Solvent- and concentration-sensitive fluorescence of 2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)benzoxazole

Article abstract of DOI:10.1002/jhet.5570380118

2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)benzoxazole (2) emits the long wavelength fluorescence around 500 nm in nonpolar solvent via the intramolecular proton transfer process in the excited state of 2 (enolform) and also emits the intermediate wavelength fluorescence around 440 nm in polar solvent, which is assumed to originate from the excited state of 2 (anion). The ease of formation of 2 (anion), compared to 2-(2-hydroxyphenyl)benzoxazole (1), is explained by the strongly inductive fluorine atoms. In a solvent with the intermediate polarity, 2 emits both fluorescences and their relative intensity is dependent on the concentration of 2, which is supposed to be caused by the high sensitivity of the intermediate wavelength emission to the concentration quenching.

Full text of DOI:10.1002/jhet.5570380118

Jan-Feb 2001
Solvent- and Concentration-sensitive Fluorescence
of 2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)benzoxazole
131
Kiyoshi Tanaka,* Makoto Deguchi, Satoru Yamaguchi,
Kouji Yamada, and Satoru Iwata
Faculty of Engineering, Seikei University, Musashino, Tokyo 180-8633, Japan
Received April 4, 2000
2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)benzoxazole (2) emits the long wavelength fluorescence around
500 nm in nonpolar solvent via the intramolecular proton transfer process in the excited state of 2 (enol-
form) and also emits the intermediate wavelength fluorescence around 440 nm in polar solvent, which is
assumed to originate from the excited state of 2 (anion). The ease of formation of 2 (anion), compared to
2-(2-hydroxyphenyl)benzoxazole (1), is explained by the strongly inductive fluorine atoms. In a solvent with
the intermediate polarity, 2 emits both fluorescences and their relative intensity is dependent on the concen-
tration of 2, which is supposed to be caused by the high sensitivity of the intermediate wavelength emission
to the concentration quenching.
J. Heterocyclic Chem., 38, 131 (2001).
The photophysics of 2-(2-hydroxyphenyl)benzoxazole
(1) has been widely studied due to its dual emitting
processes including the excited state intramolecular proton
transfer (ESIPT)[1-6]. The excited state of the intramolecu-
larly hydrogen bonded enol rapidly converts to the excited
state of the keto-form, which emits fluorescence around 500
nm and may also decay thermally back to the enol-form.
Second fluorescence around 360 nm is caused by the excited
state of the rotamer which does not exhibit the intramolecu-
lar hydrogen bonding and thus is free to be linked by hydro-
gen bonds with the medium. Dual phosphorescence has
been also observed at low temperature and is via the corre-
sponding triplet states of the enol-form and the keto-
form[7-12]. These interesting photophysical properties, par-
ticularly the large Stokes shifted fluorescence through
ESIPT process, have been highly applied in the fields of las-
ing dyes [13], scintillators [14], and devices [15].
The fluorescence of benzoxazole 1 depends largely on the
nature of the solvent. In nonpolar solvent such as cyclo-
hexane, 1 emits the long wavelength fluorescence (~500
nm) exclusively and the intensity of the short wavelength
fluorescence (~360 nm) gradually increases as the polarity
of the solvent is increased. This solvent effect is explained
by the equilibrium of the ground states of the enol-form and
the rotamer[16,17]. The third fluorescence, with intermedi-
ate wavelength (~440 nm), is described in a limited number
of papers and those indicate this fluorescence is observed in
protic solvents such as alcohol and water[18].
In a continuing research on the reactivity and function of
polyfluorobenzene derivatives[20,21], it was found that
benzoxazole 2 can be prepared by the reaction of 2-(2,3,
4,5,6-pentafluorophenyl)benzoxazole (4) with sodium
hydroxide. It was also found that the product ratio of 2 and
its para-analogue 3 is dependent on the reaction condi-
tions. When the reaction is carried out in dried dioxane, the
ortho-substituted product 2 is produced selectively,
whereas the para-analogue 3 is obtained by adding water
or 18-crown-6 to the reaction mixture (Table 1). These
results show the important role the counter cation (sodium
cation) plays. The transition state is stabilized through
coordination of the sodium cation with the oxazole-nitro-
gen atom and the attacking hydroxide anion, as shown in
Scheme 1[22]. The position of the hydroxy group in 2 and
19
3 was unequivocally determined by the F-nmr analysis.
Table 1
Product Yield and Ratio of Tetrafluorobenzoxazoles 2 and 3
Condition
Solvent
Temp/°C
Time/
hours
Total
Yield/% [a]
Ratio of
2 / 3 [a]
On the other hand, pentafluorophenol is rather acidic
dioxane
dioxane - water (9/1)
dioxane - 18-crown-6
40
40
rt
24
24
100
85
quant.
quant.
95 / 5
<1 / >99
<1 / >99
compared to phenol, with an approximate pK of 5.5 com-
a
pared with that of 10.0 for phenol [19]. As a result,
2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)benzoxazole (2)
is expected to have a high affinity to hydrogen bond. This
plays an important role in the equilibrium between the
enol- and keto-forms, thus leading to an unusual fluores-
cence characteristic of the benzoxazole 2. In this paper,
we will describe the solvent- and concentaration-sensitive
fluorescence of 2.
19
[a] Total yield and ratio of 2 and 3 based on F-nmr analysis.
In nonpolar solvents such as cyclohexane and toluene,
the absorption and fluorescence spectra of tetrafluoroben-
zoxazole 2 are essentially the same as those of the benzox-
azole 1. Slightly blue-shifted absorptions (λ = 295 and
ab

132
K. Tanaka, M. Deguchi, S. Yamaguchi, K. Yamada, and S. Iwata
Vol. 38
Figure 2. Fluorescence Spectra of 2 in a Mixture of Cyclohexane and
316 nm) and the reverse red-shifted fluorescence (λ
=
em
Ethyl Acetate in the Volume Ratio of A, 10:0; B, 8:2; C, 6:4; D, 4:6; E, 2:8;
509 nm with shoulder), compared to those of 1, are notable.
Comparable absorption properties are found for the ethyl
acetate solution of 2. However, its fluorescence properties
are quite different from those in nonpolar solvents.
-6
F, 0:10. Excitation Wavelength; 295 nm, Concentration; 6.4x10 M.
and long wavelength emissions, was noticed in solutions
consisting of various compositions of ethyl acetate and
cyclohexane. As shown in Figure 2, the intensity of the
intermediate wavelength emission increases as the ratio of
ethyl acetate to cyclohexane increases, that is, as the sol-
vent polarity increases. A characteristic of tetrafluoroben-
zoxazole 2 is that the shorter wavelength emission around
360 nm, which is detected in the case of the benzoxazole 1,
is not observed in these solvents. However the intermedi-
ate wavelength emission around 440 nm is clearly
observed in aprotic solvents such as dimethyl sulfoxide,
acetonitrile, and even in ethyl acetate.
2-(4,6-Difluoro-2-hydroxyphenyl)benzoxazole (5),
prepared by the nucleophilic aromatic substitution of 2-
(2,4,6-trifluorophenyl)benzoxazole (6) with sodium
hydroxide, was then investigated in order to determine the
electronic effects on absorption and fluorescence proper-
ties. Figure 3 indicates that the spectral properties, particu-
larly the fluorescence behavior of 5, are just intermediate
between those of 1 and 2, with the exception of the long
wavelength emission around 470 nm with 500 nm shoul-
der. A shorter wavelength emission around 360 nm is defi-
nitely detected in ethyl acetate, which shows a similar
trend to that of 1, whereas in more polar solvent such as
acetonitrile the intermediate wavelength emission is
observed, similar to that of 2.
Excitation in the main absorption band (λ = 295 nm)
ex
gave rise to the clearly detectable emission in the 433.5 nm
band, together with the long wavelength emission. The
intermediate wavelength emission was exclusively
observed in more polar solvents such as acetonitrile and
dimethyl sulfoxide. In the absorption spectra of the ace-
tonitrile solution of 2, a weak plateau at around 360 nm is
detected (Figure 1). It should also be noted that excitation
at 360 nm only gave rise to the intermediate wavelength
emission and moreover that the excitation spectra showed
a correlation between the intermediate wavelength emis-
sion and the 360 nm absorption. A pronounced solvent
polarity-dependence, of the intensity of the intermediate
The fluorescence quantum yields of benzoxazoles 2 and
5 measured in cyclohexane and in acetonitrile are summa-
rized in Table 2. To determine the effect of the introduced
fluorine atoms, quantum yields of benzoxazole 1 were also
measured under similar conditions, to give Φ = 0.019
(cyclohexane) and 0.012 (acetonitrile), respectively. The
introduction of fluorine atoms to the 2-phenyl group was
found to bring about a decrease in fluorescence quantum
Figure 1. Absorption and Fluorescence Spectra of 2 in Cyclohexane, in
Ethyl Acetate, and in Acetonitrile. Excitation Wavelength; 295 nm,
-5
-5
Concentration; 9.6x10 M for Absorption Spectra and 2.4x10 M for
Fluorescence Spectra.

Jan-Feb 2001
2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)benzoxazole
133
outstanding quenching of the intermediate wavelength
emission, compared to that of the long wavelength emis-
sion, is observed in acetonitrile solution.
Figure 3. Absorption and Fluorescence Spectra of 5 in Cyclohexane, in
Ethyl Acetate, and in Acetonitrile. Excitation Wavelength; 289 nm,
-5
Concentration; 9.6x10-5 M for Absorption Spectra and 2.4x10 M for
Fluorescence Spectra.
yield. A similar trend is realized in the starting benzoxa-
zoles; that is, 4 (Φ= 0.152 in cyclohexane) and 6 (Φ= 0.056
in cyclohexane) are substantially diminished, compared
with 2-phenylbenzoxazole (Φ= 0.786 in cyclohexane)[23].
Figure 4. Concentration Dependence of Absorption Spectra of 2 in
-6
-6
-5
Acetonitrile. Concentration: A, 3.0x10 M; B, 6.0x10 M; C, 1.2x10
Table 2
-5
-5
-5
M; D, 2.4x10 M; E, 4.8x10 M; F, 9.6x10 M.
Fluorescence Quantum Yield of 2-(2-Hydroxyphenyl)benzoxazoles 2 and 5
Quantum Yield/Φ [a]
Benzoxazole [b]
Cyclohexane
Acetonitrile
2
5
0.004
0.015
0.005
0.005
[a] Quantum yield was calculated on the basis of quinine in 0.5 M-H SO
2
4
aq (Φ = 0.51, λ = 299 nm); [b] Quantum yields of 1 were estimated to
ex
be Φ = 0.019 in cyclohexane and Φ = 0.012 in acetonitrile, respectively,
under the similar conditions.
A concentration dependence for benzoxazole 2 on the
spectral properties is very notable. The apparent validity of
Beer's law is realized in nonpolar solvents, whereas the
nonvalidity of Beer's law is noticed in acetonitrile, which is
due to the additional absorption band around 360 nm that
appears in acetonitrile. Its relative absorption compared to
that of the short wavelength bands is considerably large in
high dilution and diminishes gradually as the concentration
increases, and the absorption spectra become essentially
identical with those in nonpolar solvent (Figure 4)[24,25].
Concentration dependence of the emission-wavelength is
not observed in cyclohexane and acetonitrile, but interest-
ingly it is in ethyl acetate. The intermediate wavelength
emission, which is main emission at high dilution, seems to
be red-shifted gradually as the concentration is increased
(Figure 5). However, the change in the fluorescence spectra
may be interpreted by the difference in concentration
dependence of the fluorescence quantum efficiency of the
intermediate and long wavelength emissions. Indeed the
Figure 5. Concentration Dependence of Fluorescence Spectra of 2 in
Ethyl Acetate.Excitation Wavelength; 295 nm, Concentration: A,
-6
-6
-6
-5
-5
1.5x10 M; B, 3.0x10 M; C, 6.0x10 M; D, 1.2x10 M; E, 2.4x10
-5
-5
-4
M; F, 4.8x10 M; G, 9.6x10 M; H, 1.9x10 M.
The intermediate wavelength fluorescence emitted in apro-
tic polar solvents is characteristic of the fluorine substituted
benzoxazole 2. For benzoxazole 1, a similar emission is found
in protic solvents such as alcohols and water[16,18]. Species,

134
K. Tanaka, M. Deguchi, S. Yamaguchi, K. Yamada, and S. Iwata
Vol. 38
from which this emission originates, has been discussed for 1
and its keto isomer with trans-configuration of the quinone-
carbonyl group toward the NH group has been suggested[26].
However, its possibility may be ruled out, because reported
transient absorption spectroscopies show that the trans- and
cis-keto isomers absorb around 430 nm[27,28].
emits the intermediate wavelength fluorescence on excita-
tion of the 360 nm band. The same excitation state is formed
from the excited state of 2 (rotamer). The ease of formation
of 2 (anion) from 2 (rotamer) is due to the strongly inductive
fluorine atoms and is the reason why the shorter wavelength
emission from the excited state of 2 (rotamer) is not detected
in the case of 2. Concentration dependence of the fluores-
cence is assumed to be caused by the formation of an aggre-
gate of 2 (rotamer) and 2 (anion). The aggregate has no fluo-
rescence property and its formation is accelerated on
increasing their concentration (Scheme 2). It should be
added that the absorption spectra of 2 (enol-form), 2
(rotamer), and the aggregate are essentially identical and
cannot be distinguished from each other.
In conclusion, the benzoxazole 2 emits the long wavelength
fluorescence around 500 nm in nonpolar solvent via the
ESIPT process in the excited state of 2 (enol-form) and also
emits the intermediate wavelength fluorescence around 440
nm in polar solvent, which is assumed to originate from the
excited state of 2 (anion). In a solvent with the intermediate
polarity, 2 emits both fluorescences and their relative intensity
is dependent on the concentration of 2, which is supposed to
be caused by the high sensitivity of the intermediate wave-
length fluorescence to the concentration quenching.
The intermediate wavelength emission of 2 has the fol-
lowing features summarized as; a) its intensity is large in
polar solvents, and the wavelength is red-shifted with
increase in polarity as shown by the order of 433.5 nm in
ethyl acetate, 437.0 nm in acetonitrile, and 441.0 nm in
dimethyl sulfoxide, suggesting that it is the emission from
the excited state of an ionic molecule; b) the emission is
observed on excitation in the main absorption band of 295
nm and in the long wavelength absorption band of 360 nm;
c) the relative absorption of the 360 nm band is large in high
dilution in a polar solvent and diminishes gradually on
increasing concentration of 2, accompanied with a substan-
tial emission quenching; and d) similar ionic entity
absorbing in the 360 nm band is observed in the presence of
large amount of diazabicyclo[2.2.2]octane (DABCO) even
in nonpolar solvent and forms a 1:1 complex with DABCO.
On the basis of these features, the phenolate anion 2 (anion)
is assumed as the ionic molecule [29], because the fluo-
rophenol moiety of 2 is rather acidic and is expected to be
more acidic in the excited state [19]. Thus, the following
scenario will be assumed; the isomer 2 (rotamer), which is
hydrogen-bonded to solvent molecules under the polar envi-
ronment, equilibrates with 2 (anion). The molecule 2 (anion)
absorbs in the 360 nm band and its excitation state 2*(anion)
EXPERIMENTAL
The ir spectra were recorded on a JASCO A-100 spectrometer
1
and samples were run as potassium bromide pellets. The H- and
19
F-nmr spectra were measured with a JEOL JNM-LA400 (400
Scheme 2

Jan-Feb 2001
2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)benzoxazole
135
1
19
Compound 3 has mp 220 °C (dec.); ir: 3450 (OH) and 1650
MHz for H nmr and 376 MHz for F nmr) spectrometer in solu-
tions of deuteriochloroform. The chemical shifts are given in
δ/ppm downfield from tetramethylsilane as an internal standard
-1
1
cm (C F ); H nmr: δ 7.76 (1H, m), 7.68 (1H, m), and 7.45 (2H,
6
4
19
+
m); F nmr: δ 23.5 (2F, m) and 0.30 (2F, m); ms: m/z 283 (M ,
100%), 255 (10.6%), 92 (4.3%), 64 (12.0%), and 63 (12.3%).
Anal. Calcd for C H NF O : C, 55.12; H, 1.78; N, 4.95.
1
for H-nmr and from hexafluorobenzene as an external standard
for F-nmr, respectively; J values are given in Hz. Product ratios
were evaluated by F-nmr analysis. The uv-visible and fluores-
19
13
5
4 2
19
Found: C, 54.76; H, 1.85; N, 4.94.
cence spectra were recorded with JASCO Ubest-50 spectrometer
and JASCO FP-777 spectrometers, respectively. The ms spectra
were acquired using a HITACHI M-80B mass spectrometer.
Acetonitrile, ethyl acetate, cyclohexane, and dimethyl sulfoxide
for spectroscopy were the highest quality from KOKUSAN
Chemical Co. and were used without further purification.
Preparation of 2-(2,4,6-Trifluorophenyl)benzoxazole (6).
A mixture of 2-aminophenol (0.31 g, 2.84 mmol) and 2,4,6-tri-
fluorobenzoic acid (0.50 g, 2.84 mmol) in 25 g of polyphosphoric
acid was stirred at 150 °C for 1 hour. After the mixture was
cooled to room temperature, ice-water was added to the mixture.
The mixture was extracted with ethyl ether and the extracts were
washed with water and brine, and dried over magnesium sulfate.
The extracts were evaporated to leave a solid which was chro-
matographed on silica gel (hexane/ethyl acetate, 10:1) and then
purified by recrystallization from hexane to give a white solid
(0.26 g, 37% yield) of 6: mp 115-117 °C; ir: 3100 (CH) and 1630
Preparation of 2-(2,3,4,5,6-Pentafluorophenyl)benzoxazole (4).
Triethylamine (5.60 g, 55.4 mmol) was added dropwise to a
solution of 2-aminophenol (5.00 g, 45.9 mmol) and pentafluo-
robenzoyl chloride (12.8 g, 55.5 mmol) in 100 mL of ethyl acetate
and the mixture was stirred at room temperature for 7 hours. To the
mixture was added 10 mL of aqueous sodium hydroxide (1 M) and
the mixture was stirred at room temperature for 12 hours. The mix-
ture was extracted with ethyl acetate and the extracts were washed
with water and brine, dried over magnesiun sulfate, and evaporated
to leave a solid (14.0 g). The otained crude solid (6.50 g) was
mixed with diphosphorus pentaoxide (7.59 g, 53.5 mmol) and the
mixture was heated at 175 °C for 1 hour. After the mixture was
cooled to room temperature, ice-water was added to the mixture.
The mixture was extracted with ethyl ether and the extracts were
washed with aqueous sodium hydroxide (0.25 M), water, and brine
and dried over magnesiun sulfate. The extracts were evaporated to
leave a solid which was recrystallized from hexane to give white
crystals (3.70 g, 61% yield based on 2-aminophenol) of 4: mp 110-
-1
1
cm (C F ); H nmr: δ 7.87 (1H, m), 7.63 (1H, m), 7.42 (2H, m),
6
3
19
and 6.87 (2H, m); F nmr: δ 60.78 (1F, tt, J = 9.0 and 9.0 Hz)
and 57.71 (2F, m); uv (cyclohexane): λ (log ε) 285 nm (4.35).
max
Anal. Calcd for C H NF O: C, 62.64; H, 2.43; N, 5.62.
13
6
3
Found: C, 62.67; H, 2.30; N, 5.59.
Preparation of 2-(4,6-Difluoro-2-hydroxyphenyl)benzoxazole (5).
A mixture of 6 (0.10 g, 0.40 mmol) and crushed sodium hydrox-
ide (0.08 g, 2.0 mmol) in dried dioxane (20 mL) was stirred at 40
°C for 2.5 hours and then refluxed for another 2.5 hours. By using
a similar procedure as that described above, a crude solid was
obtained that was chromatographed on silica gel (hexane/ethyl
acetate, 20:3) to give 0.10 g (100%) of 5. Purification was per-
formed by recrystallization from hexane. Compound 5 has mp
-1
1
112 °C; ir: 1650 cm (C F ); H nmr: δ 7.89 (1H, m), 7.65 (1H,
6
5
-1
144-146 °C; ir: 3100-2600 (OH), 3100 (CH) and 1640 cm
19
m), and 7.45 (2H, m); F nmr: δ 25.49 (2F, ddd, J= 24.1, 10.5, and
4.5 Hz), 13.84 (1F, tt, J = 20.7 and 4.5 Hz), and 2.03 (2F, m); uv:
1
(C F ); H nmr: δ 12.63 (1H, s), 7.74 (1H, m), 7.67 (1H, m), 7.42
6
2
(2H, m), 6.66 (1H, dt, J = 10.0 and 2.2 Hz), and 6.53 (1H, ddd, J =
(cyclohexane) λ
(log ε) 287 nm (4.19). These spectral data are
max
19
10.7, 10.0, and 2.2 Hz); F nmr: δ 60.13 (1F, td, J = 10.0 and 10.7
consistent with those previously reported [30].
Hz) and 55.83 (1F, td, J = 10.7 and 2.2 Hz).
Preparation of 2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)benz-
oxazole (2).
Anal. Calcd for C H NF O : C, 63.15; H, 2.86; N, 5.67.
13
7
2 2
Found: C, 63.02; H, 2.82; N, 5.61.
Crushed sodium hydroxide was used for this reaction. A mix-
ture of 4 (0.10 g, 0.35 mmol) and sodium hydroxide (0.07 g, 1.75
mmol) in 10 mL of dioxane, which was dried over calcium chlo-
ride, was stirred at 40 °C for 24 hours. The mixture was acidified
with 1 M of hydrochloric acid and extracted with ethyl acetate.
The extracts were washed with water and brine, dried over mag-
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19
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13
5
4 2
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