The effect of meta-substitution on the photochemical properties of benzoxazole derivatives

Article abstract of DOI:10.1246/bcsj.80.561

The photochemistry of 2-(2-hydroxy-3-methoxyphenyl)benzoxazole (3-MHBO) and 2-(2-hydroxy-4-methoxyphenyl) benzoxazole (4-MHBO) have been investigated. The excited-state properties of 4-MHBO were similar to those for the parent compound 2-(2-hydroxyphenyl)

Full text of DOI:10.1246/bcsj.80.561

ꢀ 2007 The Chemical Society of Japan
Bull. Chem. Soc. Jpn. Vol. 80, No. 3, 561–566 (2007)
561
The Effect of meta-Substitution on the Photochemical
Properties of Benzoxazole Derivatives
ꢀ1
Asuka Ohshima,1 Masashi Ikegami,1 Yoshihiro Shinohara,2 Atsuya Momotake,1 and Tatsuo Arai
1Graduate School of Pure and Applied Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba 305-8571
2Research Facility Center for Science and Technology, University of Tsukuba, Tsukuba 305-8571
Received August 2, 2006; E-mail: arai@chem.tsukuba.ac.jp
The photochemistry of 2-(2-hydroxy-3-methoxyphenyl)benzoxazole (3-MHBO) and 2-(2-hydroxy-4-methoxy-
phenyl)benzoxazole (4-MHBO) have been investigated. The excited-state properties of 4-MHBO were similar to those
for the parent compound 2-(2-hydroxyphenyl)benzoxazole (HBO), whereas different properties were observed for
3-MHBO. 4-MHBO in benzene exhibited large Stokes-shifted fluorescence from the keto-form in the excited singlet
ꢀ
state (KE ) after excited-state intramolecular proton transfer (ESIPT), whereas 3-MHBO emitted dual fluorescence
ꢀ
ꢀ
ꢀ
from E and KE species. Fluorescence due to KE for 3-MHBO (ꢀmax ¼ 525 nm) occurred at a considerably longer
wavelength than that of 4-MHBO (ꢀmax ¼ 480 nm). The fluorescence efficiency of 3-MHBO (ꢀf ¼ 0:002) was much
smaller than those of 4-MHBO and HBO (ꢀf ¼ 0:018 and 0.02, respectively). These results are consistent with sup-
ꢀ
pression of ESIPT, stabilization of KE species, and acceleration of non-radiative decay, probably because of the for-
mation of a hydrogen bond between phenol oxygen and methoxy hydrogen at 3-position in 3-MHBO. Thus, 3-MHBO
displayed a meta-substituent effect involving the relaxation pathway of excited state HBO derivatives.
Excited-state intramolecular proton (or hydrogen atom)-
transfer reactions (ESIPT) in hydrogen-bonded molecules have
widely been investigated1 from the view points of the develop-
ment of new functional molecules, such as fluorescent metal
probes,2 laser dyes,3 photostabilizers,4 and organic light-emit-
ting diodes.5 Typical ESIPT molecules preferentially exist as
enol form (E) in their ground state. ESIPT reaction is a photo-
chemical tautomerization, which is triggered upon photoexci-
tation of the ground state E form, that yields an excited keto
form (K ) on the subpicosecond time scale.6 After decaying
ꢀ
to the ground state, K reverts back to the original thermally
stable E (Fig. 1). The structural difference between absorbing
species E and emitting species K in this reaction cycle results
in a large Stokes-shifted fluorescence, which is unusual emis-
sion at longer wavelength from a small molecule and therefore
is not reabsorbed even in high concentrations of a chromo-
phore. However, precise control of the fluorescence properties
of ESIPT is required for specific applications.7
Fig. 1. Energy diagram of enol (E) and keto (K) tautomers
of HBO.
2-(20-Hydroxyphenyl)benzoxazole (HBO) is known to un-
dergo ESIPT. Due to its chemical stability, structural simplic-
ity, and facility for chemical modification,8 the photochemistry
of many HBO analogues has been investigated experimentally
and theoretically.9 Nevertheless, to the best of our knowledge,
the practically important spectral properties that are depend-
ent on the substituent position in HBO analogues have not
been investigated. In this paper, we report the effect of substi-
tution on the photochemical properties, especially in the relax-
ation pathways, for benzoxazole derivatives 2-(2-hydroxy-3-
methoxyphenyl)benzoxazole (3-MHBO) and 2-(2-hydroxy-4-
methoxyphenyl)benzoxazole (4-MHBO). The UV-absorption
and fluorescence spectra of 4-MHBO were similar to those
for the parent compound HBO, whereas different photochem-
ical properties were observed for 3-MHBO.
Experimental
Materials. Schiff bases 1 and 2 were prepared by condensing
o-aminophenol with 2-hydroxy-3-methoxybenzaldehyde for 1 and
with 2-hydroxy-4-methoxybenzaldehyde for 2, in refluxing etha-
nol for 24 h. After the solution was cooled to room temperature,
the crude product was filtered off and washed with cold ethanol,
followed by recrystallization from ethanol to give pure product.
2-Hydroxy-N-(3-methoxysalicylidene)aniline (1): 1H NMR
(CDCl3, 200 MHz, Me4Si) ꢁ 12.58 (1H, brs, OH), 8.70 (1H, s,
CH=N), 7.24–7.16 (2H, m, ArH), 7.08–7.01 (3H, m, ArH), 6.99–
6.90 (2H, m, ArH), 5.97 (1H, brs, OH), 3.94 (3H, s, CH3). Anal.
Calcd for C14H13NO3: C, 69.12; H, 5.93; N, 5.76%. Found: C,
69.09, H, 5.46, N, 5.61%.
2-Hydroxy-N-(4-methoxysalicylidene)aniline (2): 1H NMR
(CDCl3, 200 MHz, Me4Si) ꢁ 12.69 (1H, brs, OH), 8.58 (1H, s,
Published on the web March 13, 2007; doi:10.1246/bcsj.80.561

562 Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
Photochemistry of New Benzoxazole Derivatives
CH=N), 7.32 (1H, d, J ¼ 7:4 Hz, ArH), 7.21–7.10 (2H, m, ArH),
7.02–6.93 (2H, m, ArH), 6.55–6.53 (2H, m, ArH), 5.87 (1H, brs,
OH), 3.86 (3H, s, CH3). Anal. Calcd for C14H13NO3: C, 69.12; H,
5.93; N, 5.76%. Found: C, 69.15; H, 5.47; N, 5.63%.
2-(3-Methoxy-2-hydroxyphenyl)benzoxazole (3-MHBO): A
mixture of 1 (198 mg, 0.814 mmol) and DDQ (430 mg, 1.89
mmol) in chloroform was stirred for 2 h at room temperature
(Scheme 1). After evaporation to remove solvent, the residue
was purified by silica-gel column chromatography (hexane/ethyl
acetate = 10/1 to 100% ethyl acetate) followed by recrystalliza-
tion from ethyl acetate to give a white solid (82 mg, 42%).
1H NMR (CDCl3, 400 MHz, Me4Si) ꢁ 11.70 (1H, s, OH), 7.77–
7.75 (1H, m, ArH), 7.65 (1H, d, J ¼ 8:0 Hz, ArH), 7.63–7.60 (1H,
m, ArH), 7.41–7.37 (2H, m, ArH), 7.05 (1H, d, J ¼ 8:0 Hz, ArH),
6.97 (1H, t, J ¼ 8:0 Hz, ArH), 3.97 (3H, s, CH3). Anal. Calcd for
C14H11NO3: C, 69.70; H, 4.60; N, 5.81%. Found: C, 69.55; H,
4.93; N, 5.88%.
Scheme 1.
2-(4-Methoxy-2-hydroxyphenyl)benzoxazole (4-MHBO):10
A mixture of 2 (211 mg, 0.868 mmol) and DDQ (203 mg, 0.895
mmol) in chloroform was stirred for 2 h at room temperature
(Scheme 1). After evaporation to remove the solvent, the residue
was purified by silica-gel column chromatography (hexane/ethyl
acetate = 10/1 to 100% ethyl acetate) followed by recrystalliza-
tion from ethyl acetate to give a white solid (45 mg, 22%).
1H NMR (CDCl3, 400 MHz, Me4Si) ꢁ 11.64 (1H, s, OH), 7.92
(1H, d, J ¼ 8:8 Hz, ArH), 7.70–7.67 (1H, m, ArH), 7.59–7.56
(1H, m, ArH), 7.37–7.34 (2H, m, ArH), 6.64–6.58 (2H, m, ArH),
3.87 (3H, s, CH3).
Measurements.
1H NMR spectra were measured with a
1
Bruker ARX-400 (400 MHz for H NMR) and Bruker AVANCE
500 (125 MHz for 13C NMR) spectrometer in solution of CDCl3
with tetramethylsilane as an internal standard. UV absorption
and fluorescence spectra were recorded on a Shimadzu UV-1600
UV–visible spectrophotometer and on a Hitachi F-4500 fluores-
cence spectrometer, respectively. Fluorescence lifetimes were de-
termined with Horiba NAES-1100 time-resolved spectrofluorom-
eter. Laser flash photolysis was performed by using an excimer la-
ser (Lambda Physik LPX-100, 308 nm, 20 ns fwhm) as excitation
light sources, and a pulsed xenon arc (Ushio UXL-159) was used
as a monitoring light source. A photomultiplier (Hamamatsu
R-928) and a storage oscilloscope (Iwatsu TS-123) were used
for the detection.
Fig. 2. UV absorption spectra of 3-MHBO (a) and 4-
MHBO (b) in benzene, acetonitrile, and ethanol.
Results and Discussion
Steady-State Absorption Spectra. The shapes of the ab-
sorption spectra of 4-MHBO resemble those of the parent
compound HBO,11 whereas those of 3-MHBO were different
from those of 4-MHBO or HBO. Figure 2 shows the steady-
state absorption spectra of 3-MHBO and 4-MHBO, measured
in benzene, acetonitrile, and ethanol. For 4-MHBO (Fig. 2b),
the absorption band around 330–340 nm was assigned to the
syn-enol form, and the absorption around 319–323 nm are
due to anti-enol form.12 The absorption spectrum of 3-MHBO
(Fig. 2a) is more complex.
The difference in the absorption spectra is obviously a con-
sequence of the methoxy group at the 30-position in 3-MHBO,
which changes the electronic structure, as seen from a com-
parison of the NMR spectra. In 1H NMR spectra, the peak cor-
responding to the methoxy hydrogen (methoxy-H) appeared
at 3.97 and 3.87 ppm for 3- and 4-MHBO, respectively. On
the other hand, the hydrogen at 60-position (60-H) appeared
Fig. 3. Hydrogen bonds in 3- and 4-MHBO.
at 7.65 and 7.92 ppm for 3- and 4-MHBO, respectively
(Fig. 3). The downfield shift of methoxy-H (0.10 ppm) and
the upfield shift of 60-H (0.27 ppm) in 3-MHBO suggests that:
1) a hydrogen bond between the neighboring hydroxy groups
at 20-position formed and 2) mesomeric effect (electron-donat-
ing effect) of 30-methoxy group on 60-H. These effects on the
electronic structure probably cause the considerable difference
in the absorption spectra of 3-MHBO.

A. Ohshima et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
563
Fig. 4. (a) Fluorescence (FL) and fluorescence excitation
spectra (FLE) of 3-MHBO in benzene (solid line, ꢀex
¼
307 nm for FL, ꢀem ¼ 525 nm for FLE) and in ethanol
(dotted line, ꢀex ¼ 304 nm for FL, ꢀem ¼ 388 nm for
FLE), (b) FL and FLE of 4-MHBO in benzene (solid line,
ꢀex ¼ 323 nm for FL, ꢀem ¼ 480 nm for FLE) and in
ethanol (dotted line, ꢀex ¼ 320 nm for FL, ꢀem ¼ 364
nm for FLE).
Fig. 5. Fluorescence spectra of 3-MHBO (a) and 4-MHBO
(b) in methylcyclohexane at various temperatures. The
peak at 584 nm in (a) and 632 nm in (b) are due to the ex-
citation wavelength of 292 and 321 nm, respectively.
Steady-State Fluorescence Spectra. The steady-state
fluorescence spectrum of 4-MHBO in benzene exhibited only
large Stokes-shifted fluorescence with a maximum at 480 nm
due to KE (Fig. 4b), which was again fluorescence behavior
ꢀ
similar to that of HBO. On the other hand, 3-MHBO in ben-
zene showed dual fluorescence at 365 and 525 nm (Fig. 4a).
The 365 nm band is due to E , while the band at 525 nm is as-
ꢀ
signed to KE formed by ESIPT. The substituent effects of the
ꢀ
methoxy group at the 3-position on the fluorescence properties
ꢀ
considerably longer wavelength than that for 4-MHBO or
of HBO cause fluorescence emission from KE , to occur at
HBO and the dual fluorescence in less-polar solvent, such
as benzene. The fluorescence red-shift for KE is probably
ꢀ
because hydrogen bonding between carbonyl oxygen and
ꢀ
methoxy hydrogen stabilized the KE of 3-MHBO. The dual
Fig. 6. Energy diagram of E and K tautomers of 3-MHBO.
fluorescence from 3-MHBO indicates that the methoxy group
at the 3-position works as an electron-donating group and
makes intramolecular hydrogen bonding with OH group,
which suppresses proton transfer in the excited singlet state
in benzene.
The emission spectra of both 3-MHBO and 4-MHBO in
ethanol were totally different from those in benzene. 3-MHBO
exhibited a fluorescence band at 387 nm. The shoulder around
425 nm probably originates from an anion species, observed
only in protic solvents.13 The peak with a large Stokes-shifted
fluorescence for KE was not observed in ethanol probably be-
cause ESIPT in 3-MHBO is suppressed by the solvent mole-
cules. The emission spectrum of 4-MHBO in ethanol consists
of two bands with the maxima at 360 and 473 nm, assigned to
E and KE, respectively. The appearance of the emission band
of E suggests that ESIPT partially inhibited in ethanol.
The fluorescence quantum yields in benzene were 0.002 and
0.018 for 3-MHBO and 4-MHBO, respectively. The value of
the quantum yield for 3-MHBO is determined as sum of dual
fluorescence from E and KE. The lower fluorescence quantum
yield of 3-MHBO is probably because hydrogen bonding be-
tween phenol oxygen and methoxy hydrogen promotes non-
radiative decay.
Figure 5 shows the temperature dependence of the fluores-
cence spectra in methylcyclohexane. The fluorescence intensi-
ty in the range of 480–700 nm for KE of 3-MHBO increased at
lower temperature, whereas the intensity in the range of 350–
400 nm for E was almost independent of temperature (Fig. 4a).
Usually, the rate constants for fluorescence emission are not
affected by temperature. Therefore, the lack of temperature ef-
fects on the fluorescence intensity of E indicates that the rate

564 Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
Photochemistry of New Benzoxazole Derivatives
Fig. 7. Transient absorption spectra of 3-MHBO (a), the decay curve at 420 nm (b) and at 560 nm (c) in benzene. Transient ab-
sorption spectra of 4-MHBO (d), the decay curve at 420 nm (e) and at 560 nm (f) in benzene.
ꢀ
fected by changing temperature. On the other hand, the in-
constant of ESIPT (Fig. 6) for E of 3-MHBO is scarcely af-
M
ꢁ1 cmꢁ1) solution at 308 nm. The molar extinction coeffi-
cients for KZ of 3-MHBO and 4-MHBO at 420 nm were
assumed to be the same as that of Z-6-[N-methyl-2(3H)-ben-
zothiazolylidene]cyclohexa-2,4-dienone (Z-NBT) (" at 420
nm = 7000).14 Thus, the ꢀE!Z values were estimated to be
0.04 and 0.31 for 3-MHBO and 4-MHBO, respectively. The
ꢀE!Z value for 3-MHBO was much lower than that for 4-
ꢀ
crease in the fluorescence intensity from KE with a decrease
in the temperature suggests that the relaxation pathways, espe-
cially, kE!Z is affected by temperature, since this isomeriza-
tion process is expected to have activation barrier.
The fluorescence spectra of 4-MHBO had only the large
Stokes-shifted fluorescence from KE at all temperatures. This
ꢀ
MHBO suggesting that the photoisomerization from KE to
ꢀ
crease in the temperature due to the suppression of the isomer-
fluorescence emission from KE also increased with a de-
KZ in 3-MHBO is suppressed by the intramolecular hydrogen
bond with methoxy hydrogen at the 3-position.
ꢀ
ization from KE with activation barrier.
In order to measure the T–T absorption spectra, 3- and 4-
MHBO were excited at 390 nm in the presence of Michler’s
ketone as a triplet sensitizer in benzene under argon (Fig. 8).
The T–T abosorption spectra were observed in the region from
400–800 nm as broad spectra. The lifetimes of the triplet state
were determined to be 3.4 and 2.3 ms for 3-MHBO and 4-
MHBO, respectively. Previously, we have reported the equi-
Transient Absorption Spectra. The laser transient ab-
sorption spectra of 3-MHBO and 4-MHBO in benzene after
excitation with a 308 nm wavelength laser at room temperature
under argon atmosphere are shown in Fig. 7. Both 3-MHBO
and 4-MHBO had new absorption peaks at 410 nm and at
420 nm, respectively. Since they were not quenched by oxy-
gen, they were ascribed to the ground state KZ tautomer
(Fig. 7b). In addition to the transient band for KZ, a broad sig-
nal over the entire spectral region was observed for both 3-
MHBO and 4-MHBO, which was quenched by oxygen. Thus,
it can be assigned to the excited triplet state (Fig. 7c). Because
the decay curves for 3-MHBO or 4-MHBO under argon in-
clude both KZ and the triplet state species, the curves did
not fit single-exponential function. Thus, the lifetimes for KZ
were measured under oxygen and determined to be 1.6 and
1.7 ms for 3-MHBO and 4-MHBO, respectively.
3
ꢀ
3
ꢀ
librium between E and KE for HBO.14 The existence of
both E and 3KE species has been clarified by using model
compounds, i.e., 2-(2-methoxyphenyl)benzoxazole (MBO) and
2-phenylbenzoxazole (PBO), that do not undergo ESIPT.15 On
excitation in the presence of a triplet sensitizer, both MBO and
PBO exhibit T–T absorption spectra of simple E , whereas
HBO exibit T–T absorption spectra for a mixture of 3E
and 3KE , of which the absorption band was broadened in
the range of 420–700 nm. The similarity of the T–T absorption
spectra of 3- and 4-MHBO strongly indicate that the observed
T–T absorption spectra of 3- and 4-MHBO in Fig. 8 are of a
3
ꢀ
ꢀ
3
ꢀ
ꢀ
ꢀ
The quantum yields of formation of KZ (ꢀE!Z) on the
excitation of E for 3-MHBO and 4-MHBO can be estimated
in benzene by comparing the ꢁO.D. values of the KZ for
3-MHBO and 4-MHBO with that of the T–T absorption
of an optically matched benzophenone (" at 530 nm = 7220
3
ꢀ
3
ꢀ
mixture of E and KE species.
The quantum yields for intersystem crossing (ꢀisc) involv-
ing 3- and 4-MHBO can be estimated by the triplet–triplet en-
ergy transfer from 3- or 4-MHBO to ꢂ-carotene with an opti-

A. Ohshima et al.
Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
565
ꢀ
consequence of the stabilization of KE species and non-radi-
ative decay due to the formation of a hydrogen bond between
phenol oxygen and methoxy hydrogen at 3-position in 3-
MHBO. Thus, 3-MHBO displayed a meta-substituent effect
with respect to the relaxation pathway of excited state HBO
derivatives.
This work was supported by a Grant-in-Aid for Scientific
Research on Priority Areas (417), a Grant-in-Aid for Scientific
Research (No. 16350005) and the 21st Century COE Program
from the Ministry of Education, Culture, Sports, Science and
Technology (MEXT) of the Japanese Government, by Univer-
sity of Tsukuba Research Projects, Asahi Glass Foundation,
and JSR Corporation.
References
1
a) K. Das, N. Nilmoni, A. K. Ghosh, D. Majumdar,
D. N. Nath, K. Bhattacharyya, J. Phys. Chem. 1994, 98, 9126.
b) L. Lavtchieva, V. Enchev, Z. Smedarchina, J. Phys. Chem.
1993, 97, 306. c) K. Law, J. Shoham, J. Phys. Chem. 1994, 98,
3114. d) S. Nagaoka, A. Nakamura, U. Nagashima, J. Photochem.
Photobiol., A 2002, 154, 23.
Fig. 8. Transient absorption spectra of 3-MHBO (a) and 4-
MHBO (b) on excitation at 390 nm in the presence of
Michler’s ketone in benzene under argon.
2
a) A. Sytnik, J. C. Delvalle, J. Phys. Chem. 1995, 99,
13028. b) K. Tanaka, T. Kumagai, H. Aoki, M. Deguchi, S. Iwata,
J. Org. Chem. 2001, 66, 7328. c) A. Ohshima, A. Momotake, T.
Arai, Tetrahedron Lett. 2004, 45, 9377. d) A. Sytnik, M. Kasha,
Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 8627. e) A. Sytnik, D.
Gormin, M. Kasha, Proc. Natl. Acad. Sci. U.S.A. 1994, 91, 11968.
Table 1. Observed and Calculated Spectroscopic Parame-
ters for 3-MHBO and 4-MHBO
ꢀ
fl. max
(ꢀmax)/nm
ꢃ(1KZ)
/ms
ꢃ(3KE
/ms
)
ꢀf
ꢀE!Z
ꢀisc
3
a) P. T. Chou, M. L. Martinez, J. H. Clements, Chem. Phys.
3-MHBO 365, 525
4-MHBO 480
0.002
0.018
1.6
1.7
0.04
0.31
3.4
2.3
0.02
0.03
Lett. 1993, 204, 395. b) D. A. Parthenopoulos, D. McMorrow, M.
Kasha, J. Phys. Chem. 1991, 95, 2668.
4
a) D. B. O’Connor, G. W. Scott, D. R. Coulter, A.
cally matched reference, i.e., a HBO (ꢀisc ¼ 0:03) solution, at
308 nm. Although ꢂ-carotene absorbs light at 308 nm, it did
not produce any transient species upon the excitation at 308
nm on the micro-second timescale. Therefore, one can esti-
mate the ꢀisc values by observing the intensity of T–T absorp-
tion spectra at 540 nm. The estimated values of the quantum
yields of intersystem crossing ꢀisc were 0.02 and 0.03 for 3-
and 4-MHBO, respectively.
Yavrouian, J. Phys. Chem. 1991, 95, 10252. b) J. Keck, H. E. A.
Kramer, H. Port, T. Hirsch, P. Fischer, G. Rytz, J. Phys. Chem.
1996, 100, 14468.
5
S. Kim, D. W. Jang, S. Y. Park, K. Kim, J.-I. Jin, Bull.
Korean Chem. Soc. 2001, 22, 1407.
a) N. P. Ernsting, S. A. Kovalenko, T. Senyushkina,
6
J. Saam, V. Farztdinov, J. Phys. Chem. A 2001, 105, 3443.
b) S. Y. Arzhantsev, S. Takeuchi, T. Tahara, Chem. Phys. Lett.
2000, 330, 83. c) P. T. Chou, Y.-C. Chen, W.-S. Yu, Y.-H.
Chou, C.-Y. Wei, Y.-M. Cheng, J. Phys. Chem. A 2001, 105,
1731. d) T. Fournier, S. Pommeret, J.-C. Mialocq, A. Deflandre,
R. Rozot, Chem. Phys. Lett. 2000, 325, 1671. e) S. Mitra, N.
Tamai, Phys. Chem. Chem. Phys. 2003, 5, 4647. f) O. K.
Abou-Zied, R. Jimenez, E. H. Z. Thompson, D. P. Millar, F. E.
Romesberg, J. Phys. Chem. A 2002, 106, 3665. g) C. Okabe, T.
Nakabayashi, Y. Inokuchi, N. Nishi, H. Sekiya, J. Chem. Phys.
2004, 121, 9436. h) P. T. Chou, S.-C. Pu, Y.-M. Cheng, W.-S.
Yu, Y.-C. Yu, F.-T. Hung, W.-P. Hu, J. Phys. Chem. A 2005,
109, 3777.
ꢀ
The four pathways for relaxation from KE are: fluores-
cence emission, isomerization, intersystem crossing, and non-
radiative decay. From the estimated values of ꢀf, ꢀE!Z, and
ꢀisc for 3- and 4-MHBO, the quantum yield for non-radiative
decay (ꢀnr) was calculated to be 0.94 and 0.64, for 3- and 4-
MHBO, respectively. The efficiency for non-radiative decay
ꢀ
from KE in 3-MHBO is higher than that in 4-MHBO sug-
gesting that the existence of the neighbouring methoxy group
at the 3-position promotes non-radiative decay by forming a
hydrogen bond in the excited singlet state (Table 1).
In summary, the meta-substitution effects on the photo-
chemical properties of benzoxazole derivatives were investi-
gated. The photochemical properties of 4-MHBO were similar
to those for the parent compound, HBO, whereas different
properties were observed for 3-MHBO. The fluorescence peak
7
a) A. S. Klymchenko, A. P. Demchenko, J. Am. Chem. Soc.
2002, 124, 12372. b) P. T. Chou, M. L. Martinez, J. H. Clements,
J. Phys. Chem. 1993, 97, 2618.
8
a) J. M. Kauffman, G. S. Bajwa, J. Heterocycl. Chem.
1993, 30, 1613. b) A. Mordzinski, A. Grabowska, W. Kuehnle,
A. Krowczynski, Chem. Phys. Lett. 1983, 101, 291.
ꢀ
due to KE of 3-MHBO (ꢀmax ¼ 525 nm) appeared at consid-
erably longer wavelength than that of 4-MHBO (ꢀmax ¼ 480
nm). The fluorescence quantum yield of 3-MHBO (ꢀf ¼
0:002) was much smaller than those of 4-MHBO and HBO
(ꢀf ¼ 0:018 and 0.02, respectively). These results may be a
9
a) M. A. Rios, M. C. Rios, J. Phys. Chem. 1995, 99, 12456.
b) J. Seo, S. Kim, S. Y. Park, J. Am. Chem. Soc. 2004, 126, 11154.
c) A. K. Ogawa, O. K. Abou-Zied, V. Tsui, R. Jimenez, D. A.
Case, F. E. Romesberg, J. Am. Chem. Soc. 2000, 122, 9917.

566 Bull. Chem. Soc. Jpn. Vol. 80, No. 3 (2007)
Photochemistry of New Benzoxazole Derivatives
d) G. Yang, F. Morlet-Savary, Z. Peng, S. Wu, J.-P. Fouassier,
Chem. Phys. Lett. 1996, 256, 536. e) K. Tanaka, M. Deguchi, S.
Yamaguchi, K. Yamada, S. Iwata, J. Heterocycl. Chem. 2001,
38, 131.
Phys. 1983, 77, 213. b) H. Wang, H. Zhang, O. K. Abou-Zied, C.
Yu, F. E. Romesberg, M. Glasbeek, Chem. Phys. Lett. 2003, 367,
599.
13 M. M. Henary, C. J. Fahrini, J. Phys. Chem. A 2002, 106,
10 K. Brewster, R. A. Chittenden, J. M. Harrison, T. D. Inch,
C. Brown, J. Chem. Soc., Perkin Trans. 1 1976, 1291.
5210.
14 M. Ikegami, T. Arai, J. Chem. Soc., Perkin Trans. 2 2002,
1296.
15 M. Ikegami, T. Arai, Chem. Lett. 2000, 996.
´
11 A. Mordzinsli, A. Grabowska, Chem. Phys. Lett. 1982, 90,
122.
12 a) G. J. Woolfe, M. Melzig, S. Schneider, F. Dorr, Chem.
¨