Columnar self-assembly of luminescent bent-shaped hexacatenars with a central pyridine core connected with substituted 1,3,4-oxadiazole and thiadiazoles

Article abstract of DOI:10.1039/c7nj04449f

Bent-shaped molecules with a central pyridine core flanked with substituted 1,3,4-oxadiazole and thiadiazole derivatives with a variation in the number and length of terminal tails were synthesized. Thiadiazole based compounds exhibited a wider mesophase

Full text of DOI:10.1039/c7nj04449f

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Columnar selfꢀassembly of luminescent bentꢀshaped hexacatenars
with a central pyridine core connected with substituted 1,3,4ꢀ
oxadiazole and thiadiazoles
a
a
a
b
Balaram Pradhan, Ravindra Kumar Gupta, Suraj Kumar Pathak, Joydip De,
b
a
Santanu Kumar Pal and Ammathnadu S. Achalkumar *
a
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati,
7
b
81039, Assam, India. Department of Chemical Sciences, Indian Institute of
Science Education and Research Mohali, Sectorꢀ81, Knowledge City, Manauli
40306, Punjab, India.
1
Abstract: Bentꢀshaped molecules with a central pyridine core flanked with
substituted 1,3,4ꢀoxadiazole and thiadiazole derivatives with a variation in the number
and the length of terminal tails were synthesized. Thiadiazole based compounds
exhibited higher mesophase range in comparison to oxadiazole derivatives, while the
oxadiazole derivatives exhibited higher gelation tendency. All hexacatenars exhibited
supergelation in hydrocarbon solvents along with an ability to form selfꢀstanding,
moldable gel at higher concentration. Thiadiazole based compounds exhibited
bathochromic absorption and emission in comparison to oxadiazole derivatives but a
lower quantum yield. Two of the gelators investigated exhibited aggregation induced
enhanced emission in gel and thin film state. This study shows that in addition to πꢀπ
interactions, nanoseggregation of incompatible molecular subunits like flexible tails
play a major role in gelation and liquid crystalline selfꢀassembly. Microscopic studies
and Xꢀray diffraction studies revealed a fibrillar network of several micrometer
lengths with long range molecular selfꢀassembly. They showed the ability to sense the
acid with an emission quenching/shifting mechanism, that makes it possible to detect
the acids by naked eye. Considering the dearth of solidꢀstate organic blue light
emitters that are pivotal to realize the white light emission, these polycatenars are
promising due to their wideꢀrange Col phase and aggregation induced blue emission.
Further the introduction of pyridine central unit enhanced the mesophase stability and
an acid sensing functionality in comparison to simple benzeneꢀbased bentꢀshaped
polycatenars.
Introduction
The molecules possessing an anisotropic shape along with incompatible molecular
segments exhibit the ability to selfꢀassemble into different liquid crystalline (LC)
phases. This selfꢀassembly is mainly driven by the nanoꢀseggregation of incompatible
molecular subunits and the tendency of the molecules to align with longꢀrange
1
orientational order or/and positional order. This exceptional amalgamation of order
and mobility present in LC materials gives rise to a plethora of properties that is being
2
exploited in several applications. Several nonꢀconventional molecular designs that
deviate from the conventional rod (calamitic) and discꢀlike (discotic) systems have
been prepared over the years in quest of novel mesophases and to understand the
1

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impact of different anisometric shapes and noncovalent interactions on the selfꢀ
1,3
assembly. Polycatenar liquid crystals (LCs), are one such nonconventional LCs,
which have a combination of central elongated rod like and terminal hemi disc shaped
4
units. The hemi discs may contain four to six flexible alkyl chains that provide the
necessary fluidity. As they possess the structural characters of both calamitic and
discotic LCs, they display various mesophases that are common to these classes of
LCs. This depends on the number of outer flexible tails. The columnar selfꢀassembly
of such molecules resulting in the oneꢀdimensional (1D) columnar (Col) phases with a
large πꢀoverlap are interesting, as they resemble ‘molecular wires’ with an
5
nonconductive external casing of alkyl tails. This selfꢀassembly shows anisotropic
conductivity and optical properties and thus holds promise in the development of
6
several optoelectronic devices. Polycatenar molecular design provides a seamless
synthetic flexibility in tuning the transition temperatures, optical and charge transport
behavior by proper molecular design, which is a definite advantage in comparison to
traditional discotics. Most of the polycatenars reported are known to have a central
7
linear rodꢀlike structure, while very few are known to have a bent structure. The
polycatenars with central bent structure have shown lower transition temperature and
8
enhanced mesophase range. There is a growing interest in the research community,
in the incorporation of heterocycles in the LC design, as they help in varying the
overall lateral and/or longitudinal dipole moment of the molecule, along with the
9
,10
variations in their molecular shape.
These changes affect the molecular selfꢀ
assembly and the macroscopic properties of such molecules, and therefore can be
effectively utilized to modify the material properties. 1,3,4ꢀoxadiazole and
thiadiazoles are preferential candidates among the heterocycles because of their high
10
luminescence, high thermal and photochemical stability. Incorporation of the
heterocyclic moieties has shown enhanced mesophase stability due to the increased
intermolecular interactions. Col LCs that exhibit high luminescence are of significant
importance as this combination features the 1D conductivity and luminescence in a
single material, which has a potential to be utilized as an emissive layer of OLEDs
11
along with good conductivity. Such molecular materials will be promising in
comparison to inorganic semiconductors (which are hazardous and expensive) and
polymers (which have got processability and conductivity issues). Col LCs are known
to have high intermolecular order, solution processability and selfꢀhealing ability of
structural defects. Some of our and other group’s recent works on luminescent Col
2

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LCs based on heterocyclic moieties exhibited their potential as active emissive layers
12
in OLEDs. Efficient πꢀconjugated and coplanar molecular structure is a major
requirement to achieve reduced band gaps in organic semiconductors with high
charge carrier mobility and an electroꢀoptical response. Recently, donorꢀacceptor (Dꢀ
A) molecular structures have been investigated to attain lowꢀband gap
1
3
characteristics. 1,3,4ꢀOxadiazole and thiadiazole moieties can be good electron
acceptor molecular components, because of the presence of electronegative atoms.
Their presence creates stronger polar induction and reduced molecular symmetry that
9
affects the optical and electronic behaviors. Such molecular structures also promote
1
4
the ordered selfꢀassembly in bulk as well as in the presence of solvents. Recently we
have reported such DꢀAꢀD structures that stabilized liquid crystal phases over a long
range and also showed supergelation in hydrocarbon solvents due to the enhanced πꢀπ
8cꢀe,10f
interactions.
It is always interesting to see if such organogelators show
aggregation induced enhanced emission (AIEE) effect, specially in the case of πꢀ
gelators. This is because the molecules are prepared with a view to utilize them in
organic light emitting diodes (OLEDs), where both charge carrier mobility as well as
emission in aggregated state is equally important. In most of the cases, such
aggregated systems exhibit low luminescence due to the aggregation induced
quenching (ACQ). Thus columnar selfꢀassembly and AIEE contradict each other.
However, over the years several polycatenars, including bent ones showed AIEE
14b,8d,e,10f
behavior.
Further, utilization of 2,6ꢀsubstituted pyridine will provide an
2
additional functionality for acid sensing due to the presence of an sp hybridized
8e
nitrogen atom. In addition, only a few number of pyridine based bent polycatenars
7c,d,g,h,8e
were reported in the literature, prompted us to work on this area.
As a part of our efforts to synthesize luminescent liquid crystals toward the
application in OLEDs, we have synthesized and characterized two series of bentꢀ
shaped polycatenars with a central pyridine unit flanked with unsymmetrically
substituted oxadiazole or thiadiazole unit (Fig.1). We were curious to compare the
optical, photophysical and gelation properties of polycatenars based on 1,3,4ꢀ
oxadiazole derivatives to their thiadiazole counterparts considering the alternate
arrangement of donor and acceptor moieties in the molecular system. We visualized
that the incorporation of oxadiazole or thiadiazole in the polycatenar structure
impacting the selfꢀassembly and related macroscopic properties. The number of
flexible chains and their length were modified to understand the impact on the LC
3

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r
behavior and gelation. These molecules exhibit low temperature Col and/or high
temperature Col phases over a wide thermal range. Most importantly five of the
h
polycatenars were able to gelate in different hydrocarbon solvents. Some of these
compounds exhibited aggregationꢀinduced enhancement in emission (AIEE) in thin
film and gel state.
Synthesis and characterization
The target molecules 1aꢀd and 2aꢀd (Fig.1) and their precursors were
synthesized as presented in Scheme 1. Syntheses of ethyl mono/di/trihydroxy
Scheme 1. Synthesis of oxadiazole and thiadiazoleꢀbased polycatenars.
R2
R2
R2
R1
R3
R1
R3
R1
R3
(
i)
(ii)
COOC H
COOC H5
CONHNH2
4a: R = R3 = H, R = n-OC16H33
2
5
2
8
7
6
: R1 = R3 = H, R = OH
5a: R
5b: R
1
= R
= R
= R
3
= H, R
= n-OC16
= R = n-OC16
2
= n-OC16
H
33
= H
2
1
2
: R1 = R2 = OH, R = H
: R1 = R2 = R3 = OH
1
2
2
H
33, R
3
4b: R = R2 = n-OC16H33, R = H
3
1
3
5c: R
H
4c: R = R2 = R = n-OC16H33
1 3
1
3
33
5
d
: R = R2 = R = n-OC12H25
4d: R = R2 = R3 = n-OC12H25
1
3
1
(
iii)
O
O
3
3
3
3
a
b
c
d
: R1 = R3 = H, R2 = n-OC16H33
: R = R = n-OC16H33, R3 = H
O
N
O
1
2
R1
R2
NH
HN
R1
R2
: R1 = R2 = R = n-OC16H33
N
H
N
H
3
: R = R = R = n-OC12H25
1 2 3
R3
R3
(
v)
(
iv)
2
2
2
2
a
: R1 = R = H, R2 = n-OC16H33
3
1
1
1
1
a
b
c
d
: R = R = H, R = n-OC16H33
1
3
2
b
: R1 = R = n-OC16H , R = H
: R = R = n-OC16H33, R3 = H
2 33 3
1
2
c
d
: R1 = R = R = n-OC16H
: R = R = R = n-OC16H33
2 3
33
25
1
2
3
: R1 = R = R = n-OC12H
: R = R = R = n-OC12H25
2
3
1
2
3
Reagents and conditions (yield) (i) nꢀBromoalkanes, anhydrous K
C, 24 h (70ꢀ80%); (ii) NH NH .H O, Ethanol or butanol, reflux, 48 h (70ꢀ80%); (iii)
2 2 2
2
CO
3
, DMF, 80
o
Pyridineꢀ2,6ꢀdiacid chloride, THF, Et N, 12 h, reflux; (iv) POCl , reflux, 17 h (60ꢀ
3
3
72%); (v) Lawesson's reagent, toluene, reflux, 17 h (55ꢀ60%).
4

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10e
benzoates (6ꢀ8) are carried out as reported earlier. Compounds 5aꢀd were prepared
of corresponding ethyl hydroxyl benzoates following the Williamson’s ether
synthesis protocol. These alkoxy esters (5aꢀd) were utilized to prepare the by the Oꢀ
10e
alkylation corresponding hydrazides (4aꢀd). These hydrazides on refluxing with
2
2
,6ꢀpyridine dicarboxylic acid chloride in the presence of triethylamine base provided
8
e
,6ꢀdiꢀNꢀpyridinebenzohydrazides (3aꢀd).
POCl
3
mediated dehydrocyclization
provided dioxadiazole derivatives (1aꢀd). Corresponding dithiadiazole derivatives
2aꢀd) were prepared by refluxing the compounds 3aꢀd in presence of Lawesson’s
reagent in toluene.
(
o
Temperature ( C)
Figure 1. Bargraph summarizing the thermal behavior of bentꢀshaped molecules 1aꢀd
and 2aꢀd (considering the first cooling cycle).
H
b
residual
CHCl
H
a
3
2c
residual
CHCl
H
b
3
Ha
1c
δ/ppm
1
Figure 2. Overlay of the expanded portion of the H NMR spectra (CDCl
of 2c and 1c.
3
, 600 MHz)
5

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The molecular structures of the final compounds and their precursors were confirmed
1
13
by various analytical techniques such as H NMR, C NMR, IR spectroscopy, and
HRMS or MALDIꢀTOF (see ESI for details). The electron withdrawing nature of
oxadiazole ring is more in comparison to its thiadiazole counterpart due to more
1
electronegative nature of oxygen than sulphur, which is reflected by the H NMR
1
signals. For illustration, H NMR of compound 1c with two oxadiazole units showed
the down field signal for proton H
of compound 2c (δ = 8.42 ppm) with two thiadiazole units (Fig. 2). Similarly the
proton H of compound 2c (δ = 8.13 ppm) appeared at low field in comparison, to that
b b
(δ = 8.46 ppm) and in comparison to the proton H
a
13
of compound 1c (δ = 8.04 ppm). On the other hand, C NMR of compound 1c
exhibited low field signals for the heterocyclic ring carbons of the 1,3,4ꢀoxadiazole at
171 ppm and 169 ppm in comparison to the heterocyclic ring carbons of the 1,3,4ꢀ
thiadiazole (signals at 169 ppm and 167 ppm) of compound 2c. (See the SI).
Thermal behavior
The preliminary confirmation for the occurrence of thermotropic liquid crystallinity
was the appearance of strong birefringence along with the fluidity on gradual heating
of the sample, under the polarizing optical microscopy (POM) observations.
Transition temperatures and the enthalpy changes corresponding to these phase
transitions were obtained by differential scanning calorimetry (DSC) thermograms.
Further, the mesophase assignment was carried out by performing Xꢀray diffraction
(XRD) measurements at different temperature intervals. The phase transitions and the
corresponding enthalpy changes are tabulated in table 1 and represented in Fig.1. In
comparison to normal carbocyclic LCs, heterocyclic derivatives are expected to show
higher enthalpy changes associated with phase transitions, which may be due to the
attractive hetero atom interactions and intermolecular hydrogen bonding
8,10,15
interactions.
Compound 1a and 2a with the two terminal nꢀhexadecyloxy tails and
compound 1b with the four terminal nꢀhexadecyloxy tails attached to the 3 and 4
positions of the terminal benzene rings turned out to be crystalline (Fig.S25ꢀ26). The
POM images and XRD patterns confirmed their crystalline nature (Fig.S40).
Hexacatenar 1c with six nꢀhexadecyloxy tails observed as a birefringent fluidic mass
o
under POM observation, which at 66 C showed a transition in DSC thermogram with
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a
o
ꢀ1
Table 1. Phase transition temperatures ( C) and corresponding enthalpies (kJ mol )
of the oxadiazole and thiadiazole derivatives
Entry
Phase sequence
nd
st
2
Heating
131.1 (24.6) Cr 196.6 (54) I
48.4 (9) Cr 133.1 (15.5) I
1 Cooling
I 191.5 (55.7) Cr 128.7 (23.5) Cr
I 122 (20.1) Cr 44.1 (5.9) Cr
I 75.8 (3) Colr2 45.3 (103.8) Colr1
I 92.2 (0.2) Col 42.1 (14.5) Cr
I 208.2 (27.4) Cr
I 126.7 (2.5) Col
1a
Cr
Cr
1
1
2
2
1
1b
2
2
1
b
1c
Colr1 66 (107.8) Colr2 82.1 (2.8) I
Cr 60.8 (18.1) Col 96.2 (0.9) I
Cr 127.9 (14.3) Cr
Cr 119.8 (27.2) Col
1d
h
h
2a
1
2
213.9 (22.8) I
129.2 (2) I
2
124.7 (16.6) Cr
99.1 (32.7) Cr
r
1
2b
r
2
c
Colr2 52.4 (2.2) Colr1 87 (9.7) Col
01 (11.1) I
Col 61.6 (9) Col
21.6 (3.1) I
Peak temperatures in the DSC thermograms obtained during the first cooling and second
h
I 95.6 (1.3) Col
Colr2
h
70 (8.84) Colr1 43.6 (0.6)
b
1
2d
r
r
95.2 (16.6) Col
r
I 119.7 (3.1) Col 73 (13.2) Colr1 57.3 (6)
r
b
1
Col
r
a
o
ꢀ1
b
o
heating cycles at 5 C min ; Crystallization was not observed till ꢀ20 C; Cr = crystal;
Col
h
= columnar hexagonal phase; Col
r
= columnar rectangular phase; I = isotropic liquid.
o
o
1
c: T= 70 C
1c: T= 25 C
Colr1
cooling
Colr
2
I
Colr1
Colr2
I
(
c) heating
(a)
(b)
01
(d)
01
01
11
12
12
12
10
13
24
10
13
24
23
20
(e)
(f)
o
Figure 3. POM images obtained for compound 1c at 70 C (a); at room temperature
b); DSC thermogram of compound 1c in the second heating (red trace) and first
(
o
cooling (blue trace) at a scan rate of 5 C/min (b); Intensity vs 2θ profiles obtained
from the SAXS patterns for compound 1c at 65 C for Col phase (d); from the XRD
patterns at 40 C for Col phase (e) and at 25 C (f) for Col phase. (inset shows the
o
r2
o
o
r1
r1
expanded portion in the middle angle region and the image pattern)
an enthalpy change of 107.8 kJ/mol. This mesophase was converted into an isotropic
o
liquid at around 82 C (ꢁH = 2.8 kJ/mol) (Fig.3c). In the cooling cycle, the isotropic
o
liquid showed the growth of mosaic pattern at around 76 C (Fig.3a). Further cooling
o
showed a transition in DSC at 45 C (ꢁH = 103.8 kJ/mol), but the texture did not
7

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o
show much difference (Fig.3b). Xꢀray diffractogram obtained at 65 C shown a sharp
low angle peak along with two diffused wide angle peaks (Fig.S39). The first diffused
peak with a dꢀspacing of 4.96 Å corresponds to the packing of alkyl tails, while the
second one at a dꢀspacing of 4.19 Å corresponds to the coreꢀcore stacking. As it was
difficult to identify the organization of columns in a 2Dꢀlattice with a single peak at
low angle, we decided to investigate through small angle xꢀray scattering (SAXS)
studies. Intensity vs 2θ profiles obtained from the SAXS patterns for compound 1c at
o
6
5 C (Fig.3c), showed peaks corresponding to dꢀspacings of 36.11 Å, 19.2 Å, 14.48
Å which can be indexed to Miller indices 01, 11, 12 of a rectangular lattice. From
these values, the lattice parameters were calculated and found to be a = 22.67 Å, b =
2
3
6.11 Å. The unit cell area (S) was found to be 818.6 Å and cell volume (V) was
3
found to be 3430 Å . The number of molecules (Z) in the unit rectangular cell was
found to be 1.1.
Table 2. Results of the (hkl) indexation of the XRD profiles of the compounds at a given temperature
a
(T) of the mesophases.
Compounds Phase
dobs (Å)
dcal
(Å)
Miller
indices
hk
Lattice parameters (Å), Lattice
o
2
(
D/Å)
(T/ C)
area S (Å ), Molecular volume
3
V (Å )
1
c
Colr2
(65)
36.11
19.2
36.11
19.20
14.12
01
11
12
a = 22.67, b = 36.11, S = 818.6,
V = 3430, Z = 1.1
(
59.95)
1
4
4
4
2
4.48
.96 (ha)
.19 (h
5.62
c
)
45.62
21.05
15.47
12.33
7.73
01
10
12
13
24
a = 21.05, b = 45.62,
S = 960.3, V = 4369.37,
Z = 1.45
1.05
Colr1
15.73
12.73
(
40)
7
4
.69
.55 (h )
a
4
2
1
5.55
0.96
5.69
45.55
20.96
15.42
12.30
10.48
8.62
01
10
12
13
20
23
24
a = 20.96, b = 45.55,
S = 954.73, V = 4315.37,
Z = 1.44
Colr1
12.59
10.45
8
7
4
(
25)
.85
.75
.52(h )
a
7.71
1
d
Col
(65)
h
35.03
17.47
35.03
17.52
10
20
a = 40.45, S = 1417,
V = 5653.8, Z = 2.3
(
49.77)
4
3
.86(h
.99(h
a
)
c
)
a
The 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 Col phase; c is height of the unit cell). The spacings marked h and h
h a c
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
c
slice of thickness h estimated from the lattice area S and the volume V.
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o
The XRD pattern obtained at lower temperatures (i.e. at 40 C and room
o
temperature), which is below the transition temperature 45 C in cooling cycle
o
(
Fig.3eꢀf). The XRD pattern obtained at 40 C showed five peaks at low angle
corresponding to the dꢀspacings of 45.62 Å, 21.05 Å, 15.73 Å, 12.73 Å, 7.69 Å along
o
with a relatively sharp peak at 4.55 Å (in comparison to the XRD pattern at 65 C).
The peaks at low angle can be indexed into Miller indices 01, 10, 12, 13 and 24 of a
o
rectangular lattice. Thus the transition observed at 45 C is from one Col
r
to another
r
Col phase. For the sake of identification we have named the higher temperature Col
phase as Colr2 and the low temperature one as Colr1 phase. The rectangular lattice
parameters were calculated and from that the total number of molecules (Z) present in
a unit cell was found to be 1.5. The relatively sharp peak at the wide angle
corresponds to the enhanced columnar order. Since the texture remained same even at
room temperature we were curious to know whether the phase structure remains same
or different. The XRD pattern obtained at room temperature showed several peaks in
low to mid angle region along with a relatively sharp peak in the wideꢀangle region,
which can be fit in to a rectangular lattice. Thus the Col phase remains unchanged at
r
room temperature. Compound 1d, the dodecyloxy version of compound 1c exhibited
h
a Col phase, which was confirmed by POM images, DSC and XRD studies (Fig.4
and Table 2). Here again the low angle showed a single peak, which prevented us
from the unambiguous assignment to a particular mesophase. Thus the SAXS
measurements were carried out at this temperature and the dꢀspacings obtained at low
angle were closely fitting into a hexagonal lattice (Table 2 and Fig.4c). POM texture
(Fig.4a) with large homeotropic domains supports the uniaxial nature of the Col
h
phase.
o
10
1
d: T= 55 C
(c)
(a)
Cr
Col
h
I
20
cooling
heating
Cr
Col
h
I
(b)
o
h
Figure 4. POM image obtained for the Col phase of compound 1d at 55 C (a); DSC
thermogram of compound 1d in the second heating (red trace) and first cooling (blue
o
trace) at a scan rate of 5 C/min (b); Intensity vs 2θ profiles obtained from the SAXS
o
pattern for compound 1d at 65 C for Col
h
phase (c). (inset shows the expanded
portion in the middle angle region)
9

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o
(b)
(c)
02
2
b: T= 110 C
10
11
Cr
Col
r
I
I
05
cooling
heating
Cr
Col
r
(a)
o
Figure 5. POM image obtained for the Col
r
phase of compound 2b at 110 C (a);
DSC thermogram of compound 2b in the second heating (red trace) and first cooling
o
(
blue trace) at a scan rate of 5 C/min (b); Intensity vs 2θ profiles obtained from the
o
r
SAXS pattern for compound 2b at 110 C for Col phase (c) (inset shows the
expanded portion in the middle angle region)
Figure 6. DSC thermogram of compound 2d in the second heating (red trace) and
o
first cooling (blue trace) at a scan rate of 5 C/min.
o
o
o
2
d: T= 110 C
2d: T= 69 C
2d: T= 40 C
(a)
(b)
(c)
0
1
(c)
01
(d)
(e)
0
1
2
4
10
02
0
05
5
1
05
1
03 04
35
22
4
5
23
10
o
Figure 7. POM image obtained for the Colr1 phase of compound 2d at 110 C (a);
o
o
Colr2 phase of compound 2d at 69 C (b); Colr3 phase of compound 2d at 40 C (c);
Intensity vs 2θ profiles obtained from the XRD patterns for the Colr1 phase of
o
o
compound 2d at 110 C (d); Col phase of compound 2d at 69 C (b); Col phase of
compound 2d at 40 C (c); (inset shows the expanded portion in the middle and wide
r2
r3
o
angle region as well as image patterns)
1
0

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Table 3. Results of (hk) indexation of XRD profiles of the compounds 2aꢀd, at a given temperature (T)
a
of mesophase
Compounds
Phase
(T/ C)
d
obs (Å)
d
cal(Å)
Miller
indices hk
Lattice parameters (Å), Lattice
o
2
(
D/Å)
area S (Å ), Molecular volume V
3
(
Å )
2
b
Col
(110)
r
40.83
25.60
40.83
25.60
24.43
16.33
02
10
11
05
a = 25.6, b = 81.7, c = 4.02, S =
2091.5, V = 8407.9, Z = 3.7
(
60.0)
2
1
4
4
3.79
6.97
.98 (ha)
.02 (hc)
41.48
Col
90)
h
41.48
10
a = 47.87, c = 4.99, S = 1984.47,
V = 9902.51, Z = 3.2
(
4.99 (hc)
2
c
4
5.52
45.52
16.52
14.21
7.96
03
11
15
25
a = 16.64, b = 136.56, c = 4.33,
S = 2272.36, V = 9839.32,
Z = 3.2
(
61.06)
Colr1
16.52
14.25
.95
.63(ha)
(
65)
7
4
4
.33(h
c
)
4
5.69
.88
8.09
4.63 (h )
a
5.52
45.52
15.69
9.78
01
22
42
52
1
9
a = 43.32, b = 45.52, c = 4.31,
S = 1971.93, V = 8499.02,
Z = 2.78
Colr 2
8.10
(
45)
4.31 (h
c
)
4
1
5.52
6.24
45.52
16.24
10.20
8.09
02
20
33
41
a = 32.48, b = 91.04, c = 4.35,
S = 2957, V = 12863,
Z = 4.2
Colr2
25)
10.21
8.09
(
4
4
.61 (h
.35 (h
a
)
)
c
Colr1
37.99
19.39
37.99
19.39
01
10
a = 19.39, b = 37.99, c = 4.00,
S = 736.63, V = 2946.5,
Z = 1.2
(100)
2
d
4.89 (h
a
)
(50.85)
4.00 (hc)
4
2
2
1
1.35
7.88
0.47
3.65
41.35
27.88
20.68
13.78
11.13
10.34
9.07
8.27
7.43
6.91
6.22
01
10
02
03
23
04
24
05
51
35
45
a = 37.75, b = 41.35, c = 3.99,
S = 1560.96, V = 6228.24,
Z = 2.5
Colr2
11.33
10.20
(
68.5)
8
8
7
6
6
4
3
.91
.20
.32
.83
.23
.74 (h
.99 (h
a
)
c
)
4
2
1
1.35
3.09
0.11
41.35
23.09
10.08
8.27
01
10
22
05
a = 23.09, b = 41.35, c = 4.00,
S = 954.77, V = 3819.09,
Z = 1.5
Colr3
8.11
4.55 (ha)
.00 (h
(
40)
4
c
)
a
The 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 Col phase; c is height of the unit cell). The spacings marked h and h
c
h
a
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 h estimated from the lattice area S and the volume V.
c
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1

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Thiadiazole based tetracatenar compound 2b exhibited enantiotropic Col
r
phase as
evidenced from SAXS studies (Fig.5). Hexacatenar 2c was polymesomorphic by
exhibiting one high temperature Col phase and two low temperature Col phases,
h
r
8
e
which was reported earlier. Interestingly hexacatenar 2d with six dodecyloxy chains
exhibited several phase transitions as evidenced from DSC thermograms (Fig.6).
POM images showed a mosaic texture (Fig.7aꢀc) and the XRD patterns obtained at
different temperature intervals showed that there are three different Col
Fig.7dꢀe, Table 3). The low temperature Col phase stayed upto room temperature
without any signs of crystallization. Such transitions between different Col phases
r
phases
(
r
r
15b
have been reported earlier.
Cr
Columnar rectangular phase
Columnar oblique phase
Columnar hexagonal phase
1
c
1
d
c
2
2
d
16O
1
6T
2T
1
25
40
55
70
85
100
o
Temperature ( C)
Figure 8. Comparison of the thermal behavior of pyridine based bentꢀshaped
molecules 1cꢀd and 2cꢀd with benzene based bentꢀshaped molecules reported earlier
8
a
10b
(
16O, 16T and 12T ) (considering the first cooling cycle).
It was interesting to compare the thermal behavior of pyridine based bentꢀshaped
molecules 1cꢀd and 2cꢀd with corresponding benzene based bentꢀshaped molecules
reported earlier (Fig.8). Hexacatenar 1c with hadecyloxy chains exhibited Col phase
r
over a wide range including room temperature, while the corresponding benzene
based hexacatenar 16O was crystalline at room temperature and exhibited a shortꢀ
8a
range Col phase. Similarly, compound 2c exhibited enhanced mesophase stability
h
r h
by exhibiting Col and Col phases in comparison to the monomesomorphic 16T
10b
which exhibited Colob phase. Hexacatenar 2d having shorter alkyl chains exhibited
solely Col phase, while the corresponding benzene based hexacatenar 12T exhibited
r
Colob phase. In general, the introduction of pyridine as a central core enhanced the
1
2

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mesophase stability, by lowering the melting temperature. In both cases, whether it is
pyridine or benzene; the thiadiazole based hexacatenars exhibited wider mesophase
8
cꢀe,10aꢀf
range and higher stability, which was in line with earlier reports.
Photophysical properties
Photophysical properties of oxadiazole and thiadiazole based compounds 1aꢀd
and 2aꢀd in THF (20 micromolar) solution and annealed thin film state were
investigated and the data are summarized in Table 4. It is expected that the presence
of two different heterocycles (substituted 1,3,4ꢀoxadiazole and 1,3,4ꢀthiadiazole) may
10
have a distinguishable impact on the photophysical properties. Absorption and
emission spectra of the compounds 1aꢀd and 2aꢀd were measured in micromolar THF
solution (Fig. 9a and 10a). These compounds exhibited a redꢀshifted absorption and
emission spectra, in comparison to the benzeneꢀbased polycatenars reported
8a,10b
earlier.
The solution absorption spectra of compounds 1aꢀd exhibited a single
absorption band at a wavelength of 314 nm whereas compound 2aꢀd showed red
shifted absorption band in range of 337ꢀ347 nm. Both the series of compounds 1aꢀd
and 2aꢀd show large value of molar extinction coefficients, which is the inherent
property of highly conjugated systems. The single absorption band for these
compounds can be attributed to spin allowed πꢀπ* transition of the aromatic system.
The values of molar extinction coefficients of oxadiazole derivatives 1aꢀd (ε =
ꢀ1
ꢀ1
1
3,000ꢀ22,500 M cm ) were found to be more in comparison to thiadiazole
ꢀ1
ꢀ1
derivatives 2aꢀd (ε = 4,000ꢀ11,000 M cm ). Optical bandgaps of these systems
obtained from the absorption onset values shown that the oxadiazoles derivatives 1aꢀd
1a
1b
1c
1d
(
c)
(
a)
(b)
Figure 9. Absorption (solid trace) and emission (dotted trace) spectra of compounds
aꢀd (a); Emission spectra of spin coated thin films (b); photographs taken under the
UV light (λ = 365 nm) in micromolar THF solution and thin films (c)
1
ex
1
3

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(3.39ꢀ3.46 eV) showed higher band gap in comparison to their thiadiazole counterpart
2aꢀd (3.10ꢀ3.22 eV). Likewise, emission spectra were obtained by exciting the
solutions of these compounds at their absorption maxima exhibited a single emission
maximum in the range of 385ꢀ465 nm for 1aꢀd and a red shifted emission maximum
in the range of 421ꢀ473 nm for 2aꢀd (Table 4, Fig. 9b and 10b) with large Stokes
shift. The oxadiazole derivatives 1aꢀd showed lower steady state anisotropy values
(0.02ꢀ0.09) with respect to the thiadiazole derivatives 2aꢀd (0.03ꢀ0.11). Relative
quantum yields of these compounds were measured with respect to standard quinine
sulphate solution (0.1 M in H
range of 0.48ꢀ0.58.
2 4
SO ), exhibited almost similar quantum yields in the
2a
2b
2c
2d
(c)
(
a)
(b)
Figure 10. Absorption (solid trace) and emission (dotted trace) spectra of compounds
aꢀd (a); Emission spectra of spinꢀcoated thin films (b); photographs taken under the
2
UV light (λex = 365 nm) in micromolar THF solution and thin films (c).
a
Table 4.Photophysical properties of compounds 1aꢀd and 2aꢀd in solution and film state.
Solution State
Thin film State
b
c,d
b
Absorption Emission
Stokes
shift
λ
onset
ꢁE g,opt
Steady
state
anisotropy
Relative
Absorption
(nm)
Emission
(nm)
Stokes
shift
(cm )
Entry
(
nm)
(nm)
(nm)
Quantum
ꢀ1
e
ꢀ1
(
cm )
Yield
1
1
a
314
314
385
428
5873
8483
366
367
3.39
3.39
0.015
0.015
0.58
0.48
315
316
416
461
7708
9954
b
1
c
314
314
337
341
343
347
465
452
421
446
473
10342
9723
5921
6904
8013
7359
363
359
386
389
398
401
3.42
3.46
3.22
3.19
3.12
3.10
0.088
0.018
0.031
0.027
0.111
0.50
0.48
0.55
0.56
0.53
0.48
311
318
343
345
345
345
467
479
453
478
473
464
10741
10570
7080
8065
7844
7434
1d
2a
2b
2
c
2
d
466
b
0.025
a
c
micromolar solutions in THF; excited at the respective absorption maxima; Band gap determined from the red edge of the longest
wave length (λonset) in the UVꢀvis absorption spectra; In volts (V). Quinine sulfate in 0.1 M H SO solution as the standard.
2 4
d
e
The thin films of the compounds on quartz plates were made by the spin coating of
the solutions of these compounds in chloroform. As expected, the absorption and
1
4

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emission bands of these compounds were found to be broad in comparison to their
solution spectra. The absorption spectra of the thin films of most of the oxadiazole
based compounds 1aꢀd as well as thiadiazole based compounds 2aꢀd showed a red
shift in comparison to the solution spectra, pointing towards the formation of Jꢀtype
14c
aggregates. Compound 1c and 2d showed a slight blue shift in the absorption
2
0
maximum, which may be due to the formation of Hꢀaggregates. Considering the
possibility of the several conformational isomers, it is difficult to conclude the type of
r
the aggregates. As the room temperature Col phase of compound 1c and 2c we
examined the thin films of the compounds, which were found to be transparent in
visible light and remained stable over a long period. The irradiation of these films
with UV light of long wavelength exhibited blue emission, which masked the pattern
kept behind (Fig.11). The thin film of compound 1c exhibited higher emission
intensity (2 folds) in comparison to their micromolar solution in THF, while the
emission intensity for compound 2c was reduced by half (Fig.S31). Both the solutions
and thin films of oxadiazole and thiadiazole based compounds exhibited a visually
perceivable blue emission under the UV light of long wavelength (Fig.9c and 10c).
Figure 11. Thin film of compound 1c annealed in Col
under visible light (a); the same under the UV light (λex = 365 nm) (b); Thin film of
compound 2c annealed in Col phase placed above a pattern under visible light (c); the
r
phase placed above a pattern
r
same under the UV light (λex = 365 nm) (d).
Solvatochromism and acidochromism
We investigated the effect of solvent polarity on the absorption and emission
properties of the compounds. The absorption and fluorescence spectra of
representative compounds 1c and 2c in different solvents like decane, hexane,
4 3
tetrachloromethane (CCl ), dichloromethane (DCM), chloroform (CHCl ),
tetrahydrofuran (THF), toluene were recorded and the corresponding photophysical
details are summarized in Table 5. Pictures of micromolar solution of compounds 1c
1
5

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and 2c in different solvents were depicted in Figure S28 and S29. As we know,
16
solvent polarity influences the energy levels of the absorption or emission bands.
From the absorption spectra we found that the energy levels of the absorption remain
almost unaffected indicating nonꢀpolar ground state. In contrast to the absorption
band, the emission was red shifted on increasing the solvent polarity from decane to
polar solvents like THF (Fig.12). This corresponds to the polar nature of the excited
16
state.
Table 5. Photophysical properties of compound 1c and 2c in different solvents.
Solvent
1c
2c
b
b
Absorption
nm)
Emission
(nm)
Stokes shift
(nm)
Absorption
(nm)
Emission
(nm)
Stokes
shift(nm)
(
Hexane
Decane
DCM
310
310
310
311
311
313
314
433
123
343
453
110
106
141
114
128
129
130
428
460
427
435
457
465
118
150
116
124
144
151
343
343
343
343
343
343
449
484
457
471
472
473
CCl
Toluene
4
CHCl
THF
3
a
b
micromolar solutions in different solvents; excited at the respective absorption maxima.
(a)
(b)
CCl
4
CCl
4
CHCl
3
CHCl
3
Figure 12. Normalized emission spectra of compounds 1c (a) and 2c (b) in
micromolar concentrations of different solvents
(a)
(b)
Figure 13. Emission spectra obtained for the compound 1c on gradual addition of
TFA to the in same (50 ꢂM in CHCl ) (a); the same obtained for the compound 2c on
gradual addition of TFA to the in same (50 ꢂM in CHCl3).
3
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6

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2
Considering the presence of an sp hybridized N atom of the pyridine ring, we
envisaged that these compounds might act as good proton acceptors. The protonation
may change the extent of delocalization and thus affect the optical properties of these
compounds, which can be beneficial in the detection of acids. As seen from the
Fig.13a and b, the emission intensities of compounds 1c and 2c were reduced by the
addition of trifluoroacetic acid (TFA). In the case of compound 1c the gradual
addition of TFA led to a blue shifted emission spectra, thus the solution under UV
light showed blue light emission, where the difference was not very much detectable
visually. However in the case of compound 2c, the gradual addition of TFA led to a
red shifted emission spectra, with a visually perceivable green emission under UV
light of long wave length (Fig. S32). In both cases the original blue emission was
recovered to a certain extent by the addition of an equivalent amount of triethylamine
1
(
TEA) (Fig.14 bottom panel). From the H NMR spectra, it was visualized that the
protonation of the compounds led to a large downfield shift of the protons on pyridine
ring, whereas the neutralization with the triethylamine recovered the signals back to
the original in the case of compound 1c or near to the original position in the case of
compound 2c (Fig.14 upper panel).
(
b)
(a)
residual
CHCl
3
residual
CHCl
3
1c + TFA + TEA
2c + TFA + TEA
8.33
7.95
8.43
8.13
8
.56
2c + TFA
8.30
1c + TFA
H
b
H
b
H
a
8.43
8.13
H
a
8.04
1c
8.46
2c
δ/ppm
δ/ppm
TFA
TEA
TFA
TEA
1c
2c
1
Figure 14. Expanded region of H NMR spectra of 1c (a) and 2c (b) on subsequent
addition of TFA and TEA in CDCl (The bottom panel shows the changes happen on
3
the subsequent addition of TFA and TEA, as seen under daylight and UV light of long
wavelength)
1
7

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Gelation studies
We were interested in the gelation behavior of these different series of compounds
and wanted to study the effect of different heterocycles and the number of peripheral
alkoxy tails. Gelation behavior of thiadiazoleꢀbased hexacatenar 2c has been reported
earlier, and hence we were interested to study the gelation behavior of its oxadiazole
counterpart 1c. Further the hexacatenars 1c and 2c stabilized Col phases in bulk, and
thus it is interesting to compare their aggregation behavior in presence of solvents
(Fig.15). Compounds 1c and 2c were mixed separately in different solvents of varying
polarity and proticity. Polar aprotic solvents like chloroform, dichloromethane,
tetrahydrofuran (THF), dimethylsulfoxide (DMSO), dimethylformamide (DMF);
polar protic solvents like ethanol, nꢀbutanol; nonpolar aromatic solvents like benzene,
toluene, mꢀxylene, nonpolar aliphatic solvents like hexane, decane, dodecane and
hexadecane were utilized to probe the gelation behavior of these compounds (Table
S1). Immediately after mixing, the suspensions were heated to obtain a clear solution.
Then, the solutions were allowed to cool to room temperature to form gels. Both the
compounds 1c and 2c were found to be soluble in most of the solvents like
chloroform, dichloromethane, tetrahydrofuran, toluene, benzene, mꢀxylene. These
compounds were insoluble in DMSO and DMF, and found to precipitate in ethanol
and nꢀbutanol. In the case of all aliphatic nonpolar solvents, the clear solutions of
these compounds became turbid, finally forming a translucent gel. Finally, the
formation of the gel was confirmed by “stable to inversion of the glass vial” method
(Fig. 15aꢀd). Further we checked the gelation behavior of other compounds 1a, 1b,
1d, 2a, 2b and 2d in decane (Table 6, Fig.S34). Compounds 1a, 2a and 2b
precipitated in decane after dissolving in decane. Among the oxadiazole derivatives
Table 6. Gelation behavior and CGCs of compound 1aꢀd and 2aꢀd in decane
o
Entry
Properties
P
G(O)
G(O)
G(O)
P
P
G(O)
G(O)
CGC (wt. %)
T
gel( C)
ꢀꢀꢀꢀꢀ
53
1a
ꢀꢀꢀꢀꢀ
1.5
0.6
1b
1c
56
52
ꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀ
47
1d
0.9
2a
ꢀꢀꢀꢀꢀ
ꢀꢀꢀꢀꢀ
0.5
2b
2c
2d
0.7
45
G = stable gel; P = precipitate; O = opaque. The critical gelation
concentration (wt.%) is the minimum concentration necessary for gelation.
o
Tgel ( C) is the thermal stability of the gels.
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8

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1
bꢀd showed gelation, while in the case of thiadiazole derivatives, only compounds
c and 2d have shown gelation (Fig.S34ꢀS36). Since we were interested to derive the
2
structureꢀproperty relationships we decided to investigate the compounds 1c and 2c
by extensive photophysical studies, microscopy and XRD studies. Detailed
rheological measurements were also carried out on these two representative samples,
which are described in a later section. The critical gelation concentration (CGC) and
Tgel of all the gelators were studied in different gelating solvents by ‘dropping ball
17
method’ and are tabulated in Table 6. Compounds 1cꢀd and 2cꢀd can be classified as
1
8
supergelators as their CGC was found to be lesser than 10 mg/mL or 1 wt%. Again,
thermal stability of compounds 1c and 2c with respect to concentration (Fig. 15eꢀf)
was estimated for both derivatives in decane. The plots showed a steady increase in
the thermal stability on increasing concentration. It may be noted that at higher
concentration, the gel formed could be casted into any given shape of selfꢀstanding
gel (Fig. 15gꢀj), with higher mechanical strength.
(a)
(h)
(e)
(g)
(i)
(l)
(k)
(b)
(c)
(f)
(j)
(m)
1
c
c
Shaking
Sonicating
Rest
1c
2c
1c 2c
2
Rest
(d)
Figure 15. Solution (a) and gel (b) form of compound 1c and the same for compound
c (c,d); Plots of Tgel vs concentration for compound 1c (e) and for compound 2c (f) in
decane; Images showing the moldability of both the organogels in decane
Concentrations: 1.2 wt% for 1c and 0.8 wt% for 2c) into shapes like cube for 1c (g);
trigonal prism for 1c (h); trigonal prism for 2c (j); the same under UV light (365 nm)
k); 2.5 cm long cylinder shaped gel of 2c (i); the same under UV light (365 nm) (l);
2
(
(
Images showing response to mechanical forces by 1c and 2c and their recovery (m).
It is interesting to note that, even with the lack of any specific hydrogen
bonding moieties, the compounds 1c and 2c were able to form strong gels. The
formation of stable gels can be attributed mainly to aromatic πꢀπ interaction, dipoleꢀ
1
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dipole interaction and van der Waals interaction of hydrophobic alkyl chains with
long chain hydrocarbon solvents. The gel–sol transformation can also be achieved by
the appliance of mechanical force (Fig.15m) such as vigorous shaking or sonication,
while the solutions reverted back to the original gel state on standing. Interestingly,
these organogels have also shown significant selfꢀhealing property and can be
recovered from any damage imposed on them, on standing (Fig.S35ꢀ36).
The microstructures of the representative organogels formed from the
compounds 1c and 2c in decane were investigated with field emission scanning
microscopy (FESEM) and atomic force microscopy (AFM). The xerogel films of
compounds 1c and 2c were prepared by evaporating the spinꢀcoated films of decane
ꢀ6
solutions (5 × 10 M), followed by their vacuum evaporation for 2ꢀ3 h. The AFM
images of the xerogels (Fig.16a,b), exhibited dense interwoven network of fibers
measuring several micrometers length. For compound 1c, the height of the fiber is
around 30ꢀ35 nm with a thickness of 0.8 µm. In case of compound 2c, the average
length of fibers was 40ꢀ60 nm, with a thickness of around 130 nm. These results were
again supported through SEM micrographs obtained for the xerogels of 1c and 2c
(Fig. 16c,d).
(
a)
(c)
(
d)
(
b)
Figure 16. AFM image obtained for the xerogel of compound 1c obtained from 3.33
mM decane solution (scale bar is 4.0 µm) (a); xerogel of compound 2c (scale bar is
5
50 nm) (b) (Inset shows the thickness of an individual fiber); SEM images obtained
for the xerogel of compound 1c (scale bar is 2 µm) (c); for the xerogel of compound
c (scale bar is 200 nm) (d).
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Figure 17. Intensity vs 2θ profiles obtained from the XRD pattern for the Col
of the xerogel of compound 1c.
r
phase
Table 7. Results of (hk) indexation of XRD profiles of the compounds 1c and 2c in the xerogel state at
room temperature (RT).
Compounds
state)
Phase/
(T/ C)
d
obs (Å)
dcal(Å)
Miller
indices
hk
Lattice parameters (Å), Lattice area
2 3
o
(
S (Å ), Molecular volume V (Å )
1
c
Col
r
53.05
21.11
53.05
21.11
19.61
16.52
13.56
13.26
9.06
8.26
7.48
7.04
6.22
01
10
11
12
13
04
23
24
25
30
34
35
40
45
a = 21.11, b = 53.05, c = 3.77,
S = 1119.9, V = 4212,
Z = 1.4
Xerogel
1
1
9.04
5.95
(
RT)
14.33
1
9
8
7
7
6
5
5
4
4
3
2.51
.18
.19
.57
.10
.26
.66
.31
5.86
5.28
4.73
.57
.16(h
.77(h
a
)
c
)
2
c
Col
(RT)
r
47.95
15.69
47.95
15.69
9.72
01
11
14
21
a = 16.60, b = 47.95, c = 3.82,
S = 796, V = 3040.6,
Z = 1
Xerogel
9
8
4
3
.89
.04
.58(h
.82(h
8.18
a
)
c
)
Further the arrangement of the molecules in the xerogel state was extensively probed
with the help of small angle Xꢀray diffraction studies. The XRD pattern for xerogel 1c
at room temperature showed several peaks centered at 53.05, 21.11, 19.04, 15.95,
1
4.33, 12.51, 9.18, 8.19, 7.57, 7.10, 6.26, 5.66, 5.31 and 4.57 Å. These dꢀspacings
were quite ideally fitting into a rectangular (Col ) lattice structure (Fig.17 and Table
). In the wideꢀangle region we have found two diffused reflections centered at 4.16
and 3.77 Å, could be ascribed to the packing of flexible alkyl chains and to the
r
7
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1

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packing of aromatic cores. Similar rectangular arrangement was found in case of the
8
e
xerogel of compound 2c as described earlier (Fig.S38). From the lattice parameters
we calculated the number of molecules forming a columnar slice in the case of 1c and
2c, which was found to be almost 1. It is interesting to note that these molecules
exhibit liquid crystalline behavior and organgelation, while maintaining the same
order (Columnar assembly with a rectangular symmetry) over a long range, which is
2
3
beneficial from the viewpoint of organic electronic devices.
Aggregation induced enhanced emission (AIEE) behavior
The gelation of compounds 1c and 2c in decane was followed with the help of
fluorescence spectroscopy. They showed a blueꢀshifted excitation spectra in gel state
in comparison to the excitation spectra of solution state (Fig.18a,c). There was a slight
redꢀshifted emission observed in the case of compound 1c, while the emission was not
shifted in the case compound 2c (Fig.18b,d). We were curious to know the
photophysical properties of compound 1c and 2c in thin film state. The spin coated
thin films on quartz plates showed a blue shifted absorption bands in comparison to
×10
×
6
(
a)
(b)
×
59
×
25
(
c)
(d)
Figure 18. Excitation and absorption spectra of compound 1c (a); 2c (c) in solution,
gel (at CGC) and thin film state; Emission spectra of compound 1c (b); 2c (d) in
solution, gel (at CGC) and thin film state.
the solution state excitation spectra (Fig.18a,c). This points to the formation of
aggregates in the case of thin films and gels in comparison to solution state. The blue
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shift in the absorption usually corresponds to the formation of Hꢀaggregates, where
the molecules are stacked one above the other in a cofacial or parallel manner. The
aggregated states (in thin film and gel) showed several fold enhanced luminescence in
comparison to the solution state (Fig.18b,d). Notably, the luminescence was higher in
gel state for compound 1c than in thin film state, while it is higher in thin film state
for compound 2c than in gel state.
To determine the effect of aggregate formation on the luminescence properties
as well as organogel formation, decane gels of 1c and 2c were measured at different
temperatures and times. Aggregation induced enhanced emission (AIEE) of
8
e
compound 2c was reported earlier, which was monitored with respect to temperature
and time. In both cases, the intensity of emission increases with the decrease in
temperature (Fig. 19a). A bathochromic shift in the emission of 25 nm was observed
in the case of 1c in comparison to that of compound 2c (12 nm red shift) (Fig. 19b).
In comparison to solution state, a huge increase in the fluorescence intensity in the gel
state i.e. 10 fold in case of 1c (which is higher than the emission intensity of the thin
8
e
film), whereas 25 fold in the case of 2c (which is lower than the emission intensity
of the thin film) was noted (Fig. 19c). Thus even though both the compounds points to
the formation of Hꢀaggregates in thin films and gel state, the molecular arrangement
is not same in these two different states, which results in different emission behavior.
×
10
(
(
a)
d)
(b)
(e)
(c)
(f)
Figure 19. Emission spectra of compound 1c in decane (3.33 mM) on cooling from
o
o
o
o
6
0 C to 25 C (a); Normalized emission spectra on cooling from 60 C to 25 C (b);
Comparison of emission spectra of solꢀgel transition (c); Emission spectra obtained as
a function of time (d); Plot showing the emission intensity at 415 nm as a function of
time (e); Reversible change in the emission intensity at 415 nm by repeated solꢀgel
transition (f).
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Formation of organogel was also confirmed by measuring the emission spectra of the
solution as a function of increasing time (Fig.19d). The organogel was formed within
35 minutes in the case of 1c (Fig.19e), which is little longer in comparison to that of
compound 2c (10 minutes in 2c). The reproducibility of gel formation was confirmed
after heating and cooling for many cycles as presented in Fig.19f. The increase in the
emission intensity upon aggregation, rather than the emission quenching is due to the
2
0
ability to overcome aggregation caused quenching (ACQ). In other words the
favorable molecular packing in the gel state results in the restricted intramolecular
rotations. Thus both bentꢀshaped oxadiazole and thiadiazole based bent polycatenars
1c and 2c exhibited aggregationꢀinduced enhanced emission (AIEE) effect in the
organogel state. Fluorescence lifeꢀtime measurements carried out for compounds 1c
and 2c in decane at 20 ꢂM and at their CGC showed the biexponential emission decay
with the increase in the lifeꢀtime of the excited species (Fig.S37 and Table S2). Even
though Hꢀaggregates are known to exhibit aggregation caused quenching (ACQ) of
luminescence compared to the monomer emission in solution, here completely reverse
19
behavior of the expected was observed. There are very few reports on luminescent
2
1
Hꢀaggregates, where the luminescence is attributed for the excimer species or due to
a small displacement of the two excitonꢀcoupled molecules in the excited state.
Rheological properties
The viscoelastic properties of the gels formed from compound 1c and 2c at
concentrations of 0.6 wt. % and 0.5 wt. % were measured with the help of rheological
measurements. This involved strain sweep, angular frequency sweep and step strain
measurements. For the measurement of rheological responses, a dynamic frequency
sweep experiment at constant stress (0.2 kPa) was performed, which can be described
by the equation G* (ω) = G’ (ω) + iG” (ω), where ω is the angular frequency, G’ is
2
2
the storage modulus and G” is the loss modulus. The value of G’ slightly increase
and the value of G” decrease with the frequency from 0.01 to 100 Hz and the value of
G’ is always higher than that of G” in the whole range (0.01–100 Hz), signifying that
the gels 1c and 2c are fairly tolerant to the external force. From Fig. 20a and 20d, it is
quite obvious that both 1c and 2c gels behave like a solid or viscoelastic like material
o
(
i.e. G’> G”). Both G’, G” are feebly dependent on angular frequency at 25 C using
0.2 mPa stress (Fig.20b and e). As can be observed, these are the two requirements for
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a sample to be considered as a gel that is approximately satisfied by both gels 1c and
c. In order to establish the nonꢀlinear viscoelastic regime, which is the characteristic
2
features for dynamics of the solꢀgel system, we performed stress dependent nonlinear
measurements for compound 1c (0.6 wt.%) and 2c (0.5 wt.%).
(a)
(b)
(c)
(d)
(e)
(f)
ꢀ1
Figure 20. Angular frequency (rad s ) dependence of G’ and G” obtained with small
stress amplitudes: (a) 0.6 wt. % of compound 1c in decane at 25 °C using 0.2 kPa
stress; (b) Stress dependence of G’ and G” of the same gel 1c measured at constant
frequency (at 25 °C, and frequency at 10 Hz); (c) Thixotropic nature of organogel of
1c (3.32 mM) in decane at 25 °C (Deformation; stress: 0.2 to 4 kPa, time: 300 s,
angular frequency: 10 Hz; Recovery; Stress: 0.2 kPa, time: 300 s; angular frequency:
ꢀ1
1
0 Hz.); Angular frequency (rad s ) dependence of G’ and G” obtained with small
stress amplitudes: (d) 0.5 wt. % of compound 2c in decane at 25 °C using 0.2 kPa
stress; (e) Stress dependence of G’ and G” of the same gel 2c measured at constant
frequency (at 25 °C, and frequency at 10 Hz); (f) Thixotropic nature of organogel of
2
c (2.71 mM) in decane at 25 °C (Deformation; stress: 0.2 to 4 kPa, time: 300 s,
angular frequency: 10 Hz; Recovery; Stress: 0.2 kPa, time: 300 s; angular frequency:
0 Hz.)
1
From Fig. 20b and 20e, we make two observations in case for 1c and 2c separately.
Firstly, at low strain amplitudes both G’ and G” are nearly constant and G’ > G” is
steady confirming the solid like behavior and a linear viscoelastic regime. Secondly,
both storage modulus and loss modulus became strain dependent above a stress value
of 3.27 kPa and 0.8 kPa for 1c and 2c gels respectively. This observation leads to the
formation of viscoelastic liquid behavior (G” > G’) for both cases at high stress. In
order to examine the thixotropic nature of the gels, we have studied the retrieval
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process of the damaged gels via two continuous processes of deformation and
recovery for several times. As observed from Fig. 20c for 1c and Fig. 20f for 2c, at a
particular angular frequency of 10 Hz for 300 seconds a variable stress of 0.2 to 4 kPa
was applied for the deformation process and the value of G’ vs time was recorded. It
was found that the value of G’ decrease in deformation process leading to the
formation of viscoelastic liquid at 300 seconds. The same graph was continued in the
recovery process where the value of G’ starts increasing with time leading to solid
like properties at the same angular frequency for a shear stress of 0.2 kPa in both gels.
This set of experiment was done five times for the reproducibility of the gel. It can
also be concluded that we are dealing with high elastic modulus of consistent physical
gels that can be recovered instantly, once eliminating the applied stress.
Conclusions
In this report, we have synthesized several bentꢀshaped molecules with a
central pyridine unit flanked by two unsymmetrically substituted 1,3,4ꢀoxadiazole and
thiadiazole moieties. The number and position of the terminal tails were varied. The
type of the heterocycle and the number of peripheral tails had a significant impact on
the stabilization of the liquid crystalline and organogel selfꢀassembly as well as
photophysical properties. Thiadiazole based compounds exhibited higher mesophase
stability in comparison to oxadiazole derivatives, while oxadiazole based compounds
exhibited higher gelation tendency. In the case of oxadiazole derivatives, tetracatenar
and hexacatenars exhibited gelation behavior, while in the case of thiadiazole
derivatives only hexacatenars exhibited gelation. The number of peripheral tails also
play an important role in stabilizing the gelation in hydrocarbon solvents, which is
evident from the fact that all the hexacatenars stabilized supergelation irrespective of
the type of heterocycle. The gelation is mainly promoted by attractive πꢀπ interactions,
which is in contrast to other supergelators that require strong intermolecular hydrogen
bonding. The supergelators showed the capability to form selfꢀstanding, moldable gel
at higher concentration. Thiadiazole based compounds exhibited bathochromically
shifted absorption and emission in comparison to oxadiazole derivatives but with a
lower quantum yield. In addition these compounds showed the ability to sense the
acid with a quenching/shifting in the emission spectrum, which makes it possible to
detect acids by naked eye at low concentration. Two of the hexacatenar supergelators
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investigated, exhibited aggregation induced enhanced emission with a several fold
increase in luminescence intensity in the gel state. The variation in the gelation
capability of these molecules shows that although the πꢀπ interactions plays a major
role in selfꢀassembly, the nanoseggregation of incompatible molecular segments like
peripheral flexible tails also contribute in organogelation and LC selfꢀassembly.
Scanning probe microscopic studies of the xerogels revealed a fibrillar network of
several micrometers corroborating the long range supramolecular selfꢀassembly.
Extensive rheological studies proved the viscoelastic and thixotropic nature of the
gels. Similar AIEE effect was observed in thin film state too. Stabilization of
columnar selfꢀassembly over a long range, along with AIEE behavior is significant
from the perspective of emissive solidꢀstate displays. In a nutshell, this study accounts
the effect of molecular structural variations on the nature of supramolecular selfꢀ
assembly and the resultant macroscopic properties, that is of substantial significance
to the scientific community working in the broad area of supramolecular chemistry.
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). We thank CIF, IIT Guwahati for analytical facilities.
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