Aromatic π-π Driven supergelation, aggregation induced emission and columnar self-assembly of star-shaped 1,2,4-oxadiazole derivatives

Article abstract of DOI:10.1039/c6tc01939k

A new family of star shaped 1,2,4-oxadiazole derivatives with the variation in the number of flexible peripheral tails have been synthesized and characterized. It is interesting to note that the number of flexible tails at the periphery dictated the self-

Full text of DOI:10.1039/c6tc01939k

View Article Online
View Journal
Journal of
Materials Chemistry C
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: S. K. Pathak, B.
Pradhan, R. K. Gupta, M. Gupta, S. K. PAL and A. Ammathnadu Sudhakar, J. Mater. Chem. C, 2016, DOI:
10.1039/C6TC01939K.
This is an Accepted Manuscript, which has been through the
Royal Society of Chemistry peer review process and has been
accepted for publication.
Accepted Manuscripts are published online shortly after
acceptance, before technical editing, formatting and proof reading.
Using this free service, authors can make their results available
to the community, in citable form, before we publish the edited
article. We will replace this Accepted Manuscript with the edited
and formatted Advance Article as soon as it is available.
You can find more information about Accepted Manuscripts in the
Information for Authors.
Please note that technical editing may introduce minor changes
to the text and/or graphics, which may alter content. The journal’s
standard Terms & Conditions and the Ethical guidelines still
apply. In no event shall the Royal Society of Chemistry be held
responsible for any errors or omissions in this Accepted Manuscript
or any consequences arising from the use of any information it
contains.
www.rsc.org/materialsC

Page 1 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
Aromatic πꢀπ Driven Supergelation, Aggregation Induced Emission and
Columnar Selfꢀassembly of Starꢀshaped 1,2,4ꢀOxadiazole Derivatives
Suraj Kumar Pathaka, Balaram Pradhana, Ravindra Kumar Guptaa, Monika Guptab, Santanu
Kumar Palb and Ammathnadu S. Achalkumara*
aDepartment of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781039,
Assam, India. b Indian Institute of Science Education and Research Mohali, Sectorꢀ81,
Knowledge City, Manauli 140306, Punjab, India.
Abstract: A new family of star shaped 1,2,4ꢀoxadiazole derivatives with the variation in the
number of the flexible peripheral tails have been synthesized and characterized. It is
interesting to note that the number of flexible tails in the periphery dictated the selfꢀassembly
and the photophysical behavior. The compound with three flexible tails stabilized a crystalline
state with lamellar packing, and did not show organogelation, but showed blue emission in
crystalline and thin film state. The compound with six flexible tails stabilized hexagonal
columnar liquid crystalline state and it showed the ability to gelate in nonpolar solvents at a
concentration less than 1 weight percent; qualifying it as a supergelator, where πꢀπ
interactions play a major role. This phenomenon is very rare, in comparison to earlier reports
where the supergelation is supported by Hꢀbonding interactions. Besides the capability to
form selfꢀstanding, moldable gel, this compound also exhibited aggregationꢀinduced emission
(AIE), which persisted even in xerogel state. Xꢀray diffraction studies unraveled the
rectangular columnar selfꢀassembly in the gel state. The columnar order and emissive nature
in the liquid crystal and xerogel state makes this molecule promising for the application in
emissive displays. Compounds with nine alkyl tails stabilized long range columnar hexagonal
phase. This report emphasizes the importance of various nonꢀcovalent interactions in deciding
the nature of selfꢀassembly.
Introduction
The selfꢀassembly of organic molecules into wellꢀdefined hierarchical superstructures that
perform various functions requires a ‘bottomꢀup’ approach from the molecular level.
Incorporation of the functional units in the molecular structure alone will not guarantee the
anticipated outcome. In order to have a preferred outcome, behavior of the molecular
aggregates in bulk or in a medium matters. Liquid crystals (LC) are one such example of
engineered molecular materials, which are the resultant of the spontaneous selfꢀassembly of
shape anisotropic molecules. This state of the matter possess an intermediate order and
mobility between solids and liquids.1 The driving force for such assembly is mainly
nanophase segregation between the incompatible molecular subunits in addition to other
secondary interactions like Hꢀbonding, πꢀπ interactions, ionic, hydrophilic or hydrophobic
interactions. With a careful molecular design one can predict the outcome of the selfꢀ
assembly of such shape anisotropic molecules. Gelators are another class of molecules with
1

Journal of Materials Chemistry C
Page 2 of 31
View Article Online
DOI: 10.1039/C6TC01939K
specific structural features, which immobilize large amount of solvents when they selfꢀ
assemble.2 They may gelate organic or aqueous solvents. The selfꢀassembly of such molecules
to form gels is assisted by weak forces like Hꢀbonding, πꢀπ interactions, hydrophobicꢀ
hydrophilic or van der Waals interactions. This class of materials is showing promise for the
application in optoelectronics,3 templates for the growth of inorganic nanostructures,4
fluorescent sensors5 and controlled release systems. Usually the ability of organogelators to
gelate large volume of solvent comes from the presence of certain functional groups like
amide, alcohol, carboxylic acid, aromatic units, steroid moieties and aliphatic chains etc.2
Gelation brings about drastic changes in the electronic and optical properties. For example,
fluorescent molecules that are emissive as monomers may not be emissive in aggregated state
due to aggregation induced quenching of fluorescence. Organogels which show aggregation
induced emission (AIE) due to their supramolecular selfꢀassembly in gel state form an
important class of materials from the view point of solid state lighting.6 Though there are
several reports on AIE active molecules which emit across the whole visible region,7 there is
dearth in the solid state blue emitters. Blue light emitters form an integral part of the full color
display devices and also form a vital cog in white light emitting devices.8 Hence there are lot
of efforts on building solidꢀstate blue light emitters by the introduction of functional moieties
which will help to generate AIE.9 Among gelators, there is a class of supergelators which at
very low concentration (less than 1 wt %) yield selfꢀstanding, selfꢀhealing and moldable
gels.10 A common structural feature of these molecules is the ability to form strong Hꢀ
bonding.11 There is a single report so far on a polycatenar type supergelator without such Hꢀ
bonding but mainly stabilized by πꢀπ interaction.12 The presence of two heterocyclic (1,3,4ꢀ
oxadiazole) units might have increased the intermolecular πꢀπ interactions to stabilize the
aggregation. This observation encouraged us to incorporate heterocyclic moieties in our
molecular design that might enhance the πꢀπ interactions to stabilize gelation.12 Synthesis of
such supergelators is challenging, as there is no proven design principle.
Integration of heterocyclic moieties in the molecular architecture of thermotropic
liquid crystals is a rewarding task as it can bring about a large change in their thermal and
physical properties. This stems from the polarizability of heteroatoms like nitrogen, oxygen
and sulfur atoms. There is an increased interest in the design of highly πꢀconjugated liquid
crystals bearing heterocycles13 due to the enormous possibilities they offer. Liquid crystalline
selfꢀassemblies derived from electron deficient heteroaromatic rings are potential candidates
for organic lightꢀemitting diodes (OLEDs),14 by offering good chargeꢀtransporting property
and strong fluorescence. Especially 1,3,4ꢀoxadiazole moieties are wellꢀknown for their
2

Page 3 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
excellent electronꢀaccepting behavior and fluorescence and hence been incorporated in the
core structure of calamitic and discotic LCs, to obtain the conductive smectic or columnar
mesophases respectively.13, 32 In spite of the good emissive nature and high thermal endurance
of 1,2,4ꢀoxadiazole unit, the reports on its incorporation in LC design the absorption spectra
are taken scarce, unlike its 1,3,4ꢀisomer. This is probably due to the synthetic difficulty in
comparison to 1,3,4ꢀoxadiazole derivatives. The asymmetric ring of 1,2,4ꢀoxadiazole moieties
provides a strong lateral dipole, which leads to the stabilization of different LC phases than
those obtained from corresponding 1,3,4ꢀisomers.15,17ꢀ24 Asymmetrical substitution of 1,2,4ꢀ
oxadiazole could be another tuning handle to modify their material property. In comparison to
1,2,5ꢀ and 1,3,4ꢀoxadiazole derivatives, asymmetrically substituted 1,2,4ꢀoxadiazole
derivatives are known for their high band gap and known to possess high S1 and T1 energies,
which is beneficial for the design of high triplet energy host materials for efficient
phosphorescent OLEDs (PHOLEDs).16 PHOLEDs are known for their higher efficiency in
comparison to OLEDs as they harvest both the singlet and triplet excitons. According to
literature 1,2,4ꢀoxadiazole derivatives have a moderate thermal stability (decompose around
o
300 C) than their 1,3,4ꢀoxadiazole isomers, 17 but they exhibit lower melting points and a
wider mesophase range.18 There are many reports on banana shaped, 19 bent shaped liquid
crystals, 17, 20, 21, 22 supramolecular liquid crystals,23 chiral calamitic liquid crystals 24 based on
1,2,4ꢀoxadiazole derivatives. In the case of bent compounds the bending angle provided by
the 1,2,4ꢀoxadiazole core is 140o which slightly more than that of 1,3,4ꢀoxadiazole core
(134o) and hence they are anticipated to show some interesting properties like biaxial nematic
behavior.19a Apart from the bent polycatenars reported by Gallardo et. al.,20a there are no
reports on molecules based on 1,2,4ꢀoxadiazoles exhibiting columnar phases. Though
columnar (Col) phases formed by the oneꢀdimensional stacking of discꢀlike molecules were
discovered in 1977,25 consequently many different nonꢀconventional molecular designs like
polycatenars, dendrimers, metallomesogens, bent shaped molecules, oligomers, polymers,
macrocycles and star shaped molecules were identified to stabilize Col phases.1 Columnar
phases are of technological importance in recent times because of their potential application
in organic electronic devices like solar cells, field effect transistors and OLEDs as they
provide oneꢀdimensional pathway for the movement of charge carriers along the columns. 26
Star shaped mesogens or ‘hekates’ are formed by the covalent linking of three arms
symmetrically to a central core.27 Shapeꢀpersistent star shaped mesogens are obtained on
connecting the three rigid arms to central core through linkers. Their lack of shape anisotropy
to exhibit mesophases is compensated by the nanophase segregation of chemically or
3

Journal of Materials Chemistry C
Page 4 of 31
View Article Online
DOI: 10.1039/C6TC01939K
physically different molecular subunits and their tendency of efficient space filling. Compared
to the synthetic difficulty associated with discotics this molecular design is advantageous due
to the inherent synthetic flexibility. Thus the Col phases can be incorporated with other
functional properties like hole or electron transport, nonlinear optical activity or fluorescence.
Apart from this aspect, star shaped molecules are capable of exhibiting a rich variety of
mesophases like nematic, columnar, cubic or soft crystal phases, because of their peculiar
structure and tunable molecular design. The presence of the voids between the arms of the
molecular structure helps to avoid the crystallization of mesophases and promote the glassy
state. Columnar phases with glassy nature are important as they allow the movement of
charge carriers through them with the simultaneous restriction of ionic impurities.28 In the
context of the application of Col phases to OLEDs, it is important to prepare molecules,
which stabilize Col phase along with the preserved solidꢀstate luminescence. This is because
aggregation quenching of luminescence in Col phase is detrimental to the device
performance.
In this paper we report the first examples of star shaped molecules containing three
1,2,4ꢀoxadiazole heterocyclic moieties attached to the 1, 3 and 5 positions of the central
benzene ring. These molecules exhibit Colh phase over a wide range. Most importantly one of
these molecules shows supergelation in hydrocarbon solvents. Gel and xerogel show
aggregationꢀinduced emission behavior with blue emission, while the dilute solutions
containing solvated monomers is weakly emissive.
Results and discussion
Synthesis and Characterization. The synthetic method for the preparation of the target
molecules is presented in scheme 1. The synthetic methods for ethyl gallate, 3,4,5ꢀ
trialkoxyethyl gallate are same as reported earlier.29aꢀc 3,4,5ꢀTrialkoxy ethyl gallates (6cꢀd)
and 3,4ꢀdialkoxy benzaldehyde (4b) were obtained by the Oꢀalkylation of ethyl gallate and
3,4ꢀdihydroxy benzaldehyde respectively following Williamson’s protocol. Compounds 6cꢀd
were reduced to respective 3,4,5ꢀtrialkoxy benzyl alcohols (5cꢀd) by lithium aluminium
hydride (LAH) in dry THF. Further oxidation of 3,4,5ꢀtrialkoxy benzyl alcohols (5cꢀd) using
4

Page 5 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
Scheme 1. Synthesis of star shaped 1,2,4ꢀoxadiazole derivatives. Reagents and conditions: (i)
1ꢀBromoalkanes, anhyd.K2CO3, DMF, 80 oC, 12 h (70ꢀ90%); (ii) LAH, THF, 0 oC to rt, 12 h
(75ꢀ80%); (iii) PCC, DCM, rt, 1 h (70ꢀ81%); (iv) NH2OH.HCl, DMSO, reflux, 3 h (70ꢀ80%);
(v) NH2OH.HCl, triethylamine, CH3CN, reflux, 24 h (70ꢀ85%); (vi) trimesic acid chloride,
pyridine, 6 h, reflux(55ꢀ60%).
pyridinium chlorochromate (PCC) gave 3,4,5ꢀtrialkoxy benzaldehydes (4cꢀd).29b These
benzaldehyde derivatives (4bꢀd) were converted to their respective nitriles (3bꢀd) upon
o
heating with hydroxylamine hydrochloride in DMSO at 100 C for 3h.30 pꢀHydroxy
benzonitrile was converted to pꢀalkoxy benzonitrile (3a) by Williamson’s protocol. Further
reflux of nitrile (3aꢀd) with hydroxylamine hydrochloride in ethanol led to benzamidoxime
intermediates (2aꢀd), which on further refluxing with trimesylchloride in pyridine afforded
1,3,5ꢀbenzene substituted 1,2,4ꢀoxadiazole derivatives (1aꢀd).31 The structures of all the
intermediates and target molecules were confirmed using 1H NMR, 13C NMR, IR
spectroscopy and ESIꢀHRMS or MALDIꢀTOF analysis.
5

Journal of Materials Chemistry C
Page 6 of 31
View Article Online
DOI: 10.1039/C6TC01939K
1
The H NMR spectra of oxadiazole derivatives 1aꢀd exhibited low field signals (δ =
9.15 ppm for 1a) for the protons on central benzene ring in comparison to that of the
previously reported32 star shaped molecules with 1,3,4ꢀoxadiazole arms (δ = 8.95 ppm),
which is due to the more electron withdrawing nature of 1,2,4ꢀoxadiazole units. Similarly, the
13C NMR of 1,2,4ꢀoxadiazole derivatives 1aꢀd exhibited low field signals at 173 ppm and 169
ppm for the heterocyclic carbons in comparison to the heterocyclic carbons of the 1,3,4ꢀ
oxadiazole derivatives (signals at 169 ppm and 165 ppm)(See the supporting information (SI)
for the details).
Thermal behavior. The target molecules were probed for their thermal behavior with the
help of thermogravimetric analysis (TGA), Polarizing Optical Microscopy (POM) and
Differential Scanning Calorimetry (DSC). The TGA studies of compounds 1aꢀd showed good
o
o
thermal stability at least upto ≈ 300 C and complete degradation occurred at around 600 C,
as evidenced by TGA (See the SI). The thermotropic LC behavior of the compounds is
summarized in table 1. A comparison between the thermal behaviors of the earlier reported
regioisomers32 of the present series is furnished in Fig. 1.
Table 1. Phase transition temperatures a (oC) and corresponding enthalpies (kJ/mol) of DLCs
Phase sequence
2nd Heating
1st Cooling
I 82.9 (332.3) Cr
Cr 128.8 (375.6) I
Cr 106.7 (340.2) Colh 112.4 (13.8) I
Cr 12.9 (100.2) Colh 160.0 (26.5) I
Cr1 22.0 (137.4) Cr2 42.0 (607.6) Cr3 60.0 (751.2) I 149.3 (40) Colh 12.1 (65.1) Cr
Colh 150.6 (40.0) I
1a
1b
1c
1d
I 111.4 (15.1) Colh 75.8 (349) Cr
I 158.9 (26.3) Colh 3.8 (42.8) Cr
a
Peak temperatures in the DSC thermograms obtained during the second heating and first cooling cycles at 5
oC/min. Cr = Crystal phase; Colh = Columnar hexagonal phase; I = Isotropic phase.
Figure 1. Bargraph summarizing the thermal behavior of compounds Iaꢀc32 and 1aꢀd (first
heating cycle, temperature range of the Colh phase is mentioned on the bars)
6

Page 7 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
Compound 1a with three nꢀdecyloxy tails turned out to be a crystalline compound, as in the
case of corresponding 1,3,4ꢀoxadiazole derivative Ia.32 The flexibility provided by three
alkoxy tails is not sufficient to stabilize liquid crystalllinity. Compound 1b with six nꢀ
decyloxy tails exhibited an improved thermal behavior with an enantiotropic columnar
hexagonal phase (Colh) phase over a short thermal range (6 degrees in heating cycle). On
cooling from the isotropic liquid state, a mosaic texture interspersed with homeotropic
domain (Fig.2a) was found, before reaching crystallization at ≈ 76o (Fig.2b).
Figure 2. Photomicrograph of texture as seen by POM for the Colh phase of compound 1b at
85 °C (a); DSC traces obtained for the first cooling (upper trace) and second heating (lower
trace) cycles of 1b at a rate of 5 °C min−1 (b) and XRD profiles depicting the intensity against
the 2θ obtained for the Colh phase of compound 1b at 100 oC and 85 oC (inset shows the XRD
image patterns obtained) (c)
The compound was investigated further with the help of XRD studies to understand the
o
symmetry of the mesophase. Powder XRD patterns obtained at 100 oC and at 85 C (Fig. 2b)
were almost similar. The XRD pattern obtained at 100 oC exhibited a sharp single peak at low
angle (2θ ≈ 3o), along with two diffused peaks at wide angle (2θ ≈ 16ꢀ28o). The first diffused
peak is corresponding to the packing of flexible alkyl tails, while the second one corresponds
to the packing of discs within the column, i.e. intracolumnar distance. The intracolumnar
distance was found to be 3.29 Å, which was quiet low without any secondary interactions.3 It
is to be noted that, the regioisomeric compounds Ibꢀc did not show any peak corresponding to
coreꢀcore stacking,32 thus confirming the enhanced coreꢀcore interaction in the case of 1,2,4ꢀ
oxadiazole derivatives (1bꢀd). This enhanced coreꢀcore interaction with low intracolumnar
distance is beneficial to oneꢀdimensional charge carrier mobility.26 The sharp single peak
points that the phase under investigation is Colh phase. Even though it is not unambiguous to
assign hexagonal symmetry for Col phase with a single peak in the low angle, there are many
instances in literature, where in the phase was assigned as Colh because of the minimum in the
7

Journal of Materials Chemistry C
Page 8 of 31
View Article Online
DOI: 10.1039/C6TC01939K
Figure 3. Selfꢀorganization of compound 1b in hexagonal columnar (Colh) lattice. Space
filling energy minimized (allꢀtrans) molecular model of 1b derived from molecular
mechanics (MM2) method.
Table 2. Results of (hkl) indexation of XRD profiles of the compounds at a given temperature
(T) of mesophase
Compound Phase
dobs (Å)
dcal (Å) Miller
Lattice parameters (Å), Lattice area S
(T/oC)
indices (Å2), Molecular volume V (Å3)
(D/Å)
hkl
1b
Colh
30.64
30.64
30.63
28.62
29.63
29.60
32.83
28.63
29.60
32.83
100
100
100
100
100
100
100
100
100
a = 35.38, S = 1084.1, V = 3566.5,
Z = 1.48
(100)
4.48 (ha)
3.29 (hc)
30.63
(42.21)
Colh
(85)
a = 35.37, S = 1084.4, V = 3564.3,
Z = 1.48
4.51 (ha)
3.29 (hc)
28.62
4.65 (ha)
3.29 (hc)
28.63
4.55 (ha)
3.28 (hc)
29.60
4.40 (ha)
3.29 (hc)
29.62
4.57 (ha)
3.29 (hc)
30.62
4.51 (ha)
3.29 (hc)
30.63
4.43 (ha)
3.29 (hc)
32.83
4.36 (ha)
3.30 (hc)
1c
Colh
(145)
a = 33.05, S = 945.9, V = 3112,
Z = 0.98
(42)
Colh
(80)
a = 33.06, S = 946.5, V = 3114,
Z = 0.98
Colh
(25)
a = 34.18, S = 1011.7, V = 3328.6,
Z = 1.05
1d
Colh
(130)
a = 34.2, S = 1013, V = 3332.8,
Z = 0.93
a = 34.8, S = 1053.1, V = 3464.6,
Z = 0.96
(42.8)
Colh
(100)
a = 35.4, S = 1083.1, V = 3563.3,
Z = 1
Colh
(50)
Colh
(25)
a = 37.9, S = 1244.6, V = 4107.2,
Z = 1.1
aThe diameter (D) of the disk (estimated from Chem 3D Pro 8.0 molecular model software from Cambridge Soft). dobs
:
spacing observed; dcal: spacing calculated (deduced from the lattice parameters; a for Colh phase). The spacings marked
ha and hc correspond to diffuse reflections in the wideꢀangle region arising from correlations between the alkyl chains and
core regions, respectively. Z indicates the number of molecules per columnar slice of thickness hc, estimated from the
lattice area S and the volume V.
8

Page 9 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
form factor.32, 34 Hexagonal cell parameter ‘a’ was calculated from d100 and was found to be
35.38 Å. The value of ‘a’ is 15% less than the molecular diameter of compound 1b (Table 2)
which may be due to the interdigitation of peripheral alkyl chains or folding of alkyl chains in
mesophase (Fig.3). The number of molecules forming the columnar slice, which is denoted as
Z was found to be 1.48. This is difficult to explain considering the star shaped molecular
structure.32 Such Z value (Z >1) is often found in polycatenars and explained due to the sideꢀ
byꢀside packing of two molecules to form a disc.13d
Figure 4. Photomicrographs of textures as seen by POM for the Colh phase of compound 1c
o
at 135 °C (a); at 25 C (b); XRD profiles depicting the intensity against the 2θ obtained for
the Colh phase of compound 1c at 145 oC, 80 oC and 25 oC (c).
Compound 1c with nine nꢀdecyloxy tails showed an improved mesophase range with an
isotropic temperature around 160 oC. On cooling the isotropic liquid, mosaic texture develops
from the homeotropic region (Fig. 4a). This is a regular textural pattern associated with Col
o
phases in literature. Most notably the compound crystallized at around 4 C as confirmed by
the DSC thermogams (Fig.S42c). The optical texture observed at room temperature (RT) also
showed similar textural pattern showing no signs of crystallization at RT (Fig.4b). Powder
9

Journal of Materials Chemistry C
Page 10 of 31
View Article Online
DOI: 10.1039/C6TC01939K
XRD studies carried out at higher temperature and room temperature showed similar
diffraction pattern (Table 2, Fig.4c), which shows that the Colh phase is stable for a
temperature range of 155 degrees including room temperature. The number of molecules
present in unit cell ‘Z’ in the case of compound 1c was found to be 1. It must be noted that in
the case of compound 1b; which is having six alkyl tails, the value of Z was 1.48. The
molecular diameter is almost same as that of compound 1c. This difference between the two
molecules may be due to the increase in the number of peripheral alkyl chains in the case of
compound 1c when compared to compound 1b. The last compound of the series 1d, which is
having nine dodecyloxy chains also exhibited Colh phase but with a slight decrease in the
mesophase range in comparison to its decyloxy homologue, due to an increase in the melting
and a decrease in clearing temperature (Table 1 and Fig.1). Thus the increase in the peripheral
chain length decreased the thermal range of the mesophase. The POM image was
characteristic of the Colh phase with large spherulites (Fig.5a and b), which persisted till room
o
temperature. In DSC thermograms of the first cooling scan crystallization was seen at 12 C
(Fig. S42d). The XRD patterns obtained at different temperatures corroborated the POM and
DSC observations confirming the type and thermal range of the Colh phase (Table 2)
Figure 5. Photomicrographs of textures as seen by POM for the Colh phase of compound 1d
o
at 125 °C (a); at 30 C (b); and XRD profiles depicting the intensity against the 2θ obtained
for the Colh phase of compound 1d at 130 oC, 100 oC, 50 oC and 25 oC (c).
10

Page 11 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
Photophysical and electrochemical studies
Photphysical properties of the starꢀshaped molecules 1aꢀd in micromolar THF
solution are depicted in Table 3. We expected a significant difference in their photophysical
properties due to the difference in substitution pattern. Absorbance and fluorescence spectra
of the compounds 1aꢀd were measured in micromolar THF solutions (Fig. 6). As can be seen,
the absorption spectra of hekates 1aꢀd showed absorption maximum centered around 278 nm,
which was not dependant on their substitution pattern. The absorption maxima of hekates 1aꢀ
d with respect to their 1,3,4ꢀoxadiazole counterparts Iaꢀc exhibited a hypsochromic shift
(absorption maxima 312ꢀ324 nm). 32
Table 3. Photophysical a and electrochemical c properties of starꢀshaped molecules
d, e
f
d, h
d, g
DLCs Absorption Emission
Stoke’s
shift (nm)
λ onset
(nm)
294
295
294
294
Emissioni ꢁE g, opt
(nm)
E1ox EHOMO
ELUMO
(nm)
278
278
279
278
b (nm)
299
21
36
32
31
410
484
502
496
4.23
4.21
4.22
4.23
1.97
1.83
1.97
ꢀꢀꢀꢀꢀꢀ
ꢀ6.31
ꢀ2.08
ꢀ1.96
ꢀ2.09
ꢀꢀꢀꢀꢀ
1a
1b
1c
1d
314
311
309
ꢀ6.17
ꢀ6.31
ꢀꢀꢀꢀꢀ
a Micromolar solutions in THF. b Excited at the respective absorption maxima. c Experimental conditions:
Micromolar DCM solutions, Ag/AgNO3 as reference electrode, glassy carbon as working electrode, platinum wire
as counter electrode, TBAP (0.1 M) as a supporting electrolyte, room temperature. d In electron volts (eV). e Band
gap was determined from the red edge of the longest wave length ( λonset) in the UVꢀVis absorption spectra using
h
the relation ꢁE g, opt = 1242.37/ λonset. f In volts (V). g Estimated from the formula ELUMO = EHOMO + E g, opt
.
Estimated from the onset reduction peak values by using EHOMO= ꢀ(4.8 ꢀ E1/2,Fc,Fc+ +E ox, onset) eV. i Emission
obtained by exciting the thin films at their absorption maxima in solution.
Figure 6. Normalized absorption (solid line) and emission spectra (dotted line) in micromolar
THF solution obtained for 1aꢀd (a); Normalized emission spectra of thin films of compounds
1aꢀd (excited at the absorption maxima obtained in solution state), inset shows the images of
these thin films under UV light of long wavelength (365 nm) (b).
11

Journal of Materials Chemistry C
Page 12 of 31
View Article Online
DOI: 10.1039/C6TC01939K
Based on the resemblance to the previously reported 1,2,4ꢀoxadiazole derivatives20 the
single absorption band observed with these molecules could be due to πꢀπ* transition of the
aromatic system. Optical bandgaps of these systems were estimated from the red edge of the
absorption spectra. These compounds 1aꢀd showed a larger band gap of 4.2 eV, in
comparison to their 1,3,4ꢀoxadiazole derivatives (3.4ꢀ3.5 eV).32 Emission spectra obtained by
exciting the solutions of these compounds at their absorption maxima showed emission
maxima centered around 299ꢀ314 nm. The emission spectra were red shifted on increasing the
number of peripheral alkyl chains.
The emission intensity was less in solution state and we could not measure the relative
quantum yield of these compounds with respect to standard compound. We were interested to
study the emissive nature of these molecules in solid state. The thin films of the compounds
were prepared by slow cooling of the isotropic liquids of the samples sandwiched between the
glass coverslips. They showed a redꢀshifted emission and excitation spectra (Fig. 6b; Table 2
and Fig.S47 in SI), which points to the formation of aggregates. The redꢀshifted excitation
and emission spectra of the thin films in comparison to the respective spectra in solution state
at micromolar concentration points to the formation of Jꢀaggregates 35
Energy levels of frontier molecular orbitals (HOMO and LUMO) of the star shaped
molecules were obtained by cyclic voltammetry (CV) and the data are tabulated in table 3.
We have chosen compounds 1aꢀc for carrying out CV studies. All the compounds exhibited
irreversible oxidation and reduction waves (See Fig. S49). The optical band gap Eg, opt was
estimated from the red edge of the absorption spectra. Energy levels of LUMO and HOMO
were determined by using the formulae ELUMO = EHOMO + E g, opt and EHOMO= ꢀ(4.8 ꢀ E1/2,Fc,Fc+
+E ox, onset) eV. Compounds 1aꢀc exhibited LUMO levels of ꢀ2.08 eV, ꢀ1.96 eV and ꢀ2.09 eV,
and HOMO levels of ꢀ6.31 eV, ꢀ6.17 eV and ꢀ6.31 eV. In comparison to their 1,3,4ꢀ
oxadiazole counterparts these compounds showed lower LUMO and HOMO levels and
higher band gaps. The higher band gap is the most challenging material property pursued in
the advancement of deep blue PHOLEDs.36 Considering this the present series of compounds
are good candidates for such applications.
Gelation studies. The compounds were investigated for their ability to aggregate in solutions
of hexane, decane, dodecane, hexadecane, chloroform, dichloromethane, ethanol,
dimethylsulfoxide (DMSO), nꢀbutanol, tetrahydrofuran, benzene, toluene and mꢀxylene.
Except for compound 1b with six decyloxy chains, none of the compounds exhibited gelꢀ
forming ability. Compound 1b formed gels in solvents like hexane, decane, dodecane and
12

Page 13 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
hexadecane and confirmed by the inversion of the glass vial (Fig. 7c, Table 3 in SI). This
compound was soluble in chloroform, dichloromethane and tetrahydrofuran, while it was
insoluble in DMSO. Compound 1b got precipitated in solvents like ethanol and nꢀbutanol.
Compound 1a with three decyloxy chains did not form any gel in dodecane and same with the
case of compound 1c and 1d with nine alkoxy chains. Compound 1b with six decyloxy chains
stabilized the gel formation in dodecane with very low critical gelation concentration (CGC)
of 0.52 wt %. In literature it is wellꢀknown that the molecules which undergo gelation at
concentrations lower than 1 wt% are usually classified as super gelators.10 The supergelation
capability of compound 1b was further confirmed by the formation of selfꢀstanding gels (Fig.
7e)
Figure 7. Emission spectra showing an increase in the emission intensity with time on
gelation (a); Normalized emission spectra showing a red shift on gelation (b); Images
showing the gel formation with respect to time in daylight (c); Images showing the gel
formation under the UV light (365 nm) with respect to time (d); Self standing gel in daylight
(left) and under UV light (365 nm) (e); Plot showing the change in the emission intensity at
emission maximum with respect to time (f).
The organogel was formed within 150 seconds after dissolving in dodecane (Fig.7c,d).
The formation of gel was probed by fluorescence spectroscopy by plotting the fluorescence
intensity at emission maximum against the time taken for gelation in seconds. The emission
intensity increases with the time and reaches saturation at 220 seconds (Fig. 7a,b,f). Visually
this change is apparent on irradiating the solution at these time intervals with the UV light of
13

Journal of Materials Chemistry C
Page 14 of 31
View Article Online
DOI: 10.1039/C6TC01939K
long wavelength (λ = 365 nm) (Fig. 7d). Normalized emission spectra showed a red shift in
the emission maxima from 426 nm to 442 nm (Fig.7b).
To the best of our knowledge, compound 1b is the first star shaped supergelator with
out any supportive Hꢀbonding in its molecular structure. In literature few metalꢀorganic
gels,37 organicꢀinorganic hybrids,38 polypeptides,39 sugars 40 and polymers41 show such super
gelation property. This can be explained as below. From the XRD data of the xerogel of
compound 1b it is shown that the molecules selfꢀassemble to form rectangular columnar
phase, while in the bulk state the molecules selfꢀassemble to form hexagonal columnar phase.
In both cases πꢀπ interactions play a major role in supporting columnar selfꢀassembly. The Hꢀ
bond that is observed in the case of crystalline compound 1a is between the aromatic
hydrogen of one molecule and the heterocyclic nitrogen of the other molecule. This hydrogen
bonding is only possible, if the molecules arrange in an antiparallel lamellar structure
(Fig.18a). Thus we can conclude that Hꢀbond does not play a role in the columnar selfꢀ
assemblies found in liquid crystal and gel state. Apart from πꢀπ interactions, other factors like
number and position of the peripheral alkyl chains also have vital role in the gelation, aided
by weak forces like van der Waals force. Till date there is a single report on supergelation,
where πꢀπ interactions play a major role and this is found in 1,3,4ꢀoxadiazole based
polycatenar.12 We also like to point out that rodꢀlike systems are more conducive to form
organogels than the star shaped molecules in the absence of supporting interactions like Hꢀ
bonding and one can find many examples in literature.2
Figure 8. Emission spectra showing an increase in the emission intensity on decreasing the
temperature due to gelation (a); Normalized emission spectra showing a red shift on gelation
(b); Plot showing the change in emission intensity on decreasing the temperature (c);
Reproducible reversibility of emission intensity in solution to gel interconversion (d).
14

Page 15 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
Formation of organogel was also confirmed by measuring the emission spectra of the solution
as a function of decreasing temperature. The emission intensity increased on decreasing the
o
temperature upto 40 C with a red shift from 426 nm to 452 nm (Fig. 8a,b and c).There is a
huge increase (four fold) in the luminescence intensity in gel state in comparison to the
solution state emission (Fig.7a and 8a). But we should note that this increase in luminescence
intensity is seen when the solution is above CGC (3.4 mM in dodecane). When the
concentration of the solution is very less for example at 20 ꢂM, where the monomers are
greater in number in comparison to aggregates, the difference between the luminescence
intensity is twelve fold (Fig. 9b). Thus this is a phenomenon of aggregationꢀinduced emission.
The emission spectra showed a red shift upon gelation (Fig. 8c and S46), which was similar to
the observation in Fig.7b. This gel formation was reversible for many cycles of heating and
cooling as evidenced by the change in the emission intensity at emission maximum (Fig. 8d).
Figure 9. Normalized absorption spectra of compound 1b in dodecane at 4 mM (red trace)
and 20 ꢂM (blue trace) concentration (a); Emission spectra of compound 1b in dodecane at 4
mM (red trace) and 20 ꢂM (black trace) concentration (b); The fluorescence decay of
compound 1b in dodecane at 4 mM (red trace) and 20 ꢂM (blue trace) concentrations (green
trace is instrument response function: IRF; λexc = 290 nm) (c); Dynamic light scattering curve
observed for compound 1b in dodecane in micromolar concentration (d).
15

Journal of Materials Chemistry C
Page 16 of 31
View Article Online
DOI: 10.1039/C6TC01939K
Fluorescence lifetimes of the excited species formed in the 20 ꢂM and 4 mM
dodecane solutions were measured by monitoring at their emission maxima (426 nm for dilute
solution and 442 nm for the concentrated solution). The solution with lower concentration
showed biexponential decay with two excited species [T1 = 3.4 ns (87%), T2 = 15 ns (13%)].
The solution with higher concentration showed the same set of excited state species, where
the excited state species with higher lifetime was in higher amount [T1 = 3.4 ns (10%), T2 =
15 ns (90%)] (Fig. 9c). The species with lower lifetime was solvated monomer, whereas the
species with the higher lifetime is formed due to aggregation. Dynamic light scattering
experiments carried out for compound 1b in dodecane in milimolar concentration showed the
presence of aggregates of 300 nm size (Fig.9d). The absorption spectrum obtained at higher
concentration (4 mM) has shown a blue shift compared to the solution at lower concentration
(20 ꢂM) (Fig. 9a). This shows that the aggregates formed during gelation are of Hꢀtype
aggregates where the molecules are arranged one above the other in cofacial or parallel
manner. Excitonic splitting in these aggregates leads to a highꢀenergy transition and a low
energy forbidden transition. In such systems the absorption spectra is dominated by the highꢀ
energy band leading to a blue shifted absorption band with respect to monomer absorption
band. This also leads to strong red shift in the fluorescence from low energy band.41
Eventhough Hꢀaggregates are supposed to exhibit reduced luminescence than in solution, here
the situation is exactly opposite of the expected. In literature there are limited reports on
luminescent Hꢀaggregates.43 Usually this luminescence by Hꢀaggregates is accounted for the
excimer species or due to a slight rotation of two excitonꢀcoupled molecules in the excited
state. We should note that the annealed thin films showed the formation of Jꢀaggregates. Thus
there is a Jꢀ to Hꢀtype conversion was seen on moving from solid to gel state. Similar Hꢀtype
aggregation of star shaped molecules on gelation in hydrocarbon solvents was reported by
Das et al. 44
Figure 10. Moldability of organogel of 1b in dodecane (1.5 wt%) into different shapes
like cube (a); cylinder (b); trigonal prism (c) seen under day light; under UV light (365
nm) (d); ability to sustain the weight (e); Photograph of gel after standing for 45 days in
daylight and under UV light (365 nm) (f); Plots of Tgel vs concentration for compound 1b
in dodecane (g).
16

Page 17 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
The selfꢀstanding gel showed an incredible stability on keeping inverted for a period of more
than a month (Fig.10f). When we taken the gel in small amount and kept it inverted shrinkage
of gel was observed over a period of 25 days but shape was retained. (See SI Fig. S50). This
indicated another level of selfꢀorganization of the xerogel architecture in the absence of
solvent. The luminescence remained same in spite of the change in the structure of the gel. We
have investigated the thermal stability of the gel with respect to concentration by ‘dropping
ball’ method,45 which showed a gradual increase (Fig.10g) on increasing the concentration.
The gel prepared at 1.5 wt % concentration showed higher mechanical strength and could be
molded into any shape as shown in Fig.10aꢀd and the luminescence remained unchanged.
Extensive microscopy was carried out to understand the superstructure of the gel
formed from compound 1b.The surface morphology of the xerogels of 1b obtained from the
organogel in dodecane was studied with the help of field emission scanning electron
microscopy (FEꢀSEM), atomic force microscopy (AFM) and transmission electron
microscopy (TEM). Scanning electron microscopy (SEM) images of the xerogels obtained by
evaporating the 1 mM dodecane solutions of compound 1b showed the entangled network of
fibers (Fig.11). AFM microscopy was carried out on a drop casted film (1b in dodecane,
1mM) and allowing the solvent to evaporate to get a xerogel film. The AFM images
Figure 11. SEM images obtained for compound 1b at 1mM dodecane solution (a); an
expanded region of the first image (b) (scale bar is 1ꢂm).
showed a very dense network of fibers (Fig. 12a). The same experiment carried out by drop
casting a solution at lower concentration (1 × 10ꢀ5 M), helped us to look at the individual fiber
in detail (Fig. 12b). The fibers are of several micrometers in length, with heights of 30ꢀ40 nm
and thickness of around 185 nm (Fig. 12c and d). Transmission electron microscopy (TEM)
images of the gels (1mM dodecane solution) showed interwoven network of fibers of
diameter 30ꢀ40 nm (Fig. 13). These SEM, AFM and TEM images of the gels were further
supported by fluorescence microscopy and optical microscopy studies (see the SI, Fig. S40
and S48).
17

Journal of Materials Chemistry C
Page 18 of 31
View Article Online
DOI: 10.1039/C6TC01939K
Figure 12. AFM images obtained for compound 1b at 1mM dodecane solution (scale bar is 1
ꢂm) (a); at 1 x 10ꢀ5 M (scale bar is 2ꢂm) (b); Expanded regions of figure 12b and showing the
height profiles of the individual fibers (c); Expanded regions of figure 12b and showing the
thickness of an individual fiber (d).
Figure 13. TEM images obtained for compound 1b at 1mM dodecane solution (scale bar is 5
ꢂm) (a); an expanded regions of figure 13a showing entangled fibers (scale bar is 1 ꢂm) (b)
Further the structure of the xerogel was studied with the help of powder Xꢀray
diffraction studies carried out on a thin film of the compound 1b prepared by drop casting the
1 mM solution on a glass slide. The XRD pattern of the xerogel film (Fig. 14a) showed many
peaks centered at 30.61, 24.91, 10.67, 9.11, 7.11 6.18, 5.96, 5.42, 5.05, 3.79. This may be due
to the strong intermolecular interactions of the adjacent molecules in xerogel state. The ratio
of reciprocal dꢀspacings clearly rules out a lamellar order of the molecules, but strongly
suggests columnar rectangular structure in particular. The lattice parameters ‘a’ and ‘b’ of the
unit rectangular cell was found to be 61.22 and 27.26 Å, respectively (details are in
18

Page 19 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
supporting information). The columnar arrangement of these molecules in the nanofibers of
the xerogel is also supported by the birefringent texture obtained for the xerogel state (Fig.
14b). Thus we can represent the hierarchical selfꢀassembly of these star shaped molecules in
bulk and gel state as shown in the figure 15.
Figure 14. XRD profile depicting the intensity against the 2θ obtained for the Colr phase of
compound 1b in xerogel state (inset shows the XRD image pattern obtained) (a) POM image
of the xerogel showing a birefringent texture (b).
Figure 15. Schematic representation showing the supramolecular hierarchical organization
of compound 1b.
Rheological studies
To obtain further insights into the dynamic mechanical properties of the organogel
formed from compound 1b in dodecane, we performed dynamic frequency sweep
experiments (at constant stress) to measure the rheological response, as characterized by G* (
ω) = G’(ω) + iG” (ω), where G’ is the storage modulus, G” is the loss modulus and ω is
19

Journal of Materials Chemistry C
Page 20 of 31
View Article Online
DOI: 10.1039/C6TC01939K
the angular frequency.46 These measurements are advantageous as the final details of the gel
structure remain perfect due to small dormation (i.e. small stress). Figure 16a, shows
measurements of G’ and G” for the gels containing 0.52 wt% of compound 1b in dodecane as
a function of ω (at 25 °C using 0.2 kPa stress). We make two observations from Figure 16a.
' &#$! " #
' $! " #
(
(
) *
) **
! &#$! " #
! " #
#$! " %
(a)
(b)
!
! "
! " "
0 1234. 5(6 5783719: (,5. ; <=! /
%! #
%! '
%! (
Deformation
Recovery
(c)
(d)
!
" ! !
#! !
$! !
012 3),4/
%&! !
%' ! !
Figure 16. Angular frequency (rad sꢀ1) dependence of G’ and G” obtained with small stress
amplitudes: (a) 0.52 wt% of compound 1b in dodecane at 25 °C using 0.2 kPa stress. (b)
Stress dependence of G’ and G” of the same gel 1b measured at constant frequency (at 25 °C,
and frequency at 0.1 Hz). (c) Thixotropic nature of organogel of 1b (3.4 mM) in nꢀdodecane
at 25 °C (Deformation; stress: 0.1 to 100 Pa, time: 300 s, angular frequency: 10 Hz; Recovery;
Stress: 1 Pa, time: 300 s; angular frequency: 10 Hz.) (d) Five continuous cycles of
measurement for 1b (3.4 mM) to prove that gel is thixotropic.
First, G’ is greater than G” over the entire range of frequency measurements,
indicating that the LC gel behaves as a solidꢀlike, viscoelastic material. Second, G’ and G’’
are weakly dependent on frequency. It should also be noted that even at the lowest frequency
(the limit of the instrument) we did not see liquid like behaviour of the gel (i.e. G”> G’). This
confirms that gel nature of these materials in which structural relaxation is very large.
Past studies have also demonstrated that the dynamics of the solꢀgels lead to
characteristic features in nonꢀlinear rheological measurements. To establish this nonꢀlinear
viscoelastic regime, we performed stress amplitude dependence of the storage and loss
20

Page 21 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
moduli. Figure 16b shows stressꢀdependent nonlinear measurements of the gel at a fixed
frequency of 0.1 Hz. We note two features in Figure 16b. First, at low strain amplitudes G’
is nearly constant, suggesting a linear viscoelastic regime. In this regime, G’> G” which is
consistent with the solid like behavior of the gel. Second, above a critical stress value (2.8
kPa), both G’ and G” became strain dependent and leads ultimately to the observation of
viscoelastic liquidꢀlike behaviors at high stress (G”>G’). This signifies a deformation driven
transition from a viscoelastic solid to viscoelastic liquid.
The thixotropic nature of the gel was also examined by studying the recovery of the
destroyed gel. Two continuous processes, i.e. deformation and recovery were followed step
by step to understand the thixotropic nature (Figure 16c). A varying stress from 0.2 to 3 kPa
at an angular frequency of 10 Hz for 300 s was applied for the deformation step and the
values of G’ was monitored as a function of time for 300 s. In the recovery process, the
storage modulus G’ was monitored as a function of same time interval (300 s) for a low shear
stress of 0.2 kPa while keeping the same angular frequency (10 Hz). This experiment was
repeated five times and the results are shown in Figure 16d. The results show that the gel
recovered immediately after removing the applied stress.
Single crystal XRD studies
We were curious to know how these 1,2,4ꢀoxadiazole based molecules selfꢀassemble
in crystalline state. We thought that this knowledge would help us in unraveling the
association of these molecules in liquid crystalline and gel selfꢀassemblies. Even though, we
desired the crystallization of compound 1b, unfortunately except compound 1a, none of the
compounds in this series crystallized in any solvents. Nevertheless crystal structure of
compound 1a provided some valuable insights into the selfꢀassembly of this series of
molecules, as we discuss in the following (Table 4 in SI).
Slow evaporation of CH2Cl2 solution of 1a at room temperature afforded colorless
rectangular prism like crystals suitable for single crystal Xꢀray diffraction determination
(Fig.17b); the crystallographic data are summarized in the SI section. The starꢀshaped
molecule of 1a has a diameter of 43.11Å and its oxadiazole ring is roughly coplanar with
central benzene rings and two peripheral benzene rings (Fig.17a). Neighboring pair of
21

Journal of Materials Chemistry C
Page 22 of 31
View Article Online
DOI: 10.1039/C6TC01939K
(a)
(b)
(c)
Figure 17. ORTEP representation of the compound of 1a (a); Image of a crystal of 1a under
day light (b); Image of a crystal under UV light (λ = 365 nm) (c).
molecules is aligned in an alternating antiꢀparallel manner augmented by two sets of π…π
stacking interactions.47 First one is (stacking distance of 4.50Å) between the centroid of the
oxadiazole ring of one molecule and the same of the neighboring molecule. Second one is
between the centroid of the central benzene ring of one molecule and centroid of the
peripheral benzene ring of other molecule (stacking distance of 4.03 Å) (Fig.18b). Further
there is a lateral non covalent CꢀH…N interaction between the hydrogen atom (H29) of the
peripheral benzene ring of one molecule, with the nitrogen atom (N2) of the oxadiazole ring
of another molecule (dCH….N = 2.628 Å) (Fig.18a). The restriction of the intramolecular
rotation would be restricted by the van der Waals interaction of long alkyl chains from the
neighbouring molecule leading to interdigitation48 (Fig.S54). In addition there is hydrophobic
interactions (2.394Å) between terminal nꢀdecyl chains of the next row. As a result, this
alternate packing of molecules, leads to a repeating lamellar distance of 26.95 Å (Fig 18a and
Fig.S55). The heteroatoms in the five membered rings were preferentially coꢀplanar with the
carbon and nitrogen atoms to form a planar πꢀconjugation system. The comparison of CꢀO, Nꢀ
O and CꢀN bond distances and angles of the hetrocyclic ring of 1a has been made and the
results are summarized in Table 4 (SI).
22

Page 23 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
Figure 18. Crystallographic packing structure of compound 1a, side view showing Hꢀ
bonding, hydrophobic interactions and lamellar packing (a) and simplified top view showing
πꢀstacking interactions (b).
Compound 1a is almost nonfluorescent in solution but a significant enhancement in
the emission intensity in thin film was observed, emitting blue fluorescence (Fig. 5b) upon
exposure to UV light (λex= 365nm). Interestingly, similar behavior was observed with the
single crystal too (Fig.17c). Presumably, inhibition of the intramolecular rotation helps in the
planarization of the molecule in the crystalline state of compound 1a, thus enhancing the
fluorescence due to pushꢀpull mechanism. In addition, this arrangement prevents the
aggregation quenching. Thus π…π stacking, lateral Hꢀbonding, van der Waal’s interaction and
hydrophobic interactions of alkyl tails as a whole leads to lamellar packing in the crystal
structure. In the case of compound 1b with six alkyl tails, the increased number of alkyl
chains enhances the disorder at periphery, while maintaining the planarity of the central
aromatic core; which allows the columnar stacking. Increase in the number of alkyl tails helps
23

Journal of Materials Chemistry C
Page 24 of 31
View Article Online
DOI: 10.1039/C6TC01939K
in the stabilization of liquid crystallinity and organogelation in hydrocarbons due to increased
fluidity (Fig. S56). Thus with the increase in the number of alkyl chains, molecules move
from lamellar packing in the crystalline state to columnar packing in liquid crystalline and gel
state as in the case of 1b. Further increase in the number of alkyl tails disrupts this subtle
balance, as in the case of compound 1c and 1d, which exhibit only columnar LC phase, but
lose the ability to undergo gelation. Similar behavior was observed in the recently reported
series of macrocycles, where the number of peripheral tails played a major role in deciding
the type and thermal range of the mesophase.49
Conclusion
We have reported a new class of star shaped 1,2,4ꢀoxadiazole derivatives stabilizing
hexagonal columnar phase over a broad temperature range. Their thermal behavior was
analyzed with the help of polarizing optical microscopy, differential scanning calorimetry and
Xꢀray diffraction studies. The number and length of the peripheral chains affected the
mesophase stability and the thermal range. Compound with three peripheral tails was
crystalline and showed the lamellar packing as evidenced from single crystal XRD studies.
The compounds required at least six alkyl tails at the periphery to stabilize the columnar
hexagonal mesophase. The thermal range of the mesophase increased with the number of
alkyl chains. Increase in the chain length reduced the thermal range of the columnar
hexagonal phase. All the compounds showed weak fluorescence in solution state but strong
emission in solid state. In the solidꢀstate emission the compounds showed a red shift with the
increase in the number of peripheral chains. The compound with six alkyl chains showed the
ability to selfꢀassemble into freestanding gels in nonpolar solvents. This is a first example of a
starꢀshaped supergelator, which form a selfꢀstanding gel where attractive πꢀπ interactions play
a major role. At higher concentration this compound forms a mechanically stable, moldable
gel. Another property of technological importance is that the compound shows aggregationꢀ
induced blue light emission, which is twelve times greater than the monomer emission. By
photophysical studies of thin films and gels it is revealed that in the former case this
compound forms Jꢀaggregate, while in the later case it forms Hꢀaggregates. Rheological
studies confirmed the thixotropic nature of the gel. The xerogels were characterized with the
help of scanning electron microscopy, transmission electron microscopy, atomic force
miscroscopy, fluorescence microscopy and polarizing optical microscopy. These microscopic
techniques confirmed a highly interwoven network of fibers. Fluorescent microscopy revealed
24

Page 25 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
that these fibers were fluorescent due to the aggregationꢀinduced emission phenomenon.
Polarizing optical microscopy of the xerogel films showed a birefringent texture, which is
evidence to the anisotropic nature of selfꢀassembly. Xꢀray diffraction studies revealed that the
molecules are organized form a Col phase with rectangular symmetry. Considering the dearth
of wide band gap, solidꢀstate blue light emitting organic materials, these star shaped
molecules are promising, due to their emissive nature in the aggregated state and columnar
selfꢀassembly. They have a bright potential in the field of solidꢀstate emissive displays.
Supporting Information
Synthesis and characterization details of intermediates and final compounds, 1H and 13C NMR
spectra, POM images, DSC and TGA thermograms, XRD plots, details related to
photophysical, electrochemical, gelation and single crystal XRD studies.
Acknowledgements
ASA sincerely thanks Science and Engineering Board (SERB), DST, Govt. of India and
BRNSꢀDAE for funding this work through project No. SB/S1/PCꢀ37/2012 and
No.2012/34/31/BRNS/1039 respectively. We thank the Ministry of Human Resource
Development for Centre of Excellence in FAST (F. No. 5ꢀ7/2014ꢀTSꢀVII). ASA
acknowledges CIF, IIT Guwahati for analytical facilities. We acknowledge Dr. Chandan
Mukherjee for providing his Electrochemical workstation.
References
1. (a) J. W. Goodby, P. J. Collings, T. Kato, C. Tschierske, H. Gleeson and P. Raynes,
WileyꢀVCH, Weinheim, Germany 2014, 1. Handbook of Liquid Crystals:
Fundamentals, ed. (b) T. Wöhrle, I. Wurzbach, J. Kirres, A. Kostidou, N.
Kapernaum, J. Litterscheidt, J. C. Haenle, P. Staffeld, A. Baro, F. Giesselmann,
and S. Laschat, Chem. Rev., 2016, 116 (3), 1139–1241. (c) X. Li, B. Li, L. Chen, J.
Hu, C. Wen, Q. Zheng, L. Wu, H. Zeng, B. Gong and L. Yuan, Angew.Chem. Int.Ed.
2015, 54,11147 –11152.
2. (a) P. Dastidar, Chem. Soc. Rev. 2008, 37, 2699. (b) S. S. Babu, V. K. Praveen and A.
Ajayaghosh, Chem Rev 2014, 114, 1973.
25

Journal of Materials Chemistry C
Page 26 of 31
View Article Online
DOI: 10.1039/C6TC01939K
3. (a) S. S. Babu, S. Prasanthkumar, A. Ajayaghosh, Angew. Chem., Int. Ed. 2012, 51,
1766; (b) S. Diring, F. Camerel, B. Donnio, T. Dintzer, S. Toffanin, R. Capelli, M.
Muccini and R. Ziessel, J. Am. Chem. Soc. 2009, 131, 18177.
4. (a) J. H. Jung, Y. Ono and S. Shinkai, Angew. Chem., Int. Ed. 2000, 39, 1862; (b) S.
Kobayashi, N. Hamasaki, M. Suzuki, M. Kimura, H. Shirai and K. Hanabusa, J. Am.
Chem. Soc. 2002, 124, 6550; (c) E. D. Sone, E. R. Zubarev and S. I. Stupp, Angew.
Chem., Int. Ed. 2002, 41, 1705; (d) G. Gundiah, S. Mukhopadhyay, U. G. Tumkurkar,
A. Govindaraj, U. Maitra and C.N.R. Rao, J. Mater. Chem. 2003, 13, 2118.
5. C. Wang, Q. Chen, F. Sun, D. Zhang, G. Zhang, Y. Huang, R. Zhao and D. Zhu, J.
Am. Chem. Soc. 2010, 132, 3092.
6. (a) Z. Zhao, J. W. Y. Lam and B. Z. Tang, J. Mater. Chem. 2012, 22, 23726; (b) Z.
Zhao, J. W. Y. Lam and B. Z. Tang, Soft Matter 2013, 9, 4564; (c) Y. Zhang, C.
Liang, H. Shang, Y. Ma, S. Jiang, J. Mater. Chem. C 2013, 1, 4472; (d) P. Xue, R. Lu,
G. Chen, Y. Zhang, H. Nomoto, M. Takafuji and H. Ihara, Chem. ꢀ Eur. J. 2007, 13,
8231.
7. (a) R. Hu, N. L. C. Leung and B. Z. Tang, Chem. Soc. Rev. 2014, 43, 4494; (b) Y.
Hong, J. W. Y. Lam and B. Z. Tang, Chem. Soc. Rev. 2011, 40, 5361; (c) B. Z. Tang
and A. Qin, WileyꢀVCH Verlag GMBH: Weinheim, 2013; (d) Y. Hong, J. W. Y. Lama
and B. Zhong Tang, Chem. Commun. 2009, 4332.
8. (a) P. I. Shih, C. Y. Chuang, C. H. Chien, E. W. G. Diau and C. F. Shu, Adv. Funct.
Mater. 2007, 17, 3141; (b) H. C. Li, Y. P. Lin, P. T. Chou, Y. M. Cheng and R. S. Liu,
Adv. Funct. Mater. 2007, 17, 520; (c) C. J. Tonzola, A. P. Kulkarni, A. P. Gifford, W.
Kaminsky and S. A. Jenekhe, Adv. Funct. Mater. 2007, 17, 863.
9.(a) Z. Zhao, S. Chen, J. W. Y. Lam, P. Lu, Y. Zhong, K. S. Wong, H. S. Kwok and B.
Z. Tang, Chem. Commun. 2010, 46, 2221; (b) Z. Zhao, J. W. Y. Lam and B. Z. Tang,
J. Mater. Chem. 2012, 22, 23726.
10. (a) I. Abu Bin, S. Takahiro and T. Yukikazu, Polym. J. 2010, 42, 839; (b) A. Harada,
R. Kobayashi, Y. Takashima, A. Hashidzume and H.Yamaguchi, Nat. Chem. 2011, 3,
34; (c) T. Yamamoto and M. Yoshida, Langmuir 2012, 28, 8463; (d) K. Yuichiro, T.
Yoshinori, H. Akihito, Y. Hiroyasu and H. Akira, Sci. Rep. 2013, 3; (e) M. Zhang, D.
Xu, X. Yan, J. Chen, S. Dong, B. Zheng, F. Huang, Angew. Chem., Int. Ed. 2012, 51,
7011; (f) R. Yoshida, Adv. Mater. 2010, 22, 3463.
11. L. E. Buerkle and S. Rowan, J. Chem. Soc. Rev. 2012, 41, 6089.
26

Page 27 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
12. A. P. Sivadas, N. S. S. Kumar, D. D. S. Varghese, S. K. Prasad, D. S. S. Rao and S.
Das, J. Am. Chem. Soc. 2014, 136, 5416.
13. (a) A. J. Attias, C. Cavalli, B. Donnio, D. Guillon, P. Hapiot and J. Malthête, Chem.
Mater. 2002, 14, 375; (b) B. Roy, N. De and K. C. Majumdar, Chem. Eur. J. 2012, 18,
14560; (c) C. V. Yelamaggad, A. S. Achalkumar, D. S. Shankar Rao, and S. Krishna
Prasad, J. Org. Chem. 2009, 74, 3168–3171; (d) S. K. Pathak, S. Nath, R. K. Gupta,
D. S. S. Rao, S. K. Prasad and A. S Achalkumar, J. Mater. Chem. C. 2015, 3, 8166;
(e) K. P. Gan, M. Yoshio and T. Kato, J. Mater. Chem. C. 2016, DOI:
10.1039/c6tc00808a.
14. (a) Y. Shirota, J. Mater. Chem. 2000, 10, 1. (b) D. Apreutesei and G. H. Mehl, J.
Mater. Chem. 2007, 17, 4711.
15. M. L. Parra, P. I. Hidalgo, E. A. SotoꢀBustamante, J. Barbera´, E. Y. Elgueta and V.
H.TrujilloꢀRojo, Liquid Crystals 2008, 35, 1251.
16. (a) Q. Li, L. S. Cui, C. Zhong, X. D. Yuan, S. C. Dong, Z. Q. Jiang and L. S. Liao,
Dyes and Pigments 2014, 101, 142; (b) Q. Li, L.S. Cui, C. Zhong, Z. Q. Jiang and L.
S. Liao, Org. Lett. 2014, 16, 1622.
17. H. Gallardo, R. Cristiano, A. A. Vieiraa, R. A. W. N. Filho, R. M. Srivastava and I. H.
Bechtold, Liquid Crystals 2008, 35, 857.
18. S. I. Torgova, L. A. Karamysheva, T. A. Geivandova, A. Strigazzi, Mol. Cryst. and
Liq.Cryst. 2001, 365, 99.
19. (a) G. S. Shanker, M. Nagaraj, A. Kocot, J. K. Vij, M. Prehm and C. Tschierske, Adv.
Funct. Mater. 2012, 22, 1671; (b) G. S. Shanker, M. Prehm, M. Nagaraj, J. K. Vij, M.
Weyland, A. Eremin and C. Tschierske, ChemPhysChem. 2014, 15, 1323.
20. (a) H. Gallardo, M. Ferreira, A. A. Vieira, E. Westphal, F. Molin, J. Eccher and I. H.
Bechtold, Tetrahedron 2011, 67, 9491; (b) T. E. Frizon, D. S. Rampon, H. Gallardo,
A. A. Merlo, P. H. Schneider, O. E. D. Rodrigues, A. L. Braga, Liquid Crystals 2012,
39, 769.
21. H. Gallardo, G. Conte, P. A. Tuzimoto, B. Behramand, F. Molin, J. Eccher and I. H.
Bechtold, Liquid Crystals 2012, 39, 1099.
22. (a) S. V. Subrahmanyam, P. V. Chalapathi, S. Mahabaleswara, M. Srinivasulu and D.
M. Potukuchi, Liquid Crystals 2014, 41, 1130; (b) H. Gallardo, R. Cristiano, A. A.
Vieira, R. A. W. N. Filho, R. M. Srivastav and I. H. Bechtold, Liquid Crystals 2008,
35, 857; (c) M. Parra, P. Hidalgo, E. Carrasco, J. Barbera´ and L. Silvino, Liquid
Crystals 2006, 33, 875.
27

Journal of Materials Chemistry C
Page 28 of 31
View Article Online
DOI: 10.1039/C6TC01939K
23. (a) M. Parra, P. Hidalgo and J. Alderete, Liquid Crystals 2005, 32, 449; (b) M. A.
Parra, P. Hidalgo, J. N. Barbera´ and J. Alderete, Liquid Crystals 2005, 32, 573.
24. (a) M. L. Parra, P. I. Hidalgo and E. Y. Elgueta, Liquid Crystals 2008, 35, 823; (b) M.
L. Parra, P. I. Hidalgo, E. A. SotoꢀBustamante, J. Barbera´, E. Y. Elgueta and V. H.
TrujilloꢀRojo, Liquid Crystals 2008, 35,1251.
25. S. Chandrasekhar, B. K. Sadashiva and K. A. Suresh, Pramana 1977, 9, 471.
26. Q. Li, SelfꢀOrganized Organic Semiconductors: From Materials to Device
Applications; John Wiley & Sons: New York, 2011.
27. (a) M. Lehmann, Chem. Eur. J. 2009, 15, 3638. (b) M. Lehman, Top Curr. Sci. 2012, 318,
193−224; (c) M. Lehmann, M. Jahr, Chem. Mater. 2008, 20 (17), 5453ꢀ5456; (d) M.
Lehmann, R. I. Gearba, M. H. J. Koch and D. A. Ivanov, Chem. Mater., 2004, 16, 374– 376
28.
V. Percec, M. Glodde, T. K. Bera, Y. Y. Miura, I. Shiyanovskaya, K. D. Singer,
V. S. K. Balagurusamy, P. A. Heiney, I. Schnell, A. Rapp, H. ꢀW. Spiess, S. D.
Hudsonk and H. Duank, Nature 2002, 419, 384.
29. (a) A. S. Achalkumar, U. S. Hiremath, D. S. S. Rao, S. K. Prasad and C. V.
Yelamaggad, J. Org. Chem. 2013, 78, 527; (b) A. S. Achalkumar and C. V.
Yelamaggad, Tetrahedron Letters 2012, 53, 7108; (c) A. S. Achalkumar, B. N.
Veerabhadraswamy, Uma S. Hiremath, D. S. Shankar Rao, S. Krishna Prasad and C.
V. Yelamaggad, Dyes and Pigments, 2016, 10.1016/j.dyepig.2016.05.010
30. S. T. Chill and R. C. Mebane, Synthetic Communications 2009, 39, 3601.
31. (a) D. Conole, T. M. Beck, M. JayꢀSmith, M. D. Tingle, C. T. Eason, M. A. Brimble
and D. Rennison, Bioorganic & Medicinal Chemistry 2014, 22, 2220.
32. S. K. Pathak, R. K. Gupta, S. Nath, D. S. S. Rao, S. K. Prasad and A. S Achalkumar,
J. Mater. Chem. C. 2015, 3, 2940.
33. R. I. Gearba, M. Lehmann, J. Levin, D. A. Ivanov, M. H. J. Koch, J. Barbera, M. G.
Debije, J. Piris, Y. H. Geerts, Adv. Mater. 2003, 15, 1614.
34. a) S. Ito, M. Ando, A. Nomura, N. Morita, C. Kabuto, H. Mukai, K. Ohta, J.
Kawakami, A. Yoshizawa and A. Tajiri, J. Org. Chem. 2005, 70, 3939; b) A. Hayer,
V. D. Halleux, A. Kohler, A. ElꢀGaroughy, E. W. Meijer, J. Barbera, J. Tant, J. Levin,
M. Lehmann, J. Gierschner, J. Cornil and Y. H. J. Phys. Chem. B. 2006, 110, 7653; (c)
C. V. Yelamaggad, A. S. Achalkumar, D. S. S. Rao and S. K. Prasad, J. Mater. Chem.
2007, 17, 4521; (d) C. V. Yelamaggad, A. S. Achalkumar, D. S. S. Rao and S. K.
Prasad, J. Org. Chem. 2007, 72, 8308; (e) C. V. Yelamaggad, A. S. Achalkumar, D. S.
S. Rao, M. Nobusawa, H. Akutsu, J. Yamada, S. Nakatsuji, J. Mater. Chem. 2008, 18,
28

Page 29 of 31
Journal of Materials Chemistry C
View Article Online
DOI: 10.1039/C6TC01939K
3433; (f) A. S. Achalkumar, U. S. Hiremath, D. S. S. Rao, S. K. Prasad and C. V.
Yelamaggad, J. Org. Chem. 2013, 78, 527. (g) R. K. Gupta, S. K. Pathak, B. Pradhan,
M. Gupta, S. K. Pal, A. S. Achalkumar, ChemPhysChem. 2016, 17, 859.
35. (a) F. Wu
50, 3376. (b) F. Wu
2008, 20, 1695; (c) F. Wu
̈
̈ller, Angew. Chem., Int. Ed. 2011,
̈
̈
Chem.–Eur. J. 2001, 7, 2245.
36. (a) K.ꢀR. Wee, Y. –J. Cho, S. Jeong, S. Kwon, J. D. Lee, I. H. Suh and S. O Kang, J.
Am. Chem. Soc. 2012, 134, 17982; (b) P. Venkatakrishnan, P. Natarajan, J. N.
Moorthy, Z. Lin and T. J. Chow, Tetrahedron 2012, 68, 7502.
37. S. Suman and B. Kumar, Chem. Mater. 2012, 24, 1165.
38. Q. Wang, J. Mynar, M. Yoshida, E. Lee, M. Lee, K. Okuro, K. Kinbara and T. Aida,
Nature 2010, 463, 339.
39. E. R. Zubarev, M. U. Pralle, E. D. Sone and S. I. Stupp, Adv. Mater. 2002, 14, 198.
40. (a) G. Di, X. Min, P. Junxia, L. Jing, Y. Ni, H. Panli and F. Yu, Tetrahedron 2010, 66,
2961; (b) Y. Kenji, A. Natsuki, O. Yoshiyuki, A. Tetsuyuki, S. Hideyuki, T.
Masayuki, S. Seiji, N. R. David, Chem.Eur. J. 1999, 5, 2722; (c) A. Vidyasagar, K.
Handore and K. M. Sureshan, Angew.Chem. Int. Ed. 2011, 50, 8021.
41. (a) A. Mahler, M. Reches, M. Rechter, S. Cohen and E. Gazit, Adv. Mater. 2006, 18,
1365; (b) L. E. R. O’Leary, J. A. Fallas, E. L. Bakota, M. K. Kang and J. D.
Hartgerink, Nat. Chem. 2011, 3, 821.
42. (a) B.W. Messmore, J. F. Hulvat, E. D. Sone and S. I. Stupp, J. Am. Chem. Soc. 2004,
126, 14452; (b) M. Kasha, H. R. Rawls and M. A. ElꢀBayoumi, Pure Appl. Chem.
1965, 11, 371; (c) D. G. Whitten, Acc. Chem. Res. 1993, 26, 502; (d) K. C. Hannah
and B. A. Armitage, Acc. Chem. Res. 2004, 37, 845; (e) S. Y. Ryu, S. Kim, J. Seo, Y. ꢀ
W. Kim, O. –H. Kwon, D. ꢀJ. Jang and S. Y. Park, Chem. Commun. 2004, 70; (f) H.
H. Yao, K. Domoto, T. Isohashi, K. Kimura, Langmuir 2005, 21, 1067.
43. (a) S. Basak, S. Bhattacharya, A. Datta and A. Banerjee, Chem.– Eur. J. 2014, 20, 1;
(b) F. Alexandre, D. J. Marc, G. D. Todor, I. G. Nikolai, A. V. Aleksey and V. Eric, J.
Am. Chem. Soc. 2006, 128, 7661; (c) M. Cigan, J. Donovalova, V. Szocs, J. Gaspar,
K. Jakusova and A. Gaplovsky, J. Phys. Chem. A 2013, 117, 4870; (d) Q. Fang, F.
Wang, H. Zhao, X. Liu, R. Tu, D. Wang and Z. Zhang, J. Phys. Chem. B 2008, 112,
2837; (e) V. Lau and B. Heyne, Chem. Commun. 2010, 46, 3595. 22; (f) U. Rosch, S.
Yao, R. Wortmann and F. Wurthner, Angew. Chem., Int. Ed. 2006, 45, 7026; (g) L.
29