,
9798.
12 T. Nakanishi, H. Murakami and N. Nakashima, Chem. Lett., 1998,
1219.
13 H. Imahori, H. Norieda, S. Ozawa, K. Ushida, H. Yamada, T. Azuma,
K. Tamaki and Y. Sakata, Langmuir, 1998, 14, 5335.
14 C. Luo, C. Huang, L. Gan, D. Zhou, W. Xia, Q. Zhuang, Y. Zhao and
Y. Huang, J. Phys. Chem., 1996, 100, 16 685.
15 H. Imahori and Y. Sakata, Adv. Mater., 1997, 9, 537.
Communication 8/09918I
558
Chem. Commun., 1999, 557–558