Photoinduced intramolecular hydrogen atom transfer in 2-(2-hydroxyphenyl)benzoxazole and 2-(2-hydroxyphenyl)benzothiazole studied by laser flash photolysis

Article abstract of DOI:10.1039/b202559k

2-(2-Hydroxyphenyl)benzoxazole (HBO) and 2-(2-hydroxyphenyl)benzothiazole (HBT) underwent hydrogen atom transfer to give the tautomer in both the singlet excited state and the triplet excited state. In the singlet excited state, the keto form produced by excited state hydrogen atom transfer underwent isomerization around the quasi-double bond to give the trans-keto form. In the triplet excited state, HBO and HBT were equilibrated between the enol form and the cis-keto form and the equilibrium constant was determined by triplet sensitization and quenching experiment by using laser flash photolysis in benzene at room temperature. From these results, we have revealed the energy diagram of the hydrogen atom transfer reaction of HBO and HBT both in the singlet excited state and the triplet excited state.

Full text of DOI:10.1039/b202559k

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Photoinduced intramolecular hydrogen atom transfer in
2-(2-hydroxyphenyl)benzoxazole and 2-(2-hydroxyphenyl)-
benzothiazole studied by laser flash photolysis
2
Masashi Ikegami and Tatsuo Arai*
Department of Chemistry, University of Tsukuba, Tsukuba, Ibaraki, 305-8571, Japan.
E-mail: arai@chem.tsukuba.ac.jp; Fax: ϩ81-298-53-6503; Tel: ϩ81-298-53-4315
Received (in Cambridge, UK) 13th March 2002, Accepted 24th April 2002
First published as an Advance Article on the web 14th May 2002
2-(2-Hydroxyphenyl)benzoxazole (HBO) and 2-(2-hydroxyphenyl)benzothiazole (HBT) underwent hydrogen atom
transfer to give the tautomer in both the singlet excited state and the triplet excited state. In the singlet excited state,
the keto form produced by excited state hydrogen atom transfer underwent isomerization around the quasi-double
bond to give the trans-keto form. In the triplet excited state, HBO and HBT were equilibrated between the enol form
and the cis-keto form and the equilibrium constant was determined by triplet sensitization and quenching experiment
by using laser flash photolysis in benzene at room temperature. From these results, we have revealed the energy
diagram of the hydrogen atom transfer reaction of HBO and HBT both in the singlet excited state and the triplet
excited state.
spectra with a maximum wavelength of 430 and 460 nm,
Introduction
respectively.6,8,11
The excited state intramolecular proton or hydrogen atom
transfer (ESIPT) reaction has received considerable attention
from the viewpoint of reaction dynamics and the possibility
of the applications of photostabilizers, proton transfer laser,
From the results of two-step laser excitation (TSLE)
fluorescence and the effect of temperature on the intensity of
tautomer emission, the observed transient species in the
ground state was assigned to the trans-keto form, which was
etc.1–5 The mechanism of the ESIPT reaction is usually
produced by cis-to-trans isomerization of the cis-keto form in
described by a four level scheme involving the singlet excited
the singlet excited state.10–14
state of the normal form and the tautomer form and those
As to the photochemical behavior in the triplet excited
forms in the ground state.
state, HBO exhibited T–T absorption in non-polar solvents. In
The ESIPT reaction of 2-(2-hydroxyphenyl)benzoxazole
addition, dual phosphorescence spectra were observed in HBO
(HBO) and 2-(2-hydroxyphenyl)benzothiazole (HBT) (Fig. 1)
and HBT at cryogenic temperatures.13,16,17 In HBO, the observ-
ation of a nearly temperature-independent ratio of two phos-
phorescences was explained by the approximately isoenergetic
relation of the enol and keto form in the triplet excited state.16
However, the potential energy diagram of ESIPT of HBO
and HBT at room temperature was still uncertain.
In this study, the equilibrium constant between the enol and
keto form in the triplet excited state of HBO and HBT was
estimated by laser flash photolysis on triplet sensitization and
quenching method. In addition, we have synthesized a model
compound of the tautomer of HBT, and its photochemical
behavior was examined. Thus, we can clearly observe the cis–
trans isomerization of the keto form of HBO and HBT in the
excited state and can estimate the quantum yield of formation
of the trans-keto form in the ground state. Furthermore, we
have revealed the potential energy diagram of hydrogen atom
transfer (Fig. 2) of HBO and HBT.
Experimental
Fig. 1 Compounds investigated in this study.
Materials and solvents
was extensively investigated.6–19 HBO and HBT underwent
intramolecular hydrogen atom transfer not only in the singlet
excited state but also in the triplet excited state.14–16
In the singlet excited state, HBO and HBT gave a tautomer
exhibiting fluorescence emission with a large Stokes shift, where
the maximum wavelength was observed at 500 and 520 nm
for HBO and HBT, respectively in non-polar solvents.10–14
In addition, HBO and HBT exhibited transient absorption
2-(2-Hydroxyphenyl)benzoxazole (HBO) and 2-(2-hydroxy-
phenyl)benzothiazole (HBT) were obtained from TCI (Tokyo
Kasei) and purified by silica gel column chromatography and
recrystallization from ethanol.
2-(2-Methoxyphenyl)benzoxazole (MBO) and 2-(2-methoxy-
phenyl)benzothiazole (MBT) were prepared from HBO and
HBT by reaction with dimethyl sulfate in dioxane in the
presence of aqueous NaOH.
1296
J. Chem. Soc., Perkin Trans. 2, 2002, 1296–1301
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b202559k

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Fig. 2 Excited state intramolecular hydrogen atom transfer of HBO
and HBT.
2-Phenylbenzoxazole (BO) and 2-phenylbenzothiazole (BT)
were purchased from Aldrich and purified by silica gel column
chromatography and recrystallization from ethanol.
trans-6-[N-Methyl-2(3H )-benzothiazolylidene]cyclohexa-2,4-
dienone (trans-NBT) was synthesized according to a procedure
similar to that reported by Nagaoka et al.18 trans-NBT: 1H
NMR (methanol-d4, 600 MHz) δ 4.29 (s, 3H; N-CH3), 6.66 (m,
1H; Hb), 6.86 (d, 1H, J = 8.6 Hz; Hd), 7.39 (m, 1H; Hc), 7.67 (m,
1H; Hf), 7.70 (d, 1H, J = 8.2 Hz; Ha), 7.80 (m, 1H; Hg), 8.05
(d, 1H, J = 8.5 Hz; Hh), 8.13 (d, 1H, J = 7.8 Hz; He) ppm.
Fig. 3 Absorption and fluorescence spectra of HBO (a) and HBT
(b) in benzene.
Transient absorption spectra on direct irradiation
HBO and HBT exhibited transient absorption spectra with
a maximum wavelength of 430 and 465 nm, respectively on
308 nm excitation as shown in Fig. 4. The transient absorption
In spectroscopy, Dotite Spectrosol and Luminasol were used
as solvents without further purification.
Measurement
Absorption and fluorescence spectra were measured on a
JASCO Ubest-55 and on a Hitachi F-4000 fluorescence spec-
trometer, respectively. Quantum yields of fluorescence were
determined by using anthracene (Φf = 0.27)21 as an emission
standard.
Laser flash photolysis was performed by using an excimer
laser (Lambda Physik LPX-100, 308 nm, 20 ns fwhm) or
excimer laser pumped dye laser (Lambda Physik LEXtra-100,
308 nm, 20 ns fwhm and Lambda Physik Scanmate; Lambda
Physik LPX-100, 308 nm, 20 ns fwhm and Lambda Physik
FL-3002, 10 ns fwhm). Stilbene 3, DMQ, and QUI were used
as laser dyes to obtain a 425, 366 and 390 nm laser pulse,
respectively. A pulsed xenon arc (Ushio UXL-159) was used as
a monitoring light source.
Fig. 4 Transient absorption spectra of HBO (a) and HBT (b) in
benzene on direct irradiation.
Results
spectra of HBO shifted to the shorter wavelength with time and
the maximum wavelength appeared around 400 nm at 1.87 µs
after the laser pulse, which had a lifetime of 2.5 µs at room
temperature and was not quenched by oxygen. Therefore, this
transient species can be assigned to the tautomer form. We have
observed a negative temperature effect on the lifetime of this
transient in methylcyclohexane as shown in Fig. 5. Thus, the
lifetime increased with increasing temperature. In addition, a
transient absorption with a maximum of 400 nm was observed
in benzene (Fig. 4a), which was assigned to the triplet state by
oxygen quenching experiments, where the quenching rate con-
stant was determined to be 3 × 109 MϪ1 sϪ1. The spectra and the
lifetimes of transient absorption were similar to those in other
hydrocarbon solvents.6
Absorption and fluorescence spectra
Absorption and fluorescence spectra of HBO and HBT in
benzene are shown in Fig. 3. The quantum yield of the taut-
omer fluorescence was determined to be 0.02 and 5 × 10Ϫ3 in
benzene for HBO and HBT, respectively.
The effect of temperature on the quantum yield of fluor-
escence emission in benzene was examined. The fluorescence
quantum yield of the tautomer emission increased with
decreasing temperature in benzene from 0.013 at 40 ЊC to
0.035 at 7 ЊC indicating that the deactivation pathway with
activation energy exists in the singlet excited state of the taut-
omer. The temperature effect on the fluorescence intensity was
also reported in some hydrocarbon solvents.7,8
J. Chem. Soc., Perkin Trans. 2, 2002, 1296–1301
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time constant of 110 ns. This result indicates that trans-NBT
underwent trans-to-cis isomerization photochemically followed
by reverse cis-to-trans isomerization thermally in benzene with
a time constant of 110 ns.
Two step laser excitation of HBO
After we produced the tautomer of HBO by 308 nm laser pulse
with laser flash photolysis, the second laser pulse of 425 nm was
irradiated with an appropriate delay time. In the time profile of
transient absorption, we can observe the bleaching of the trans-
keto form at 430 nm, with the concomitant recovery of the enol
form at 344 nm under argon as shown in Fig. 8.
Fig. 5 Arrhenius plot for decay constants of HBO tautomer in
methylcyclohexane.
Conformation and absorption spectrum of NBT, a model
tautomer compound
In order to assign the conformation of the model compound
NBT, COSY and NOESY spectra were measured in methanol-
d4. The two peaks at 8.05 and 7.70 ppm showed NOE with
N-CH3 proton (4.29 ppm) indicating that NBT exists in the
trans conformation in the ground state.
The absorption spectrum of trans-NBT appeared with the
maximum wavelength at 466 nm with a molar extinction co-
efficient of 7000 in benzene as shown in Fig. 6. trans-NBT did
not give fluorescence emission in benzene at room temperature.
Fig. 8 Transient absorption spectra (a) and time profiles (b) observed
by two step laser excitation (308 and 425 nm) monitored at 344 (᭺) and
430 nm (•) and by single laser excitation (308 nm) monitored at 430 nm
(ϩ) of HBO in benzene.
Fig. 6 Absorption spectrum of trans-NBT in benzene at room
temperature.
Laser photolysis of trans-NBT
Determination of the quantum yield of formation of the tautomer
form
Laser flash photolysis of trans-NBT was performed in benzene.
Immediately after excitation with a 308 nm laser pulse, bleach-
ing of the absorption maximum at 465 nm and an intense
absorption band at 510 nm were observed as shown in Fig. 7.
The recovery at 465 nm and the decay at 510 nm gave the same
The quantum yield of formation of the tautomer form was
estimated in benzene by comparing the ∆OD value of the
tautomer form of HBO and HBT with ∆OD of the T–T
absorption of optically matched benzophenone solution at
308 nm, assuming that the molar extinction coefficient of the
tautomer was the same as that of trans-NBT. The molar extinc-
tion coefficient of benzophenone in the triplet excited state at
530 nm was reported to be 7220.20 Thus, the quantum yield of
formation of the tautomer was estimated to be 0.2 and 0.4 for
HBO and HBT, respectively at room temperature in benzene.
Transient absorption spectra of BO, BT, MBO, MBT and
quantum yield of intersystem crossing
Transient absorption spectra of BO, BT, MBO, and MBT are
summarized in Fig. 9. The intense band was observed at around
400 nm and the lifetimes of transient absorption spectra were
determined to be 10 µs under argon atmosphere. The transient
absorption spectra were assigned to the enol forms, since they
could not undergo tautomerization.
The triplet energies of these compounds were estimated by
the rate constant of the triplet energy transfer from sensitizer
to quencher. The rate constants (kq) from Michler’s ketone
Fig. 7 Transient absorption spectra of trans-NBT in benzene at room
temperature.
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Table 1 Photochemical properties of benzoxazole and benzothiazole
derivatives at room temperature in benzene
a
b
b
Φf
τf/ns
Φisc
ES
ET
BO
MBO
BT
MBT
HBO
HBT
0.66
0.51
0.01
0.05
1.5
1.2
0.13
0.16
0.57
0.69
0.03
89
85
87
83
84
80
(60)d
66c
6021
(60)d
66c
0.02
5 × 10Ϫ3
(60)d
a 2-Phenylbenzoxazole (Φf = 0.75 in cyclohexane)9 used as a fluor-
escence standard. b In kcal molϪ1
.
c From the quenching rate constant
21
of 4,4Ј-bis(dimethylamino)benzophenone (1) (ET = 65.8 kcal molϪ1
)
triplet by MBO and MBT. d Triplet state of 1 was quenched by BO,
MBT and HBT with a diffusion controlled rate constant. However, the
lifetime of biacetyl triplet (ET = 56 kcal molϪ1) was not affected by BO,
MBT and HBT.
0.01 M and one can expect that triplet energy transfer occurred
within the laser pulse of 20 ns. Triplet energies and quantum
yields of intersystem crossing are summarized in Table 1.
We should mention the previous reports about the lack of
intersystem crossing for HBT and BT. Thus, both HBO and
HBT underwent hydrogen atom transfer to give the cis-keto
form in the excited singlet state. The cis-keto form of HBT did
not undergo intersystem crossing to the triplet state,8 while
HBO underwent intersystem crossing to the triplet state at
room temperature. It was reported that only a weak T–T
absorption spectrum with a maximum at 420–440 nm was
detected at room temperature on direct irradiation of BT.9
Therefore, the lack of intersystem crossing in HBT was
explained in connection with the low efficiency of intersystem
crossing in BT by the importance of torsional motion around
the single bond to the radiationless deactivation.8,9 However, we
have observed the T–T absorption spectra (λmax = 400 nm) of
BT with quantum efficiency as high as 0.6.
Transient absorption spectra of HBO and HBT on triplet
sensitization and the quenching rate constant
The transient absorption spectra observed on benzophenone
and Michler’s ketone sensitization of HBO and HBT on excit-
ation at 366 and 390 nm, respectively at room temperature
under argon atmosphere in benzene are shown in Fig. 10. In
this sensitization experiment, the concentration of HBO and
HBT was adjusted to 0.01 M and that of benzophenone and
Michler’s ketone was adjusted to 0.03 and 0.006 M, respect-
ively. In these experimental conditions, the triplet energy trans-
fer should take place within the duration of the laser pulse and
just after the laser pulse we can observe the triplet state of HBO
and HBT. The lifetime of the triplet state was determined to be
7.4 and 7.5 µs for HBO and HBT, respectively under argon
atmosphere in benzene. These transient absorption spectra were
Fig. 9 Transient absorption spectra of BO, BT, MBO and MBT in
benzene.
(ET = 65.8 kcal molϪ1 21 to HBO and MBO were determined to
)
be 3.7 × 109 and 3.6 × 109 MϪ1 sϪ1, respectively, thus the triplet
energy was estimated to be 66 kcal molϪ1 for both HBO and
MBO by using the Sandros equation (eqn. (1)),28 which can be
applied in the case that the energy transfer process is iso-
energetic or slightly endothermic. The value of ∆Ea stands for
the energy difference between the sensitizer and the quencher
and the kdiff was estimated to be 6.3 × 109 MϪ1 sϪ1 in the
case.15,22–27
quenched by oxygen with the rate constant of 3 × 109 MϪ1 sϪ1
.
The spectral profiles of MBO and MBT are similar to those
of BO and BT, but the absorption maximum slightly shifted to
a longer wavelength and appeared at 420 nm. However, the T–T
absorption spectra of HBO and HBT observed on triplet sensi-
tization (Fig. 10) are different from those of MBO and MBT
(Fig. 9) indicating that the observed triplet states for HBO and
HBT are not simply the enol triplet, but a mixture of the enol
form (3E*) and the cis-keto form (3K*). The observed transient
absorption spectra can be assigned to either the enol form or
the cis-keto form in the triplet state.
kq = kdiff exp(Ϫ∆Ea/RT)/[1 ϩ exp(Ϫ∆Ea/RT)]
(1)
The triplet state of Michler’s ketone was quenched by BO,
MBT and HBT with a diffusion controlled rate constant.
21
However, the lifetime of biacetyl triplet (ET = 56 kcal molϪ1
)
was not affected by BO, MBT and HBT. On the basis of
these results, the triplet energy of the enol form of HBT was
The quenching rate constants for the HBO triplet by cis- (ET
estimated to be ca. 60 kcal molϪ1
.
= 54.3 kcal molϪ1) and trans-stilbene (ET = 49.2 kcal molϪ1) are
The quantum yields of intersystem crossing (Φisc) were
estimated by comparing ∆OD of the T–T absorption spectrum
of an optically matched solution on benzophenone sensitiz-
ation (λex = 360 nm). The concentration of quencher was
determined to be 3.3 × 109 and 4.7 × 109 MϪ1 Ϫ1, respectively
s
for HBO. The quenching rate constants for HBT by cis- and
trans-stilbene are 3.2 × 108 and 1.7 × 109 MϪ1 Ϫ1, respectively
s
and are lower than the values for HBO.
J. Chem. Soc., Perkin Trans. 2, 2002, 1296–1301
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excited state hydrogen atom transfer in the singlet excited state
followed by cis-to-trans isomerization.
Conformation of the tautomer
Since the absorption maximum of trans-NBT, the model
compound of the trans-keto form, was observed at 466 nm and
was shorter than that of cis-NBT, the transient observed on
laser excitation of HBT (Fig. 4b) with the maximum at 465 nm
was reasonably assigned to the trans-keto form. We can also
assign the 430 nm transient observed for HBO (Fig. 4a) to the
trans-keto form of HBO. Two step laser excitation also sup-
ports the assignment of the transient species to the trans-keto
form in the ground state. The cis-keto form could not be
observed since fast reverse hydrogen atom transfer can take
place within our time resolution of 20 ns in the cis-keto form
due to the presence of intramolecular hydrogen bonding. The
quantum yield of trans-keto tautomer formation was thus
estimated to be 0.2 and 0.4 for HBO and HBT, respectively.
Equilibrium constant between enol form and cis-keto form in the
triplet excited state
We can estimate the equilibrium constant (K = [3K*]/[3E*])
by measuring the rate constant of energy transfer from the
transient species to several quenchers.
Fig. 10 Transient absorption spectra of HBO (a) and HBT (b) on
The triplet energies of the enol form and the cis-keto form for
HBO are estimated to be 63.8 and 50.0 kcal molϪ1 and those for
HBT are estimated to be 59.3 and 49.2 kcal molϪ1, respectively
from the phosphorescence spectra.13,16 These values were con-
sistent with the triplet energies estimated from the quenching
rate constant of the triplet excited state of HBO and HBT as
shown in Table 1. Thus, the triplet energies of the enol forms of
HBO and HBT are higher than those of cis- and trans-stilbene,
while those of the cis-keto form are comparable to the triplet
energy of trans-stilbene.
triplet sensitization in benzene.
Discussion
Photochemistry of trans-NBT, as a model compound for the
tautomer of HBT
The result of laser flash photolysis of trans-NBT (Fig. 7)
indicates that trans-NBT underwent photochemical trans-to-cis
isomerization followed by reverse cis-to-trans isomerization
thermally in benzene with the time constant of 110 ns.
The absorption maximum of cis-NBT appeared at 510 nm
which is longer than that of trans-NBT by 50 nm. The change
of the absorption maximum may be due to the change of the
dipole between cis- and trans-NBT and/or change of conjug-
ation character of the N-CH3 group between cis- and trans-
NBT by steric effect.
The observed quenching rate constants of HBT triplet by
trans-stilbene (kq = 1.7 × 109 MϪ1 sϪ1) as well as cis-stilbene (kq =
0.32 × 109 MϪ1 Ϫ1) were smaller than the diffusion controlled
s
rate constant, while the values for HBO quenched by cis-
stilbene are nearly 1/2 of the diffusion controlled rate constant
(kdiff = 6.3 × 109 MϪ1 Ϫ1). Since cis-stilbene can quench the
s
triplet enol form with nearly the diffusion controlled rate con-
stant, but can scarcely quench the triplet cis-keto form, we can
estimate the equilibrium constant between the enol form and
the cis-keto form (K = [3K*]/[3E*]) by using eqn. (2). Thus, K is
estimated to be 0.91 and 20 for HBO and HBT, respectively.
The potential energy surface of isomerization of trans-NBT
is shown in Fig. 11. The absorption spectrum of the ground
kq = kdiff[3E*]/([3E*] ϩ [3K*]) = kdiff/(1 ϩ K)
(2)
Potential energy surfaces of hydrogen atom transfer of HBO and
HBT
HBO and HBT exhibited fluorescence spectra with a large
Stokes shift of 10000 cmϪ1, which were assigned to the tauto-
mer produced by hydrogen atom transfer in the singlet excited
state. In addition, transient absorption spectra with a maximum
wavelength of 430 and 465 nm respectively were observed in
benzene. The fluorescence quantum yield of the tautomer
emission increased with decreasing temperature. This result
indicates that the deactivation pathway with activation energy
exists in the singlet excited state of the tautomer, therefore the
cis-keto form would undergo cis-to-trans isomerization to give
the trans-keto form.
As to the deactivation from the triplet state, one can expect
the cis–trans isomerization of the cis-keto form to give the
trans-keto form as well as in the singlet excited state. If the
trans-keto form was produced through the deactivation pro-
cesses, one could observe its absorption spectra with the
absorption maximum at 430 nm and the lifetime of 7.5 µs.
However, we have observed only the T–T absorption spectra
Fig. 11 Potential energy surfaces of isomerization of trans-NBT in
benzene at room temperature.
state transients observed on laser excitation appearing at
465 nm (Fig. 4) is quite similar to that of trans-NBT. These
results strongly indicate that HBT gave the trans-keto form by
1300
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under oxygen atmosphere as well as argon atmosphere in
benzene at room temperature. Therefore, the deactivation of the
triplet state of HBO and HBT took place to give the enol form
or the cis-keto form: the cis-keto form underwent fast reverse
hydrogen atom transfer within our time resolution (∼20 ns) to
give the enol form. Thus, the diabatic as well as adiabatic cis–
trans isomerization around the double bond in the cis-keto
form did not take place in the triplet state. These results are
summarized in Fig. 12.
transfer within 20 ns. This reverse hydrogen atom transfer to
give the enol form in the ground state was also supported by
the effect of temperature on the lifetime of the ground state
tautomer exhibiting negative Arrhenius activation energy in
methylcyclohexane.
In conclusion, we have revealed the potential energy surface
of hydrogen atom transfer of HBO and HBT in the excited
state.
Acknowledgements
The authors thank Professor H. Itoh, Yamagata University, for
a preliminary PM3 calculation. This work was supported by a
Grant-in-Aid for Scientific Research (No. 10440166) from the
Ministry of Education, Science, Sports and Culture, Japan, by
the Research Foundation for Opto-Science and Technology and
by the Asahi Glass Foundation.
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HBO and HBT underwent excited state intramolecular hydro-
gen atom transfer followed by cis-to-trans isomerization in the
singlet excited state to give the trans-keto tautomer with the
quantum yield of 0.2 and 0.4 for HBO and HBT, respectively.
On the other hand, cis–trans isomerization did not occur in the
cis-keto form in the triplet excited state. These results indicate
that intramolecular hydrogen bonding is broken in the singlet
excited state, but may be present in the triplet excited state.
The trans-keto form produced by ESIPT followed by cis–
trans isomerization in the ground state isomerizes thermally to
the cis-keto form, which undergoes reverse hydrogen atom
J. Chem. Soc., Perkin Trans. 2, 2002, 1296–1301
1301