Novel fluorescent 2-(2-aminofluorophenyl)benzoxazoles: Syntheses and photophysical properties

Article abstract of DOI:10.1002/jhet.5570390629

2-(2-Amino-3,4,5,6-tetrafluorophenyl)benzoxazole (2) absorbs in long wavelength band (λ_abs^max = 346 nm in methanol) and in the normal wavelength band (λ_abs^max = 285.5 nm), and emits blue fluorescence. The emissi

Full text of DOI:10.1002/jhet.5570390629

Nov-Dec 2002
Novel Fluorescent 2-(2-Aminofluorophenyl)benzoxazoles:
Syntheses and Photophysical Properties
1299
Kiyoshi Tanaka* and Tomohiro Komiya
Faculty of Engineering, Seikei University, Musashino, Tokyo 180-8633, Japan
Received May 7, 2002
max
2-(2-Amino-3,4,5,6-tetrafluorophenyl)benzoxazole (2) absorbs in long wavelength band (l abs
= 346
nm in methanol) and in the normal wavelength band (l absmax = 285.5 nm), and emits blue fluorescence. The
emission intensity is highly affected by the solvent polarity and is large in a polar solvent such as methanol.
2-(2-Pentafluorobenzamido-3,4,5,6- tetrafluorophenyl)benzoxazole (5) emits green fluorescence along with
the short wavelength emission around 380 nm and their relative intensity depends on the solvent polarity.
Green fluorescence is enhanced in nonpolar solvents such as chloroform and toluene, resulting in the con-
siderably large Stokes shift.
J. Heterocyclic Chem., 39, 1299(2002).
Photophysical properties of 2-(2-hydroxyphenyl)ben-
atom and oxazole-nitrogen atom in the intermediate
Meisenheimer complex, compared to the corresponding
interaction in the case of pentafluoronitrobenzene[14]. It
is interesting that the amino substituted benzoxazole 2
accepts the second nucleophilic substitution with methox-
ide at the para-position to produce the disubstitued ben-
zoxazole 6. Electrophilic substitution of 2 with acetic
anhydride or pentafluorobenzoyl chloride takes place at
the amino group to give the corresponding amide benzoxa-
zole 4 or 5. The selective introduction of the substituted
amino group into theortho-position is attained by the reac-
tion with 2-(2,3,5,6-tetrafluoro-4-methoxyphenyl)benzox-
azole (7), which is selectively obtained by the reaction of 1
with sodium methoxide in methanol. This strategy was
applied to the preparation of 8, 9, and 10.
zoxazole have been widely explored because of its dual
fluorescence including the excited state intramolecular
proton transfer (ESIPT)[1-7]. The excited state of the
intramolecularly hydrogen bonded enol rapidly converts to
the excited state of the keto-form, which emits fluores-
cence around 500 nm. Large Stokes shifted fluorescence
through ESIPT process has been highly applied in the
fields of lasing dyes, scintillators, and devices[8-10].
Photophysics of 2-aminophenyl analogue, 2-(2-
aminophenyl)benzoxazole, has been also investigated and
emission (415 nm in methanol) with large fluorescence
quantum yield, which is not from ESIPT process, are clar-
ified[11]. ESIPT process in 2-(2-aminophenyl)benzoxa-
zoles is limited to a few cases where strongly electron-
withdrawing group such as sufonyl group is attached to the
amino group[12,13]. In a continuing research on the reac-
tivity and the function of polyfluorobenzene deriva-
tives[14,15], we have reported photophysical properties of
2-(2-hydroxy-3,4,5,6-tetrafluorophenyl)benzoxazole
which emits not only the long wavelength fluorescence
around 500 nm in nonpolar solvent via ESIPT process but
also the intermediate wavelength fluorescence around 440
nm in polar solvent through the excited state of phenolate
anion, the easy formation of which is explained by the
strongly inductive fluorine atoms[16,17]. In this paper, we
focus on fluorinated 2-amino analogues, 2-amino or amide
substituted 2-(3,4,5,6-tetrafluorophenyl)benzoxazoles and
describe their convenient syntheses and photophysical
properties.
The absorption spectra of 2 and 3 are essentially the
same as those of the corresponding non-fluorinated 2-(2-
aminophenyl)benzoxazole; that is, the absorption of 3 is
max
notable at l
= 305 nm in methanol whereas, in the
abs
max
case of 2, a large red-shifted band at l
= 346 nm in
abs
addition to the normal absorption band is observed. It is
interesting that the red-shifted band observed in 2 is not
present in 4, N-acetyl substiuted 2, which absorbs in the
normal wavelength band. The fluorescent wavelength of 2
and 3 is comparable to the corresponding non-fluorinated
(aminophenyl)benzoxazoles. Excitation of 2 in both nor-
mal and red-shifted absorption bands (l = 285.5 and 346
ex
nm) gave rise to the same blue emission at 426 nm (Figure
1). It is noted that the excitation spectrum showed a corre-
spondence between the blue emission and the long wave-
length absorption. The shorter wavelength emission was
observed in 3, and its fluorescence quantum yield was
evaluated to be very small (F = 0.002 in methanol)[20],
which is in sharp contrast to almost unity of 2-(4-
aminophenyl)benzoxazole. The small fluorescence quan-
tum yield of 3 is ascribed to the fluorine atoms substituted
at the ortho-positions of 2-phenyl group [16]. The fluores-
cence quantum yield of 2 is almost thirty times that of 3
but still smaller than that of 2-(2-aminophenyl)benzoxa-
zole, the quantum yield of which is reported to be F = 0.75
Since the preferential ortho-attack is reported in the sub-
stitution of pentafluoronitrobenzene with ammonia under
mild conditions [18,19], the reaction of 2-(2,3,4,5,6-penta-
fluorophenyl)benzoxazole (1) with ammonia was carried
out to obtain 2-(2-amino-3,4,5,6-tetrafluorophenyl)ben-
zoxazole (2). Yield of the reaction was moderately good,
but the product ratio of ortho-substituted benzoxazole 2
and para-analogue 3 was 13/87. The decrease in ortho-
selectivity in the reaction of 1 seems to reflect the rela-
tively weak interaction between the ammonium hydrogen

1300
K. Tanaka and T. Komiya
Vol. 39
in methanol (Table 1)[11]. Large solvent-dependence of
blue emission of 2 was noticed, although its absorption
spectra in various solvents showed no significant change
(Figure 2). It is clearly depicted that, in a polar solvent
such as mehanol, the emission intensity is large and the
wavelength is red-shifted, compared to those in nonpolar
solvents such as toluene and ethyl acetate.
Effect of N-alkyl group on the absorption and fluores-
cence spectra was next investigated in the family of 2-
(2-amino-3,5,6-trifluoro-4-methoxyphenyl)benzoxazole
(6). The absorption and fluorescence behavior of 6 is
similar to that of 2 except for the relatively large flu
o-
rescence quantum yield (F = 0.11 in methanol). The
introduced propyl or L-alanyl group causes slight red-
shifts of the normal and long wavelength absorption
bands. However, the introduction of two propyl groups
brings about the outstanding change in the absorption
and, particularly, the long wavelength absorption band
is almost disappeared and is shown as only a weak
plateau. The similar red-shifts of the emissions, with
the decrease in the fluorescence quantum yields, are
caused by the introduction of propyl and L-alanyl
groups. More appreciably red-shifted emission, appear-
ing as green light, is observed in dipropylaminobenzox-
azole 10 (Figure 3).
Figure 1. Absorption and Fluorescence Spectra of 2 (¡), 3 (o), and 4 ( x
) in Methanol. [2] = [3] = [4] = 1x10 M, Excitation Wavelength; 285.5
nm (2), 305 nm (3), and 287 nm (4).
-5

Nov-Dec 2002
Novel Fluorescent 2-(2-Aminofluorophenyl)benzoxazoles
1301
Table 1
Fluorescence Quantum Yields of Benzoxazoles [a]
Benzoxazole
2
3
4
5
6
8
9
10
Quantum Yield [b]
0.062
0.002
0.003 [c]
0.031 [c]
0.11
0.025
0.002
0.010
(0.001 [d])
(0.017 [d])
[a] Quantum yield was calculated on the basis of 2-phenylbenzoxazole in cyclohexane (f = 0.78, l = 300 nm), see ref. 20; [b] Measured in methanol,
ex
unless otherwise noted; [c] Quantum yield as total emission in chloroform; [d] Quantum yield as total emission in toluene.
Figure 2. Absorption and Fluorescence Spectra of 2 in Methanol (¡), Ethyl
-5
Acetate (o), and Toluene ( x ). [2] = 1x10 M, Excitation Wavelength;
Figure 4. Absorption and Fluorescence Spectra of 5 in Methanol (¡),
285.5 nm (Methanol), 286 nm (Ethyl Acetate), and 288 nm (Toluene).
Acetonitrile (¯), Ethyl Acetate (o), Toluene ( x ), and Chloroform (r).
-5
[5] = 1x10 M, Excitation Wavelength; 286.5 nm (Methanol), 290.5 nm
(Ethyl Acetate), 283 nm (Acetonitrile), 304.5 nm (Toluene), and 300 nm
(Chloroform).
length emission at l = 380.5 nm along with the long wave-
em
length one at l
= 522.5 nm, the intensity of the former
em
being larger than that of the latter in methanol. The relative
intensity of the both emissions is found to be highly depen-
dent on the solvent polarity. The long wavelength emission
is enhanced and the short one is reversely quenched, with a
decrease in the polarity of the solvent (Figure 4). The similar
trend of the relative fluorescence intensity is observed in 4
but its change is quite small. The considerably large Stokes
shift of 5 in the solvent with small polarity such as chloro-
form or toluene may come from the ESIPT process, which is
accelerated by the strong intramolecular hydrogen-bonding
of the amide hydrogen atom of 5 with the oxazole nitrogen
atom in less polar solvent [21]. The fluorescence quantum
yields as total emission of 4 and5 are summarized in Table 1,
and it is noted that 5 emits green light at the solid state, as
observed with 2-(2-hydroxyphenyl)benzoxazoles.
Figure 3. Absorption and Fluorescence Spectra of 6 (¡), 8 (¯), 9 (o),
and 10 ( x ) in Methanol. [6] = [8] = [9] = [10] = 1x10 M, Excitation
Wavelength; 289.5 nm (6), 293.5 nm (8), 292.5 nm (9), and 248.5 nm (10).
-5
The absorption spectrum of pentafluorobenzamido-
phenylbenzoxazole 5 shows only the normal absorption
without a long wavelength one, which is a similar trend to
that of acetamidophenylbenzoxazole 4, and its absorption
band is slightly red-shifted as the solvent polarity decreases.
The fluorescence spectrum of 5 indicates the short wave-
Emission of 2-(2-aminophenyl)benzoxazole is reported
to be sensitive to a pH change of surrounds and reveal a
large blue-shift with considerable quenching in acidic con-
ditions, which is due to the formation of its monocation
[11]. So the effect of pH on the fluorescence spectra along
with absorption spectra of 2 was investigated in

1302
K. Tanaka and T. Komiya
Vol. 39
methanol/water (9:1) with the constant ionic strength (0.1
M NaCl). Observation was performed at pH 1 and pH 13
and, however, 2 showed no change in fluorescence and
absorption, which pointed out its high acid and alkali resis-
nonpolar solvents, which supports that green emission comes
from the ESIPT process caused by the strong intramolecular
hydrogen-bonding formation. The ESIPT process along
with the relatively short wavelength absorption of 5 gives
rise to the considerably large Stokes shift.
tance in fluorescence behavior.
In the case of 2, a large red-shifted band at l
max
= 346
abs
nm along with the normal absorption band is observed.
EXPERIMENTAL
max
The similar red-shifted band at l
= 354 nm
abs
(methanol) is reported in 2-(2-aminophenyl)benzoxazole
and this is suggested by a result of intramolecular hydro-
gen-bonding formation between the amino hydrogen atom
and the benzoxazole nitrogen atom [11]. This hydrogen-
bonding is also supported by our results that the N-mono-
substituted aminobenzoxazoles 8 and 9 have a similarl red-
shifted band whereas the N,N- disubstituted amino-
benzoxazole 10 has no such a band. Based on the consider-
ation of the absence of a red-shifted band in amidobenzox-
azoles 4 and 5, where the strong intramolecular hydrogen-
bonding formation is expected, a red-shifted band is limited
to aminobenzoxazoles capable of both intramolecular
hydrogen-bonding formation and polarization between two
nitrogens, as depicted in Scheme 2. Blue emission is
thought to come from the polarized excited state. On the
other hand, the ESIPT process leading to green emission is
caused by the strong intramolecular hydrogen-bonding for-
mation of the amido hydrogen atom of 5.
The ir spectra were recorded on a JASCO A-100 spectrometer
and samples were run as pottasium bromide pellets, unless other-
1
wise noted. The H and 19F nmr spectra were measured with a
JEOL JNM-LA400 (400 MHz for 1H nmr and 376 MHz for 19
F
nmr) spectrometer for solutions of CDCl3. The chemical shifts
are given in d/ppm downfield from tetramethylsilane as an inter-
nal standard for 1H nmr and from trifluoroacetic acid as an exter-
nal standard for 19F nmr, respectively; J values are given in Hz.
The uv-visible and fluorescence spectra were recorded with
JASCO Ubest-50 and JASCO FP-777 spectrometers, respec-
tively. Acetonitrile, ethyl acetate, cyclohexane, chloroform, and
methanol for spectroscopy were the highest quality from KOKU-
SAN Chemical Co. and were used without further purification.
Benzoxazole 1 was prepared by our previously reported method
and 7 was obtained by the reaction of 1 with sodium methoxide in
methanol [16,17].
Preparation of 2-(2-Amino-3,4,5,6-tetrafluorophenyl)benzoxa-
zole (2) and Its 4-Aminophenyl Analogue 3.
Ammonia gas was bubbled into a solution of 1 (4.0 g, 14.0
mmol) and triethylamine (10.0 g, 0.10 mol) in 50 cm3 of dioxane
at room temperature for 14 days. The mixture was extracted with
ethyl acetate and the extracts were washed with water and brine,
dried over sodium sulfate, and evaporated to leave a residue.
Chromatography on silica gel (hexane-ethyl acetate, 5/1) of the
residue produced 0.55 g (14%) of 2 and 2.88 g (73%) of 3. Each
compound was then purified by recrystallization from hexane-
ethyl acetate to give white crystals.
Compound 2 has mp 172-173 °C; ir: 3490, 3310 (NH2), 1660
(C6F4), and 1250 cm-1 (CF); 1H nmr: d = 7.76 (1H, m), 7.64 (1H,
m), 7.40(2H, m), and 6.50 (2H, br.s); 19F nmr: d = -61.2 (1F, ddd,
J=20.3, 10.2, and 6.0 Hz), -74.8 (1F, td, J=20.3 and 6.0 Hz), -84.9
(1F, ddd, J=20.3, 10.2, and 7.1 Hz), and -92.8 (1F, td, J=20.3 and
7.1 Hz).
Anal. Calcd for C13H6F4N2O: C, 55.33; H, 2.14; N, 9.93.
Found: C, 55.06; H, 1.91; N, 9.81.
Compound 3 has mp 195-196 °C; ir: 3460, 3400, 3320, 3200
(NH2), 1670, 1650 (C6F4), and 1240 cm-1 (CF); 1H nmr: d = 7.85
(1H, m), 7.62 (1H, m), 7.35 (2H, m), and 4.44 (2H, br.s); 19F
nmr: d = -62.3 (2F, A2X2) and -83.8 (2F, A2X2).
Anal. Calcd for C13H6F4N2O: C, 55.33; H, 2.14; N, 9.93.
Found: C, 55.18; H, 1.83; N, 9.78.
In conclusion, a number of fluorinated (2-amino- and
amidophenyl)benzoxazoles are synthesized through the
nucleophilic substitutions of the pentafluorophenyl group
and their photophysical properties are disclosed.
Aminobenzoxazole 2 absorbs in long wavelength band
max
(l
= 346 nm in methanol) and emits blue fluores-
abs
Preparation of 2-(2-Acetamido-3,4,5,6-tetrafluorophenyl)benz-
cence. The red-shifted absorption is ascribed to both the
intramolecular hydrogen-bonding of the amino hydrogen
atom with the benzoxazole nitrogen atom and the polariza-
tion between two nitrogen atoms, its blue emission being due
to the polarized excited state. On the other hand, amidoben-
zoxazole 5 emits green fluorescence along with the short
wavelength emission and green fluorescence is enhanced in
oxazole (4).
A solution of 2 (0.20 g, 0.71 mmol) in 10 cm3 of acetic anhy-
dride was stirred at 80 °C for 20 hours. The mixture was
extracted with ethyl acetate and the extracts were washed with
water, 5%- aqueous solution of sodium hydrogencarbonate, and
brine. After drying over sodium sulfate, the extracts were evapo-
rated to leave a residue. A mixture of the residue in dioxane (20

Nov-Dec 2002
Novel Fluorescent 2-(2-Aminofluorophenyl)benzoxazoles
1303
cm3) and 1%- aqueous solution of sodium hydrogencarbonate
(25 cm3) was stirred at room temperature for 2 days. The mixture
was extracted with ethyl acetate and the extracts were washed
with water and brine. After drying over sodium sulfate, the
extracts were evaporated to leave a residue which was chro-
matographed on silica gel (hexane-ethyl acetate, 5/1) to give 4
(0.09 g, 39%). The compound was purified by recrystallization
from hexane-ethyl acetate to give white crystals of 4: mp 158-
159 °C; ir: 3260 (NH), 1680 (C6F4 and C=O), and 1250 cm-1
(CF); 1H nmr: d = 9.92 (1H, s), 7.82 (1H, m), 7.70 (1H, m), 7.48
(2H, m), and 2.27 (3H, s); 19F nmr: d = -58.6 (1F, m), -59.3 (1F,
ddd, J=21.4, 8.6, and 5.6 Hz), -70.9 (1F, td, J=21.4 and 5.6 Hz), -
81.0 (1F, t, J=21.4 Hz).
pound was recrystallized from hexane to give pale yellow crys-
tals of 8: mp 61-62 °C; ir: 3250 (NH), 1650 (C6F3), and 1250
1
cm-1 (CF); H nmr: d = 8.20 (1H, br.s), 7.73 (1H, m), 7.61 (1H,
m), 7.37 (2H, m), 4.12 (3H, t, J=1.2 Hz), 3.46 (2H, m), 1.68 (2H,
qt, J=7.3 and 7.3 Hz), and 0.99 (3H, t, J=7.3 Hz); 19F nmr: d =
-61.9 (1F, dd, J=21.4 and 10.1 Hz), -74.8 (1F, d, J=10.1 Hz), and
-91.1 (1F, d, J=21.4 Hz).
Anal. Calcd for C17H15F3N2O2: C, 60.71; H, 4.50; N, 8.33.
Found: C, 60.66; H, 4.57; N, 8.33.
Preparation of 2-[3,5,6-Trifluoro-4-methoxy-2-(1-methoxycar-
bonylethyl)aminophenyl]benzoxazole (9).
A mixture of 7 (0.50 g, 1.7 mmol), L-alanine methyl ester
hydrochloride (2.3 g, 16.6 mmol), and triethylamine (3 cm3) in
25 cm3 of dioxane was stirred at 50 °C for 5 days and then at 80
°C for 20 days. The mixture was extracted with ethyl acetate and
the extracts were washed with water and brine. After drying over
sodium sulfate, the extracts were evaporated to leave a residue
which was chromatographed on silica gel (hexane-ethyl acetate,
5/1) to give 9 (0.27 g, 42%). The compound was recrystallized
from hexane-ethyl acetate to give white needles of 9: mp 105-106
°C; ir: 3230 (NH), 1740 (C=O), 1640 (C6F3), and 1250 cm-1
(CF); 1H nmr: d = 8.59 (1H, d,J=7.6 Hz), 7.77 (1H, m), 7.63 (1H,
m), 7.39 (2H, m), 4.58 (1H, qdd, J=7.6, 7.6, and 3.0 Hz), 4.11
(3H, t, J=1.5 Hz), 3.70 (3H, s), and 1.58 (3H, d, J=7.6 Hz); 19F
nmr: d = -61.2 (1F, dd, J=21.8 and 8.6 Hz), -74.3 (1F, m), and -
89.1 (1F, d, J=21.8 Hz).
Anal. Calcd for C15H8F4N2O2: C, 55.57; H, 2.49; N, 8.64.
Found: C, 55.15; H, 2.50; N, 8.48.
Preparation of 2-(2-Pentafluorobenzamido-3,4,5,6-tetrafluo-
rophenyl)benzoxazole (5).
A mixture of 2 (0.06 g, 0.21 mmol), pentafluorobenzoyl chlo-
ride (0.16 g, 0.70 mmol), and triethylamine (0.07 g, 0.69 mmol)
in 10 cm3 of ethyl acetate was stirred at room temperature for 5
days. The mixture was washed with water and brine, dried over
sodium sulfate, and evaporated to leave a residue which was
chromatographed on silica gel (hexane-ethyl acetate, 5/1) to give
5 (0.05 g, 50%). The compound was recrystallized from hexane-
ethyl acetate to give white crystals of 5: mp 204-205 °C; ir: 3250
1
(NH), 1680 (C6F4 and C=O), and 1240 cm-1 (CF); H nmr: d =
Anal. Calcd for C18H15F3N2O4: C, 56.85; H, 3.98; N, 7.37.
Found: C, 56.96; H, 4.19; N, 7.28.
10.6 (1H, s), 7.73 (2H, m), and 7.48 (2H, m); 19F nmr: d = -56.2
(1F, m), -58.5 (1F, m), -61.2 (2F, m), -70.3 (1F, td, J=20.7 and 5.6
Hz), -70.7 (1F, m), -79.0 (1F, t, J=20.7 Hz), and -81.6 (2F, m).
Anal. Calcd for C20H5F9N2O2: C, 50.44; H, 1.06; N, 5.88.
Found: C, 50.30; H, 1.01; N, 5.87.
Preparation of 2-(3,5,6-Trifluoro-4-methoxy-2-dipropylamino-
phenyl)benzoxazole (10).
A solution of 7 (0.30 g, 1.0 mmol) and dipropylamine (3.0 g,
29.7 mmol) in 25 cm3 of dioxane was stirred at 50 °C for 20
hours and then at 80 °C for 30 hours. The mixture was extracted
with ethyl acetate and the extracts were washed with water and
brine. After drying over sodium sulfate, the extracts were evapo-
rated to leave a residue which was chromatographed on silica gel
(hexane-ethyl acetate, 5/1) to give pale yellow oil (0.17 g, 45%)
Preparation of 2-(2-Amino-3,5,6-trifluoro-4-methoxyphenyl)-
benzoxazole (6).
A mixture of 2 (0.05 g, 0.18 mmol) and sodium methoxide
(0.02 g, 0.37 mmol) in 30 cm3 of methanol was stirred at room
temperature for 6 days and then at 40 °C for 7 days. After the sol-
vent was removed, the mixture was extracted with ethyl acetate
and the extracts were washed with water and brine, dried over
sodium sulfate, and evaporated to leave a residue.
Chromatography on silica gel (hexane-ethyl acetate, 5/1) of the
residue produced 0.04 g (75%) of 6. The compound was then
purified by recrystallization from hexane to give pale brown
crystals of 6: mp 126-127 °C; ir: 3480, 3300 (NH2), 1650 (C6F3),
1
of 10: ir (film): 1630 (C6F3) and 1240 cm-1 (CF); H nmr: d =
7.83 (1H, m), 7.59 (1H, m), 7.40 (2H, m), 4.13 (3H, t, J=1.2 Hz),
2.88 (4H, td, J=7.3 and 1.2 Hz), 1.38 (4H, qt, J=7.3 and 7.3 Hz),
and 0.74 (6H, t, J=7.3 Hz); 19F nmr: d = -62.1 (1F, m), -62.4 (1F,
dd, J=21.4 and 10.1 Hz), and -78.8 (1F, dd, J=21.4 and 4.5 Hz).
Anal. Calcd for C20H21F3N2O2: C, 63.48; H, 5.59; N, 7.40.
Found: C, 63.29; H, 5.60; N, 7.37.
1
and 1245 cm-1 (CF); H nmr: d = 7.74 (1H, m), 7.64 (1H, m),
7.38 (2H, m), 6.34 (2H, br.s), and 4.15 (3H, t, J=1.5 Hz); 19F
nmr: d = -63.1 (1F, dd, J=20.7 and 10.5 Hz), -80.0 (1F, d, J=10.5
Hz), and -92.0 (1F, d, J=20.7 Hz).
REFERENCES AND NOTES
Anal. Calcd for C14H9F3N2O2: C, 57.15; H, 3.08; N, 9.52.
Found: C, 56.93; H, 3.05; N, 9.46.
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1304
K. Tanaka and T. Komiya
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