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 Takamasu b 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 solutions 5 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 13 C-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 10 9 –10 8 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 a Department of Chemistry, Graduate School of Science and Engineering, Saitama University, 255 Shimo-okubo, Sakura-ku, Saitama-shi, Saitama 338-8570, Japan. E-mail: [email protected]b National Institute for Materials Science (NIMS), 3-13 Sakura, Tsukuba, Ibaraki 305-0003, Japan PAPER www.rsc.org/pccp | Physical Chemistry Chemical Physics Downloaded by Saitama Daigaku Fuzoku Toshokan on 20 January 2011 Published on 03 November 2010 on http://pubs.rsc.org | doi:10.1039/C0CP00077A View Online
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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
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.
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