Efficient bidirectional photocurrent generation by self-assembled monolayer of penta(aryl)[60]fullerene phosphonic acid

Full text of DOI:10.1002/asia.200900155

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DOI: 10.1002/asia.200900155
Efficient Bidirectional Photocurrent Generation by Self-Assembled
Monolayer of PentaACTHNUTRGNEUNG(aryl)[60]fullerene Phosphonic Acid
Aiko Sakamoto,[a] Yutaka Matsuo,*[a, b] Keiko Matsuo,[b] and Eiichi Nakamura*[a, b]
Photocurrent-generating
self-assembled
monolayer
(SAM) of photoactive molecules on an electrode[1,2] is an ar-
chetypal device to achieve photoelectric conversion.[3] Each
molecule in the assembly represents a molecular device that
absorbs light and generates photocurrent by giving or re-
ceiving electrons to or from the electrode. Among such mol-
ecules, fullerene derivatives[2] are particularly interesting for
their high electron affinity[4] for a long-lived triplet excited
state, small reorganization energy,[5] and good electron-
transporting ability.[6] However, the propensity of fullerene
derivatives to aggregate poses a problem in SAM formation,
because it causes excited state annihilation and low efficien-
cy of photocurrent generation. We recently reported a
simple molecular design that effectively solves this problem
Figure 1. SAMs of penta
ACHTUNGTREN(NUNG organo)[60]fullerenes on a metal oxide elec-
trode. a) Penta(biphenyl)[60]fullerene pentacarboxylic acid 1 (pentapod
AHCTUNGTRENNUNG
fullerene) attached to the electrode generates anodic photocurrent only
and not cathodic current. b) Phosphonic acid 2a generates both anodic
and cathodic photocurrent under reductive and oxidative environment,
respectively. The hexyl linker was chosen in consideration of the size of
the aryl spacer to secure stable binding of the molecule to the substrate
surface.
(Figure 1a); that is, a pentaACTHNURTGNEU(GN biphenyl)[60]fullerene pentacar-
boxylic acid 1 resembling an Apollo lunar landing module
generated an anodic (and not cathodic) photocurrent with a
quantum efficiency of up to 18% under 400 nm irradia-
tion.[7] The unidirectional photocurrent generation is unique
among other fullerenes that are generally known to be bidir-
ectional, and are thus worthy of careful investigations. In ad-
dition, the role of the five aryl groups as a space- and orien-
tation-controlling anchor needs to be confirmed. One practi-
cal problem was that the SAM on ITO is not very stable
and did not tolerate the use of organic medium. To investi-
gate these issues, we have studied phosphonic compounds
(2a and 2b, Scheme 1) that bear aryl spacer groups but lack
the carboxylic acid groups. We found that these molecules
form stable SAM on ITO and SnO2 both in an aqueous and
organic medium, and effectively generate both anodic and
cathodic currents. The results suggest that polarization of
the pentaarylfullerene molecule 1 caused by the five acid
[a] Dr. A. Sakamoto, Prof. Y. Matsuo, Prof. E. Nakamura
Nakamura Functional Carbon Cluster Project, ERATO
Japan Science and Technology Agency
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 (Japan)
Fax : (+81)3-5800-6889
E-mail: matsuo@chem.s.u-tokyo.ac.jp
nakamura@chem.s.u-tokyo.ac.jp
[b] Prof. Y. Matsuo, Dr. K. Matsuo, Prof. E. Nakamura
Department of Chemistry
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-0033 (Japan)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.200900155.
Scheme 1. Synthesis of [60]fullerene phosphonic acid derivatives.
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Chem. Asian J. 2009, 4, 1208 – 1212

groups is a factor responsible for the unidirectional photo-
current generation property of this molecule.
The phosphonic acid fullerene derivatives 2a and 2b start-
ed from the penta-adducts, C60Ar5H (Ar=biphenyl and
phenyl) as shown in Scheme 1.[8] Deprotonation of the
penta-adduct with KOtBu in THF generated a cyclopenta-
dienyl anion,[9] which was alkylated with excess 1,6-diiodo
hexane or 1,4-diiodo butane to obtain the iodoalkyl adducts
3a and 3b. Michaelis–Arbuzov reaction converted the ter-
minal iodine atom into the dimethyl phosphate group in de-
rivatives 4a and 4b. Demethylation of the methyl esters of
4a and 4b with bromotrimethylsilane followed by methanol-
ysis afforded the desired phosphonic acid derivatives 2a and
2b as an air-stable orange powder in good overall yield. The
products were characterized by NMR and atmospheric-pres-
sure chemical ionization mass spectrometry (APCI-MS),
and the molecular structure of 4b was determined unambig-
uously by X-ray crystallography (Figure 2). To our knowl-
Figure 3. Photophysical properties. a) UV/Vis absorption spectra of solu-
tions of compounds 2a in 1,2-Cl2C6H4 (dotted line) and 2b in 1,2-
Cl2C6H4/ethanol (1:1 v/v%, solid line). b) Emission spectra of solutions
of dimethyl phosphates 4a (dotted line) and 4b (solid line).
small
[(1.5Æ0.1)ꢁ103]
as
for
the
penta-
AHCTUNGTRENNUNG
(organo)[60]fullerenes in general,[10] because of extremely
efficient intersystem crossing from the singlet excited state
to the triplet excited state. The electrochemistry of the di-
methyl phosphates 4a and 4b instead of the phosphonic
acid 2a and 2b were performed, because the acidic hydro-
gen atom interferes with the measurements. The cyclic vol-
tammograms exhibited two reversible one-electron reduc-
tion processes at E=À1.49 V and À1.94 V (4a) versus Fc/
Fc+ (Figure S1 in the Supporting Information).
Photoelectrochemical studies of the SAMs of the penta-
AHCTUNGTREG(NNNU aryl)[60]fullerenes 2a and 2b were performed by irradia-
tion at lex =400 nm (244 mW). The SAMs were prepared on
ITO and SnO2 by immersion of the electrode into a solution
of the compound. Quartz crystal microbalance measure-
ments[11] indicated that the SAM deposition saturates after
10 min immersion time. As shown by the data in Table 1,
the phosphonic acid molecules generated both anodic and
cathodic photocurrents under reductive and oxidative envi-
ronments, respectively.
In the presence of the sacrificial donor ascorbic acid
(AsA), the SAMs of the biphenyl compound 2a showed
anodic photocurrent with quantum yields of 4.5% and 11%
without and with an applied bias voltage of 0.1 V, respec-
tively (Table 1, entries 1 and 2). The phenyl compound 2b
showed 0.5% and 1.9% yield under the same conditions
Figure 2. Crystal structure of dimethylphosphonate 4b. a) The ORTEP
drawing with 30% probability level ellipsoids. b) Top view of the CPK
model. c) Side view of the CPK model.
edge, this is the first phosphonic acid derivative of a fuller-
ene to be used for photocurrent generation on an electrode
surface.
The compounds 2a and 2b
Table 1. Photocurrent generation characteristics of the SAMs of 2a or 2b.[a]
showed essentially the same
photophysical and electrochem-
ical properties in solution as the
carboxylic acid 1. The UV/Vis
absorption maximum appears
at 397 nm (e=16800mÀ1 cmÀ1,
Figure 3a, assigned to the p–p*
excitation of the fullerene
core). The emission peak wave-
length (excitation wavelength
400 nm) was 630 and 627 nm
for 4a and 4b, respectively
(Figure 3b). The photolumines-
Entry
Cmpd
Sacrificer
Solv.
Substrate
Bias[b]
Direction
Quantum
yield [%]
1
2
3
4
5
6
7
8
9
2a
2a
2b
2b
2a
2b
2a
2a
2a
2a
AsA
AsA
AsA
AsA
MV2+
MV2+
AsA
MV2+
TEA
TEA
H2O
H2O
H2O
H2O
H2O
H2O
H2O
H2O
ITO
ITO
ITO
ITO
ITO
ITO
SnO2
SnO2
ITO
ITO
0
0.1
0
0.1
À0.1
À0.1
0.1
anodic
anodic
anodic
anodic
cathodic
cathodic
anodic
cathodic
anodic
anodic
4.5
11.0
0.5
1.9
1.5
0.2
2.1
5.8
17.6
11.8
À0.1
CH3CN
CH3CN
0
10
À0.1
[a] Conditions: sacrificing reagents, 50 mm; excitation wavelength, 400 nm. [b] Applied bias potentials vs an
Ag/AgCl reference electrode (filled with saturated aqueous KCl solution). Applied bias potentials vs a Ag/
cence quantum yield was very Ag+ reference electrode for the non-aqueous systems.
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Y. Matsuo, E. Nakamura et al.
COMMUNICATION
(entries 3 and 4), and was found to be generally less effec-
tive (data not shown). Figure 4a shows an on–off profile of
the anodic photocurrent generation (150 nAcmÀ2). The
tentials, respectively (Table 1, entries 9 and 10). Note that
the À0.2 V bias potential is effectively comparable to the
0 V bias in an aqueous system since the Ag/Ag+ couple
(non-aqueous) is about 0.2 V lower than the Ag/AgCl
couple (aqueous). The observed efficiency is comparable to
the best value reported for fullerene derivatives on the ITO
system (19.5%).[2c]
We suggest a rationale for the anodic and cathodic photo-
current generation in Figure 5. The redox potential of the
excited state of the pentaACTHUNRGTNE(NUG organo)[60]fullerene can be calcu-
lated by the equation E0’ex =E0’+Egap, where E0’ is the re-
ex
Figure 4. Photoelectrochemical measurements were performed upon irra-
diation at lex =400 nm (244 mW) in an argon-saturated 0.1m Na2SO4
aqueous solution containing AsA (50 mm) or oxygen/MV2+ (50 mm) as
sacrificing reagents using ITO/2a as a working electrode, a platinum
counterelectrode, and a Ag/AgCl (KCl saturated) reference electrode.
a) Photocurrent response for anodic photocurrent in the presence of
AsA under 0.1 V bias potential vs Ag/AgCl (KCl saturated). b) Corre-
sponding action spectrum. c) Photocurrent response for cathodic photo-
current in the presence of MV2+ under À0.1 V bias potential vs Ag/AgCl
(KCl saturated). d) Corresponding action spectrum.
action spectrum of the ITO/2a/AsA/Pt system (Figure 4b) is
in good agreement with the absorption spectrum of com-
pound 2a (Figure 3a), indicating that 2a is responsible for
the anodic photocurrent generation. In the presence of the
sacrificial acceptor methyl viologen dication (MV2+), the
SAMs of compounds 2a and 2b generated cathodic photo-
current with 1.5% and 0.2%, respectively under À0.1 V ap-
plied bias potential (Table 1, entries 5 and 6).
The SAMs of compound 2a on SnO2 were prepared with
the same immersion procedures. The SnO2/2a system exhib-
ited higher cathodic (quantum yield, 5.8%, entry 8) than
anodic photocurrent (2.1%, entry 7), likely because of the
lower work function of SnO2 (4.75 eV)[12] than that of ITO
(4.95 eV),[13] which facilitates electron transfer from the
electrodes.
Figure 5. Mechanisms of photocurrent generation. a) Anodic current.
b) Cathodic current. The fullerene molecules are simplified as C60.
The phosphonic acid compounds bind tightly to the ITO
surface and can be used in an organic solvent system as op-
posed to the carboxylic acid 1, which is easily depleted in an
organic solvent. Triethanolamine (TEA) was found to be a
good sacrificial donor to be used in an organic solvent
(CH3CN). Photocurrent generation measurements of the
ITO/2a system in CH3CN resulted in 17.6% and 11.8%
photocurrent quantum yields under 0 V and À0.1 V bias po-
duction potential of the excited state (i.e., redox couple of
C60*R5), E0 is the reduction potential of the ground state
(redox couple of C60R5; Figure S1 in the Supporting Infor-
mation), and Egap is the energy gap for excitation. The
energy gap between the ground state and the singlet excited
state was derived from the peak top of the emission spectra
of compound 2a in solution (Figure 3b). The energy gap be-
tween the triplet state and the ground state was taken from
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Chem. Asian J. 2009, 4, 1208 – 1212

Self-Assembled Monolayer of PentaACTHNUTRGNE(UNG aryl)[60]fullerene Phosphonic Acid
the literature.[10] Arrows indicate the electron flows after
photo-excitation. The first step is the generation of the sin-
glet excited state of the fullerene. This singlet species is
known to be rapidly and quantitatively converted into the
triplet species.[10] The excited fullerene is naturally easier to
reduce than the neutral molecule. The triplet fullerene
(+0.6 V) receives an electron from the donor AsA
(À0.19 V), and the resulting fullerene radical anion (À1.0 V)
then donates its electron to the ITO electrode, leading to
the anodic photocurrent generation.
Figure 5b illustrates the cathodic photocurrent generation
process. Upon photoirradiation, the triplet fullerene receives
an electron from the ITO, and the resulting fullerene radical
anion (redox couple: À1.0 V) donates an electron to MV2+
(redox couple: À0.62 V) or molecular oxygen (redox
couple: À0.48 V) to generate the cathodic photocurrent.[2b]
This energy diagram accounts for the bidirectional photocur-
rent generation by compound 2. As the effect of the carbox-
ylic acid groups on the electronic properties of the fullerene
core is known to be rather small (e.g., ca. 0.1 eV lowering of
the fullerene LUMO),[7] we consider that the polarization
effect of these electron-withdrawing groups caused the shut-
down of the cathodic current generation in 1. A related po-
larization effect was suggested for Grꢂtzel cells and for or-
ganic modulation of the work function.[14]
utions containing Na2SO4 (50 mm) electrolyte and sacrificing reagents
(50 mm; AsA and MV2+) in a quartz photoelectrochemical cell equipped
with an Ag/AgCl (filled with saturated aqueous KCl solution) reference
electrode and a platinum wire counterelectrode (diameter 0.5 mm). The
SAM-modified ITO electrodes were used as working electrodes. The sur-
face area of the electrode was 0.20 cm2. The light intensity at 400 nm was
244 mW. Quantum yields (f) of anodic and cathodic photocurrent were
obtained by the following equation: f=(i/e)/[I ACTHNUTRGNEUNG
(1–10A)], where I=(Wl/
hc). I is the number of photons per unit area and unit time, A is the ab-
sorbance of the adsorbed dyes at l nm, i is the photocurrent density, e is
the elementary charge, W is the light power irradiated at l nm, l is the
wavelength of light irradiation, h is the Planck constant, and c is the
speed of light. A was estimated from the next equation, A=ec’l, where e
was measured from the UV/Vis spectrum of the compound in solution
and the surface coverage c’l was estimated from cyclic voltammetry.
Acknowledgements
This work was partially supported by KAKENHI (#18105004) and the
Global COE Program for Chemistry Innovation of the MEXT, Japan.
K.M. thanks the Japan Society for the Promotion of Science (JSPS) for a
Research Fellowship for Young Scientists.
Keywords: fullerenes · molecular devices · phosphonic acid ·
photocurrent generation · self-assembled monolayers
In summary, we have synthesized fullerene phosphonic
acids C60(Ar)5ACHTUNGTRENNUNG(CH2)nPO(OH)2 (Ar=Ph, biphenyl; n=4 and
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Experimental Section
Preparation of self-assembled monolayers on ITO: The ITO electrodes,
which were treated beforehand with O3/UV, were immersed in the 1,2-di-
chlorobenzene solution of fullerene derivatives (0.02 mm) for 10 min. The
substrates were taken from the solution and rinsed with 1,2-dichloroben-
zene to remove excess molecules from the surface. Then they were rinsed
also with fresh dichloromethane to remove 1,2-dichlorobenzene and fi-
nally dried in an argon stream. The immersion time was determined to
10 min from the data of quartz crystal microbalance measurements (Affi-
nix-Q, Initium).[11]
Typical measurements of photocurrent generation: Photocurrent was
measured by a HOKUTO DENKO electroanalyzer HZ-5000, and irradi-
ation was performed by a HITACHI fluorescence analyzer F-4500. Pho-
tocurrent measurements were conducted in argon-saturated aqueous sol-
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Received: April 23, 2009
Published online: June 17, 2009
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Chem. Asian J. 2009, 4, 1208 – 1212