Fluorescent behavior of 2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)imidazo-[1, 2-a]pyridine in the presence of metal perchlorate

Article abstract of DOI:10.1002/jhet.5570440204

(Chemical Equation Presented) 2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl) imidazo[1,2-a]pyridine (1) emits long wavelength light around 540 nm both in polar and in nonpolar solvents. Zn²⁺ perchlorate in acetonitrile causes the intermediate waveleng

Full text of DOI:10.1002/jhet.5570440204

Mar-Apr 2007 Fluorescent Behavior of 2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)imidazo-
303
[1,2-a]pyridine in the Presence of Metal Perchlorate
Kiyoshi Tanaka*, Tohru Kurushima, Satoru Iwata, and Syunsuke Shimada
Laboratory of Molecular Control, Faculty of Science and Technology
Seikei University, Musashino-shi, Tokyo 180-8633, Japan
Received March 27, 2006
HO
F
F
F
Zn2+
N
green emission
via ESIPT
F
blue emission
N
perchlorate
in CH3CN
polar and nonpolar
solvents
2-(3,4,5,6-Tetrafluoro-2-hydroxyphenyl)imidazo[1,2-a]pyridine (1) emits long wavelength
light around 540 nm both in polar and in nonpolar solvents. Zn2+ perchlorate in acetonitrile
causes the intermediate wavelength emission around 430 nm, which is ascribed to the species
where the imidazole nitrogen atom and the phenolate oxygen atom bridge Zn2+. In the presence
of Hg2+ and Al3+ perchlorates, short wavelength emission around 370 nm is strongly increased
and this fluorescent enhancement is attributable not to the coordination of Hg2+ and Al3+ to 1 but
to the formation of the salt of perchloric acid of 1.
J. Heterocyclic Chem., 44, 303 (2006).
fluorescence via the ESIPT process, where the 1-
nitrogen atom plays a role as strong acceptor of the
intramolecular hydrogen bonding [6,7]. Therefore, it is
INTRODUCTION
Fluorescence of 2-(2-hydroxyphenyl)benzazoles via
the excited state intramolecular proton transfer (ESIPT)
has been widely studied from the viewpoint of
photophysics [1]. The benzazoles emit long wave-
length fluorescence around >480 nm via the ESIPT
process of the excited state of the intramolecularly
hydrogen bonded enol in nonpolar solvent. The
intensity of short wavelength fluorescence around 380
nm, which arises from the excited state of the
intermolecularly hydrogen bonded rotamer, gradually
increases in accordance with increase of the polarity of
solvent [2,3]. In our continuing research on the
function of polyfluoro-benzene derivatives [4], we have
expected
that
2-(3,4,5,6-tetrafluoro-2-hydroxy-
phenyl)-imidazo[1,2-a]pyridine (1) strongly captures a
metal cation, because it has strong recognition-sites:
tetrafluorophenolate and the 1-nitrogen atom of the
imidazopyridine ring. In this paper, we demonstrate
the fluorescent behavior of 1 in the presence/absence of
metal perchlorate.
RESULTS AND DISCUSSION
The hydroxyimidazopyridine 1 was synthesized by
hydroxylation of 2-(pentafluorophenyl)imidazo[1,2-a]-
pyridine (2) with sodium hydroxide in 73% yield. The
reported
that
2-(3,5,6-trifluoro-2-hydroxy-4-
compound
2
was prepared by condensation of
methoxyphenyl)-benzoxazole and -benzothiazole emit
similar long wavelength fluorescence in nonpolar
solvents and exclusively emit the intermediate
wavelength fluorescence around 450 nm in a polar
solvent such as acetonitrile and methanol, which is
assumed to emit from the excited state of the
corresponding phenolate anions of these fluorophores.
The strongly electron-withdrawing fluorine atoms of
these fluorophores make it possible to exchange the
phenolic proton with a metal cation in a neutral
methanol solution to result in the enhancement of the
bromomethyl pentafluorophenyl ketone and 2-amino-
pyridine. The ortho-substituted position in hydroxylation
was determined by 19F NMR analysis. Substitution at
para position, as was observed in hydroxylation of 2-(-
pentafluoro-phenyl)-benz-oxazole, was anticipated, but
no formation of any para-substituted product was
detected. The selective ortho substitution seems to be
ascribed to the relatively strong coordination of sodium
cation to the imidazole nitrogen atom of 2 to result in the
acceleration of the ortho attack by the accompanying
1
hydroxide. In the H NMR spectrum of 1 in CDCl3, the
intermediate wavelength fluorescence.
These
hydroxy proton appears highly deshielded (13.97 ppm)
and the 3-hydrogen atom couples with the 6-fluorine atom
at 4.0 Hz, suggesting a planar ground-state conformation
with an intramolecular hydrogen bond, as shown in
properties can be applied to the fluorescent probes that
can sense magnesium and zinc cations [5]. On the
other hand, 2-(2-hydroxyphenyl)imidazo[1,2-a]-
pyridine is well established to have
a similar

304
K. Tanaka, T. Kurushima, S. Iwata, and S. Shimada
Vol 44
0.2
Scheme 1. It should be noted that no similar H-F coupling
0. 1
appears in the compound 2.
0. 08
0.15
0.1
0. 06
0. 04
0. 02
Scheme 1
F
F
F
F
F
F
HO
N
N
i) NaOH
ii) HCl
F
F
0
N
N
0.05
10
0 2 4 6 8
12
pH
F
0
250
2
1
300
350
400
450
wavelength / nm
In the fluorescence spectra of 1, long wavelength
emission via the ESIPT process in a nonpolar solvent
such as toluene is blue-shifted to λem = 540 nm, compared
with that (λem = 588 nm in cyclohexane) of the analogous
non-fluorinated 2-(2-hydroxyphenyl)imidazo[1,2-a]-
pyridine 3 [6]. Similarly, long wavelength emission of 1
via the ESIPT process is exclusively observed in dioxane.
This is in sharp contrast to that of 3, which in dioxane
fluoresces short wavelength emission at λem = 379 nm
from the excited state of the intermolecularly hydrogen
bonded rotamer, together with the emission via the ESIPT
process. These fluorescent properties suggest the rather
rigid intramolecular hydrogen bonding at an excited state
of 1. Weak emission via the ESIPT process appears also
in acetonitrile and its fluorescence quantum yield was
estimated to be Φf = 0.015. However, no intermediate
wavelength emission attributable to the phenolate anion
of 1, such as is observed in the cases of the fluorinated 2-
(2-hydroxyphenyl)benzoxazoles, is detected (Figure 1).
But in protic solvent such as methanol-water (9/1),
intermediate wavelength emission appears faintly, with no
emission via the ESIPT process.
Figure 2. Spectral change of 1 between pH 1 and pH 12.
[1] = 1.0x10-5 M, solvent: MeOH / H2O = 9 / 1, [NaCl] = 0.1 M.
The inset shows the absorbance change at 350 nm.
The effect of metal cation on the emission of 1 was
investigated in acetonitrile using metal perchlorates,
which are conventionally used as sources of metal
cations. Alkali metal cations (Li+, Na+, K+, Rb+, Cs+),
alkaline earth metal cations (Mg2+, Ca2+, Ba2+), Cd2+, and
Ag+ cations do not cause any meaningful changes in
fluorescence or absorption, even though 100 molar ratio
of cations were added. However, Zn2+, Hg2+, and Al3+
cations bring about fluorescence enhancement; it was
found that the fluorescence wavelength is dependent on
the added metal perchlorate. Zn2+ perchlorate causes a
new fluorescence of the intermediate wavelength
emission (λem = 430 nm) (Figure 3b) and an increase in
absorbance at longer wavelength around 350 nm (Figure
3a). From these spectral behaviors, we assume that the
species where the imidazole nitrogen atom and the
phenolate oxygen atom bridge Zn2+ is formed and that the
coplanarity causes the 430 nm emission [1c,5,8]. In the
case of Hg2+ perchlorate, absorbance around 350 nm
gradually increases, as the concentration of Hg2+
perchlorate increases in the range of 0 to 0.45 molar ratio
of 1. But in the more concentrated range, it reverses
direction and decreases with the increase of the
concentration of Hg2+ perchlorate (Figures 4a and 4'). In
contrast, the absorbance around 280 nm and the 370 nm
emission simply increase in these concentrations (Figure
4b). Such spectral behavior suggests the formation of two
species: one is a species similar to that of Zn2+, which
absorbs at longer wavelength, but in this case has rather
low quantum yield of the intermediate wavelength
emission [5a], and the other is a species that causes the
370 nm emission. Two similar species seem to be formed
with Al3+ perchlorate, but the formation of the species
causing the red-shifted absorption is only detected in
quite low concentrations of Al3+ perchlorate (Figure 5a).
Another species absorbing around 280 nm emits the
strong 370 nm fluorescence (Figure 5b). It should be
0.2
20
dioxane
toluene
0.15
0.1
toluene
CH3CN
10
dioxane
0.05
CH3CN
0
0
400
500
600
400
wavelength / nm
300
450
350
wavelength / nm]
Figure 1. Absorption and fluorescence spectra of 1 in toluene, in dioxane, and in
acetonitrile. [1] = 1.0x10-5 M, λex = 326 nm.
The pH dependence of the absorption was measured in
the methanol-water solution (Figure 2). Absorbance at a
longer wavelength around 350 nm is noticed under basic
conditions; this is attributable to the phenolate species of 1.
The pKa value was estimated to be 8.6±0.1 based on the
curve fitting method using the absorbance change at 350
nm. Excitation at 350 nm under basic conditions does not
show any meaningful enhancement of intermediate
wavelength emission. These facts indicate that the
quantum yield of the phenolate species of 1 is small.

Mar-Apr 2007
Fluorescence of 2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)imidazo[1,2-a]pyridine
305
added that the intensity of the 370 nm emission is very
sensitive to the water-proportion of the acetonitrile
solution; 1.75% of water reduces the intensity to 1/7.5.
pyridine (2) as a model of 1. Similarly to 1, increases in
fluorescence around 350 nm and absorbance around 280 nm
occur in acetonitrile due to the increase of Al3+ perchlorate
(Figure 6: 2 and Al3+). The Job’s plot seems to support the
formation of a 2:1 complex of 2 and Al3+ (Figure 7a). Isolation
and crystallization of the product were attained by standing a
mixture with a 2:1 ratio of 2 and Al(ClO4)3·9H2O in hexane-
acetone. The UV-vis spectra of the isolated compound 4 are
the same as those of a mixture of 2 and Al(ClO4)3·9H2O and
the fluoresecent spectra also show the strong 350 nm emission
(Figure 6). The ORTEP drawing taken in X-ray analysis
shows that the ClO4 moiety is sandwiched between two
protonated imidazopyridines 2 (Figure 8) [9]. Thus, it is
confirmed that the coordination of Al3+ is not participated to
the complexation but the salt of perchloric acid is formed. The
Job’s plot (Figure 7a) will be explained by the assumption that
the following hydrolysis is involved in the solvent.
(a)
(b)
2
1.5
20
1
0.5
0
10
0
250
450
300
350
400
400
500
600
wavelength / nm
wavelength / nm
Figure 3. Absorption (a) and fluorescence (b) spectral changes with increase of Zn2+
.
]
[1] = 1.0x10-4 M for absorption spectra and 1.0x10-5 M for fluorescence spectra, [Zn2+
= 0~10 equivalent of 1, Zn2+: Zn(ClO4)2·6H2O, solvent: acetonitrile, λex = 350 nm.
(a)
(a')
2
2
Al(ClO4)3·9H2O → 2HClO4 + Al(ClO4)(OH)2·7H2O
0~0.45 equiv. of Hg2+
0.75~10 equiv. of Hg2+
1.5
1
1.5
1
The protonation will contribute to the coplanarity of the
imidazopyridine ring and the pentafluorophenyl group to
result in enhancement of the 350 nm emission.
0.5
0
0.5
0
250
250
300
350
400
450
300
350
400
450
2 and Al3+
wavelength / nm
wavelength / nm
(b)
2
1.5
1
0eq
0.25eq
0.5eq
1eq
0eq
0.25eq
0.5eq
1eq
200
600
400
200
2
100
2
0.5
0
0
300
400
500
600
250
300
350
400
450
0
Wavelength [nm
wavelength /]nm
Wave ngth [nm]
wavelelength / nm
500
300
400
wavelength / nm
Isolated complex 4
Figure 4. Absorption (a and a') and fluorescence (b) spectral changes with
increase of Hg2+. [1] = 1.0x10-4 M for absorption spectra and 1.0x10-5 M for
fluorescence spectra, [Hg2+] = 0~0.45 (a), 0.75~10 (a'), and 0~10 (b)
equivalent of 1, Hg2+: Hg(ClO4)2·3H2O, solvent: acetonitrile, λex = 284 nm.
5
1.0 10-
M
2
1.5
1
2. 5
2
×
600
2.0 10- 5 M
×
5.0 10- 5 M
×
1. 5
1
400
200
0. 5
0
(a)
(b)
0
2
4
6
8
10
concentration of 2
/ 10-5
0.5
2
1.5
1
150
100
M
0
250
0
300
350
400
450
300
400
500
600
Wavelength [nm]
Wavelength [nm
]
wavelength / nm
wavelength / nm
Figure 6. 2 and Al3+; Spectral change of 2 with increase of Al3+
Al(ClO4)3•9H2O, solvent: acetonitrile. Isolated complex 4; Concentration dependence on
.
[2] = 1.0x10-4 M, Al3+
:
50
0
0.5
0
spectra of isolated complex.
300
400
500
450
350
wavelength / nm]
250
300
400
(a)
wavelength / nm]
(b)
0.8
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
Figure 5. Absorption (a) and fluorescence (b) spectral changes with increase of Al3+
.
0.6
[1] = 1.0x10-4
M
for absorption spectra and 1.0x10-5 M for fluorescence spectra,
[Al3+] = 0~10 equivalent of 1 for absorption spectra and 0~2 equivalent of 1 for
fluorescence spectra, Al3+: Al(ClO4)3·9H2O, solvent: acetonitrile, λex = 284 nm.
0.4
0.2
0
0
0.2
0.4
0.6
0.8
1.0
0
0.2
0.4
0.6
0.8
1
To determine the structure of the species causing the 370
nm emission, we studied the association behavior of Al3+
[2] / ([2]+[Al3+])
[1] / ([1]+[Al3+])
Figure 7. Job plots of (a) 2 and Al3+; (b) 1 and Al3+. [2] = [1] = 1.0x10-4 M, Al3+
:
perchlorate
using
2-(pentafluorophenyl)imidazo[1,2-a]-
Al(ClO4)3·9H2O, solvent: acetonitrile.

306
K. Tanaka, T. Kurushima, S. Iwata, and S. Shimada
Vol 44
respectively, are detected. In the presence of Li+, Na+, K+,
Rb+, Cs+, Mg2+, Ca2+, Ba2+, Cd2+, Ag+, Zn2+, Hg2+, or Al3+
perchlorate in acetonitrile, only Zn2+ perchlorate causesan
enhancement of the intermediate wavelength emission
(λem = 430 nm), which is ascribed to the species where the
imidazole nitrogen atom and the phenolate oxygen atom
bridge Zn2+. In the presence of Hg2+ and Al3+ perchlorates,
short wavelength emission (λem = 370 nm) was strongly
increased and this fluorescent enhancement is attributable
not to the coordination of Hg2+ and Al3+ to 1 but to the
formation of the salt of perchloric acid of 1.
EXPERIMENTAL
General Methods. The FTIR spectra were run as KBr
pellets. The UV-vis and fluorescent spectra were recorded in
toluene, dioxane, or acetonitrile of the highest quality for
spectroscopy (KOKUSAN Chemical Co.) without further
purification. The chemical shifts are given in δ/ppm downfield
from tetramethylsilane as an internal standard for 1H nmr
spectra and downfield from trifluoroacetic acid as an external
standard for 19F nmr spectra, respectively; J values are given
in Hz. The fluorescence quantum yields were estimated
relatively, on the basis of Φf = 0.78 for 2-phenylbenzoxazole
in cyclohexane [10].
Based on the spectral behavior and the Job’s plot
(Figure 7b), a species similar to 4 is deduced for the case
of 1 and Al(ClO4)3·9H2O. Preferential formation of the
similar species is assumed to be dependent on the acidity
of metal perchlorates. So the pH values of the methanol-
water solution of Zn2+, Hg2+, and Al3+ perchlorates were
measured and are summarized in Table 1. The pH value
of 1 ([1] = 1.0x10-4 M) under the same conditions is 7.1.
If these values are assumed to be comparable to those in
acetonitrile, acidic Al3+ perchlorate forms the salt of
perchloric acid of 1 even in rather low concentration,
while the neutral Zn2+ perchlorate forms the species in
which the phenolate participates even in higher
concentrations. Hg2+ perchlorate provides an intermediate
acidity between Zn2+ and Al3+ perchlorates and so both
species can coexist under low concentration, but under
higher concentration the medium becomes more acidic to
result in the formation of the salt of perchloric acid of 1,
leading to the enhancement of the 370 nm emission.
2-(Pentafluorophenyl)imidazo[1,2-a]pyridine (2).
A
solution of bromomethyl pentafluorophenyl ketone (3.00 g, 10.4
mmol) in 30 mL of DMF was added dropwise to a solution of 2-
aminopyridine (0.98 g, 10.4 mmol) in 30 mL of DMF. The
mixture was stirred at 60 °C for 2 h and then ethyl acetate was
added to the reaction mixture. The mixture was washed with
water, with 5% aqueous sodium hydrogen-carbonate, and with
brine. The mixture was dried over anhydrous magnesiun sulfate
and evaporated. The resulting residue was chromatographed on
silica gel (hexane/ethyl acetate = 1/1) to give 2.51 g (85% yield)
of 2. Recrystallization from hexane-ethyl acetate was carried
out to give white needles of 2: mp 153-154 °C; FT/IR (KBr): ν
3163, 3043, 1655, 1636, 1548, 1528, 1520, 1510, 1505, 1471,
1366, 1088, 985, 973, 756 cm-1; UV-vis (CH3CN): λmax (log ε/L
mol-1 cm-1) 275.5 (4.04), 310 nm (4.04); 1H nmr (400 MHz,
CD3CN): δ 6.92 (td, 1H, J = 6.8, 1.2 Hz), 7.30 (ddd, 1H, J = 9.2,
6.8, 1.2 Hz), 7.58 (1H, d, J = 9.2 Hz), 8.14 (s, 1H), 8.38 (dt, 1H,
J = 6.8, 1.2 Hz); 19F nmr (376 MHz, CD3CN): δ -87.8 (m, 2F), -
81.3 (t, 1F, J = 21.8 Hz), -65.6 (m, 2F). Anal. Calcd for
C13H5F5N2: C, 54.94; H, 1.77; N, 9.86. Found : C, 54.85; H,
1.52; N, 9.87.
2-(2,3,5,6-Tetrafluoro-2-hydroxyphenyl)imidazo[1,2-a]py-
ridine (1). A mixture of 2 (250 mg, 0.88 mmol) and crushed
sodium hydroxide (350 mg, 8.75 mmol) in 10 mL of dioxane
was stirred at 80 °C for 44 h. Water was added to the reaction
mixture and the aqueous mixture was washed with ethyl acetate
and, after being acidified with 1 M of hydrochloric acid, the
mixture was extracted with ethyl acetate. The extracts were
washed with water and brine, and dried over anhydrous sodium
sulfate. The extracts were evaporated to leave a residue which
was chromatographed on silica gel with eluents of hexane/ethyl
acetate (1/1) to give 180 mg (73%) of white solid 1. Further
Table 1
The pH values of aqueous methanol solutions of metal perchlorates1)
concentration of perchlorate
1.0 x 10-4 M 1.0 x 10-3
M
Al(ClO4)3·9H2O
Hg(ClO4)2·3H2O
Zn(ClO4)2·6H2O
4.1
6.8
7.0
3.7
4.4
7.1
1) Solvent: MeOH / H2O = 9 / 1, [NaCl] = 0.1 M.
As a conclusion, long wavelength emission of 1 via the
ESIPT process is exclusively observed even in polar
solvent such as acetonitrile and no intermediate and short
wavelength emissions attributable to the phenolate anion
and the intermolecularly hydrogen bonded rotamer of 1,

Mar-Apr 2007
Fluorescence of 2-(3,4,5,6-tetrafluoro-2-hydroxyphenyl)imidazo[1,2-a]pyridine
307
purification was performed by recrystallization from hexane-
acetone to give white crytals of 1: mp 262-263 °C; FT/IR (KBr):
ν 3173, 3152, 1638, 1517, 1483, 1430, 1366, 1083, 991, 860,
755, 739 cm-1; UV-vis (toluene): λmax (log ε/L mol-1 cm-1) 313
(sh, 4.03), 326 (4.12), 340 nm (3.97); 1H nmr (400 MHz,
CDCl3): δ 6.95 (dd, 1H, J = 6.8, 7.2 Hz), 7.34 (dd, 1H, J = 9.2,
6.8 Hz), 7.63 (d, 1H, J = 9.2 Hz), 8.03 (d, 1H, J = 4.0 Hz), 8.22
(d, 1H, J = 7.2 Hz), 13.97 (s, 1H); 19F nmr (376 MHz, CDCl3): δ
–96.6 (td, 1F, J = 20.3, 5.6 Hz), -87.9 (ddd, 1F, J = 20.3, 7.8, 5.6
Hz), -81.6 (td, 1F, J = 20.3, 2.3 Hz). –66.3 (dddd, 1F, J = 20.3,
7.8, 4.0, 2.3 Hz). Anal. Calcd for C13H6F4N2O: C, 55.33; H,
2.14; N, 9.93. Found: C, 55.21; H, 2.04; N, 9.84.
“High-Tech Research Center” Project for Private Universities:
matching fund subsidy from the Japanese Ministry of Education,
Culture, Sports, Science and Technology. We thank Dr. T.
Tsukuda and Professor T. Tsubomura for the measurements of
X-ray crystallography data and for their helpful discussions.
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Compound 4 from 2 and Al(ClO4)3·9H2O. A mixture of 2
(200 mg, 0.70 mmol) and Al(ClO4)3·9H2O (170 mg, 0.35 mmol)
was dissolved in a minimum amount of acetone. After a slightly
excess amount of hexane was added to the mixture, the resulting
solution was allowed to stand at room temperature for 5 days.
The formed crystals were collected on a filter and washed with a
cooled mixture of acetone and hexane (1/3) several times, giving
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(KBr): ν 3393, 1659, 1521, 1505, 1116, 1089, 986 cm-1; UV-vis
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1
(CH3CN): λmax (log ε/L mol-1 cm-1) 284 nm (4.37); H nmr (400
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(2001).
MHz, CD3CN): δ 7.54 (td, 1H, J = 6.4, 1.2 Hz), 8.00 (d, 1H, J =
9.2 Hz), 8.04 (ddd, 1H, J = 9.2, 6.4, 1.2 Hz), 8.46 (s, 1H), 8.67
(dt, 1H, J = 6.4, 1.2 Hz); 19F nmr (376 MHz, CD3CN): δ -85.1
(m, 2F), -74.4 (tt, 1F, J = 20.7, 4.0 Hz), -63.6 (m, 2F). Anal.
Calcd for C13H6ClF5N2O4: C, 40.59; H, 1.57; N, 7.28. Found:
C, 40.78; H, 1.56; N, 7.30.
Crystal data for 4: C26H12Cl2F10N4O8,
M = 769.29,
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monoclinic, space group P 1 21/c 1, a = 5.2084(11), b =
º
27.495(6), c = 139.252(4) Å, α = 90 , β = 91.0712(9), γ = 90, V
= 2756.5(10) Å , T = 123.1 K, Z = 4, Dc = 1.854 g cm-3, λ =
0.7107 Å, 5905 reflections measured, 16800 unique (Rint
0.028). R1 = 0.055 and Rw = 0.139.
=
Acknowledgments. This research was supported by a Grant-
in-Aid for Scientific Research (No. 14550811) and partially by
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