Uphill photooxidation of NADH analogues by hexyl viologen catalyzed by zinc porphyrin-linked fullerenes

Article abstract of DOI:10.1021/jp011613g

In the absence of oxygen, the photolytically generated C₆₀^.- moiety in ZnP^.+-C₆₀^.- and ZnP^.+-H₂P-C₆₀^.- radical ion pairs undergoes one-electron oxidation by hexyl viologen (HV²⁺), whereas the ZnP^.+ moiety is reduced by NADH analogues (1-benzyl-1,4-dihydronicotinamide and 10-methyl-9,10-dihydroacridine). Thus, both ZnP-C₆₀ and ZnP-H₂P-C₆₀ donor-acceptor ensembles act in benzonitrile as efficient photocatalysts for the uphill oxidation of NADH analogues by HV²⁺. In the case of ZnP-C₆₀, the quantum yield of the photocatalytic reaction increases with increasing concentration of HV²⁺ or an NADH analogue to reach a limiting value of 0.99. The limiting quantum yields of ZnP-C₆₀ and ZnP-H₂P-C₆₀ agree well with the quantum yields of radical ion pair formation, ZnP^.+-C₆₀^.- and ZnP^.+-H₂P-C₆₀^.-, respectively. In the presence of oxygen, the lifetimes of the radical ion pairs are, however, markedly reduced because of an oxygen-catalyzed back electron transfer process between C₆₀^.- and ZnP^.+. Such an impact on the radical ion pair lifetime consequences a significant decrease in the photocatalytic reactivity of the dyad (i.e., ZnP-C₆₀) in the overall photooxidation of an NADH analogue by HV²⁺. By contrast, the reactivity of the triad (i.e., ZnP-H₂P-C₆₀) shows little effects upon admitting O₂.

Full text of DOI:10.1021/jp011613g

J. Phys. Chem. A 2002, 106, 1903-1908
1903
ARTICLES
Uphill Photooxidation of NADH Analogues by Hexyl Viologen Catalyzed by Zinc
Porphyrin-Linked Fullerenes†
,
‡
,‡
‡
‡
Shunichi Fukuzumi,* Hiroshi Imahori,* Ken Okamoto, Hiroko Yamada,
§
,§
,|
Mamoru Fujitsuka, Osamu Ito,* and Dirk M. Guldi*
Department of Material and Life Science, Graduate School of Engineering, Osaka UniVersity, CREST, Japan
Science and Technology Corporation (JST), Suita, Osaka 565-0871, Japan, and Institute of Multidisciplinary
Research for AdVanced Materials, Tohoku UniVersity, CREST, Japan Science and Technology Corporation
(
JST), Sendai, Miyagi 980-8577, Japan, and Radiation Laboratory, UniVersity of Notre Dame,
Notre Dame, Indiana 46556
ReceiVed: April 27, 2001; In Final Form: June 27, 2001
In the absence of oxygen, the photolytically generated C60 moiety in ZnP -C60 and ZnP -H
P-C60•-
•
-
•+
•-
•+
2
2
+
•+
radical ion pairs undergoes one-electron oxidation by hexyl viologen (HV ), whereas the ZnP moiety is
reduced by NADH analogues (1-benzyl-1,4-dihydronicotinamide and 10-methyl-9,10-dihydroacridine). Thus,
both ZnP-C60 and ZnP-H
2
P-C60 donor-acceptor ensembles act in benzonitrile as efficient photocatalysts
2
+
for the uphill oxidation of NADH analogues by HV . In the case of ZnP-C60, the quantum yield of the
2
+
photocatalytic reaction increases with increasing concentration of HV or an NADH analogue to reach a
limiting value of 0.99. The limiting quantum yields of ZnP-C60 and ZnP-H P-C60 agree well with the
quantum yields of radical ion pair formation, ZnP -C60 and ZnP -H
of oxygen, the lifetimes of the radical ion pairs are, however, markedly reduced because of an oxygen-
2
•+
•-
•+
•-
2
P-C60 , respectively. In the presence
•
-
•+
catalyzed back electron transfer process between C60 and ZnP . Such an impact on the radical ion pair
lifetime consequences a significant decrease in the photocatalytic reactivity of the dyad (i.e., ZnP-C60) in
2
+
the overall photooxidation of an NADH analogue by HV . By contrast, the reactivity of the triad (i.e., ZnP-
H
2
2
P-C60) shows little effects upon admitting O .
+
Introduction
In the biological redox systems, the NADH/NAD system
+
(
NAD ) nicotinamide adenine dinuclotide; NADH ) reduced
form of NAD ) plays a key role in energy conversion and
The spherical shape of C60, containing sixty delocalized π
electrons, renders these carbon allotropes ideal components for
the construction of efficient electron transfer model systems.
Most importantly, the small reorganization energy, found for
C60 in electron transfer processes, results, on one hand, in an
overall acceleration of the charge separation (CS) step, whereas
on the other hand, a deceleration was noted for the energy-
+
9
-11
storage.
In fact, NADH is a source of two electrons and a
9
-11
proton, which is the equivalent to a hydride ion.
Once
+
NADH is oxidized to NAD , it is, however, rather difficult to
reduce NAD back to NADH by a one-electron reducing entity.
The intrinsic stability of NAD relates to its low reduction
+
+
wasting charge recombination (CR) step.1
-3
potential (-1.1 V vs. SCE) although the overall two-electron
Thus, efficient
1
0,11
process is thermodynamically favorable.
An NADH ana-
stepwise CS, for example, in fullerene-based dyad, triad, and
tetrad ensembles, has been accomplished along well-designed
logue, 9-phenyl-10-methyl-9,10-dihydroacridine (AcrHPh) has
previously been used as an electron source in the direct
photochemical oxidation of AcrHPh by N,N'-dimethyl-4,4'-
bipyridinium dication (methyl viologen, MV ) without a
photocatalyst. The amount of energy stored in this photo-
and fine-tuned redox gradients.1
-3
The resulting high-energy
CS state is converted into electrical energy by constructing
integrated artificial photosynthetic assemblies on gold electrodes
2
+
1
2
with the use of self-assembled monolayers (SAMs).4,5 However,
-
1 12
chemical oxidation was estimated as 0.76 eV (73 kJ mol ).
the conversion of the high-energy CS state in fullerene-
containing donor-acceptor linked systems into chemical energy
has yet to be reported, although there have been some reports
on uphill photochemical reactions catalyzed by other photo-
In this study, we wish to report on highly efficient energy
conversion systems, based on several C -containing donor
6
0
acceptor ensembles, zinc porphyrin-C60 (ZnP-C ) and zinc
60
6
-8
sensitizers.
porphyrin-free base porphyrin-fullerene triad (ZnP-H P-
2
C60). These systems act as novel photocatalysts for the uphill
†
Part of the special issue “Noboru Mataga Festschrift”.
2+
oxidation of NADH analogues by hexyl viologen (HV ) in
the absence of oxygen. By contrast, upon admitting oxygen,
the rate limiting back electron transfer (BET) process, that is,
CR between the fullerene radical anion and the zinc porphyrin
radical cation, was accelerated profoundly without forming
*
To whom correspondence should be addressed. E-mail: fukuzumi@
ap.chem.eng.osaka-u.ac.jp. E-mail: imahori@ap.chem.eng.osaka-u.ac.jp.
E-mail: ito@tagen.tohoku.ac.jp. E-mail: guldi.1@nd.edu.
‡
Osaka University.
Tohoku University.
University of Notre Dame.
§
|
1
0.1021/jp011613g CCC: $22.00 © 2002 American Chemical Society
Published on Web 08/04/2001

1904 J. Phys. Chem. A, Vol. 106, No. 10, 2002
Fukuzumi et al.
SCHEME 1
Figure 1. Structures of ZnP-C60 and ZnP-H P-C60.
2
reactive oxygen species, such as singlet oxygen or superoxide
anion.13 The effects of O2 on the photocatalytic reactivity of
ZnP-C60 and ZnP-H2P-C60 are also examined in relation with
the catalytic effects of O2 on the BET reactions.1
+
-1
-1 20
of AcrH at 358 nm (ꢀ ) 18 000 M cm ). In order to avoid
the contribution of light absorption of the products, only the
initial rates were used for determination of the quantum yields.
Laser Flash Photolysis. Nanosecond transient absorption
measurements were carried out using SHG (532 nm) of a Nd:
YAG laser (Spectra-Physics, Quanta-Ray GCR-130, fwhm 6
ns) as an excitation source. For transient absorption spectra in
the near-IR region (600-1600 nm), monitoring light from a
pulsed Xe lamp was detected with a Ge-avalanche photodiode
3
Experimental Section
Materials. The synthesis and characterization of the por-
3,14
phyrin-fullerene linked molecules (ZnP-C60 and ZnP-H2P-
3
C60 ) have been described previously (see Figure 1). Preparation
(
Hamamatsu Photonics, B2834). Photoinduced events in micro-
and millisecond time regions were estimated by using a
continuous Xe lamp (150 W) and an InGaAs-PIN photodiode
Hamamatsu Photonics, G5125-10) as a probe light and a
detector, respectively. Details of the transient absorption
of 1-benzyl-1,4-dihydronicotinamide (BNAH) and the BNA
1
5
dimer was described previously. 10-Methyl-9,10-dihydro-
acridine (AcrH2) was prepared from 10-methylacridinium iodide
(
+
-
(
AcrH I ) by the reduction with NaBH4 in methanol and
15
+ -
purified by recrystallization from ethanol. AcrH I was
prepared by the reaction of acridine with methyl iodide in
3
measurements were described elsewhere. All of the samples
in a quartz cell (1 × 1 cm) were deaerated by bubbling argon
+
-
acetone and was converted to the perchlorate salt (AcrH ClO4 )
by the addition of magnesium perchlorate to the iodide salt
through the solution for 15 min.
+
-
16
Electrochemical Measurements. The cyclic voltammetry
(
AcrH I ) and purified by recrystallization from methanol.
(
CV) measurements were performed on a BAS 50 W electro-
chemical analyzer in a deoxygenated PhCN solution containing
.10 M n-Bu4NPF6 as a supporting electrolyte at 298 K. The
differential pulse voltammetry measurements were also per-
formed on a BAS 50 W electrochemical analyzer in a deaerated
PhCN solution containing 0.10 M n-Bu4NPF6 as a supporting
electrolyte at 298 K (10 mV s ). The glassy carbon working
electrode was polished with BAS polishing alumina suspension
and rinsed with acetone before use. The counter electrode was
a platinum wire. The measured potentials were recorded with
respect to an Ag/AgCl (saturated KCl) reference electrode.
Ferrocene/ferricenium was used as an external standard.
Tetrabutylammonium hexafluorophosphate used as a supporting
electrolyte for the electrochemical measurements was obtained
from Tokyo Kasei Organic Chemicals. Benzonitrile was pur-
chased from Wako Pure Chemical Ind., Ltd. and purified by
successive distillation over calcium hydride. The synthesized
0
1
,1-dihexyl-4,4'-dipyridinium dibromide and the corresponding
-
1
2+
13,17
diperchlorate salt (HV ) were described previously.
Quantum Yield Determinations. A standard actinometer
(
potassium ferrioxalate)18 was used for the quantum yield
determination of the photochemical reactions of NADH ana-
logues with hexyl viologen in the presence of ZnP-C60 or ZnP-
H2P-C60. Typically, a square quartz cuvette (10 mm i.d.) which
3
contained a deaerated PhCN solution (3.0 cm ) of ZnP-C60
Results and Discussion
-
6
-4
2+
(
1
3.0 × 10 M), BNAH (4.0 × 10 M), and HV (8.0 ×
-4
2+
0
M) was irradiated with monochromatized light of λ ) 433
Photocatalytic Oxidation of BNAH by HV . The energy
nm from a Shimadzu RF-5000 fluorescence spectrophotometer.
Under the conditions of actinometry experiments, the actinom-
eter and ZnP-C60 absorbed essentially all of the incident light
of λ ) 433 nm. The light intensity of monochromatized light
levels of the singlet and triplet excited states of the investigated
ZnP-C60 dyad in PhCN are summarized in Scheme 1, illustrat-
ing the different relaxation pathways following the initial
3
photoexcitation event of ZnP-C60. Time-resolved techniques,
-
9
-1
of λ ) 433 nm was determined as 9.22 × 10 einstein s
including fluorescence lifetime and transient absorption meas-
urements, have been employed to probe the CS and CR
dynamics in this donor-acceptor array, disclosing the formation
with the slit width of 20 nm. The photochemical reaction was
monitored using a Hewlett Packard 8452A diode-array spectro-
photometer. The quantum yields in the absence of oxygen were
•
+
•-
3,13
of the ZnP -C60 pair in a variety of solvents.
With the
•
+
determined from increase in absorbance of HV at 615 nm (ꢀ
help of these time-resolved studies we determined the quantum
yield of the radical ion pair formation as Φ1 )0.99. The rate
of intramolecular back electron transfer (kBET) from C60 to
ZnP in ZnP -C60 , is 1.3 × 10 s in oxygen-free PhCN
-
1
-1
3
)
10 000 M cm ). The ꢀ value was determined by the
2+
•-
quantitative reduction of HV by tetramethylsemiquinone
radical anion which was prepared by the dispoportionation
reaction of tetramethylhydroquinone and the dianion obtained
by deprotonation with tetrabutylammonium hydroxide. In the
presence of oxygen, the quantum yields of the photooxidation
of AcrH2 by HV2 were determined from increase in absorbance
•
+
•+
•-
6
-1
3
solutions.
1
9
The photophysics of the second compound, namely, ZnP-
3
H2P-C60, in PhCN are well-studied. An initial singlet-singlet
+
1
*
energy transfer from the ZnP moiety (2.04 eV) to the H2P

Uphill Photooxidation of NADH Analogues
J. Phys. Chem. A, Vol. 106, No. 10, 2002 1905
Figure 2. Transient absorption spectra of a PhCN solution containing
-
4
2+
-3
ZnP-C60 (1.0 × 10 M) and HV (3.5 × 10 M) after laser
excitation (0.1 and 1 µs) in deaerated PhCN. Inset: Absorption-time
profiles at 620 and 1000 nm.
moiety (1.89 eV) is followed by a sequential ET relay starting
1
•+
•-
with the ZnP- H2P*-C60 via the transient ZnP-H2P -C60
•+
•-
(
1.63 eV) to yield finally the ZnP -H2P-C60 (1.34 eV). The
individual charge shift reactions are, however, in competition
with the back ET to the ground state and triplet excited states,
resulting in only a moderate quantum yield (0.40) for the final
radical ion pair formation (ZnP -H2P-C60 ). On the other
hand, the large distance, separating the donor and acceptor
moieties, is beneficial with respect to increasing the lifetime
Figure 3. (a) Spectral change observed in the steady-state photolysis
-
4
2+
-4
of a PhCN solution of BNAH (4.0 × 10 M), HV (8.0 × 10 M),
-6
and ZnP-C60 (3.0 × 10 M) under irradiation with monochromatized
light of λ ) 433 nm. (b) Spectral change observed in the steady-state
•
+
•- 3
-
4
2+
photolysis of a PhCN solution of BNAH (4.0 × 10 M), HV (8.0
-4
-6
×
10 M), and ZnP-H
2
P-C60 (3.0 × 10 M) under irradiation with
monochromatized light of λ ) 433 nm.
4
-1
•+
•-
(
kBET ) 4.8 × 10 s ) of the ZnP -H2P-C60 pair by a factor
of 27 relative to that noted for the ZnP -C60 pair.
A comparison between the one-electron oxidation potential
•
+
•-
3,13
SCHEME 2
•
-
0
of C60 (E ox ) -0.67 V vs SCE, which is equivalent to the
one-electron reduction potential of C60) and the one-electron
reduction potential of HV2 (E ) -0.42 V vs SCE) unravels
+
0
red
•
-
2+
that an electron transfer from C60 to HV is exergonic by
+
0
.25 eV (Scheme 1). In fact, when HV2 is added to the ZnP-
•
-
C60 system, a direct electron transfer from C60 in the radical
•
+
•-
2+
•+
ion pair (ZnP -C60 ) to HV occurs, producing HV (λmax
615 nm) as shown in Figure 2. Importantly, the decay of the
C60 absorption at 1000 nm coincides with the appearance
of HV at 615 nm (see inset of Figure 2), and furthermore,
the decay rate varies linearly with HV concentration. These
findings serve as crucial testimony for the existence of a one-
electron transfer between the photolytically generated C60 (i.e.,
ZnP -C60 ) and HV to form the stable HV .
The transient features of ZnP are not affected by this
reaction and remain virtually stable on the monitored time scale
shown in Figure 2.13 The second-order rate constant of the
intermolecular electron transfer is determined from the formation
)
•
-
21
via photoirradiation of ZnP-C60 or ZnP-H P-C , the oxida-
2
60
•
+
2+
tion of BNAH and the reduction of HV should be ac-
2+
complished by a reaction with ZnP and C60•-, respectively
•+
22
•+
(Scheme 2). Hereby, the longer lifetime of the ZnP -spacer-
C60 state in the triad as compared to that in the dyad (vide
•-
•-
•
+
•-
2+
•+
supra) may result in facilitated redox reactions involving the
•
+
2+
CS state, BNAH and HV .
In fact, this pathway was experimentally confirmed by
2+
photolysis of the ZnP-C /BNAH/HV and ZnP-H P-C /
6
0
2
60
2+
BNAH/HV systems with visible light (433 nm) in deoxygen-
ated PhCN. For instance, Figure 3a,b depicts the steady state
photolysis in deoxygenated PhCN, in which the HV absorption
•
+
8
-1 -1
rate of HV as (7 ( 2) × 10 M s . Thus, the rate of
intermolecular electron transfer from C60 to HV (2.5 × 10
at 3.5 × 10 M of HV ) competes well with the rate of
•
-
2+
6
•+
-1
-3
2+
s
band (λmax ) 402 and 615 nm) increases progressively with
irradiation time. By contrast, no reaction occurs in the dark or
in the absence of the photocatalyst (i.e., ZnP-C60 or ZnP-
•
-
•+
intramolecular back electron transfer from C60 to ZnP in
•
+
•-
6
-1 3
ZnP -C60 (kBET ) 1.3 × 10 s ).
•+
•+
If ZnP is indeed reduced by an external electron donor such
H P-C ) under photoirradiation. Once HV is generated in
2
60
2+
as BNAH, separately added to the ZnP-C60/HV or ZnP-
the photochemical reaction, it was found to be stable in
deoxygenated PhCN. The stoichiometry of the reaction is
established by employing eq 1,
H2P-C60/HV2 systems, ZnP-C60 or ZnP-H2P-C60 may, in
+
2
+
fact, photocatalyze the overall oxidation of BNAH by HV
Scheme 2). An electron transfer from BNAH to ZnP•+ is
expected to be exothermic, considering the one-electron oxida-
(
hν
2
+
+
•+
+
BNAH + 2HV
8 BNA + 2HV + H (1)
ZnP-C60
0
15
tion potential of BNAH (E ox ) 0.57 V vs SCE), which is
.14 V less positive than the one-electron reduction potential
of ZnP (0.71 V vs SCE, which is equivalent to the one-electron
ZnP-H2P-C60
0
•+
where BNAH acts as a two-electron donor to reduce two
13
•+
oxidation potential of ZnP). Thus, once the CS state is obtained
equivalents of HV .

1906 J. Phys. Chem. A, Vol. 106, No. 10, 2002
Fukuzumi et al.
TABLE 1: Quantum Yields (Φobs) of Photocatalytic
-
3
2+
Oxidation of BNAH (4.0 × 10 M) by HV in the Presence
M) in Deoxygenated PhCN
-
6
-6
of ZnP-C60 (3.0 × 10 M) or ZnP-H P-C60 (3.0 × 10
2
catalyst
ZnP-C60
[HV2+]/mM
Φ
obs
2
4
8
0
2
8
0.65
0.76
0.87
0.90
0.40
0.40
1
ZnP-H P-C60
2
SCHEME 3
Figure 4. Plot of Φobs/(Φ
1
- Φobs) vs [HV2+] for the photocatalytic
-
4
2+
oxidation of BNAH (4.0 × 10 M) by HV with ZnP-C60 (3.0 ×
-
6
1
0
M).
The quantum yields (Φobs) for the photochemical reaction of
2
+
BNAH with HV in the presence of ZnP-C60 or ZnP-H2P-
1
8
C60 were determined using a ferrioxalate actinometer under
irradiation with monochromatic light of λ ) 433 nm. The Φobs
-
6
value in the presence of ZnP-C60 (3.0 × 10 M) increases
2+
with increasing HV concentration at a fixed BNAH concen-
-
3
tration (4.0 × 10 M). Conversely, the Φobs value in ZnP-
2+
H2P-C60 is invariant, irrespective of the chosen HV concen-
tration (Table 1). We ascribe this difference in concentration
dependence to effects that evolve from the different lifetimes
of radical ion pair in ZnP -C60 and ZnP -H2P-C60 (vide
infra).
Figure 5. Spectral change observed in the steady-state photolysis of
-
4
2+
-4
a PhCN solution of AcrH
2
-6
(4.0 × 10 M), HV (8.0 × 10 M), and
•+
•-
•+
•-
ZnP-H P-C (3.0 × 10 M) under irradiation with monochromatized
2
60
light of λ ) 433 nm.
•
+
BNAH is oxidized by the ZnP moiety in the radical ion
pair (k1), whereas HV2 is reduced by the C60•- moiety (k2) as
shown in Scheme 3. These individual electron transfer processes
compete, however, with the BET in the radical ion pair (kBET).
6
-1
3
confirmed for the ZnP-C60 system. With kBET (1.3 × 10 s ),
k2 is derived from
+
Φobs
k2
k1[BNAH]
2+
•
+
BNAH thus produced may disproportionate to give BNAH,
BNA , and H , when the overall redox reaction is given by
eq 1.
)
[HV ] +
(3)
Φ - Φ
kBET
kBET
+
+ 23
1
obs
9
-1 -1 24
the slope as (1.0 ( 0.1) × 10 M s . This value agrees
By applying the steady-state approximation to the concentra-
tions of the ZnP•+ and C60 moieties in Scheme 3, the
dependence of Φobs on [BNAH] and [HV ] can be derived as
within experimental error with the value determined directly
•-
8
from the time-resolved experiment in Figure 2 [(7( 2) × 10
2+
-1 -1
M
s ].
given by eq 2
2+
Photocatalytic Oxidation of AcrH2 by HV . Upon replac-
-
3
ing BNAH with AcrH2 (4.0 × 10 M), photoinitiated (λ )
2+
2
+
-3
+
Φ (k [BNAH] + k [HV ])
433 nm) oxidation by HV (8.0 × 10 M) produces AcrH
•+
1
1
2
Φobs
)
(2)
(λmax ) 358 nm) and HV (λmax ) 402 and 615 nm) in the
presence of ZnP-H2P-C60, as shown in Figure 5. The stoi-
chiometry of the photocatalytic AcrH2 oxidation (eq 4) is the
same as established above for the BNAH oxidation (eq 1).
2+
kBET + k [BNAH] + k [HV ]
1
2
where Φ1 is the quantum yield of the radical ion pair formation.
Under the experimental conditions (i.e., k1[BNAH] or k2[HV ]
2+
.
kBET), the limiting quantum yield (Φ∞) corresponds to Φ1.
The dependence of Φobs on [HV ] in the case of ZnP-C60
2+
(Table 1) agrees with eq 2 from which the Φ∞ value is
determined as 0.99. This value agrees well with the Φ1 value
obtained independently from the direct kinetic analysis of the
3
radical ion pair formation (0.99). In the case of ZnP-H2P-
As is the case of the photocatalytic BNAH oxidation, the
quantum yields were determined from an increase in absorbance
C60, the Φ∞ value agrees also with the Φ1 value (0.40) obtained
from the transient absorption spectrum.3 Such agreements
confirm the validity of Scheme 3.
a
•+
due to HV at 615 nm (Table 2). Hereby, the Φ values (Φ )
0
.25 (ZnP-C60) and 0.15 (ZnP-H2P-C60)) are markedly
Equation 2 is reformulated to eq 3, which predicts, in
principle, a linear correlation between Φobs/(Φ1 - Φobs) and
HV ]. With the help of Figure 4 this linear correlation was
reduced relative to the corresponding values obtained for the
oxidation of BNAH (Φ ) 0.87 (ZnP-C60) and 0.40 (ZnP-
H2P-C60)). We ascribe this decrease to the higher oxidation
2
+
[

Uphill Photooxidation of NADH Analogues
J. Phys. Chem. A, Vol. 106, No. 10, 2002 1907
molecular electron transfer from C60 to HV2+ (5.6 × 10 s
•
-
6 -1
TABLE 2: Quantum Yields (Φobs) of Photocatalytic
-
3
2+
-3
Oxidation of AcrH
2
(4.0 × 10 M) by HV (8.0 × 10 M)
-3
2+
at 8.0 × 10 M HV ) is much faster than the BET rate (3.2
-
6
in the Presence of ZnP-C60 (3.0 × 10 M) or
5 -1
×
10 s ) in an O2-saturated PhCN. This can be regarded as a
-
6
ZnP-H
2
P-C60 (3.0 × 10 M) in the Absence and Presence
potential rationale for the observation that the Φobs value of
the triad (ZnP-H2P-C60) system is little affected by the
presence of oxygen. Conversely, in the case of dyad (ZnP-
C60) the BET rate, especially in an O2-saturated PhCN solution
of O
2
in PhCN
Φ
obs
-
3
catlyst
O
2
) 0 M
O
2
) 8.5 × 10
M
7
-1 13
ZnP-C60
ZnP-H P-C60
0.25
0.15
0.02
0.14
(1.5 × 10 s ) is much faster than the electron transfer rate,
•- 2+
2
C60 / HV .
In conclusion, we have demonstrated that ZnP-linked-C60
systems act as efficient photocatalysts for the uphill oxidation
of NADH analogues by HV . The catalytic performance of
the triad (ZnP-H2P-C60) system, which exhibits a much longer
lifetime of the radical ion pair, is little affected by O2, whereas
the catalytic reactivity of the dyad (ZnP-C60) is reduced
significantly by the pure presence of O2.
0
ox
23
potential of AcrH2 (E vs SCE ) 0.81 V) compared to that
0
13
of BNAH (E vs SCE ) 0.57 V). In this case, electron
transfer from AcrH2 to ZnP is endergonic by 0.10 eV and, in
turn, thermodynamically unfavorable. However, AcrH2 is
known to undergo a rapid deprotonation to produce subsequently
2+
ox
•
+
•
+
•
23
AcrH . Following this deprotonation step, an electron transfer
•
•+
+
from AcrH to ZnP , forming AcrH , becomes now highly
•
exergonic judging from the low oxidation potential of the AcrH
species (E ) -0.46 V, which is equivalent to the one-
electron reduction potential of AcrH ).
stoichiometry implies also a two-electron oxidation of AcrH2
by two equivalents of HV2+ (eq 4), although the actual electron
transfer event involving AcrH2 is much slower than the
corresponding oxidation of BNAH, leading consequently to the
smaller quantum yields.
0
Acknowledgment. This work was supported by a grant-in-
aid for Scientific Research Priority Area (No. 11228205) from
the Ministry of Education, Science, Sports and Culture, Japan,
the Sumitomo Foundation, and the Office of Basic Energy
Sciences of the U.S. Department of Energy (contribution No.
NDRL-4295 from the Notre Dame Radiation Laboratory).
ox
+
25,26
Thus, the overall
The amount of free energy (∆G), stored in the photochemical
References and Notes
2+
oxidation of AcrH2 by HV , is obtained as 1.28 eV (124 kJ
-
1
(1) (a) Imahori, H.; Y. Sakata, Y. AdV. Mater. 1997, 9, 537. (b) Guldi,
D. M. Chem. Commun. 2000, 321. (c) Imahori, H.; Sakata, Y. Eur. J. Org.
Chem. 1999, 2445. (d) Fukuzumi, S.; Guldi, D. M. In Electron Transfer in
Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol.
2, pp 270-326.
mol ) based on the difference between the redox potentials of
+
26
2+
•+
the AcrH2/AcrH (0.22 V vs SCE) and HV /HV (-0.42
V vs SCE) redox couples (eq 5). In the case of BNAH
0
+
27
(
E (BNAH/BNA ) ) 0.02 V), the corresponding value is
(
2) (a) Page, C. C.; Moser, C. C.; Chen, X.; Dutton, P. L. Nature 1999,
-1
obtained as 0.88 eV (85 kJ mol ). In the dark, the back reaction
4
02, 47. (b) Gust, D.; Moore, T. A. In The Porphyrin Handbook; Kadish,
K. M., Smith, K. M., Guilard, R., Eds.; Academic: San Diego, CA, 2000;
Vol. 8, pp 153-190. (c) Gust, D.; Moore, T. A.; Moore, A. L. In Electron
Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany,
0
+
0
2+
•+
∆
G ) 2F[E (AcrH /AcrH )] - E (HV /HV )] (5)
2
2
001; Vol. 3, pp 272-336.
is extremely slow because of the kinetic barrier for a highly
(3) (a) Imahori, H.; Tamaki, K.; Guldi, D. M.; Luo, C.; Fujitsuka, M.;
•
+
0
endergonic electron transfer from HV (E ) -0.42 V vs
Ito, O.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 2607. (b)
Luo, C.; Guldi, D. M.; Imahori, H.; Tamaki, K.; Sakata, Y. J. Am. Chem.
Soc. 2000, 122, 6535. (c) Imahori, H.; El-Khouly, M. E.; Fujitsuka, M.;
Ito, O.; Sakata, Y.; Fukuzumi, S. J. Phys. Chem. A 2001, 105, 325.
(4) Fukuzumi, S.; Imahori, H. In Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 2, pp 927-
ox
+
0
red
15
SCE) to BNA (E ) -1.08 V vs SCE), although the
2
8,29
overall two-electron process is exergonic.
Effects of O2 on the Photocatalytic Systems. In the presence
•
+
of oxygen, the lifetimes of both radical ion pairs (i.e., ZnP -
966.
•
-
•+
•-
C60 and ZnP -H2P-C60 ) are reduced significantly because
(5) (a) Imahori, H.; Norieda, H.; Yamada, H.; Nishimura, Y.; Yamazaki,
of an oxygen-catalyzed back electron transfer (BET) processes
I.; Sakata, Y.; Fukuzumi, S. J. Am. Chem. Soc. 2001, 123, 100. (b) Imahori,
H.; Yamada, H.; Ozawa, S.; Ushida, K.; Sakata, Y. Chem. Commun. 1999,
1165. (c) Imahori, H.; Yamada, H.; Nishimura, Y.; Yamazaki, I.; Sakata,
Y. J. Phys. Chem. B 2000, 104, 2099.
•
-
•+ 13
•-
•-
between C60 and ZnP . An intermolecular ET from C60
to O2 occurs via the coordination of O2 to ZnP to yield O2
bound to ZnP , followed by a rapid intramolecular ET from
O2 to ZnP in the O2 -ZnP complex to regenerate O2.
•
+
•
+
(6) (a) Vaidyalingam, A. S.; Coutant, M. A.; Dutta, P. K. In Electron
•
-
•+
•-
•+
13
Transfer in Chemistry; Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany,
2
4
001; Vol. 3, pp 412-486. (b) Borja, M.; Dutta, P. K. Nature 1993, 362,
3.
Such a binding of the radical anion to metal ion is known to
accelerate the ET process.30,31 The impact that such a reduction
in lifetimes of the radical ion pairs has on the photocatalytic
AcrH2 oxidation was examined by deriving the quantum yield
(7) (a) Hurst, J. K.; Khairutdinov. In Electron Transfer in Chemistry;
Balzani, V., Ed.; Wiley-VCH: Weinheim, Germany, 2001; Vol. 3, pp 578-
623. (b) Tabushi, I.; Kugimiya, S.; Mizutani, T. J. Am. Chem. Soc. 1983,
105, 1658. (c) Handman, J.; Harriman, A.; Porter, G. Nature 1984, 307,
+
of formation of AcrH from O2-saturated PhCN solutions (see
534.
32
the Experimental Section). The quantum yields under O2-
(8) (a) Steinberg-Yfrach, G.; Liddell, P. A.; Hung, S.-C.; Moore, A.
saturated conditions are also listed in Table 2.33
L.; Gust, D.; Moore, T. A. Nature 1997, 385, 239. (b) Steinberg-Yfrach,
G.; Rigaud, J.-L.; Durantini, E. N.; Moore, A. L.; Gust, D.; Moore, T. A.
Nature 1998, 392, 479.
9) Stryer, L. Biochemistry, 3rd ed.; Freeman: New York, 1988;
Chapter 17.
(10) Fukuzumi, S. In AdVances in Electron Transfer Chemistry; Mariano,
P. S., Ed.; JAI Press: Greenwich, CT, 1992; pp 67-175.
11) Fukuzumi, S.; Tanaka, T. Photoinduced Electron Transfer; Fox,
M. A., Chanon, M., Eds.; Elsevier: Amsterdam, 1988; Part C, Chap. 10.
(12) Koper, N. W.; Jonker, S. A.; Verhoeven, J. W.; van Dijk, C. Recl.
TraV. Chim. Pays-Bas 1985, 104, 296.
13) Fukuzumi, S.; Imahori, H.; Yamada, H.; El-Khouly, M. E.;
Fujitsuka, M.; Ito, O.; Guldi, D. M. J. Am. Chem. Soc. 2001, 123, 2571.
14) Yamada, K.; Imahori, H.; Nishimura, Y.; Yamazaki, I.; Sakata, Y.
Chem. Lett. 1999, 895.
Surprisingly, the Φobs value (0.14) of the triad (ZnP-H2P-
C60) system is little affected by molecular oxygen as compared
to the value (0.15) found in the absence of O2. In contrast to
the triad, the Φobs value (0.02) of the dyad (ZnP-C60) system
becomes significantly smaller than the corresponding value
(
(
(
0.25) in the absence of O2. It should be noted that the kBET
4
-1
5 -1
value of the triad increases from 4.8 × 10 s to 3.2 × 10 s
in the absence and presence of O2, respectively. This value is,
(
however, still significantly smaller than the kBET value of the
6
-1 13
dyad, even in the absence of O2 (1.3 × 10 s ). Furthermore,
(
in the case of the triad (ZnP-H2P-C60), the rate of inter-

1908 J. Phys. Chem. A, Vol. 106, No. 10, 2002
Fukuzumi et al.
(
15) Fukuzumi, S.; Koumitsu, S.; Hironaka, K.; Tanaka, T. J. Am. Chem.
Soc. 1987, 109, 305.
16) Roberts, R. M. G.; Ostovic, D.; Kreevoy, M. M. Faraday Discuss.
Chem. Soc. 1982, 74, 257.
17) Bruinink, J.; Kregting, C. G. A.; Ponjee, J. J. J. Electrochem. Soc.
977, 124, 1854.
18) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956,
35, 518.
19) Fukuzumi, S.; Nakanishi, I.; Suenobu, T.; Kadish, K. M. J. Am.
Chem. Soc. 1999, 121, 3468.
20) Fukuzumi, S.; Tokuda, Y.; Kitano, T.; Okamoto, T.; Otera, J. J.
Am. Chem. Soc. 1993, 115, 8960.
21) (a) Kadish, K. M.; Gao, X.; Van Caemelbecke, E.; Suenobu, T.;
(26) Hapiot, P.; Moiroux, J.; Sav e´ ant, J.-M. J. Am. Chem. Soc. 1990,
112, 1337.
(27) Anne, A.; Moiroux, J. J. Org. Chem. 1990, 55, 4608.
28) An appropriate catalyst is thereby required to undergo the back
(
(
(
12
reaction. As noted by Verhoeven et al., the energy stored in the
photocatalytic reaction can, in principle, be made available via a photo-
galvanic device, employing one electrode which specifically catalzyed
1
(
29
2
+
the two-electron reduction of an NAD analogue.
(
(
29) Koper, N. W.; Verhoeven, J. W. Proc. K. Ned. Akad. Wet., Ser. B
983, 86, 79.
30) Fukuzumi, S.; Itoh, S. In AdVances in Photochemistry; Neckers,
D. C., Volman, D. H., von B u¨ nau, G., Eds.; Wiley: New York, 1998; Vol.
5, pp 107-172.
31) Fukuzumi, S. In Electron Transfer in Chemistry; Balzani, V., Ed.;
1
(
(
(
2
Fukuzumi, S. J. Phys. Chem. A 2000, 104, 3878. (b) Guldi, D. M.; Asmus,
K.-D. J. Phys. Chem. A 1997, 101, 1472.
(
•
+
Wiley-VCH: Weinheim, Germany, 2001; Vol. 4, pp 3-59.
(22) The transient formation of BNAH by electron transfer from
•
+
(32) Because HV•+ is unstable in the presence of oxygen, the quantum
BNAH to ZnP was not examined because of the spectral overlap.
23) Fukuzumi, S.; Kondo, Y.; Tanaka, T. J. Chem. Soc., Perkin Trans.
1984, 673.
24) The k1 vlaue was determined from the intercept as k1 ) 1.0 × 10
+
(
yield was determined from increase in absorbance that is due to AcrH at
2
358 nm.
9
(
(33) The O2 concentrations in air- and O2-saturated PhCN solutions were
determined by the spectroscopic titration for the photooxidation of 10-
methyl-9,10-dihydroacridine by O2; see: Fukuzumi, S.; Ishikawa, M.;
Tanaka, T. J. Chem. Soc., Perkin Trans. 2 1989, 1037.
M-
1
s .
-1
(
25) Fukuzumi, S.; Ohkubo, K.; Tokuda, Y.; Suenobu, T. J. Am. Chem.
Soc. 2000, 122, 4286.