Bis(2-hydroxyphenyl)-1,3,4-oxadiazole derivative for anion sensing and fluorescent patterning

Article abstract of DOI:10.1080/15421400601028054

We have attempted to synthesize an oxadiazole compound which has ability of anion sensing and of fluorescent pattern construction. Organic soluble oxadiazole molecule with adjacent hydroxyphenyl groups for excited-state intramolecular proton transfer (ESI

Full text of DOI:10.1080/15421400601028054

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Bis(2-hydroxyphenyl)-1,3,4-
oxadiazole Derivative for
Anion Sensing and Fluorescent
Patterning
a
a
a
Tae Hoon Kim , Chi-Han Lee , Chan Gyu Kwak
,
a
a
a
Moon Soo Choi , Won Ho Park & Taek Seung Lee
a
Organic and Optoelectronic Materials Laboratory,
Department of Advanced Organic Materials and
Textile System Engineering (BK 21) , Chungnam
National University , Daejeon, Korea
Published online: 12 Mar 2007.
To cite this article: Tae Hoon Kim , Chi-Han Lee , Chan Gyu Kwak , Moon Soo
Choi , Won Ho Park & Taek Seung Lee (2007) Bis(2-hydroxyphenyl)-1,3,4-oxadiazole
Derivative for Anion Sensing and Fluorescent Patterning, Molecular Crystals and Liquid
Crystals, 463:1, 255/[537]-261/[543], DOI: 10.1080/15421400601028054
To link to this article: http://dx.doi.org/10.1080/15421400601028054
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Mol. Cryst. Liq. Cryst., Vol. 463, pp. 255=[537]–261=[543], 2007
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400601028054
Bis(2-hydroxyphenyl)-1,3,4-oxadiazole Derivative for
Anion Sensing and Fluorescent Patterning
Tae Hoon Kim
Chi-Han Lee
Chan Gyu Kwak
Moon Soo Choi
Won Ho Park
Taek Seung Lee
Organic and Optoelectronic Materials Laboratory, Department of
Advanced Organic Materials and Textile System Engineering (BK 21),
Chungnam National University, Daejeon, Korea
We have attempted to synthesize an oxadiazole compound which has ability of
anion sensing and of fluorescent pattern construction. Organic soluble oxadiazole
molecule with adjacent hydroxyphenyl groups for excited-state intramolecular
proton transfer (ESIPT) was synthesized. Significant UV-vis absorption and fluor-
À
escence changes were observed upon addition of F . UV-irradiation on the poly
(methyl methacrylate) (PMMA) film doped with oxadiazole compound in the pres-
ence of photoacid generator (PAG) induced novel fluorescence quenching-induced
patterning.
Keywords: anion sensor; bishydroxyphenyloxadiazole; ESIPT; fluorescence patterning
INTRODUCTION
As the fluorescence is one of the most sensitive signals for the environ-
mental changes, a great deal of attention have been focused on the
fluorometric sensing of inorganic anions because of their importance
in numerous biological and chemical processes for past decades [1,2].
Among these chemosensors for anions, phosphate and fluoride ion
Financial support from the Korea Research Foundation is gratefully acknowledged
(Grant No. 2005-005-J00402).
Address correspondence to Taek Seung Lee, Organic and Optoelectronic Materials
Laboratory, Textile Engineering Department (BK 21), Chungnam National University,
Daejeon 305-764, Korea. E-mail: tslee@cnu.ac.kr
255=[537]

2
56=[538]
T. H. Kim et al.
sensing are of special interest due to their importance in a number of
disease states and environmental pollutions [3]. However, simple and
easily prepared anion optical sensors are rather rare because optical sig-
nal transduction might be limited by many factors, and thus there is still
a need for good anion sensors with a high sensitivity and selectivity [4].
The oxadiazoles with ortho-hydroxyphenyl group have been studied
in their abnormally red-shifted fluorescence due to excited-state intra-
molecular proton transfer (ESIPT) [5]. Main advantage of this large
Stokes shift is the increase in emission output as a result of low self-
absorption of emitted light. The ESIPT plays an important role within
the limited quantity of known primary photochemical and photophysi-
cal processes. The product of the ESIPT reaction, excited phototautomer
form (keto form), emits light in a longer wavelength region than the
emission of the initial ‘normal’ form (enol form) [6,7]. Among variety
types of compounds with ESIPT, oxadiazole compounds with ESIPT
have been widely studied from the view point of photophysics [8].
Herein, we are reporting a new chemosensor for phosphate and flu-
oride anions containing ortho-hydroxyphenyl oxadiazole with large
Stokes shift. Besides, fluorescence quenching-based patterning system
by addition of an acid was examined for the application of optical data
storage. It is thought that the patterned image without lithography
process has an advantage in the cost and production time.
EXPERIMENTAL
Reagents and Measurements
5
-Bromosalicylic acid and semicarbazide hydrochloride was purchased
from Acros and Aldrich, respectively. PMMA and triphenylsulfonium
1
triflate as a PAG were purchased from Aldrich. H-NMR spectrum
was obtained with a Bruker DRX-300 spectrometer (Korea Basic
Science Institute). Elemental analysis was performed with CE Instru-
ments EA-1110 Elemental Analyzer. All UV-vis spectra were mea-
sured with a Perkin Elmer Lambda 35 spectrometer. Steady-stated
fluorescence was taken from a Perkin Elmer LS 45 spectrofluorom-
eter. Chloride, fluoride and phosphate anions were purchased from
Aldrich as their tetrabutylammonium salt forms. Photographs of fluor-
escence image were taken from Cannon A80 digital camera.
Synthesis of 2,5-Bis(5-bromo-2-hydroxyphenyl)-1,3,
4
-oxadiazole 2
To polyphosphoric acid solution 5-bromo-2-salicylic acid 1 (5 g,
3.03 mmol) and semicarbazide hydrochloride (2.54 g, 23.03 mmol)
2

Anion Sensing and Fluorescent Patterning
257=[539]
ꢀ
were added. The resulting mixture was stirred at 150–160 C for 3 hr
and then was poured into ice water [9]. The residual solid was filtered
and washed with water, and then crystallized with acetic acid to give
white crystals (2.41 g, 51%).
1
H-NMR (CDCl ): d 9.88 (s, 2H), 8.03 (d, 2H), 7.6 (q, 2H), 7.1 (d, 2H)
3
ppm; C H Br N O Found: C, 40.33; H, 1.83; N, 6.60. Calculated C,
1
2
8
2
2
4
0.80; H, 1.96; N, 6.80%.
Measurement of Sensing Ability and Fluorescent
Image Fabrication
À5
UV-vis absorption and emission changes of 2 solution [5 Â 10 M in
N,N-dimethylformamide (DMF)] were measured upon exposure to
anions. The anions used in the ion-responsive property measurements
À
À
À
were H PO , F and Cl in water and methanol solution. The PMMA
2
4
film was obtained from the THF solution composed of 1 wt% of 2 with
PAG on a silicon wafer after filtration with a 0.25 mm syringe filter.
ꢀ
The cast film was thoroughly dried in a vacuum oven at 40 C for 12
hr and exposed under UV light of 254 nm for 2 hr to investigate the
construction of fluorescence pattern.
RESULTS AND DISCUSSION
2
,5-Bis(5-bromo-2-hydroxyphenyl)-1,3,4-oxadiazole 2 was obtained
from the simple condensation reaction with 1 and semicarbazide
hydrochloride by an one-step reaction as shown in Scheme 1. The
1
chemical structure of 2 was confirmed with H-NMR in CDCl and
3
FT-IR, which showed the presence of phenolic hydrogen at 9.88 ppm
À1
and 3400 cm , respectively. Compound 2 was a off-white needle-
shaped crystal and was easily soluble in common organic solvents such
as THF, chloroform, and DMF. The DMF solution of 2 showed strong
absorption at 330 nm and weak one at 420 nm, and emission at 380 nm
(
from enol form) and around 500 nm (from keto form). The film
exhibited a similar behavior to that in the solution, however, the weak
SCHEME 1 Synthesis scheme of 2,5-bis(5-bromo-2-hydroxyphenyl)-1,3,4-
oxadiazole.

2
58=[540]
T. H. Kim et al.
FIGURE 1 Fluorescence changes of 2 in DMF (25.0 mM) upon addition of
À
H PO4 (0, 0.15, 0.30, 0.45, 0.60, 1.05 mM) excitation wavelength 333 nm.
2
absorption at 420 nm and weak emission at 380 nm were not observed
mainly due to energetically preferred keto-form freezing in solid state
[
10,11]. The emission spectrum of the film was centered at 500 nm
which corresponded to green fluorescence at excitation wavelength
65 nm. The absence of emission band at short wavelength indicated
3
the compound 2 formed keto form easily in the film.
Figure 1 shows the fluorescence spectra of 2 in the presence of vari-
À
À
ous amounts of H PO in DMF solution. The addition of H PO4 leads
2
4
2
to strong quenching and slight red-shift of the emission to around
20 nm indicating that the possible formation of intermolecular hydro-
5
À
À
gen bond between H PO and hydroxyl group in 2. In the case of F ,
2
4
decrease in emission intensity observed under 0.3 mM of fluoride ion,
however, above this concentration, the formation of new intermediate
band between original emission bands of enol and keto forms was
observed around at 450 nm (Fig. 2). The new emission was the result
of a new species generated under basic condition, anionic form of 2
À
in DMF solution. In contrast, the addition of Cl showed almost
unchanged fluorescence spectra. It is presumed that the difference
in emission changes between fluoride anion and other anions lies in
the difference in the anion size as well as anionic nucleophilicity.
À
À
As shown in Figure 3, in the presence of H PO4 or F (3 mM),
2
the original absorption band at 330 nm decreased with increase in

Anion Sensing and Fluorescent Patterning
259=[541]
À
FIGURE 2 Fluorescence changes of 2 in DMF (25.0 mM) upon addition of F
0, 0.15, 0.30, 0.45, 0.60, 0.75, 0.90, 1.05 mM) excitation wavelength 333 nm.
(
long-wavelength absorption around 420 nm. The maximum intensity of
À
this new absorption peak was largest for the F ion. Accordingly the color
À
of DMF solution changed from colorless to yellow upon exposure to F .
FIGURE 3 Absorption spectra of 2 in DMF (50.0 mM) after addition of 3.0 mM
of anions.

2
60=[542]
T. H. Kim et al.
FIGURE 4 Fluorescence pattern produced with a 1 wt% of 2=PMMA blend
film with photoacid generator upon irradiation of a wavelength of 254 nm
through a photomask; (a) under sunlight and (b) under UV light. The area
of green fluorescence was an unexposed portion (Film size 1.5 cm  5 cm).
Fluorescence quenching of 2 by addition of an acid was investigated.
It can be expected that fluorescence quenching was facilitated by
addition of acid due to the blocking of keto formation. 1 wt% of
2
=PMMA solution with PAG (10 wt% of 2) was spin-coated on a silicon
wafer (1.5 cm  5 cm). Monochromatic UV light of 254 nm was irra-
diated on the film through photomask and well-patterned fluorescence
ꢀ
image was obtained after baking at 100 C for 60 sec. In the exposed
area, it is presumed that the proton from reductive PAG interrupts
the ESIPT of 2, resulting in fluorescence quenching as shown in
Figure 4. It is expected that this oxadiazole compound can be used
as a potential candidate for the optical data storage.
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7