Synthesis of organogelling, fluoride ion-responsive, cholesteryl-based benzoxazole containing intra-and intermolecular hydrogen-bonding sites

Article abstract of DOI:10.1016/j.tetlet.2010.08.053

A cholesteryl-based 2-(2′-hydroxyphenyl)benzoxazole (HPB) derivative 3 linked with an amide bond was prepared through an efficient synthetic pathway. The HPB, amide, and cholesteryl groups play important roles in constructing the supramolecular gel structure. UV-vis and fluorescence spectroscopy also showed that HPB and amide groups, which provide intra-and intermolecular hydrogen bonding, respectively, also contribute the recognition of fluoride anions.

Full text of DOI:10.1016/j.tetlet.2010.08.053

Tetrahedron Letters 51 (2010) 5596–5600
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Synthesis of organogelling, fluoride ion-responsive, cholesteryl-based
benzoxazole containing intra- and intermolecular hydrogen-bonding sites
Tae Hyeon Kim a,b, Na Young Kwon a, Taek Seung Lee a,c,
⇑
a Organic and Optoelectronic Materials Laboratory, Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University,
Daejeon 305-764, Korea
b Film Research Institute, Kolon Central Research Park, Kolon Industries, Inc., Gyeongbuk 730-030, Korea
c Graduate School of Analytical Science and Technology, Chungnam National University, Daejeon 305-764, Korea
a r t i c l e i n f o
a b s t r a c t
A cholesteryl-based 2-(20-hydroxyphenyl)benzoxazole (HPB) derivative 3 linked with an amide bond was
prepared through an efficient synthetic pathway. The HPB, amide, and cholesteryl groups play important
roles in constructing the supramolecular gel structure. UV–vis and fluorescence spectroscopy also
showed that HPB and amide groups, which provide intra- and intermolecular hydrogen bonding, respec-
tively, also contribute the recognition of fluoride anions.
Article history:
Received 13 July 2010
Revised 11 August 2010
Accepted 18 August 2010
Available online 21 August 2010
Ó 2010 Elsevier Ltd. All rights reserved.
Keywords:
Organogel
Cholesteryl group
Fluoride anion
Recognition
The self-assembly of functional small molecules into the supra-
molecular structure is an interesting approach to the development
of new nano-scale materials.1 Recently, low molecular weight gels
organized in nano-architectures through specific noncovalent inter-
lar structuring.13 However, recent reports on the organogelling sys-
tems containing HPB are rare. HPB molecule generally exists as an
enol tautomer with a short-wavelength emission. Once irradiated,
the HPB molecule transforms into a keto tautomer with a long-
wavelength emission via an excited state intramolecular proton
transfer (ESIPT) process (see Scheme S1 in the Supplementary
data).14 Intramolecular hydrogen bonding in HPB and intermolecu-
lar hydrogen bonding in the amide group can be altered by the pres-
ence of fluoride anions regardless of its monomeric or polymeric
form, which showed a visually noticeable color change.
In this study, organogelators 3 containing a cholesteryl group
with a fluorophore (HPB) linked with an amide group was designed
and synthesized, as illustrated in Scheme 1. The synthetic strategy
stems from the fact that a cholesteryl-based amide with a strong
intermolecular hydrogen bond and van der Waals forces may pro-
vide powerful driving forces to form an organogel.14d Moreover,
the introduction of an HPB unit into such a system can help to facil-
actions, such as hydrogen bonds,
p–p interactions, and van der
Waals forces have attracted considerable attention as an elegant
class of self-assembled materials.2 In particular, organogelators
have attracted significant interest for their potential applications
including sol-to-gel transcription,3 switching properties,4 organic
light emitting diodes,5 transistors,6 and light harvesting materials.7
Interestingly, organogels based on low molecular weight com-
pounds with the receptor moiety or photo-functional groups have
been the target of increasing attention because of the wide range of
potential field-responsive applications, such as light-responsive
properties,8 complexation,9 and sensing.10 Regarding the sensing
properties, photo-responsive organogels are subjected to change
in their suprastructure and fluorescence in response to external
physical and chemical stimuli.11
itate intermolecular forces via p–p interactions. Such interactions
As a type of intriguing building block, urea and amide groups are
used widely as elements for fluorescent receptors as well as for the
supramolecular architecture based on intermolecular hydrogen
bonding.12 Similar to urea and amide groups in the gelling system,
HPB exhibits exciting anion-responsive properties and provides
intra- and intermolecular hydrogen bonding sites for supramolecu-
might induce the self-assembled gelation leading to a supramolec-
ular architecture.
The details of the synthesis of compound 3 are described in the
Supplementary data. Organogelator 3 was synthesized using a sim-
ple and easy method. Compound 3 can form a gel only in a mixture
of cyclohexanone and cyclohexane or cyclohexane and p-xylene.
Solvent mixtures were employed as the gelation medium because
no sole solvent was found to be efficient for gelation. A feature of
compound 3 is the readily-induced gelation in solvent mixtures
through a heating–cooling method, providing a new fluorescent
supramolecular system (Fig. 1). The organogel of compound 3
⇑
Corresponding author. Address: Organic and Optoelectronic Materials Labora-
tory, Department of Advanced Organic Materials and Textile System Engineering,
Chungnam National University, Daejeon 305-764, Korea. Tel.: +82 42 821 6615; fax:
+82 42 823 3736.
E-mail address: tslee@cnu.kr (T.S. Lee).
0040-4039/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2010.08.053

T. H. Kim et al. / Tetrahedron Letters 51 (2010) 5596–5600
5597
OH
OH
NH2
H2N
HO
NH2
N
O
PPA
+
COOH
1
O
O
HO
O
NH2
HO
NH
Cl
O
O
O
2
OH
N
O
DCC, DMAP
HN
O
1
+ 2
NH
O
O
3
Scheme 1. Synthetic route of organogelator 3.
Figure 1. Photographs of the sol-to-gel process of compound 3 under (a) ambient light and (b) UV-irradiation and the effect of fluoride anion.
was stable for months and the sol-to-gel transitions were ther-
mally reversible. The critical gelation concentration and gel-to-
sol transition temperature of compound 3 in the cyclohexanone
and cyclohexane mixture were 28 mg/mL and 53 °C, respectively.
The self-assembled gel structure of xerogel of compound 3 from
the solvent mixtures was examined by field-emission scanning
electron microscopy (FE-SEM). The FE-SEM images of the gel from
cyclohexanone and cyclohexane (1:3, v/v) show that the gel had

5598
T. H. Kim et al. / Tetrahedron Letters 51 (2010) 5596–5600
Figure 2. FE-SEM images of the xerogel of compound 3 from (a) cyclohexanone/cyclohexane (1:3, v/v) and (b) cyclohexanone/p-xylene (1:3, v/v).
leaf-like aggregation with a length and width of approximately
m and 0.3–0.8 m, respectively (Fig. 2a). The xerogel from the
group was inhibited by the formation of a planar keto tautomer
at a more concentrated condition, resulting in the spontaneous
2
l
l
mixture of cyclohexanone and p-xylene (1:3, v/v) has a similar
morphological structure and size to that from the mixture of
cyclohexanone and cyclohexane (Fig. 2b). Hence, cooperative inter-
actions between molecules, such as hydrogen bonding, van der
gelation through enhanced
structures in Fig. 3b).
p–p intermolecular interactions (inset
Based on the role of intramolecular hydrogen bonding in the
ESIPT mechanism, compound 3 showed a spectroscopic response
upon the addition of fluoride ions with compound 3 in the gel
state. The addition of tetrabutylammonium fluoride resulted in a
rapid transition from an opaque gel to a homogeneous solution
with an altered fluorescence color (Fig. 1). The solution did not re-
turn to a stable gel. This means that some driving forces for gela-
tion were affected by the presence of a proton-attracting fluoride
anion. As already reported, the N–H bond in the amide group inter-
acted with a fluoride ion via hydrogen bonding or was deproto-
nated by the presence of fluoride ions resulting in a disruption of
the gel state.16
The interaction between compound 3 and fluoride anions in a
DMF solution was examined by UV–vis and emission spectroscopy.
As shown in Figure 4a, compound 3 has a high selectivity toward
fluoride ions, as determined from the colorless to yellow color
change that could be observed by the naked eye, exhibiting a
red-shift in absorption from 332 to 410 nm. In contrast, the UV–
vis spectra showed a negligible change after the addition of other
anions, such as chloride, bromide, iodide, and acetate.
Waals force, and
p–p interactions facilitate the formation of the
self-assembled gel architecture.
The UV–vis absorption and emission spectra of compound 3 in a
dilute chloroform solution (1.0 ꢀ 10ꢁ5 M) were investigated
(Fig. 3a). Compound 3 emits a bluish green fluorescence at the
maximum wavelength of 484 nm resulting from the keto form
via ESIPT under excitation of 330 nm in a chloroform solution.
Along with the emission at 484 nm, short wavelength emission
at 388 nm was also observed due to the enol tautomer of 3. This
means that the ESIPT can develop in a chloroform solution,
whereas it is inefficient for gelation because of the excellent
solubility.
The ESIPT process in compound 3 was examined by the concen-
tration-dependent emission spectra in a mixture of cyclohexanone
and cyclohexane (1:3, v/v) as shown in Figure 3b. As the concentra-
tion of compound 3 increased from the solution to gel state
through an aggregated state, the long-wavelength emission at
491 nm was intensified due to the facilitated ESIPT process in the
solid state. Compared to the emission spectra in chloroform shown
in Figure 3a (keto form dominant), the enol tautomer is dominant
in the dilute solution (cyclohexanone and cyclohexane) with a con-
centration of 0.01 wt %, presumably due to the effect of the solvent
polarity.14c,15 Based on this, free rotation between the benzoxazole
and phenol group is possible resulting in dominant enol tautomer.
However, the free rotation between the benzoxazole and phenol
The emission spectra were obtained with the excitation of com-
pound 3 at 333 nm. As shown in Figure 4b, the fluorescence spectra
changed considerably upon exposure to fluoride ions. A unique
emission band appeared at 450 nm with a shoulder at 418 nm,
while other ions did not show any noticeable change in emission
band except for acetate ions. It was assumed that the conjugation
and electronic environment for intramolecular hydrogen bonding

T. H. Kim et al. / Tetrahedron Letters 51 (2010) 5596–5600
5599
0.3
0.4
0.2
0.0
a
a
3
3 + Cl-
3 + Br-
3 + I-
200
0.2
3 + F-
100
0.1
3 + AcO-
0.0
0
300
400
500
600
300
400
500
600
700
Wavelength (nm)
Wavelength (nm)
b
H
O
b
H O
R
R
N
O
600
400
200
0
N
600
400
200
0
R
O
3 + F-
R
3 + AcO-
3
3 + Cl-
3 + Br-
3 + I-
400
500
Wavelength (nm)
600
400
500
Wavelength (nm)
600
Figure 4. (a) Absorption and (b) emission spectra of compound 3 in a DMF solution
upon the addition of variuos anions; [3] = 1.0 ꢀ 10ꢁ5 M and [anion] = 1.0 ꢀ 10ꢁ3 M;
excitation wavelength 333 nm.
Figure 3. (a) Absorption (solid line) and emission spectra (dashed line) for
compound 3 in chloroform (1.0 ꢀ 10ꢁ5 M, excitation wavelength at 330 nm); (b)
emission spectra of compound 3 at different concentrations in cyclohexanone/
cyclohexane (1:3, v/v) solution (kex = 332 nm); concentration of 3 = 0.01 wt %
(solution, j), 0.1 wt % (aggregated state, ), and 10 wt % (gel state, ).
(No. R01-2007-000-10740-0 and 2009-008146) were greatly
acknowledged.
between N–H and O@C might be affected by fluoride anions, which
is responsible for the shift in the two emission bands. This suggests
that intramolecular hydrogen bonding-induced ESIPT is practically
disabled due to the interruption of fluoride ions.
The fluoride ion-responsive property of compound 3 can be ex-
plained by the following: (i) a change in intermolecular hydrogen
bonding between the amide bonds, which is an intriguing driving
force for gelation, resulting in a gel-to-sol transition; and (ii) a
change in intramolecular hydrogen bonding between N–H in bez-
oxazole ring and O@C in phenylene unit, which are important con-
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tetlet.2010.08.053.
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superimposed emission.
a disabled ESIPT-exhibiting
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The financial support from the National Research Foundation
(NRF) and the grant funded by the Korea government (MEST)

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