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structural isomers. The positive Cotton effect for gelator 1 in the
diluted gel state was observed at 295 and negative at 212 nm,
while 2 showed the positive Cotton effect at 356, 341 (combined)
and 290 with negative at 312, 301 (combined) and 243 nm
(Fig. S5, ESI†). The difference in UV-vis and CD spectra of 1
and 2 may arise due to the presence and absence of the chelation
induced conformational change as well as aggregation.6f
Variable temperature CD of the diluted gel showed that the
intensity decreases with increasing temperature indicating the
presence of aggregation.
The fluorescence spectrum of the Li+ containing gel displayed
a broad band at 475 nm (0.7% w/v, lex = 325 nm, Stokes shift =
150 nm) while isomer 2 is non-fluorescent under similar condi-
tions (Fig. 1 and Fig. S6, ESI†). Dilution of the gel (1 ꢁ 10ꢂ5 M,
MeOH) led to a blue shift of B25 nm with a decrease in the
fluorescence intensity by 60%. It may be attributed to segrega-
tion of the gel matrix, in turn, indicating the J-type aggregation
(Fig. 1). The occurrence of fluorescence at lower concentration in
such a simple system may be due to chelation-enhanced fluores-
Scheme 1 Structures of two isomers 1, 2 and their cartoon representation
of conformational changes before and after Li+ chelation. Photographs of
the gel in an inverted vial at naked eye and UV light, 2 forms non-fluorescent
solution under similar conditions (right side). Photograph showing the gel
lifting up and down upon heating at 75 1C and cooling in a reversible manner
(bottom).
cence (CHEF) with a high quantum yield (QY) of 35% (lem
=
450 nm, 1 ꢁ 10ꢂ5 M, MeOH).7a Under analogous conditions, 2
showed a fluorescence QY of merely 0.2% which suggested that
the –OH group located at the ortho-position in 1 provides the
chelation site for Li+ while it is absent in 2. A red shift of the
band at 450 to 475 nm (Dl = 25 nm) in the gel state may be due to
p–p stacking between salen rings like excimer formation
(Fig. 1).4 Furthermore, a remarkable enhancement in the fluores-
cence intensity (B60%) for the band at 475 nm can be ascribed
to the aggregation induced enhanced emission (AIEE) effect
(a combined effect of the electrostatic interaction between Li+
and deprotonated O atoms of the gelator, p–p stacking and various
hydrogen bonds created by CH3OH as well as solvated Li+).7b
The large Stokes shift in the gel (B150 nm) and the diluted state
(1 ꢁ 10ꢂ5 M, 125 nm) suggested that there is segregation of p–p
stacking on going from the gel state to its diluted state. Average
lifetimes determined for the gel and its diluted state were found to
be t = 1.304 and 0.948 ns, respectively, which are in good
agreement with the Stokes shift difference for the two states
(Fig. S7, ESI†). It implies that aggregation restricts the molecular
rotation which leads to enhanced fluorescence emission
effectively.7c Furthermore, a variable temperature experiment
was performed on gels (0.7% w/v) in the temperature range of
15–70 1C. It was observed that an increase in the temperature
leads to an incessant blue shift (Dl = 14 nm), which also
supported J-type aggregation (Fig. S7, ESI†).
depends upon the ratio of the gelator and LiOH. At 0.15% w/v
LiOH concentration, gelator requirement 0.2% w/v which is in
the lower range of minimum gelation concentration among the
reported LMWGs.6b Surprisingly, a methanolic gel is thermally
stable up to 80 1C, which signifies that the solvent is tightly
entrapped in the gel matrix, above which it slowly dried up.6c The
tight packing of the gel matrix was further demonstrated by
heating the vial containing gel. Exceptionally, with an increase in
temperature the gel lifted up and went down upon cooling in a
reversible manner (Scheme 1, Video S1, Fig. S1, ESI†).
Transmission electron microscopy (TEM) was performed to
gain deep insight into the morphology of the gel. Furthermore,
to avoid any possible artefacts we executed experiments with a
lowest LiOH concentration containing diluted gel. It revealed
typical nano-cluster morphology with alignment of the small
granules of uniform size (230 nm) (Fig. 1). The AFM images,
which vary with the gel dilution, directly revealed development
of a gel network, i.e., the discontinuous granular aggregates
sequentially align and connect with each other to form a
complete gel network. SEM also revealed the intricately nano-
cluster morphology like cauliflower (Fig. S2–S4, ESI†). Such a
type of morphology has not been reported for chiral low
molecular weight metallo-organic gels.6d,e The circular dichroism
(CD) data analysis confirmed the chiral nature of these two
FT-IR spectra of 1 and its gel were acquired to gain some
information about the mechanism of gelation and conse-
quently nanostructure formation by probing the interaction
of Li+ with 4CQO groups. The band at 1673 cmꢂ1 corres-
ponding to 4CQO stretching in the vibrational spectra of
gelator 1 shifted to 1620 cmꢂ1 (Dn = 53 cmꢂ1) in the gel state
confirming weakening of the 4CQO bond due to 4CQOꢀ ꢀ ꢀLi+
interaction (Fig. S8, ESI†).4 Deprotonation of all the six labile
protons (Ar–OH, NH and –OH) was explicitly confirmed by
Fig. 1 (A) TEM image of the diluted gel (1 ꢁ 10ꢂ4 M), (B) magnified image
of A, (C) fusion of two nano grains and (D) fluorescence spectra (lex
=
325 nm, methanol); gel state (line 1, 0.7% w/v, solution 1 at 1 ꢁ 10ꢂ3 M),
diluted gel (solution 2, line 2, 1 ꢁ 10ꢂ5 M, MeOH), gel at temperature 70 1C
(line 3), solution of 2 (line 4, deprotonated with LiOH, 1 ꢁ 10ꢂ3 M, MeOH).
1
comparative H NMR experiments on 1 with and without the
treatment of LiOH in d6-DMSO (Fig. S10, ESI†). To avoid any
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Chem. Commun., 2014, 50, 8144--8147 | 8145