Investigation of the Photoelectrochemistry of C60 and Its Pyrrolidine Derivatives by Monolayer-Modified SnO2 Electrodes

Article abstract of DOI:10.1021/jp961428g

A monolayer of a C60 mixture with arachidic acid (1:1) and C60-pyrrolidine derivatives were successfully obtained at the air/water interface and deposited onto solid substrates with tra

Full text of DOI:10.1021/jp961428g

J. Phys. Chem. 1996, 100, 16685-16689
16685
Investigation of the Photoelectrochemistry of C60 and Its Pyrrolidine Derivatives by
Monolayer-Modified SnO2 Electrodes
Chuping Luo, Chunhui Huang,* Liangbing Gan, Dejian Zhou, Wensheng Xia,
Qiankun Zhuang, Yilei Zhao, and Yanyi Huang
State Key Laboratory of Rare Earth Material Chemistry and Applications, Peking UniVersity,
Beijing 100871, P. R. China
ReceiVed: May 16, 1996; In Final Form: August 5, 1996X
A monolayer of a C60 mixture with arachidic acid (1:1) and C60-pyrrolidine derivatives [C60(C3H6NR); R )
H (1), C6H5 (2), o-C6H4NO2 (3), and o-C6H4NMe2 (4)] were successfully obtained at the air/water interface
and deposited onto solid substrates with transfer ratios of about 0.90 ( 0.05 at 30 mN‚m-1 by the Langmuir-
Blodgett (LB) technique. The photoelectrochemical investigation of these compounds on SnO2 electrodes
was carried out. The photocurrent was determined under nitrogen atmosphere because oxygen can suppress
the photocurrents. Results showed that electrons flow from the electrolyte through LB film to SnO2 and a
chemical energy conversion occurs together with the photoelectric conversion. Positive bias voltage, electron
donor, the incident light intensity, and the electron-repulsion groups in the derivatives are beneficial factors
for generating higher photocurrent. The quantum yields vary from 0.015 to 0.069 for C60 and its derivatives
1-4.
Introduction
onto hydrophilically pretreated quartz substrates to form high-
quality LB films, which has been demonstrated in previous
In the past 6 years, a rich “three-dimensional” chemistry of
spherical and polyfunctional all-carbon molecules has been
developed.1-3 Investigations reveal that fullerenes and their
derivatives, so-called fulleroids or organofullerenes, exhibit a
variety of remarkable properties, for instance, superconductivity
and molecular magnetism as well as catalytic and biological
activity.2,8 One of the most realistic applications seems to be
the development of specific electronic devices.3c When the cast
films on metal electrode surfaces, C60 can act as an n-type
semiconductor though the dark current is at least 1 order of
magnitude larger than the photocurrent.4 The bandgap of solid
C60 as a semiconductor is about 2 eV, and the conductivity can
be varied between <10-7 and 10 S/cm by doping.3b On the
other hand, fullerenes and their derivatives are good electron
acceptors and can be successively reduced up to six steps.5
Polyvinyl carbazole doped with C60 was shown to be a photo-
conductor, in which holes in the carbazole valence band act as
the charge carriers and the C60 molecules act as the photoactive
electron acceptors.6 When C60 and C70 are embedded within a
lipid membrane, they act both as photosensitizers for efficient
electron transfer from a donor and mediators for electron
transport across the membrane.7 In this process, electron transfer
from the fullerene anions formed at the reductant interface
occurs through fullerene aggregates in which the charges transfer
by hopping and ultimately reach the oxidizing entities at the
opposite interface.8
work.11 Besides, fullerenes and their derivatives can also form
stable mixed Langmuir films with long-chain fatty acids or
alcohols.9d,10a,c These mixed films could be easily transferred
onto solid substrates with a uniform transfer ratio. However,
the limiting area of the 1:1 mixed Langmuir films was much
smaller than that it should be (around 93 Å2/molecule). In
addition, solvents and the concentrations of spreading solutions
also play an important role in making high-quality LB
monolayers.10d
Here we wish to report the film formation properties of a
series of C60-pyrrolidines derivatives C60(C3H6NR) (where R
) H (1), C6H5 (2), o-C6H4NO2 (3), and o-C6H4NMe2 (4)) and
the photocurrent generation from the monolayer modified SnO2
electrodes of these compounds (abbreviated as Fuller-SnO2
electrodes). The dependencies of the photocurrents on some
factors have been studied which may enhance or decrease the
magnitude of the observed photocurrent, mainly, the bias
voltage, the electron donor and acceptor in the electrolyte, and
the light intensity as well as the electron attracting-repulsing
effect of substituent groups bridged to C60 by pyrrolidines. The
mechanism of generating photocurrent is proposed, and a
chemical energy conversion should take place together with the
photoelectronic conversion.
The monolayer film of the fullerenes is very sensitive to
vibration, easy to aggregate, and very difficult to be transferred
onto solid substrates9 due to the very high hydrophobicity of
the rigid ball-shape molecules.10 It has been discovered that
the introduction of polar atoms, such as nitrogen or oxygen,
into the close-caged molecule can not only enhance their
Langmuir film stability but also improve their monolayer
transformation ability, in which the derivatives molecules really
stand at the air/water interface. The monolayers were transferred
Experimental Section
Materials. The C60/C70 mixture was prepared in our lab
according to the graphite-arc vaporization method.12 Pure C60
was obtained by a chromatographic separation method, in
a purity of >99.9% (detected by HPLC13). The pyrrolidine
* To whom all correspondence to be addressed.
X Abstract published in AdVance ACS Abstracts, September 15, 1996.
S0022-3654(96)01428-1 CCC: $12.00 © 1996 American Chemical Society

16686 J. Phys. Chem., Vol. 100, No. 41, 1996
Luo et al.
SCHEME 1
derivatives 1-4 were synthesized by a 1,3-dipolar addition to
C60 of the decarboxylated iminium ion formed from aldehyde
(RCHO, R ) H, C6H5, o-C6H4NO2, and o-C6H4NMe2) and
N-methylglycine by following a procedure slightly modified
from that described by Maggini et al.14a The structures of 1-4
were characterized by 1H NMR and 13C NMR (400 MHz
Bruker, CS2, 310 K)14b as shown in Scheme 1.
Methylviologen diiodide (MV2+) was synthesized by the
reaction of 4,4′-bipyridyl with excess methyl iodine in refluxing
ethanol for 6 h. The product was filtered and washed with
ethanol four times, then dried in vacuum. The NMR spectrum
indicated that the product was highly pure. Arachidic acid (AA)
was purchased from Aldrich Co. The electrolytes were (n-
Bu)4NPF6 and KCl for electrochemical and photoelectrochemi-
cal experiments, respectively. The former was purchased from
(+)
Figure 1. Surface pressure-area isotherms of C60/arachidic acid (1)
and the derivative 4 [C60(C11H16N2)] (2) at the air/water surface.
Aldrich Co. and the later from Beijing Chemical Factory.
L
-
Ascorbic acid (AsA) was from Medicine Manufacture in
Shanghai and was recrystallized from water before use. The
solvents, dichloromethane and chloroform (AR), were purchased
from the Chemical Factory of Beijing. Deionized water purified
by passing through an EASYpure RF compact ultrapure water
system (Barnstead Co., U.S.) was used in all experiments.
Preparation of LB Films. A NIMA 622 computer-
controlled Langmuir trough (U.K.) was used for the formation
of the fullerenes monolayers. It took about 1 h to spread a
chloroform solution of C60/AA or its derivatives (10-6-10-5
mol‚L-1) onto an ultrapure water subphase (20 ( 1 °C, pH 5.60,
>18 Ω). After the evaporation of the solvent over 15 min, the
surface pressure-area (π-A) isotherms were recorded. The
monolayer was deposited onto the hydrophilic pretreated
transparent SnO2 glass substrates with a lateral resistance of 50
Ω at a rate of 5 mm/min (vertical dipping) at a constant surface
pressure of 30 mN‚m-1. Typical transfer ratios were 0.90 (
0.05.
Figure 2. Possible packing arrangement of C60 and arachidic acid on
the air/water surface.
Results and Discussions
Electrochemical Measurements. Cyclic voltammetry was
recorded on a PAR-270 Electrochemical Analysis System (U.S.).
Fabrication of Fuller-SnO2 Electrodes. The π-A isotherm
of C60/AA (Figure 1) shows that the film can sustain more than
70 mN‚m-1 surface pressure and has a limiting molecular area
of 87.9 Å2/molecule. Expansion and recompression of a
previously compressed film reveals very little hysteresis up to
the point at which the expansion initiated (for less than about
55 mN‚m-1) without visible patchy domains on subphase. This
suggests that the film on the aqueous subphase is a uniform
and high-quality monolayer. Supposing that the molecules of
C60 and arachidic acid were in an ideal arrangement on the air/
water interface (Figure 2), then the projection of the molecules
onto the water surface forms a square lattice with a lateral length
a; the calculated area per molecule is a2. This yields nearest-
neighbor distances of 9.4 ( 0.6 Å for C60 or arachidic acid
molecules. Considering that arachidic acid has limiting area
of 20 Å2/molecule, the calculated nearest-neighbor distance of
9.4 ( 0.6 Å is slightly smaller than of the ideal monolayer of
C60/AA (11.2 ( 0.4 Å). This fact suggests that the C60
molecules are squeezed into the hydrophobic long chains in a
certain extent at such a surface pressure.
A three-electrode configuration was used throughout.
A
polished platinum electrode (0.5 mm diameter) was used as
working electrode. The counter electrode and the reference were
a platinum wire and a Ag/AgCl electrode, respectively. All
measurements were performed in a 0.1 mol‚L-1 dichlo-
romethane solution of (n-Bu)4NPF6 at -10 °C under a nitrogen
atmosphere. The concentrations of C60 and its derivatives
ranged from 10-4 to 10-3 mol‚L-1
.
Photoelectrochemical Measurements. The photoelectro-
chemical studies was performed by using a model 600 volta-
mmetric analyzer (CH Instruments Inc., U.S.) and a 500 W
xenon lamp (Ushio Electric, Japan). A variety of filters
(Toshiba, Japan) with certain band passes were used to obtain
different wavelengths. The intensities of incident beams were
standardized by a power and energy meter (Scientech 372,
Boulder Co., U.S.). The IR light was filtered throughout the
experiment with a Toshiba IRA-25S filter (Japan). To eliminate
the influence of oxygen, all experiments were carried out under
a nitrogen atmosphere. At least eight Fuller-SnO2 electrodes
were fabricated to test the reproducibility of photocurrent data
under each conditions.
The π-A isotherms of all fullerene derivatives indicate that
these molecules can form stable monolayers on an aqueous

Photoelectrochemistry of C60
J. Phys. Chem., Vol. 100, No. 41, 1996 16687
TABLE 1: Comparison of Interfacial Properties of C60 and Its Derivatives
deposited condition
transferability
b
c
spreading solution
A0
πmax
compd
(mmol‚L-1
)
(Å2)
(mN‚m-1
)
model
treatment
or transfer ratio
ref
C60
CS2/CH2Cl2 (0.1)
benzene
CS2/CH2Cl2 (<0.1)
94.2
7
95.9
∼73
>45
∼73
vertical
horizontal
vertical
horizontal
vertical
vertical
vertical
vertical
vertical
vertical
either
x
yes
x
yes
0.85 ( 0.05
0.90 ( 0.05
0.90 ( 0.05
0.90 ( 0.05
0.90 ( 0.05
0.90 ( 0.05
9c
10b
9c
9c
C60/IAa
C60O
hydrophobic
hydrophobic
hydrophobic
hydrophilic
hydrophilic
hydrophilic
hydrophilic
hydrophilic
hydrophilic
C60(C4H8N2)
C60/AA
C60(C3H7N) (1)
C60(C9H11N) (2)
C60(C9H10N2O2) (3)
C60(C11H16N2) (4)
CS2/CH2Cl2 (7.6 × 10-2
)
89
>40
>70
>60
>60
>60
>60
11a
CHCl3 (8.8 × 10-3
)
87.9
93.4
97.8
96.6
96.7
this work
this work
this work
this work
this work
CHCl3 (7.8 × 10-3
CHCl3 (6.8 × 10-3
CHCl3 (8.5 × 10-3
)
)
)
CHCl3 (9.5 × 10-3
)
a IA: icosanoic acid. b A0: the limiting molecular area. c πmax: the collapse pressure.
subphase due to the introduction of hydrophilic functional
groups. The limiting molecular area of compound 1 is in good
agreement with that for C60O,9c but the others are slightly larger
(Table 1). The nearest-neighbor distances are calculated as
follows. Assuming that the substituents of the fullerene
derivatives are attracted toward the water and that the projection
of the molecules onto the water surface forms a triangular lattice
of circular objects of radius r, the calculated area per molecule
2
x
is 2 3r , thus the nearest-neighbor distances of the derivatives
1, 2, 3, and 4 are 10.4 ( 0.5, 10.6 ( 0.5, 10.6 ( 0.5, and 10.6
( 0.5 Å, respectively. They are slightly larger than that of
C60.8a,c This indicates that the films are really monolayers and
the phenyl group occupied a small area on the monolayers. The
π-A isotherms of expansion and recompression also show the
same results as C60/AA, suggesting that the induced hydrophilic
Figure 3. Representative actions spectrum of the Fuller-SnO2 electrode
(deriviative 4). The photocurrents and the intensities of different
wavelengths are all normalized.
groups really enhance the stability of these Langmuir films.
Photocurrent Generation from Fuller-SnO2 Electrodes.
The experimental results show that C60 and its derivatives have
very similar photoelectric responsibilities. To describe conve-
niently, the derivative 4 (C60(C11H16N2)) is chosen as a
representative to discuss below because its SnO2 electrode can
give the largest photocurrent among them.
The WaVelength of Incident Light. The photocurrents are very
low when visible incident light is used, and the absorbency of
the SnO2 electrode increases very sharply at wavelengths below
320 nm. Since the electronic absorption spectra of Fuller-films
on quartz substrate show that the absorbencies in the near-
ultraviolet part are several times higher than those in the visible
part,11a,c the light beam of the wavelength at 355 ( 30 nm was
chosen as the exciting source. The action spectra prove that
the photocurrent with the wavelength of 355 ( 30 nm as
incident light is really several times higher than that with visible
light (Figure 3).
Figure 4. Representative photocurrent for the derivative 4 obtained
under N2 atmosphere without bubbling when the Fuller-SnO2 electrode
was first illuminated.
Effect of Oxygen. Oxygen can quench the triplet state of the
fullerenes8b,15a,b and suppress >95% of the photocurrent.7b
These processes occur for the derivatives since the electronic
structure of C60 is retained in its derivatives and the similar
photoelectrochemical responses of C60 and its derivatives were
observed when oxygen exists. From our experimental results,
oxygen can not only suppress the magnitude of photocurrent
but also redirect the flow of electrons. Consequently, all
experiments were carried out under nitrogen.
Under the condition of a nitrogen atmosphere, an anodic pulse
photocurrent up to 120 nA was observed when the Fuller-SnO2
electrode was illuminated under a light intensity of 3.4
mW‚cm-2 with a 355 ( 30 nm light beam as the exciting source
(after transmitting the SnO2 electrode). Also, the photocurrent
rose gradually to an equilibrium value ranging from 19 to 38
nA in a 1.0 mol‚L-1 KCl electrolyte solution without any bias
voltage (Figure 4). The equilibrium values can increase to 65-
92 nA if nitrogen is kept bubbling when the photocurrent is
being determined.
The monolayer film shows a stable photoelectronic response
for tens of times switching on and off the light, and the
attenuation is not very large compared with the initial photo-
current (ca. 0.01 of decay rate) after keeping in 1.0 mol‚L-1
KCl electrolyte for 3 days. The photoelectronic responses of
uncoated SnO2 electrodes and more than eight SnO2 electrodes
coated by arachidic acid were studied to probe their contributions
to the photocurrent. A stable anodic photocurrent ranging from
12 to 20 nA under a 355 ( 30 nm light beam was obtained.
These values are smaller than the photocurrent generated from
the Fuller-SnO2 electrode, revealing that C60 and its derivatives
are the main contributors causing the change of photocurrent.
Effect of Bias Voltage and Light Intensity. To probe the
electron-transfer process between the SnO2 electrode and the
Fuller film, the effect of bias voltage was investigated. There
is a linear relationship with a slope of 0.27 nA‚mV-1 between
the observed photocurrent and the bias voltage in the range
-0.10 to 0.20 V vs SCE (Figure 5). The anodic photocurrent

16688 J. Phys. Chem., Vol. 100, No. 41, 1996
Luo et al.
The photocurrent can be repeated for tens of times switching
on and off the light, and the attenuation is also very small,
coincident with the results obtained under no AsA and no bias
voltage. It is worthy to notice that the equilibrium values of
photocurrent are almost the same regardless of aeration or not.
An opposite effect is observed for the addition of the electron
acceptor methylviologen diiodide (MV2+) to the electrolyte.
Without bias voltage, the photocurrents decrease sharply when
the concentration is less than 2 mmol‚L-1. Results indicate that
MV2+ not only attenuates the anodic photocurrent but also
redirects the flow of electrons when the concentration is up to
20 mmol‚L-1
.
From the above results, it can be concluded that positive bias
voltage and electron donor as well as light intensity are
beneficial factors for generating photocurrent and that the
electrons flow from the electrolyte through the LB films to the
electrodes.
Figure 5. Effluence of bias voltage to the photocurrent generation
from the Fuller-SnO2 electrode of the derivative 4 with N2 bubbling
and an irradiation of 3.4 mW/cm-2
.
Photocurrent Generation Mechanism. Photoinduced elec-
tron transfer investigations show that C60 and its derivatives
give very high yields of the triplet state under irradiation at
355 nm as a result of very high rate of intersystem crossing
3
from the singlet state.8,15 C60 is a powerful electron acceptor
with a reduction potential near 1.14 V vs SCE.16 The reduction
potential of compounds 1-4 should also be near 1.14 V vs SCE.
The reasons are that (1) the redox potentials of compound 1-4
are ca. 0.1 V negative shifts with C60 (as shown in Table 2),
which means that derivatives 1-4 retain the electronegative
property of its parent fullerene, and (2) the triplet energies of
the derivatives (1.50 eV) are very close to that of C60 (1.57
eV).8b,15a,b,17 Another important aspect of the photoinduced
electron-transfer process of fullerenes is that the triplet state
can be efficiently quenched by oxygen.8,15
Therefore, when the Fuller-SnO2 electrode is illuminated
without an electron donor, the Fuller molecules are excited to
the singlet and transfer to the triplet very quickly. Then the
excited Fuller molecules are reduced to anions by water and
oxygen is produced because the 3Ered (the reduced potential of
the triplet state) of the Fuller molecules is higher than the EO /H O
(0.66 VSCE as pH is 5.60). After that the electrons transfer from
the Fuller triplet-state anions to the holes in the valence band
of SnO2. As a result, the electrons transfer from electrolyte
through film to the SnO2 electrode. More details of mechanism
are still being investigated.
The oxygen molecules produced in the photoelectronic
response process quench the photoactive Fuller-molecules, and
a suppressed photocurrent is thus obtained. This causes the
equilibrium values of photocurrent without bubbling nitrogen
to be lower due to the concentration of oxygen in the electrolyte
rising up. As the electron donor, AsA, is added, the molecules
of AsA take precedence to give electrons to the Fuller molecules
because of the lower potential E(ox)/(red) (-0.21 VSCE). In this
case, the equilibrium photocurrent is independent of aerating
or not since there is no suppressed factor.
Figure 6. Representative photocurrent obtained from the Fuller-SnO2
electrode of the derivative 4 under N2 atmosphere with 0.1 V bias
voltage and 15 mmol‚L-1 ascorbic acid in the electrolyte solution.
increases as the positive bias of the electrode increase, and vice
versa. This shows that the electrons flow from the electrolyte
through the LB film to the electrode.
2
2
A good linear relationship between the photocurrent and the
light intensities (from 0.9 to 10.3 mW/cm2) is observed. The
photocurrent increases along with the light intensity increase.
No saturation state was observed because the filter and the SnO2
glass absorbed more than 90% of the intensity of the excitating
light and such an intensity cannot supply enough photons to
make all molecules active and contribute to the photocurrent.
Effect of Electron Donor and Acceptor. The effect of the
electron acceptor (AsA) and donor (MV2+) also certifies the
direction of electron transfer. As the well-known electron donor
ascorbic acid was added into the electrolyte solution (1.0
mol‚L-1 KCl solution), the photocurrent increases in direct
proportion with the concentration of AsA in lower range (<1.9
mmol‚L-1) with a slope of 120-200 nA‚(mmol‚L-1 -1
) , then
turns to an nonlinear relationship, and finally levels off at a
higher concentration (started at ca. 15.6 mmol‚L-1). A hori-
zontal photocurrent of the modified electrode of compound 4
as well as C60 and other derivatives was obtained with 15.6
mmol‚L-1 AsA and a 0.1 V bias voltage as shown in Figure 6.
The Effect of Different Substituent Groups on Photocur-
rent Generation. Due to the fact that arachidic acid was used
for improving the stability of C60 film, part of the SnO2 electrode
surface is occupied by the arachidic acid molecules and some
C60 molecules are squeezed into the hydrophobic long chains
TABLE 2: E1/2 a (V vs Ag/AgCl) by Cyclic Voltammetryb of C60 and Its Derivatives 1-4
compd
C60
C60(C3H7N) (1)
C60(C9H11N) (2)
C60(C9H10N2O2) (3)
C60(C11H16N2) (4)
E0/1-
-0.50
-0.88
-1.33
-0.61
-0.96
-1.51
-0.61
-0.99
-1.51
-0.64
-1.03
-1.42
-0.63
-1.00
-1.52
E1-/2-
E2-/3-
a E1/2 is the reduced potential from state 1 to state 2. b vs Ag/AgCl in a 0.1 mol‚L-1 dichloromethane solution of Bun NPF6 at -10 °C. Scan rates
4
) 200 mV/s.

Photoelectrochemistry of C60
J. Phys. Chem., Vol. 100, No. 41, 1996 16689
TABLE 3: Values of Photocurrent (I, nA) and the Quantum
Yields of the Fuller-SnO2 Electrodes with the Electrolytes
Containing 15 mmol‚L-1 AsA and Saturated N2 under 0.1 V
Bias Voltage
Acknowledgment. Financial support from the Climbing
Program (a National Fundamental Research Key Project of
China) and the National Natural Science Foundation of China
is greatly acknowledged.
electrode
Iobs(nA)
Iunit
C60/AA
1
2
3
4
270-430 450-600 550-750 500-700 1300-1900
3.4-5.4 5.6-7.5 6.9-9.4 6.3-8.8 16.3-23.8
References and Notes
(nA‚mm-2
quantum
yield (%)
)
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The photocurrent ranges of C60/AA and its derivatives without
bias voltage and electron donor are overlapped to quite an extent.
This may due to the complex influence of the produced oxygen,
the released rate of oxygen from the electrolyte solution, and
the different adsorption abilities for oxygen on the surfaces of
the Fuller-SnO2 electrodes. Results show that positive bias
voltage and electron donor as well as light intensity enhance
generating stable photocurrents. Therefore, in order to test the
capacities of generating photocurrents of each compound, the
determining conditions were chosen as follows: a 1.0 mol‚L-1
KCl electrolyte solution containing 15 mmol‚L-1 ascorbic acid
and saturated N2 under 0.1 V bias voltage. Table 3 shows the
values of photocurrent and the quantum yields of C60/AA and
its derivatives with these conditions. The quantum yields were
calculated according to the following formula:
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quantum yield )
electron number of photocurrent
photon number absorbed by the Fuller-SnO2 electrode
Results in Table 3 indicate that the photocurrent and the
quantum yield obtained from C60-SnO2 electrode are higher
than what were reported by Hwang et al.7 This difference results
from the beneficial factors used here. What need to be
emphasized is the electron attracting-repulsing effect of dif-
ferent substituent groups. The introduction of the ring of
pyrrolidine can increase the values of photocurrent as compared
with that of C60 because the methyl on the nitrogen can act as
an electron donor and cause more efficient electric charge
separation than the parent fullerene. Values also show that the
phenyl-substituted pyrrolidines promote photocurrent more
effectively. Particularly for the compound 4, the photocurrent
is more than twice that of other systems. This strongly supports
that an efficient electron-donating group can efficiently enhance
photoelectronic response. But, the electron-attracting group,
-NO2 in compound 3, does not show the effect expected on
generating photocurrent as compared with compound 2.
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Conclusion
We have successfully transferred the monolayer films of C60
and its pyrrolidines derivatives onto SnO2 electrodes by the LB
technique; their photoelectrochemistry behavior were observed.
The photocurrents indicate that electrons flow from the elec-
trolyte through the LB film to SnO2; therefore, a chemical
energy conversion occurs together with the photoelectronic
response. The derivatives can give higher photocurrents than
C60 itself, especially when efficient electron-donating groups
exist. More investigation is in progress on the electron-transfer
process and photocurrent generation of other Fuller-systems.
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