Effect of Atomic-Scale Differences on the Self-Assembly of Thiophene-based Polycatenars in Liquid Crystalline and Organogel States

Article abstract of DOI:10.1002/chem.201603678

Two series of polycatenars are reported that contain a central thiophene moiety connected to two substituted oxadiazole or thiadiazole units. The number, position, and length of the peripheral chains connected to these molecules were varied. The oxadiazol

Full text of DOI:10.1002/chem.201603678

DOI: 10.1002/chem.201603678
Full Paper
&
Liquid Crystals
Effect of Atomic-Scale Differences on the Self-Assembly of
Thiophene-based Polycatenars in Liquid Crystalline and
Organogel States
Balaram Pradhan,[a] V. M. Vaisakh,[b] Geetha G. Nair,[b] D. S. Shankar Rao,[b] S. Krishna Prasad,[b]
and Achalkumar Ammathnadu Sudhakar*[a]
Dedicated to Professor Takuzo Aida
Abstract: Two series of polycatenars are reported that con-
tain a central thiophene moiety connected to two substitut-
ed oxadiazole or thiadiazole units. The number, position,
and length of the peripheral chains connected to these mol-
ecules were varied. The oxadiazole-based polycatenars ex-
hibited columnar phases with rectangular and hexagonal or
oblique symmetry, whereas the thiadiazole-based polycate-
nars exhibited columnar phases with rectangular and/or hex-
agonal symmetry. All of the compounds exhibited bright
emission in the solution and thin-film states. Two oxadia-
zole-based molecules and one thiadiazole-based molecule
exhibited supergelation ability in hydrocarbon solvents,
which is mainly supported by attractive p–p interactions.
These gels showed aggregation-induced enhanced emission,
which is of high technological importance for applications in
solid-state emissive displays. X-ray diffraction studies of the
xerogel fibers of oxadiazole-based polycatenars revealed
a columnar rectangular organization, whereas a hexagonal
columnar arrangement was observed for thiadiazole-based
polycatenars. Rheological measurements carried out on the
samples quantitatively confirmed the formation of gels and
showed that these gels are mechanically robust. The impact
of an atomic-scale difference (oxygen to sulfur, <2% of the
molecular weight) on the self-assembly and the macroscopic
properties of those self-assembled structures are clearly vi-
sualized.
smectic phases.[5] A few oligomers with a higher number of
flexible terminal tails were reported to stabilize columnar (Col)
phases.[6] Stabilization of Col phases in this class of materials is
beneficial because the stacking of molecules one above the
other to form columns of indefinite length provides a pathway
for one-dimensional charge migration. As mentioned earlier,
the ease of processability makes the Col phase an ideal substi-
tute for conjugated polymers or organic single crystals from
the viewpoint of optoelectronic devices.[7] p-Conjugated mole-
cules that contain strong donor (D) and acceptor (A) systems
show intramolecular charge transfer and low bandgap proper-
ties, which make them attractive.[8] Kato et al. reported star-
shaped molecules containing three donor thiophene moieties
connected to a central electron-accepting triazine moiety,
which exhibited a Col phase with ambipolar conductivity.[6b]
1,3,4-Oxadiazole derivatives, which are known for their elec-
tron-deficient and fluorescence nature, are finding applications
in OLEDs as electron transport and electroluminescent layers.[9]
These have been integrated into the liquid-crystal design to
obtain conductive smectic or columnar mesophases.[10] 1,3,4-
Thiadiazole derivatives are less often reported, but they also
exhibit promising thermal and photophysical behavior.[11] The
self-assembly of thiophene-based molecules into organogels
has been studied extensively because of the enhanced charge-
transport behavior and optoelectronic properties due to mo-
lecular packing in the self-assembled systems.[3a,12] In most
Introduction
Thiophene-based molecular materials are an extensively stud-
ied type of heterocyclic materials because of their synthetic
flexibility and applications as p-type (electron-rich) semicon-
ductor systems.[1] Thiophene-based oligomers and polymers
are known for their unique optoelectronic properties and have
found utility in the fabrication of field-effect transistors,[2] light-
emitting diodes,[3] and photovoltaic cells.[4] Most oligothio-
phenes must be purified by vacuum-sublimation techniques
due to their poor solubility. Thus, liquid-crystalline (LC) thio-
phene derivatives seem to be good alternatives due to their
good solubility because of which they can be easily purified.
Furthermore, good-quality thin films can be obtained by
simple solution-processing techniques and further molecular
alignment can be addressed through annealing. Most of the
thiophene-based liquid crystals reported stabilize nematic and
[a] B. Pradhan, Dr. A. A. Sudhakar
Department of Chemistry, Indian Institute of Technology Guwahati
Guwahati, 781039, Assam (India)
E-mail: achalkumar@iitg.ernet.in
[b] V. M. Vaisakh, Dr. G. G. Nair, Dr. D. S. S. Rao, Dr. S. K. Prasad
Centre for Nano and Soft Matter Sciences
Bangalore (India)
Supporting information for this article, including color images, is available
on the WWW under http://dx.doi.org/10.1002/chem.201603678.
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cases, such self-assembled systems are assisted by intermolec-
ular hydrogen bonding augmented by peptide or amide units.
Recently, a few liquid crystalline polycatenars with 1,3,4-oxadia-
zole units have exhibited gelation, which is exclusively sup-
ported by p–p interactions.[13] However, there are no reports
on thiophene-containing organogels, which are stabilized
mainly by p–p interactions. Tailoring the molecular structure to
obtain these long-range-ordered self-assembled structures is
important, given their potential applications in optoelectronic
devices.
Considering all these aspects, we aspired to design a new
polycatenar system that contains a central electron-rich thio-
phene unit connected to two electron-deficient fluorophores,
such as substituted 1,3,4-oxadiazole or 1,3,4-thiadiazole units.
We expected that these compounds would show good lumi-
nescence, thermal behavior, and ability to undergo organoge-
lation. The alternate arrangement of donor and acceptor moi-
eties may modulate the bandgap of these molecules. We en-
visaged that the presence of oxadiazole and thiadiazole could
result in a drastic difference in the self-assembly and related
macroscopic properties. The number of flexible chains, their
positions, and their length were modified to understand the
impact on the self-assembly and photophysical properties of
these polycatenar systems.
Scheme 1. Reagents and conditions: i) n-Bromoalkanes, anhydrous K2CO3,
DMF, 808C, 24 h (70–80%); ii) NH2NH2·H2O, ethanol or butanol, reflux, 48 h
(70–80%); iii) thiophene-2,5-acid chloride, THF, Et3N, 12 h, reflux (70–80%);
iv) POCl3, reflux, 17 h (60–70%); v) Lawesson’s reagent, toluene, reflux, 17 h
(40–50%).
Results and Discussion
Synthesis and characterization
The synthetic route is elucidated in Scheme 1. Common meth-
odologies for the synthesis were same as reported else-
where.[11] Alkoxyethyl benzoates 5a–d were prepared by using
the Williamson’s ether synthesis. Alkylation of ethyl-3,4-dihy-
droxy benzoate gave compound 5a. Ethyl-3,5-dihydroxy ben-
zoate was O-alkylated to give compound 5b, whereas alkyla-
tion of ethyl-3,4,5-trihydroxy benzoate gave compounds 5c
and 5d. On treatment with hydrazine hydrate, these esters
gave respective hydrazides 4a–d, which were treated with
thiophene-2,5-diacid chloride under basic conditions to give
the corresponding hydrazides (3a–d). These hydrazides were
treated with phosphoryl chloride to give the corresponding di-
oxadiazoles (1a–d) or with Lawesson’s reagent to give the
thiadiazole derivatives (2a–d). A molecular structural character-
1
Figure 1. Overlay of the expanded portion of the H NMR spectra (CDCl3,
600 MHz) of 2c and 1c.
1
ization was carried out by using H and 13C NMR spectroscopy,
Thermal behavior
IR spectroscopy, and MALDI-TOF mass spectrometry (see the
Supporting Information for details). The electron-withdrawing
nature of the oxadiazole units compared with the thiadiazole
All the compounds were investigated by using thermogravi-
metric analysis (TGA), polarizing optical microscopy (POM), and
differential scanning calorimetry (DSC). Polycatenars were
stable up to at least 3158C, except for compounds 1b and 2b.
These compounds, which have alkoxy groups at the 3,5-posi-
tions, were stable only up to approximately 2308C, as shown
by the TGA results (Figure S36). The mesophase-type, phase-
transition temperatures, and associated enthalpy changes are
1
units is reflected by the low-field H NMR signals obtained for
compounds 1a–d with respect to compounds 2a–d (see the
1
Supporting Information). For example, the H NMR spectrum of
compound 1c with two oxadiazole units showed low-field sig-
nals for protons Ha and Hb compared with those of compound
2c with two thiadiazole units (Figure 1).
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113 8C, after passing through a few crystal-to-crystal transitions
as observed in the DSC scans. The birefringent fluid was shear-
able and converted to an isotropic liquid at a temperature of
about 1388C. On cooling (58Cminꢀ1), the isotropic liquid
showed a transition to a birefringent texture comprised of
a mosaic pattern interspersed with homeotropic domains (Fig-
ure 3a). The mesophase existed up to 1298C over a tempera-
Table 1. The phase-transition temperatures[a] and corresponding enthal-
pies of the polycatenars.
Phase sequence
2nd heating
1st cooling
T [8C]
Phase[c]
T [8C]
E [kJmolꢀ1
]
Phase[c]
E [kJmolꢀ1
]
1a
1b
1c
Cr
113.0
130.2
138.1
138.1
28.4
25.4
I
138.2
129.3
87.9
24.8
27.3
125.7
Colob2
Colob1
I
Cr1
Cr2
Cr3
I
Colr2
Colr1
Colh
I
Colob1
Colob2
Cr
I
Cr
29.1
73.5
77.3
203.1
124.1
129.4
20.6
142.38
58.8
65.7
81.1
56.1
127.07
2.8
I
77
53.5
48.7
3.9
127.5
71.7
Colh
Colr1
Colr2
[b]
1d
2a
Cr
Colob
I
Cr1
Cr2
Cr3
Cr4
I
24.24
78.8
28.7
10.5
I
75.4
16.3
10.3
16
Colob
Cr
I
Cr3
Cr2
Cr1
64.2
101.3
105.8
125.8
10.4
19.8
7.1
100.6
91.2
55.4
33.2
99.3
10.6
17.9
2b
2c
Cr
I
88.3
277.6
I
Cr
I
Colh1
Colh2
Colr1
65.5
278.5
Colr2
Colr1
Colh2
Colh1
I
55.8
59.7
93
11.3
65.4
26.7
20.2
96.1
90.9
55.1
48.1
20.9
25.4
75.2
10.9
Figure 3. POM images of 1a a) at 132 (Colob1 phase) and b) 1228C (Colob2
phase). c) DSC thermograms of 1a showing the first cooling (upper trace)
and second heating (lower trace). XRD patterns obtained for 1a at d) 136
(Colob1 phase) and e) 1228C (Colob2 phase).
97.2
[b]
Colr2
2d
Colh2
Colh1
I
71.5
120.5
16.0
71.6
I
116.6
67.4
69.5
15.8
Colh1
Colh2
[b]
ture range of 98C and then transformed into another meso-
phase, as shown by a sudden change in the birefringence ac-
companied by an enthalpy change in the DSC (Figure 3b,c).
This mesophase existed up to around 888C, and then crystal-
lized as observed with a loss of shearability and change in the
textural pattern. The XRD studies were carried out at two dif-
ferent temperatures, that is, 136 and 1228C (Table 2). The XRD
pattern obtained at 1368C showed several peaks in the low-
angle region with d spacings of 44.41, 42.34, 41.28, 40.36, and
36.12 ꢁ, along with a diffuse peak at a wide angle with a d
spacing of 4.61 ꢁ. The wide-angle diffuse peak corresponds to
the packing of flexible tails. The low-angle d spacings (0<2q<
58) correspond to the Miller indices (02), (12), (20), (21), and
(ꢀ12) of a columnar oblique (Colob) lattice. The lattice parame-
ters derived from these reflections were found to be a=83.9
and b=90.2 ꢁ with a tilt angle of g=79.98. The XRD pattern
obtained at 1228C showed a complex diffraction pattern with
many peaks at low angles, but they could be assigned to an-
other Colob phase with a tilt angle of g=688, and the lattice
parameters a and b were reduced to almost half compared
with the high-temperature Colob phase. The number of mole-
cules in a unit cell (Z) was found to be 1.3. These two Colob
[a] Peak temperatures in the DSC thermograms obtained during the first
heating and cooling cycles at 58Cminꢀ1. [b] The mesophase freezes in
glassy state and does not crystallize up to ꢀ208C. [c] Colob =columnar
oblique phase, Colr =columnar rectangular phase, Colh =columnar hexag-
onal phase, Cr=crystalline, I=isotropic.
Figure 2. Bar graph summarizing the thermal behavior of 1a–d and 2a–d
(second heating cycle); Colob =Columnar oblique phase, Colr =columnar rec-
tangular phase, Colh =columnar hexagonal phase.
presented in Table 1. Figure 2 graphically represents the ther-
mal behavior of compounds in the second heating scans.
A crystalline sample of oxadiazole-based polycatenar 1a,
with four hexadecyloxy tails connected to the terminal ben-
zene rings at the 3,4-positions, exhibited enantiotropic meso-
morphism; on heating, the crystalline sample showed a transi-
tion to a birefringent fluid at a temperature of approximately
phases are denoted as Colob1 and Colob2
.
Compound 1b, with four alkoxy tails connected to the ter-
minal benzene rings at the 3- and 5-positions, proved to be
crystalline, which shows the delicate balance involved in the
LC self-assembly. Compound 1c, with six hexadecyloxy tails
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Table 2. Results of indexation of XRD profiles of compounds 1a–d, at
a given temperature (T) of the mesophase.[a]
Compound Phase
dobsd [ꢁ][b] dcalcd [ꢁ][b] Miller
Lattice
(D [ꢁ])
symmetry
indices parameters[c]
(T [8C])
hk
1a (56.24)
Colob1, P2
(136)
44.41
42.34
41.28
40.36
44.41
42.34
41.28
40.24
36.53
02
12
20
21
ꢀ12
a=83.9 ꢁ
b=90.2 ꢁ
g=79.98
36.12
4.61(ha)[d]
35.24
Colob2, P2
(122)
35.24
22.80
20.44
17.62
15.36
14.70
13.50
10
ꢀ11
02
20
ꢀ12
13
a=38.0 ꢁ
b=44.1 ꢁ
g=68.08
22.80
20.44
17.83
15.29
14.53
13.38
4.50 (ha)[d]
Figure 4. POM images of 1c at a) 65 (Colh phase), b) 50 (Colr1 phase), and
c) 288C (Colr2 phase). d) DSC thermograms of compound 1c showing first
cooling (upper trace) and second heating (lower trace). XRD patterns ob-
tained for 1c at e) 65 (Colh phase) and f) 258C (Colr2 phase).
23
1c (60.30)
Colh, P6mm 35.47
35.47
20.48
10
11
a=41 ꢁ
(65)
20.53
4.48 (ha)[d]
Colr2, P2mm 37.63
37.63
21.00
18.33
14.01
01
10
11
12
a=21.0 ꢁ
b=37.6 ꢁ
(40)
21.00
18.28
13.64
4.20 (ha)[d]
4.19 (hc)[d]
sition showed the growth of a needle-like texture on the exist-
ing mosaic pattern (Figure 4c). This pattern remained un-
changed up to room temperature. We carried out XRD studies
at 65 and 408C and room temperature (Table 2). The diffraction
pattern at 658C exhibited two sharp reflections at low angles
with d spacings of 35.47 and 20.53 ꢁ, along with a diffuse peak
at wide angle with a d spacing of 4.48 ꢁ. The first two d spac-
ings in the low-angle region correspond to Miller indices (10)
and (11) of a columnar hexagonal (Colh) lattice. The lattice pa-
rameters were found to be a=41 ꢁ. The spacing ratio is
1:0.578, which corresponds to a Colh phase. The number of
molecules derived from this lattice parameter in the unit hex-
agonal cell was found to be approximately 2. On cooling, this
phase passed through a transition that spanned 58C, as seen
in the DSC scans (Figure 4d), with a slight change in the color
of the textural pattern. We could not carry out XRD studies in
this short interval. The XRD studies carried out at 40 and 258C
were almost identical. For example, the XRD pattern at 258C
exhibited several peaks in the low-to-mid angle region with d
spacings of 37.52, 21.02, 18.27, and 13.68 ꢁ that corresponded
to Miller indices (01), (10), (11), and (12) of a rectangular lattice.
The spacing ratios were 1:0.56:0.49:0.36, which vary slightly
from the spacing ratio in the higher-temperature XRD pattern
obtained for Colh phase. This could indicate the continuation
of a higher-temperature Colh phase. However, in view of the
signatures from the DSC scans and a better agreement be-
tween the measured and calculated spacing values, we sug-
gest that the lattices could be rectangular in nature. The wide-
angle region showed two diffuse peaks at 4.15 and 4.17 ꢁ.
Here, the first peak corresponds to the packing of flexible alkyl
tails, whereas the second diffuse peak originates from the
stacking of the cores. There was not much change in the lat-
tice parameters and the number of molecules within the rec-
tangular unit cell. The number of molecules in a unit cell was
found to be one irrespective of temperature. We denote this
phase as Colr2, whereas the phase present between Colh and
Colr2, P2mm 37.52
37.52
21.02
18.34
13.99
01
10
11
12
a=21.0 ꢁ
b=37.5 ꢁ
(25)
21.02
18.27
13.68
4.15 (ha)[d]
4.17 (hc)[d]
33.78
1d (45.35)
Colob, P2
33.78
33.01
29.24
01
11
10
a=35.2 ꢁ
b=40.7 ꢁ
g=56.18
(72)
33.01
29.24
4.50 (ha)[d]
3.61 (hc)[d]
32.59
Colob, P2
32.59
30.71
30.05
01
11
10
a=34.4 ꢁ
b=37.2 ꢁ
g=61.08
(50)
30.71
30.05
4.48 (ha)[d]
3.59 (hc)[d]
32.09
Colob, P2
32.09
31.18
15.57
11
01
12
a=33.06 ꢁ
b=32.12 ꢁ
g=76.18
(25)
31.18
15.57
4.41 (ha)[d]
3.52 (hc)[d]
[a] The average diameter (D) of the polycatenars (estimated by using the
Chem 3D Pro 8.0 molecular modeling software from Cambridge Soft).
[b] dobsd =observed spacing, dcalcd =calculated spacing (deduced from the
lattice parameters; a and b for Colob and Colr). [c] g is the column tilt
angle. [d] Spacings marked ha and hc correspond to diffuse reflections in
the wide-angle region that arise from correlations between the alkyl
chains and core regions, respectively.
connected to the terminal benzene rings at the 3,4,5-positions,
showed a mixture of pseudo-focal conic fans with mosaic pat-
terns (Figure 4a) on slow cooling of the isotropic liquid at
a rate of 58Cminꢀ1. DSC scans during cooling showed transi-
tions at approximately 54 (DH=127.5 kJmolꢀ1) and 498C
(DH=71.7 kJmolꢀ1; Figure 4d). The first transition did not
show a major difference in the optical texture, except for a de-
crease in the birefringence (Figure 4b), whereas the next tran-
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Colr2 that spans a thermal range of 58C is denoted as the Colr1
phase because of the high enthalpy change associated with
the phase transition from the high-temperature Colh phase
(Figure 4a–c).
Compound 1d with six n-decyloxy chains showed a mosaic
texture on cooling from the isotropic liquid state, and the tex-
ture remained unchanged to room temperature (Fig-
ure S34a,b). This compound showed crystallization at 168C, as
determined from the DSC cooling scan (DH=16 kJmolꢀ1; Fig-
ure S34c). From the XRD studies, it was found that the Colob
phase was stabilized throughout the thermal range (Fig-
ure S34d,e). Thus polycatenars with shorter alkyl chains stabi-
lized the Colob phase, whereas polycatenars with longer alkyl
chains (compound 1c) stabilized the Colh and Colr phases.
The thermal behavior of the next series of polycatenars, in
which the central thiophene ring was connected to two sym-
metrically substituted 1,3,4-thiadiazole rings, is discussed
below. Compounds 2a and 2b, with four n-hexadecyloxy tails
connected to the terminal benzene rings at the 3,4- and 3,5-
positions, proved to be crystalline. On cooling from the iso-
tropic temperature, hexacatenar 2c, with six n-hexadecyloxy
tails, showed a mosaic pattern interspersed with homeotropic
domains, which is characteristic of a uniaxial phase (Figure 5a).
Figure 6. a) DSC thermograms of 2c showing first cooling (upper trace) and
second heating (lower trace). XRD patterns obtained for 2c at b) 90 (Colh2
phase), c) 50 (Colr1 phase), and d) 258C (Colr2 phase).
tern obtained at 508C showed d spacings of 42.01, 25.63,
10.31, 5.77, 4.27, and 4.22 ꢁ (Figure 6c). The relatively diffuse
peak at 4.27 ꢁ can be assigned to the packing of alkyl tails,
whereas the other peaks correspond to Miller indices (01), (11),
(31), and (45) of a rectangular lattice. The lattice parameters of
the rectangular cell are a=32.3 and b=42 ꢁ, with approxi-
mately two molecules (Z=1.9) in the unit cell. We denoted
this phase as Colr1. The DSC scan showed a further transition
at approximately 488C (DH=38.5 kJmolꢀ1), whereas the tex-
ture remained same without showing signs of crystallization to
room temperature. The XRD pattern obtained at room temper-
ature (Figure 6d) exhibited additional reflections in the mid-
angle region, but these reflections fit to a rectangular lattice
and we denoted this phase as Colr2. The two phases differ
from each other with respect to the value of a. The Colr2 phase
shows a reduced value of a, whereas the value of b remains
almost same.
Figure 5. POM images of 2c at a) 95 (Colh1 phase), b) 88 (Colh2 phase), c) 50
(Colr1 phase), and d) 258C (Colr2 phase).
At approximately 918C (DH=25.4 kJmolꢀ1), a transition was
observed in the DSC scan, whereas this change was very hard
to detect in the POM (Figures 6a and 5b). The XRD pattern ob-
tained at 908C shows two peaks at low angle with d spacings
of 35.51 and 20.49 ꢁ, along with two diffuse peaks at wide
angle with d spacings of 4.58 and 3.7 ꢁ (Table 3, Figure 6b).
The first two low-angle peaks can be indexed to (10) and (11)
reflections from a hexagonal lattice. The wide-angle spacings
correspond to the packing of alkyl tails and the distance be-
tween the cores. The calculated lattice parameter a was 41 ꢁ,
which is 30% less than the molecular length (60.4 ꢁ) of the
polycatenar in the all-trans conformation. This difference indi-
cates an intercalation of the peripheral tails of neighboring
molecules in the Colh phase. We denote this columnar phase
as Colh2, whereas the one at higher temperature (>918C) is de-
noted as the Colh1 phase, based on the textural similarity. The
number of molecules in the unit hexagonal cell was found to
be two. This columnar phase spanned a thermal range of 368C
before showing a textural change at approximately 488C, cou-
pled with an enthalpy change of 104.9 kJmolꢀ1. The XRD pat-
On cooling, the isotropic liquid of compound 2d, which is
the lower homologue of compound 2c (with six n-decyloxy
chains), showed the growth of spherulites that originated from
the dark field of view (Figure S35a). Further cooling resulted in
a transformation of these spherulites into a mosaic pattern at
around 678C (Figure S35b), which remained unchanged to
room temperature (Figure S35c). This transition was confirmed
by the DSC scans, which showed an enthalpy change of
15.8 kJmolꢀ1 (Figure S35d, Table 1). There was no sign of crys-
tallization up to ꢀ208C, as revealed by the DSC results. The
XRD patterns obtained at 110, 60, and 258C confirmed a Colh
phase (Figure S35e–g, Table 3). The DSC scans showed that
there is a transition between two Colh phases, which we de-
noted as Colh1, and Colh2. It is surprising that the lower homo-
logue exhibited a Colh phase in contrast to the oxadiazole ana-
logue (1d), which exhibited a Colob phase.
Looking at the thermal behavior, we can conclude that the
length of the peripheral chain and pattern of substitution mat-
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tives 2c–d exhibited Colr and Colh phases. Liquid crystallinity
and the width of the mesophase exhibited by compounds 1a,
1c, 1d, and 2c, 2d is a summation of bent-angle, dipole-
moment, and space-filling effects and nanophase segregation.
It is difficult to pinpoint a certain factor that establishes a rela-
tionship between the structure and the thermal behavior. This
shows that a delicate ratio of incompatible subunits in
a shape-anisotropic molecule is required to affect the nano-
phase segregation, which results in LC self-assembly. A general
comparison of oxadiazole- and thiadiazole-based polycatenars
based on a central thiophene leads to the following generali-
zations: Compounds with thiadiazole linkage show a rich
phase sequence compared with oxadiazole derivatives. More
importantly, the longer the chain length, the richer the phase
sequence for both linking groups (oxadiazole or thiadiazole).
For example, the higher (C16) homologue of the thiadiazole
linking group (2c) shows a Iso–Colh1–Colh2–Colr1–Colr2 phase
sequence, whereas the lower (C10) homologue (2d) has only
two mesophases. Similarly, the C16 homologue of the oxadia-
zole linking group (1c) exhibits a Colh phase and two rectan-
gular columnar phases, whereas the C10 homologue (1d)
shows only the oblique columnar mesophase. A possible
reason for this difference could be better isolation of the cen-
tral cores in a columnar structure, provided by the longer pe-
ripheral chains.
Table 3. Results of indexation of XRD profiles of compounds 2c–d, at
a given temperature (T) of the mesophase.[a]
Compound Phase
dobsd [ꢁ][b] dcalcd [ꢁ][b] Miller
indices parameters
Lattice
(D [ꢁ])
symmetry
(T [8C])
hk
[ꢁ]
2c (60.40)
Colh2, P6mm 35.51
35.51
20.43
10
11
a=41.0
(90)
20.49
4.58 (ha)[c]
3.70 (hc)[c]
Colr1, P6mm 42.01
42.01
25.63
10.44
5.83
01
11
31
45
a=32.3
b=42.0
(50)
25.63
10.31
5.77
4.27 (ha)[c]
4.22 (hc)[c]
Colr2, P2mm 41.76
41.76
25.60
20.88
13.92
10.44
5.81
01
10
02
03
04
43
a=25.6
b=41.8
(25)
25.60
21.06
13.83
10.32
5.74
4.19 (ha)[c]
4.17 (hc)[c]
2d (46.49)
Colh1, P6mm 29.82
(110)
5.77 (ha)[c]
4.34 (hc)[c]
Colh2, P6mm 32.68
29.82
10
a=34.4
a=37.7
32.68
16.34
12.35
10
20
21
(60)
16.38
11.74
5.78 (ha)[c]
4.26 (hc)[c]
Another point to be noted is that replacing a benzene ring
with a thiophene ring leads to a richer mesophase scenario.
We can compare the thermal behavior of these polycatenars
(Figure 2) with previously reported polycatenars with a central
benzene ring and substituted 1,3,4-oxadiazole[10c,d] or 1,3,4-thia-
diazole[11b] as side rings. Figure 7 shows the phase-transition
temperatures and mesophase widths of these polycatenars. To
make a clear comparison, we chose the hexacatenar with n-de-
cyloxy tails from each series. The bent angles of the central
cores and other heterocycles help us to understand their
impact on the self-assembly (Figure 8). The bent angle of poly-
catenars with a central benzene ring can be either 180 (for p/
10 and pT/10, p-substituted compounds) or 1208 (for m/10
Colh2, P6mm 32.52
32.52
16.26
12.29
10
20
21
a=37.6
(25)
16.27
12.19
5.74 (ha)[c]
4.18 (hc)[c]
[a] The average diameter (D) of the polycatenars (estimated by using the
Chem 3D Pro 8.0 molecular modeling software from Cambridge Soft).
[b] dobsd =observed spacing, dcalcd =calculated spacing (deduced from the
lattice parameters; a for Colh, a and b for Colr). [c] The spacings marked
ha and hc correspond to diffuse reflections in the wide-angle region that
arise from correlations between the alkyl chains and core regions, respec-
tively.
ters in determining the thermal range of the mesophase. Oxa-
diazole derivative 1b and thiadiazole-based polycatenar com-
pound 2b were crystalline in nature and showed that periph-
eral chain substitution at the 3,5-positions is not conducive for
mesophase stabilization. This may be due to the lack of space
filling and nanosegregation. Compound 1a, with four periph-
eral alkyl chains substituted at the 3- and 4-positions of the
terminal benzene ring, was liquid crystalline, whereas corre-
sponding thiadiazole compound 2a was crystalline, which indi-
cated the effect of the thiadiazole ring on the self-assembly.
Compounds 1c and 2c showed an enhanced mesophase
range with respect to 1a,b and 2a,b, which showed that sub-
stitution at the 3,4, and 5-positions stabilized the columnar
packing. Furthermore, a reduction in the length of the periph-
eral chain from n-hexadecyloxy to n-decyloxy (for compounds
1d and 2d) led to the broadest thermal range in the respec-
tive series. Oxadiazole derivatives 1a, 1c, and 1d showed sta-
bilized Colr, Colh, or Colob phases, whereas thiadiazole deriva-
Figure 7. Bar graph summarizing the thermal behavior of 1,3,4-oxadiazole-
based polycatenars[10c,d] with a central benzene ring substituted at the p-
and m- positions and 1,3,4-thiadiazole-based polycatenars[11b] with a central
benzene ring substituted at the p- and m- positions (second heating cycle);
Colob =columnar oblique phase, Colh =columnar hexagonal phase.
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phene unit, which is very favorable for mesophase stabilization
compared with previously reported benzene-centered polyca-
tenars.
Photophysical studies
Figure 8. Comparison of the bent angles of different central and side rings
of the polycatenars.[23]
The photophysical properties of polycatenars 1a–d and 2a–d
were investigated in the solution and thin-film states (Table 4).
Absorption and emission spectra of these compounds were re-
corded for micromolar solutions in THF (Figure 9). As can be
and mT/10, m-substituted compounds). Here, for both series
of oxadiazole- and thiadiazole-based polycatenars, m-substitut-
ed compounds show higher mesophase stabilization. Note
that oxadiazole derivative p/10 was crystalline, whereas m/10
exhibited a Colh phase. For the thiadiazole-based polycatenars,
again the m-substituted compound mT/10 showed a wider
thermal range than p-substituted compound pT/10. Compared
with polycatenars with oxadiazole side rings (bent angle ca.
1348), polycatenars based on thiadiazole side rings (bent angle
ca. 1608) show higher mesophase stability. Therefore, an in-
crease in the bent angle of the side rings helps to enhance the
mesophase width.
Figure 9. Absorption (solid trace) and emission (dotted trace) spectra of
a) 1a–d and b) 2a–d. c) Photographs taken under UV light (lex =365 nm; mi-
cromolar solutions in THF).
Herein, the bent angle of the central thiophene ring is
around 1508.[23] This is an intermediate angle compared with p-
or m-substituted benzene. In this case, polycatenars with oxa-
diazole and thiadiazole side rings exhibited liquid crystallinity.
The mesophase width is larger than m- and p-substituted poly-
catenars (both oxadiazole and thiadiazole) with a central ben-
zene ring. It should be noted that, as in the case of benzene-
centered polycatenars, thiadiazole-based polycatenar 2d
showed a wider thermal range than its oxadiazole counterpart
(1d). Molecular models of hexacatenars with n-decyloxy chains
are provided in the Supporting Information (Figure S42). This
study confirms that this very small deviation in the bent angle
(which is the effect of the hetero atom of the constituent rings
in the polycatenar) leads to a huge difference in the self-as-
sembly in the bulk. Smaller bent angles lead to a higher dipole
moment, as in the case of oxadiazole derivatives, whereas
larger bent angles reduce the overall dipole moment, as in the
case of thiadiazole or thiophene derivatives. This also has
a direct consequence on the mesophase stability. Another im-
portant factor is the bent angle provided by the central thio-
seen, the absorption spectra for the solutions of oxadiazole-
based compounds 1a–d showed absorption maxima in the
range of l=362 to 365 nm (Figure 9a), whereas the thiadia-
zole-based compounds 2a–d showed a slightly redshifted ab-
sorption spectrum at l=368 to 394 nm (Figure 9b). Oxadia-
zole derivatives 1a–d showed higher bandgaps (3.07–3.2 eV)
than thiadiazole derivatives 2a–d (2.73–2.92 eV). Similarly, the
emission maxima of compounds 1a–d were in the range of
l=442 to 476 nm, whereas the emission maxima of com-
pounds 2a–d were in the range of l=476 to 490 nm. Oxadia-
zole derivatives 1a–d showed lower steady-state anisotropy
values (0.071–0.088) than thiadiazole derivatives 2a–d (0.122–
0.142). The molar extinction coefficients of oxadiazole deriva-
tives 1a–d (e=28800–31150mꢀ1 cmꢀ1) were found to be
higher than thiadiazole derivatives 2a–d (e=20650–
22900mꢀ1 cmꢀ1). The absorption bands mainly correspond to
the p–p* transition of this molecular system. Thin films of
Table 4. Photophysical properties in the solution and thin-film states.
Solution state[a]
Absorption Emission Stokes shift [nm] lonset DEg,opt
Thin-film state
[c]
Steady-state anisotropy Relative quantum yield[d] Absorption Emission Stokes shift
[nm]
[nm][b]
[nm] [eV]
[nm]
[nm][b]
[nm]
1a
1b
1c
1d
2a
2b
2c
2d
365
442
476
472
472
476
476
487
490
77
405
388
405
405
455
425
455
441
3.07
0.087
0.086
0.088
0.071
0.130
0.137
0.142
0.122
0.72
0.65
0.57
0.56
0.50
0.50
0.54
0.61
365
473
453
478
501
486
473
512
507
108
85
353
362
362
368
374
383
394
123
110
110
108
102
104
96
3.20
3.07
3.07
2.73
2.92
2.73
2.82
368
367
367
373
371
365
369
111
134
113
102
147
138
[a] As micromolar solutions in THF. [b] Excited at the respective absorption maxima; [c] Bandgap determined from the red edge of the longest wavelength
(lonset) in the UV/Vis absorption spectra. [d] Standard: quinine sulfate in 0.1m H2SO4.
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these compounds showed small shifts in their absorption and
emission spectra due to aggregation. Oxadiazole-based com-
pounds 1a–d showed higher quantum yields of 0.56 to 0.72,
whereas thiadiazole-based compounds 2a–d exhibited slightly
lower quantum yields in the range of 0.5 of 0.61 (Figure S37).
The observed quantum yields are higher in comparison to
polycatenars with a central benzene ring instead of thiophe-
ne.[11b]
cause of the possibility of several diastereomeric conforma-
tions for these polycatenars.
The photophysical properties of representative compounds
1c and 2c were studied in solution with different solvents to
examine their behavior with different solvent polarities
(Figure 11, Table 5). The absorption spectra did not vary much
with respect to solvent polarity, which indicates the nonpolari-
The thin films of these compounds were prepared by drop
casting micromolar solutions of these compounds in toluene
onto glass slides. The absorption spectra and emission spectra
of these compounds were found to be broad compared with
their solution spectra (Figure 10). The absorption spectra of
Figure 11. a) Absorption and b) emission spectra of 1c (20 mmol) in different
solvents and c) absorption and d) emission spectra of 2c (20 mmol) in differ-
ent solvents.
Figure 10. a) Absorption and b) emission spectra of 1a–d and d) absorption
and e) emission spectra of 2a–d. c,f) Photographs of thin films of 1a–d and
2a–d taken under UV light (lex =365 nm).
the thin films of oxadiazole-based compounds 1a–d show
a redshift compared with the solution spectra; even though
the absorption maximum of compound 1a in the thin film
shows the same value as in solution, the difference spectra
shows the redshift (Figure S38; Figure 10a). Other than com-
pound 1b, the emission spectra of 1a, 1c, and 1d showed
a redshift (Figure 10b). In the case of thiadiazole-based com-
pounds, all the compounds exhibited blueshifted absorption
spectra except for compound 2a (Figure 10d). The emission
spectra of the films of thiadiazole derivatives 2a and 2c–d
showed a redshift, whereas that of compound 2b was blue-
shifted (Figure 10e). All the compounds showed a visually per-
ceivable bright emission in the thin-film state on irradiation
with long-wavelength UV light (Figure 10c,f).
Table 5. Photophysical properties in different solvents.[a]
Solvent
Absorption [nm] Emission [nm][b] Stokes shift [nm]
1c decane
362
362
431
438
468
472
470
477
495
487
69
76
106
110
91
91
101
112
toluene
chloroform 362
THF
2c decane
toluene
362
379
386
chloroform 386
THF 383
[a] As micromolar solutions in decane, toluene, chloroform, or THF. [b] Ex-
cited at the respective absorption maxima.
It is evident from previous reports that the redshifted ab-
sorption spectra in the aggregated state points to the forma-
tion of J-type aggregates,[14] in which the molecules are ar-
ranged in a slipped-stack position, whereas the blueshifted ab-
sorption spectra indicates the formation of H-type aggregates,
in which the molecules are arranged in a cofacial manner. J-ag-
gregates are supposed to be highly luminescent compared
with H-aggregates, in which luminescence is quenched. How-
ever, there are a few reports of fluorescent H-type aggre-
gates.[15] Here we cannot provide conclusive proof of the for-
mation of a particular type of aggregate in the thin films be-
ty of the molecules in the ground state. In contrast to the ab-
sorption spectra, the emission spectra showed a redshift,
which indicates the polar nature of the excited state. The pres-
ence of donor and acceptor moieties in conjugation leads to
the localization of p electrons towards the acceptor unit in
polar solvents. This leads to an internal charge-transfer state in
addition to the locally excited state. The internal charge-trans-
fer state is affected by solvent polarity. Thus the energy gap
between the ground and excited states is reduced in polar sol-
vents, which leads to a redshifted emission.
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Electrochemical studies
Table 7. Gelation behavior and CGCs of 1a–d and 2a–d in decane.
Energy levels of frontier molecular orbitals (HOMO and LUMO)
of the polycatenars were obtained by using cyclic voltammetry
(CV) and the data are tabulated in Table 6. All the compounds
exhibited irreversible oxidation and reduction waves (see Fig-
ure S39). The optical band gap (Eg,opt) was estimated from the
Properties[a]
CGC [wt.%][b]
Tgel [8C][c]
1a
1b
1c
1d
2a
2b
2c
2d
G(O)
P
G(O)
S
P
P
0.45
–
1.00
–
–
–
63
–
71
–
–
–
G(O)
P
0.70
–
47
–
Table 6. Electrochemical properties.[a,b]
[a] G=stable gel, P=precipitate, I=insoluble, S=soluble, O=opaque.
[b] The critical gelation concentration is the minimum concentration nec-
essary for gelation. [c] Tgel is the thermal stability of the gels.
E1red [V][d] E1ox [V] EHOMO [eV][c] ELUMO [eV][d] DECV [eV][e] DEg,opt [eV][f]
1a ꢀ1.16
1b ꢀ1.30
1c ꢀ1.06
1d ꢀ1.13
2a ꢀ1.04
2b ꢀ1.13
2c ꢀ1.03
2d ꢀ1.17
1.61
1.74
1.77
1.66
1.60
1.61
1.56
1.62
ꢀ5.95
ꢀ6.08
ꢀ6.11
ꢀ6.00
ꢀ5.94
ꢀ5.95
ꢀ5.90
ꢀ5.96
ꢀ3.18
ꢀ3.04
ꢀ3.28
ꢀ3.21
ꢀ3.30
ꢀ3.21
ꢀ3.31
ꢀ3.17
2.77
3.04
2.83
2.79
2.64
2.74
2.59
2.79
3.07
3.20
3.07
3.07
2.73
2.92
2.73
2.82
diazole derivatives none of the compounds except 2c showed
gelation in decane. Oxadiazole-based compound 1a, with four
alkyl tails, showed gelation at a very low critical gel concentra-
tion (CGC) of 0.45 wt.%. This compound was gelated within
6 min after heat-assisted dissolution. Immediately after adding
the compound to decane, the suspension was heated to
obtain a clear solution. The resultant solution was allowed to
cool for 10 minutes to form a gel at room temperature. Gel
formation was confirmed by using the “stable to inversion of
the glass vial” method, that is, the gel was unable to flow on
inversion of the container, which confirmed the formation of
a stable gel. Oxadiazole-based compound 1c, with six alkoxy
tails, showed a CGC of 1 wt.% and took longer to gelate (ap-
proximately 3 h), whereas thiadiazole-based compound 2c
(again with six alkyl tails) took almost 50 min from the point of
complete dissolution with a CGC of 0.7 wt.% respectively.
Therefore, compounds 1a and 2c qualify as supergelators be-
cause they are able to gelate at a very low CGC, that is, well
below 1 wt.%.[16] All the gels were opaque and fluorescent
(Figure 12a). Compound 1a showed the ability to form self-
standing and moldable gels of any given shape (Figure 12b). It
is apparent that the gelation depends on a very delicate bal-
ance of the number and pattern of substitution of alkyl tails
and the type of heterocycle present in the polycatenar.
[a] As micromolar solutions in dichloromethane. [b] Experimental condi-
tions: Ag/AgNO3 as the reference electrode, glassy carbon as the working
electrode, platinum rod as the counter electrode, TBAP (0.1m) as the sup-
porting electrolyte, RT, scanning rate 0.05 mVsꢀ1. [c] Estimated from the
onset oxidation peak values by using the formula EHOMO
=
ꢀ(4.8ꢀE1=2,Fc,Fcþ +Eox,onset). [d] Estimated from the onset reduction peak
values by using ELUMO =ꢀ(4.8ꢀE1=2,Fc,Fcþ +Ered,onset); E1=2,Fc,Fcþ =0.46 V. [e] Es-
timated from the formula DECV =ELUMOꢀEHOMO. [f] Bandgap determined
from the red edge of the longest wavelength in the UV/Vis absorption
spectrum.
red edge of the absorption spectra. Energy levels of the LUMO
and HOMO were determined by using the formulae DECV
ELUMOꢀEHOMO HOMO =ꢀ(4.8ꢀE1=2,Fc,Fcþ +Eox,onset), and ELUMO
=
=
,
E
ꢀ(4.8ꢀE1=2,Fc,Fcþ +Ered,onset). Compounds 1a–d exhibited LUMO
levels of ꢀ3.04 to ꢀ3.21 eV and HOMO levels of ꢀ5.95 to
ꢀ6.11 eV. Compounds 2a–d exhibited LUMO levels of ꢀ3.17 to
ꢀ3.31 eV and HOMO levels of ꢀ5.90 to ꢀ5.96 eV. Oxadiazole
derivatives 1–d exhibited a higher bandgap of 3.07 to 3.2 eV
compared with thiadiazole derivatives 2a–d, which showed
a bandgap of 2.73 to 2.92 eV. Compounds 2a–d can be com-
pared with polycatenars reported previously, in which the cen-
tral benzene ring is connected to thiadiazole rings at the 3,5-
positions.11b These compounds show reduced LUMO, HOMO
levels and bandgaps compared with their benzene analogues.
Gelation studies
Compounds 1c and 2c were investigated for their ability to
undergo organogelation in solution in hexane, decane, dodec-
ane, hexadecane, chloroform, dichloromethane, ethanol, dime-
thylsulfoxide (DMSO), n-butanol, tetrahydrofuran, benzene, tol-
uene, and m-xylene. Both of these compounds showed gela-
tion in hydrocarbon solvents, such as hexane, decane, dodec-
ane, and hexadecane (Table S1). We also checked the gelation
of other compounds in decane (Table 7). Compounds 1b, 2a,
2b, and 2d precipitated after being dissolved in decane,
whereas compound 1d dissolved in decane. Of the oxadiazole
derivatives, 1a and 1c showed gelation whereas for the thia-
Figure 12. a) Photographs of 1a, 1c, and 2c in the solution and gel states
under long-wavelength UV light (l=365 nm) at the CGC. b) Formation of
a self-standing gel of compound 1a at the CGC.
Because we were interested in deriving the structure–prop-
erty relationships, we decided to investigate compounds 1c
and 2c by using extensive photophysical studies, microscopy,
and XRD studies. Detailed rheological measurements were also
carried out on the two representative samples, as described
below. Formation of the organogel was monitored by measur-
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ing the emission spectra of the solution with respect to tem-
perature and time. For compound 1c the emission intensity in-
creased as the temperature was decreased from 70 to 208C,
with
a concomitant redshift from l=461 to 471 nm
(Figure 13). A huge fourfold increase in the emission intensity
was observed on gelation, compared with the solution-state
emission (5.51 mm in decane; Figure 14a). Similarly, the intensi-
Figure 15. a) Emission spectra showing an increase in the emission intensity
with the decrease in temperature from solution to the gel state for 2c.
b) Normalized emission spectra of 2c, showing a redshift on gelation.
c) Emission spectra showing an increase in the emission intensity with time
during the transformation from solution to gel. d) Plot of the change in
emission intensity at the emission maximum (l=383 nm) vs. time (concen-
tration: 3.8 mm, decane).
upon gelation (Figure 15a–b. The increase in emission intensity
upon gelation from solution at the CGC (3.8 mm, decane) was
found to be fivefold, which confirmed the AIEE phenomenon
(Figure 16a). From solution to the gel state, a redshift was ob-
served from l=520 to 538 nm. The gelation process, moni-
tored by fluorescence spectroscopy with respect to time and
temperature, showed similar behavior (Figure 15). The gelation
was complete in 45 min and the intensity of the emission re-
mained steady (Figure 15d). Similar to 1c, the gel formation
was reversible over many cycles of heating and cooling (Fig-
ure 16b).
Figure 13. a) Emission spectra showing an increase in the emission intensity
with a decrease in temperature from solution to the gel state for 1c. b) Nor-
malized emission spectra of 1c, showing a redshift on gelation. c) Emission
spectra showing an increase in the emission intensity with time during the
transformation from solution to gel. d) Plot showing the change in emission
intensity at the emission maximum (l=362 nm) vs. time (concentration:
5.51 mm, decane).
Figure 14. a) The increase in intensity from the solution state to gelation,
showing aggregation-induced emission for 1c at the CGC. b) Reversible
change in the emission intensity at l=362 nm on repeated sol–gel transi-
tions (concentration: 5.51 mm, decane).
Figure 16. a) The increase in intensity from the solution state to gelation,
showing aggregation-induced emission for 2c at the CGC. b) Reversible
change in the emission intensity at l=383 nm on repeated sol–gel transi-
tions (concentration: 3.8 mm, decane).
ty of the emission was highest at 115 min and thereafter re-
mained steady (Figure 13d). This is a phenomenon of aggrega-
tion-induced enhanced emission (AIEE). The emission spectra
showed a redshift upon gelation (Figure 13c), which was simi-
lar to the observation in Figure 13b. This gel formation was re-
versible for many cycles of heating and cooling, as shown by
the change in the emission intensity at its emission maximum
(Figure 14b).
Fluorescence lifetimes of the excited species formed in very
dilute (20 mm) or concentrated (5.51 mm) solutions of 1c in
decane (at CGC) were measured by monitoring at the emission
maxima (l=472 nm for the dilute solution and l=471 nm for
the concentrated solution). The solution with lower concentra-
tion showed monoexponential decay with a single excited spe-
cies (T1 =1.05 ns (100%)). The solution with higher concentra-
Compound 2c exhibited similar behavior. An increase in the
emission intensity and a redshift in the emission was observed
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Figure 17. a) The fluorescence decay of 1c in decane; IRF is the instrument-
response function; lexc =375 nm. b) The fluorescence decay of 2c in decane;
lexc =375 nm.
Figure 18. a) Emission spectra of 1c in solution, gel (5.51 mm, decane), and
thin-film states and b) emission spectra of 2c in solution, gel (3.8 mm,
decane), and thin-film states.
tion showed the presence of two excited-state species, one
with a higher lifetime (T1 =1.6 ns (49%)) and the other with
a lower lifetime (T2 =0.93 ns (51%); Figure 17a). Similar behav-
ior was observed for 2c. The fluorescence lifetimes of the ex-
cited species formed in the 20 mm and 5.51 mm solutions of 2c
in decane were measured by monitoring at their emission
maxima (l=487 nm for the dilute solution and l=538 nm for
the concentrated solution). The 20 mm solution showed
a single species (T1 =0.55 ns (100%)). The solution at higher
concentration exhibited biexponential decay with two species.
One species showed a lower lifetime (T1 =0.57 ns (22%)),
which can be attributed to the solvated monomer, whereas
the other showed a higher lifetime (T2 =3.79 ns (78%)), which
is due to aggregation (Figure 17b). We measured the excita-
tion spectra of 1c and 2c at 20 mm and also at their CGC. The
excitation spectra of 1c at the gelation concentration showed
a large blueshift compared with the excitation spectra ob-
tained for the 20 mm solution. For compound 2c, a slight red-
shift was observed with associated broadening of the excita-
tion spectrum (Figure S40). An additional band was also ob-
served at lower wavelengths. This shows that although the ag-
gregation takes place in both compounds, the extent and
nature of aggregation is different in each compound. The blue-
shifted excitation spectra of 1c hint at the formation of H-ag-
gregates. For compound 2c, both blueshifted and slightly red-
shifted bands are present in the excitation spectra, which cor-
responds to the formation of both H- and J-aggregates, with
the latter being the major product.[14,22]
Figure 19. a),c) AFM images obtained for the xerogel of 1c obtained from
a 5.51 mm solution in decane (scale bar 0.5 mm). b) Expanded region of (a)
showing the height profiles of the individual fibers and d) expanded region
of (b) showing the thickness of an individual fiber. e),g) AFM images ob-
tained for the xerogel of 2c obtained from a 3.8 mm solution in decane
(scale bar 0.5 mm). f) Expanded region of e) showing the height profiles of
the individual fibers and h) expanded region of (g) showing the thickness of
an individual fiber.
meters in length, with an average height of 30 to 40 nm and
an average thickness of around 150 nm. The SEM images of
the xerogels of 1c and 2c also show similar entangled net-
works of fibers.
POM of the xerogels of 1c and 2c show a birefringent pat-
tern as shown in Figure 20, inset, which suggests the presence
of anisotropic order (Table 8). Therefore, a powder XRD study
of the xerogel was carried out to investigate the structure of
this self-assembly. The XRD pattern of the xerogel of 1c shows
several peaks at low angles that can be indexed to the Colr
phase with lattice constants a=26.5 and b=36.5 ꢁ (Fig-
At this point, we were curious to compare the emission of
the LC aggregates in the thin films and gels of 1c and 2c. An
overlay of the emission spectra of these compounds in the so-
lution, gel, and thin-film states showed that the emission in-
tensity of the LC thin films was quite high compared with the
solution and gel states (Figure 18). It is surprising to see that
the thin film of 1c exhibited fivefold-enhanced emission (Fig-
ure 18a), whereas 2c exhibited ninefold-enhanced emission
with respect to the solution state (Figure 18b). From an appli-
cation point of view, this is of high technological importance
in emissive displays.
These gels were further characterized extensively through
atomic force microscopy (AFM; Figure 19) and scanning elec-
tron microscopy (SEM; Figure S41). The AFM microscopic
images of the xerogels of 1c and 2c show entangled networks
of fibers. The fibers found in the xerogels were several micro-
Figure 20. a) XRD patterns obtained for the xerogel of 1c (Colr phase); inset:
the obtained POM image of 1c. b) XRD patterns obtained for the xerogel of
2c (Colh phase); inset: the obtained POM image of 2c.
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with the columns arranged in a hexagonal lattice, as revealed
by the XRD studies (Figure 21b). This shows the impact of an
atomic-scale difference (<2% of the molecular weight) on the
self-assembly of polycatenars in the gel state. It is also impor-
tant to note that the long nanofibers are mainly formed by p–
p interactions and can be termed as molecular nanowires with
a central conductive core and a peripheral insulating sheath.
This long-range self-assembly is helpful from the viewpoint of
applications in organic electronic devices.
Table 8. Results of indexation of XRD profiles of xerogels of 1c and 2c at
RT.
Compound Phase dobsd [ꢁ] dcalcd [ꢁ] Miller indices hk Lattice
(D [ꢁ])
parameters [ꢁ]
1c (60.3)
Colr
26.5
18.3
15.1
12.0
9.6
7.7
4.2
3.7
3.5
54.2
21.9
16.4
13.0
4.68
3.98
26.5
18.3
15.0
12.2
9.1
7.5
4.3
3.7
3.6
54.2
21.5
16.9
13.4
10
02
12
03
04
24
55
57
66
11
32
51
53
a=26.5
b=36.5
Rheological studies
2c (60.4)
Colh
a=108.3
To establish the elastic nature of the gel samples, dynamic
rheological measurements that involved strain sweep, angular
frequency sweep, and step-strain measurements were carried
out. The dependence on the strain amplitude (g) of the stor-
age or elastic modulus (G’) and loss or viscous modulus (G’’)
obtained for representative gels 1c and 2c is shown in Fig-
ure 22a. At low g values, both the samples show strain-inde-
pendent behavior for G’ and G’’, with G’>G’’. This solid-like be-
ure 20a, Table 8). The presence of several peaks in the low-
and mid-angle regions suggested higher intercolumnar order
in the columnar self-assembly. Therefore, compound 1c forms
a columnar self-assembly in LC and gel states. Similarly the d
spacings obtained from the diffraction pattern (Figure 20b) of
the xerogel of 2c could be fitted to a Colh phase with the lat-
tice constant a=108.3 ꢁ. Thus the self-assembly of the two
polycatenars in the LC and gel states is dependent on the type
of hetero atom present in the adjacent rings of the thiophene
moiety. This can be schematically represented as shown in Fig-
ure 21a,b. Compound 1c, which is in the form of a discoid,
Figure 22. a) The dependence of storage (G’) and loss (G’’) moduli on strain
(g) for 1c and 2c. The temperature was kept constant at 208C and the an-
gular frequency (w) at 1 radsꢀ1. b) Angular frequency (w) vs. G’ and G’’ for
1c and 2c. The temperature was kept constant at 208C. The strain ampli-
tude (g) for both samples was kept constant at a value that corresponded
to the LVR.
havior is typical of a gel structure.[17] Above a critical strain am-
plitude (gc =0.004 for 2c and 0.03 for 1c), which defines the
upper limit of the linear viscoelastic regime (LVR), both G’ and
G’’ become strain-dependent. G’ shows a monotonic decrease
for both samples, whereas G’’ for 2c passes through a maxi-
mum before decreasing. The latter feature is associated with
soft glassy rheological (SGR) behavior[18] universally seen in ma-
terials such as foams, slurries, and pastes. The magnitude of G’,
a direct measure of the mechanical stiffness, was found to be
lower and the LVR longer for 1c. These features indicate that
although 2c is a stiff gel (higher G’), it is less robust than 1c. In
both gels, G’ crosses over G’’ at higher g values (above gc),
which signals a deformation-driven transition from a viscoelas-
tic solid to a viscoelastic liquid.[19] The variation in G’ and G’’
with angular frequency (w) obtained by keeping the strain am-
plitude constant in the LVR is shown in Figure 22b. Both G’
and G’’ show almost no variation with w, with an elastic re-
sponse (G’>G’’) over the entire frequency range studied,
which confirms the gel nature of the samples.[17] A further gel
Figure 21. Diagram of the self-assembly of a) 1c and b) 2c in LC and gel
states.
self-assembles to form columns, and these columns further
self-organize into a Colh phase, which on further cooling forms
a Colr phase that is stable at room temperature. In the pres-
ence of a hydrocarbon solvent, the compound forms nanofib-
ers of several micrometers in length, which further entangle to
form a dense network of fibers that entrap a huge amount of
solvent (Figure 21a). In compound 2c, two molecules organize
to form a disc that self-assembles to form columns, and these
columns further self-organize into a Colh phase. On cooling,
the arrangement of columns changes to a rectangular lattice
that is stable at room temperature. Interestingly, on interaction
with the hydrocarbon solvent this compound forms nanofibers
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behavior reappeared (region III). Regions I and III both show
a power-law behavior with an identical exponent of 0.8ꢁ0.06,
which is lower than the power-law exponent (0.95ꢁ0.01) ob-
tained for 1c. The neat shear-thinning behavior of 1c with in-
creased shearing conditions makes it more suitable for applica-
tions that require uniform spreadability.[21]
Conclusion
Figure 23. a) The step-strain with w=1 radsꢀ1 for a gel of 1c showing the
collapse of the gel structure when the gel was subjected to a high oscillato-
ry strain (g) of 5 (regions indicated as high). On application of a g of 0.001
(regions marked low), gel recovery took place within about 20 s and was re-
produced over repeated cycles of measurement. b) The data obtained for
the same experiment for 2c.
Two series of polycatenar mesogens that contain a central
thiophene moiety connected to two substituted oxadiazole or
thiadiazole units have been prepared. These heterocycles are
connected to peripherally substituted phenyl groups. The
number, length, and position of substitution of the peripheral
chains were varied. The oxadiazole-based polycatenars exhibit-
ed a columnar phase with rectangular and hexagonal or obli-
que symmetry, whereas thiadiazole-based polycatenars exhibit-
ed columnar phases with rectangular and/or hexagonal sym-
metry. All the compounds exhibited bright emission in the so-
lution and thin-film states. Two oxadiazole-based molecules
and one thiadiazole-based molecule exhibited good ability to
undergo gelation in hydrocarbon solvents at very low concen-
trations, which qualifies them as supergelators. The supergela-
tion is mainly supported by attractive p–p interactions, along
with other weak forces. Of these gels, the oxadiazole- and thia-
diazole-containing hexacatenars with n-hexadecyloxy chains
were studied extensively. These gels showed aggregation-in-
duced enhanced emission, which is of high technological im-
portance for applications in solid-state emissive displays. X-ray
diffraction studies showed that the fibers of the xerogels
formed by oxadiazole-based polycatenars self-assembled in
a rectangular columnar phase, whereas those of the thiadia-
zole-based polycatenars self-assembled in a hexagonal colum-
nar phase. The number, length, and position of substitution of
the peripheral tails and the type of heterocycle moiety adja-
cent to the thiophene ring greatly affected the self-assembly
of the molecules in the LC and gel states. Rheological meas-
urements carried out on the samples confirmed the formation
of gels quantitatively and showed that these gels are mechani-
cally robust. The shear-rate dependence of bulk viscosity ex-
hibited an overall shear-thinning behavior, with the exception
of an anomaly of shear thickening at a smaller range of shear
rates in one of the gels studied. The impact of an atomic-scale
difference (oxygen to sulfur, <2% of the molecular weight) on
the self-assembly and the macroscopic properties of these self-
assembled compounds have been clearly visualized. This mo-
lecular design helps in the development of long molecular
nanowires with a strong overlap of central conducting cores
and an insulating peripheral sheath, which will be helpful for
applications in organic electronic devices.
collapse and recovery test was done by performing step-strain
measurements (Figure 23). For this, the samples were subject-
ed to a large amplitude strain (g>gc), which resulted in the
breakdown of the gel structure; subsequently, the strain was
reduced to a smaller amplitude (g<gc,), with G’ and G’’ moni-
tored throughout. The results obtained for 1c (Figure 23a)
show that on application of high g, G’ decreased by orders of
magnitude and became smaller than G’’, which resulted in
a viscous state. When the strain was reduced to g<gc, the
elastic state was recovered. The gel collapse and recovery was
found to be instantaneous, with a response time of less than
20 s, and was reproducible over repeated cycles of measure-
ment, which indicated the mechanical robustness of both sam-
ples.[19]
In the steady-state measurements the shear viscosity (h) was
determined as the samples were subjected to increasing shear
rate (g˙). The obtained flow curves (h vs. g˙) are shown in Fig-
ure 24a,b for 2c and 1c, respectively. Although both samples
show an overall shear-thinning be-
havior, 2c exhibits an anomaly in
the data that gives rise to three
distinct regions. In region I at low
shear rates, the viscosity decreased
as the shear rate was increased,
which is typical shear-thinning be-
havior. In region II, h started in-
creasing as g˙ was increased and
exhibited a shear-thickening be-
havior. This unusual behavior is in-
dicative that structural changes are
taking place in the gel, and needs
extensive optorheological meas-
urements to explain the phenom-
ena. Similar flow behavior has
Figure 24. The dependence of
viscosity (h) on shear rate for
a) 2c and b) 1c. Whereas 1c
been observed in triblock-copoly-
mer-decorated systems in which
exhibits simple shear-thinning region II (termed the transition Acknowledgements
behavior, 2c shows an anom-
aly with regions I and III
showing shear-thinning be-
havior and region II showing
shear-thickening behavior.
region) corresponds to the coexis-
tence of lamellar and onion
phases.[20] On further increasing
the shear rate, the shear-thinning
A.A.S. sincerely thanks the Science and Engineering Board
(SERB), DST, Govt. of India, and BRNS-DAE for funding this
work through project no. SB/S1/PC-37/2012 and no. 2012/34/
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Kumar Pal from IISER Mohali for providing the XRD data for xe-
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Keywords: columnar phases
polycatenars · self-assembly
·
gels
·
liquid crystals
·
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Received: August 2, 2016
Revised: September 2, 2016
Published online on && &&, 0000
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FULL PAPER
Small difference, large impact: Two
series of polycatenars composed of
a central thiophene moiety substituted
with either oxadiazole or thiadiazole
units have been prepared. A detailed
account of the impact of structural var-
iations on liquid-crystalline and organo-
gel self-assemblies is provided.
&
Liquid Crystals
B. Pradhan, V. M. Vaisakh, G. G. Nair,
D. S. S. Rao, S. K. Prasad, A. A. Sudhakar*
&& – &&
Effect of Atomic-Scale Differences on
the Self-Assembly of Thiophene-based
Polycatenars in Liquid Crystalline and
Organogel States
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