Scandium ion-promoted photoinduced electron transfer from electron donors to acridine and pyrene. Essential role of scandium ion in photocatalytic oxygenation of hexamethylbenzene

Article abstract of DOI:10.1021/ja031649h

Photoinduced electron transfer from a variety of electron donors including alkylbenzenes to the singlet excited state of acridine and pyrene is accelerated significantly by the presence of scandium trifiate [Sc(OTf)₃] in acetonitrile, whereas no photoinduced electron transfer from alkylbenzenes to the singlet excited state of acridine or pyrene takes place in the absence of Sc(OTf)₃. The rate constants of the Sc(OTf)₃-promoted photoinduced electron-transfer reactions (k_et) of acridine to afford the complex between acridine radical anion and Sc(OTf)₃ remain constant under the conditions such that all the acridine molecules form the complex with Sc(OTf)₃. In contrast to the case of acridine, the ket value of the Sc(OTf)₃-promoted photoinduced electron transfer of pyrene increases with an increase in concentration of Sc(OTf)₃ to exhibit first-order dependence on [Sc(OTf)₃] at low concentrations, changing to second-order dependence at high concentrations. The first-order and second-order dependence of k_et on [Sc(OTf)₃] is ascribed to the 1:1 and 1:2 complexes formation between pyrene radical anion and Sc(OTf)₃. The positive shifts of the one-electron redox potentials for the couple between the singlet excited state and the ground-state radical anion of acridine and pyrene in the presence of Sc(OTf)₃ as compared to those in the absence of Sc(OTf)₃ have been determined by adapting the free energy relationship for the photoinduced electron-transfer reactions. The Sc(OTf)₃-promoted photoinduced electron transfer from hexamethylbenzene to the singlet excited state of acridine or pyrene leads to efficient oxygenation of hexamethylbenzene to produce pentamethylbenzyl alcohol which is further oxygenated under prolonged photoirradiation of an O ₂-saturated acetonitrile solution of hexamethylbenzene in the presence of acridine or pyrene which acts as a photocatalyst together with Sc(OTf)₃. The photocatalytic oxygenation mechanism has been proposed based on the studies on the quantum yields, the fluorescence quenching, and direct detection of the reaction intermediates by ESR and laser flash photolysis.

Full text of DOI:10.1021/ja031649h

Published on Web 05/28/2004
Scandium Ion-Promoted Photoinduced Electron Transfer from
Electron Donors to Acridine and Pyrene. Essential Role of
Scandium Ion in Photocatalytic Oxygenation of
Hexamethylbenzene
Shunichi Fukuzumi,* Junpei Yuasa, Naoya Satoh, and Tomoyoshi Suenobu
Contribution from the Department of Material and Life Science, Graduate School of
Engineering, Osaka UniVersity, CREST, Japan Science and Technology Agency (JST),
Suita, Osaka 565-0871, Japan
Received December 11, 2003; E-mail: fukuzumi@chem.eng.osaka-u.ac.jp
Abstract: Photoinduced electron transfer from a variety of electron donors including alkylbenzenes to the
singlet excited state of acridine and pyrene is accelerated significantly by the presence of scandium triflate
[Sc(OTf)3] in acetonitrile, whereas no photoinduced electron transfer from alkylbenzenes to the singlet excited
state of acridine or pyrene takes place in the absence of Sc(OTf)3. The rate constants of the Sc(OTf)3-
promoted photoinduced electron-transfer reactions (ket) of acridine to afford the complex between acridine
radical anion and Sc(OTf)3 remain constant under the conditions such that all the acridine molecules form
the complex with Sc(OTf)3. In contrast to the case of acridine, the ket value of the Sc(OTf)3-promoted
photoinduced electron transfer of pyrene increases with an increase in concentration of Sc(OTf)3 to exhibit
first-order dependence on [Sc(OTf)3] at low concentrations, changing to second-order dependence at high
concentrations. The first-order and second-order dependence of ket on [Sc(OTf)3] is ascribed to the 1:1
and 1:2 complexes formation between pyrene radical anion and Sc(OTf)3. The positive shifts of the one-
electron redox potentials for the couple between the singlet excited state and the ground-state radical
anion of acridine and pyrene in the presence of Sc(OTf)3 as compared to those in the absence of Sc(OTf)3
have been determined by adapting the free energy relationship for the photoinduced electron-transfer
reactions. The Sc(OTf)3-promoted photoinduced electron transfer from hexamethylbenzene to the singlet
excited state of acridine or pyrene leads to efficient oxygenation of hexamethylbenzene to produce
pentamethylbenzyl alcohol which is further oxygenated under prolonged photoirradiation of an O2-saturated
acetonitrile solution of hexamethylbenzene in the presence of acridine or pyrene which acts as a
photocatalyst together with Sc(OTf)3. The photocatalytic oxygenation mechanism has been proposed based
on the studies on the quantum yields, the fluorescence quenching, and direct detection of the reaction
intermediates by ESR and laser flash photolysis.
duced electron-transfer reactions of carbonyl compounds.9-15
Introduction
Photoexcitation of carbonyl compounds with lowest n-π* singlet
Since photoexcitation induces significant enhancement in the
reactivity of electron transfer, photochemical reactions via
photoinduced electron transfer have been explored extensively.1-15
In particular, there have been a number of reports on photoin-
excited states generally results in formation of the nonfluorescent
triplet excited state via the fast intersystem crossing.16 Thus,
the most photochemical reactions of carbonyl compounds are
known to undergo via the triplet excited states.9-19 In the case
(1) Photoinduced Electron Transfer; Fox, M. A., Chanon, M., Eds.; Elsevier:
Amsterdam, 1988; Part A-D.
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Verlag: Berlin, 1992; Vol. 163, pp 131-245. (c) Saeva, F. D. In AdVances
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9
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J. AM. CHEM. SOC. 2004, 126, 7585-7594
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A R T I C L E S
Fukuzumi et al.
of photoinduced electron transfer reactions, however, the
oxidizing power of the triplet excited state is generally lower
than that of the singlet excited state because of the lower
excitation energy. Accordingly, electron donors which can be
employed in the photoinduced electron transfer reactions of
carbonyl compounds have been limited to relatively strong
reductants such as ketene silyl acetals,17 allylic stannanes,11,18
and amines.19 The promoting effects of metal ions on the
photoinduced electron transfer have made it possible to expand
the scope of photoinduced electron-transfer reactions of carbonyl
compounds.20-23 However, such promoting effects of metal ions
on photoinduced electron transfer have so far been limited to
carbonyl compounds which can form the ground-state complexes
with metal ions. As such, no photocatalytic system using
sensitizers which have no interaction with metal ions at the
ground state has ever been reported.
experiments, and direct detection of the reaction intermediates
by ESR and laser flash photolysis.
Experimental Section
Materials. 10-Methyl-9,10-dihydroacridine (AcrH2) was prepared
from 10-methyl-acridinium iodide (AcrH+I-), which was obtained by
the reaction of acridine with methyl iodide in acetone, by reduction
with NaBH4 in methanol and purified by recrystallization from ethanol.25
p-MeO-N,N-dimethylaniline was prepared according to the literature.26
The dimeric 1-benzyl-1,4-dihydronicotinamide dimer [(BNA)2] was
prepared according to the literature.27 Pyrene (Py), acridine (AcrN),
ferrocene, N,N-dimethylaniline, 1,4-dimethoxybenzene, 1,3-dimethoxy-
benzene, 1,2,4-trimethoxybenzene, hexamethylbenzene, pentamethyl-
benzene, naphthalene, 1,2,4,5-tetramethylbenzene and 1,2,3,5-tetra-
methylbenzene, 1,2,3,4-tetramethylbenzene, 1,2,4-trimethylbenzene,
1,2,3-trimethylbenzene, p-xylene, o-xylene, m-xylene, ethylbenzene,
toluene, and benzene were obtained commercially and purified by the
standard methods.28 1,1′-Dimethylferrocene, 1,2,4,5-tetramethyl-1,4-cy-
clohexadiene, pentamethylbenzyl alcohol and 1,2,4-trimethoxybenzene
were obtained from Aldrich. Sodium iodide (NaI) was obtained from
Nakalai Tesque, Inc. Scandium trifluoromethanesulfonate, Sc(OTf)3
(99%, F. W. ) 492.16) was obtained from Pacific Metals Co., Ltd.
(Taiheiyo Kinzoku). 1,4-Dioxane was obtained commercially from To-
kyo Kasei Organic Chemicals. Tetra-n-butylammonium hexafluoro-
phosphate used as a supporting electrolyte for the electrochemical
measurements also obtained commercially from Tokyo Kasei Organic
Chemicals. Acetonitrile (MeCN) and propionitrile (EtCN) used as
solvents were purified and dried by the standard procedure.28
[2H3]Acetonitrile (CD3CN) was obtained from EURI SO-TOP, CEA,
France.
We report herein remarkable promoting effects of scandium
triflate, Sc(OTf)3, on photoinduced electron transfer from a
variety of electron donors to the singlet excited state of not only
acridine which can form the ground-state complex with Sc(OTf)3
but also pyrene which has no interaction with Sc(OTf)3 in the
ground state. Although the oxidizing ability of the singlet excited
state of pyrene or acridine is too weak to oxidize alkylben-
zenes,24 the Sc(OTf)3-promoted photoinduced electron transfer
from hexamethylbenzene to the singlet excited state of pyrene
and acridine leads to efficient oxygenation of hexamethylben-
zene to produce pentamethylbenzyl alcohol as the primary
oxygenated product. Without Sc(OTf)3, pyrene or acridine has
no ability to photocatalyze the oxygenation of hexamethylben-
zene. Thus, the essential role of Sc(OTf)3 in the photocatalytic
reaction found in this study will expand the scope of photo-
catalytic reactions via photoinduced electron transfer. The
photocatalytic mechanism is examined in detail based on the
studies on the quantum yields, the fluorescence quenching
Reaction Procedure. Typically, hexamethylbenzene (2.0 × 10-3
M) was added to an NMR tube that contained a CD3CN solution (600
µL) of Py (2.0 × 10-5 M) or AcrN (2.0 × 10-5 M) in the presence of
Sc(OTf)3 (4.0 × 10-2 M) under atmospheric pressure of oxygen. The
solution was flushed with oxygen gas for 8 min, and the NMR tube
was sealed with a rubber septum. Then, the solution was irradiated
with UV-visible light from a xenon lamp (Ushio Model V1-501C)
through a cut-off filter (Toshiba UV-31) transmitting λ > 300 nm at
298 K for 5 h. The reaction solution was analyzed by 1H NMR
spectroscopy. The product from photooxygenation of hexamethylben-
zene with O2 in the presence of Sc(OTf)3 was identified by comparing
(12) (a) Fukuzumi, S.; Fujita, M.; Otera, J. J. Org. Chem. 1993, 58, 5405. (b)
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Chem. Soc., Perkin Trans. 2 1993, 1395.
1
the H NMR spectra with that of a CD3CN solution of pentamethyl-
benzyl alcohol in the presence of Sc(OTf)3. The yield of the reaction
was determined based on the concentration of the internal standard,
1,4-dioxane (5.0 × 10-3 M). 1H NMR measurements were performed
with a Japan Electron Optics JNM-GSX-400 (400 MHz) NMR
1
spectrometer at 298 K. H NMR (CD3CN, 298 K); δ(Me4Si, ppm):
4.60 (d, J ) 4.0 Hz, 2H), 2.23 (s, 6H), 2.21 (s, 9H). The isolation of
the preparative scale photocatalytic oxygenation product of hexameth-
ylbenzene was performed as follows. Typically, an MeCN solution of
hexamethylbenzene (1.0 × 10-2 M, 50 mL) was added to the reaction
vessel containing an MeCN solution (50 mL) of AcrN (4.0 × 10-5 M)
in the presence of Sc(OTf)3 (2.0 × 10-2 M). The solution was flushed
with oxygen gas for 20 min. Then, the solution was irradiated with
UV-visible light from a xenon lamp (Ushio Model V1-501C) through
a cut-off filter (Toshiba UV-31) transmitting λ > 300 nm at 298 K for
7 h. Deionized water (100 mL) was added to the resulting solution
containing the products, and the MeCN was removed by vacuum
evaporation. The resulting suspension was washed with chloroform.
The organic layer was separated and the solvent was removed from
(16) (a) Boldridge, D. W.; Justus, B. L.; Scott, G. W. J. Chem. Phys. 1984, 80,
3179. (b) Samanta, A.; Fessenden, R. W. Chem. Phys. Lett. 1988, 153,
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114, 10271.
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(19) Pandey, G. In Topics in Current Chemistry; Photoinduced Electron-Transfer
V; Springer-Verlag: Berlin, 1993; Vol. 168, pp 175-221.
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Wiley-VCH: New York, 2001; Vol. 4, pp 3-67. (b) Fukuzumi, S. Org.
Biomol. Chem. 2003, 1, 609. (c) Fukuzumi, S. Pure Appl. Chem. 2003,
75, 577.
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Volman, D. H. Eds.; John Wiley & Sons: New York, 1999; Vol. 25, p
107. (b) Fukuzumi, S. Bull. Chem. Soc. Jpn. 1997, 70, 1. (c) Fukuzumi, S.
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Fujitsuka, M.; Ito, O. J. Am. Chem. Soc. 2001, 123, 7756.
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Role of Scandium Ion in Photocatalytic Oxygenation
A R T I C L E S
the reaction mixture by distillation at 313 K under vacuum. The residue
was purified by silica gel column chromatography (chloroform as an
eluent) to afford pentamethylbenzyl alcohol as a white solid. The
isolated yield of pentamethylbenzyl alcohol was determined to be 30%.
presence of Sc(OTf)3 were carried out with a BAS 100B electrochemical
analyzer in deaerated MeCN containing 0.10 M n-Bu4N+PF6- (TBAPF6)
as a supporting electrolyte at 298 K. The gold 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/AgNO3 (0.01
M) reference electrode. The E0red values (vs Ag/AgNO3) are converted
into those vs SCE by addition of 0.29 V.32
Quantum Yield Determination. A standard actinometer (potassium
ferrioxalate)29 was used for the quantum yield determination of the
photooxygenation of hexamethylbenzene by O2 in the presence of Sc-
(OTf)3 and Py. A square quartz cuvette (10 mm i.d.) which contained
an O2-saturated MeCN (3.0 cm3) of hexamethylbenzene (2.0 × 10-2
- 5.0 × 10-2 M), Py (1.0 × 10-4 M), and Sc(OTf)3 (4.0 × 10-2 M)
was irradiated by monochromatized light of λ ) 335 nm from a
Shimadzu spectrofluorophotometer (RF-5000). Under the condition of
actinometry experiments, both the actinometer and Py absorbed es-
sentially all the incident light of λ ) 335 nm. The light intensity of
monochromatized light of λ ) 335 nm was determined as 9.3 × 10-7
einstein dm-3 s-1, 1.8 × 10-6 einstein dm-3 s-1 and 5.7 × 10-6 einstein
dm-3 s-1 with the slit width of 10, 15, and 20 nm. The photochemical
reaction was monitored using a Shimadzu GC-17A gas chromatograph
and Shimadzu MS-QP5000 mass spectrometer. The quantum yields
were determined from the decrease in the amount of hexamethylben-
zene.
Laser Flash Photolysis. A deaerated MeCN solution containing
AcrN and hexamethylbenzene was excited by a Nd:YAG laser
(Continuum, SLII-10, 4-6 ns fwhm) at λ ) 355 nm with the power of
20 mJ per pulse. The transient absorption spectra in the photoinduced
electron-transfer reactions were measured by using a continuous Xe-
lamp (150 W) and a photomultiplier tube (Hamamatsu 2949) as a probe
light and a detector, respectively. The output from the photomultiplier
tube was recorded with a digitizing oscilloscope (Tektronix, TDS3032,
300 MHz). The transient spectra were recorded using fresh solutions
in each laser excitation at 298 K.
ESR Measurements. The ESR spectra of the scandium complexes
of acridine radical anion (AcrN•-) and pyrene radical anion (Py•-) were
recorded on a JEOL JES-FA100 spectrometer under irradiation of a
high-pressure mercury lamp (USH-1005D) which is focused at the
sample cell containing a deaerated EtCN solution of AcrN or Py,
(BNA)2, and Sc(OTf)3 (400 µL) in the ESR cavity at 203 K. ESR
spectra of electrochemically generated Py•- were taken on a JEOL JES-
FA100 by using an electrolysis cell designed for ESR measurements.33
The ESR cell is composed of a helical gold wire with large surface
area (12 cm2) as the working electrode, which can generate enough
Py•- to detect the ESR spectra, a platinum wire as a counter electrode,
and a silver wire as a reference electrode. The controlled potential
electrolysis of Py was carried out in anhydrous deaerated EtCN con-
taining 0.20 M n-Bu4NPF6 as supporting electrolyte in the ESR cavity.
The ESR spectrum of the Sc(OTf)3 complex of superoxide radical anion
(O2•-) was measured at 203 K by using a JEOL TES-FA100 with
low-temperature cooling apparatus. The mixed solution of hexameth-
ylbenzene (6.2 × 10-2 M), Py (1.0 × 10-1 M), and Sc(OTf)3 (5.0 ×
10-2 M) in O2-saturated EtCN at 203 K was irradiated by using a high-
pressure mercury lamp (USH-1005D) focusing at the sample cell in
the ESR cavity. All the ESR spectra were recorded under nonsaturating
microwave power conditions. The magnitude of modulation was chosen
to optimize the resolution and signal-to-noise (S/N) ratio of the observed
spectra. The g values and hyperfine splitting constants (hfs) were
calibrated by using an Mn2+ marker. Computer simulation of the ESR
spectra was carried out by using ESRaII Version 1.2 (Calleo Scientific
Software Publisher) on a Macintosh personal computer.
Fluorescence Quenching. Quenching experiments of the fluores-
cence of AcrN and Py were carried out on a Shimadzu spectrofluoro-
photometer (RF-5000). The excitation wavelength of AcrN and
(AcrN)2-Sc(OTf)3 complex was λ ) 355 nm. The monitoring
wavelengths were those corresponding to the maxima of the emission
band at λmax ) 397 nm (AcrN) and λmax ) 480 nm [(AcrN)2-Sc(OTf)3].
The excitation wavelength of Py was λ ) 335 nm in MeCN. The
monitoring wavelength was that corresponding to the maximum of the
emission band at λmax ) 390 nm. Typically, an MeCN solution of AcrN
or Py was deaerated by argon purging for 8 min prior to the
measurements. Relative fluorescence intensities were measured for
MeCN solutions containing Py (1.3 × 10-6 M) with various electron
donors (6.5 × 10-5 - 4.0 × 10-1 M) in the presence of Sc(OTf)3 (0-
1.5 × 10-1 M). Relative fluorescence intensities were also measured
for MeCN solutions containing AcrN (1.0 × 10-4 M) with various
electron donors (2.5 × 10-3 - 1.5 M) in the presence of Sc(OTf)3 (4.0
× 10-2 M). There was no change in the shape but there was a change
in the intensity of the fluorescence spectrum by the addition of a
quencher (D). The Stern-Volmer relationship (eq 1) was obtained for
I0/I ) 1 + KSV[D]
(1)
the ratio of the emission intensities in the absence and presence of
Sc(OTf)3 and concentrations of various electron donors (D) used as
quenchers. The fluorescence lifetimes of AcrN30 and (AcrN)2-Sc(OTf)3
complex were determined as τ ) 31.0 and 32.2 ns, respectively in
deaerated MeCN solution at 298 K by single photon counting using a
Horiba NAES-1100 time-resolved spectrofluorophotometer. The fluo-
rescence lifetimes of Py in the absence31 and in the presence of Sc-
(OTf)3 (4.0 × 10-2, 8.0 × 10-2, 1.2 × 10-1, and 1.5 × 10-1 M) were
determined as τ ) 295, 256, 171, 163, and 104 ns, respectively, in a
deaerated MeCN solution at 298 K. The fluorescence lifetime of Py in
the presence of Sc(OTf)3 (4.0 × 10-2 M) was also determined as τ )
4.1 ns in O2-saturated MeCN at 298 K. The observed quenching rate
constants kq (KSVτ-1) were obtained from the Stern-Volmer constants
(KSV) and the fluorescence lifetime τ.
Spectral Measurements. The amount of hydrogen peroxide (H2O2)
was determined by titration with iodide ion.34 The aliquots of the product
-
mixture in MeCN was treated with excess NaI and the amount of I3
formed was determined by the UV-visible spectrum (λmax ) 365 nm,
ꢀmax ) 28 000 M-1 cm-1 22 using a Hewlett-Packard 8453 diode array
)
spectrophotometer with a quartz cuvette (path length ) 2 mm) at 298
K.
Results and Discussion
Sc(OTf)3-Promoted Photoinduced Electron Transfer. Acri-
dine (AcrN) forms a 2:1 complex with Sc(OTf)3 (eq 2) as
indicated by the spectral titration in MeCN (Figure 1a).
Electrochemical Measurements. The second harmonic ac volta-
mmetry (SHACV) measurements of Py and AcrN in the absence and
(29) (a) Hatchard, C. G.; Parker, C. A. Proc. R. Soc. London, Ser. A 1956, 235,
518. (b) Calvert, J. C.; Pitts, J. N. Photochemistry; Willy: New York, 1966;
p 786.
(30) Williams, J. O.; Clarke, B. P.; Shaw, M. J. Chem. Phys. Lett. 1976, 39,
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(31) Baggotto, J. E.; Pilling, M. J. J. Chem. Soc., Faraday Trans. 1 1983, 79,
221.
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A R T I C L E S
Fukuzumi et al.
Table 1. Oxidation Potentials (E0ox) of Various Electron Donors
and Fluorescence Quenching Rate Constants (kq) of AcrN (1.0 ×
10-4 M) by Various Electron Donors in the Absence and Presence
of Sc(OTf)3 (4.0 × 10-2 M) in Deaerated MeCN at 298 K
in the absence in the presence
E0ox, V
vs SCE
of Sc(OTf)3
of Sc(OTf)3
kq, M-1 s-1
no.
electron donor
kq, M-1 s-1
1
2
3
4
5
6
7
8
9
1,1′-dimethylferrocene
ferrocene
10-methyl-9,10-dihydroacridine 0.81
1,2,4-trimethoxybenzene
1,4-dimethoxybenzene
hexamethylbenzene
1,2,3,4-tetramethylbenzene
1,2,4-trimethylbenzene
1,2,3-trimethylbenzene
0.26
0.37
1.2 × 1010 2.6 × 1010
1.2 × 1010 2.9 × 1010
1.0 × 1010 1.5 × 1010
a
1.12
1.34
1.49
1.71
1.79
1.88
1.93
1.98
2.02
2.14
2.20
2.35
7.7 × 108
a
4.6 × 107
1.5 × 1010
1.4 × 1010
1.3 × 1010
1.3 × 1010
1.4 × 1010
1.2 × 1010
9.6 × 109
7.0 × 108
6.0 × 108
7.4 × 106
10 p-xylene
11 o-xylene
12 m-xylene
13 ethylbenzene
14 toluene
15 benzene
a Not determined due to complex formation between electron donors and
Sc3+
.
quenched by benzene and other alkylbenzene derivatives (see
Supporting Information S2). The fluorescence quenching occurs
by photoinduced electron transfer from benzene to singlet
excited state of the (AcrN)2-Sc(OTf)3 complex (eq 3) as
discussed below.
Figure 1. (a) UV-visible absorption spectral changes and (b) fluorescence
spectral changes observed upon addition of Sc(OTf)3 to an MeCN solution
of AcrN (3.5 × 10-5 M) at 298 K. Inset: (a) absorption changes at λ )
356 nm and (b) fluorescence changes at λ ) 479 nm.
There is weak fluorescence of AcrN at 380 nm (Supporting
Information S1). When AcrN forms a complex with Sc(OTf)3,
(AcrN)2-Sc(OTf)3, strong fluorescence of the Sc(OTf)3 complex
of AcrN appears at 480 nm (Figure 1b). The formation of a 2:1
complex is confirmed by the titration of the fluorescence spectra
(inset of Figure 1b).
The fluorescence quenching rate constants (kq) in the absence
and presence of Sc(OTf)3 (4.0 × 10-2 M) are summarized in
Table 1, together with the one-electron oxidation potentials of
the electron donors (E0ox) employed in this study. A plot of log
In contrast to the case of AcrN, there was no change in the
absorption and fluorescence spectra of pyrene (Py) in the
presence of Sc(OTf)3. This indicates that there is no interaction
of Sc(OTf)3 with Py and the singlet excited state (1Py*).35
kq vs E0 is shown in Figure 2, where the kq value increases
ox
with decreasing the E0ox value to reach a diffusion-limited value
(2.0 × 1010 M-1 s-1).24 This indicates that the fluorescence
quenching proceeds via electron transfer from the electron
No fluorescence quenching of AcrN by benzene is observed
in MeCN. When AcrN forms the complex with Sc(OTf)3, the
fluorescence maximum is shifted to λ ) 480 nm (Figure 1b)
and the fluorescence lifetime of the (AcrN)2-Sc(OTf)3 complex
(32.2 ns) becomes slightly longer as compared to that of the
singlet excited-state AcrN (1AcrN*) (31.0 ns) (see Experimental
Section). In contrast to the case in the absence of Sc(OTf)3, the
fluorescence of the (AcrN)2-Sc(OTf)3 complex is efficiently
1
1
donors to AcrN* and [(AcrN)2-Sc(OTf)3]* in the absence
and presence of Sc(OTf)3, respectively.
The dependence of the activation Gibbs energy of photoin-
duced electron transfer (∆G*) on the Gibbs energy change of
electron transfer (∆G0et) has well been established as given
by the Gibbs energy relationship (eq 4),24 where ∆G* is the
0
(32) Mann, C. K.; Barnes, K. K. In Electrochemical Reaction in Nonaqueous
Systems; Marcel Dekker: New York, 1970.
(33) Ohya-Nishiguchi, H. Bull. Chem. Soc. Jpn. 1979, 52, 2064.
(34) Mair, R. D.; Graupner, A. J. Anal. Chem. 1964, 36, 194.
(35) Sc3+ probably promotes intersystem crossing process of 1Py* and diminishes
the fluorescence lifetime of the Py. However such interaction is not so
strong to form a complex.
∆G* ) (∆G0 /2) + [(∆G0 /2)2 + (∆G* )2]1/2
(4)
et
et
0
intrinsic barrier that represents the activation Gibbs energy when
the driving force of electron transfer is zero, i.e., ∆G* ) ∆G*
0
9
7588 J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004

Role of Scandium Ion in Photocatalytic Oxygenation
A R T I C L E S
Table 2. Oxidation Potentials (E0ox) of Various Electron Donors
and Fluorescence Quenching Rate Constants (kq) of Py (1.3 ×
10-6 M) by Various Electron Donors in the Absence and Presence
of Sc(OTf)3 (4.0 × 10-2 M) in Deaerated MeCN at 298 K
in the absence in the presence
E0ox, V
vs SCE
of Sc(OTf)3
of Sc(OTf)3
kq, M-1 s-1
no.
electron donor
kq, M-1 s-1
1
2
3
4
5
6
7
1,1′-dimethylferrocene
ferrocene
N,N-dimethylaniline
0.26
0.37
0.71
2.0 × 1010 1.9 × 1010
1.9 × 1010 1.9 × 1010
a
1.7 × 1010
10-methyl-9,10-dihydroacridine 0.81
1.3 × 1010 6.8 × 109
a
1,2,4-trimethoxybenzene
1,4-dimethoxybenzene
1,2,4,5-tetramethyl
-1,4-cyclohexadiene
1,2-dimethoxybenzene
1,3-dimethoxybenzene
1.12
1.34
6.4 × 109
a
1.6 × 108
b
1.38
1.45
1.49
1.49
1.58
1.60
1.63
1.71
2.2 × 107
a
8
9
3.7 × 106
a
2.0 × 106
10 hexamethylbenzene
11 pentamethylbenzene
12 naphthalene
13 1,2,4,5-tetramethylbenzene
14 1,2,3,5-tetramethylbenzene
7.3 × 107
2.7 × 107
2.6 × 107
2.1 × 107
5.9 × 106
Figure 2. Plots of log kq vs E0 for the fluorescence quenching of AcrN
ox
(1.0 × 10-4 M) by various electron donors in the absence (O) and presence
of Sc(OTf)3 (4.0 × 10-2 M, 9) in deaerated MeCN at 298 K. Numbers at
each point correspond to those in Table 1.
a Not determined due to complex formation between electron donors and
Sc3+ b Not determined.
.
at ∆G0 ) 0. On the otherhand ∆G* values are related to the
rate constant of electron transfer (ket) as given by eq 5, where
et
efficiently by 1,2,3,5-tetramethylbenzene in the presence of Sc-
(OTf)3 (eq 6).
∆G* ) (2.3RT/F) log[Z(ket-1 - kdiff-1)]
(5)
Z is the collision frequency that is taken as 1 × 1011 M-1 s-1
,
F is the Faraday constant, and kdiff is the diffusion rate constant
in MeCN (2.0 × 1010 M-1 s-1).24 The ∆G0 value is obtained
et
from the one-electron oxidation potentials of electron donors
(E0ox) and the one-electron reduction potential of 1AcrN* in the
absence and presence of Sc(OTf)3 (E0red*). The plot of Figure
2 is fitted using eqs 4 and 5 as shown by the solid line, which
agrees with the experimental results. The best fit lines shown
in Figure 2 give the E0red* values (1.29 V vs SCE in the absence
of Sc(OTf)3 and 2.23 V vs SCE in the presence of Sc(OTf)3)
Other weak electron donors can also quench the Py fluores-
cence in the presence of Sc(OTf)3 (see Supporting Information
S3). The kq values in the absence and presence of a constant
concentration of Sc(OTf)3 (4.0 × 10-2 M) are summarized in
Table 2, where the one-electron oxidation potentials of the
electron donors (E0ox) employed in this study are also listed.
and ∆G* value (3.94 kcal mol-1 in the absence of Sc(OTf)3
0
and 2.64 kcal mol-1 in the presence of Sc(OTf)3). As compared
to the one-electron reduction potential of 1AcrN* in the absence
of Sc(OTf)3, the E0red* value of 1[(AcrN)2-Sc(OTf)3]* is
significantly shifted to the positive direction (0.94 V). Such a
large positive shift in the one-electron reduction potential
indicates that the electron acceptor ability of 1AcrN* is
drastically enhanced by the complex formation with Sc-
(OTf)3.
The plots of log kq vs E0ox are shown in Figure 3, where the
kq value increases with decreasing the E0 value to reach a
ox
diffusion-limited value (2.0 × 1010 M-1 s-1).24 Such dependence
of kq on E0 is typical for photoinduced electron-transfer
ox
reactions,24 and thus, the fluorescence quenching may proceed
via electron transfer from the electron donors to the 1Py* in the
absence and presence of Sc(OTf)3.
Such a large positive shift in the one-electron reduction
potential of 1AcrN* in the presence of Sc(OTf)3 is also observed
in the case of Py*. Irradiation of the absorption band of Py
The plots of Figure 3 can also be fitted using eqs 4 and 5 as
shown by the solid line, which agrees with the experimental
results. The best fit lines shown in Figure 3 give the E0red* values
(1.24 V vs SCE in the absence of Sc(OTf)3 and 1.83 V vs SCE
in the presence of Sc(OTf)3) and ∆G*0 (2.46 kcal mol-1 in the
absence of Sc(OTf)3 and 7.08 kcal mol-1 in the presence of
Sc(OTf)3). As compared with the one-electron reduction po-
tential of 1Py* (1.24 V), the E0red* in the presence of Sc(OTf)3
(4.0 × 10-2 M) is significantly shifted to the positive direction
(0.59 V). Such a large positive shift in the one-electron reduction
1
results in fluorescence at λ ) 391 nm in MeCN. The
fluorescence of Py is quenched efficiently by relatively strong
electron donors such as methoxybenzenes, N,N-dimethylanilines
and ferrocenes (see typical Stern-Volmer plots in Supporting
Information S3). When relatively weak electron donors such
as 1,2,3,5-tetramethylbenzene are employed, no fluorescence
quenching is observed. When Sc(OTf)3 (4.0 × 10-2 M) is added
to this system, however, the Py fluorescence is quenched
1
potential indicates that the electron acceptor ability of Py* as
9
J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004 7589

A R T I C L E S
Fukuzumi et al.
Figure 3. Plots of log kq vs E0ox for the fluorescence quenching of Py (1.3
× 10-6 M) by various electron donors in the absence (O) and presence of
Sc(OTf)3 (4.0 × 10-2 M, 9) in deaerated MeCN at 298 K. Numbers
correspond to those in Table 2.
well as 1[(AcrN)2-Sc(OTf)3]* is significantly enhanced as
compared to that in the absence of Sc(OTf)3.36
The kq value for photoinduced electron transfer from ethyl-
benzene to 1AcrN* in the presence of excess Sc(OTf)3 relative
to AcrN remains constant independent of [Sc(OTf)3] as shown
in Figure 4a. This indicates the photoinduced electron transfer
occurs from the singlet excited state of the 2:1 complex with
Sc3+ ((AcrN)2-Sc(OTf)3). In the case of photoinduced electron
transfer from 1,2,3,5-tetramethylbenzene and the other electron
donors to 1Py*, the dependence of kq on [Sc3+] are quite different
from the case of 1AcrN*. The kq value for photoinduced electron
transfer from 1,2,3,5-tetramethylbenzene and the other electron
Figure 4. (a) Dependence of kq on [Sc(OTf)3] for the fluorescence
quenching of AcrN (3.5 × 10-5 M) by ethylbenzene in the presence of
Sc(OTf)3 in deaerated MeCN at 298 K. (b) Dependence of kq on [Sc(OTf)3]
for the fluorescence quenching of Py (1.3 × 10-6 M) by 1,2,3,5-
tetramethylbenzene (O), 1,2,4,5-tetramethylbenzene (b), and pentameth-
ylbenzene (4) in the presence of Sc(OTf)3 in deaerated MeCN at 298 K.
1
donors to Py* increases with an increase in [Sc(OTf)3] to
exhibit first-order dependence on [Sc(OTf)3] at low concentra-
tion changing to second-order dependence on [Sc(OTf)3] as
shown in Figure 4b. Such first-order and second-order depen-
dence of kq on [Sc(OTf)3] can be explained by the 1:1 and 1:2
complex formation between Py•- and Sc(OTf)3 as shown in eqs
7 and 8, respectively. In such a case, the one-electron reduction
potential of Py in the presence of Sc(OTf)3 (Ered*) is shifted to
positive direction according to the Nernst equation (eq 9), where
1
E0red* is the one-electron reduction potential of Py* in the
absence of Sc(OTf)3, K1 and K2 are the formation constants of
1:1 and 1:2 complexes of Py•- and Sc(OTf)3, respectively. If
such a change in the energetics of electron transfer (eq 9)
Ered* ) E0red* + (2.3RT/F)log{K1[Sc(OTf)3](1 +
K2[Sc(OTf)3])} (9)
is directly reflected in the transition state of electron transfer,
the dependence of the fluorescence quenching rate constant of
Sc(OTf)3-promoted electron transfer (kq) on [Sc(OTf)3] may be
given by eq 10, where k0 is the rate constant in the absence of
Sc(OTf)3.
kq - k0 ) k0K1[Sc(OTf)3](1 +K2[Sc(OTf)3]) (10)
From the slopes and intercepts of linear plots of (kq-k0)/[Sc-
(OTf)3] vs [Sc(OTf)3] (Supporting Information S4) are obtained
the K2 values which are listed in Table 3.
Direct Detection of Sc(OTf)3 Complexes with AcrN•- and
Py•-. The Sc(OTf)3-promoted electron transfer from electron
donors to AcrN is expected to produce the Sc(OTf)3 complex
(36) A large positive shift in the one-electron reduction potentials of the ground-
state AcrN and Py in the presence of Sc(OTf)3 is also observed by the
electrochemical study.
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7590 J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004

Role of Scandium Ion in Photocatalytic Oxygenation
A R T I C L E S
Scheme 1
Table 3. Fluorescence Quenching Rate Constants (kq) of Py (1.3
× 10-6 M) by 1,2,3,5-Tetramethylbenzene, 1,2,4,5-Tetramethyl-
benzene, and Pentamethylbenzene in the Presence of Sc(OTf)3
(4.0 × 10-2 M), the Rate Constants (k0K1), and the Formation
Constants of Py•--2Sc(OTf)3 (K2) in Deaerated MeCN at 298 K
-1a
electron donor
kq, M-1 s-1
k0K , M-2 s-1a
K , M
1
2
1,2,3,5-tetramethylbenzene
1,2,4,5-tetramethylbenzene
pentamethylbenzene
5.9 × 106
2.1 × 107
2.7 × 107
1.1 × 108
3.7 × 108
5.0 × 108
13.3
14.8
11.6
a Determined from the dependence of kq on [Sc(OTf)3].
with AcrN•-, that is (AcrN)2•--Sc(OTf)3 (eq 3). The ESR
detection of (AcrN)2•--Sc(OTf)3 would provide valuable
information of the binding of Sc(OTf)3 with AcrN. The
(AcrN)2•--Sc(OTf)3 complex was produced by electron transfer
from dimeric 1-benzyl-1,4-dihydronicotinamide [(BNA)2]37 to
AcrN in EtCN in the presence of Sc(OTf)3 at 203 K (Scheme
1). The (BNA)2 is known to act as a unique electron donor to
produce the radical anions of electron acceptors.38 The addition
of Sc(OTf)3 (4.4 × 10-2 M) to the (BNA)2-AcrN system results
in a drastic change in the hyperfine splitting pattern of AcrN•-
due to the complexation with Sc3+ (Scheme 1) as shown in
Figure 5a.39 The ESR spectrum is well reproduced by the
computer simulation spectrum with the hfc values of a(2N) )
6.95 G, a(2H) ) 5.03 G, a(4H) ) 0.45 G, a(4H) ) 0.50 G,
and a(Sc3+) ) 0.63 G (Figure 5b).40 The complete agreement
of the observed ESR spectrum (Figure 5a) with the computer
simulation spectrum (Figure 5b) indicates that AcrN•- forms a
2:1 complex with Sc3+, which is bridged by Sc(OTf)3.
Figure 5. (a) ESR spectrum of a propionitrile solution containing (BNA)2
(1.6 × 10-2 M) and AcrN (5.1 × 10-3 M) in the presence of Sc(OTf)3 (1.6
× 10-1 M) at 203 K. (b) Computer simulation spectrum.
with the hyperfine splitting (hfs) values: a(4H) ) 4.9 G, a(4H)
) 2.0 G, and a(2H) ) 1.1 G (Figure 6b), which agree with the
literature values.41 In the presence of Sc3+, a drastic change in
the hyperfine pattern of Py•- due to the complexation with Sc3+
is observed as shown in Figure 6c. The broad triplet signal
indicates that two equivalent protons have a large hfs value of
9.4 G. The ESR spectrum is well reproduced by the computer
simulation spectrum with the hfc values of a(2H) ) 9.4 G
(Figure 6d). This indicates that the unpaired electron is localized
largely on the C2 and C7 positions and that the negative charges
are located on the C5 and C10 positions, where two Sc3+ ions
can bind as shown in Figure 6c. Thus, carbon atoms which
exhibit the smallest spin density in Py•- accommodate the largest
spin density in the Py•--2Sc(OTf)3 complex.42
The 1:2 complex formation between Py•- and Sc3+ (Py•--
2Sc(OTf)3) can also be confirmed by ESR in the (BNA)2-Py
system. The ESR spectrum of Py•- produced by the electro-
chemical reduction of Py was measured using the electrolysis
cell designed for ESR measurements (see Experimental Section)
as shown in Figure 6a. The hyperfine structures of the radical
anion species are well reproduced by the computer simulation
The formation of (AcrN)2•--Sc(OTf)3 in the Sc(OTf)3-
promoted electron transfer from an electron donor to AcrN is
also confirmed by the laser flash photolysis (vide infra). The
transient absorption spectrum obtained after the laser pulse
excitation of the AcrN-hexamethylbenzene (HMB) system in
(37) Patz, M.; Kuwahara, Y.; Suenobu, T.; Fukuzumi, S. Chem. Lett. 1997, 567.
(38) (a) Fukuzumi, S.; Suenobu, T.; Patz, M.; Hirasaka, T.; Itoh, S.; Fujitsuka,
M.; Ito, O. J. Am. Chem. Soc. 1998, 120, 8060. (b) Fukuzumi, S.; Yuasa,
J.; Suenobu, T. J. Am. Chem. Soc. 2002, 124, 12566. (c) Fukuzumi, S.;
Ohkubo, K.; Okamoto, T. J. Am. Chem. Soc. 2002, 124, 14147.
(39) In the presence of Sc(OTf)3, electron transfer from (BNA)2 to AcrN occurs
under dark via the Sc(OTf)3-promoted thermal electron-transfer process.
(40) The ESR spectrum of AcrN•- was reported previously, see: Chaudhuri,
J.; Kume, S.; Jagur-Grodzinski, J.; Szwarc, M. J. Am. Chem. Soc. 1968,
90, 6421.
(41) Hammerich, O.; Nielsen, M. F.; Zuilhof, H.; Mulder, P. P. J.; Lodder, G.;
Reiter, R. C.; Kage, D. E.; Rice, C. V.; Stevenson, C. D. J. Phys. Chem.
1996, 100, 3454.
(42) The example of rare earth metal ion-Py complexes, see: Thiele, K.-H.;
Bambirra, S.; Sieler, J.; Yelonek, S. Angew. Chem., Int. Ed. Engl. 1998,
37, 2886.
9
J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004 7591

A R T I C L E S
Fukuzumi et al.
Figure 8. Plots of the conversion of HMB (O) and the yield of
1
pentamethylbenzyl alcohol (0) vs time determined based on the H NMR
spectral change observed in the photosensitized oxygenation of HMB (2.0
× 10-3 M) in the presence of AcrN (2.0 × 10-5 M) and Sc(OTf)3 (4.0 ×
10-2 M) under irradiation of UV-visible light (λ > 300 nm) in O2-saturated
MeCN at 298 K.
(AcrN)2•--Sc(OTf)3, respectively (Figure 7). The transient
spectrum of the (AcrN)2•--Sc(OTf)3 complex is independently
detected in the photoinduced electron transfer from ferrocene
to AcrN in the presence of Sc(OTf)3 (see Supporting Informa-
tion, S5).
Figure 6. (a) ESR spectrum of a propionitrile solution containing Py (3.3
× 10-3 M) in the presence of n-Bu4NPF6 (0.2 M) at 203 K. (c) ESR
spectrum of a propionitrile solution containing (BNA)2 (2.0 × 10-2 M)
and Py (6.6 × 10-2 M) in the presence of Sc(OTf)3 (0.5 M) irradiated with
a high-pressure mercury lamp at 203 K. The computer simulation spectra
are shown in (b) and (d).
Photocatalytic Oxygenation of Hexamethylbenzene via Sc-
(OTf)3-Promoted Photoinduced Electron Transfer. The Sc-
(OTf)3-promoted photoinduced electron transfer from HMB to
AcrN and Py (vide supra) has led us to develop a new
photocatalytic system using AcrN or Py as a photocatalyst in
the presence of Sc(OTf)3. Irradiation of the absorption band with
UV-visible light (λ > 300 nm) of AcrN in an O2-saturated
CD3CN solution containing hexamethylbenzene (HMB) and
AcrN results in no change in the 1H NMR spectrum. When Sc-
(OTf)3 is added to an O2-saturated CD3CN solution of HMB
and AcrN, however, the 1H NMR signals due to HMB decreased
1
and new H NMR signals due to the oxygenated product, i.e.,
pentamethylbenzyl alcohol appear during the photoirradiation
(eq 11).46 The time course of the photochemical reaction is
Figure 7. Transient absorption spectra in the photoreduction of AcrN (6.0
× 10-5 M) by HMB (5.6 × 10-2 M) in the absence (b) and presence (O)
of Sc(OTf)3 (9.6 × 10-3 M) at 2.0 µs (b) and 8.0 µs (O) after laser excitation
at λ ) 355 nm in deaerated MeCN at 298 K.
the absence and presence of Sc(OTf)3 is shown in Figure 7.
The transient absorption band at λmax ) 438 nm is observed in
the absence of Sc(OTf)3 (closed circles). This is assigned to
the triplet excited state of AcrN.43 Thus, it is confirmed that no
photoinduced electron transfer occurs from HMB to 1AcrN* in
the absence of Sc(OTf)3. In the presence of Sc(OTf)3, however,
the transient absorption bands are observed at λ ) 485 and λ
) 510 nm, which are assignable to HMB radical cation44,45 and
shown in Figure 8, where the disappearance of HMB is
accompanied by the formation of pentamethylbenzyl alcohol
which is further converted to the more oxygenated products
(44) Masnovi, J. M.; Sankararaman, S.; Kochi, J. K. J. Am. Chem. Soc. 1989,
111, 2263.
(45) (a) Peacock, N. J.; Schuster, G. B. J. Am. Chem. Soc. 1983, 105, 3632. (b)
Teng, H. H.; Dunbar, R. C. J. Chem. Phys. 1978, 68, 3133.
(43) Ezuml, K.; Kubota, T.; Miyazaki, H.; Yamakawa, M. J. Phys. Chem. 1976,
80, 980.
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7592 J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004

Role of Scandium Ion in Photocatalytic Oxygenation
A R T I C L E S
in Figure 9 can only be explained by the radical chain reactions
initiated by the Sc(OTf)3-promoted photoinduced electron
1
transfer from HMB to Py* (Scheme 2).
The dramatically enhanced electron acceptor ability of 1Py*
in the presence of Sc(OTf)3 as compared to 1Py* in the absence
of Sc(OTf)3 (vide supra) makes it possible that electron transfer
1
from HMB to Py* occurs efficiently to produce the radical
ion pair [HMB•+ Py•--n Sc(OTf)3 (n ) 1,2)]. In the presence
of O2, the Py•--nSc(OTf)3 complex may be rapidly oxidized
by O2 to produce the O2•--Sc(OTf)3 complex which has been
reported to be relatively stable.48 In fact, the ESR signal due to
the O2•--Sc(OTf)3 complex is observed during the photoirra-
diation of an O2-saturated EtCN solution of HMB and Py in
•-
the presence of Sc(OTf)3 as shown in Figure 10. The O2
-
Sc(OTf)3 complex generates H2O2 via disproportionation reac-
tion (Supporting Information S7). Pentamethylbenzyl radical
produced by the deprotonation of HMB•+ may also be rapidly
trapped by O2 to give pentamethylbenzylperoxyl radical which
abstracts a hydrogen from HMB to produce pentamethylbenzyl
hydroperoxide, accompanied by regeneration of pentamethyl-
benzyl radical. The bimolecular reaction of pentamethylbenzyl-
peroxyl radicals affords penta-methylbenzyloxyl radicals, ac-
companied by generation of O2.49 The pentamethylbenzyloxyl
radical can abstract a hydrogen atom from HMB to yield penta-
methylbenzyl alcohol, accompanied by regeneration of pen-
tamethylbenzyl radical to constitute the radical chain cycle
(Scheme 2). The bimolecular reaction of pentamethylbenzyl-
peroxyl radicals may also produce pentamethylbenzyl alcohol,
pentamethylbenzaldehyde, and oxygen, which is the termination
step in the radical chain reaction in Scheme 2.50,51 Since the
aldehyde has hardly been detected, the termination step is
negligible as compared with the efficient radical chain process
in Scheme 2.
Figure 9. (a) Dependence of the quantum yields (Φ) on the light intensity
(In-1/2) for the photosensitized oxygenation of HMB (4.0 × 10-2 M) in
the presence of Py (1.0 × 10-4 M) and Sc(OTf)3 (4.0 × 10-2 M) in O2-
saturated MeCN at 298 K. (b) Dependence of the quantum yields (Φ) on
[HMB]3/2 for the photosensitized oxygenation of HMB in the presence of
Py (1.0 × 10-4 M) and Sc(OTf)3 (4.0 × 10-2 M) in O2-saturated MeCN at
298 K.
By applying the steady-state approximation to the radical
species (pentamethylbenzyl radical cation, Py•--nSc(OTf)3,
pentamethylbenzylperoxyl radical, etc.) in Scheme 2, the
dependence of Φ on [HMB] can be derived as given by eq 12.
Φ ) kp[(Φ0kqτ[HMB])/2ktIn(1 + kqτ[HMB])]1/2[HMB] (12)
which were not analyzed here. The preparative scale photo-
catalytic oxygenation of HMB afforded pentamethylbenzyl
alcohol (isolated yield: 30%, see Experimental Section).47 In
the case of Py, virtually the same results are obtained (see
Supporting Information S6).
Under the experimental conditions such that kqτ[HMB] , 1(kqτ
) 3.0 × 10-1 M-1, [HMB] ) 5.0 × 10-2 M), eq 12 is reduced
to eq 13
Φ ) C[HMB]3/2In-1/2
(13)
The quantum yields (Φ) of the photocatalytic oxygenation
of HMB using Py as a photocatalyst in the presence of Sc-
(OTf)3 were determined from a decrease in the HMB concentra-
tion by using GC-MS (see Experimental Section). The Φ value
for the photooxygenation of HMB is found to vary depending
on the light intensity at fixed HMB concentration as shown in
Figure 9a, where the Φ value is proportional to the reciprocal
of the square root of light intensity (In-1/2). The dependence of
Φ on the HMB concentration was also examined and the Φ
value increases with increasing [HMB] exhibiting the 3/2-order
dependence on [HMB] as shown in Figure 9b, where a linear
correlation is seen between Φ and [HMB]3/2. The Φ value much
larger than unity and unusual dependence of Φ on In and [HMB]
where C ) kp(Φ0kqτ/2kt)1/2. The dependence of the Φ on [HMB]
and in Figure 9 agrees with eq 13. Such agreement confirms
the validity of Scheme 2.
Virtually the same radical chain mechanism can be applied
to the photocatalytic oxygenation of HMB via Sc(OTf)3-
Rigaudy, J.; Schmidt, R. Acc. Chem. Res. 2003, 36, 668. (c) Albini, A.;
Maurizio, F. In CRC Handbook of Organic Photochemistry and Photobi-
ology (2nd Ed.); Wiliam, H., Francesco, L., Eds.; CRC Press: Boca Raton;
2004, 45/1-45/19.
(47) Hexamethylbenzene was converted to a mixture of oxidized products in
dye-sensitized photooxidation in benzene/methanol mixed solvent. However,
no pentamethylbenzyl alcohol was obtained, see: Wasserman, H. H.;
Mariano, P. S.; Keehn, P. M. J. Org. Chem. 1971, 36, 1765.
(48) Fukuzumi, S.; Patz, M.; Suenobu, T.; Kuwahara, Y.; Itoh, S. J. Am. Chem.
Soc. 1999, 121, 1605.
(46) Photooxygenation of alkylbenzenes using 9, 10-dicyanoanthracene which
is a much stronger oxidant than pyrene or acridine has previously been
reported: (a) Saito, I.; Tamoto, K.; Matsuura, T. Tetrahedron. Lett. 1979,
2889. In general, photosensitized oxygenation of aromatic compounds
involves generation of singlet oxygen, see: (b) Aubry, J.-M.; Pierlot, C.;
(49) Fukuzumi, S.; Ishikawa, M.; Tanaka, T. J. Chem. Soc., Perkin Trans. 2
1989, 1037.
(50) (a) Howard. J. A. AdV. Free-Radical Chem. 1972, 4, 49. (b) Howard, J. A.
In Peroxyl Radicals; Alfassi, Z. B., Ed.; Wiley: Chichester, 1997; pp 283-
334.
9
J. AM. CHEM. SOC. VOL. 126, NO. 24, 2004 7593

A R T I C L E S
Fukuzumi et al.
Scheme 2
and AcrN significantly by promoting photoinduced electron
transfer from HMB to the singlet excited state of not only AcrN
which can form the ground-state complex with Sc(OTf)3 but
also Py which has no interaction with Sc(OTf)3 in the ground
state due to complexation of Sc(OTf)3 with Py•- and AcrN•-,
respectively. Such remarkable promoting effects of Sc(OTf)3
in photoinduced electron-transfer reactions of photosensitizers
will expand the scope of photocatalytic reactions via photoin-
duced electron transfer.52
Acknowledgment. This work was partially supported by
Grants-in-Aid for Scientific Research on Priority Area (Nos.
13440216 and 14045249) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
Supporting Information Available: Fluorescence spectra of
AcrN in the absence and presence of Sc(OTf)3 (S1), Stern-
Volmer plots for the fluorescence quenching of AcrN (S2),
Stern-Volmer plots for the fluorescence quenching of Py (S3),
plots of (kq-k0)/[Sc(OTf)3] vs [Sc(OTf)3] for the fluorescence
•-
quenching of Py (S4), transient spectrum of the (AcrN)2
-
Figure 10. ESR spectrum of (a) O2•--Sc(OTf)3 complex produced in
photosensitized oxygenation of HMB (6.2 × 10-2 M) in the presence of
Py (1.0 × 10-1 M) and Sc(OTf)3 (5.0 × 10-2 M) in O2-saturated EtCN at
203 K and (b) the computer simulation spectrum.
Sc(OTf)3 complex (S5), conversion of HMB and the yield of
pentamethylbenzyl alcohol in the photosensitized oxygen-
ation of HMB in the presence of Py and Sc(OTf)3 (S6),
determination of the concentration of hydrogen peroxide (H2O2)
(S7). This material is available free of charge via the Internet
at http://pubs.acs.org.
promoted photoinduced electron transfer from HMB to the
(AcrN)2-Sc(OTf)3 complex. It should be emphasized that with-
out Sc(OTf)3 no photocatalytic oxygenation of HMB has occur-
red using Py or AcrN as a photosensitizer. Thus, Sc(OTf)3 plays
an essential role to improve the photocatalytic activity of Py
JA031649H
(52) Electrochemical reactions provide complementary methods for photocata-
lytic reactions via photoinduced electron transfer, see for example: (a)
Grimshaw, J. Electrochemical Reaction and Mechanisms in Organic
Chemistry; Elsevier: London, 2000. (b) Swenton, J. S. Acc. Chem. Res.
1983, 16, 74. (c) Ronla´n, A.; Hammerich, O.; Parker, V. D. J. Am. Chem.
Soc. 1973, 95, 7132.
(51) Lindsay, D.; Howard, J. A.; Horswill, E. C.; Iton, L.; Ingold, K. U.; Cobbley,
T.; Ll, A. Can. J. Chem. 1973, 51, 870.
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