Gelation-induced fluorescence enhancement of benzoxazole-based organogel and its naked-eye fluoride detection

Article abstract of DOI:10.1039/b800813b

The benzoxazole derivative gelator 1 forms a stable DMF/toluene cosolvent gel with dramatically enhanced fluorescence emission compared to its mother solution. The translucent colorless gel was changed to a solution with strong greenish fluorescence in th

Full text of DOI:10.1039/b800813b

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Gelation-induced fluorescence enhancement of benzoxazole-based
organogel and its naked-eye fluoride detectionw
Tae Hyeon Kim,a Moon Soo Choi,a Byeong-Hyeok Sohn,b Soo-Young Park,c
Won Seok Lyood and Taek Seung Lee*a
Received (in Cambridge, UK) 17th January 2008, Accepted 21st February 2008
First published as an Advance Article on the web 20th March 2008
DOI: 10.1039/b800813b
The benzoxazole derivative gelator 1 forms a stable DMF/toluene
cosolvent gel with dramatically enhanced fluorescence emission
compared to its mother solution. The translucent colorless gel was
changed to a solution with strong greenish fluorescence in the
presence of fluoride anion with disruption of the gel structure.
function as p–p interactions, hydrogen bonding, and van der
Waals interactions, respectively [Fig. 1(A)]. As already reported
in detail, HPB has two tautomers with two emission maxima of
the enol (weak fluorescence) and keto (strong fluorescence)
forms by excited state intramolecular proton transfer (ESIPT)
(see Scheme S2 in the ESIw).9 The intramolecular hydrogen
bonding in HPB can be altered by the presence of fluoride anion
regardless of its monomeric or polymeric form, which showed a
visually noticeable color change as reported earlier.9b,c Our
interest is focused on the following: (1) gelator 1 adopts a flat
conformation in the aggregation state originating from intra-
molecular hydrogen bonding in the HPB unit, which may
induce a facilitated proton transfer between the hydroxyl pro-
ton of benzene ring and the nitrogen of the benzoxazole group;
(2) the hydroxyl groups in HPB are able to selectively interact
with fluoride anions, which may provide the color changes; (3)
the urea moieties are able to bind with fluoride anions; there-
fore, intermolecular hydrogen bonding between neighboring
urea moieties would be disrupted in the presence of fluoride
anion, which may bring about the gel-to-sol transition.
Supramolecular structures composed of low molecular mass
molecules have attracted much interest due to their unique
characteristics and wide range of potential applications as
templates of nano-scale inorganic materials,1 organic soft
materials,2 and optical sensors.3 Gelation is an intriguing
phenomenon demonstrated by small molecules in aqueous or
organic solvents resulting from weak secondary interactions,
leading to the formation of three-dimensional supramolecular
structures of nanometer to micrometer dimensions.4
Self-assembled gelation for supramolecular structure is or-
ganized by intermolecular physical interaction facilitated by
hydrogen bonding and other subsequent weak interactions
such as the p–p interaction of heterocyclic rings, van der Waals
forces of long alkyl chains, and electrostatic forces.2a,5 Along
with a number of reports on the gelation of small molecules,
many efforts have been devoted to the development of gels
with optical absorption or fluorescence.6
Gelator 1 was synthesized from the reaction of 5-amino-2-(50-
amino-20-hydroxyphenyl)benzoxazole and octyl isocyanate in
THF at ambient conditions. Powdery 1 was dissolved comple-
tely in hot DMF, and cold toluene was added dropwise to the
solution [0.5 wt%, DMF–toluene = 1 : 9 (v/v)] giving a stable
and highly fluorescent gel as shown in Fig. 1(B). Gel formation
Fluorescent sensors for anions have been extensively studied
since the sensing of fluoride anions is of special interest due to
their importance in biological and industrial systems.7 Re-
cently, Zinic and co-workers reported on fluoride sensing with
a gel based on oxalamide-derived anthraquinone.8 The gel
showed the color change and the gel-to-sol phase transition in
the presence of fluoride anion, which may provide the basis for
the development of the sensing of fluoride with the naked eye.
In this communication, we designed a low molecular mass
molecule 1 composed of the 2-(20-hydroxyphenyl)benzoxazole
(HPB) unit, urea groups, and long alkyl chains, which could
a Organic and Optoelectronic Materials Laboratory, Department of
Advanced Organic Materials and Textile System Engineering,
Chungnam National University, Daejeon 305-764, Korea. E-mail:
tslee@cnu.ac.kr; Fax: +82 42 823 3736; Tel: +82 42 821 6615
b School of Chemistry, NANO Systems Institute, Seoul National
University, Seoul 151-747, Korea
c Department of Polymer Science, Kyungpook National University,
Daegu 702-701, Korea
d School of Textiles, Yeungnam University, Gyeongsan, Gyeongbuk
712-749, Korea
w Electronic supplementary information (ESI) available: Experimental
information including synthesis and analytical data. See DOI: 10.1039/
b800813b
Fig. 1 (A) Structure of the benzoxazole derivatives 1 and 2; (B)
photograph of fluorescent gel, and (C) FE-SEM images of xerogel 1
(scale bar: left = 1 mm; right = 100 nm).
ꢀc
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was not observed in other solvents or solvent mixtures such as
aliphatic solvents, ethers, dioxanes, and tetrahydrofuran due to
the poor solubility of 1. The FE-SEM image reveals that the
gelator 1 was self-assembled with three-dimensional networks
composed of fibrous aggregates, which are approximately 30
nm in diameter and a few mm long [Fig. 1(C)].
addition to the blue-shift of the lmax, the emission maximum
was also blue-shifted (from 414 to 402 nm for the enol form
and from 541 to 519 nm for the keto form) with remarkable
emission enhancement of the keto tautomer upon aggregation.
Generally, in the heterocyclic compound, the aggregation-
induced emission quenching is accompanied with a red-shift. A
few reports have been published on the abnormal AIE arising
from spectral blue-shifted aggregation, further study of which is
needed for elucidation of the meachanism.11 Intramolecular
rotation of the benzoxazole group is more restricted by aggrega-
tion and thus ESIPT can be more easily developed in the solid gel
state than in solution. Thus the planar conformation favored the
increase in the fluorescence intensity of its keto form as well as in
efficient p–p stacking for gel formation. Therefore, it is regarded
that the planarity of HPB is one of the main driving forces for
aggregation and the large increase in emission intensity.
In this organogel structure, the p–p stacking interactions
among the HPB moieties, the hydrogen-bonding interactions
between the urea moieties, and the van der Waals forces
between the long alkyl chains cooperatively stabilized the
aggregate structure; these are well-known interactions in gel
systems.2a,5 Specifically to clarify the effect of the hydroxyl
group in the HPB unit on gelation, we also designed and
synthesized reference compound 2, which does not contain an
adjacent hydroxyl group and thereby cannot induce its planar
keto tautomer upon excitation. As expected, 2 was not gelled
but precipitated under such conditions that brought about the
gelation of 1 because the molecular stacking between the HPB
units was interfered with due to the lack of planarity. Therefore,
it is presumed that the planarity of HPB from intramolecular
hydrogen bonding is one of the driving forces for gelation.
Compound 1 is weakly luminescent in DMF solution [the
absolute quantum yield measured in integrating sphere (FF)
was 1.4%, at 1.71 ꢁ 10ꢂ5 M]; however, it becomes highly
fluorescent in the gel state (FF = 34.7%) by aggregation-
induced emission (AIE) as shown in Fig. 2(A).10 When 1
aggregated in the DMF/toluene mixture, its fluorescence
increased significantly [Fig. 2(B)]. Interestingly, absorption
of the aggregates (343 nm) in a such solution mixture was
blue-shifted from that of the solution (360 nm). The blue-shift
is unusual because aggregation usually induces a red-shift. In
Further evidence for enhanced proton transfer by aggrega-
tion was provided by time-resolved fluorescence spectroscopic
analysis (see Fig. S1 in the ESIw). Compound 1 showed through
the double-exponential decay that the long-lived excited state
was a major contribution [t1 = 7.2 ns (82.3%) and t2 = 0.5 ns
(17.7%) at 519 nm] in the wet-gel state, whereas a short-lived
excited state was a major component (t1 = 0.2 ns (89.1%) and
t2 = 3.1 ns (10.9%) at 541 nm) in the solution state, which
demonstrated a delayed fluorescence decay due to facilitated
proton transfer. Furthermore, the dried xerogel of 1 exhibited a
much longer lifetime, such that the major contribution was
composed of the long-lived excited state [t1 = 3.1 ns (16.9%)
and t2 = 7.9 ns (83.1%) at 519 nm]. Considering these lifetime
results, p–p stacking of the heterocyclic rings was occurred to
form aggregates during the drying process as well as during
gelation, which showed increased lifetime.
The UV-Vis and fluorescence spectral change of 1 was re-
corded in the presence of 100 equiv. of anions (Fꢂ, Clꢂ, Brꢂ, Iꢂ,
CH3COOꢂ, H2PO4ꢂ) in DMSO (Fig. 3 and see Fig. S2 in the
ESIw). After the addition of fluoride anion, the absorption and
emission maxima were increased and significantly shifted to
longer wavelength (Fig. 3 and see Fig. S3 in the ESIw), while
other anions did not show any shift. The anion-binding proper-
ties of 2 were also determined with the same target anions (see
Fig. S4 in the ESIw). A noticeable spectral shift upon addition of
Fig. 2 (A) Photographs of the aggregation-induced gel formation of
1 (9.52 ꢁ 10ꢂ5 M); vial 1, DMF–toluene = 1 : 58 (v/v); vial 2,
DMF–toluene = 1 : 10.8 (v/v); vial 3, DMF only; (B) UV-Vis (solid)
and fluorescence (dashed) spectra of 1 (9.52 ꢁ 10ꢂ5 M); blue line (vial
1), DMF–toluene = 1 : 58 (v/v); and red line (vial 3), DMF.
Fig. 3 UV-Vis (solid) and fluorescence (dashed) spectra of 1 (7.5 ꢁ
10ꢂ6 M) in DMSO before (red) and after (blue) addition of 100 equiv.
of fluoride anion.
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the target anions was not observed in the UV-Vis spectrum [see
Fig. S4(A) in the ESIw], while the fluorescence intensity was
quenched or increased upon exposure to anions. The amidic NH
groups linked directly to the HPB unit can be responsible for the
emission intensity change.8 Taking into account these results,
absorption and emission maximum shifts are induced by inter-
action between the hydroxyl group in HPB and the target anion.
Thus the presence of the hydroxyl group in HPB is crucial in
gelation as well as in detecting anionic species.
than that with fluoride anion. Therefore, we believe that gel 1 can
be used as a selective naked-eye sensor system for fluoride anion.
In conclusion, we have demonstrated the self-assembled gel
formation which showed fluorescent properties of the gelator
bearing HPB. As expected, the preferred planar conformation
of the HPB core (caused by intramolecular proton transfer)
played a crucial role in gel formation through its strong p–p
interactions. We observed strong emission from gel 1, but
observed hardly any from the solution, which was induced by
intramolecular proton transfer from the enol to the keto
tautomer. Furthermore, we have described the naked-eye
fluoride sensor which showed the gel-to-sol transition and a
visibly noticeable color change, which were induced by the
interaction between fluoride anion and amidic NH and hydro-
xyl proton in the HPB, respectively.
1
In the H NMR spectra of 1, the hydroxyl proton in HPB
showed peak splitting and the integral decreased by the addition
of fluoride anions, which indicates the possible interaction
between the hydroxyl proton in HPB and the fluoride anion
(see Fig. S5 in the ESIw). The spectral shift and decreased
integral of the proton signals from the urea moieties were due to
the deprotonation of the urea groups by fluoride anions accord-
ing to previous reports.12 In the 1H NMR spectra of 2, all NH
peaks disappeared upon addition of fluoride anion, which
means that almost all NH was deprotonated by fluoride anion.
Furthermore, the major decay time of the wet-gel state was
decreased from 7.2 to 5.8 ns after the addition of 100 equiv. of
fluoride anion [see Fig. S1(D) in the ESIw].
The naked-eye anion detection of 1 toward a number of
selected target anions was examined in a DMF/toluene mixture
as shown in Fig. 4(A). As expected, the color change was
observed only in the presence of the fluoride anion. The presence
of fluoride not only changes the color of the gel, but actively
disrupts a preformed gel, as shown in Fig. 4(B). As discussed
above, the urea moieties are able to bind with fluoride anions,
thus, the intermolecular hydrogen bonding between neighboring
urea moieties was disrupted in the presence of the fluoride
anion.13 Placing fluoride anion on top of the DMF/toluene gel
immediately produces a gel-to-sol transition with a color change
from a translucent colorless gel to a solution with a strong
greenish emission. Completion of the gel-to-sol transition oc-
curred within 30 min. Though the disruption of the gel structure
appeared in the presence of other anions, the dramatic color
change and rapid gel-to-sol transition were observed only in the
case of the fluoride anion. The acetate anion, because a high
concentration was used, exhibited a weakly greenish yellow color
with the gel-to-sol transition; however, the color was even weaker
This work was supported by Korea Science and Engineering
Foundation (KOSEF) grant funded by the Korea Govern-
ment (MOST) (No. R01-2007-000-10740-0).
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Fig. 4 Photographs of 1 (A) in DMF–toluene mixture (9.52 ꢁ 10ꢂ5
M, 1 : 58 (v/v)]; and (B) gel upon addition of 100 equiv. of each anion.
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2366 | Chem. Commun., 2008, 2364–2366