Efficient catalysis of rare-earth metal ions in photoinduced electron-transfer oxidation of benzyl alcohols by a flavin analogue

Article abstract of DOI:10.1021/jp012709d

A flavin analogue (riboflavin-2′,3′,4′,5′-tetraacetate, Fl) forms the 1:1 and 1:2 complexes with rare-earth metal ions. The largest formation constants K₁ and K₂ for the 1:1 and 1:2 complexes between Fl and Sc³⁺ are determined as K₁ = 3.1 × 10⁴ M^-1 and K₂ = 1.4 × 10³ M^-1, respectively. The complexation of Fl with rare-earth metal ions results in blue shifts of the fluorescence maximum, shortening of the fluorescence lifetime, and more importantly the change in the lowest excited state from the n,π* triplet state of Fl to the π,π* singlet states of Fl-rare-earth metal ion complexes as indicated by the disappearance of the triplet-triplet (T-T) absorption spectrum of Fl by the complexation with metal ions. The strong complex formation between Fl and rare-earth metal ions enhances the oxidizing ability of the excited state of Fl as indicated by the significant acceleration in the fluorescence quenching rates of Fl-rare earth metal ion complexes via electron transfer from electron donors (e.g., alkylbenzenes) as compared to those of uncomplexed Fl. The one-electron reduction potential of the singlet excited state of the 1:2 complex between Fl and Sc³⁺, ¹(Fl-2Sc³⁺)* (* denotes the excited state), is positively shifted by 780 mV as compared to ¹Fl*. Such a remarkable enhancement of the redox reactivity of ¹(Fl-2Sc³⁺)* as compared to that of ¹Fl* makes it possible to oxidize efficiently p-chlorobenzyl alcohol to p-chlorobenzaldehyde by ¹(Fl-2Sc³⁺)*, although no photooxidation of p-chlorobenzyl alcohol by Fl occurred in deaerated MeCN. The quantum yield for the photooxidation of p-chlorobenzyl alcohol by Fl-2Sc³⁺ is the largest among various Fl-metal ion complexes. A comparison of the observed rate constant derived from the dependence of the quantum yield on the concentration of p-chlorobenzyl alcohol with the fluorescence quenching rate constant by electron transfer from the alcohol and the direct detection of radical intermediates reveal that the photooxidation proceeds via electron transfer from p-chlorobenzyl alcohol to ¹(Fl-2Sc³⁺)*. Under an atmospheric pressure of oxygen, the photooxidation of p-methoxybenzyl alcohol by oxygen proceeds efficiently in the presence of Fl-Lu³⁺ which acts as an efficient photocatalyst. No photodegradation was observed in the case of the Fl-Lu³⁺ complex, whereas the facile photodegradation of Fl-Mg²⁺ has precluded the efficient photocatalytic oxidation of the alcohol by oxygen.

Full text of DOI:10.1021/jp012709d

J. Phys. Chem. A 2001, 105, 10501-10510
10501
ARTICLES
Efficient Catalysis of Rare-Earth Metal Ions in Photoinduced Electron-Transfer Oxidation
of Benzyl Alcohols by a Flavin Analogue
,
†
†
†
†
Shunichi Fukuzumi,* Kiyomi Yasui, Tomoyoshi Suenobu, Kei Ohkubo,
‡
‡
Mamoru Fujitsuka, 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 13, 2001
A flavin analogue (riboflavin-2′,3′,4′,5′-tetraacetate, Fl) forms the 1:1 and 1:2 complexes with rare-earth
3+
1 2
metal ions. The largest formation constants K and K for the 1:1 and 1:2 complexes between Fl and Sc are
4
-1
3
-1
determined as K
1
) 3.1 × 10 M and K ) 1.4 × 10 M , respectively. The complexation of Fl with
2
rare-earth metal ions results in blue shifts of the fluorescence maximum, shortening of the fluorescence lifetime,
and more importantly the change in the lowest excited state from the n,π* triplet state of Fl to the π,π*
singlet states of Fl-rare-earth metal ion complexes as indicated by the disappearance of the triplet-triplet
(
T-T) absorption spectrum of Fl by the complexation with metal ions. The strong complex formation between
Fl and rare-earth metal ions enhances the oxidizing ability of the excited state of Fl as indicated by the
significant acceleration in the fluorescence quenching rates of Fl-rare earth metal ion complexes via electron
transfer from electron donors (e.g., alkylbenzenes) as compared to those of uncomplexed Fl. The one-electron
3
+
1
3+
reduction potential of the singlet excited state of the 1:2 complex between Fl and Sc , (Fl-2Sc )*
1
(
* denotes the excited state), is positively shifted by 780 mV as compared to Fl*. Such a remarkable
enhancement of the redox reactivity of (Fl-2Sc )* as compared to that of Fl* makes it possible to oxidize
1
3+
1
1
3+
efficiently p-chlorobenzyl alcohol to p-chlorobenzaldehyde by (Fl-2Sc )*, although no photooxidation of
p-chlorobenzyl alcohol by Fl occurred in deaerated MeCN. The quantum yield for the photooxidation of
p-chlorobenzyl alcohol by Fl-2Sc is the largest among various Fl-metal ion complexes. A comparison of
the observed rate constant derived from the dependence of the quantum yield on the concentration of
p-chlorobenzyl alcohol with the fluorescence quenching rate constant by electron transfer from the alcohol
and the direct detection of radical intermediates reveal that the photooxidation proceeds via electron transfer
3
+
1
3+
from p-chlorobenzyl alcohol to (Fl-2Sc )*. Under an atmospheric pressure of oxygen, the photooxidation
3
+
of p-methoxybenzyl alcohol by oxygen proceeds efficiently in the presence of Fl-Lu which acts as an
3
+
efficient photocatalyst. No photodegradation was observed in the case of the Fl-Lu complex, whereas the
2
+
facile photodegradation of Fl-Mg has precluded the efficient photocatalytic oxidation of the alcohol by
oxygen.
Introduction
determination of the roles of these noncovalent interactions in
biological systems has been complicated by the multiple forces
Flavoenzymes are a ubiquitous and structurally and function-
ally diverse class of redox-active proteins that use the redox
active isoalloxazine ring of flavins to mediate a variety of
electron-transfer processes over a wide range of redox poten-
involved, model studies have provided valuable information to
7
probe and isolate each of these interactions. The redox reactivity
of flavins is the most drastically changed by the photoexcitation
as compared to the ground state. Thus, photochemistry of
flavoenzymes and flavin analogues has been the subject of
intense research in photocatalysts for the photobiological redox
1
tials. The redox reactivity of flavins is modulated by the
interaction with apoenzymes, that is affected by hydrogen-
1,2
3
4
bonding, π-π stacking, donor-π interaction, conformational
8
-10
effects, and coordination to metal ions.6 Although direct
5
processes.
The redox reactivity of the photoexcited states
of flavins has been further modulated by coordination to metal
1
1-13
2+
2+
2+
2+
2+
ions.
Divalent metal cations (Mg , Ca , Sr , Ba , Zn ,
*
Author to whom correspondence should be addressed. E-mail:
2
+
fukuzumi@ap.chem.eng.osaka-u.ac.jp.
or Cd ) have been reported to be indispensable for the flavin-
dependent photocleavage of RNA at particular base pairs via
oxidative cleavage processes. The effect of metal ions on the
photocatalytic reactivity has also been studied using flavin
†
Osaka University, CREST, Japan Science and Technology Corporation
(
JST).
1
4
‡
Tohoku University, CREST, Japan Science and Technology Corporation
JST).
(
1
0.1021/jp012709d CCC: $20.00 © 2001 American Chemical Society
Published on Web 10/27/2001

1
0502 J. Phys. Chem. A, Vol. 105, No. 46, 2001
Fukuzumi et al.
analogues with the crown ether moiety as binding site for metal
ions.15
solution of trifluoromethanesulfonic acid. Lanthanum triflate was
obtained from Aldrich as hexahydrate form and used after drying
under vacuum evacuation for 40 h. Magnesium triflate [Mg-
(OTf)2] obtained from Aldrich was used as received. 1-Benzyl-
1,4-dihydronicotinamide dimer [(BNA)2] was prepared accord-
Among metal ions, rare-earth metal ions have attracted
considerable interest as mild and selective Lewis acids in organic
synthesis.16 Trivalent rare-earth (Sc , Yb , etc.) trifluoro-
3+
3+
22
methanesulfonates (triflates) have been utilized as Lewis acids
ing to the literature. Acetonitrile used as a solvent was purified
17
23
in promoting various carbon-carbon bond forming reactions.
and dried by the standard procedure. Acetonitrile-d3 and
In particular, scandium(III) triflate [Sc(OTf)3] has attracted much
attention due to its hard character as well as strong affinity to
carbonyl oxygen.18 However, there has been no report on the
effects of rare-earth metal ions on the redox reactivity of
photoexcited states of flavins and the photocatalytic reactivity.
We report herein that the riboflavin-2′,3′,4′,5′-tetraacetate (Fl)
forms not only 1:1 but also 1:2 complexes with rare-earth metal
ions and that the Fl-rare-earth metal ion complexes can act as
efficient photocatalysts possessing much stronger oxidizing
ability as well as much improved stability against the photo-
chloroform-d were obtained from EURISO-TOP, CEA, France.
Reaction Procedures and Analysis. Typically, Fl (1.0 ×
-2
1
0
M) was added to an NMR tube that contained an
acetonitrile-d3 (CD3CN) solution (0.6 mL) of p-ClC6H4CH2-
-2
-2
OH (2.0 × 10 M) in the presence of Sc(OTf)3 (3.0 × 10
M). Then the solution was deaerated by bubbling with argon
gas for 5 min, and the NMR tube was sealed with a rubber
septum. The solution was irradiated with a xenon lamp (USHIO-
UI-501C) through Pyrex filter transmitting λ > 420 nm for 9
h. The oxidation product of p-ClC H CH OH was identified
by comparing the H NMR spectrum with that of an authentic
sample of p-ClC6H4CHO. The yield of the reaction was
determined on the basis of the concentration of the internal
6
4
2
2
+
1
degradation as compared to the Fl-divalent metal ion (Mg )
complex in the photocatalytic oxidation of benzyl alcohol
derivatives by oxygen. Remarkable enhancement in the redox
reactivity of the photoexcited state of Fl-rare-earth metal ion
complexes has been evaluated quantitatively as compared to
-3
1
standard, 1,4-dioxane (2.0 × 10 M). H NMR measurements
were performed with a Japan Electron Optics JNM-GSX-400
2
+
the Fl-Mg complex and the uncomplexed Fl by analyzing
the free energy relationships for the photoinduced electron-
transfer reactions. Since we have found that the 1:2 complex
1
(
400 MHz) NMR spectrometer at 300 K. H NMR (CD3CN,
98 K); δ(Me4Si, ppm): p-ClC6H4CHO, δ 7.5 (m, 2H), 7.8
m, 2H), 9.9 (s, 1H, CHO). The photooxidation of p-MeOC6H4-
2
(
3+
3+
formed between Fl and Sc (Fl-2Sc ) shows the largest
reactivity among the Fl-metal ion complexes, the reaction
mechanisms for the photooxidation of benzyl alcohol derivatives
by Fl-2Sc3 and the photocatalytic oxidation by oxygen are
revealed based on the dependence of quantum yields on the
alcohol concentration and the direct detection of the reactive
intermediates in the photochemical reaction by the laser flash
CH2OH with Fl in the presence of Lu(OTf)3 in oxygen-saturated
MeCN was performed as follows. Typically, p-MeOC6H4CH2-
-3
OH (3.0 × 10 M) was added to the MeCN solution of Fl
+
-4
-2
(
2.0 × 10 M) in the presence of Lu(OTf)3 (1.0 × 10 M) in
quartz cuvette (i.d. 10 mm). The solution was purged with
oxygen gas for 5 min and irradiated with the monochromatized
light (λ ) 430 nm, Slit width: 20 nm) from a SHIMADZU
spectrofluorophotometer (RF-5000). The aliquot of reaction
solution was diluted and introduced to a SHIMADZU gas
chromatography-mass spectrometer system (GC 17A-QP
3+
photolysis of the Fl-2Sc /benzyl alcohol derivative system.
Experimental Section
5
000) to measure the amount of the oxidation product,
Materials. Flavin analogue (riboflavin-2′,3′,4′,5′-tetraacetate,
Fl) was prepared by the reaction of riboflavin obtained from
Wako Pure Chemicals with acetic anhydride in pyridine and
p-MeOC6H4CHO and the reactant, p-MeOC6H4CH2OH. The
amount of reduction product H2O2 was determined by the
2
4
1
9
standard method (titration by iodide ion); the diluted MeCN
solution of the product mixture was treated with excess amounts
purified by recrystallization from ethanol-chloroform. Ben-
zene derivatives (toluene, ethylbenzene, m-xylene, o-xylene,
p-cymene, p-xylene, 1,2,3-trimethylbenzene, 1,2,4-trimethyl-
benzene, 1,2,3,5-tetramethylbenzene, 1,2,3,4-tetramethylben-
zene, pentamethylbenzene, and m-dimethoxybenzene) used as
electron donors in fluorescence quenching experiments were
obtained from Tokyo Kasei Organic Chemicals. p-Methoxy-
benzyl alcohol, p-chlorobenzyl alcohol, p-methoxybenzaldehyde,
and p-chlorobenzaldehyde were obtained from Tokyo Kasei
Organic Chemicals. Potassium ferrioxalate used as an actinom-
eter was prepared according to the literature,20 and purified by
recrystallization from hot water. Anhydrous magnesium per-
chlorate was obtained from Nacalai Tesque. Scandium triflate
-
of NaI and the amount of I3 formed was determined by the
4
-1
-1
visible spectrum (λmax ) 362 nm, ꢀ ) 1.3 × 10 M cm )
with use of a Hewlett-Packard 8452A diode array spectropho-
tometer.
Spectral Measurements. The formation of Fl-rare-earth ion
complexes was examined from the change in the UV-Vis
-
4
spectra of Fl (1.0 × 10 M) in the presence of various
concentrations of Yb(OTf)3 (6.6 × 10 - 2.7 × 10 M), Lu-
(OTf)3 (6.6 × 10 - 2.7 × 10 M), La(OTf)3 (6.6 × 10
-
5
-3
-5
-3
-5
-
-
3
-4
-1
2.8 × 10 M), Mg(OTf)2 (1.0 × 10 - 5.0 × 10 M), and
-
6
-4
Sc(OTf)3 (6.5 × 10 - 7.3 × 10 M) with use of a Hewlett-
Packard 8452A diode array spectrophotometer. The formation
constants were obtained from the change in the UV-Vis spectra
[
Sc(OTf)3] was prepared by the following procedure according
21
to the literature. A deionized aqueous solution was mixed (1:1
v/v) with trifluoromethanesulfonic acid (>99.5%, 10.6 mL)
obtained from Central Glass, Co., Ltd., Japan. The trifluoro-
methanesulfonic acid solution was slowly added to a flask which
contained scandium oxide (Sc2O3) (>99.9%, 30 mmol) obtained
from Shin-Etsu Chemical, Co., Ltd., Japan. The mixture was
refluxed at 100 °C for 3 days. After centrifugation of the reaction
mixture, the solution containing scandium triflate was separated
and water was removed by vacuum evaporation. Scandium
triflate was dried under vacuum evacuation for 40 h. Similarly,
lutetium triflate and ytterbium triflate were prepared by the
reaction of lutetium oxide and ytterbium oxide with an aqueous
3+
due to the formation of the 1:1 complexes: Fl-Yb (λmax )
3
+
376 and 430 nm), Fl-Lu (λmax ) 362 and 438 nm), Fl-
3
+
2+
La (λmax ) 370 and 434 nm), Fl-Mg (λmax ) 360 and 436
3
+
nm), and the 1:2 complex, Fl-2Sc (λmax ) 384 and λmax )
426 nm). The measurements of IR spectra of rare-earth ion
3
+
3+
2+
complexes with Fl (Fl-2Sc , Fl-Yb , and Fl-Mg ) in
MeCN were performed on a SHIMADZU-FTIR8200PC spec-
trophotometer. A standard actinometer (potassium ferrioxalate)20
was used for the quantum yield determinations on the photo-
chemical reaction of Fl with p-ClC6H4CH2OH under a deaerated
3
+
condition in the presence of Sc ion as well as the photooxi-

Efficient Catalysis of Rare-Earth Metal Ions
J. Phys. Chem. A, Vol. 105, No. 46, 2001 10503
dation of p-MeOC6H4CH2OH by oxygen catalyzed by Fl in the
M TBAP as a supporting electrolyte were determined at room
temperature by SHACV (second harmonic ac voltammetry)
method under deaerated conditions using a three-electrode
system and a BAS 100B electrochemical analyzer. The working
and counter electrodes were platinum, while Ag/AgNO3 (0.01
M) was used as a reference electrode. All potentials (vs Ag/
3
+
presence of Lu ion in MeCN. In the case of photooxidation
of p-ClC6H4CH2OH with Fl, a square quartz cuvette (10 mm
i.d.) which contained a deaerated MeCN solution of Fl (1.0 ×
2
5
-
2
-2
-1
1
0
M) and p-ClC6H4CH2OH (1.0 × 10 - 8.0 × 10 M)
-2
in the presence of Sc(OTf)3 (1.0 × 10 M) was irradiated with
+
26
the monochromatized light (λ ) 430 nm) with the slit width of
Ag ) were converted to values vs SCE by adding 0.29 V.
ESR Measurements. Fl was dissolved in MeCN (3.3 mg;
2
0 nm from a SHIMADZU spectrofluorophotometer (RF-5000)
-
3
at 298 K. The light intensity of monochromatized light of λ )
6.0 × 10 M in 1.0 mL) and purged with argon for 10 min.
-
8
-1
-2
4
30 nm was determined as 4.7 × 10 einstein s with the slit
Sc(OTf) (9.8 mg; 2.0 × 10 M in 1.0 mL) was also dissolved
3
width of 20 nm. The photochemical reaction was monitored by
a Hewlett-Packard 8452A diode array spectrophotometer. The
quantum yields in deaerated condition were determined from
the decrease in absorbance due to Fl at λmax ) 430 nm in MeCN.
The quantum yields for the photooxidation of p-MeOC6H4CH2-
OH by oxygen in the presence of Lu3 ion in MeCN were
determined from the increase in absorption band at λ ) 350
nm due to the product p-MeOC6H4CHO which is identified by
the GC-MS analysis.
in deaerated MeCN. The Fl solution (200 µL) and Sc(OTf)3
solution (200 µL) were introduced into the ESR cell (0.8 mm
i.d.) containing (BNA) (1.0 mg) and mixed by bubbling with
2
Ar gas through a syringe with a long needle. The ESR spectra
•
-
3+
of Fl -2Sc were recorded on a JEOL JES-RE1XE spec-
trometer under irradiation of a high-pressure mercury lamp
(USH-1005D) focusing at the sample cell in the ESR cavity at
+
•
-
298 K. The ESR spectra of Fl were also measured in the
absence of Sc(OTf) under otherwise the same experimental
3
Fluorescence Quenching Experiments. Quenching experi-
ments of the fluorescence of Fl-metal ion complexes were
carried out on a SHIMADZU spectrofluorophotometer (RF-
conditions. The magnitude of modulation was chosen to
optimize the resolution and signal-to-noise (S/N) ratio of the
observed spectra under nonsaturating microwave power condi-
2
+
5
000). The excitation wavelength of Fl and Fl-metal ion
tions. The g values were calibrated using an Mn marker.
Theoretical Calculations. Density functional calculation was
performed on a COMPAQ DS20E computer using the spin-
restricted B3LYP functional for the open shell molecule.27
B3LYP geometries for N(10)-methyl flavin radical anion were
determined using the 6-311++G** basis and the Gaussian 98
3+
3+
2+
complexes (Fl-2Sc , Fl-Yb , and Fl-Mg ) was 460 nm
in deaerated and aerated MeCN. The monitoring wavelengths
were those corresponding to the maxima of the emission band
3
+
3+
at λmax ) 486 nm (F-2Sc ), λmax ) 500 nm (Fl-Yb ),
2+
λmax ) 504 nm (Fl-Mg ), and λmax ) 506 nm (Fl). The MeCN
solutions were deaerated by argon purging for 7 min prior to
the measurements. Relative fluorescence intensities were mea-
2
8
2
program. The 〈S 〉 value was determined as 0.763, indicating
a good representation of the doublet state.
-5
sured for MeCN solutions containing Fl (1.0 × 10 M) with
-
2
-1
various alkylbenzenes (1.0 × 10 - 9.6 × 10 M) in the
Results and Discussion
-
2
absence and presence of metal ions (1.0 × 10 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. The
Stern-Volmer relationship (eq 1)
Complex Formation of Fl with Metal Ions. The UV-Vis
absorption spectra of Fl in MeCN are significantly affected by
the addition of metal ions. The absorption band of Fl at 362
nm is red shifted and the absorption band of Fl at 438 nm is
blue shifted in the presence of metal ions. As shown in Figure
I /I ) 1 + K [D]
(1)
0
q
3
+
1
3
for the Fl-Sc complex, isosbestic points are observed at
00, 354, and 425 nm at low concentrations of Sc(OTf)3.
was obtained for the ratio of the fluorescence intensities (I0/I)
in the absence and presence of electron donors and the
concentrations of donors used as quenchers [D]. The fluores-
cence lifetimes (τ) of the metal ion complexes of Fl were
determined in deaerated MeCN containing metal ions at 298 K
by single photon counting using a Horiba NAES-1100 time-
resolved spectrofluorophotometer.
However, crossover points observed at the low Sc(OTf)3
concentrations spread over slightly in a progressive manner
when the Sc(OTf)3 concentration was increased and then, new
isosbestic points are observed at 302, 362, and 467 nm at higher
Sc(OTf)3 concentration. Such spectroscopic changes may be
interpreted as due to the formation of complexes between Fl
Laser Flash Photolysis. Triplet-triplet transient absorption
spectra of Fl and Fl-Yb3 complex were measured by laser
+
-
5
flash photolysis of the MeCN solution containing Fl (1.0 × 10
-
2
M) in the absence and presence of Yb(OTf)3 (1.0 × 10 M).
For the detection of transient absorption spectra in the photo-
chemical reaction of the Fl-Sc(OTf)3 complex with p-ClC6H4-
-
4
CH2OH, a deaerated MeCN solution containing Fl (1.0 × 10
-
2
M), p-ClC6H4CH2OH (1.0 M), and Sc(OTf)3 (1.0 × 10 M)
was excited by an optical parametric oscillation (Continuum
Surelite OPO, fwhm 4 ns, 440 nm) pumped by a Nd:YAG laser
and Sc(OTf)3 via two steps. The first step is the formation of a
1
:1 complex (eq 2)
(Continuum, Surelite II-10) with the power of 10 mJ. For the
photoinduced electron transfer from (BNA)2 to the Fl-Sc(OTf)3
K1
-
4
complex, (BNA)2 (1.0 × 10 M) was employed in place of
p-ClC6H4CH2OH under otherwise the same experimental condi-
tions. The transient spectra were recorded using fresh solutions
in each laser excitation. All experiments were performed at
3+
3+
Fl + Sc
{\ } Fl-Sc
(2)
and the second step is the additional Sc3+ coordination to Fl
for the formation of a 1:2 complex (eq 3).
298 K.
Electrochemical Measurement. Redox potentials of various
benzene derivatives (2.0 × 10 M) in MeCN containing 0.10
K2
3
+
3+
3+
-
3
Fl-Sc + Sc
{\ } Fl-2Sc
(3)

10504 J. Phys. Chem. A, Vol. 105, No. 46, 2001
Fukuzumi et al.
-
4
Figure 1. Electronic absorption spectra of Fl (1.0 × 10 M) in the
3
+
-5
presence of various concentrations of (a) Sc (0, 1.3 × 10 , 2.5 ×
-
5
-5
-5
-5
-5
-4
1
1
3
2
0
, 3.8 × 10 , 5.0 × 10 , 6.3 × 10 , 7.5 × 10 , 1.0 × 10
,
,
-
4
-4
3+
-4
-4
.1 × 10 , 1.3 × 10 M), and (b) Sc (1.3 × 10 , 2.5 × 10
-
4
-4
-4
-4
.7 × 10 , 4.9 × 10 , 6.1 × 10 , 7.3 × 10 M) in MeCN at
98 K.
Figure 2. Plots for determination of the stepwise complex formation
constants (a) K and (b) K for the complex formation of Fl (1.0 ×
Similar two-step spectroscopic changes were also observed when
Sc(OTf)3 is replaced by La(OTf)3 or Mg(ClO4)2.
1
2
-
4
3+
10 M) with Sc in MeCN.
The equilibrium constant (K1) in eq 2 is determined by
eq 4,29
1 2
TABLE 1: Formation Constants K and K of the Fl
2
Complexes with Various Metal Ions, Mg , Lu , Yb3+,
+
3+
3+
3+
La , and Sc in MeCN at 298 K
-1
-1
3+
(R
- 1) ) K ([Sc ] - R[Fl] )
(4)
1
0
0
Mg2+
Lu3+
Yb3+
La3+
Sc3+
3+
-1
2
2
2
3
4
3
where [Sc ]0 and [Fl]0 are the initial concentrations, and R )
A - A0)/(A∞ - A0); A is the absorbance at 384 nm in the
K
K
1
2
, M
, M
2.2 × 10 4.1 × 10 8.8 × 10 4.5 × 10 3.1 × 10
-
1
2
0.6
1.6 × 10 1.4 × 10
(
presence of Sc(OTf)3, A0 and A∞ are the initial and final
absorbances at the same wavelength in the absence and presence
of an excess of Sc(OTf)3 such that all the Fl molecules form
2
TABLE 2: The ν(CdO) Frequencies of the C - and
4
C -Carbonyl Groups of Fl in the Absence and Presence
of Sc3 , Yb or Mg Ion in MeCN
+
3+
2+
3+
the 1:1 complex (Fl-Sc ), respectively. The linear plot between
2
-1
4
-1
ν(C dO), cm
ν(C dO), cm
-1
-1
3+
(R
- 1) and ([Sc ]0 - R[[Fl]0) is shown in Figure 2a.
nonea
1689
1645
1620
1606
1718
1650
1699
1677
The K1 value is determined from the slope of the linear plot in
Figure 2a.
2+ b
[
Mg ]
Yb
3+
c
[
]
[Sc3+]
d
The K2 value in eq 3 is determined using the K1 value as
3
+
follows. The absorbance A′ at 452 nm due to the Fl-2Sc
complex is expressed by eq 5,
a
In the absence of metal ion. b 1.0 × 10 M. c 1.0 × 10 M.
-1
-2
d
-2
3
.0 × 10 M.
3
+
3+
{
(1 + K [Sc ] )A′ - A ′}/K [Sc ] )
1 0 0 1 0
3+
The K1 value increases in order: Mg < Lu3+ < Yb3+
2+
<
K (A ′ - A′)[Sc ] + A (5)
2
∞
0
1
La3 < Sc . The K2 value increases in the same order, although
+
3+
3+
3+
the K2 values of the 1:2 complexes of Fl with Yb or Lu
where A0′ and A1 are the absorbances at 452 nm due to Fl in
the absence of Sc(OTf)3 and due to Fl-Sc3+ in the presence of
Sc(OTf)3, respectively, and A∞′ is the absorbance due to Fl-
could not be determined because of the low solubility of
Yb(OTf)3 and Lu(OTf)3 in MeCN.
3
+
2
Sc in the presence of a large excess of Sc(OTf)3 such that
IR spectra of Fl in MeCN show three CdO stretching bands
assignable to the C - and C - and acetate carbonyl groups as
reported in the literature. When an excess amount of a metal
ion such that most Fl molecules form the metal ion complex is
added to the MeCN solution of Fl, all the CdO stretching bands
are significantly red shifted as listed in Table 2. Especially, the
3+
2
4
all Fl molecules form the 1:2 complex (Fl-2Sc ) with Sc-
OTf)3. Since the value of the left-hand side in eq 5 is obtained
3
1
(
using the K1 value, the K2 value can be determined from the
slope of a linear plot between the left-hand side in eq 5 and
3
+
(
A∞′ - A′)[Sc ]0. Such a linear plot is in Figure 2b, in
30
2
3+
3+
agreement with eq 5. The K1 and K2 values thus obtained for
the Fl-metal ion complexes are listed in Table 1.
CdO stretching band due to the C -carbonyl group of Fl-2Sc
is largely red shifted by ca. 80 cm , suggesting that Sc
-
1

Efficient Catalysis of Rare-Earth Metal Ions
J. Phys. Chem. A, Vol. 105, No. 46, 2001 10505
TABLE 3: Fluorescence Maxima (λmax), Lifetimes (τ),
One-Electron Reduction Potentials (E0red*) of the Singlet
Excited States of Fl and Fl-Metal Ion Complexes and
Intrinsic Barriers of Photoinduced Electron Transfer (∆G
The observed triplet state quenching in Figure 3 by the metal
ion may also be caused by heavy atom quenching. However,
35
the fluorescence intensity and lifetime are changed depending
on the metal ion concentration in accordance with the complex
formation between Fl and the metal ion (vide infra). In the
q
0
)
E0red*,
V vs SCE
∆G
q
,
0
metal iona
none
λ
max, nm
τ, ns
kcal mol
-1
3+
concentration range of Sc where Fl forms a 1:1 complex with
506
504
500
486
6.5
1.3
0.9
2.4
1.67
2.06
2.25
2.45
3.4
4.0
4.5
4.7
3+
3+
Sc , the fluorescence intensity decreases with increasing Sc
2
+
Mg
concentration to reach a constant value without change in the
fluorescence lifetime (see Supporting Information, S1). The
decrease in the fluorescence intensity without change in the
lifetime may come from increase in the nonradiative decay rate
3
+
Yb
3
+
Sc
a
0
.01 M.
3+
due to the complex formation between Fl and Sc . The
4
-1
formation constant K1 (3.1 × 10 M ) determined from the
dependence of the initial fluorescence intensity on Sc3
concentration (S1) agrees with the value determined from the
+
4
-1 31
absorption spectral change in Figure 2a (3.1 × 10 M ). At
3
+
higher Sc concentrations, the fluorescence lifetime decreases
3
+
with increasing Sc concentration to reach a constant value.
The formation constants K1 and K2 for 1:1 and 1:2 complexes
3
+
4
-1
3
-1
between Fl and Sc (3.2 × 10 M and 1.4 × 10 M ,
respectively) determined from the dependence of the steady-
3
+
state fluorescence intensity on Sc concentration (see Sup-
porting Information, S2 and S3) also agree within the experi-
mental error with the values determined from the absorption
-
4
-1
spectral change in Figures 2a and 2b (3.1 × 10
M
and
3
-1
30
1
.4 × 10 M , respectively). Such agreement of the K1 and
K2 values between the fluorescence and absorption spectra
confirms that the change in the fluorescence spectrum in the
presence of the metal ion is caused by the complex formation
Figure 3. Transient absorption spectra obtained by laser flash
-
5
photolysis of Fl (1.0 × 10 M) in the absence (b) and presence of
3
+
-2
36
Yb (2, 1.0 × 10 M) in deaerated MeCN at 3.0 µs after laser
between Fl and the metal ion.
excitation at 355 nm.
The fluorescence of Fl is quenched via photoinduced electron
1
transfer from electron donors to the singlet excited state ( Fl*)
2
4
12,37
interacts more strongly with C -carbonyl group than the C -
carbonyl group.
in MeCN.
The effects of metal ions on the oxidizing ability
of the singlet excited state of Fl are examined by comparing
Fluorescence Quenching of Fl-Metal Ion Complexes. The
complexation of Fl with metal ions results in blue shifts of the
fluorescence maximum and shortening of the fluorescence
lifetime. The fluorescence maxima and the lifetimes of the
singlet excited state of Fl in the absence and presence of metal
the quenching rate constants (k ) in the presence of a metal ion
q
with those in its absence.
1
3+ *
The fluorescence of (Fl-2Sc ) is efficiently quenched by
various electron donors including a relatively weak electron
donor such as toluene (eq 6). No fluorescence quenching by
-
2
ions (1.0 × 10 M) are determined as listed in Table 3. In the
3+
case of Sc , the largest blue shift is observed when Fl forms
3+
3+
the 1:2 complex (Fl-2Sc ) with 0.01 M Sc (Table 2). The
complexation of Fl with metal ions also results in the change
in the lowest excited state from the n,π* triplet state to the π,π*
singlet state as indicated by the disappearance of the triplet-
triplet (T-T) absorption spectrum of Fl by the complexation
toluene occurs in the case of the Fl-Mg2 complex or Fl in
the absence of metal ion. From the slope of the Stern-Volmer
plots (eq 1) are obtained quenching constants Kq () kqτ; where
τ ) the fluorescence lifetime) for the fluorescence quenching
of Fl by alkyl- or methoxy-substituted benzenes in the absence
and presence of Sc(OTf)3, Yb(OTf)3, and Mg(ClO4)2 (see
Supporting Information, S4). The kq values are listed in Table
+
3
+
with Yb (Figure 3). The T-T absorption spectrum of Fl in
MeCN in Figure 3 shows absorption maxima at λ ) 395, 475,
3
2
5
85, and 655 nm which agree with the literature values in the
absence of metal ions. The T-T absorption spectrum diminishes
-
2
0
completely in the presence of Yb(OTf)3 (1.0 × 10 M) under
otherwise the same experimental conditions. Such a change in
the lowest excited state may be caused by the complexation of
Fl with the metal ion. The nonbonding orbital is more stabilized
by the complex formation with the metal ion than the π-orbital
due to the stronger interaction between nonbonding electrons
and the metal ion. On the other hand, the π,π* singlet excited
state is more stabilized by the interaction with the metal ion,
that is singlet, than the π,π* triplet excited state. Thus, the π,π*
excited state in the Fl-metal ion complex becomes lower in
energy than the n,π* singlet and triplet excited states in the
uncomplexed Fl. The change in the lowest excited state from
the n,π* triplet excited state to the π,π* singlet excited state
has also been observed when aromatic carbonyl compounds
4 together with the one-electron oxidation potentials (E ox) of
the electron donors (see Experimental Section). The quenching
rate constants (kq) increase with decreasing the one-electron
oxidation potentials of electron donors to reach a diffusion-
limited value as shown in Figure 4. This confirms that the
fluorescence quenching occurs by photoinduced electron transfer
from electron donors to the singlet excited states of Fl-metal
ion complexes.
0
The one-electron reduction potential (E red* vs SCE) of the
singlet excited state of the Fl-metal ion complex can be
determined by adaptation of the free energy relationship for
photoinduced electron transfer from a series of electron donors
to the singlet excited state of the Fl-metal ion complex in the
-
2
presence of a metal ion (1.0 × 10 M) in MeCN at 298 K.
form the complexes with Mg2 ion.
+
13b,33,34
The free energy change of photoinduced electron transfer from

10506 J. Phys. Chem. A, Vol. 105, No. 46, 2001
Fukuzumi et al.
TABLE 4. Quenching Rate Constants (k
q
) for the Fluorescence Quenching of Fl by Alkyl- or Methoxy-Substituted Benzenes
0
-2
and the One-Electron Oxidation Potentials (E ox) in the Absence and Presence of 1.0 × 10 M Metal Ions in MeCN
-
1 -1
k
q
, M
Yb3+
s
in the presence of
Mg2+
E0ox,a
V vs SCE
Sc3
+
noneb
no.
substituted benzene
toluene
ethylbenzene
m-xylene
o-xylene
9
9
9
9
9
9
9
9
9
9
8
1
2
3
4
5
6
7
8
9
2.20
2.14
2.02
1.98
1.96
1.93
1.88
1.79
1.71
1.71
1.58
1.50
1.0 × 10
1.5 × 10
3.8 × 10
4.8 × 10
4.4 × 10
5.7 × 10
5.5 × 10
6.5 × 10
7.6 × 10
7.8 × 10
1.3 × 10
3.8 × 10
c
c
c
c
c
c
c
c
c
c
8
8
6.3 × 10
9
8
1.9 × 10
7.0 × 10
9
p-cymene
p-xylene
1.3 × 10
9
9
9
9
2.6 × 10
2.8 × 10
4.1 × 10
1.4 × 10
9
1,2,3-trimethylbenzene
1,2,4-trimethylbenzene
1,2,3,5-tetramethylbenzene
1,2,3,4-tetramethylbenzene
pentamethylbenzene
m-dimethoxybenzene
1.8 × 10
9
2.3 × 10
9
8
8
9
9
4.2 × 10
1.2 × 10
1.5 × 10
1.3 × 10
3.2 × 10
9
9
1
1
1
0
1
2
6.4 × 10
9.3 × 10
10
1.3 × 10
10
1.1× 10
a
Determined using the SHACV method in MeCN containing 0.1 M TBAP. b In the absence of metal ion. c Too small to be determined accurately.
q
Figure 4. Plots of the logarithm of the quenching rate constant (k )
-
5
for the fluorescence quenching of Fl (1.0 × 10 M) in the absence
3
+
3+
2+
-2
(
b) and presence of Sc (2), Yb (9), and Mg (O) (1.0 × 10
Figure 5. Plots of E0ox - (∆G et/e) vs (e/∆G et) for the data in
q
q
M) by substituted benzenes: toluene (1), ethylbenzene (2), m-xylene
Figure 4.
(3), o-xylene (4), p-cymene (5), p-xylene (6), 1,2,3-trimethylbenzene
(
7), 1,2,4-trimethylbenzene (8), 1,2,3,4-tetramethylbenzene (9), 1,2,3,5-
tetramethylbenzene (10), pentamethylbenzene (11), and m-dimethoxy-
benzene (12) vs one-electron oxidation potential (E ox) of the substituted
benzenes in MeCN at 298 K.
From eqs 7 and 8 is derived a linear relationship between
E ox - (∆G et/e) and (e/∆G et) as given by eq 10.
0
0
q
q
39
0
q
0
q
2
q
et
E
- (∆G /e) ) E * - (∆G /e) /(∆G /e) (10)
ox
et
red
0
electron donors to the singlet excited state of the Fl-metal ion
complex (∆G et) is given by eq 7,
0
q
The ∆G et values are obtained from the kq values using eq 9.
0
q
0
et
0
ox
0
Thus, the unknown values of E red* and ∆G 0 can be determined
∆
G
) e(E - E red*)
(7)
0
from the intercept and slope of the linear plots of E ox -
(∆G et/e) vs (e/∆G et) as shown in Figure 5. The order of E red*
q
q
0
0
where e is the elementary charge, and E ox is the one-electron
oxidation potential of an electron donor. The dependence of
the activation free energy of photoinduced electron transfer
1
3+
(
vs SCE) values of Fl-metal ion complexes is (Fl-2Sc )*
1
3+
1
2+
(
2.45 V) > (Fl-Yb )* (2.25 V) > (Fl-Mg )* (2.06 V) >
1
q
0
Fl* (1.67 V), and this order is consistent with that of the
(
∆G et) on the free energy change of electron transfer (∆G et)
40
formation constants (K1) of Fl-metal ion complexes. The
has well been established as given by the free energy relationship
(
0
1
3+
3
8
comparison of the E red* value of (Fl-2Sc )* with that of
eq 8),
1
Fl* reveals the remarkable positive shifts (ca. 780 mV) of the
q
G
0
et
0
et
2
q
0
2 1/2
0
1
3+
1
E red* value of (Fl-2Sc )* as compared to that of Fl*. Such
∆
) (∆G /2) + [(∆G /2) + (∆G ) ]
(8)
et
0
a large positive shift of the E red* value results in a significant
1
3+
q
increase in the reactivity of (Fl-2Sc )* vs uncomplexed Fl
in the photoinduced electron-transfer reactions in Figure 5.
where∆G 0 is the intrinsic barrier that represents the activation
free energy when the driving force of electron transfer is zero,
i.e., ∆G et ) ∆G 0 at ∆G et ) 0. On the other hand, the ∆G et
values are related to the fluorescence quenching rate constant
3+
q
q
0
q
Photooxidation of p-Chlorobenzyl Alcohol by Fl-2Sc .
1
The remarkable enhancement of the redox reactivity of (Fl-
3
+
1
1
2+
2
Sc )* as compared to that of Fl* and (Fl-Mg )* makes it
(
kq) for the photoinduced electron transfer as given by eq 9,
1
possible to oxidize efficiently p-chlorobenzyl alcohol by (Fl-
q
G
) 2.3k T log[Z(kq - kdiff-1)]
-1
3+
∆
(9)
2Sc )*. Irradiation of a deaerated MeCN solution containing
et
B
Fl, p-chlorobenzyl alcohol, and Sc(OTf)3 with visible light at
λ > 420 nm results in the formation of p-chlorobenzaldehyde
and FlH2 (eq 11). No photooxidation of p-chlorobenzyl alcohol
was observed in the absence of the metal ion under otherwise
where kB is the Boltzmann constant, Z is the collision frequency
1
1
-1 -1
that is taken as 1 × 10
M
s , and kdiff is the diffusion rate
1
0
-1 -1
38
constant (2.0 × 10
M
s ) in MeCN.

Efficient Catalysis of Rare-Earth Metal Ions
J. Phys. Chem. A, Vol. 105, No. 46, 2001 10507
TABLE 5: Quantum Yields (Φ) of the Photooxidation of
p-Chloro- and Methoxybenzyl Alcohol by Fl-Metal Ion
Complexes in Deaerated MeCN at 298 K
metal iona
Φ(p-ClC
H CH
4
OH)b
Φ(p-MeOC
H
CH
OH)c
6
2
6
4
2
Sc3
La
Lu
+
1.7 × 10
2.4 × 10
6.2 × 10
3.3 × 10
1.9 × 10
-1
-2
-3
-3
-3
1.2 × 10
5.7 × 10
1.7 × 10
6.0 × 10
6.6 × 10
-1
3
+
-2
-1
-3
-2
3
+
3
+
Yb
2
+
Mg
a
-2
Metal ion is used as a triflate salt; [metal ion] ) 1.0 × 10 M.
b
-4
OH] ) 8.0 × 10 M. c [Fl] )
-1
[
Fl] ) 2.0 × 10 M, [p-ClC
6 4 2
H CH
-
4
-2
3
.0 × 10 M, [p-MeOC
6 4
H
CH
2
OH] ) 2.7 × 10 M.
the same experimental conditions. Similarly, the photooxidation
of p-chlorobenzyl alcohol by Fl proceeds in the presence of
other metal triflates such as La(OTf)3, Lu(OTf)3, Sc(OTf)3, and
Mg(OTf)2. The quantum yields (Φ) for the photooxidation of
-
1
p-chlorobenzyl alcohol (8.0 × 10 M) in the presence of Fl
-
4
-2
(
2.0 × 10 M) and metal triflates (1.0 × 10 M) were
determined from the rate of disappearance of the absorption
band due to the Fl-metal ion complexes under irradiation of
monochromatized light of λ ) 430 nm (see Supporting
-
2
Information, S5). In the presence of 1.0 × 10 M metal ion,
Fl forms the 1:2 complex with Sc3 and La and the 1:1
complex with other metal ions (Table 1). The Φ values are listed
in Table 5. The Φ value is largest in the case of Sc(OTf)3
+
3+
3+
3+
3+
Figure 6. (a) Dependence of the quantum yield (Φ) on [p-ClC
6
H
H
4
-
-
(
Φ ) 0.17) and decreases in order: Sc > La > Lu
>
-
4
CH
CH
2
OH] for the photoreduction of Fl (1.5 × 10 M) with p-ClC
6
4
3
+
2+
Yb > Mg . When p-chlorobenzyl alcohol is replaced by
p-methoxybenzyl alcohol that is a stronger electron donor, the
Φ values of Fl-metal ion complexes become larger except for
the case of Sc(OTf)3 (Table 5).
3+ -2
2
OH in the presence of Sc (1.0 × 10 M) in deaerated MeCN at
-1
-1
2
6 4 2
98 K. (b) Plot of Φ vs [p-ClC H CH OH] .
was obtained from the Stern-Volmer relationship (vide supra).
The Φ value for the photooxidation of p-chlorobenzyl alcohol
to p-chlorobenzaldehyde in MeCN increases with an increase
in the concentration of p-chlorobenzyl alcohol [p-ClC6H4CH2-
OH] to approach a limiting value (Φ∞) as shown in Figure 6a.
The dependence of quantum yields on [p-ClC6H4CH2OH] is
expressed by eq 12,
9
-1 -1
The kq value thus obtained is 1.8 × 10 M
s
which agrees
with the kobs value. Such an agreement indicates that the
photooxidation of p-chlorobenzyl alcohol by Fl in the presence
of Sc(OTf)3 proceeds via electron transfer from p-chlorobenzyl
1
3+
alcohol to (Fl-2Sc )* which has much stronger oxidizing
1
ability than Fl*.
The occurrence of photoinduced electron transfer in the
photooxidation of p-chlorobenzyl alcohol by Fl-2Sc has been
confirmed by the laser flash photolysis study as follows. The
transient absorption spectra in the visible region (λmax ) 560
nm) are observed by the laser flash photolysis of a deaerated
Φ ) Φ K [p-ClC H CH OH]/
3+
∞
obs
6
4
2
(1 + Kobs[p-ClC H CH OH]) (12)
6
4
2
-
1
which is rewritten as a linear correlation between Φ vs
p-ClC6H4CH2OH]-1 (eq 13),
[
MeCN solution containing Fl, Sc(OTf) , and p-chlorobenzyl
3
alcohol with 440 nm laser light as shown in Figure 7a. To assign
Φ- ) Φ∞ [1 + (Kobs[p-ClC H CH OH]) ] (13)
1
-1
-1
•
-
3+
the absorption bands in Figure 7a, Fl -2Sc was produced
6
4
2
by electron transfer from a dimeric 1-benzyl-1,4-dihydronico-
where Kobs is the quenching constant of 1(Fl-2Sc )* by
3+
tinamide [(BNA)2] to Fl in the presence of Sc(OTf)3 in MeCN.
42
p-ClC6H4CH2OH. The validity of eq 13 is confirmed by the
The (BNA)2 is known to act as a unique two-electron donor to
produce two equivalents of the radical anions of electron
-
1
-1
plot of Φ vs [p-ClC6H4CH2OH] as shown in Figure 6b.
From the slope and the intercept are obtained the values of Φ∞
4
2-44
•-
3+
acceptors.
The absorption spectrum due to Fl -2Sc is
-
1
-1
(
(
M
2.3 × 10 ) and Kobs (4.0 M ). The quenching constant Kobs
shown in Figure 7b, where the absorption band at 510 nm is
red shifted as compared to the reported band of the radical anion
9
) kobsτ) is converted to the rate constant (kobs ) 1.6 × 10
-1
-1
1
3+
45
s ) of the reaction of (Fl-2Sc )* with p-chlorobenzyl
of N(10)-isobutyl-N(3)-methyl flavin (λmax ) 478 nm).
•
-
3+
alcohol using the fluorescence lifetime (2.4 ns). Since the one-
The formation of Fl -2Sc is confirmed by the ESR
spectrum as shown in Figure 8a which exhibits apparent 13 line
signals centered at g ) 2.0033. The hyperfine coupling constants
are determined as given in Figure 8b by the computer simulation
of the ESR spectrum. A similar but slightly different ESR
0
electron oxidation potential of p-chlorobenzyl alcohol (E ox vs
SCE in MeCN ) 1.88 V)41 lies within the range of the oxidation
potentials of substituted benzenes used for the fluorescence
1
3+
quenching of (Fl-2Sc )* via electron transfer (Table 4),
1
3+
•-
fluorescence quenching of (Fl-2Sc )* by p-chlorobenzyl
spectrum was obtained for Fl which was produced by
alcohol also occurred efficiently. The fluorescence quenching
photoinduced electron transfer from (BNA)2 to Fl in the absence
1
3+
3+
rate constant (kq) of (Fl-2Sc )* by p-chlorobenzyl alcohol
of Sc ion in MeCN (see Supporting Information, S6). The

10508 J. Phys. Chem. A, Vol. 105, No. 46, 2001
Fukuzumi et al.
SCHEME 1
SCHEME 2
spin densities of the radical anion of N(10)-methyl flavin were
calculated using density functional theory at the Becke3LYP/
Figure 7. (a) Transient absorption spectra observed in the photochemi-
cal reaction of the Fl-2Sc(OTf)
3
complex formed between Fl (1.0 ×
-2
6 4 2
6
-311++G** level (see Experimental Section) as shown in
-
4
1
0
M) and Sc(OTf)
3
(1.0 × 10 M) with p-ClC
H CH OH (1.0 M)
4
6,47
Figure 8b.
Since there is essentially no spin density on the
at 25 µs (b) and 250 µs (2) after laser excitation in deaerated MeCN
3+
carbonyl oxygens which can bind with Sc , the ESR spectrum
of Fl -2Sc is only slightly different from that of Fl .
at 298 K. (b) Visible spectra observed in the electron-transfer reduction
-
4
-3
•-
3+
•-
of Fl (1.0 × 10 M) by (BNA)
2
(1.0 × 10 M) in the presence of
-
3
Sc(OTf)
3
(3.0 × 10 M) at 5 s (broken line) and 120 s (solid line) in
The transient absorption spectra in Figure 7a (λmax ) 560
deaerated MeCN at 298 K.
•-
3+
nm) are clearly different from that of Fl -2Sc (λmax ) 510
•
-
nm) or Fl (λmax ) 478 nm). The absorption bands in the long
wavelength region between 500 and 600 nm are rather similar
to those reported for the neutral radical (FlH ). Thus, the
•
32a
transient absorption band at λmax ) 560 nm in Figure 7a may
•
3+
be assigned due to FlH -2Sc which is produced by proton
•+
•-
3+
transfer from p-ClC6H4CH2OH to Fl -2Sc . The absorption
band at 520 nm observed at 25 µs after the laser excitation may
be overlapped with that due to Fl -2Sc (λmax ) 510 nm).
•
-
3+
On the basis of the results and discussion described above,
the reaction mechanism of photooxidation of p-chlorobenzyl
3
+
alcohol by Fl-2Sc is summarized as shown in Scheme 1.
1
3+
The singlet excited state (Fl-2Sc )* is quenched by electron
transfer from p-chlorobenzyl alcohol (ket) to give the radical
•
+
•-
3+
ion pair [p-ClC6H4CH2OH Fl -2Sc ] in competition with
the decay to the ground state. Then, a proton is transferred from
•
-
3+
p-chlorobenzyl alcohol radical cation to Fl -2Sc in the
•
•
radical ion pair to give the radical pair [p-ClC6H4CHOH FlH -
3
+
48
2
Sc ] as observed in the transient spectra in Figure 7a. The
•
•
3+
hydrogen transfer from p-ClC6H4CHOH to FlH -2Sc yields
3
+ 12
•
-
3+
the product p-ClC6H4CHO and FlH2-2Sc . The proton-
transfer step (kp) may compete well with the back electron-
transfer step to the reactant pair (kb).
Figure 8. (a) ESR spectrum of Fl -2Sc generated in the photo-
-
3
-3
induced reaction of Fl (3.0 × 10 M) with (BNA)
2
(6.0 × 10 M)
-
2
and Sc(OTf)
3
(1.0 × 10 M) in deaerated MeCN at 298 K. (b)
5
Computer simulation spectrum with g ) 2.0033, a(N ) ) 7.2 G,
By applying the steady-state approximation to the reactive
1
0
6
7
8
a(N ) ) 3.8 G, a(H ) ) 3.6 G, a(3H ) ) 0.7 G, a(3H ) ) 3.6 G,
1
3+
1
0
species, (Fl-2Sc )* and the radical ion and radical pairs in
Scheme 2, the dependence of Φ on the p-chlorobenzyl alcohol
concentration [p-ClC6H4CH2OH] can be derived as given by
eq 14,
a(N -CH
2
) ) 3.8 G, ∆Hmsl ) 2.2 G. The spin densities of the radical
anion of N(10)-methyl flavin calculated using density functional theory
5
10
at the Becke3LYP/ 6-311++G** level are 0.378 (N ), 0.077 (N ),
.186 (C ), -0.104 (C ), 0.201 (C ), 0.039 (C dO), 0.0628 (C dO).
6
7
8
2
4
0
ESR spectrum of Fl•- is essentially the same as that reported
Φ ) [k /(k + k )]k τ[p-ClC H CH OH]/
p
p
b
et
6
4
2
for the radical anion of N(10)-isobutyl-N(3)-methyl flavin which
4
5
(1 + k τ[p-ClC H CH OH]) (14)
et 6 4 2
exhibits apparent 12 line signals centered at g ) 2.0038. The

Efficient Catalysis of Rare-Earth Metal Ions
J. Phys. Chem. A, Vol. 105, No. 46, 2001 10509
tration of p-methoxybenzyl alcohol to reach a constant value
-
2
Φ∞ ) 0.17 at [p-MeOC6H4CH2OH] ) 2.7 × 10 M, which
agrees with the corresponding value (0.17) for the photooxi-
3
+
dation of p-methoxybenzyl alcohol by Fl-Lu in deaerated
MeCN (Table 5). Such an agreement indicates that oxidation
3+
of the reduced flavin, FlH2-Lu , by oxygen to regenerate Fl-
3
+
Lu is too fast to affect the Φ value as shown in Scheme 2.
The rare-earth metal ion may accelerate the oxidation of the
reduced flavin by oxygen to yield H2O2 as reported for the
2
+
49
Mg -catalyzed oxidation of reduced flavin by oxygen.
Although the triplet excited state of Fl is quenched by oxygen
5
0
that is a well-known triplet quencher, the Fl-metal ion
complexes can act as efficient and stable photocatalysts in the
photooxidation of benzyl alcohol derivatives by oxygen due to
the change in the spin state from the n,π* triplet to the π,π*
singlet excited state by the complexation with metal ions
(Figure 3).
6 4 2
Figure 9. Time dependence of the concentration of p-MeOC H CH -
OH (9), p-MeOC
6
H
4
CHO (b), and H
2
O
2
(2) for the photooxidation
-
3
of p-MeOC CH
6
H
4
2
OH (3.0 × 10 M) with oxygen catalyzed by Fl
-
4
3+
-2
(
2.0 × 10 M) in the presence of Lu (1.0 × 10 M) under visible
light irradiation (λ ) 430 nm) in MeCN at 298 K.
Acknowledgment. We are grateful to Dr. A. Ishida for the
measurements of laser flash photolysis at the early stage of this
study. This work was partially supported by a Grant-in-Aid for
Scientific Research Priority Area (Nos. 11228205 and 413/
which agrees with the observed dependence of Φ on [p-ClC6H4-
CH2OH] in eq 12. The limiting quantum yield Φ∞ corresponds
to kp/(kp + kb). Thus, the Φ∞ values being smaller than unity
may be ascribed to the competition of the proton-transfer process
1
3031059) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan.
(kp) with the back electron-transfer process (kb).
Photocatalytic Oxidation of p-Methoxybenzyl Alcohol by
Supporting Information Available: Change in the fluo-
Oxygen. Since the Fl-Lu3 complex gives the largest Φ value
+
3+
rescence decay curve of Fl with Sc concentration (S1), change
(
0.17) for the photooxidation of p-methoxybenzyl alcohol in
deaerated MeCN (Table 5), the photocatalytic oxidation of
3+
in the steady-state fluorescence spectra on Sc concentration
(
S2,S3), Stern-Volmer plots for the fluorescence quenching
p-methoxybenzyl alcohol by oxygen is examined using the Fl-
3+
of Fl by various alkylbenzenes in the presence of Sc (S4);
electronic spectra observed in the photochemical reaction of Fl
3+
Lu complex. When an oxygen-saturated MeCN solution
-3
containing p-methoxybenzyl alcohol (3.0 × 10 M), Lu(OTf)3
3+
with p-ClC6H4CH2OH in the presence of Sc (S5); ESR
-
2
-4
(
1.0 × 10 M) and the catalytic amount of Fl (2.0 × 10 M)
•-
spectrum of Fl generated in photoinduced electron transfer
was irradiated with visible monochromatized light (λ ) 430
nm), the decrease in the concentration of p-methoxybenzyl
alcohol was accompanied by the increase in the concentration
of p-methoxybenzaldehyde and H2O2 (eq 15). Yields of p-
from (BNA)2 to Fl, and the computer simulation spectrum (S6).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References and Notes
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1
400% in 50 min irradiation, demonstrating an efficient
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6
1
1
(
(
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•
•+
(44) The BNA radical produced by the C-C bond cleavage in (BNA)2
can reduce the electron acceptor, resulting in formation of two equivalents
of the radical anion in the photoinduced electron transfer from (BNA)2 to
the electron acceptor.4
2,43
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5
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1
0
6
7
8
9
and N ) and carbon atoms (C , C , C , and C ) where the hyperfine splitting
due to the nitrogens and protons bound to the carbons is observed. It should
be noted that there is almost no spin density on carbonyl oxygens with
which Sc3 binds.
+
(
(
19) Kyogoku, Y.; Yu, B. S. Bull. Chem. Soc. Jpn. 1969, 42, 1387.
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•
-
(47) The main difference between the ESR spectra of Fl and
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3+
9
•-
3+
1
956, 235, 518. (b) Calvert, J. G.; Pitts, J. N. In Photochemistry; Wiley:
New York, 1966; p 783.
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•-
compared to the value in Fl
.
•
(
(48) The transient absorption band due to p-ClC6H4CHOH does not
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