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band at 518 nm was observed (Figure 2a). Similarly, a decrease
in temperature resulted in quenching of the fluorescence in-
tensity due to the formation of OPV-Ph aggregates (Figure S1a
in the Supporting Information). Contrary to the behavior of
OPV-Ph, no considerable shift in the absorption maximum was
noted for OPV-Py with an increase in temperature in toluene
(Figure 2b). However, a strong red-shifted vibronic shoulder
band at 520 nm was observed with relatively less decrease in
the intensity of the p–p* transition band at 448 nm. The emis-
sion of OPV-Py aggregates in toluene at 208C exhibited a shift
to lower wavelengths with an increase in intensity at higher
temperatures, indicating the breaking of the self-assembly (Fig-
Results and Discussion
The phenyl-amide functionalized gelator, OPV-Ph and the pyr-
idyl-amide functionalized OPV derivative, OPV-Py (Figure 1),
were synthesized as depicted in Scheme S1 in the Supporting
Information. For this purpose, the OPV-bisester (OPV2) was
synthesized using a Wittig–Horner reaction of the phospho-
ure S1b in the Supporting Information). In addition, we found
À5
that OPV-Py in CHCl (510 m; Figure S2 in the Supporting
3
Information) has the tendency to form aggregates, whereas
OPV-Ph was found to be highly soluble and exists as the mon-
omeric species (Figure S3 in the Supporting Information).
The stability of the aggregates was determined from the
plot of the fraction of aggregate (aagg) versus temperature (Fig-
ure 2insets). A melting transition temperature (T , temperature
m
at which aagg = 0.50) of 598C was obtained for OPV-Ph in tolu-
ene, whereas OPV-Py aggregates showed a Tm of 54 and 338C
in toluene and chloroform, respectively. These observations
suggest that the aggregation modes of OPV-Ph and OPV-Py
are significantly different from each other, which emphasize
the importance of the terminal functional moiety in the amide
linkage in controlling the aggregation properties. Having seen
a difference in the aggregation modes of the two molecules,
we further examined their gel formation in toluene. As expect-
ed, OPV-Ph formed a gel in toluene at a critical gelator con-
centration (CGC) of 0.45 mm (Figure S4 in the Supporting Infor-
mation). Surprisingly, OPV-Py did not form a gel, even at
higher concentrations, underlining that the difference in the
mode of aggregation has a significant influence on the gela-
tion behavior.
Figure 1. Molecular structures of phenyl-amide and pyridyl-amide functional-
ized OPV derivatives.
nate ester of ethyl 2-bromoacetate with the OPV-bisaldehyde
(
OPV1), which was prepared according to a known procedur-
[12b,16]
e.
Hydrolysis of OPV2 using KOH in methanol yielded
OPV3. The final reaction between OPV3 and the amine deriva-
tive was conducted using 1[bis(dimethylamino)methylene]-1H-
1
,2,3-triazolo[4,5-b]pyridinium 3-oxid hexafluorophosphate
(
HATU) in the presence of DiPEA. All these compounds were
1
13
characterized using FT-IR, H NMR, and C NMR spectroscopies
and MALDI-TOF mass spectrometry.
Variable-temperature absorption spectral changes of OPV-Ph
and OPV-Py in toluene (510 m) exhibited a distinct behav-
The above observations indicate that OPV-Ph preferably
forms an H-type assembly through the amide H-bonding (Fig-
ure 3a), whereas OPV-Py likely forms a random assembly (Fig-
ure 3b). Formation of such non-gelling random assembly can
be explained by the interference of the terminal pyridyl moiety
with the amide H-bonding in OPV-Py. If so, protonation of the
pyridyl moiety should facilitate the aggregation of
À5
ior as shown in Figure 2. OPV-Ph in toluene at 708C showed
an absorption maximum at 445 nm, corresponding to the
monomeric species. At 208C, a blue-shifted absorption band at
À1
lmax =420 nm (Dl=1338 cm ) with a small vibronic shoulder
the OPV-Py through amide H-bonding, leading to
the gelation of the solvent. As hypothesized, addition
of 1 equivalent of TFA to the solution of OPV-Py in
À5
CHCl (510 m) resulted in a red-shift in the ab-
3
sorption maximum from 448 to 474 nm with a slight
decrease in the intensity (Figure 4a). In addition, the
emission was significantly quenched and became
broad with loss of vibronic features (Figure 4ainset).
Addition of 2 equivalents of TFA induced a further
shift of the absorption maximum to 492 nm and the
emission maximum to 610 nm (Figure 4a). These
Figure 2. Variable-temperature absorption changes of (a) OPV-Ph and (b) OPV-Py in tolu-
ene (510 m). Arrows indicates relative changes in absorption with increase in temper-
ature from 20 to 708C. Insets show plots of fraction of aggregate (aagg) vs. temperature.
Figure 2a inset: OPV-Ph in toluene (*). Figure 2b inset: OPV-Py in toluene (~) and
changes in absorption and emission wavelengths are
also visible in the photographs (Figure S5a and S5b
À5
in the Supporting Information) of the CHCl solutions
3
chloroform (
&
).
of OPV-Py before (greenish-yellow color) and after
Chem. Asian J. 2015, 10, 2250 – 2256
2251
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