Dehydrogenation vs oxygenation in photosensitized oxidation of 9-substituted 10-methyl-9,10-dihydroacridine in the presence of scandium ion

Article abstract of DOI:10.1021/jp0128729

Photooxidation of 9-substituted 10-methyl-9,10-dihydroacridine (AcrHR) with oxygen occurs efficiently in the presence of 9,10-dicyanoanthracene (DCA) and scandium triflate [Sc(OTf)₃] under visible light irradiation in oxygen-saturated acetonitr

Full text of DOI:10.1021/jp0128729

J. Phys. Chem. A 2002, 106, 1465-1472
1465
Dehydrogenation vs Oxygenation in Photosensitized Oxidation of 9-Substituted
10-Methyl-9,10-dihydroacridine in the Presence of Scandium Ion
Shunichi Fukuzumi,*,† Shunsuke Fujita,† Tomoyoshi Suenobu,† Hiroshi Imahori,†
Yasuyuki Araki,‡ and Osamu Ito*,‡
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
ReceiVed: July 25, 2001; In Final Form: NoVember 5, 2001
Photooxidation of 9-substituted 10-methyl-9,10-dihydroacridine (AcrHR) with oxygen occurs efficiently in
the presence of 9,10-dicyanoanthracene (DCA) and scandium triflate [Sc(OTf)3] under visible light irradiation
in oxygen-saturated acetonitrile (MeCN) to yield the 9-substituted 10-methylacridinium ion (AcrR+) and H2O2
or the 10-methylacridinium ion (AcrH+) and the oxygenated products of R such as ROOH, depending on the
type of substitutent R. No DCA-photosensitized oxidation of AcrHR occurs in the absence of Sc3+ under
otherwise the same experimental conditions. The observed selectivities for the C(9)-H vs C(9)-C bond
cleavage of AcrHR in the DCA-photosensitized oxidation of AcrHR in the presence of Sc(OTf)3 agree with
those for the cleavage of radical cations of AcrHR (AcrHR•+) depending on the type of substituent R. Such
product selectivities, being consistent with the electron-transfer oxidation of AcrHR, combined with quantum
yield determination, the 1O2 phosphorescence decay dynamics, and the detection of radical ion intermediates
in the laser-flash photolysis experiments reveal the electron-transfer radical chain mechanism for the DCA-
photosensitized oxidation of AcrHR initiated by photoinduced electron transfer from AcrHR to the singlet
excited state of DCA.
Introduction
oxidation reactions with oxygen. Both transient dynamics of
the intermediate radical ions and the careful product analysis
Reduced nicotinamide adenine dinucleotide (NADH) plays
a vital role as the electron source in the respiratory chain to
reduce oxygen to water.1 NADH itself is stable to oxygen, and
thus, either oxygen or NADH should be activated to undergo
the reduction of oxygen by NADH.2,3 In the former case,
activated oxygen species such as singlet oxygen have been
reported to oxidize NADH to yield the two-electron oxidized
form, i.e., NAD+.4,5 In the latter case, photoinduced electron
transfer from the singlet excited state of NADH and analogues
to oxygen initiates the reduction of oxygen in the protic media
to hydrogen peroxide.6,7
Singlet oxygen is normally produced by photosensitization,8
and a variety of substrates are oxygenated by the reactions with
singlet oxygen.9-12 Cyanoaromatic compounds such as 9,10-
dicyanoanthracene (DCA) have frequently been used as sensitiz-
ers for the formation of singlet oxygen.13-16 However, a
sensitizer for the formation of singlet oxygen is often converted
to the radical ion by the photoinduced electron-transfer reactions
are required to disclose the operating reaction mechanism. No
such study has ever been performed for photosensitized oxida-
tion of NADH analogues with oxygen.
We have previously found that the electron-transfer oxidation
of 9-substituted 10-methyl-9,10-dihydroacridine (AcrHR), which
is an acid stable NADH analogue, results in cleavage of the
C(9)-H or C(9)-C bond depending on the type of the
substituent R to yield the 9-substituted 10-methylacridinium ion
(AcrR+) or 10-methylacridinium ion (AcrH+).20 We report
herein that the DCA-photosensitized oxidation of AcrHR by
O2 occurs efficiently in the presence of scandium triflate [Sc-
(OTf)3] leading to the cleavage of the C(9)-H or C(9)-C bond
of AcrHR to yield AcrR+ and H2O2 (dehydrogenation) or AcrH+
and the oxygenated products of R such as ROOH (oxygenation).
The scandium ion (Sc3+) is chosen as an effective promoter for
the reduction of O2, because Sc3+ has been reported to be the
most effective catalyst among a variety of metal ions in the
electron-transfer reduction of O2.21,22 The ratio of dehydroge-
nation vs oxygenation varies depending on the type of R. No
photosensitized oxidation of AcrHR was observed in the absence
•-
in the presence of an electron donor or acceptor.17-19 Both O2
1
and O2 can be formed under the same reaction conditions,
resulting in a competition between two different types of
oxidation reactions.15-17 To further complicate matters, 1O2 can
•-
be reduced to O2 by thermal and photoinduced electron
of Sc3+ under otherwise the same experimental conditions. The
transfer from electron donors.16 Thus, considerable care must
be taken in assigning the mechanisms of photosensitized
1
possible role of O2 and the detailed mechanism of the Sc3+
-
promoted photosensitized oxidation of AcrHR by O2 (dehydro-
genation vs oxygenation) are carefully examined for the first
time based on the product analysis, quantum yield determination,
and the detection of radical ion intermediates in the laser-flash
photolysis experiments.
* To whom correspondence should be addressed. E-mail: fukuzumi@ap.
chem.eng.osaka-u.ac.jp.
† Osaka University.
‡ Tohoku University.
10.1021/jp0128729 CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/29/2002

1466 J. Phys. Chem. A, Vol. 106, No. 7, 2002
Fukuzumi et al.
Experimental Section
intensity of monochromatized light of λ ) 422 nm was
determined as 1.6 × 10-9, 2.5 × 10-9, and 8.8 × 10-9 einstein
s-1 with the slit width of 10, 15, and 20 nm, respectively. The
photochemical reactions were monitored using a Hewlett
Packard 8453 diode array spectrophotometer. The quantum
yields were determined from the increase in absorbance due to
AcrH+ (λmax ) 358 nm, ꢀmax ) 1.8 × 104 M-1 cm-1), AcrMe+
Materials. AcrHR was prepared from 10-methylacridinium
iodide (AcrH+I-) by reduction with NaBH4 in methanol and
purified by recrystallization from ethanol.23 AcrH+I- was
prepared by the reaction of acridine with methyl iodide in
-
acetone and was converted to the perchlorate salt (AcrH+ClO4
)
by the addition of magnesium perchlorate to the iodide salt
(AcrH+I-) and purified by recrystallization from methanol.24
[9,9′-2H2]-10-Methyl-9,10-dihydroacridine (AcrD2) was prepared
from 10-methylacridone by reduction with LiAlD4, which was
obtained from Aldrich. 9-Alkyl (or phenyl)-10-methyl-9,10-
dihydroacridines (AcrHR; R ) Me, Et, CH2Ph, But and Ph)
were prepared by the reduction of AcrH+I- with the corre-
sponding Grignard reagents (RMgX).20 10,10′-Dimethyl-9,9′,-
(λmax ) 358 nm, ꢀmax ) 1.8 × 104 M-1 cm-1), AcrEt+ (λmax
)
358 nm, ꢀmax ) 1.8 × 104 M-1 cm-1), AcrPh+ (λmax ) 360
nm, ꢀmax ) 1.8 × 104 M-1 cm-1), and AcrCH2Ph+ (λmax
)
360 nm, ꢀmax ) 1.8 × 104 M-1 cm-1), respectively, where the
DCA absorbance at 422 nm has been taken into account. To
avoid the contribution of light absorption of the products, only
the initial rates were determined for determination of the
quantum yields.
10,10′-tetrahydro-9,9′-biacridine [(AcrH)2] was prepared from
1O2 Phosphorescence Measurements. An O2-saturated CD3-
CN solution containing DCA (2.5 × 10-4 M) in quartz cells
(optical path length 10 mm) was excited at λ ) 422 nm using
a Cosmo System LVU-200S spectrometer. Nanosecond pulse
for sample excitation (λ ) 422 nm) was obtained by the dye
laser (Usho Optical Systems DL50) with the coumarin 120 in
methanol (0.12 g/L), where the dye was excited by the frequency
tripled output (λ ) 355 nm) of the Q-switched Nd:YAG laser
(Spectra Physics GCR-150-10) operating at 10 Hz. Typical
pulse energies at the sample were in the range of 1-2 mJ. A
photomultiplier (Hamamatsu Photonics, R5509-72) cooled by
liquid nitrogen was used to detect emission from the sample
cell in the near-infrared region through the slit with 2 mm width
and a monochromator equipped with diffraction gratings ap-
propriate for the infrared light. The signal was captured and
averaged by a digital storage oscilloscope (Tektronix TDS3032)
and transferred to a personal computer for analysis. Typical
decays were recorded as the average of 32 laser shots. The
decays were analyzed with nonlinear least-squares analysis of
the observed time profile with a single-exponential function and
in all cases gave good quality fits.
the reduction of 10-methylacridinium perchlorate (AcrH+ClO4
)
-
with Me3SnSnMe3 in MeCN at 333 K25 and purified by
recrystallization from the mixture of acetonitrile (MeCN) and
chloroform. Anal. Calcd for C28H24N2: C, 86.56; H, 6.23; N,
7.21. Found: C, 86.56; H, 6.29; N, 7.15. 9,10-Dicyanoan-
thracene (DCA) was obtained from Tokyo Kasei Organic
Chemicals. Scandium trifluoromethanesulfonate, Sc(OTf)3 (99%,
F.W. ) 492.16) was obtained from Pacific Metals Co., Ltd.
(Taiheiyo Kinzoku). 7-Amino-4-methylcoumarin (coumarin
120) for the laser dye was purchased from Lambda Physik. tert-
Butyl alcohol and benzaldehyde were obtained from Wako Pure
Chemicals. ButOOH was obtained from Nacalai Tesque Co.,
Ltd. Potassium ferrioxalate used as an actinometer was prepared
according to the literature and purified by recrystallization from
hot water.26 MeCN and propionitrile (EtCN) as solvents were
purified and dried by the standard procedure.27 Deuterated
[2H3]acetonitrile (CD3CN, 99.8%) was obtained from EURI SO-
TOP, CEA, France and used as received.
Reaction Procedure. Typically, AcrHR (6.0 × 10-3 M),
DCA (3.0 × 10-4 M), Sc(OTf)3 (3.0 × 10-4 M), and CF3COOH
(1.0 × 10-2 M) were added to an NMR tube which contained
an O2-saturated CD3CN solution (0.60 cm3). The solution was
irradiated for 1 h with a Xe lamp (λ > 380 nm) equipped with
a Toshiba UV-37 cut off filter. The oxidized products of AcrHR
were identified by the 1H NMR spectra by comparing with those
of authentic samples. The 1H NMR measurements were
performed using a JEOL JNM-AL-300 (300 MHz) NMR
spectrometer. 1H NMR (CD3CN): AcrH+ClO4-: δ 4.76 (s, 3H),
7.9-8.8 (m, 8H), 9.87 (s, 1H); AcrMe+ClO4-: δ 3.48 (s, 3H),
4.74 (s, 3H), 7.9-8.9 (m, 8H); AcrEt+ClO4-: δ 1.52 (t, 3H, J
) 7.5 Hz), 3.95 (q, 2H, J ) 7.5 Hz), 4.71 (s, 3H), 7.9-8.9 (m,
8H); AcrPh+ClO4-: δ 4.83 (s, 3H), 7.5-8.6 (m, 13H); AcrCH2-
Ph+ClO4-: δ 4.79 (s, 3H), 5.35 (s, 2H), 7.5-8.9 (m, 13H);
ButOOH: δ 1.18 (s, 9H); ButOH: δ 1.17 (s, 9H); PhCHO: δ
10.00 (s, 1H), 7.5-8.0 (m, 5H); PhCH2OOH: δ 4.89 (s, 2H),
7.5-8.0 (m, 5H). The amounts of H2O2 formed in the photo-
oxidation of AcrHR were determined by the titration with iodide
ion according to the procedure as described previously.28
Quantum Yield Determination. A standard actinometer
(potassium ferrioxalate)26 was used for the quantum yield
determination of the photooxidation of AcrHR in the presence
of Sc(OTf)3 and DCA in O2-saturated MeCN. A 10 mm square
quartz cuvette which contained an O2-saturated MeCN solution
(3.0 cm3) of AcrHR, DCA (1.0 × 10-4 M), and Sc(OTf)3 was
irradiated with monochromatized light of λ ) 422 nm using a
Shimadzu spectrofluorophotometer (RF-5000). Under the condi-
tion of actinometry experiments, the actinometer absorbed
essentially all the incident light of λ ) 422 nm and DCA
absorbed 95% of incident light of λ ) 422 nm. The light
The 1O2 phosphorescence lifetimes were measured for CD3-
CN solutions containing DCA (2.5 × 10-4 M) with AcrH2 and
AcrD2 in the absence and presence of Sc(OTf)3. There was no
change in the shape of the phosphorescence spectra, but there
was a change in the phosphorescence lifetime by the addition
of a quencher. The Stern-Volmer relationship (eq 1) was
obtained for the ratio of the phosphorescence lifetimes in
τ0/τ ) 1 + kqτ0[D]
(1)
the absence and presence of Sc(OTf)3 and the concentrations
of AcrH2 and AcrD2 used as quenchers (τ0 is the lifetime in the
absence of a quencher).
Fluorescence Quenching. Quenching experiments of the
fluorescence of DCA were carried out on a Shimadzu spec-
trofluorophotometer (RF-5000). The excitation wavelength of
DCA was 422 nm in MeCN. The monitoring wavelength was
corresponding to the maximum of the emission band at λmax
)
435 nm. Typically, an MeCN solution (3.0 cm3) was deaerated
by argon purging for 8 min prior to the measurements. Relative
fluorescence intensities were measured for MeCN solutions
containing DCA (1.0 × 10-5 M), electron donors and O2. There
was no change in the shape, but there was a change in the
intensity of the fluorescence peak by the addition of a quencher.
The Stern-Volmer relationship (eq 2) was obtained for the ratio
of the emission intensities and the concentrations of electron
I0/I ) 1 + KSV[D]
(2)

Dehydrogenation vs Oxygenation
J. Phys. Chem. A, Vol. 106, No. 7, 2002 1467
TABLE 1: Photooxidation of AcrHR (6.0 × 10-3 M) in the
Presence of DCA (3.0 × 10-4 M), CF3COOH (1.0 × 10-2 M),
and Sc3+ (3.0 × 10-4 M) in O2 Saturated CD3CN under
Irradiation of Visible Light (λ > 380 nm) for 1 h at 298 K
AcrHR
product (yield, %)
AcrH2
AcrHMe
AcrHEt
AcrHPh
AcrHCH2Ph
AcrH+ (100)
H2O2 (95)
H2O2 (90)
H2O2 (97)
H2O2 (87)
AcrHMe+ (100)
AcrHEt+ (100)
AcrPh+ (100)
AcrHCH2Ph+ (75)
AcrH+ (25)
H2O2 (75)
PhCHO (10), PhCH2COOH (15)
ButOOH (76), ButOH (24)
H2O2 (86)
AcrHBut
(AcrH)2
AcrH+ (100)
AcrH+ (100)
proximately equivalent amount of hydrogen peroxide (95%
based on the initial amount of AcrH2) was formed (see the
Experimental Section). Thus, the stoichiometry of the photo-
chemical reaction is given by eq 3. Trifluoroacetic acid was
added as a proton source (see the Experimental Section). The
DCA absorbance remains unchanged during the photooxidation
of AcrH2. This indicates that DCA acts as an efficient and stable
photocatalyst for the photooxidation of AcrH2 in the presence
of Sc3+. In the absence of Sc3+, however, no photooxidation of
AcrH2 was observed with or without O2 (λ > 380 nm). Without
DCA no photooxidation of AcrH2 occurs even in the presence
of Sc3+ under otherwise the same experimental conditions. Thus,
both DCA and Sc3+ are required for the photooxidation of
AcrH2.
Other 9-substituted AcrHR can also be oxidized by the DCA
photosensitization in the presence of Sc3+ in MeCN. As is the
case of AcrH2, AcrHR with R ) Me, Et, and Ph is dehydro-
genated to yield AcrR+ accompanied by formation of H2O2.
These product yields are listed in Table 1. In contrast to these
dehydrogenation reactions, the DCA-sensitized photooxidation
of AcrHR containing tertiary alkyl group (R ) But) yields the
oxygenated product of the tert-butyl group, i.e, ButOOH and
ButOH, together with AcrH+ (eq 4). In this case, the C(9)-C
bond is cleaved to undergo the oxygenation reaction rather than
the dehydrogenation via the C(9)-H bond cleavage. In the case
of the dimer (R ) AcrH), the C(9)-C bond is also cleaved
selectively to yield AcrH+ accompanied by formation of H2O2
(Table 1). In the case of a secondary alkyl group (R ) CH2-
Ph), both the C(9)-H and C(9)-C bonds are cleaved to yield
the two types of products shown in eqs 3 and 4 (Table 1).
Figure 1. UV-visible spectral change in the photooxidation of AcrH2
(5.0 × 10-5 M) in the presence of DCA (1.0 × 10-4 M) and Sc3+ (2.0
× 10-4 M) in O2 saturated MeCN under irradiation of visible light (λ
> 380 nm) at 298 K; interval 150 s.
donors and O2 used as quenchers.29 The observed quenching
rate constants kq ()KSVτ-1) were obtained from the Stern-
Volmer constants KSV and the fluorescence lifetime τ (19.6 ns).30
ESR Measurements. The ESR spectra of the O2•--Sc3+
complex were measured using a JEOL JES-FA 100 X-band
spectrometer with a low-temperature cooling apparatus. An O2-
saturated propionitrile solution of AcrH2 (3.6 × 10-5 M), DCA
(2.5 × 10-4 M), and Sc(OTf)3 (1.0 × 10-2 M) was irradiated
at 193 K with a high-pressure mercury lamp (USH-1005D)
through a cut off filter (Toshiba UV-37) transmitting λ > 380
nm focusing at the sample cell in the ESR cavity. 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 using an Mn2+ marker.
Laser Flash Photolysis. The measurements of transient
absorption spectra in the photooxidation of AcrH2 (2.8 × 10-4
M) in the presence of DCA (1.2 × 10-4 M) and Sc(OTf)3 (0,
1.0 × 10-3 M) were performed according to the following
procedures. The O2 saturated MeCN solution containing DCA
(1.2 × 10-4 M), AcrH2 (2.8 × 10-4 M), and Sc(OTf)3 (0, 1.0
× 10-3 M) was excited by a Nd:YAG laser (Quanta-Ray, GCR-
130, 6 ns fwhm) at λ ) 425 nm with the power of 10 mJ per
pulse. A Xe flash lamp was used for the probe beam, which
was detected with a Si-PIN module (Hamamatsu, S5343) after
passing through the photochemical quartz vessel (10 mm × 10
mm) and a monochromator. The output from the Si-PIN module
was recorded with a digitizing oscilloscope (HP 54510B, 300
MHz). The transient spectra were recorded using fresh solutions
in each laser excitation. All experiments were performed at 298
K.
Results and Discussion
The observed selectivities for the C(9)-H vs C(9)-C bond
cleavage in Table 1 agree with those reported for the cleavage
of radical cations of AcrHR (AcrHR•+) depending on the type
of substituent R.20 Such an agreement indicates that the DCA-
photosensitized oxidation of AcrHR occurs via AcrHR•+ which
may be produced by electron transfer from AcrHR to the singlet
excited state of DCA (1DCA*). Alternatively, AcrHR•+ may
be produced via electron transfer from AcrHR to singlet oxygen
(1O2), because singlet oxygen (1O2) is known to be produced
Sc3+-Promoted DCA-Photosensitized Oxidation of AcrHR
with Oxygen. When an O2-saturated MeCN solution containing
AcrH2 (5.0 × 10-5 M), DCA (1.0 × 10-4 M), and Sc(OTf)3
(2.0 × 10-4 M) is irradiated with the visible light (λ > 380
nm), the absorption band due to AcrH+ (λmax ) 358 nm) appears
and the absorbance increases with the concomitant disappearance
of the absorption band due to AcrH2 (λmax ) 285 nm), as shown
in Figure 1. The quantitative formation of AcrH+ has also been
confirmed by the H NMR (see the Experimental Section).31
efficiently via an energy transfer from DCA* to O2.14 Thus,
1
1
After the completion of the photochemical reaction, an ap-
the possibility of involvement of 1O2 in the DCA-photosensitized

1468 J. Phys. Chem. A, Vol. 106, No. 7, 2002
Fukuzumi et al.
Figure 3. Transient absorption spectrum in the photooxidation of
AcrH2 (2.8 × 10-4 M) in the presence of DCA (1.2 × 10-4 M) in O2
saturated MeCN at 298 K (laser excitation at λ ) 425 nm).
and 710 nm of DCA•-,41 an absorption due to AcrH2•+ (λmax
)
640 nm)20 may be overlapped with the absorption due to DCA•-
•+
in Figure 3. The formation of AcrH2 is completed within 1
•+
µs, and no further rise of the absorption due to AcrH2 is
observed.
The rate constant of electron transfer from AcrH2 to 1DCA*
is determined by the fluorescence quenching (see the Experi-
mental Section) as 2.8 × 1010 M-1 s-1 which is a diffusion-
limited value. Such a fast electron transfer is consistent with
the highly exergonic nature of the reaction: the free energy
change of electron transfer (∆G0et) is determined as -1.10 eV
from the E0ox value of AcrH2 (0.81 V)20 and the E0red value of
1DCA* (1.91 V).30 On the other hand, the DCA fluorescence
(τ0 ) 19.6 ns)30 is also quenched by oxygen via an energy
transfer with the rate constant of 3.2 × 109 M-1 s-1 (see the
Experimental Section).42 Thus, an energy transfer and electron
transfer are competing and the relative ratio depends on the
concentration ratio of AcrH2 and O2. In any case, the formation
Figure 2. (a) Decay curves of phosphorescence of 1O2 at λmax ) 1270
nm in the absence and presence of AcrH2 after laser excitation (λ )
422 nm) in CD3CN at 298 K. (b) Stern-Volmer plot for the
1
phosphorescence quenching of O2 by AcrH2 in CD3CN at 298 K.
oxidation of AcrHR in the presence of Sc3+ was examined in
detail (vide infra).
1
Singlet Oxygen vs Electron Transfer. The O2 phospho-
rescence at 1270 nm is observed upon laser excitation of an
O2-saturated CD3CN solution containing DCA (2.5 × 10-4 M).
1
The decay of O2 phosphorescence obeys first-order kinetics,
and the lifetime (1.0 ms) agreed with the literature data.32,33
The addition of AcrH2 as a quencher results in a decrease in
the 1O2 lifetime as shown in Figure 2a. The Stern-Volmer plot
(eq 1) gives a good linear correlation between τ0/τ and the
quencher concentration (Figure 2b). The quenching rate constant
(kq) is determined as 7.0 × 107 M-1 s-1 from the slope of the
linear plot. It was confirmed that the kq value was independent
of the Sc3+ concentration.
of AcrH2 via photoinduced electron transfer from AcrH2 to
•+
1DCA* would be prompt and readily distinguishable from the
microsecond or millisecond time scale for the reaction of AcrH2
with 1O2 (Figure 2). Thus, it can be concluded that AcrH2•+ is
formed via photoinduced electron transfer from AcrH2 to 1DCA*
1
and not from AcrH2 to O2.
The quantum yields (Φ) for the formation of AcrH+ in the
DCA-photosensitized oxidation of AcrH2 by O2 in the presence
of Sc(OTf)3 in O2-saturated MeCN are determined by varying
the AcrH2 and Sc(OTf)3 concentrations (see the Experimental
Section). The Φ value at a fixed concentration is independent
of the Sc(OTf)3 concentration as shown in Figure 4, although
there is no photooxidation of AcrH2 without Sc(OTf)3 (vide
supra). Thus, a small concentration of Sc(OTf)3 (1.0 × 10-4
M) is enough to make it possible to undergo the DCA-
photosensitized oxidation of AcrH2 with O2.43 The effects of
other metal ions on the Φ value are also examined using Mg-
(ClO4)2 and Lu(OTf)3 (Figure 4). The Sc(OTf)3 gives the higher
Φ values as compared to the weaker Lewis acids, i.e., Mg-
(ClO4)2 and Lu(OTf)3. The Φ value for the formation of AcrH+
is proportional to the concentration of AcrH2 at low concentra-
tions (ca. 10-4 M) as shown in Figure 5. This is also consistent
with the electron-transfer mechanism since the quenching
When AcrH2 is replaced by the dideuterated compound
(AcrD2), eventually, the same kq value is obtained as shown in
Figure 2b (compare the open circle with triangle for AcrH2 and
AcrD2, respectively). Thus, there is no kinetic isotope effect in
1
the O2 quenching by AcrH2 and AcrD2. The absence of the
kinetic isotope effect indicates that no transfer of hydrogen is
1
involved in the rate-determining step of the O2 quenching by
AcrH2. This is consistent with a number of reported examples
of the 1O2 quenching via formation of exciplexes with electron
donors in which only a partial charge transfer occurs.34-38 The
1
full electron transfer from AcrH2 to O2 is unlikely to occur
judging from the more positive one-electron oxidation potential
of AcrH2 (E0 vs SCE ) 0.81 V)20 than the one-electron
ox
reduction potential of O2 in MeCN (E0* vs SCE ) 0.11
1
red
V).39,40
1
New absorption bands appear at 510, 640, and 710 nm after
the laser excitation (425 nm) of the absorption band of DCA
(1.2 × 10-4 M) in MeCN containing AcrH2 (2.8 × 10-4 M) at
1.4 µs as shown in Figure 3. Because the absorption at 640 nm
is significantly large as compared to other absorptions at 510
efficiency of DCA* in the presence of 1 × 10-4 M AcrH2 is
3% and increases linearly with increasing [AcrH2]. In contrast
1
to the case of the O2 quenching by AcrH2, an appreciable
kinetic isotope effect (ΦH/ΦD ) 1.9) is observed for the Φ value
when AcrH2 is replaced by AcrD2 (open triangle in Figure 5).

Dehydrogenation vs Oxygenation
J. Phys. Chem. A, Vol. 106, No. 7, 2002 1469
SCHEME 1
Figure 4. Dependence of the quantum yield (Φ) on [Mn+] (Mn+
)
Sc3+ (b), Lu3+ (9), Mg2+ (2)) for the photooxidation of AcrH2 (1.8 ×
10-4 M) in MeCN at 298 K.
In competition with the back electron transfer, a hydrogen
transfer from AcrH2 to O2•--Sc3+ occurs to yield the final
•+
product, AcrH+ and H2O2 (after the protonation in Scheme 1).44
In the absence of Sc3+, O2•- is produced instead of O2•--Sc3+
•-
in Scheme 1, and the back electron transfer from O2 to
•+
AcrH2 may be much faster than a hydrogen transfer from
•+
AcrH2 to O2•-, because the back electron transfer is highly
•-
exergonic (∆G0 ) -1.68 eV),45 and the reactivity of O2 is
et
Figure 5. Dependence of the quantum yield (Φ) for the formation of
AcrH+ on [AcrH2, AcrD2] (AcrH2 (O), AcrD2 (4)) for the DCA-
photosensitized oxidation of AcrH2 and AcrD2 in the presence of Sc3+
(1.0 × 10-3 M) in MeCN at 298 K.
much smaller than O2•--Sc3+ in which the unpaired electron
is more localized on the terminal oxygen atom.22 In the presence
of Sc3+, O2 is known to form the complex O2•--Sc3+ 22
.
•-
Because of the strong binding of Sc3+ to O2•-, O2•--Sc3+ is
harder to oxidize than O2•-, thus retarding the back electron
•+ 46,47
transfer to AcrH2
.
The formation of O2•--Sc3+ as a reactive intermediate for
the DCA-photosensitized oxidation of AcrH2 is confirmed by
the ESR measurements of an O2-saturated propionitrile solution
containing AcrH2 (3.6 × 10-5 M), DCA (2.5 × 10-4 M), and
Sc(OTf)3 (1.0 × 10-2 M) at 193 K under photoirradiation with
a Xe lamp (λ > 380 nm).48 Figure 6 shows the observed ESR
spectrum under the photoirradiation. The clear eight-line
isotropic spectrum at the center is ascribed to the superhyperfine
•-
7
coupling of O2 with the /2 nuclear spin of the scandium
nucleus (aSc ) 4.21 G) in O2•--Sc3+ as reported previously.39
The isotropic g value (2.0157) is appreciably smaller than the
corresponding value (2.030) of uncomplexed O2•- at 77 K, being
consistent with the spin delocalization to the scandium nucleus
as demonstrated by observation of superhyperfine.22
Figure 6. ESR spectrum of O2 radical anion-Sc3+ complex produced
in photooxidation of AcrH2 (3.6 × 10-5 M) in the presence of DCA
(2.5 × 10-4 M) and Sc3+ (1.0 × 10-2 M) in O2 saturated EtCN at 193
K.
The observed kinetic isotope effect may be ascribed to a
hydrogen transfer step from AcrH2 to O2•--Sc3+ produced
The quantum yields of DCA-photosensitized oxidation of
other substituted compounds (AcrHR) also exhibit linear
dependence on [AcrHR] as shown in Figure 7. The quantum
yield increases in order: R ) Ph < Et < Me < H < CH2Ph <
But < AcrH. This order agrees with the reactivity order of
AcrHR•+,20 supporting the electron-transfer mechanism in
Scheme 1. In the case of AcrHBut, photoinduced electron
•+
via an electron transfer from DCA•- to O2 in the presence of
Sc3+ as shown in Scheme 1. The singlet excited state (1DCA*)
is quenched by AcrH2 and O2 by an electron transfer and energy
•+
1
1
transfer to produce AcrH2 and O2, respectively. The O2 is
quenched by AcrH2 via an exciplex formation which decays to
the ground state. The radical ion pair produced by photoinduced
electron transfer decays to the reactant pair via the back electron
transfer from DCA•- to AcrH2•+. In competition with the back
electron transfer, DCA•- in the radical ion pair is oxidized by
O2 to produce (AcrH2•+ O2•--Sc3+) which also undergoes back
electron transfer from O2•--Sc3+ to AcrH2•+ to the ground state.
1
transfer from AcrHBut to DCA* in the presence of O2 and
•-
Sc3+ leads to formation of the radical ion pair (AcrHBut•+ O2
-
Sc3+) as in the case of AcrH2 (Scheme 1). The But group of
AcrHBut•+ is transferred to O2•--Sc3+ via cleavage of the
C(9)-C bond in competition with the back electron transfer

1470 J. Phys. Chem. A, Vol. 106, No. 7, 2002
Fukuzumi et al.
Figure 7. Dependence of the quantum yield (Φ) on [AcrHR] (R ) H
(O), But (4), Et (3), Ph (0), Me (b), CH2Ph (9), AcrH (2)) for the
photooxidation of AcrHR in the presence of DCA (1.0 × 10-4 M) and
Sc3+ (1.0 × 10-3 M) in O2 saturated MeCN at 298 K.
SCHEME 2
Figure 8. (a) Dependence of the quantum yield (Φ) on [AcrH2, AcrD2]
(AcrH2 (O), AcrD2 (4)) for the DCA-photosensitized oxidation of
AcrH2 and AcrD2 in the presence of DCA (1.0 × 10-4 M) and Sc3+
(1.0 × 10-3 M) in O2 saturated MeCN at 298 K; the irradiation light
intensity: 1.6 × 10-9 einstein s-1. (b) Dependence of the quantum yield
(Φ) on light intensity (In) for the DCA-photosensitized oxidation of
AcrH2 (3.0 × 10-2 M) in the presence of DCA (1.0 × 10-4 M) and
Sc3+ (1.0 × 10-3 M) in O2 saturated MeCN at 298 K.
from O2•--Sc3+ to AcrHBut•+ to yield ButOOH (Scheme 2).
The ButOOH decomposes further to ButOH (Table 1). The
C(9)-C bond is also cleaved in (AcrH)2•+, resulting in formation
of AcrH• which can further reduce O2•--Sc3+ to yield AcrH+
and H2O2.
SCHEME 3
Electron-Transfer Radical Chain Mechanism. According
to Scheme 1, the quantum yield would reach the limiting value
which is determined by the competition between the hydrogen
•+
transfer from AcrH2 to O2•--Sc3+ and the back electron
transfer. However, the quantum yield increases linearly with a
further increase in the AcrH2 concentration, exceeding unity as
shown in Figure 8a.49 The kinetic isotope effect ΦH/ΦD is also
determined using AcrD2 as 3.0 (Figure 8a), which is different
from the value (1.9) determined at the low concentration range
(Figure 5). Such a difference in the kinetic isotope effects
suggests the change in the reaction mechanism depending on
the AcrH2 concentration. The dependence of the Φ value on
the light intensity at fixed AcrH2 concentrations is also examined
as shown in Figure 8b, where the Φ value is proportional to
reciprocal of the square root of light intensity. The large quantum
yields exceeding unity, combined with the dependence of Φ
on the AcrH2 concentration (Figure 8a) and on the light intensity
(Figure 8b), indicate strong involvement of radical chain
processes which are initiated by the photoinduced electron-
transfer reactions as shown in Scheme 3.
a hydrogen from AcrH2 to yield H2O2, accompanied by
regeneration of AcrH• (Scheme 3). Such electron-transfer radical
chain reactions are also started by O2•--Sc3+ escaped from the
radical ion pair (AcrH2•+ O2•--Sc3+). The rate-determining step
may be the hydrogen transfer from AcrH2 to HO2• because the
observed kinetic isotope effect (3.0) agrees with that reported
The AcrH2•+ in the radical ion pair (AcrH2•+ O2•--Sc3+) in
Scheme 1 may escape from the cage to give the deprotonated
radical (AcrH•) which can start the radical chain propagation
in Scheme 3. First, an electron transfer from AcrH• to O2 occurs
efficiently in the presence of Sc3+ to give O2•--Sc3+ which is
for the hydrogen abstraction reaction of HO2 from AcrH2.7 This
•
may be the reason the kinetic isotope effect is changed from
the low AcrH2 concentration range (Figure 5) where the
contribution of the radical chain reactions is negligible and the
• 21,50
•
converted to HO2 .
In the propagation step, HO2 abstracts

Dehydrogenation vs Oxygenation
J. Phys. Chem. A, Vol. 106, No. 7, 2002 1471
AcrH+ and the oxygenated products of R such as ROOH,
depending on the type of substituent R. The observed selectivi-
ties for the C(9)-H vs C(9)-C bond cleavage of AcrHR in the
DCA-photosensitized oxidation of AcrHR in the presence of
Sc3+ result from those for the cleavage of the C(9)-H vs
C(9)-C bond of AcrHR•+ depending on the type of substituent
1
R. The photoinduced electron transfer from AcrHR to DCA*
starts the electron-transfer radical chain reactions (Schemes 3
and 4) to yield the oxidation products. The essential role of
Sc3+ for the DCA-photosensitized oxidation of AcrHR may be
the strong binding of Sc3+ to O2•-, which retards the back
electron transfer leading to the oxidation of AcrHR. Singlet
oxygen plays no essential role in the oxidation of AcrHR.
Acknowledgment. This work was partially supported by a
Grant-in-Aid for Scientific Research Priority Area (No. 11228205)
from the Ministry of Education, Science, Culture and Sports,
Japan.
Figure 9. Dependence of the quantum yield (Φ) on [AcrHBut] for
the photooxidation of AcrHBut in the presence of DCA (1.0 × 10-4
M) and Sc3+ (1.0 × 10-3 M) in O2 saturated MeCN at 298 K.
Supporting Information Available: Derivation of eq 5 (S1).
This material is available free of charge via the Internet at http://
pubs.acs.org.
SCHEME 4
References and Notes
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Trans. 2 1989, 1037.
•+
ΦH/ΦD value is ascribed to the hydrogen transfer from AcrH2
to O2•--Sc3+ (Scheme 1).
(8) (a) Foote, C. S. Acc. Chem. Res. 1968, 1, 104. (b) Kearns, D. R.
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By applying the steady-state approximation to the radical
species (HO2• and AcrH•) in Scheme 1, the quantum yield (Φ)
is given as the function of the light intensity (In) and the
concentration of AcrH2 (eq 5)
Φ ) kp(Φ0/ktIn)1/2[AcrH2]
(5)
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where Φ0 is the quantum yield of the initiation step, kp is the
rate constant of the rate-determining propagation step, and kt is
the rate constant of the termination step (see the Supporting
Information S1 for the derivation of eq 5). The derived
dependence of Φ on [AcrH2] agrees with the experimental
results that the Φ value is proportional to the AcrH2 concentra-
tion (Figure 8a) and the reciprocal of the square root of the
light intensity (Figure 8b) and independent of the Sc3+
concentration (Figure 4).
In the case of AcrHBut as well, the Φ value increases linearly
with a further increase in the AcrHBut concentration, exceeding
unity (Figure 9). Eventually the same radical chain mechanism
as Scheme 3 may be applied to the DCA-photosensitized
oxygenation of AcrHBut. In the propagation step, however, the
C(9)-C bond of AcrHBut is cleaved rather than the C(9)-H
bond to yield ButOOH as shown in Scheme 4.
Conclusions
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In summary, photooxidation of AcrHR with O2 occurs
efficiently in the presence of DCA and Sc3+ under visible light
irradiation in O2-saturated MeCN to yield AcrR+ and H2O2 or

1472 J. Phys. Chem. A, Vol. 106, No. 7, 2002
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of the quantum yield on the Sc3+ concentration, see Figure 4).
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(39) The E0red value is obtained by adding the excitation energy of 1O2
3
(0.98 eV) to the E0 value of O2 in the same solvent (-0.87 V).40
red
(40) Sawyer, D. T.; Chiericato, G., Jr.; Angelis, C. T.; Nanni, E. J., Jr.;
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(43) It was confirmed that the UV-visible spectral change at [Sc3+] )
2.0 × 10-4 M (Figure 1) was virtually the same as the change at higher
concentrations.
(44) The proton source here is H2O (∼10-2 M) contained in MeCN.
The presence of additional proton source such as CF3COOH (Table 1) does
not affect the stoichiometry of the photochemical reaction (eqs 3 and 4).
(45) The ∆G0 value is obtained from the difference between the E0
et
ox
value of AcrH2 (0.81 V)22 and the E0 value of O2 (-0.87 V).40
red
(46) Because the reorganization energy of the electron transfer of O2 is
know to be large (1.97 eV),47 the back electron transfer may be in the Marcus
normal region, when the rate of back electron transfer decreases with the
decreasing the driving force of electron transfer.
(47) Lind, J.; Shen, X.; Mere´nyi, G.; Jonsson, B. O¨ . J. Am. Chem. Soc.
1989, 111, 7654.
(48) The slopes in Figure 8a are larger than the corresponding apparent
slopes in Figure 5 because of the contribution of the nonchain process at
low AcrH2 or AcrD2 concentrations. However, the degree of the contribution
of the nonchain process is not determined quantitatively in this study. At
high AcrH2 or AcrD2 concentrations in Figure 8a, the chain process certainly
dominates as compared to the nonchain process.
(49) For the measurement of ESR, a high concentration of Sc3+ (1.0 ×
10-2 M) is required to obtain the clear signal due to the O2•--Sc3+ complex
(Figure 6).
(33) A longer lifetime around 1.5 ms has been reported previously.
See: Clough, R. L.; Dillon, M. P.; Iu, K.-K.; Ogilby, P. R. Macromolecules
1
1989, 22, 3620. This may indicate the presence of some O2 quenching in
(50) The electron transfer from AcrH• to O2 is endothermic in MeCN,
because the oxidation potential of AcrH• (-0.43 V)24 is more positive than
the reduction potential of O2 (-0.87 V).40 In the presence of Sc3+; however,
the reduction potential of O2 may be shifted in the positive direction,22
making it possible that AcrH• transfers an electron to O2 to yield AcrH+.
the present system.
(34) (a) Darmanyan, A. P.; Jenks, W. S.; Jardon, P. J. Phys. Chem. A
1998, 102, 7420. (b) Darmanyan, A. P.; Jenks, W. S.; Jardon, P. J. Phys.
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