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Cite this: RSC Adv., 2021, 11, 19856
Blue-emissive two-component supergelator with
aggregation-induced enhanced emission†
Swathi Vanaja Chandrasekharan, Nithiyanandan Krishnan, Siriki Atchimnaidu,
Gowtham Raj, Anusree Krishna P. K., Soumya Sagar, Suresh Das and Reji Varghese
*
Two-component organogels offer several advantages over one-component gels, but their design is highly
challenging. Hence, it is extremely important to design new approaches for the crafting of two-component
organogels with interesting optical and mechanical properties. Herein, we report the design of a new class
of
two-component
supergelators
obtained
from
the
assembly
between
acid
functionalized
tetraphenylethylene (TPE)-based dendrons and alkylated melamine. No gelation behaviour is observed
for the individual components, but interestingly, remarkable gelation behaviour is observed for their
hydrogen-bonded complex. The primary driving force responsible for the gelation is the strong p–p
stacking interaction of TPE units. Because of the strong p-stacking of TPEs in the gel state, the C(sp2)–
C(sp2) bond rotation of the TPE segment is completely arrested in the gel state, which results in intense
fluorescence emission of the gels. Furthermore, excellent elastic response is observed for the gels as
Received 13th May 2021
Accepted 28th May 2021
evident from their high storage modulus compared to loss modulus values. Our results clearly
demonstrate that by the appropriate selection of the molecular components, this approach can be
DOI: 10.1039/d1ra03751j
applied for the creation of functional nanomaterials with emergent properties absent in the individual
rsc.li/rsc-advances
blocks.
1. Introduction
The study of uorescent supramolecular gels derived from the
self-assembly of low molecular weight organogelators has
received considerable attention in recent years due to their
numerous applications as so emissive materials in nanoscience and nanotechnology.1 Supramolecular gels are typically
composed of three dimensional (3D) entangled network of the
gelator molecules assembled via various non-covalent interactions such as hydrogen bonding,2 p–p stacking,3 donor–
acceptor4 and hydrophobic interactions.5 Depending upon the
chemical structure and functionality tethered on the gelator,
various superstructures are possible for the self-assembled
nanostructures such as vesicles,6 nanobers,7 micelles8 and
helical structures.9 Two-component organogelators10 are
particularly interesting, wherein two complementary and individually non-gelating molecules undergo initial association to
form an intermolecular complex and its subsequent selfassembly leads to the formation of entangled network and
gelation. Two-component gels offer several advantages over
one-component gels. Firstly, since the intermolecular complex
formation between the two non-gelating molecules is the
School of Chemistry, Indian Institute of Science Education and Research (IISER),
Thiruvananthapuram, India. E-mail: reji@iisrtvm.ac.in
† Electronic supplementary
10.1039/d1ra03751j
information
19856 | RSC Adv., 2021, 11, 19856–19863
(ESI)
available.
See
DOI:
primary association step responsible for the gelation, they offer
an additional level of control and tunability for the gelation
process, which is not conceivable with one-component gelators.
Secondly, functional domains can be introduced into the gelator by structural modication of either one of the components
and hence it permits the creation of multifunctional organogelators. Moreover, the morphology and material behaviours of
the gels can be ne-tuned by appropriate variation of the ratio of
the two components. Though there are several reports available
for the design of two-component organogels,11 their unique
functional properties and promising applications always
demands for the proposal of new strategies for the creation of
two-component organogels.
Herein, we report the design of a novel class of twocomponent organogels obtained from the supramolecular
complex between acid functionalized tetraphenylethylene
(TPE)-based dendron (1) and an alkylated melamine (2) as the
two components (Scheme 1a). Tetraphenylethylene-based
derivative is selected as one of the components in our study
due to its high self-assembly propensity via p–p stacking
interaction and enhanced emission behaviour in the aggregated
state.12 Melamine moiety is functionalized with long hydrocarbon chains to improve its solubility in non-polar solvents.
Individually both the components are unable to gelate any of
the organic solvents investigated. However, they form a strong
intermolecular non-covalent complex (1@2) through complementary hydrogen bonding interaction between carboxylic acid
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(a) Scheme for the synthesis of 1 and the chemical structures of 1 and 2. (b) Scheme depicting the supramolecular association of 1and
2 through complementary H-bonding interaction to the intermolecular complex 1@2. Self-assembly of 1@2into blue-emissive nanofibers and
their gelation is also shown.
Scheme 1
and melamine in 3 : 1 molar ratio, respectively.13 Interestingly,
the hydrogen-bonded complex 1@2 is able to gelate non-polar
solvents such as toluene, xylene etc. One of the most remarkable features of the gel is the enhanced emission of the complex
in the gel state compared to its non-assembled monomeric
state. Microscopic analyses reveal that 1@2 complex undergoes
self-assembly via strong p–p stacking interaction of TPE units
into brous nanostructures and at higher concentration the
hierarchical cross-linking and entanglement of the bers
occurs, which results in the formation of blue-emissive organogel. The strong aggregation induced enhanced emission
(AIEE) observed for the gel is due to the restricted rotation of
C(sp2)–C(sp2) bonds of the TPE segment of 1 in the selfassembled gel state, which is a characteristic property of TPEbased molecular assemblies (Scheme 1b).
2.
2.1
Results and discussion
Synthesis of 1 and 2
Synthesis of 1 was achieved through multi-step organic reactions. Alcohol functionalized TPE dendron (1b) was synthesized
by following our reported procedure.14 Subsequently, alcohol
derivative was converted into the corresponding azide (1a) by
© 2021 The Author(s). Published by the Royal Society of Chemistry
the treatment with diphenylphosphoryl azide (DPPA) and 1,8diazabicyclo[5.4.0]undec-7-ene (DBU) in THF as the solvent.
Copper-catalysed alkyne–azide cycloaddition (CuAAC) reaction
of 1a with 6-heptynoic acid (1b) then furnished the acid functionalized TPE dendron (1) in reasonable yield (Scheme 1a). On
the other hand, the alkylated melamine derivative (2) was
synthesized by the reaction between melamine (2a) and 1-bromooctane in DMF with potassium carbonate (K2CO3) as the
base. All the intermediates, 1 and 2 are fully characterized using
spectroscopic and mass analyses. Synthesis and characterization details are provided in the ESI.†
2.2
Photophysical and aggregation studies of 1
Detailed optical studies of 1 (10 mM) were carried in various
polar and non-polar solvents. UV-vis electronic absorption
spectra of 1 in polar (DMSO), moderately polar (DCM) and nonpolar solvent (toluene) revealed the characteristic absorption of
monomeric TPE units centred at 310 nm (Fig. 1a). However, the
corresponding spectrum in THF:water (5 : 95) showed a broad
absorption band for TPE unit ranging from 545 to 310 nm,
indicating the aggregation of 1 with the addition of water into
THF solution. In accordance with the absorption spectrum,
emission spectrum of 1 in DCM exhibited the characteristic
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(a) Absorption and (b) emission spectra of 1 in different solvents.
Temperature dependent (c) absorption and (d) emission spectra of 1 in
THF:water (5 : 95). [1] ¼ 10 mM.
Fig. 1
monomeric emission of TPE centered at 375 nm (lexc ¼ 300 nm)
(Fig. 1b). However, the emission spectra of 1 in toluene, THF
and DMSO showed the emergence of aggregate peak at 475 nm
in addition to the monomeric peak at 375 nm. This clearly
reveals that 1 undergoes self-assembly via p–p stacking interaction of TPE units to the self-assembled species and the
emissive nature of the aggregated species is due to the AIEE
phenomenon, which is characteristic for the aggregation of
TPE-based systems. More interestingly, the emission spectrum
of 1 in THF:water showed intense emission peak corresponding
to the aggregated species at 475 nm with the complete disappearance of monomeric peak at 375 nm (lexc ¼ 324 nm).
Temperature-dependent optical studies were further performed
to shed more light into the aggregation behaviour of 1 in
THF:water. Variable temperature absorption spectra showed
the gradual decrease in the intensity of the aggregate peak with
the rise in temperature from 20 C to 70 C (Fig. 1c). In accordance with this, temperature-dependent emission spectra also
revealed decrease in intensity of the emission band corresponds
to the aggregate peak without the concomitant formation of
monomeric emission peak (Fig. 1d). As expected, temperaturedependent uorescence changes were found to be reversible
in nature. These results suggest that the aggregates of 1 are
thermally stable, at least stable up to 70 C, and the observed
decrease in emission intensity with the rise in temperature is
the mere effect of temperature on uorescence.15 These results
unequivocally conclude that 1 forms thermally stable aggregated species in THF:water mixture with strong AIEE behaviour.
However, it must also be noted that 1 and 2 were unable to
gelate any of the solvents including THF:water mixture.
Paper
for 10 minutes and subsequently the solvent was removed by
slow evaporation. The solid complex obtained in powder form
was then dried under vacuum and used for further experiments
without any further purication. The complex formation was
characterized by using Fourier-Transform Infrared (FT-IR),
Differential Scanning Calorimetry (DSC) and Nuclear
Magnetic Resonance (NMR) experiments. Initially, the complex
formation was characterized by using solution state 1H-NMR
spectroscopic studies (Fig. 2a). Typically, protons involved in
hydrogen bonding exhibit chemical shi change or attenuation
of the corresponding peaks.16 Accordingly, we followed the
changes associated with the carboxylic acid proton of 1 and NH
proton of 2 in order to conrm the hydrogen bonded complex
formation between 1 and 2. 1H-NMR spectra of 1 showed the
carboxylic acid proton at 12.04 ppm, whereas 2 showed the –NH
protons at 4.86 ppm. Interestingly, 1@2 complex revealed
signicant broadening and almost complete disappearance of
carboxylic acid peak at 12.04 ppm and the –NH peak at
4.86 ppm. FT-IR analyses of 1@2 revealed a shi in carbonyl
stretching frequency of 1 from 1732 cm 1 to 1722 cm 1
(Fig. 2b). The red-shi in the wavenumber for the carbonyl
stretch is attributed to the intermolecular hydrogen bonding
between the carboxylic acid group of 1 and melamine hydrogen,
which causes weakening of the carbonyl bond. The formation of
the intermolecular complex was further supported by the DSC
experiments. Heating thermograms of 1 and 2 clearly showed
2.3 Synthesis and characterization of intermolecular
complex 1@2
The supramolecular complex 1@2 was prepared by annealing
3 : 1 molar ratio of 1 (810 mM) and 2 (270 mM) in DCM at 40 C
19858 | RSC Adv., 2021, 11, 19856–19863
Fig. 2 Comparison of (a) 1H-NMR spectra (b) IR spectra and (c) DSC
thermograms of 1, 2 and 1@2.
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sharp transitions at 50 C and 156 C, respectively, which
correspond to their characteristic melting points (Fig. 2c).
Interestingly, the complex 1@2 exhibited a new transition at
130 C, which is distinct from the melting points of 1 and 2.
This is a clear indication for the formation of intermolecular
complex between 1 and 2. These results unequivocally conrm
the formation of thermally stable 1@2 complex through the
complementary intermolecular hydrogen bonding between the
–COOH group of 1 and melamine part of 2 in 3 : 1 stoichiometry
(Scheme 1b).
2.4
Gelation studies of 1@2
Gelation of the solvents by 1@2 was achieved by following
a reported procedure.17 Typically, a weighed amount of 1@2 was
added into a vial, followed by the addition of respective solvent
and the mixture was heated until a clear solution was formed.
Slow cooling of the solution resulted in the formation of gels,
which was conrmed by the lack of ow upon inversion of the
vial. Accordingly, 1@2 formed a stable and transparent gel in
toluene with a critical gelation concentration (CGC) of 2.5 mg
mL 1 (0.98 mM, 0.25 wt%) (Fig. 3a). The gelation ability of 1@2
was investigated in other aromatic solvents such as o-xylene, pxylene and mesitylene as well, and it was observed that 1@2 was
capable of gelating all the solvents. The CGCs for the gels in oxylene, p-xylene and mesitylene were 2.7 mg mL 1 (1.07 mM,
0.27 wt%), 3 mg mL 1 (1.19 mM, 0.3 wt%) and 3.1 mg mL 1
(1.23 mM, 0.31 wt%), respectively. It is worth noting that CGC of
all the gels are well below 1 wt%, which suggests that this twocomponent gels belong to the class of supergelators.18 The
melting points of the gels (Tg) in toluene, o-xylene, p-xylene and
RSC Advances
mesitylene are 85 C, 70 C, 66 C and 50 C, respectively. Most
importantly, the gels are highly emissive in nature. The uorescence maximum of the gel in toluene is centred at 485 nm
(lexc ¼ 324 nm) (Fig. 3b). In order to check whether the preforming of the complex (1@2) is necessary for the gelation, we
have carried out mixing of individual components (1 and 2) at 1
(1) to 3 (2) molar ratios in toluene and annealed the sample at
90 C for 10 minutes followed by slow colling to room temperature. Interestingly, gelation was observed in this case also
(Fig. S5†), indicating that the pre-forming of the complex is not
indeed necessary for the gelation process. It is important to note
that 1@2 was unable to gelate non-aromatic solvents (Table 1).
This implies that p-stacking interaction between the gelator
(1@2) and solvent molecule is also crucial for the gelation
process.19
In order to study the role of stoichiometry of the complex for
the gelation process, we have carried gelation studies with 1 : 1
and 1 : 2 complex of 1 and 2 in toluene, respectively. Interestingly, gelation was observed in all the cases (Fig. S6†). However,
gel melting studies revealed a gradual decrease in the gel
meting temperature. The Tg values for 1 : 3, 1 : 2 and 1 : 1 gels
are 85 C, 82 C, 80 C, respectively. This was further supported
by DSC analyses as well (Fig. S7†). These results suggest that the
hydrophobic and p-stacking interactions are crucial for gelation
and stability of the gel increases with the increase in the
number of p-stacking component (1) and the number of alkyl
chains (2).
2.5
Microscopic and PXRD analyses of the gels
In order to understand the morphology of the aggregated
species of 1@2 formed in the gel state, detailed microscopic
analyses were carried out. Transmission electron microscopic
(TEM) analyses of diluted solutions of the gel in toluene, oxylene, p-xylene and mesitylene showed the formation of
brous assemblies. A representative TEM image for the diluted
solution of toluene gel is provided in Fig. 4a, which clearly
reveals the formation of micrometre long bers. Width of the
bers is in the range of few nanometres. Similar brous
Table 1
Gelation behaviours of 1@2 in different solventsa,b,c
Solvent
1
2
1@2 (wt%)
Tg ( C)
CH2Cl2
CHCl3
EtOAc
THF
Cyclohexane
Hexane
Toluene
o-Xylene
p-Xylene
Mesitylene
S
S
S
S
P
P
P
P
P
P
S
S
S
S
S
S
S
S
S
S
S
S
S
S
P
P
G (0.25%)
G (0.27%)
G (0.30%)
G (0.31%)
—
—
—
—
—
—
85
70
66
50
a
(a) Photographs of the gel in toluene under (left) daylight and
(right) UV irradiation. (b) Fluorescence spectrum of the gel (lexc ¼ 324
nm) and the inset shows the corresponding absorption spectrum of
the gel.
Fig. 3
© 2021 The Author(s). Published by the Royal Society of Chemistry
Complex was taken in respective solvents and heated until the
formation of a clear solution and was allowed to cool slowly. b S ¼
Solution, P¼ Precipitate, G¼ Gel. c Critical gelation concentration
(CGC) in wt%.
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2.6
Rheological studies of the gels
The mechanical property of the organogels is very vital for their
practical applications. Hence, the viscoelastic properties of the
gels were studied through rheology experiments. The storage
(G0 ) and loss (G00 ) moduli were evaluated by the amplitude sweep
and frequency sweep at 25 C. For any viscoelastic material, the
storage modulus G0 represents the elastic component and loss
modulus G00 represents the viscous component, respectively. For
viscoelastic solids like gels, which has extensive intermolecular
physical cross-linking are supposed to have G0 > G00 owing to
their superior elastic component over the viscous component.
The amplitude sweep experiment for all the gels showed G0 > G00
with shear storage moduli values of 3126, 2589, 1074 and 312 Pa
for the gels in toluene (Fig. 5a), o-xylene (Fig. S9a†), p-xylene
(Fig. S9b†) and mesitylene (Fig. S9c†), respectively. From the G0
values it is evident that the gel formed in toluene is the most
and that formed in mesitylene is the least stable. Dynamic
frequency sweep experiment for all the gels, except for the gel in
mesitylene, exhibited G0 > G00 over the entire frequency range
from 0.1 rad s 1 to 100 rad s 1, which implies the obvious
elastic response of all the gels over a substantial range of
frequencies (Fig. 5b).
2.7
(a) TEM image of the diluted solution of the gel in toluene (b)
SEM image of the toluene gel. (c) PXRD pattern of the xerogel of the
gel obtained from toluene.
Fig. 4
assemblies were observed for the diluted solutions of the gels
of 1@2 in other solvents as well (Fig. S8†). Subsequently, the
morphology of the self-assembled species in the gel state was
also investigated. For this purpose, xerogels of the respective
gels were prepared by freeze-drying the respective gels. The
scanning electron microscopic (SEM) analyses of the xerogel
obtained from toluene showed the formation of micrometer
sized rod-like nanostructures (Fig. 4b). Similar morphology
was observed for the xerogels in other solvents as well (Fig. S9–
S12†). The difference in the morphology observed for the gel
network in the SEM when compared to the TEM can be
attributed to the drying effect during SEM sample preparation.
Confocal microscopic analyses of the gels revealed blueemissive gel network (Fig. S13†). Furthermore, powder X-ray
diffraction (PXRD) pattern of the xerogel obtained from
toluene exhibited a broad diffraction peak at 2q ¼ 16 –28 ,
which corresponds to a d-spacing of 5.5–3.2 Å, respectively
(Fig. 4c). This can be assigned to the different p-stacking
distances of TPE units in the gels state. These results collectively conclude that 1@2 complex initially undergoes selfassembly via p–p stacking interaction of TPE units of 1 into
brous nanostructure, which then undergoes hierarchical
assembly to entangled rod-like network and leads to the
formation of blue emissive organogels.
19860 | RSC Adv., 2021, 11, 19856–19863
Photophysical properties of the gel
One of the most remarkable features of the gels is their highly
emissive nature due to the AIEE phenomenon of TPE part of
1@2. In order to study their optical properties, gels were initially
formed in a quartz cuvette having a path length of 1 mm.
Absorption spectrum of the gel in toluene at 20 C revealed
a broad absorption peak with maximum centred at 330 nm,
which is a characteristic spectral feature of aggregated TPE
units in the gel state (Fig. 5c). Similarly, emission spectrum of
the gel in toluene at 20 C showed an intense emission peak at
485 nm (lex ¼ 324 nm) (Fig. 5d). The uorescence quantum
Rheological studies of gel in toluene. (a) Amplitude and (b)
frequency sweep plots of 0.25 wt% of the gel. Temperature dependent
(c) absorption and (d) emission (lexc ¼ 324 nm) spectra of the gel in
toluene (c ¼ 0.25 wt%).
Fig. 5
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yield (ff) of the gel in toluene is 35%. Temperature-dependent
absorption spectra of the gel showed no signicant changes
until 80 C, but a sudden rise in absorbance was observed at
90 C. This can be attributed to the melting of the gel at 85 C,
which results in the breaking of the aggregated species into the
monomeric species. Temperature-dependent emission spectra
of the gel, however, revealed a gradual decrease in emission
intensity with the rise in temperature from 20 C to 90 C and
become almost non-emissive (ff ¼ 0.08%) at 90 C (Fig. 5b). It is
to be noted that the spectral nature of monomeric species obtained at 90 C is distinct from the monomeric species of 1 in
toluene (Fig. 1b), indicating that the spectrum obtained at 90 C
corresponds to the emission spectrum of 1@2 complex and not
of 1. This discloses the excellent thermal stability of the intermolecular complex. Furthermore, these results clearly imply
that highly emissive nature of the gel is due to the restricted
rotation of C(sp2)–C(sp2) bonds TPE in the gel state. Whereas in
the monomeric state of 1@2 (at high temperature), nonradiative relaxation of the excited state, most likely the C(sp2)–
C(sp2) bond rotation of TPE units, leads to the signicant
quenching of uorescence. The same optical behaviours were
observed for the gels of 1@2 in other solvents as well (Fig. S10†).
It is also important to note that the gels are stable up to several
months with the emission behaviours intact, which is very
important for their practical applications.
3.
Conclusions
In summary, we have reported the design of a novel class of
two-component, highly emissive, low-molecular weight
supergelator. Individual components (1 and 2) are unable to
gelate any of the solvents. However, they form strong 3 : 1
intermolecular complex (1@2) via complementary hydrogen
bonding interaction between the carboxylic acid moiety of 1
(3 moles) and amine group of 2 (1 mole). Infrared and 1HNMR spectroscopies validated the strong hydrogen bonding
between the two components through complimentary acid–
amine interaction. The supramolecular complex was further
characterized using DSC analyses, which revealed distinct
melting point for the complex. Unlike the individual building
blocks, which were incapable of forming gels in any of the
solvents, the complex (1@2) exhibited remarkable gelation
properties in aromatic solvents such as toluene, o-xylene, pxylene and mesitylene. Selective gelation towards aromatic
solvents was attributed to the possible p-stacking between
the gelator complex and the solvent molecules. Microscopic
analyses of diluted solutions of the gels revealed the formation of micrometre long nanobers. On the other hand, rodlike network assembly was observed for the gel state. These
results suggest that 1@2 complex undergoes self-assembly
via strong p–p stacking interaction of TPE segment to
brous nanostructures and at high concentration the hierarchical assembly of the bres resulted in the formation of
organogel. One of the unique structural features of the gel is
its intense emission in the blue region. This is attributed to
the restricted rotation of C(sp2)–C(sp2) bonds of TPE units in
the gel state, which facilitates the radiative relaxation of the
© 2021 The Author(s). Published by the Royal Society of Chemistry
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excited state and leads to high uorescence quantum yields
for the gel. Furthermore, the gels were found to have excellent
elastic response as evident from the high storage modulus
compared to loss modulus values. We hope the twocomponent nature of gelation, superior gelating ability
(supergelator), blue emission with AIEE and excellent
mechanical behaviour will nd potential applications for this
class of gels in various elds including material science,
nanotechnology and medicine.
4. Experimental section
4.1
General methods
Chemicals used for the syntheses were purchased from Sigma
Aldrich and TCI, and were used as received. TLC analyses were
performed on aluminium plates coated with silica gel 60 F254,
and column chromatography was performed on 200–400 mesh
silica gel. Melting points were measured on Stuart SMP50
automated melting point apparatus and are uncorrected. 1H
and 13C NMR spectra were recorded on a Bruker Avance 500
MHz DPX spectrometer using 1,1,1,1-tetramethylsilane (TMS)
as the internal standard. Water used for all studies was Milli Q
deionised water (18.2 MU cm). FT-IR spectra were recorded on
a IR Prestige-21 (Shimadzu) spectrometer using KBr pellet
method. Mass measurements were performed on a Shimadzu
GCMSQP-2010 in EI mode. ESI-MS analyses were carried out
on Thermo Scientic Q-Exactive plus orbitrap mass spectrometer in positive mode. Matrix-assisted laser desorption
ionization time-of-ight (MALDI-TOF) mass spectra were obtained on a Bruker Daltonics Autoex MALDI-TOF mass
spectrometer. TEM analyses were carried out on FEI Tecnai
F20 (120 kV). Samples were prepared by depositing 2 mL of the
sample on a 400-mesh carbon coated copper grid (Ted Pella,
Inc.). Samples were allowed to adsorb on the grid for 2 min
and then the excess sample was wicked with a piece of lter
paper. FE-SEM were carried out on FEI Nova NanoSEM 450
(FEG type). For FE-SEM, xerogels of respective gels were
sprinkled on the carbon tape stuck on the specimen stub and
was spin coated with platinum and gold. The PXRD experiments were conducted using slow and continuous scan rate
mode using Cu as the anode material (Ka1 ¼ 1.540598 Å). DSC
analyses were done at a heating rate of 5 C min 1 using a DSC
Q2000 differential scanning calorimeter with nitrogen as
purge gas with a steady ow rate of 100 mL min 1. Absorption
spectra were recorded using a quartz cuvette of 10 mm and
1 mm path length on a ShimadzuUV-3600Vis-NIR spectrophotometer having a Peltier controlled cell holder. Steadystate uorescence spectra were recorded on a Horiba Jobin
Yvon Fluorimeter equipped with a thermostat Peltier cell
holder, in a quartz cuvette of 10 mm or 1 mm path length.
Temperature dependent emission spectra were recorded from
20–90 C at an interval of 10 C and the sample was equilibrated for 3 or 10 min at each temperature before measurement. Rheology analyses of the gels were performed using an
Anton Paar Modular Compact Rheometer (MCR 302) using
parallel plate conguration (PP25).
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4.2
Paper
Synthesis of 1
3
A 250 mL two-neck round-bottom ask equipped with
a magnetic bead was charged with 1a (2 g, 1.97 mmol) and dried
under high vacuum using a Schlenk line followed by which the
ask was ushed with inert nitrogen gas several times. Anhydrous THF was added to the ask and degassed with inert
nitrogen gas. 6-Heptynoic acid (1b) (0.323 g, 2.560 mmol) and
[Cu(CH3CN)4]PF6 (0.150 g, 0.394 mmol) was added to the ask
under inert condition and was stirred at 70 C for 24 hours
during which the reaction progress was monitored using TLC.
Upon completion of the reaction, solvent was removed under
reduced pressure and crude mixture was puried using silica
gel column chromatography using 5% MeOH in DCM as the
eluent to yield the desired product as off-white solid (yield:
90%). Rf ¼ 0.45 (MeOH : DCM 5 : 95); mp ¼ 150 C; 1H NMR
(500 MHz, DMSO-d6) d (ppm): 12.04 (s, 1H), 8.25 (s, 2H), 7.93 (s,
1H), 7.10–7.17 (m, 22H), 6.99–7.03 (m, 16H), 6.75–6.76 (m, 1H),
6.59 (d, J ¼ 2 Hz, 2H), 5.56 (s, 4H), 5.48 (s, 2H), 5.14 (s, 4H), 2.64
(t, J ¼ 7 Hz, 2H), 2.27 (t, J ¼ 7 Hz, 2H), 1.55–1.65 (m, 4H); 13C
NMR (125 MHz, DMSO-d6) d (ppm): 174.85, 159.82, 147.46,
143.52, 143.47, 143.45, 143.13, 141.42, 140.44, 138.89, 134.47,
131.45, 131.07, 131.04, 128.35, 128.32, 128.26, 127.83, 127.12,
127.04, 125.21, 122.47, 107.62, 101.35, 61.70, 55.37, 53.08,
52.92, 33.86, 28.87, 25.22, 24.58; MALDI-TOF m/z value for
C74H63N9NaO4 ¼ 1164.49 (calcd) 1164.46 (expt.)
4
Conflicts of interest
There are no conicts to declare.
5
Acknowledgements
Financial support from DBT (BT/PR30172/NNT/28/1593/2018)
and CSIR is gratefully acknowledged.
6
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