Solvent assisted fluorescence modulation of a C3-symmetric organogelator

Article abstract of DOI:10.1039/c4tc01008f

The synthesis and self-assembling properties of C₃ symmetric donor-acceptor molecules containing 1,3,4-oxadiazole and bisthiophene moieties in the core functionalized with octyl (BTOX8) and dodecyl (BTOX12) substituted phenyl acetylene units at

Full text of DOI:10.1039/c4tc01008f

Journal of
Materials Chemistry C
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Solvent assisted fluorescence modulation of a
C3-symmetric organogelator†
Cite this: J. Mater. Chem. C, 2014, 2,
7039
*
Deepak D. Prabhu, Aneesh P. Sivadas and Suresh Das
The synthesis and self-assembling properties of C3 symmetric donor–acceptor molecules containing 1,3,4-
oxadiazole and bisthiophene moieties in the core functionalized with octyl (BTOX8) and dodecyl (BTOX12)
substituted phenyl acetylene units at the periphery are reported. BTOX8 was found to form gels only in
aliphatic solvents, whereas BTOX12 formed gels in both aliphatic and aromatic solvents. Photophysical
analysis of BTOX12 solutions showed a striking effect of the solvent on the nature of the self-assembled
aggregate formed. Our studies indicate that in aliphatic solvents such as n-decane the solvent molecules
interact mainly with the alkyl regions of BTOX12. As a result the p–p interaction between the
neighbouring molecules becomes feasible resulting in strong excitonic coupling between the
neighbouring molecules leading to excimer type emission. In aromatic solvents the solvent molecules
interact mainly with the chromophoric part of BTOX12 resulting in reduced p-stacking between the
molecules in the aggregate leading to monomer type emission. Films prepared from aliphatic and
aromatic solvents exhibited photophysical properties significantly similar to those observed in the
respective solvents. Photophysical studies of the films indicated that the films prepared from n-decane
exhibited an H-type molecular arrangement whereas the films prepared from toluene exhibited a slipped
stack J-type arrangement.
Received 15th May 2014
Accepted 13th June 2014
DOI: 10.1039/c4tc01008f
www.rsc.org/MaterialsC
formation of highly luminescent materials.3 Hence by creating a
difference in the mode of aggregation it would be possible to
Introduction
control the optical properties of the bulk materials formed from
the same set of molecules. Although there are several reports in
the literature on controlling the preferential formation of a
particular type of aggregate in solution using ionic liquids,4
host–guest interactions,5 ultrasound,6 hydrogen bonding inter-
actions7 or by making subtle changes in the molecular struc-
ture,8 only a restricted number of reports discuss the role of
solvents in such processes.9 Moreover the control of aggregation
using solvents does not normally translate into control of
organization in the bulk materials or lms drawn from them.
We have been interested in the optical and self-assembling
properties of multi-arm oxadiazole based donor–acceptor
systems.10 There is a signicant amount of growing interest in
developing this class of materials since they can help to increase
the dimensionality of conjugation compared to conventional
one dimensional polymers and oligomers.11 The ability of such
molecules to self-organize in a columnar fashion can result in
materials wherein anisotropic conduction of excitons, charges
and ions is possible. In this work we describe the synthesis, self-
assembly and optical properties of a class of three arm mole-
cules (BTOX8 and BTOX12) incorporating a bisthiophene
moiety between the oxadiazole and alkoxy substituted phenyl
acetylene chromophore of the p-conjugated arm (Chart 1).
Among the various heterocyclic building blocks thiophene
chromophores have been most widely used for the generation of
The increasing interest in p-conjugated materials, driven
mainly by their potential for application in devices such as light-
emitting diodes, photovoltaic devices, and eld effect transis-
tors, has led researchers to design polymers and small mole-
cules possessing novel architectures and study their
supramolecular organization in the bulk state with the inten-
tion of optimizing their optical and electronic properties.1 In
spite of these efforts, ne control of the supramolecular orga-
nization of these molecules which are dependent on various
weak non-covalent interactions remains a major challenge.2
Depending upon the nature of the application, preferential
organization of the molecules can vary, such as for example the
formation of H-aggregates formed by face-to-face arrangements
of molecules which facilitates better charge transport is desir-
able for OFETs and solar cells, whereas formation of J-aggre-
gates formed by head-to-tail arrangements is desirable for the
Photosciences and Photonics Section, Chemical Sciences and Technology Division,
National Institute for Interdisciplinary Science and Technology and Network of
Institutes for Solar Energy, CSIR–NIIST, Trivandrum-695 019, India. E-mail:
sureshdas@niist.res.in
† Electronic supplementary information (ESI) available: Experimental details,
syntheses, characterization details, dynamic light scattering spectra, preliminary
photophysical data in different solvents, excitation, emission, uorescence
lifetime decay curves and data of BTOX12 in the gel state in toluene and
n-decane. See DOI: 10.1039/c4tc01008f
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prepared by dissolving small amounts of the gelator molecules
in the respective solvents at higher temperatures and allowing
the solution to cool to room temperature in a closed vial. Gel
formation was conrmed by the failure of so mass to ow on
inverting the glass vial.14 It was found that BTOX8 could form
gels only in aliphatic solvents such as n-decane and hexadecane
whereas BTOX12 formed gels in all solvents mentioned above.
The thermal stability of the gels was measured using a
“dropping ball” method.15 The critical gelation concentrations
and Tgel values for both derivatives in different solvents were
estimated and are summarized in Table 1. It may be noted that
both BTOX8 and BTOX12 do not possess any specic hydrogen
bonding moieties and the strong self-assembly leading to gel
formation could be mainly through the synergistic effect of p–p
stacking, dipole–dipole interactions and hydrophobic interac-
tions between the long alkyl chains in these donor–acceptor
molecules. Since BTOX12 formed gels in both aliphatic as well
as aromatic solvents, the aggregation behavior and photo-
physical properties of this molecule were studied in detail.
Dynamic light scattering measurements of 2 ꢀ 10ꢁ4 M solu-
tions of BTOX12 revealed the presence of ꢂ0.5 and 1 micro-
metre sized aggregates in toluene and n-decane, respectively
(Fig. S1†).
Chart 1 Molecular structure of BTOX derivatives.
conducting materials since they can impart favourable charge
transport and redox properties.12 Here, we report on the pho-
tophysical properties of the gels and thin lms of these mole-
cules formed from solutions in aliphatic and aromatic solvents.
It was observed that BTOX8 could gelate only aliphatic solvents
whereas BTOX12 formed stable gels in both aliphatic as well as
aromatic solvents. Moreover, the luminescent properties of the
BTOX12 organogels were observed to be very sensitive to the
nature of the solvent used, which could be attributed to the
differences in the nature of the aggregates formed. Thin lms
drawn from these solvents also exhibited similar differences in
their luminescence behavior. The nature of the different
aggregates formed and their impact on the luminescence
behavior are discussed.
The structures of the aggregates formed in these solutions
were investigated using microscopic techniques. Fig. 1 shows
the AFM and TEM images observed for BTOX12 (2 ꢀ 10ꢁ4 M) in
n-decane and toluene which indicated an interlocked network
structure of bres. In order to elucidate the molecular
arrangements in the bres we have carried out small angle X-ray
scattering (SAXS) and wide angle X-ray scattering (WAXS)
measurements on the xerogels obtained by drying 5 wt% (3 ꢀ
10ꢁ2 M) of n-decane and toluene gels. Both the xerogels showed
only one peak in the small angle region and a broad halo in
the wide angle region. For the n-decane xerogel a SAXS peak at
2q ¼ 1.29ꢃ and for the toluene xerogel a peak at 2q ¼ 1.38ꢃ
˚
˚
corresponding to d-spacing values of 67.97 A and 63.90 A,
respectively (Fig. 2a), were observed. This is indicative of one
dimensional columnar arrangement in bres.16 The absence of
higher order reections in the SAXS pattern suggests that long
range inter-columnar structural ordering is absent in these
systems. In the wide angle region only a broad halo was
Results and discussion
Syntheses
BTOX8 and BTOX12 were synthesized via multistep reactions as
shown in Scheme S1.† The nal step involved conversion of the
corresponding tetrazole derivatives into oxadiazoles by a reac-
tion involving the Huisgen mechanism.13 The tetrazole deriva-
tives (8a and b) were synthesized by reacting sodium azide with
ꢃ
˚
observed corresponding to 4.45 A (2q ¼ 19.94 ) for the toluene
xerogel indicative of liquid like arrangements of the peripheral
alkyl chains (Fig. 2b).17 For the bres observed in the n-decane
xerogel, in addition to the reection peak corresponding to
the
corresponding
50-((3,4-bis(alkoxy)phenyl)ethynyl)-2,20-
bithiophene-5-carbonitrile (7a and b) which were prepared by
performing the Sonogashira coupling reaction between the 5-
iodo-2,20-bithiophene-5-carbonitrile (6) and 1,2-bis(alkoxy)-4-
ethynylbenzene (4a and b). All the intermediates as well as the
nal products were isolated in good yields using chromato-
graphic techniques and were characterized using 1H, 13C NMR,
IR spectroscopy and MALDI-TOF analysis.
Table 1 Critical gelation concentrations (CGCs) and Tgel values of
BTOX12 and BTOX8 in different solvents
BTOX8
BTOX12
Solvent
CGC
Tgel
CGC
Tgel
Gelation properties
n-Decane
Hexadecane
Benzene
16.2 mM
12.1 mM
Soluble
78 ꢃC
87 ꢃC
—
17.6 mM
14.3 mM
18.6 mM
21.5 mM
51 ꢃ
56 ꢃ
65 ꢃ
68 ꢃ
C
C
C
C
The gelation behavior of BTOX8 and BTOX12 was investigated
in aliphatic solvents such as n-decane, hexadecane and
aromatic solvents such as benzene and toluene. The gels were
Toluene
Soluble
—
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Fig. 1 AFM and TEM images of 2 ꢀ 10ꢁ4 M solution of BTOX12 in (a
and c) toluene and (b and d) n-decane.
˚
˚
4.5 A, a peak corresponding to 3.7 A indicative of the p–p
interaction was also observed. This indicates more ordered
packing of the molecules in the nano-bres formed in n-decane
compared to that formed in toluene. The differences in
molecular ordering of the gels in the two solvents may be
attributed to the aliphatic solvent interacting more strongly
with the alkyl chains of the gelator molecule without hindering
the p–p interaction between the molecules, and the aromatic
solvent interacting with the aromatic part of the gelator mole-
cule which would minimize the effective p–p interaction
between the gelator molecules.
Fig. 2 (a) SAXS and (b) WAXS patterns of the BTOX12 xerogel in n-
decane and toluene.
aggregates. Except for a slight broadening the absorption
spectrum of the aggregate is very similar to that of the mono-
mer. In contrast the emission spectra of the monomer and the
aggregated forms were signicantly different. In comparison to
the structured monomer emission in the 450–600 nm region the
aggregates formed in n-decane exhibited a broad, structureless
and highly red shied emission spectrum (Stokes shi ꢂ 200
nm) with a maximum centered at 610 nm. The presence of
aggregates was conrmed by studying the effect of temperature
on the absorption spectrum (Fig. 3b). With decrease in
temperature, formation of aggregates is indicated by the hypo-
chromism and slight broadening of the absorption spectrum.
Such changes in the absorption spectrum on formation of
aggregates are normally attributed to the formation of H-type
aggregates.19 Fig. 4a shows the excitation spectrum measured in
1 ꢀ 10ꢁ7 M and 2 ꢀ 10ꢁ4 M solutions of BTOX12 in n-decane. In
the dilute solution, BTOX12 exhibited a narrow excitation
spectrum with a maximum centered at 402 nm which could be
attributed to the solvated molecules. The excitation spectrum
measured in aggregated solutions however showed consider-
able broadening with a slight blue shi in the excitation
maximum (389 nm) suggesting the presence of exciton coupled
aggregates. Fluorescence lifetimes of the excited species formed
Photophysical properties
The photophysical properties of the BTOX12 derivative in
different solvents under dilute conditions were measured
(Fig. S2 and Table S1†). A minimal shi in the absorption
maxima with solvent polarity (Fig. S2a†) indicated that the
dipole moment values for both ground and Franck–Condon
(FC) excited states are the same, whereas a bathochromic shi
with increasing polarity (Fig. S2b†) indicated that luminescence
occurred from a non-Frank–Condon polar intramolecular
charge-transfer (ICT) state.18 Since the X-ray measurements
indicated clear differences in the nature of the self-assembly of
BTOX12 molecules in the xerogels formed from n-decane and
toluene detailed photophysical properties of the aggregates
formed in these solvents were compared.
Aggregation behavior in n-decane
Fig. 3a shows the normalized absorption and emission spectra in the 1 ꢀ 10ꢁ7 M and 2 ꢀ 10ꢁ4 M solutions were measured by
of BTOX12 in a dilute (1 ꢀ 10ꢁ7 M) solution containing solvated monitoring at their emission maxima (500 nm for dilute solu-
molecules and in 2 ꢀ 10ꢁ4 M solution containing the tion and 610 nm for the concentrated solution; Fig. 4b). The
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non-emissive or weakly emissive due to rapid intraband relax-
ation processes there are a few reports in the literature
describing luminescent H-aggregates where the observed
emission was ascribed to the formation of either excimer type
species or due to slight rotation of the two exciton coupled
molecules in the excited state.20 The photophysical properties of
the gel formed in n-decane were very similar to those observed
in the concentrated solution (Fig. S3 and Table S2†).
Aggregation behavior in toluene
Fig. 5a shows the normalized absorption and emission spectra
of BTOX12 in 1 ꢀ 10ꢁ7 M and 2 ꢀ 10ꢁ4 M solutions in toluene.
Unlike in n-decane the absorption and emission spectra were
nearly the same for the two solutions, although dynamic light
scattering measurements of the concentrated solution had
revealed the presence of aggregates with an average size of
ꢂ0.5 micrometer (Fig. S1b†). The absence of signicant spectral
changes suggests that the excitonic coupling between the
neighboring molecules is very weak in these aggregates.
Temperature dependent absorption changes however indicated
the break-up of the aggregates with increasing temperature
(Fig. 5b). Except for a slight broadening, the excitation spectra
of the aggregates were very similar to those of the monomers
Fig. 3 (a) Absorption and emission spectrum (lex ¼ 400 nm) of
BTOX12 in dilute (C ¼ 1 ꢀ 10ꢁ7 M; black dashed line) and aggregated
solutions (C ¼ 2 ꢀ 10ꢁ4 M; orange line) in n-decane and (b) temper-
ature dependent changes in the absorption spectrum of 2 ꢀ 10ꢁ4
solution of BTOX12 in n-decane.
M
Fig. 4 (a) Excitation spectrum (monitored at emission maximum) and
(b) fluorescence lifetime decay profile of BTOX12 in 1 ꢀ 10ꢁ7 M (black)
and 2 ꢀ 10ꢁ4 M (orange) solution in n-decane.
uorescence lifetime decay prole for the 1 ꢀ 10ꢁ7 M solution
exhibited a mono-exponential decay with a lifetime value of 0.3
ns attributable to the solvated monomer species present under
these conditions. For the concentrated solution a triexponential
decay was observed with all the species having higher lifetimes
[T1 ¼ 0.5 ns (36%), T2 ¼ 1.8 ns (47%), T3 ¼ 7.4 ns (17%)].
The broadening of the absorption spectrum coupled with the
red-shied emission spectrum and long lifetimes conrm the
formation of H-type aggregates in the concentrated solutions.
Fig. 5 (a) Normalized absorption and emission spectra (lex ¼ 400 nm)
of BTOX12 in dilute (C ¼ 10ꢁ7 M; black short dashed line) and aggre-
gated solutions (C ¼ 2 ꢀ 10ꢁ4 M; green line) in toluene and (b)
temperature dependent changes in the absorption spectrum of
Although such types of aggregates are usually reported to be 2 ꢀ 10ꢁ4 M solution of BTOX12 in toluene.
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(Fig. 6a). Fluorescence lifetime measurements of the aggregated
solution exhibited a triexponential decay with the majority
component (83%) exhibiting a lifetime (0.4 ns) very similar to
that observed for the monomer (0.3 ns) (Fig. 6b). These results
conrm that although the molecules are aggregated, the exci-
tonic coupling between the molecules within the aggregates
formed in toluene is signicantly weaker than that observed in
n-decane. The photophysical properties of the toluene gel were
nearly similar to those observed in the concentrated toluene
solution (Fig. S4 and Table S2†).
An interesting observation was made while preparing solu-
tions of higher concentration ($10ꢁ3 M) and gels of BTOX12 in
toluene. BTOX12 solutions of concentrations $10ꢁ3 M were
prepared by dissolving appropriate amounts of the compound
at high temperature and subsequently allowing the solution to
cool to room temperature. Initially the emission spectra of these
solutions were very similar to those observed in n-decane, with
the emission maximum centered at 610 nm. Over a period of
time (ꢂ30 minutes) the intensity of the 610 nm emission band
decreased and a concomitant increase in the intensity of an
emission band centered at 500 nm was observed (Fig. 7a). The
change in the uorescence intensities at the two wavelengths
(500 nm and 610 nm) plotted as a function of time showed that
the initial uorescence spectrum was stable for up to 8 minutes
following which transformation of one band to the other took
place over a period of ꢂ30 minutes (Fig. 7b). This change could
be visually perceived as a change in the color of emission from
orange to green (Fig. 7c).
Based on the spectral changes it could be summarized that
in the concentrated BTOX12 solutions in toluene, excitonically
coupled p-stacked H-type aggregates are initially formed. Over a
period of time, the interaction of toluene with the aromatic part
of the BTOX12 molecules leads to disruption of the p-stacked
H-type aggregates yielding aggregates in which excitonic
coupling between neighbouring BTOX12 molecules is weak.
To conrm that the toluene solvent molecules disrupt the
formation of exciton coupled aggregates the emission spectra of
1 ꢀ 10ꢁ3 M solutions of BTOX12 in toluene–n-decane solvent
mixtures of varying proportions were studied (Fig. 8) by diluting
Fig.
7
(a) Time dependent changes in the emission spectrum
(lex ¼ 400 nm; 1 mm cuvette) of 1 ꢀ 10ꢁ3 M solutions of BTOX12 in
toluene and (b) secondary plot showing the changes in the intensity at
610 nm and 500 nm with time and (c) photographs showing the time
dependent changes in fluorescence colour of 1 ꢀ 10ꢁ3 M solution of
BTOX12 in toluene under 365 nm UV light.
studies we can conclude that toluene being an aromatic mole-
cule is capable of forming weak complexes with the chromo-
phoric part of BTOX12 and thereby interferes with the excitonic
coupling between the molecules in the aggregates by disrupting
the p-stacking between them. Recently, Banerjee and co-
workers have reported the involvement of solvent molecules in
forming charge transfer complexes with gelator molecules.
Using spectroscopic and computational studies, the authors
have shown that different aromatic solvents (toluene, xylenes
and mesitylene) form CT complexes with a naphthalene diimide
based gelator molecule. The strength of the CT complex formed
was found to depend on the electron donating ability of the
solvent molecule.21 In order to test the hypothesis that the
aromatic solvent molecules could interact with the chromo-
phoric unit of BTOX12 and prevent the formation of H-stacked
a
concentrated solution of BTOX12 in n-decane. Upon
increasing the percentage composition of toluene in the
mixture, the intensity of the uorescence band centered at
610 nm decreased and a concomitant increase in the intensity
of the 500 nm emission band was observed. Based on these
Fig. 6 (a) Excitation spectrum (monitored at emission maximum) and Fig. 8 Changes in the emission spectrum (lex ¼ 400 nm; 1 mm
(b) fluorescence lifetime decay profile of BTOX12 in 1 ꢀ 10ꢁ7 M (black) cuvette) of 1 ꢀ 10ꢁ3 M solution of BTOX12 in n-decane–toluene
and 2 ꢀ 10ꢁ4 M (green) solutions in toluene.
solvent mixtures.
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Scheme 1 Schematic representation of the aggregate formation of
BTOX12 in n-decane and toluene.
exhibited a broad band centered at 360 nm, whereas the lm
prepared from toluene exhibited a sharp red shied peak with
the maximum at 480 nm (Fig. 10b). In comparison with the
spectra observed in solutions it is clear that in the BTOX12 lms
drawn from n-decane the molecules are arranged in a face to
face arrangement. The red-shied excitation and emission
peaks observed for the lms drawn from toluene compared to
the respective spectra measured in solution indicate that in the
Fig. 9 (a) Emission spectrum of BTOX12 in the gel state in different
solvents (lex ¼ 400 nm; 1 mm cuvette) and (b) photograph of various
gels under 365 nm UV illumination.
aggregates, we have investigated the gels formed in different
aromatic solvents differing in their bulkiness with respect to
toluene (Fig. 9). The emission spectra showed an increasing red-
shi (toluene < o-xylene < mesitylene) with increasing bulkiness
of the solvent. This conrms that the aromatic solvents can
disrupt p-stacking between the molecules in the aggregates
leading to a reduction in exciton coupling between them. As the
bulkiness of the aromatic solvent increases, its ability to inter-
fere with the p-stacking between neighbouring molecules
decreases leading to improved exciton coupling between the
molecules.
Based on these results a schematic representation of the
mode of disruption of the p-stacking within the aggregates is
shown in Scheme 1. The alkyl solvent n-decane will mainly
interact with the alkyl regions of the molecule and hence will
not interfere with the excitonic coupling of the aromatic core of
the BTOX12 molecules. It was interesting to note that lms cast
from toluene and n-decane retained to some extent the differ-
ences in photophysical characteristics observed in the respec-
tive solvents. Fig. 10a shows the emission spectra of lms of
BTOX12 prepared by drop casting 5 ꢀ 10ꢁ3 M solution in
n-decane and toluene on a quartz plate. Although the uores-
cence spectrum of the lm drawn from n-decane was identical
to that observed in solution (Fig. 10a and S6†) the maximum of
the uorescence band of the lm drawn from toluene was red
shied by 50 nm compared to that observed in the toluene
Fig. 10 (a) Emission spectrum (lex ¼ 400 nm) and (b) excitation
spectrum (emission monitored at lmax) of films of BTOX12 cast from n-
solution. The excitation spectra of the lm from n-decane decane (orange) and toluene (green).
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aliphatic and aromatic solvents. Photophysical studies indi-
cated that the nature of the aggregates of BTOX12 formed in
these two types of solvents were remarkably different. Absorp-
tion, uorescence and lifetime analysis of the aggregated and
gel states indicated that the aliphatic solvent molecules interact
mainly with the alkyl regions of the BTOX12 molecule, allowing
strong p–p interactions to occur between the chromophoric
units of the molecules. This led to the formation of strongly
excitonic coupled H-aggregates as indicated by the broadening
of the absorption spectrum accompanied by highly red shied
excimer type emission with signicantly long uorescence
lifetimes. The aromatic solvents on the other hand interact
more strongly with the chromophoric part of BTOX12 resulting
in reduced p-stacking between the molecules in the aggregate
leading to monomer type emission. It was interesting to note
that lms prepared from aliphatic and aromatic solvents
retained the photophysical characteristics seen in the respective
solvent to a signicant extent. Films prepared from n-decane
exhibited an H-type molecular arrangement whereas the lms
prepared from toluene exhibited
a slipped stack J-type
arrangement. Such control on the nature of the aggregate
formed by changing the type of solvent is highly desirable since
it would be possible to control the optoelectronic properties of
the bulk materials formed from the same set of molecules using
simple techniques.
Fig. 11 (a) Emission spectrum (lex ¼ 400 nm) and (b) excitation
spectrum (lem monitored at 610 nm) of the toluene film of BTOX12
before and after thermal treatment.
Acknowledgements
This work is supported by the Council of Scientic and Indus-
trial Research (CSIR) under the Project NWP 55. D.D.P. and
A.P.S. are grateful to CSIR for fellowships (contribution no.
lms the molecules are organized in a slip-stacked J-type NIIST-PPG-358).
arrangement.22
Removal of the aromatic solvent complexed with the
Notes and references
aromatic core of BTOX12 would lead to improved coupling
between the chromophores resulting in the formation of a J-type
aggregate. On annealing the BTOX12 lms prepared from
toluene by holding it at 120 ꢃC for 20 minutes the emission
changed from yellow (ꢂ550 nm) to orange red (ꢂ610 nm)
(Fig. 11a), which can be attributed to the transformation of the
J-type aggregate into the H-type aggregate. It is to be noted that
there was no phase transition and the compound was stable at
this temperature as evidenced from the DSC and TG analyses
(Fig. S5†). The excitation spectra also showed a broadening,
with increased intensity of emission at lower excitation wave-
lengths which is in support of the J- to H-type transformation on
annealing (Fig. 11b). The lms drawn from n-decane however
did not show any change on similar treatment, conrming that
the face-to-face H-type aggregates are the thermodynamically
more stable form for this molecule.
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Conclusion
The synthesis, self-assembly and gelation behavior of two C3
symmetric donor–acceptor molecules BTOX8 and BTOX12 have
been investigated. BTOX8 was observed to form gels only in
aliphatic solvents, whereas BTOX12 formed gels in both
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