photochemical primary process of photo-fries …wakasa-lab.chem.saitama-u.ac.jp/paper/99.pdfthis...
TRANSCRIPT
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 755–761 755
Photochemical primary process of photo-Fries rearrangement reaction of
1-naphthyl acetate as studied by MFE probe
Masao Gohdo,a Tadashi Takamasub and Masanobu Wakasa*a
Received 31st March 2010, Accepted 30th September 2010
DOI: 10.1039/c0cp00077a
Photo-Fries rearrangement reactions of 1-naphthyl acetate (NA) in n-hexane and in cyclohexane
were studied by the magnetic field effect probe (MFE probe) under magnetic fields (B) of 0 to 7 T.
Transient absorptions of the 1-naphthoxyl radical, T–T absorption of NA, and a short-lifetime
intermediate (t = 24 ns) were observed by a nanosecond laser flash photolysis technique.
In n-hexane, the yield of escaped 1-naphthoxyl radicals dropped dramatically upon application of
a 3 mT field, but then the yield increased with increasing B for 3 mT o B r 7 T. These observed
MFEs can be explained by the hyperfine coupling and the Dg mechanisms through the singlet
radical pair. The fact that MFEs were observed for the present photo-Fries rearrangement
reaction indicates the presence of a singlet radical pair intermediate with a lifetime as long as
several tens of nanoseconds.
1. Introduction
Magnetic field effects (MFEs) on photochemical reactions
through radical pairs (RPs) and biradicals have received
considerable attention during the past three decades, and the
mechanism of MFEs has been well clarified experimentally
and theoretically.1–3 In RPs generated by photochemical
reactions, the unpaired electron spins on each radical are
coupled, giving two different spin states: singlet (S) and triplet
(T). According to the Pauli principle, singlet RPs can react to
form a recombination product, whereas triplet RPs cannot
react with each other but instead form escaped radicals.
Magnetic fields interact with these spins and affect the reaction
of the RPs without changing other parameters such as reaction
rate of singlet RPs, activation barrier, and diffusion motion of
the radicals. Because the interaction between magnetic fields
and spins can be defined by quantum chemistry, MFE studies
on RPs provide valuable information on their kinetics and
dynamics and, in particular, on aspects of the reaction
mechanism such as the presence of precursors and intermediates.
Therefore, we refer to an MFE study on RPs as a magnetic
field effect probe (MFE probe).4 Recently, using the MFE
probe, we have reported the microviscosity of alcoholic
solutions5 and the nanoscale heterogeneous structure of ionic
liquids.6–8
Photo-Fries rearrangement reactions are believed to occur
via geminate RPs formed by C–O bond cleavage in the ps*state of the aryl esters that form the key intermediate of
cyclohexadienone.9–13 The reaction kinetics and intermediates
of photo-Fries reactions have been continuously studied by
means of product analysis, chemically induced dynamic nuclear
polarization (CIDNP), laser flash photolysis, and magnetic
isotope effects (MIEs).14–19 Nakagaki et al. reported that
MIEs were observed for a photo-Fries reaction of 13C-labeled
1-naphthyl acetate (NA) in acetonitrile under a magnetic field
of 0.64 T.14 Shine and Subotkowski reported an MIE of a
photo-Fries reaction of 4-methoxyphenyl acetate in ethanol.15
In contrast, Lochbrunner et al. measured broad and weak
femtosecond transient absorption spectra of a photo-Fries
reaction of 4-tert-butylphenyl acetate in cyclohexane and
reported a cyclohexadienone formation time of 13 ps.18
Because the rate constants for spin conversion of RPs are
109–108 s�1, MIEs should not be observed for the photo-Fries
reactions with a cyclohexadienone formation time of 13 ps.
Moreover, in non-viscous solvents, such as acetonitrile,
ethanol, and cyclohexane, the RPs should disappear within
several hundreds of picoseconds by diffusion, but the spin
conversion of the RPs is not fast enough to compete with the
diffusion process.20–22 As such, the mechanisms of photo-Fries
reactions are still unclear and, therefore, it is worthwhile
to study photochemical primary processes of photo-Fries
rearrangement reactions in non-viscous homogeneous
solutions. Recently, we reported preliminary results for the
photochemical primary process of the photo-Fries rearrangement
reaction of NA in n-hexane.4 In this paper, we provide a full
investigation of this reaction by means of the MFE probe.
2. Experimental
1-Naphthyl acetate (NA, cica, SP grade) was purified by
passing through silica gel with n-hexane solution containing
5% of dichloromethane, and then the filtrate was evaporated
to obtain a white solid. The white solid was recrystallized twice
from n-hexane, yielding white needles. n-Hexane (cica, SP
grade), cyclohexane (cica, SP grade) and methylcyclohexane
(cica, SP grade) solvents were used as received. Viscosity was
measured with a viscometer (CBC, VM-10A-L) at 298 K.
Water content was measured by a Karl Fischer coulometer
(Metrohm, 831 KF Coulometer). The properties of n-hexane
and cyclohexane are listed in Table 1.23
aDepartment of Chemistry, Graduate School of Science andEngineering, Saitama University, 255 Shimo-okubo, Sakura-ku,Saitama-shi, Saitama 338-8570, Japan.E-mail: [email protected]
bNational Institute for Materials Science (NIMS), 3-13 Sakura,Tsukuba, Ibaraki 305-0003, Japan
PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics
Dow
nloa
ded
by S
aita
ma
Dai
gaku
Fuz
oku
Tos
hoka
n on
20
Janu
ary
2011
Publ
ishe
d on
03
Nov
embe
r 20
10 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C0C
P000
77A
View Online
756 Phys. Chem. Chem. Phys., 2011, 13, 755–761 This journal is c the Owner Societies 2011
A laser flash photolysis apparatus with a superconducting
magnet (Toshiba), which can generate magnetic fields up to
7 T, was newly constructed to accurately measure MFEs. The
excitation light source was the fourth harmonic of a Nd:YAG
laser (Spectra Physics, GCR-130-10, fwhm E 7 ns, 6.5 mJ per
pulse). A quartz cell was placed at the center of the magnet,
and a laser beam was introduced to the cell at an angle
perpendicular to a monitoring light beam. The optical path
of the monitoring light was 5 or 10 mm. The energy of the laser
beam was recorded for each excitation pulse by an energy
meter (Gentec, PRJ-M, with ED-100 thermopile head). The
fluctuation of the excitation energy was less than 3% during
each experiment. To measure the MFEs at lower magnetic
fields of 0 to 1.65 T, we used an electromagnet (Tokin,
SEE-10W). The laser flash photolysis apparatus equipped with
this electromagnet has been described previously.24 All laser
flash photolysis experiments were carried out at 298 K. The
sample solution was deoxygenated by bubbling with pure
nitrogen for 30 min and was pumped through a quartz flow
cell. The concentration of NA was 0.50 � 10�3 mol dm�3.
The fluorescence quantum yield was measured by an
absolute photoluminescence quantum yield measurement
system (Hamamatsu, C9920-02G) at room temperature. The
excitation wavelength for fluorescence measurements was 266 nm,
and the concentration of NA was 0.50 � 10�3 mol dm�3 in
n-hexane.
3. Results and discussion
3.1 Transient absorption spectra
Transient absorption spectra observed for the reaction of
1-naphthyl acetate (NA) in n-hexane at delay times of 0.03,
0.01, 1.0, 10, 20 and 50 ms after laser excitation are shown in
Fig. 1. The spectrum observed at a delay time of 1.0 ms afterlaser excitation had three peaks at 320, 390 and 410 nm,
whereas that observed at 50 ms had broad bands around 320
and 390 nm. The spectra observed at delay times of 1.0 ms andlater were essentially the same as those previously reported.14,16
Nakagaki et al. reported that the 1-naphthoxyl radical exhibits
transient absorption around 380–410 nm.14 However, Gritsan
et al. assigned this band to T–T absorption of NA, whereas the
absorption of 1-naphthoxyl radical was reported to exhibit a
slow decay around 380 nm.16 Thus the assignments of the
transient absorption spectra observed for the photo-Fries
rearrangement reaction of NA are somewhat complicated at
present.
Time profiles of the transient absorption, A(t), of NA
observed in n-hexane at 320, 380, 410, and 430 nm are shown
in Fig. 2. The A(t) curves observed at 380 and 410 nm agreed
well with an exponential fit (k = 1.41 � 105 s�1) combined
with a slow second-order component. Since the exponential
decay component was magnetically inactive, it was assigned to
the T–T absorption of NA. As discussed below, the second-order
component was magnetically active; thus, this component
could be safely assigned to the 1-naphthoxyl radical. The
second-order rate constant (k/el) was calculated to be
2.65 � 107 s�1 mol�1 dm3, where e is the extinction coefficient
and l is the illuminated path length. Using data reported by
Nakagaki et al. for NA in acetonitrile,14 we estimated the
second-order rate constant to be 1 � 107 s�1 mol�1 dm3. This
estimated value is similar to the present experimental data
obtained with an illuminated path length of about 5 mm.
The A(t) curve observed at 320 nm in n-hexane has an
exponential decay component and almost constant component.
Table 1 Properties of solvent
Solvent ea Z/mPa s Water content/ppm
n-Hexane 1.8799 0.2942a 22.4Cyclohexane 2.02 0.94 17.0
a Ref. 23.
Fig. 1 Transient absorption spectra observed for the photo-Fries
reaction of 1-naphthyl acetate (NA) in n-hexane (0.50� 10�3 mol dm�3).
Emissions observed around 320 nm are fluorescence of NA.
Fig. 2 Time profiles of transient absorption observed for NA in
n-hexane (0.50 � 10�3 mol dm�3) at 320, 380, 410, and 430 nm.
Dow
nloa
ded
by S
aita
ma
Dai
gaku
Fuz
oku
Tos
hoka
n on
20
Janu
ary
2011
Publ
ishe
d on
03
Nov
embe
r 20
10 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C0C
P000
77A
View Online
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 755–761 757
Since the exponential decay component is magnetically
inactive and has a rate constant (1.32 � 105 s�1) similar to
that observed at 410 nm, it can be assigned to the T–T
absorption of NA. However, almost constant one may be
assigned to cyclohexadienone intermediate, because the life-
time of cyclohexadienone intermediates observed for the
photo-Claisen rearrangement reactions, which are similar
rearrangement reactions of allyl phenyl ethers, was reported
as ms time region.25,26
As shown in Fig. 1, the spectrum observed at a delay time of
0.03 ms had only a single peak at 410 nm, and the shape of the
spectrum, especially in the longer wavelength region, was
different from that of the spectrum observed at a delay time
of 1.0 ms. Moreover, each A(t) curve observed at 430–600 nm
had a fast decay component, as shown in Fig. 3. The lifetime
of the fast decay component was observed to be 24 ns
(k = 4.10 � 107 s�1), and this component may be attributed
to the RP complex of 1-naphthoxyl and acetyl radicals. This
assignment is discussed further in the next section.
The time profiles of the transient absorption observed in
n-hexane and cyclohexane at 410 and 430 nm are shown in
Fig. 4. As shown in Fig. 4a, the decay profile observed at
410 nm was dependent on the solvent composition: the decay
observed in cyclohexane (upper trace) was slower than that
observed in n-hexane (lower trace). It is reasonable to conclude
from these data that the lifetime of the triplet state and the
second-order reaction were strongly affected by the solvent
viscosity. In contrast, no change in the fast decay component
was observed for different solvents at 430 nm (Fig. 4b). Since
the first decay component was almost independent of the
solvent viscosity, this component may be ascribable to an
intra-molecular reaction.
3.2 Magnetic field effects
A(t) curves were measured at 410 nm for 1-naphthoxyl radical
in the absence and presence of magnetic fields up to 7 T. The
A(t) curves observed at 0 and 1.65 T in n-hexane are shown in
Fig. 5. The A(t) curves observed both at 0 and 1.65 T exhibit
exponential and second-order decay components due to the
T–T absorption of NA and 1-naphthoxyl radical, respectively.
Fig. 5 shows an appreciable MFE on the second-order decay
component of 1-naphthoxyl radical. The fact that MFEs were
observed for the present reaction in n-hexane strongly indicates
that the lifetime of the RPs of 1-naphthoxyl and acetyl radicals
are as long as several nanoseconds, because the spin conversion
process should compete with the disappearance of the RPs.
To elucidate the mechanism of the MFEs, we measured the
magnetic field dependence under magnetic fields up to 7 T.
Using the escaped radical yield Y(B) at a delay time of 3 msafter laser excitation, we calculated the relative radical yield as
R(B) = Y(B)/Y(0 T) = A(B)/A(0 T). The obtained R(B)Fig. 3 Time profiles of transient absorption observed at 380, 410, and
430 nm in the time range of 0–0.9 ms.
Fig. 4 Time profiles of transient absorption observed in n-hexane and
cyclohexane (a) at 410 nm in the time range of 0–70 ms, and (b) at
430 nm in the time range of 0–0.9 ms.
Fig. 5 Time profiles of transient absorption observed at 410 nm in the
absence and presence of a magnetic field of 1.65 T. Inset: plots in the
time range of 2–4 ms.
Dow
nloa
ded
by S
aita
ma
Dai
gaku
Fuz
oku
Tos
hoka
n on
20
Janu
ary
2011
Publ
ishe
d on
03
Nov
embe
r 20
10 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C0C
P000
77A
View Online
758 Phys. Chem. Chem. Phys., 2011, 13, 755–761 This journal is c the Owner Societies 2011
values are plotted against B in Fig. 6. The magnetic field
dependence on R(B) has an inverted feature. R(B) dropped
dramatically upon application of a 3 mT field, giving the
minimum observed R(B) of 0.97 � 0.02. Then R(B) gradually
increased with increasing B to 7 T. A cross field of R(B) = 1
was observed at 0.1 T. No saturation of R(B) was observed at
the maximum field of 7 T (R(7 T) = 1.07 � 0.02).
These observed MFEs can be classified by several
mechanisms as follows:1–3 (1) the Dgmechanism (DgM), which
is due to the difference between the isotropic g factors of two
radicals in a pair; (2) the hyperfine coupling mechanism
(HFCM), which is due to the isotropic hyperfine interaction
between electron and nuclear spins; (3) the level crossing
mechanism (LCM), which is due to crossing between the S
and T+1(or T�1) levels; (4) the relaxation mechanism (RM),
which is due to the anisotropic g tensor (dg), HFC (dHFC),
and the spin–spin dipolar interactions of RPs.
Among these mechanisms, the inverted MFEs on R(B) can
be explained by two possible mechanisms. One possible
mechanism is the MFEs due to the RM. In the present
reaction in n-hexane, however, the solvent viscosity was too
small to observe the MFEs by the RM. In other words, the
escaping rate of the RPs was larger than the spin relaxation
rate. Therefore, we can exclude the possibility of the RM.
Another possible mechanism to explain the inverted MFEs
on R(B) is the HFCM associated with the DgM. The
rate constant of S–T conversion by the HFCM (kHFCM) is
given by1–3
kHFCM = (1/2)gmBB1/2/h. (1)
Here, g and mB are the g-value of the free electron (2.00231)
and the Bohr magneton, respectively. B1/2 is the effective
hyperfine coupling (Weller’s half-field of the saturation) in
the RP, which can be expressed by27
B1/2 = 2 (B12 + B2
2)/(B1 + B2), (2)
Bi ¼Xk
a2ikIkðIk þ 1Þ( )1=2
ð3Þ
Here, aik is the isotropic hyperfine coupling constant of the kth
nuclei in radical (i = 1, 2). Ik is the nuclear spin. Using the
reported hyperfine coupling constants of 1-naphthoxyl28 and
acetyl radicals,29 kHFCM in the absence of a magnetic field was
estimated to be 9.16 � 107 s�1. In the presence of a magnetic
field, the triplet sublevel splits into three levels (T0, T+1, and
T�1) by means of the Zeeman interaction, and S–T+1
and S–T�1 spin conversions are obstructed. Thus the S–T
conversion rate by the HFCM decreases with increasing B and
reaches a value that is 3 fold of that observed in the zero field.
Since the obstruction of the spin conversions S–T+1 and
S–T�1 occurs at higher magnetic fields of the HFC, the
decrease in R(B) is saturated at those fields. The half-field of
the saturation (B1/2) for the present RP was calculated by
eqn (2) and (3) to be 1.04 mT.27,28 As shown in Fig. 6b, the
R(B) value reached its minimum at 3 mT, which may have
been the saturation field for the HFCM. Therefore, B1/2 was
roughly estimated to be 1–2 mT. This estimated value agrees
well with our calculated value of 1.04 mT. These results
indicate that the sharp drop in R(B) observed at 3 mT can
safely be explained by the HFCM.
The rate constant of S–T conversion by the DgM (kDgM) is
given by
kDgM = (1/2)DgmBB/h. (4)
Here, Dg is the difference between the g-values of two radicals.
The kDgM value at 0 T is zero, but this value increases
proportionally with increasing B. Since Dg was estimated to
be 0.00381 from the reported g-values of 1-naphthoxyl radical
(2.00431)30 and acetyl radical (2.0005),29 kDgM at 7 T was
calculated to be 1.17 � 109 s�1. From these calculations, we
can estimate the magnetic field dependence of the overall S–T
conversion (kS�T) as follows:
kS�T = kDgM + (1/3) kHFCM. (5)
The rate constant of S–T conversion decreased dramatically
at low fields and then increased with increasing B to reach
1.20� 109 s�1 at 7 T. This observed behavior indicates that the
present MFEs can be qualitatively interpreted by the HFCM
associated with the DgM.
This spin conversion process by the HFCM and the DgMcompetes with the escaping process of the RPs. Therefore, we
also considered the rate constant of the escaping process. The
rate of escape (kesc) from a solvent cage depends on the
solvent’s viscosity. The simplest expression of the kesc is
described by the Stokes–Einstein relationship as follows:
kesc = 1/t = kBT/12pr3Z. (6)
Fig. 6 (a) Magnetic field dependence on the relative radical yield
(R(B)) of 1-naphthoxyl radical observed at 410 nm. Circles and
diamonds denote the used transient absorption apparatus with an
electromagnet (EM) and that with a superconducting magnet (SCM),
respectively. (b) Plots in the lower magnetic field range of 0–0.12 T.
Dow
nloa
ded
by S
aita
ma
Dai
gaku
Fuz
oku
Tos
hoka
n on
20
Janu
ary
2011
Publ
ishe
d on
03
Nov
embe
r 20
10 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C0C
P000
77A
View Online
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 755–761 759
Here, kB is the Boltzmann constant, T is the temperature, and
Z is the viscosity of the solvent. The lifetime (t) of a neutral RP
is defined as t = 4r2/D, where r and D are the van der Waals
radii of the component radicals and the sum of the diffusion
coefficients of each radical, respectively.31 In the present study,
kesc was estimated to be 3.0 � 109 s�1 at 298 K using
Z = 0.2942 � 10�3 Pa s, rnaphthoxyl = 0.71 nm, and racetyl =
0.29 nm.32,33 Comparison of this value with the calculated S–T
conversion rate of 1.20 � 109 s�1 at 7 T reveals that the escape
rate is much larger than the S–T conversion rate. Therefore,
the MFEs observed in this study cannot be explained by the
normal diffusion process calculated from the Stokes–Einstein
relationship. Instead, a much slower escape process involving
the RP complex is suggested.
To clarify the formation of such an RP complex, the A(t)
curves observed in the time range of 0–0.9 ms were examined in
detail. As shown in Fig. 3, in the time range of 0–0.1 ms, a fast
decay component was observed at 430 nm concomitant with a
fast rise component observed at 380 and 410 nm. Such fast
decay component was also observed at 430–600 nm. The rate
constants of both decay (at 430 nm) and rise (at 410 nm) were
4.10 � 107 s�1. Since the transient absorption observed at
380 and 410 nm consists of both the 1-naphthoxyl radical and
the T–T absorption of NA, the fast decay component observed
at 430–600 nm could be assigned to the singlet–singlet (S–S)
absorption of NA or to an absorption of a parent intermediate
of the 1-naphthoxyl radical, such as an RP complex. To clarify
the assignment of the fast decay component, the rate
constant of fluorescence (kF) was measured in n-hexane and
in cyclohexane at 323 nm under the same conditions as those
used for the transient absorption measurement. The kF values
observed in n-hexane and cyclohexane were 3.99 � 107 and
5.29 � 107 s�1, respectively. As shown in Fig. 4b, the fast
decay component was independent of solvent composition.
Therefore, the fast decay component cannot be assigned to the
S–S absorption of NA, because the kF values in n-hexane
and in cyclohexane were different. From these results, we
concluded that the fast kinetics of decaying at 430–600 nm
could be safely assigned to the disappearance of the
1-naphthoxyl radical’s parent intermediate, namely an RP
complex. We further concluded that the fast kinetics of rising
observed at 380 and 410 nm could be attributed to the
formation of free 1-naphthoxyl radicals.
Among the published reports on the MFEs of photo-
chemical reactions, there are several reports of MFEs in
non-viscous homogeneous solvents; such MFEs are classified
as the d-type triplet mechanism (TM).34–36 This class of MFEs
is observed only from the triplet precursor and is often
observed for RPs containing heavy atoms such as d-block
metals,34 sulfur35 or phosphorus.36 The key intermediates for
the d-type TM are excited triplet molecules or RP complexes.
For the present reaction, although this class of MFEs can
safely be excluded, it is worthwhile to consider the character of
the RP complex. In this study, the MFEs due to the LCMwere
not observed. Therefore, the exchange interaction should have
been small enough to observe MFEs due to the HFCM and
the DgM. Even in a non-viscous solvent, such complexation
enables long-lifetime RPs. Moreover, since the fast decayed
transient absorption observed at 430–600 nm was very broad
and not very strong, the RP complex may be characterized as
similar to a charge transfer complex or an electron-donor–
acceptor complex. Theoretical calculations may be useful to
elucidate the ultraviolet-visible spectrum of such complex.
However, those calculations are beyond the scope of the
present experimental study of the photochemical primary
process.
3.3 Time-resolved EPR measurements
Next, to directly measure the RP complex, we carried out
time-resolved electron paramagnetic resonance (EPR)
measurements on an n-hexane solution of NA. The concentration
of the NA solution was 2.0 � 10�3 mol dm�3 and the solution
was pumped through a quartz cell at a flow rate of 4 mL min�1.
No signal of spin-correlated RPs was observed under
excitation by the fourth harmonic of a Nd:YAG laser. This
lack of signal might have been due to the fast rearrangement
reaction of the RP complex. The time resolution of our
apparatus is ca. 100 ns, and the spin polarization of the RP
is generated within a similar time range. If the lifetime of the
RP complex was as short as 24 ns (k = 4.10 � 107 s�1),
it would be difficult to observe the signal of the RPs by
time-resolved EPR.
3.4 Photochemical primary process of the photo-Fries
rearrangement reaction
Upon irradiation of a degassed n-hexane solution of NA
(4.0 � 10�3 mol dm�3) with an Ushio 500 W Xe lamp,
2-acetyl-1-naphthol was obtained as the main product. For
an aerated solution of NA, the yield of 2-acetyl-1-naphthol
decreased by only 8 � 2%. Such a small decrease in yield
indicates that the present reaction occurred fast enough to
avoid quenching by oxygen. Nakagaki et al. have also
reported that dissolved oxygen has no effect upon the initial
transient absorption during laser flash photolysis of NA.14
From the results of the present study, the reaction mechanism
can be described as follows (Scheme 1):
Here, 1,3NA*, NpO�, Ac�, and 1,3(NpO� �Ac) denote the
singlet or triplet excited state of NA, 1-naphthoxyl radical,
acetyl radical, and the singlet or triplet RP, respectively. Upon
irradiation of NA by the fourth harmonic of the nanosecond
Nd:YAG laser, 1NA* is immediately generated. Fluorescence
Scheme 1 Mechanism of the photo-Fries rearrangement reaction.
Dow
nloa
ded
by S
aita
ma
Dai
gaku
Fuz
oku
Tos
hoka
n on
20
Janu
ary
2011
Publ
ishe
d on
03
Nov
embe
r 20
10 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C0C
P000
77A
View Online
760 Phys. Chem. Chem. Phys., 2011, 13, 755–761 This journal is c the Owner Societies 2011
and phosphorescence of NA were observed in n-hexane and in
methylcyclohexane around 310–390 nm (peaks: 317.8, 322.6,
328.0, 332.2, 337.6, 342.0, 348.0 nm) at room temperature and
around 470–630 nm (peaks: 479.0, 491.0, 514.6, 555.0, 603.2 nm)
at 77 K, respectively. The excitation energies estimated from
the 0–0 band were 384 kJ mol�1 for S1 and 255 kJ mol�1 for
T1. Fluorescence is emitted with a quantum yield of 0.17 at
room temperature, and intersystem crossing spontaneously
occurs to form 3NA*. T–T absorption was observed at
380–410 nm. Since no decomposition reaction occurs from3NA*,37–39 we concluded that 3NA* is likely deactivated by
thermal relaxation at room temperature or by phosphorescence
radiation at 77 K. 1NA* generates the singlet RP complex of1(NpO� �Ac). The RP complex of 1(NpO� �Ac) can convert to
a triplet RP complex of 3(NpO� �Ac), and this spin conversion
process is affected by the magnetic field.1–3 From the singlet
RP complex of 1(NpO� �Ac), rearrangement products (in-cage
products) are generated. In contrast, 3(NpO� �Ac) generates
escaped radicals that subsequently form escaped products,
because the triplet RP cannot recombine.
Some authors have reported a cyclohexadienone inter-
mediate as the key species of photo-Fries rearrangement
reaction, and they have further reported that a subsequent
proton shift is generally slow even if the cyclohexadienone
intermediate is unstable.26
1NA* - 1(NpO� Ac�), (7)
1(NpO� Ac�) - cyclohexadienone(s), (8)
Cyclohexadienone(s) - acetyl naphthol(s). (9)
Considering the results obtained from the present MFE
measurements, we concluded that the key species are the
RPs, because magnetic fields only affected the spin conversion
process of the RPs. According to Lochbrunner et al., the
formation time of cyclohexadienone is 13 ps for 4-tert-
butylphenyl acetate in cyclohexane.18 If the geminate singlet
RPs are depleted by the formation of cyclohexadienone within
several tens of picoseconds, then no MFEs would be observed
for the RPs. Therefore, because MFEs were indeed observed
for the RPs, we concluded that the RP complex rather than the
cyclohexadienone intermediate should be considered the
primary intermediate in this reaction. In the previous section,
we assigned the fast-decaying component observed at 430 nm
to the disappearance of the RP complex. It is reasonable to
conclude that the transient signal of the 1-naphthoxyl radical
observed at 410 nm increased as the signal at 430 nm from the
RP complex decreased.
4. Conclusion
Laser flash photolysis under magnetic fields up to 7 T was
carried out for the photo-Fries rearrangement reactions of
1-naphthyl acetate in n-hexane and cyclohexane. The transient
absorption of the 1-naphthoxyl radical was observed at
380–410 nm in both solvents, and appreciable MFEs on its
escaped yields were observed for the first time. Observation of
these MFEs shows the existence of the RP intermediates in
the present reaction, but the escape of the RP cannot be
explained by the normal diffusive process. The presence of
an RP complex, which had a lifetime of 24 ns, was strongly
suggested by the slow escaping process of the RP.
Consequently, we concluded that the observed MFEs can be
explained by the HFCM associated with DgM.
Acknowledgements
We thank Professor Yoshio Sakaguchi of RIKEN for
measuring time-resolved EPR spectrum. This work was
partially supported by a Grant-in-Aid for Scientific Research
(No. 2003002) in the Priority Area ‘‘High Field Spin Science in
100 T’’ (No. 451) from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT) of Japan.
References
1 U. E. Steiner and T. Ulrich, Chem. Rev., 1989, 89, 51–147.2 S. Nagakura, H. Hayashi and T. Azumi, Dynamic Spin Chemistry,Kodansha-Wiley, Tokyo, NY, 1998.
3 H. Hayashi, Introduction to Dynamic Spin Chemistry, WorldScientific, Singapore, 2004.
4 M. Gohdo and M. Wakasa, Chem. Lett., 2010, 39, 106–107.5 A. Hamasaki, T. Yago and M. Wakasa, J. Phys. Chem. B, 2008,112, 14185–14192.
6 M. Wakasa, J. Phys. Chem. B, 2007, 111, 9434–9436.7 A. Hamasaki, T. Yago, T. Takamasu, G. Kido and M. Wakasa,J. Phys. Chem. B, 2008, 112, 3375–3379.
8 M. Wakasa, T. Yago and H. Hamasaki, J. Phys. Chem. B, 2009,113, 10559–10561.
9 S. Grimme, Chem. Phys., 1992, 163, 313–330.10 M. A. Miranda and F. Galindo, CRC Handbook of Organic
Photochemistry and Photobiology, CRC Press, Boca Raton,2nd edn, 2004, ch. 42, pp. 42-1.
11 V. I. Stenberg, Org. Photochem., 1967, 1, 127.12 D. Bellus, Adv. Photochem., 1971, 8, 109–159.13 R. Martin, Org. Prep. Proced. Int., 1992, 24, 369–435.14 R. Nakagaki, M. Hiramatsu, T. Watanabe, Y. Tanimoto and
S. Nagakura, J. Phys. Chem., 1985, 89, 3222–3226.15 H. J. Shine and W. Subotkowski, J. Org. Chem., 1987, 52,
3815–3821.16 N. P. Gritsan, Y. P. Tsentalovich, A. V. Yurkovskaya and
R. Z. Sagdeev, J. Phys. Chem., 1996, 100, 4448–4458.17 I. F. Molokov, Y. P. Tsentalovich, A. V. Yurkovskaya and
R. Z. Sagdeev, J. Photochem. Photobiol., A, 1997, 110, 159–165.18 S. Lochbrunner, M. Zissler, J. Piel, E. Riedle, A. Spiegel and
T. Bach, J. Chem. Phys., 2004, 120, 11634–11639.19 T. Mori, M. Takamoto, H. Saito, T. Furo, T. Wada and Y. Inoue,
Chem. Lett., 2004, 254–255.20 R. M. Noyes, J. Am. Chem. Soc., 1956, 78, 5486–5490.21 E. N. Step, A. L. Buchachenko and N. J. Turro, J. Org. Chem.,
1992, 57, 7018–7024.22 N. J. Turro, A. L. Buchachenko and V. F. Tarasov, Acc. Chem.
Res., 1995, 28, 69–80.23 Organic Solvents, ed. J. A. Riddick, W. B. Bunger and
T. K. Sakano, Wiley, New York, 4th edn, 1986.24 M. Wakasa, K. Nishizawa, H. Abe, G. Kido and H. Hayashi,
J. Am. Chem. Soc., 1999, 121, 9191–9197 and references citedtherein.
25 F. Galindo, J. Photochem. Photobiol., C, 2005, 6, 123–138.26 M. C. Jimenez, M. A. Miranda, J. C. Scaiano and R. Tormos,
Chem. Commun., 1997, 1487–1488.27 A. Weller, F. Nolting and H. Staerk, Chem. Phys. Lett., 1983, 96,
24–27.28 W. T. Dixon, W. E. J. Foster and D. Murphy, J. Chem. Soc.,
Perkin Trans. 2, 1973, 2124–2127.29 J. Benett and B. Mile, Trans. Faraday Soc., 1971, 67, 1587–1597.30 M. Adams, M. S. Blois Jr. and R. H. Sands, J. Chem. Phys., 1958,
28, 774–776.31 K. M. Salikhov, Magnetic Isotope Effect in Radical Reactions,
Springer-Verlag, Weinheim, Germany, 1996.
Dow
nloa
ded
by S
aita
ma
Dai
gaku
Fuz
oku
Tos
hoka
n on
20
Janu
ary
2011
Publ
ishe
d on
03
Nov
embe
r 20
10 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C0C
P000
77A
View Online
This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 755–761 761
32 W. Gu, D. J. Abdallah and R. G. Weiss, J. Photochem. Photobiol.,A, 2001, 139, 79–87.
33 Each van der Waals radius was estimated with the reportedcrystallographic data of NA in ref. 32, assuming that bondcleavage will not cause big change to molecular structure. Fromthe EPR study reported in ref. 29, the structure of acetyl radical is abend form and C–C–O angle of the radical (1301 by EPR) is similarto that of NA (127.141 by XRD). Estimated radii were 0.71 nm fornaphthoxyl radical and 0.29 nm for acetyl radical defined by themaximum length of molecule.
34 U. E. Steiner, Chem. Phys. Lett., 1980, 74, 108–112.
35 P. Gilch, M. Linsenmann, W. Haas and U. E. Steiner, Chem. Phys.Lett., 1996, 254, 384–390.
36 Y. Sakaguchi and H. Hayashi, J. Phys. Chem. A, 2004, 108,3421–3429.
37 C. E. Kalmus and D. M. Hercules, J. Am. Chem. Soc., 1974, 96,449–456.
38 P. Subramanian, D. Creed, A. C. Griffin, C. E. Hoyle andK. Venkataram, J. Photochem. Photobiol., A, 1991, 61,317–327.
39 S. Grimme and H. Dreeskamp, J. Photochem. Photobiol., A, 1992,65, 371–382.
Dow
nloa
ded
by S
aita
ma
Dai
gaku
Fuz
oku
Tos
hoka
n on
20
Janu
ary
2011
Publ
ishe
d on
03
Nov
embe
r 20
10 o
n ht
tp://
pubs
.rsc
.org
| do
i:10.
1039
/C0C
P000
77A
View Online