Synthesis of fluorescent probe sensitive to amine

Article abstract of DOI:10.1080/15421400490506252

5-Bromo-2-(5′-bromo-2′-hydroxyphenyl)benzoxazole 4 is prepared from the Sandmeyer reaction of 5-amino-2-(5′-amino-2′-hydroxyphenyl)benzoxazole 3 with copper bromide. Two kinds of the (hydroxyphenyl)benzoxazole compounds are investigated for application in

Full text of DOI:10.1080/15421400490506252

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Synthesis of Fluorescent Probe
Sensitive to Amine
Jin Lee a , Jongho Na a , Won Park a , Taek Lee a &
Yong Kwon b
a Organic and Optoelectronic Materials Laboratory,
Textile Engineering Department , Chungnam
National University , Daejeon, Korea
b Department of Polymer Science and Engineering ,
Inha University , Incheon, Korea
Published online: 18 Oct 2010.
To cite this article: Jin Lee , Jongho Na , Won Park , Taek Lee & Yong Kwon (2004)
Synthesis of Fluorescent Probe Sensitive to Amine, Molecular Crystals and Liquid
Crystals, 424:1, 245-251, DOI: 10.1080/15421400490506252
To link to this article: http://dx.doi.org/10.1080/15421400490506252
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Mol. Cryst. Liq. Cryst., Vol. 424, pp. 245–251, 2004
Copyright # Taylor & Francis Inc.
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080=15421400490506252
SYNTHESIS OF FLUORESCENT PROBE SENSITIVE
TO AMINE
Jin Koo Lee, Jongho Na, Won Ho Park, and Taek Seung Lee
Organic and Optoelectronic Materials Laboratory, Textile Engineering
Department, Chungnam National University, Daejeon 305-764, Korea
Yong Ku Kwon
Department of Polymer Science and Engineering, Inha University,
Incheon 402-751, Korea
5-Bromo-2-(50-bromo-20-hydroxyphenyl)benzoxazole 4 is prepared from the
Sandmeyer reaction of 5-amino-2-(50-amino-20-hydroxyphenyl)benzoxazole 3
with copper bromide. Two kinds of the (hydroxyphenyl)benzoxazole
compounds are investigated for application in fluorescent probe sensing
amino compounds and compound 4 is proved to be more suitable for sensing
hexamethylenediamine and hydrazine with different fluorescent colors.
INTRODUCTION
The sensitive and selective binding of chemical species upon molecular
recognition has received considerable attention in applications for sensors,
switches, triggers, logic gates, and molecular level machines [1–3]. The
enormous potential of fluorometric methods for the analytical detection
of environmentally and biologically relevant organic species requires the
development of highly selective and highly sensitive fluorescent probes.
Due to the sensitivity reasons, probes showing fluorescence enhancement
as a result of binding event are to be favored over those which exhibit fluor-
escence quenching upon binding [4,5].
It is well-known that 2-(20-hydroxyphenyl)benzoxazole has been widely
studied from the viewpoint of photophysics because of its high Stokes shift
Received 1 October 2003; accepted 16 January 2004.
Financial support from Korea Research Foundation is gratefully acknowledged (Grant
No. 2001-005-E00037).
Address correspondence to Taek Seung Lee, Organic and Optoelectronic Materials Labora-
tory, Textile Engineering Department, Chungnam National University, Daejeon 305-764,
Korea. Tel.: þ 82-42-821-6615, Fax: þ 82-42-823-3736, E-mail: tslee@cnu.ac.kr
245

246
J. K. Lee et al.
and dual emission via the excited-state intramolecular proton transfer
(ESIPT) for use as laser dyes, polymer UV stabilizers, and organic NLO
materials [6–8]. In nonpolar solvent, it emits long-wavelength fluorescence
(ꢀ500 nm) exclusively via the ESIPT process of the excited state of the
intramolecularly hydrogen-bonded keto form, and the intensity of short
wavelength fluorescence (ꢀ360 nm), which arises from the excited state
of the intramolecularly hydrogen-bonded enol form [9]. Several investiga-
tions on probe or chemosensor with 2-(20-hydroxyphenyl)benzoxazole
have been reported for metal cations and anions [9–11]. However, to our
knowledge, description on sensing other organic compounds such as
amines is rare.
Therefore, in this paper we are reporting newly synthesized fluorescent
chemosensors composed of 2-(20-hydroxyphenyl)benzoxazole, 3 and 4,
which demonstrate selective fluorescence enhancing effects with specific
amines.
EXPERIMENTAL
Reagents and Measurements
2,4-Diaminophenol dihydrochloride and 5-aminosalicylic acid were purchased
from Aldrich Chemical and used without further purification. Tetrahydro-
furan (THF) was purified via conventional sodium=benzophenone method.
1H NMR spectra were obtained on a Bruker ARX-300 spectrometer.
UV-visible spectra were recorded on a Perkin Elmer Lambda 35 spectro-
meter. Steady-state fluorescence measurement was carried out using a
Perkin Elmer LS 45 spectrofluorimeter.
Synthesis of 5-amino-2-(50-amino-20-hydroxyphenyl)
benzoxazole 3
A solution of 2,4-diaminophenol dihydrochloride 1 (6.43 g, 32.65 mmol)
and 5-aminosalicylic acid 2 (5.0 g, 32.65 mmol) in polyphosphoric acid
(150 ml) was heated under reflux for 15 hr. The reaction mixture was
cooled to room temperature and poured into ice water. The mixture was
controlled to pH 7 by addition of sodium bicarbonate. The precipitate
was isolated by filtration and the brown solid was recrystallized in ethanol.
The product was further purified by flash column chromatography on silica
gel with acetone. Yield (4.10 g, 52%).
1H NMR (acetone-d6): 10.7 (s, hydroxyl), 7.66, 7.45, 7.38, 7.02, 6.85 (m,
aromatic), 4.80 (s, amine), 4.47 (s, amine) ppm.

Synthesis of Fluorescent Probe Sensitive to Amine
247
Synthesis of 5-bromo-2-(50-bromo-20-hydroxyphenyl)
benzoxazole 4
Three-necked round-bottomed flask was charged with 2.4g (10 mmol) of 3
and 32% hydrobromic acid (50 ml) at 0ꢁC. Aqueous solution (5 ml) of sodium
nitrate (1.73 g, 25 mmol) was slowly added into the flask and mixture was stir-
red for 10 min. During the stirring, 11.48 g (40 mmol) of copper bromide dis-
solved in 48% hydrobromic acid was added dropwise with dropping funnel.
After 10 min stirring at 0ꢁC, the mixture was allowed to raise 100ꢁC and was
further stirring for 5 hr. The mixture was extracted with chloroform (1:1 by
volume) four times and dried over magnesium sulfate. The crude product
was purified by flash column chromatography on silica gel with n-hexane
and ethyl acetate (2:1 by volume) yielding 2.10 g (57%).
1H NMR (DMSO-d6): 11.1 (s, hydroxyl), 8.11, 7.82, 7.69, 7.13 (m,
aromatic) ppm.
RESULTS AND DISCUSSION
For the synthesis of 5-bromo-2-(50-bromo-20-hydroxyphenyl)benzoxazole 4,
5-amino-2-(50-amino-20-hydroxyphenyl)benzoxazole 3 was first synthesized
following the procedure of well-known ring closure reaction in the presence
of polyphosphoric acid. Treatment of 3 with sodium nitrate and HBr followed
by reaction with CuBr known as Sandmeyer reaction allows yielding dibromo-
compound 4. The compound 4 was obtained in 57% yield (Scheme 1). Con-
version from amine to bromine was confirmed with NMR spectrum, which
exhibited disappearance of amine contribution at 4.47 and 4.80 ppm.
Absorption spectra of 3 and 4 in chloroform solutions are essentially
different, as that of 3 is red-shifted as shown in Figure 1(a). The origin
SCHEME 1 Synthesis of 3 and 4.

248
J. K. Lee et al.
FIGURE 1 (a) Absorption spectra of 3 and 4 in chloroform solution and fluores-
cence spectra in chloroform (b) and in THF (c) solution; 3 (dotted line), 4 (solid
line).

Synthesis of Fluorescent Probe Sensitive to Amine
249
of red-shift of absorption in diamino compound 3 is attributed to the fact that
electron-donating and withdrawing properties of substituents (amines and
bromines) play a decisive role in structural planarization or conjugation via
electron movement. Fluorescence spectra of 3 in chloroform solution show
appreciable two emissions at 438 and 546 nm resulted from enol and keto form
by ESIPT, respectively (Fig. 1(b)). However, only single broad emission cen-
tered at 489 nm was observed in the case of 4 in chloroform solution, which is
an intermediate emission wavelength of 3 and implying ESIPT in 4 was not
brought out in chloroform. On the contrary, two emissions by means of ESIPT
can be observed in fluorescence spectra of 4 in THF solution at 405 and
around 500 nm, but the same result could not be drawn in 3 in THF, which
shows a strong emission at 455 nm and a weak shoulder at around 580 nm
as shown in Figure 1(c). Fluorescence intensity of 4 in chloroform or THF sol-
ution is much stronger than 3 at the same concentration and the solvents
effects on the fluorometric behaviors such as ESIPT seems to be tremendous.
It is presumed that the solvent interaction as well as substituents on each end
affect the hydroxyl group of 3 and 4, which is strongly related to ESIPT.
We investigated the effect of amines on the emission of 3 and 4 in THF
and the change in the luminescence intensity in the presence of an amine is
summarized in Figure 2. Compared to 3, dibromo-terminated (hydroxyphe-
nyl)benzoxazole 4 recognizes hexamethylenediamine (HMDA) and hydra-
zine to bring about considerable fluorescence enhancement and recognizes
triethylamine (TEA) to cause negligible fluorescence enhancement. Along
with fluorescence intensity increase, emission from keto form (from around
500 nm to 466 nm) was much more affected by hydrazine rather than that
from enol form (403 nm). For the case of 3, a meaningful luminescence
change is not detected for three amines such as HMDA, hydrazine, and
TEA as shown in Figure 2(a). From the observation described above, 4
can be regarded as a probe sensing HMDA sensitively and selectively.
Noticeable fluorometric changes of 3 and 4 by addition of amines were
not observed when chloroform was used as a solvent.
It is generally accepted that dramatic fluorescence enhancement of
(hydroxyphenyl)benzoxazole compound with interacting chemical species
was considered by the formation of a complex, where the benzoxazole and
phenolate ring lies on the same plane by the bridging, and this coplanarity
and the anionic property of the phenolate ring bring about large fluores-
cence enhancement [12,13].
CONCLUSION
We have synthesized (hydroxypheyl)benzoxazole-based molecules 3 and 4
for amine sensing measurement. The substituents (amine and bromine) in

250
J. K. Lee et al.
FIGURE 2 Fluorescence spectra of (a) 3 and (b) 4 in THF solution with addition
of amines; original polymer solution (6.94 ꢂ 10ꢃ4 mM, solid line); HMDA
(4.3 ꢂ 10ꢃ1 M); hydrazine (0.10 ꢂ 10ꢃ1 M); TEA (3.6 ꢂ 10ꢃ1 M).
3 and 4 as well as solvent used have considerable effects on ESIPT beha-
vior and interaction with amines which will be transduced into optical
signals. Generally dibromo-terminated compound 4 exhibited noticeable
and appreciable fluorometric changes such as fluorescence enhancement

Synthesis of Fluorescent Probe Sensitive to Amine
251
and emission shift compared to amino-terminated compound 3. Presum-
ably this resulted from electron density increase in electron-poor benzox-
azole ring by electron-donating property of the amines.
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