Effect of oxygen on the formation and decay of stilbene radical cation during the resonant two-photon ionization

Article abstract of DOI:10.1021/jo050317n

Formation and decay of radical cations of trans-stilbene and p-substituted trans-stilbenes (S^.+) during the resonant two-photon ionization (TPI) of S in acetonitrile in the presence and absence of O₂ have been studied with laser flash photolysis using a XeCl excimer laser (308 nm, fwhm 25 ns). The transient absorption spectra of S^.+ were observed with a peak around 470-490 nm. The formation quantum yield of S^.+ (0.06-0.29) increased with decreasing oxidation potential CE^ox) and increasing fluorescence lifetime (τ_f) of S, except for trans-4-methoxystilbene which has the lowest E^ox and longer τ_f among S. The considerable low yield and fast decay in a few tens of nanoseconds time scale were observed for trans-4-methoxystilbene ^.+ in the presence of O₂, but not for other S ^.+. It is suggested that formation of the ground-state complex between trans-4-methoxystilbene and O₂ and the distonic character of trans-4-methoxystilbene^.+ with separation and localization of the positive charge on the oxygen of the p-methoxyl group and an unpaired electron on the β-olefinic carbon are responsible for the fast reaction of trans-4-methoxystilbene^.+ with O₂ or superoxide anion, leading to the considerable low yield and fast decay of trans-4- methoxystilbene^.+. The mechanism based on the transient absorption measurement of S^.+ during the TPI is consistent with the relatively high oxidation efficiency of trans-4-methoxystilbene among S based on the product analysis during the photoinduced electron transfer in the presence of a photosensitizer such as 9,10-dicyanoanthracene and O₂ in acetonitrile.

Full text of DOI:10.1021/jo050317n

Effect of Oxygen on the Formation and Decay of Stilbene Radical
Cation during the Resonant Two-Photon Ionization
Michihiro Hara, Shingo Samori, Cai Xichen, Mamoru Fujitsuka, and Tetsuro Majima*
The Institute of Scientific and Industrial Research (SANKEN), Osaka University, Mihogaoka 8-1, Ibaraki,
Osaka 567-0047, Japan
majima@sanken.osaka-u.ac.jp
Received February 19, 2005
Formation and decay of radical cations of trans-stilbene and p-substituted trans-stilbenes (S•+)
during the resonant two-photon ionization (TPI) of S in acetonitrile in the presence and absence of
O2 have been studied with laser flash photolysis using a XeCl excimer laser (308 nm, fwhm 25 ns).
The transient absorption spectra of S•+ were observed with a peak around 470-490 nm. The
formation quantum yield of S•+ (0.06-0.29) increased with decreasing oxidation potential (Eox) and
increasing fluorescence lifetime (τf) of S, except for trans-4-methoxystilbene which has the lowest
Eox and longer τf among S. The considerable low yield and fast decay in a few tens of nanoseconds
time scale were observed for trans-4-methoxystilbene•+ in the presence of O2, but not for other S•+.
It is suggested that formation of the ground-state complex between trans-4-methoxystilbene and
O2 and the distonic character of trans-4-methoxystilbene•+ with separation and localization of the
positive charge on the oxygen of the p-methoxyl group and an unpaired electron on the â-olefinic
carbon are responsible for the fast reaction of trans-4-methoxystilbene•+ with O2 or superoxide anion,
leading to the considerable low yield and fast decay of trans-4-methoxystilbene•+. The mechanism
based on the transient absorption measurement of S•+ during the TPI is consistent with the relatively
high oxidation efficiency of trans-4-methoxystilbene among S based on the product analysis during
the photoinduced electron transfer in the presence of a photosensitizer such as 9,10-dicyano-
anthracene and O2 in acetonitrile.
Introduction
Since most of radical cations are highly reactive inter-
mediates, they decompose unimolecularly or react bimo-
lecularly with other molecules such as hole transfer,
addition, and neutralization. For example, the nucleo-
philic addition to radical cations is well-known to occur
to give the corresponding adducts because of the cationic
character. On the other hand, the bimolecular reactivity
of radical cations as radicals is rather rare.
Radical cations can be generated from one-electron
oxidation such as electrochemical reaction, chemical
oxidation using one-electron oxidants, photoinduced elec-
tron transfer, resonant two-photon ionization (TPI), and
radiation chemical reaction. The properties and reactions
of radical cations have been extensively investigated.1-5
Radical cations of stilbene (St) and stilbene derivatives
(S)1,4-7 react with various necleophiles. On the other
hand, most S•+ react slowly with O2 at the bimolecular
* Corresponding author. Tel: +81-6-6879-8495. Fax: +81-6-6879-
8499.
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10.1021/jo050317n CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/20/2005
4370
J. Org. Chem. 2005, 70, 4370-4374

Resonant Two-Photon Ionization
TABLE 1. Properties of S and S•+ and Formation of S•+ during the TPI Using the 308-nm XeCl Excimer Lasera
absorption
ꢀmax/M-1 cm-1
fluorescence
λmax/nm τf /ps
formation of S•+
ꢀmax/M-1 cm-1
S
Eox/V
λmax/nm
λmax/nm
Φion
slope
St
1.27
1.11
0.90
307
299
317
3.3 × 104
2.2 × 104
2.5 × 104
349
352
373
44
45
64
472
485
480
6.0 × 104
6.7 × 104
6.1 × 104
2.0 × 10-3
1.9 × 10-3
0.6 × 10-3
2.4 × 10-3b
2.9 × 10-3
1.1 × 10-3
1.9
2.0
2.0
MeSt
MeOSt
ClSt
CNSt
1.23
1.39
296
323
3.7 × 104
374
378
66
32
490
485
6.9 × 104
1.7
1.9
3.7 × 104
5.1 × 104
a Peak voltage for the irreversible oxidation wave vs Ag/Ag+ was taken as oxidation potentials (Eox), absorption peak wavelength (λmax),
molar absorption coefficient of the peak (ꢀmax), fluorescence peak (λmax), and lifetime (τf) of S, λmax and ꢀmax of the transient absorption of
S
•+, formation quantum yields of S•+ (Φion), and slope of log-log plots of concentration of S•+ against F in J cm-2 b In Ar-saturated AN
.
solution.
rate constant of <106 M-1 s-1 8,9
.
However, trans-4-
presence and absence of O2 using a XeCl laser flash (308
nm, 50-200 mJ pulse-1, F) 128-512 J cm-2, 25-ns
fwhm), and found that the trans-4-methoxystilbene radi-
cal cation is considerably quenched by O2 or O2 in a
few tens ns time scale, but not for other S.
methoxystilbene•+ reacts much faster with O2 at the
bimolecular rate constant of (0.9-4.5) × 107 M-1 s-1
because of its distonic character.9 Several S•+ react with
superoxide anion (O2•-), generated from photoinduced
electron-transfer reaction of an acetonitrile (AN) solution
of S and a photosensitizer such as 9-cyanoanthracene,
9,10-dicyanoanthracene, and chloranil in the presence of
O2, at almost diffusion-controlled rate, to yield two
aromatic aldehydes as the stable products via cyclorever-
sion of the dioxetane intermediates.8-12
•-
Experimental Section
St, trans-4-methylstilbene (MeSt), trans-4-methoxystilbene
(MeOSt), trans-4-chlorostilbene (ClSt), and trans-4-cyano-
stilbene (CNSt) were prepared according to the procedures
previously reported.26 AN (spectroscopic grade) was used
without further purification. Measurements of UV/vis spectra,
fluorescence spectra, and oxidation potentials (Eox) of S were
carried out as described in our previous report.26 Such spectral
data and Eox are summarized in Table 1.
The τf values of S were measured by a single photon
counting method using a streak scope. The samples were
excited with third harmonic generation (287 nm) of a Ti:
sapphire laser (fwhm 100 fs) equipped with a pulse selector
and harmonic generator.
Laser flash photolysis was carried out using a XeCl excimer
laser (308 nm, 25 ns, 200 mJ pulse-1) or Nd:YAG laser (355
nm, 5 ns, 60 mJ pulse-1) as an excitation source. The monitor
light was obtained from a 450-W Xe lamp synchronized with
the laser flash. The irradiation volume of the laser beam was
identical with that of the monitor light source. The intensity
of the monitor light source was detected using a photomulti-
plier. The signal from the photomultiplier was digitized by an
oscilloscope and transmitted to a personal computer via the
RS 232C interface. Transient absorption spectra were mea-
sured by a multichannel analyzer with an image intensifier
having a 30-ns gate width and transmitted to a personal
computer via the RS 232C interface. Air-saturated AN solu-
tions containing S were prepared in a transparent rectangular
cell made of quartz (1.0 × 1.0 × 4.0 cm, optical path length of
1.0 cm). The concentration of S was adjusted to have an
absorbance of 1.0 at 308 nm of the excitation laser wavelength.
All data during the laser flash photolysis were measured with
a single laser shot on a fresh sample in order to avoid sample
degradation. All spectral measurements, transient absorption
measurements, and oxidation potential measurements were
carried out at room temperature.
The TPI of organic molecules (M) occurs to give M•+
and an electron with irradiation at a high laser fluence
(F in J cm-2) using an intense short-pulsed laser with a
wavelength tuned to the M absorption,13-25 when M has
relatively low oxidation potentials (Eox) or ionization
potential (IP) and strong absorption at the laser wave-
length. We have studied the TPI of various M26,27 and
elucidated the dependence of the formation efficiency of
M•+ on solvents, fluorescence lifetime (τf), and Eox of M
and effect of cyclodextrines including M.26
In the present paper, we have studied the formation
and decay of S•+ during the TPI of S in AN in the
(8) Tsuchiya, M.; Ebbesen, T. W.; Nishimura, Y.; Sakuragi, H.;
Tokumaru, K. Chem. Lett. 1987, 2121.
(9) Majima, T.; Tojo, S.; Ishida, A.; Takamuku, S. J. Org. Chem.
1996, 61, 1, 7793.
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I. R.; Farid, S. J. Am. Chem. Soc. 1990, 112, 8055.
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1997, 106, 119.
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1990, 94, 244.
(15) Shafirovich, V.; Dourandin, A.; Huang, W.; Luneva, N. P.;
Geacintov, N. E. J. Phys. Chem. B 1999, 103, 10924.
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1986, 90, 6065.
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74, 4137.
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Chem. A 2001, 105, 8605.
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Lett. 1991, 183, 584.
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(25) Johnston, L. J.; Schepp, N. P. J. Am. Chem. Soc. 1993, 115,
6564.
(26) Hara, M.; Tojo, S.; Majima, T. J. Photochem. Photobiol. A 2004,
162, 121.
(27) Hara, M.; Tojo, S.; Kawai, K.; Majima, T. Phys. Chem. Chem.
Phys. 2004, 6, 3215.
Results
Formation of S•+ during the TPI with a 308-nm
25-ns Laser Flash. A transient absorption spectrum
with a peak at 472 nm was observed during the 308-nm
XeCl laser flash photolysis of St in air-saturated AN (3.8
× 10-5 M) at F ) 512 mJ cm-2 (Figure 1). The optical
density (∆OD) of the transient absorption increased with
increasing F. The transient absorption spectrum was
reasonably assigned to St•+ according to our previous
J. Org. Chem, Vol. 70, No. 11, 2005 4371

Hara et al.
FIGURE 1. Transient absorption spectra of S•+ observed at 500 ns after a laser flash with F ) 512 mJ cm-2 pulse-1 during the
TPI of S using a 308-nm XeCl excimer laser in air-saturated AN. Concentrations of St, MeSt, MeOSt, ClSt, and CNSt were 3.8
× 10-5, 5.0 × 10-5, 4.0 × 10-5, 3.1 × 10-5, and 3.3 × 10-5 M, respectively.
report.12,26 From the relationship between the ionization
potential (IP) of St (7.8 eV)28 and the photon energy of
XeCl laser (4.0 eV), St is ionized by two-photon excitation
during the 308-nm laser flash photolysis. Similar tran-
sient absorption spectra of S•+ with a peak around 470-
490 nm were observed during the TPI of S with a 308-
nm laser flash in air-saturated AN (eq 1)
S + 2hv (308 nm) f S•+ + e-
(1)
An electron generated together with S•+ reacts with
AN to give an AN radical anion (AN•-) and the dimer
radical anion ((AN)2•-), which have a weak absorption
in the range of 400-600 nm.29 Even when the solvated
electron reacts partly with S (10-5 M) at the diffusion-
FIGURE 2. Log-log plots of [S•+] (in M) vs laser fluence (F
in J cm-2 pulse-1) during the TPI using the 308-nm XeCl
excimer laser. [S•+] values were calculated from ∆OD at the
absorption peak observed immediately after the laser flash:
St•+(O), MeSt•+(b), MeOSt•+(0), ClSt•+(2), and CNSt•+(0).
controlled rate constant of (2-5) × 1010 M-1 s-1 30
,
S
radical anion can be negligible in the time scale of 100
ns. In fact, no transient absorption of St radical anion
with a peak at 530 nm was observed at 100 ns after the
laser flash during the TPI of St (3.8 × 10-5 M) in air-
saturated AN as shown in Figure 1.
9,10-dicyanoanthracene (1 mM), and biphenyl (0.1 M) in
air-saturated AN, the concentration of St•+ ([St•+]) was
calculated to be 2.0 × 10-6 M. The number of laser
photons absorbed by St (Np) was measured to be 2.8 ×
10-4 einstein L-1 from the laser intensity.31 The formation
quantum yield of St•+ (Φion ) [St•+]/Np) under the
All S•+ have a relatively sharp absorption peak around
470-490 nm except for MeOSt•+. The broad absorption
of MeOSt•+ may correspond to the distonic character with
separation and localization of the positive charge on the
oxygen of the p-methoxyl group and an unpaired electron
on the â-olefinic carbon.9
conditions of Figure 1 was calculated to be 2.0 × 10-3
.
Similarly, Φion values for other S were obtained as shown
in Table 1.
The log-log plots of [St•+] vs F showed linear relation-
ships with the slope of approximately 2 (Table 1),
indicating that the yield of St•+ is proportional to F2
(Figure 2). The slope of approximately 2 was also
obtained for other S, indicating two-photon excitation
during the TPI of S.
From ∆OD480 ) 0.15 at 500 ns after the laser shot and
the molar absorption coefficient of St•+ at 480 nm (ꢀ480
)
4.7 × 104 M-1 cm-1), which was estimated from the hole
transfer from biphenyl radical cation to St during the
laser flash photolysis of a mixture of St (3.8 × 10-5 M),
Relationship between Φion and Properties of S
(28) Murov, S. L.; Carmichael, I.; Hug, G. L. Handbook of Photo-
chemistry, 2nd ed.; Marcel Dekker: New York, 1993.
(29) Bell, I. P.; Rodgers, M. A. J.; Burrows, H. D. J. Chem. Soc.,
Faraday Trans. 1 1977, 73, 315.
(30) Majima, T.; Fukui, M.; Ishida, A.; Takamuku, S. J. Phys. Chem.
1996, 100, 8913.
such as Eox and τf. Effects of properties of S such as Eox
(31) Hara, M.; Sunagawa, T.; Oseki, Y.; Majima, T. J. Photochem.
Photobiol. A 2004, 163, 153.
4372 J. Org. Chem., Vol. 70, No. 11, 2005

Resonant Two-Photon Ionization
withdrawing (chloro or cyano) substituent. According to
the two-step two-photon absorption during the TPI, S is
excited to S in the lowest excited singlet state (S(S1)) by
the first photon excitation at 308 nm, and S(S1) is then
excited to S(Sn) by the second photon excitation at 308
nm within 25-ns duration of the XeCl excimer laser flash.
Since the energy of S(Sn) is higher than the IP of S,
electron detachment from S(Sn) occurs to give S•+ and
an electron, competitively with the internal conversion
to S(S1). The transient absorption of S•+ with a maximum
peak at 470-490 nm increased with increasing F. Φion
was proportional to F2, and slopes of the linear log-log
plots of concentration of S•+ generated initially against
F were approximately 2 as shown in Table 1. This
relation is usually observed for the stepwise excitation
mechanism of the TPI because two-photon absorption is
necessary for ionization of S to give S•+.
FIGURE 3. Plots of log Φion vs log Eox (a) and vs log τf (b)
during the TPI of S in air- (O) and Ar-saturated (4) AN
solutions using a 25-ns 308-nm XeCl excimer laser.
Relationships between Φion and Eox of S or τf of S(S1)
have been reported for various substituted trans-stil-
benes.26 The dependence of Φion on Eox can be explained
by the excess energy of S(Sn), which decreases with
increasing IP. Because the relatively large [S•+] was
observed for S with long τf, Φion increases with increasing
τf, except for MeOSt in air-saturated solution. In other
words, Φion depends on Eox and τf, except for MeOSt in
air-saturated AN solution.
Effects of O2 Concentration on the TPI of MeOSt.
To explain the exceptional behavior of MeOSt in air-
saturated solution during the TPI of S, the following
should be addressed. First, properties of MeOSt(S1)
generated from the first one-photon excitation may be
different from those of other S(S1). However, the absorp-
tion and emission spectra of S and τf of all S(S1)
resembled each other; therefore, MeOSt(S1) has proper-
ties similar to those of other S. On the other hand, the
absorption spectrum of MeOSt•+ was different from those
of other S•+. It has been suggested that MeOSt•+ has the
distonic character separation and localization of the
positive charge on the oxygen of the p-methoxyl group
and an unpaired electron on the â-olefinic carbon.9
Because of the localization of an unpaired electron on the
â-olefinic carbon, MeOSt•+ must have high reactivity
against O2. The distonic character is probably responsible
to the different absorption spectrum of MeOSt•+ com-
pared with other S•+.
FIGURE 4. Time profiles of the transient absorption of
MEOST•+ at 480 nm during the 355-nm TPI of MeOST in
Ar-, air-, and O2-saturated AN (O2 concentration of 0, 2, and
9 mM, respectively) using a 355-nm Nd3+:YAG laser (5-ns, 60
mJ pulse-1, 212 mJ cm-2).
and τf of S in the lowest singlet excited state (S(S1)) on
Φion were examined. Linear relations between the logΦion
(Φion ) 0.06-0.29) and logEox of S or logτf of S(S1) were
observed (Figure 3), except for MeOSt in air-saturated
AN solution. In other words, Φion increased with decreas-
ing Eox and increasing τf, except for MeOSt in air-
saturated AN solution.
Decay of S•+ during the TPI with a 355-nm 5-ns
Laser Flash. The decay profiles of the transient absorp-
tion of MeOSt•+ at 480 nm were measured in Ar-, air-,
and O2-saturated AN (5.6 × 10-3 M) during the 355-nm
TPI of MeOSt using a Nd3+:YAG laser (Figure 4). The
∆OD480 values observed immediately after the 5-ns laser
flash depended on the concentration of O2. The decay of
∆OD480 in the time scale of 1 µs in the absence of O2 was
analyzed by the second-order kinetics for the reaction of
Next, we examined the TPI of S with the 355-nm 5-ns
laser flash photolysis in Ar-, air-, and O2-saturated AN
to determine the concentration of S•+ immediately after
the laser flash during the TPI of S (Figure 4). The
concentration of MeOSt•+ immediately after the laser
flash and the decay profile depended on the concentration
of O2. Although S•+ has no or less reactivity to O2,
•-
MeOSt•+ with AN•- and (AN)2 at the rate constant of
approximately 1013 M-1 s-1. The decay of ∆OD480 in the
time scale of a few microseconds in the absence and
presence of O2 were analyzed by the pseudo-first-order
kinetics for the dimerization of MeOSt•+ with MeOSt
and bimolecular reaction of MeOSt•+ with O2 at the rate
constant of approximately 107 M-1 s-1.9 In contrast to
MeOSt•+, no effect of O2 was observed for the formation
and decay profiles of the transient absorption of other
S•+.
MeOSt•+ reacts with O2 at the rate constant of kO2
(0.9-1.4) × 107 M-1 s-1(eq 2).9
)
Discussion
Important Factors of the TPI Efficiency. Forma-
tion of S•+ was observed during the TPI of S bearing an
electron-donating (methyl or methoxy) or an electron-
Because of the distonic character of MeOSt•+, radical
addition of O2 to the â-olefinic carbon of MeOSt•+ occurs
J. Org. Chem, Vol. 70, No. 11, 2005 4373

Hara et al.
to give peroxy adduct ((MeOSt-O2)•+), which leads to (4-
methoxylphenyl)methyl phenyl ketone as a stable prod-
uct. Since the concentrations of O2 in air- and O2-
saturated AN are 2 and 9 mM, respectively, MeOSt•+
generated immediately after the laser flash cannot be
bimolecularly quenched by O2.
ported by the fact that the fluorescence quantum yield
of MeOSt in O2-saturated AN (0.015) was lower than
that in Ar-saturated AN (0.02). The deactivation pathway
in MeOSt(S1)/O2 proceeds faster than that in free MeO-
St(S1). This may relate to the low Φion value of MeOSt•+
in the presence of O2.
Finally, it should be mentioned that Φion of S depends
not only on Eox of S and τf of S(S1) but also ground-state
complex formation between MeOSt with O2 and reactiv-
ity of MeOSt•+ during the TPI of S using lasers having
a long duration such as excimer lasers. The decay of
MeOSt•+ corresponds to bimolecular reactions such as
addition reactions between MeOSt•+ and O2 or O2•- (eqs
2 and 3, neutralization of MeOSt•+ with AN•- and
(AN)2•-, and dimerization of MeOSt•+ with MeOSt.
Thererfore, it is clear that the concentration of MeOSt•+
depends on the time when MeOSt•+ is monitored after
the laser flash.
In the previous paper we reported that radical cations
of St and MeOSt generated oxidized products in high
yields (64 and 68%, respectively) in O2-saturated AN.
From the present results, TPI yield of MeOSt is much
smaller than St. Thus, the complex MeOSt with O2
generated oxidized product efficiently with a laser pulse
duration, while St•+ forms the oxidized products with
bimolecular manner.
It has been reported that MeOSt•+ reacts with O2•- at
almost diffusion-controlled rate (eq 3)9,10,12 to give a
dioxetane as the initial adduct which decomposes into
4-methoxylbenzaldehyde and benzaldehyde at room tem-
•-
perature. If O2 is generated from the electron capture
by O2 during the TPI of MeOSt in the presence of O2,
O2 may react with MeOSt•+. However, the concentra-
•-
tion of O2•- was less than 10-6 M. Therefore, the quench-
ing of MeOSt•+ cannot be explained by the bimolecular
collisional reaction between MeOSt•+ and O2•-. Conse-
quently, the decay must correspond to a much faster
reaction such as an intramolecular reaction or neutral-
ization.
Conclusions
Ground-State Complex between MeOSt and O2.
It is suggested that a ground-state complex between
MeOSt and O2 (MeOSt/O2) may play a key roll for the
transient behavior of MeOSt•+ in the presence of O2.
When a apart of MeOSt forms the ground-state complex
with O2 in air- or O2-saturated AN, reactions of MeOSt•+
S•+ was generated from the TPI of S in AN with
irradiation of a XeCl laser flash with high F. Φion was
proportional to F2. Linear relationships between Φion
(0.06-0.29) and Eox of S and τf were observed, except for
MeOSt. The TPI proceeds via the two-step two-photon
excitation of the S(S0) f S(S1) f S(Sn) transition, and
ionization from S(Sn) to give S•+ and electron. It is found
that Φion of S•+ during the TPI depends not only on Eox
of S and τf of S(S1), except for MeOSt•+, but also on the
time for the detection of MeOSt•+ after the laser flash.
It is suggested that the ground-state complex between
MeOSt and O2 and the distonic character of MeOSt•+
with separation and localization of the positive charge
on the oxygen of the p-methoxyl group and an unpaired
electron on the â-olefinic carbon, are responsible to the
fast reaction of MeOSt•+ with O2 or O2•- , leading to the
considerable low yield and fast decay of MeOSt•+.
•-
and O2 or O2 in complex occur much faster than the
bimolecular reaction rate, explaining the efficient quench-
ing of MeOSt•+ by O2 or O2 (eqs 4 and 5).
•-
MeOSt + O2 a MeOSt/O2
(4)
MeOSt/O2 + 2hν (308 nm) f MeOSt•+/O2 +
e-, MeOSt•+/O2 (5)
•-
Thus, the low Φion value of MeOSt•+ during the TPI of
MeOSt can be explained by the fast reaction between
MeOSt•+ and O2 or O2 in the complex before dissocia-
•-
•-
tion to free MeOSt•+ and O2 or O2 . The ground-state
Acknowledgment. This work has been partly sup-
ported by a Grant-in-Aid for Scientific Research on
Priority Area (417), 21st Century COE Research, and
others from the Ministry of Education, Culture, Sports,
Science and Technology (MEXT) of the Japanese
Government.
complexes with various compounds such as benzophe-
none32 and cyclobutane-type cis,syn-dimer of dimethyl-
thymine.33 The complex formation of MeOSt/O2 is sup-
(32) Hutchinson, J. A.; DiBenedetto, J.; Rentzepis, P. M. J. Am.
Chem. Soc. 1986, 108, 6517.
(33) Majima, T.; Pac, C.; Kubo, J.; Sakurai, H. Tetrahedron Lett.
1980, 21, 377.
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