Effect of regioisomerism on the self-assembly and photophysical behavior of 1,3,4-thiadiazole-based polycatenars

Article abstract of DOI:10.1039/c5tc01437a

A new class of polycatenars, where the central benzene ring is connected to two arms derived from substituted 1,3,4-thiadiazoles at the 1,3- and 1,4-positions, was synthesized and characterized. These thiadiazole-based molecules are promising as they stabilize columnar phases over a wide temperature range, in comparison to their oxadiazole analogues. The para-substituted polycatenars exhibited a columnar hexagonal and/or a columnar oblique phase, while the meta-substituted polycatenars exhibited solely a columnar oblique phase. The para-substituted polycatenars exhibited green emission, while the meta-substituted polycatenars exhibited blue emission in solution and film states. Stabilization of a broad range columnar phase and luminescence in the solid state make these new compounds promising from the viewpoint of applications in emissive displays. The self-assembly and luminescence of these regioisomers was greatly influenced by the molecular structure.

Full text of DOI:10.1039/c5tc01437a

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DOI: 10.1039/C5TC01437A
Dr. A. S. Achalkumar
Assistant Professor
Department of Chemistry
Indian Institute of Technology Guwahati
Guwahati – 781039, Assam, India
Phone: +91-361-258-2329
Fax: +91-361-258-2349
E-mail: achalkumar@iitg.ernet.in
achalkumar78@gmail.com
Table of Contents Graphic
Effect of Regioisomerism on the Self-assembly and Luminescence of Polycatenars

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DOI: 10.1039/C5TC01437A
Effect of Regioisomerism on the Self-assembly and
Photophysical Behavior of 1,3,4-Thiadiazole based Polycatenars
Suraj Kumar Pathaka, Subrata Natha, Ravindra Kumar Gupta,aD. S. Shankar Raob,
S. Krishna Prasadb and Ammathnadu S. Achalkumara,*
aDepartment of Chemistry, Indian Institute of Technology Guwahati, Guwahati, 781039, Assam,
India b Centre for Nano and Soft Matter Sciences, Jalahalli, P. B. No. 1329, Bangalore, 560013,
Karnataka, India.
Abstract: A new class of polycatenars where the central benzene ring is connected to two arms derived
from substituted 1,3,4ꢀthiadiazoles at 1,3ꢀ and 1,4ꢀpositions was synthesized and characterized. These
thiadiazole based molecules are promising as they stabilize columnar phases over a wide range in
comparison to their oxadiazole analogues. paraꢀsubstituted polycatenars exhibited columnar hexagonal
and/or columnar oblique phase, while metaꢀsubstituted polycatenars exhibited solely columnar oblique
phase. paraꢀSubstituted polycatenars exhibit green emission, while the metaꢀsubstituted polycatenars
exhibited blue emission in solution and film state. Stabilization of broad range columnar phase and
luminescence in solid state make these new compounds promising from the view point of applications
in emissive displays. The selfꢀassembly and luminescence of these regioisomers was greatly influenced
by the molecular structure.
Table of contents (TOC) Graphic
I. Introduction
Liquid crystal (LC) selfꢀassembly of shape anisotropic molecules is a subject that is attracting a
huge interest in recent years. This unique state of matter with a combination of the order and fluidity
leads to special inherent properties that are important from the view points of basic and applied
research.1 Liquid crystals can be broadly classified as conventional and nonꢀconventional, based on their
shape.2 Conventional LCs are comprised of two classes, that are calamitic (rodꢀlike) 3 or discotic (discꢀ
like).4 Shape anisotropic molecules which stabilize mesophases, that are neither calamitic nor discotic
fall under the class of nonꢀconventional LCs.2 Scientific curiosity in a quest to explore novel
mesophases and applications, catapulted a flurry of activities in the design and syntheses of several
different shape aniotropic molecules which deviate from the conventional rod or disc shapes.2, 5ꢀ12
.
Some
1

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of the important examples of nonꢀconventional systems are oligomers,5 bentꢀcore molecules,6
polyhydroxy amphiphiles,7, 2cꢀe octahedral complexes,8 star shaped molecules,9 rodꢀcoil molecules,10
dendrimers11 and polycatenars.12
Polycatenars or phasmidꢀlike LCs are one such class of nonꢀconventional LCs comprising of
central long aromatic rod connected to two terminal semiꢀdiscs with multiple flexible chains.12aꢀc These
molecules selfꢀassemble into different LC phases due to the nanosegregation of aromatic units and
flexible chains. Since polycatenars share the structural features of both calamitic and discotic LCs they
exhibit various LC phases like nematic, smectic (lamellar), cubic, and columnar phases depending on
the number of peripheral tails. Recently it was reported that some polycatenars based on indigoid system
even exhibited mesophases similar to bent core LC phases.13 This synthetically flexible molecular motif
can incorporate many functional moieties so as to develop multifunctional LCs. Incorporation of
extended πꢀconjugated aromatic system in the polycatenar molecular structure is very important from
the viewpoint of charge carrier and luminescence properties.14 Gin et. al. reoprted a polycatenar system
with photophysical functions.12e Yelamaggad et. al. incorporated hydrogen bonding oligopeptides in the
polycatenar structure that stabilizes Columnar phase.15 Prasad et.al. introduced photoisomerizable
azobenzene unit in the polycatenar structure.16 There are many reports on luminescent polycatenar
LCs.14dꢀi
Discovery of columnar selfꢀassembly of discotics4 opened up a fascinating research area because
of the various potential applications.18ꢀ22 This is because Col phases formed from discotics resemble
molecular wires due to the one dimensional stacking of central rigid discs with an insulating sheath of
peripheral tails, which help in one dimensional carrier migration.17 Columnar phase has the potential to
realize many optoelectronic devices like photovoltaics,18 light emmitting diodes,19 field effect
transistors,20 gas sensors21 and lubricants.22 Considering the synthetic difficulty and the rigidity for the
structural tuning in DLCs, polycatenar LCs seems to be simpler alternative, as they can also stabilize
Col phases.
Recently there is a renewed interest in the incorporation of heterocyles in the molecular design
of mesogens due to the wide variety of structures and hence the resultant properties.23 Hetero atoms like
nitrogen, oxygen and sulfur provide a reduced molecular symmetry, strong lateral and/or longitudinal
dipoles and a donorꢀacceptor interaction within the molecule, which in turn affects the LC selfꢀassembly
and electronic behavior of the mesogens.23 Mesogens with heterocyclic moieties in their molecular
structures, provide emission color tunability and also enable a polarized emission. This is an important
property which finds application in the area of OLEDs.24 Thus many polycatenar LCs bearing a
heterocyclic moiety in their molecular structures are reported. There are relatively more reports on
1,3,4ꢀOxadiazole derivatives in comparison to their sulfur analogues 1,3,4ꢀthiadiazole derivatives.26, 27
2

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1,3,4ꢀOxadiazole based molecules are known for their high luminescenceDeOfI:f1i0c.i10e3n9c/Cy5,TCn0ꢀ14ty37pAe
behavior (electron transporting), resistance to oxidative degradation, hydrolytic and thermal stability.
Hence they found application as nꢀtype electroluminescent layers in organic light emitting diodes.23aꢀc, 25
Unfortunately, oxadiazole based mesogens have few drawbacks like high melting and clearing
temperatures, narrow mesophase range and poor solubility which limits their applications.26 Reports on
their sulfur analogues, i.e. 1,3,4ꢀthiadiazole based mesogens are limited in number. It is expected that,
substitution of oxygen with sulfur increases the dipole moments, packing of molecules, viscocity,
melting and clearing temperatures.23c, 27 In addition to this notion, partly the synthetic difficulty coupled
with low yield curtailed the attempts in this direction. Calamitic LCs,28 banana shaped LCs,29 hydrogen
bonded LCs,30 and polymeric LCs31 based on 1,3,4ꢀthiadiazole are reported. Very recently few
polycatenar LCs26, 32 and star shaped mesogens33a stabilizing columnar phase are reported. Our group
reported the star shaped thiadiazole derivatives stabilizing hexagonal columnar mesophase.33b
OR
N
OR
RO
RO
N
N
S
OR
OR
N
OR
OR
S
S
S
N
N
N
RO
N
RO
OR
RO
2a: R1 = R2 = R3 = n-C10H21
2b: R1 = R2 = R3 = n-C12H25
2c: R1 = R2 = R3 = n-C16H33
1a: R1 = R2 = R3 = n-C10H21
1b: R1 = R2 = R3 = n-C12H25
1c: R1 = R2 = R3 = n-C16H33
Figure 1. The moleular structures of comounds 1a-c and 2a-c.
Figure 2. Polycatenars based on 1,3,4ꢀoxadiazoles (Ia-c34b and IIa-b34a) that are comparable to 1,3,4ꢀ
thiadiazole based polycatenars 1a-c and 2a-c of the present work; Bargraph showing the thermal
behavior of compounds Ia-c and IIa-b (in heating cycle)
3

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Scheme 1. Synthesis of 1,3,4-thiadiazole based liquid crystals
OH
OR
OR
HO
OH
RO
OR
RO
OR
(ii)
(i)
COOC2H5
CONHNH2
COOC2H5
6a: R = n-C10H21
6b: R = n-C12H25
6c: R= n-C16H33
5a: R = n-C10H21
5b: R = n-C12H25
5c: R= n-C16H33
(iii)
OR
OR
RO
RO
NH
O
H
N
H
N
O
NH
O
O
RO
HN
NH
HN NH
O
O
OR
RO
OR
O
O
RO
OR
OR
OR
4a: R1 = R2 = R3 = n-C10H21
4b: R1 = R2 = R3 = n-C12H25
4c: R1 = R2 = R3 = n-C16H33
3a: R1 = R2 = R3 = n-C10H21
3b: R1 = R2 = R3 = n-C12H25
3c: R1 = R2 = R3 = n-C16H33
2a: R1 = R2 = R3 = n-C10H21
2b: R1 = R2 = R3 = n-C12H25
2c: R1 = R2 = R3 = n-C16H33
1a: R1 = R2 = R3 = n-C10H21
1b: R1 = R2 = R3 = n-C12H25
1c: R1 = R2 = R3 = n-C16H33
Scheme 1. Synthesis of 1,3,4ꢀthiadiazole based polycatenars. Reagents and conditions: (i) nꢀ
bromoalkanes, anhydrous K2CO3, DMF, 80 oC, 24 h (70ꢀ90 %); (ii) NH2NH2.H2O, nꢀbutanol, reflux, 48
h (71ꢀ80 %); (iii) (a) 1,4ꢀbenzene dicarboxylic acid, SOCl2, reflux, 6 h; (b) 5a-c, THF, triethylamine, 12
h, reflux; (iv) (a) 1,3ꢀbenzene dicarboxylic acid, SOCl2, reflux, 6 h; (b) 5a-c,THF, triethylamine, 12 h,
reflux; (v) Lawesson's reagent, toluene, reflux, 17 h (40ꢀ45 %).
As part of our efforts to combine luminescence with liquid crystalline order, we were interested to
synthesize polycatenar liquid crystals, in particular hexacatenars based on 1,3,4ꢀthiadiazole derivatives
(Fig.1). We were also interested to compare their properties with the polycatenars based on 1,3,4ꢀ
oxadiazole derivatives (Fig.2).34 Additionally this also gives information on the effect of structure on the
selfꢀassembly process on going from hexacatenars to star shaped molecules. In this paper we report the
synthesis of hexacatenars comprising five rings, among which two are 1,3,4ꢀthiadiazoles. These two
4

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thiadiazole rings are interconnected by a paraꢀ and metaꢀsubstituted benzene rings and having sixꢀalkyl
chains attached equally to two terminal benzene rings.
II. Results and discussion
II.1 Synthesis and molecular structural characterization (reference)
The synthetic strategy to prepare the hexacatenars is presented in scheme 1. Ethyl gallate was Oꢀ
alkylated by heating with appropriate nꢀbromoalkanes in presence of anhydrous K2CO3 and anhydrous
DMF. The synthetic methods of ethyl gallate and its alkoxy derivatives are reported prior.35a These
alkoxy esters (6a-c) were refluxed with hydrazine hydrate in nꢀbutanol to obtain respective hydrazides
(5a-c).33b, 34 Hydrazides 5a-c were then coupled with terephthalic acid chloride to get 1,4ꢀdiꢀNꢀ
benzoylbenzohydrazides 4a-c.34 Similarly hydrazides 5a-c on coupling with isophthalic acid chloride
yielded 1,3ꢀdiꢀNꢀbenzoylbenzohydrazides 3a-c. Compounds 4a-c and 3a-c on refluxing with
Lawesson’s reagent35b in toluene yielded polycatenars 1a-c and 2a-c.33b Molecular structural
characterizations were carried out using 1H NMR, 13C NMR, IR spectroscopy and ESIꢀHRMS. (See the
supporting information (SI) for the details).
II. 2. Thermal behavior
The liquid crystalline properties of these new hexacatenars belonging to two different series was
investigated by polarizing optical microscopy (POM), thermogravimetric analysis (TGA), differential
scanning calorimetry (DSC), and Xꢀray diffraction (XRD) studies. Liquid crystalline nature of
polycatenar LCs was identified by the observation of strong birefringent and fluidic nature of the
materials under POM. Further, the assignment of LC phase was made by correlating the typical textural
pattern with the data obtained from XRD studies. The peak temperatures obtained in DSC traces due to
phase transitions were found to be in accordance with POM observations. The transition temperatures,
mesophase sequence and the corresponding enthalpy changes are summarized in table 1. The bargraph
provided in Fig.3 accounts the thermal behavior of these compounds in first heating cycle. The thermal
stability of the compounds were analysed by TGA showed that all the compounds were thermally stable
upto ≈ 250 oC and complete degradation occured at around 600 oC. (See the SI).
In the following sections, we discuss the LC behavior of the thiadiazole based polycatenars
using the aboveꢀmentioned complementary techniques. From the bargraph shown in Fig. 3, it is evident
that in the case of pꢀsubstituted polycatenars melting temperature decreases with the increase in chain
length, while decrease in clearing point is not regular. The mesophase range is wider for compound 1b
5

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with medium chain length, while it is short for compound 1c with highest chain length. In the case of mꢀ
substituted polycatenars, there is no such trend in the variation of melting points with respect to chain
length, but show a constant decrease in the clearing point. Again as in the case of pꢀsubstituted
polycatenars, compound 2b with medium chain length shows a wide range of mesophase, while
compound 2c shows the shortest mesophase width. Thus from these information we can infer that for
Table 1. Phase transition temperatures a (oC) and corresponding enthalpies (kJ/mol) of DLCs
Compound
Phase sequence
Heating
Cooling
Cr1 45.3 (10.3) Cr2 51.5 (120.7) Cr3 79.5
(266.5) Colh 95.9 (11.3) I
I 94.6 (11.3) Colh 28.4 (13.7) Cr
1a
Cr1 34.8 (205.3)c Cr2 78.3 (358.1) Colh 95.9
(20.2) I
I 94.6 (20.2) Colh 28.5 (24.8) Cr
Cr1 55.6 (76.5) Cr2 67.6 (14.8) Cr3 78.1 (131) I 97.3 (7.2) Colob 44 (19.1) Cr
Colob 98.9 (7.1) I
1b
Cr1 53 (269)c Cr2 76.4 (365) Colob 98.4 (15.7) I 97.3 (16.4) Colob 44 (45.4) Cr
I
b
Cr 70.2 (202.5) Colh 83.5 (1.5) I
Cr1 55.5 (502) Cr2 63.99 (78.1) Cr3 73.2 (6.2) I 80.6 (17.6) Colh 51 Colob 46.1 (262) Cr
Colh 81.7 (17) I
I 80.4 (5.3) Colh 51 Colob 46.1 (142.3) Cr
1c
2a
b
Cr1 42.2 (10.6) Cr2 62 (22.5) Colob 97.9
(12.6) I
I 96.9 (11.5) Colob 53.4 (25) Cr2 19.7 (29.9) Cr1
Cr1 25.03 (62.2) Cr2 62 (49) Colob 97.8 (20.2) I 96.9 (20.1) Colob 53.51 (45.2) Cr2 19.8 (54.5)
I
Cr1
Cr 57.2 (135.7) Colob 94.4 (11.7) I
I 93.4 (10.8) Colob 54.9 (20.4) Cr1 41.8 (71.7)
2b
Cr2
Cr1 48.4 (97.7) Cr2 55.8 (103.3) Cr3 59.9
(24.8) Colob 94.3 (25) I
I 93.4 (24.6) Colob 54.9 (48.6) Cr2 41.8 (161.9)
Cr1
Cr1 75.6 (147.3) Colob 87 (8) I
Cr1 75.7 (499.6) Colob 87 (27) I
I 86 (8.2) Colob 64.6 (151.7) Cr
I 86 (28.4) Colob 64.5 (516.5) Cr
2c
a Peak temperatures in the DSC thermograms obtained during the first and second heatingꢀcooling
cycles at 5 oC/min. b The phase observed is monotropic. Colh = Columnar hexagonal phase; Colob =
Columnar oblique phase.c exothermic crystallization.
Figure 3. Bargraph summarizing the thermal behavior of compounds 1a-c and 2a-c (heating cycle)
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this class of polycatenars, the appropriate length of peripheral chains is an important factor in deciding
the mesophase width. In cooling cycle, both the series of compounds showed a common trend, i.e. a
constant decrease in the mesophase width with the increase in chain length.
Thiadiazole based pꢀsubstituted polycatenar molecule 1a with shortest chain, exhibited
enantiotropic columnar mesophase spanning a thermal range of 16 degrees in heating cycle after passing
o
o
through two CrꢀCr transitions at ≈ 45 C and 52 C with an enthalpy change of 10.3 kJ/mol and 120.7
o
kJ/mol respectively as evidenced by DSC thermograms. Further heating shows that around 80 C, a
o
transition to birefringent fluidic liquid crystalline phase was observed, which stays upto 96 C before
passing to an isotropic liquid. On cooling from isotropic liquid state small spherulites developed from
o
the homeotropic dark field of view (Fig. 4a and b), persisted till 28 C. The compound crystallizes
below this temperature (Fig. 4c).
Figure 4. Photomicrographs of textures as seen by POM for the Colh phase of compound 1a (a) at
o
90 °C; (b) at 70 C; DSC traces obtained for first cooling (blue trace) and second heating (red trace)
o
cycles of compound 1a at a rate of 5 C/min. (c); XRD profiles depicting the intensity against the 2θ
obtained for the Colh phase of compound 1a at 70 °C (d)
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Powder Xꢀray diffraction studies on compound 1a at different temperatures was done to determine the
symmetry of the enantiotropic columnar mesophase. The results of indexing the reflections of the XRD
profiles are tabulated in table 2. The Xꢀray profile of compound 1a at 90 oC showed three reflections at
small angle region corresponding to the dꢀspacings 29.94 Å, 17.27 Å and 15 Å that can be indexed into
Miller indices 100, 110 and 200 respectively (SI). In the wide angle region a relatively diffused peak
Table 2. Results of (hkl) indexation of XRD profiles of the compounds at a given temperature (T) of
mesophasesa
Compounds
Phase
(T/oC)
dobs (Å)
dcal (Å)
Miller
indices
(hkl)
100
110
200
(D/Å)
Colh
(90)
29.94
17.27
15.00
29.95
17.29
14.97
1a
(48.3)
4.47 (ha)
30.30
17.49
Colh
(70)
30.31
17.50
15.16
100
110
020
15.21
4.43 (ha)
Colob
(90)
34.23
33.39
32.21
34.23
33.39
32.21
010
100
110
1b
(53.3)
4.49 (ha)
Colh
(70)
35.88
20.75
4.51 (ha)
35.90
20.73
100
110
1c
(63.4)
40.00
37.81
21.89
18.71
13.92
10.42
7.51
40.00
37.81
21.89
18.91
14.02
10.40
7.56
010
110
ꢀ110
220
ꢀ210
420
550
Colob
(50)
6.07
6.17
ꢀ430
4.20 (ha)
4.19 (hc)
aThe average diameter (D) of the polycatenars (estimated from Chem 3D Pro 8.0 molecular model software from
Cambridge Soft). dobs: spacing observed; dcal: spacing calculated (deduced from the lattice parameters; a for Colh
phase; a and b for Colob phase; γ is the column tilt angle). The spacings marked ha and hc correspond to diffuse
reflections in the wideꢀangle region arising from correlations between the alkyl chains and core regions,
respectively.
corresponding to the d spacing of 4.47 Å was observed. The first three reflections follow the reciprocal
spacing ratio of 1:0.58:0.5 of a hexagonal lattice. The diffuse peak at the wide angle corresponds to the
packing of flexible alkyl tails in the columnar phase. The peak corresponding to the stacking of cores
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was not observed showing that the intermolecular interactions are not that strong. ThDeOhI:e10x.1a0g39o/nC5aTlCl0a14tt3i7cAe
parameter ‘a’ calculated was found to be 34.58 Å, while the lattice area and molecular volume was
found to be 1035.04 Å2, and 4627.7 Å3 respectively. From these values, the number of molecules
present in an unit cell was derived and found to be 2.09. This can be explained as below. In principle we
can consider both dumbellꢀlike and hemidiscꢀlike conformations of pꢀsubstituted polycatenar 1a formed
due to the different orientations of two thiadiazole rings (Fig.5). Among these conformations dumbellꢀ
like conformer (Fig. 5a) is somewhat staright, while hemidiscꢀlike conformer (Fig. 5b) is little bent and
all akyl chains are towards one side. In spite of these conformations, these molecules do not differ much
Figure 5. Molecular models showing ‘dumbellꢀlike’ conformation (a) and hemidiscꢀlike conformation
(b) for compound 1a.
Figure 6. Schematic showing the selfꢀorganization of hexacatenar 1a into hexagonal columnar (Colh)
lattice with interꢀdigitation of alkyl tails. Space filling energy minimized (allꢀtrans) molecular model of
1a derived from molecular mechanics (MM2) method.
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in their molecular diameter as obtained from the molecular models (Table 2). This is because of the
larger atomic size of sulfur which leads to a larger bending angle. From the XRD studies, it is evident
that a columnar slice of thickness of 4.47 Å consists of two molecules.
Thus among the two conformations, it is reasonable to assume that two hemidisc shaped
conformers (Fig. 5b) can pack side by side to form a disc with efficient space filling. Further the voids
were filled by the felxible chains (interdigitation) of polycatenars from the adjacent columns or by chain
folding. Eventhough the discs formed by these polycatenars look slight elliptical in shape, their time and
space averaging results in a circular shape (Fig. 6). Similar packing arrangement is reported for
analogous oxadiazole based polycatenars.34b These discs in turn selfꢀassemble to form a columnar phase
with hexagonal symmetry (Fig. 6). The hexagonal cell parameter ‘a’ obtained from XRD studies was
found to be 30% less than the molecular diameter calculated from Chem 3D Pro 8.0 software. This
points to the interdigitaion or folding of alkyl tails of polycatenars in adjacent columns. Xꢀray
diffraction carried out at lower temperature (70 oC) also showed the similar pattern with a slight increase
in the lattice parameter ‘a’ (Fig. 4d and table 2).
Compound 1b also showed slightly higher isotropic temperature than compound 1a, which on cooling
from the isotropic melt showed the formation of Columnar phase with the appearance of a mosaic
texture (Fig. 7a). At some regions spherulitic texture was also seen (Fig. 7b). Both of these textures in
some cases reported for Colh phase but mosaic texture is often seen for Colr phase.36, 37 Even Columnar
oblique phase (Colob) also sometime show fan shaped textures.35a Since there are minor differences in
the structures of these different columnar phases, assignment of the symmetry of the mesophase
exclusively based on textures is often misleading.
Figure 7. Photomicrographs of textures as seen by POM for the Colob phase of compound 1b (a) at
o
90 °C; (b) at 65 C; XRD profiles depicting the intensity against the 2θ obtained for the Colob phase of
compound 1b at 90 °C (c)
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The diffraction pattern obtained from the powder XRD studies of the unaligned sample cooled from
o
isotropic liquid state at 90 C is explained below. Indexing of oneꢀdimensional (1D) intensity vs 2θ
o
profile deduced from the powder 2D pattern at 90 C shows three strong reflections at low angle region
(0 < 2θ < 5 o) with the respective spacing ratio of 1: 0.97: 0.94, which could be assigned to (010), (100)
and (110) reflections from a Colob lattice (Fig. 7c). This belongs to a primitive planar space group P1
and hence there are no reflection conditions as in the case of Colh or Colr. In the wide angle region, a
broad diffused peak corresponding to the packing of flexible alkyl tails was noticed at a d spacing of
4.49 Å.The oblique lattice had the cell parameters a = 37.38 Å and b = 38.31 Å with the angle between
o
the two dimensional (2D) lattice directions, i.e. γ = 26.7 . Thus the shape of the 2D lattice is a
parallelogram.
Figure 8. Photomicrographs of textures as seen by POM for the Colh phase of compound 1c (a) at
70 °C; (b) Colob phase at 50 oC; XRD profiles depicting the intensity against the 2θ obtained for the Colh
phase of compound 1c at 70 °C (c) and Colob phase at 50 °C
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Polycatenar molecule 1c with longest chains in this series showed a reduced mesophase range of around
o
13 degrees in heating cycle with an isotropic temperature of 84 C. Crystal to mesophase transition
observed as a broad peak in DSC thermogram with an enthalpy change of 202.5 kJ/mol. In the cooling
cycle Colh phase appeared with a characteristic texture of large homeotropic regions with some bright
o
defects interspersed (Fig. 8a). This was also corraborated by XRD studies carried out at 70 C, with a
characteristic pattern commonly observed for Colh phase. Around 51 oC the small needles grow over the
mosaic pattern and covers the entire area (Fig. 8b). Such needle like textures are reported for Colr
phase.37 XRD studies carried out at 50 oC, showed several peaks from low angle to mid angle region (0
o
< 2θ < 15 ), in addition to two diffused peaks at wide angle region. The d spacings obtained for these
reflections are corresponding to the Miller indices (010), (110), (ꢀ110), (220), (ꢀ210), (420), (550) and (ꢀ
430). Lattice parameters derived from these reflections were found to be a = 41.68 Å and b = 46.21 Å
with the angle between them γ = 30.04 o. Since the value of γ ≠ 90 o, we can conclude that the phase
under investigation is Colob.
It is interesting to note that all the three molecules showed different thermal behavior, i.e. compound 1a
showed Colh phase exclusively, compound 1b showed only Colob phase, while compound 1c showed
both the columnar phases, eventhough Colob phase observed was a monotropic phase. Except the chain
length, there is no other modification, thus the chain length is an imortant factor in deciding the
symmetry of Col phase observed here. In the case of discotics with the increase in peripheral chain
length, it is usual to predict a cross over from rectangular to hexagonal columnar phase.36,38 But such
tendency is not seen in this class of polycatenars. It is to be noted that coresponding oxadiazole based
polycatenars exhibited either crystalline or Colh phase, and the mesophase stabilization is limited to
certain chain length. In the case of present series of thiadiazoles all the compounds were liquid
crystalline and the medium chain length compound 1b exhibited broad mesophase range in heating
cycle. In the cooling cycle a different trend was observed, i.e.,with the increase in chain length a
decrease in the mesophase width was observed. Oxygen to sulphur substitution in the molecular
structure definitely improved the mesophase width of these compounds with respect to their oxadiazole
counterparts.34b
o
Thiadiazole based mꢀsubstituted polycatenar 2a on heating showed CrꢀCr transition at ≈ 42 C
before passing to a birefringent fluidic mesophase at 62 oC. This compound on further heating showed a
o
mesophase to isotropic state transition at ≈ 98 C with an enthalpy change of 13 kJ/mol. On cooling
from the isotropic melt, large pseudofocal fan shaped texture with spherulitic domains developed with
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the concurrent occurrence of homeotropic domains (Fig. 9a). This on further cooling remains unchanged
except a change in color and loss of fluidity; eventhough a mesophase to crystal transition with an
o
enthalpy change of 25 kJ/mol at ≈ 54 C was observed in DSC (Fig. 9b). Powder XRD studies carried
out at 90 oC and 70 oC (Fig.9c) showed that the phase under investigation is Colob phase (Table 3). XRD
o
studies at 90 C, showed three relatively sharp peaks at low angle followed by two diffused peaks at
wide angle. The first three reflections with d spacings of 32.41 Å, 31.03 Å and 15.52 Å, were
corresponding to Miller indices (010), (110) and (ꢀ110) respectively. These d spacings were in the ratio
of 1: 0.96: 0.48, with lattice parameters a and b found to be 31.52 Å and 41.39 Å respectively with
γ = 38.47 Å. The first diffused peak observed with a d spacing of 4.57 Å corresponds to the packing of
flexible alkyl tails, while the second peak with a d spacing of 3.77 Å corresponds to the coreꢀcore
distance. The number of molecules present in a unit cell was found to be 2.2. The XRD studies carried
out at 70 oC also showed a similar pattern as shown in figure 9c.
Figure 9. Photomicrograph of texture as seen by POM for the Colr phase of compound 2a at 70 °C (a);
DSC traces obtained for first cooling (blue trace) and second heating (red trace) cycles of compound 2a
at a rate of 5 oC/min (b); XRD profiles depicting the intensity against the 2θ obtained for the Colh phase
of compound 2a at 70 °C (c).
The packing of two molecules in a columnar slice can be described as below. The mꢀsubstituted
polycatenar 2a can exist in two possible conformations as shown in figure 10. As the XRD data suggests
that on an average two molecules must be packed inside the unit cell, then they must arrange side by
side to ensure the space filling. These sets of two molecules forming a disc, later selfꢀassemble to form
columns, and these columns in turn selfꢀorganize to form a 2D lattice of parallelogram.
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Figure 10. Molecular models obtained from Chem 3D showing the possible molecular conformations of
compound 2a.
Figure 11. Schematic showing the selfꢀorganization of hexacatenar 2a into columnar oblique (Colob)
lattice. Space filling energy minimized (allꢀtrans) molecular model of 2a was derived from molecular
mechanics (MM2) method. Ellipses denote tilted discs with respect to columnar axis.
Polycatenars 2b and 2c showed enantiotropic monomesomorphic behavior. Pseudofocal conic fan
shaped texture observed for both of these compounds (Fig. 12). Powder XRD investigations suggested
that these compounds stabilize Colob phase (Table 3).
In the first series of pꢀsubstituted polycatenars the reduction in isotropic temperature is not
regular in heating cycle, while in the second series of mꢀsubstituted polycatenars there is a gradual
reduction in the isotropic temperature with the increase in chain length (Fig. 3). Both the series of
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Figure 12. Photomicrograph of texture as seen by POM for the Colob phase of compound 2b at 80 °C
(a); and for compound 2c at 84 oC (b)
Table 3. Results of (hkl) indexation of XRD profiles of the compounds at a given temperature (T) of
mesophasesa
Compounds
(D/Å)
Phase
(T/oC)
Colob
(90)
dobs (Å)
dcal (Å)
Miller
indices hkl
010
110
ꢀ110
32.41
31.03
15.52
32.41
31.03
15.52
2a
44.01
4.57 (ha)
3.77 (hc)
001
Colob
(70)
34.74
31.90
18.42
15.96
34.74
31.90
18.42
15.95
010
110
ꢀ110
020
4.54 (ha)
3.72 (hc)
36.10
34.82
33.55
001
Colob
(80)
36.10
34.82
33.55
010
110
100
2b
48.5
4.55 (ha)
3.71 (hc)
001
Colob
(82)
40.98
38.40
37.03
21.38
40.98
38.40
37.03
21.23
010
100
110
210
2c
58.8
4.59 (ha)
3.75 (hc)
001
aThe diameter (D) of the polycatenar (estimated from Chem 3D Pro 8.0 molecular model software from
Cambridge Soft). dobs: spacing observed; dcal: spacing calculated (deduced from the lattice parameters; a for Colh
phase; a and b for Colob phase; γ is the column tilt angle). The spacings marked ha and hc correspond to diffuse
reflections in the wideꢀangle region arising from correlations between the alkyl chains and core regions,
respectively.
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molecules showed reduction in the thermal range with the increase in chain length. When compared to
pꢀsubstituted polycatenars 1a-c, which exhibited Colh and/or Colob phase, mꢀsubstituted polycatenars 2a-
c showed broader mesophase range and stabilized Colob phase exclusively. We need to recall here that
among the pꢀsubstituted polycatenars from first series, the peak corresponding to coreꢀcore stacking was
absent. On the contrary all the mꢀsubstituted polycatenars show the reflection corresponding to coreꢀ
core stacking suggesting that, there is an enhanced coreꢀcore interaction in this series of compounds. It
is understandable that for the stabilization of Colob phase, enhanced coreꢀcore interactions are essential
because the tilt of the cores in one column must be related to the tilt of cores in another column in
certain way.36
Mesomorphic behavior of thiadiazoleꢀbased polycatenars 1a-c, 2a and 2c of the present work
can be compared to their oxadiazole counterparts Ia-c34b and IIa-b34a, which are reported earlier (Fig. 2).
On comparing thermal behavior it is evident that thiadiazoleꢀbased pꢀsubstituted polycatenars stabilize
Colh and/or Colob phase, while their oxadiazole counterparts Ia-c34b stabilized crystal/ Colh phase. It is to
be noted that analogous mꢀsubstituted polycatenars based on 1,3,4ꢀoxdiazoles exhibited Colh phase
exclusively, irrespective of the chain length (Fig. 2).34a This also supports the enhanced coreꢀcore
interactions in the case of these thiadiazole based mꢀsubstituted polycatenars. Hetero atom sulphur in the
molecular structure is another contributing factor due to the attractive S…S interactions, which is a wellꢀ
known fact in the case of chalcogen based semiconductors.3
Mesophase width was increased in the case of thiadiazole derivatives in comparison to their
oxadiazole analogues. This behavior is in line with the behavior of star shaped 1,3,4ꢀthiadiazole
derivatives recently reported, where star shaped 1,3,4ꢀthiadiazoles exhibited wide range enantiotropic
Colh phase in comparison to their oxadiazole counterparts.33b Thus thiadiazole moiety enhances the
mesogenic behavior due to favorable interactions. Polycatenar thiadiazoles exhibited reduced
mesophase range when compared to the star shaped thiadiazole derivatives.
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III. Photophysical properties
Photphysical properties of the polycatenars 1a-c and 2a-c in solution are depicted in
table 4. It is expected that due to the difference in substitution pattern between the two series,
there must be a considerable difference in the photophysical properties. Absorption and
fluorescence spectra of the compounds 1a-c and 2a-c were taken in THF (Fig. 13).
As can be seen, the absorption spectra for the solutions of pꢀsubstituted polycatenars
1a-c showed single absorption maxima in a range of 360ꢀ365 nm. The absorption maxima of
mꢀsubstituted polycatenars 2a-c compounds exhibited a hypsochromic shift with an
absorption maxima centered at around 335ꢀ337 nm. Large values of molar extinction
coefficients showed that these are highly delocalized electronic systems (ε = >18,650 Mꢀ1 cmꢀ
1). Based on the similarity to reported 1,3,4ꢀoxadiazole and thiadiazole based systems,33b the
single absorption band of these systems is attributed to πꢀπ* transition of the aromatic system.
Optical bandgaps of these systems were calculated from the absorption onset values.
Compounds 1a-c showed a band gap of 3 eV, while compounds 2a-c showed a slightly
higher bandgap of 3.23 eV. This shows that π electron cloud is delocalized to a higher extent
in the case of pꢀsubstituted compounds (1a-c) in comparison to mꢀsubstituted compounds
(2a-c). Compared to their star shaped analogue i.e.1,3,5ꢀbenzene trisubstituted thiadiazole
derivative (optical band gap of 3.1 eV) with nine decyloxy chains, compounds 1a-c showed a
bathochromic shift of 20 nm in the absorption maximum, while 2a-c showed a hypsochromic
shift of 10 nm. When compared to the corresponding oxadiazole derivatives pꢀ and mꢀ
substituted thiadiazole derivatives, showed a red shift of 26 nm.34
Emission spectra of compounds 1a-c exhibited a single emission maximum centered
around 480ꢀ483 nm with large Stoke’s shift of 115ꢀ121 nm. Compounds 2a-c showed
emission with their maxima centered around 446ꢀ450 nm, with a Stoke’s shift of 111ꢀ114 nm.
As can be seen from the table, there was no much difference in the absorption and emission
properties with respect to chain length.33b, 34, 37 When compared to their star shaped analogue
with nine decyloxy chains, compounds 1a-c showed a bathochromic shift of 9 nm in the
emission maximum, while 2a-c showed a hypsochromic shift of 24 nm. As can be seen in the
Fig.13, green light was observed in the emissive state for compounds 1a-c, while blue light
was observed for compounds 2a-c on irradiation with the UV light of 365 nm wavelength.
When compared to the corresponding oxadiazole derivatives pꢀsubstituted thiadiazole
derivatives showed a red shift of 15 nm, while mꢀsubstituted thiadiazole derivatives, showed
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a red shift of 21 nm. 34 As a representative case we measured the relative quantum yield for
compounds 1a and 2a with respect to quinine sulphate solution (0.1 M H2SO4 with a quantum
yield of 0.54). Compound 1a showed a relative quantum yield of 0.4, while compound 2a
with a silghtly lesser value (0.35) (see SI). These compounds showed slightly higher quantum
yield with respect to their star shaped analogue III (0.31).33b
Table 4. Photophysical properties of polycatenarsa
Entry
Absorptio Emissionb Stokes shift λonset (nm) ΔEc,d
g, opt
n (nm)
365
(nm)
480
483
481
nm (cm-1)
115(6564)
118(7075)
121 (6987)
111 (7429)
114 (7540)
112 (7402)
1a
1b
1c
414
414
412
383
385
385
3.0
360
360
335
3.0
3.02
3.24
3.23
3.23
2a
2b
2c
446
450
449
336
337
amicromolar solutions in THF; b excited at the respective absorption maxima; e Band gap determined from the red edge of the longest wave
length (λonset) in the UVꢀvis absorption spectra; d In volts (V)
Figure 13. Normalized absorption (solid trace) and emission spectra (dotted trace) in THF
solution obtained for 1a (green trace) and 2a (blue trace). Pictures of micromolar solutions of
compounds 1a and 2a in THF as seen with the illumination of 365 nm UV light (inset).
Dilution of these micromolar solutions in THF did not show any variation in the absorption
and emission maxima, showing that these polycatenars are not aggregating at these
concentrations (SI). When the concentration is gradually increased from micromolar solution
to milimolar solution absorption maximum shows a slight blue shift (Fig. 14a). The emission
spectra in solution with the increase in concentrations did not show any shift in the emission
maxima (Fig.14b), but showed a gradual increase in the fluorescence intensity upto certain
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concentration (Fig. 14c). Further increase in concentration reduced the fluorescence intensity
due to the aggregation quenching (Fig. 14d). Similar behavior was observed in the case of
Figure 14. Normalized absorption (a) and emission (b) spectra obtained for compound 1a as
a function of concentration; Enhancement of luminescence in solution on increasing the
concentration (c); A graph showing the variation of luminescence intensity with respect to
concentration (d).
Figure 15. Normalized absorption (a) and emission (b) spectra obtained for compound 2a as
a function of concentration; Enhancement of luminescence in solution on increasing the
concentration (c); A graph showing the variation of luminescence intensity with respect to
concentration (d).
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compound 2a (Fig. 15). At higher concentration the absorption spectra showed a slight blue
shift, while the emission spectra did not show any change. As in the case of compound 1a,
upto a certain concentration there was an increase in the intensity of fluorescence, after which
there was a decrease in the intensity (Fig. 15d). This decrease may be due to the aggregation
quenching. Fluorescence lifeꢀtime decay profiles obtained for compounds 1a and 2a at
concentrations corresponding to their highest fluorescence intensity exhibited
a
monoexponential decay with the lifeꢀtime values of 1.73 and 1.44 ns respectively, confirming
the presence of solvated monomers (Fig. 16).
Figure 16. (a) The fluorescence decay of compound 1a in THF (6.25 × 10ꢀ5M, λexc = 375 nm)
(b) fluorescence decay of compound 2a in THF (1.25 × 10ꢀ4M, λexc = 336 nm). (Red trace
corresponding to the decay profile, black trace corresponding to instrument response
function; the residual plot is shown below the decay plot)
We were interested to study the emissive nature of these molecules in solid state. Thin fims
of these molecules were prepared on glass slides by drop casting the millimolar solutions in
toluene. The absorption spectra of compounds 1a and 2a showed structured absorption bands.
Compound 1a showed two absorption maxima centered at 301 and 385 nm, while compound
2a also showed two absorption maxima centered at 301 and 371 nm. In comparison to their
structureless solution state absorption spectra, these absorption spectra showed two
absorption maxima and a blue shift. Emission spectra obtained by exciting the solutions at
their absorption maxima were found to be broadened with a slight red shift. Compound 1a
showed an emission maximum centered at 485 nm, while compound 2a showed an emission
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maximum centered at 463 nm. Thus compound 1a showed a red shift of 5 nm, while
compound 2a showed a red shift of 13 nm with respect to their solution state spectra. But
under UV light of long wavelength (365 nm) these films showed similar visually perceivable
emisssion as in the solution state for compounds 1a and 2a (Fig. 13 and Fig. 17b). The blue
shift in the absorption bands observed for thin films in comparison to the solution state
indicates that the emission is arising from the molecular aggregates, in which the molecules
Figure 17. Normalized absorption spectra obtained for compounds 1a (green trace) and 2a
(blue trace) in the thin film state prepared from drop casting (a); Normalized emission spectra
obtained for compounds 1a and 2a in the thin film state (b). Inset shows the thin films of
compounds 1a and 2a on illumination of 365 nm UV light (inset).
are stacked one above the other in a cofacial manner which is denoted as Hꢀaggregates.40 We
have also attempted to see whether these compounds form gel in various solvents, but found
that the πꢀπ interaction between the aromatic cores alone was not sufficient for gelation. We
have also prepared thin films of the sample by heating the sandwiched sample between glass
slides to isotropic state. These films were suddenly cooled in their liquid crystalline state.
Polarizing optical images have shown that compound 1a was crystallized during this sudden
cooling (Fig. 18c), while compound 2a retained the texture of Colob phase (Fig. 18f). Both of
the films showed good transparency (Fig.18a and d). It is interesting to note that the thin film
prepared from compound 1a turned opaque and formed a green colored film slowly, while
comound 2a still retained the transparency and liquid crystalline texture even after 7 days
(Fig.18g). Thus it can be understood that the molecular structure of pꢀsubstituted polycatenars
are more conducive to crystallization, while that of mꢀsubstituted polycatenars prevent
crystallization and favors glassy state. Glassy columnar structure helps in charge migration
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with the simultaneous freezing of ionic impurities, which is a favorable feature for
applications in organic electronic devices.41
Figure 18. Thin film prepared from compound 1a by annealing the isotropic liquid under (a)
day light and (b) 365 nm UV light; POM image of the same film showing crystallization (c);
Thin film prepared from compound 2a by annealing the isotropic liquid under (d) day light
and (e) 365 nm UV light; POM image of the same film showing the freezing of Colob phase
(f); POM image of the film after 7 days showing the stable glassy state (g).
IV. Electrochemical properties
Energy levels of HOMO and LUMO and the band gap of the polycatenars were
obtained by cyclic voltammetry (CV) and the data are tabulated in table 5. We have chosen
compound 1a and 2a for carrying out CV studies. Both the compounds exhibited irreversible
oxidation and reduction waves (See SI). The optical band gap Eg, opt was estimated from the
red edge of the absorption spectra. pꢀSubstituted compounds 1a exhibited lower band gap
when compared to mꢀsubstituted compounds 2a-c. Energy levels of LUMO and HOMO were
determined by using the formulae ELUMO = – (4.8–E1/2, Fc,Fc+ + Ered, onset) eV, EHOMO = (4.8–
E1/2, Fc,Fc+ + Eox, onset) eV. pꢀSubstituted compounds 1a exhibited LUMO level ꢀ3.31 eV, and a
HOMO level at ꢀ5.87 eV, while mꢀsubstituted compounds 2a exhibited LUMO levels ꢀ3.29
eV, and a HOMO level at ꢀ5.98 eV. Thus the band gaps estimated from CV by using the
above formula give the band gap values of ꢀ2.56 eV for compound 1a and ꢀ2.69 eV for
compound 2a respectively. These values are slightly lesser than their respective optical band
gap values (Table 4). When compared to their star shaped analogues, polycatenars exhibit
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slightly lower LUMO and higher HOMO levels. As a result there is a reduction in the band
gap of polycatenars when compared to their star shaped analogue33b (Fig. 19).
Table 5. Electrochemicala,b properties of polycatenars
Entry Ed
Ed
Ec,e
Ec,f
ΔEc,g
LUMO CV
1red
1oxd
HOMO
1a
-0.93
-0.95
1.63
1.74
-5.87
-5.98
-3.31
-3.29
2.56
2.69
2a
amicromolar solutions in THF; b Experimental conditions: Ag/AgNO3 as reference electrode, Glassy carbon working electrode, Platinum
rod counter electrode, TBAP (0.1 M) as a supporting electrolyte, room temperature, scanning rate of 0.05 mVsꢀ1; cIn electron volts (eV), d In
e
volts (V), Estimated from the onset oxidation peak values by using from the formula EHOMO = (4.8 –E1/2, Fc,Fc+ + Eox, onset) eV, fEstimated
from the onset reduction peak values by using ELUMO = – (4.8 –E1/2, Fc,Fc+ + Ered, onset) eV. gEstimated from the formula ꢁECV = EHOMO ꢀ ELUMO
eV.
Figure 19. Comparison of frontier molecular orbitals for compounds 1a, 2a and III.
V. Experimental Section
Commercially available chemicals were used without any purification; solvents were dried
following the standard procedures. Chromatography was performed using either silica gel
(60ꢀ120 and 100ꢀ200) or neutral aluminium oxide. For thin layer chromatography, aluminium
sheets preꢀcoated with silica gel were employed. IR spectra were recorded on a Perkin Elmer
IR spectrometer at normal temperature by using KBr pellet. The spectral positions are given
in wave number (cmꢀ1) unit. NMR spectra were recorded using Varian Mercury 400 MHz or
Bruker 600 MHz NMR spectrometer (at 298K). For 1H NMR spectra, the chemical shifts are
reported in ppm relative to TMS as an internal standard. Coupling constants are given in Hz.
Mass spectra were obtained from MALDIꢀTOF mass spectrometer in Laser Desorption
positive mode using αꢀcyanoꢀ4ꢀhydroxycinnamic acid matrix or from High Resolution Mass
Spectrometer.
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The mesogenic compounds were investigated for their liquid crystalline behavior
(birefringence and fluidity) by employing a polarizing optical microscope (POM) (Nikon
Eclipse LV100POL) equipped with a programmable hot stage (Mettler Toledo FP90). Clean
glass slides and coverslips were employed for the polarizing optical microscopic
observations. The transition temperatures and associated enthalpy changes were determined
by differential scanning calorimeter (Mettler Toledo DSC1) under nitrogen atmosphere. Peak
temperatures obtained in DSC corresponding to transitions were in agreement with the POM
observations. The transition temperatures obtained from calorimetric measurements of the
first heating and cooling cycles at a rate of 5 °C/min are tabulated. In the cases where the
DSC signatures are not observed for the phase transitions, the transition temperatures have
been taken from POM observations. Temperature dependent Xꢀray diffraction studies were
carried on unaligned powder samples in Lindemann capillaries (1 mm diameter) held in
programmable hot stage and irradiated with CuKα radiation (λ = 1.5418 Å). The samples
were filled in the capillary tube in their isotropic state and their both ends were flame sealed.
The apparatus essentially consisted of a PANalytical X'Pert PRO MP Xꢀray diffractometer
consisting of a focusing elliptical mirror and a fast high resolution detector (PIXCEL).
Thermogravimetric analysis (TGA) was performed using thermogravimetric analyzer
(Mettler Toledo, model TG/SDTA 851e) under constant nitrogen flow at a heating rate of 10
°C/min. UVꢀVis spectra were obtained by using PerkinꢀElmer Lambda 750, UV/VIS/NIR
spectrometer. Fluorescence emission spectra in solution state were recorded with Horiba
Fluoromaxꢀ4 fluorescence spectrophotometer or Perkin Elmer LS 50B spectrometer. Cyclic
Voltammetry studies were carried out using a PAR Model 700D series Electrochemical
workstation. Time resolved fluorescence was measured using an Edinburgh Instruments Life
Spec II instrument. Here the fluorescence lifetimes were determined by a timeꢀcorrelated
single photon counting method. A Hamamatsu microchannel plate (MCP) detector that has a
response time of 50 ps was used in the abovementioned instrument. A picosecond 375 nm
laser diode and 336 LED were used as the light sources. The full width at half maxima for the
laser diode and the LED are 90 ps and 570 ps respectively. The fluorescence emission
maxima for the corresponding species has been chosen as the monitoring wavelength in the
time resolved fluorescence measurements. The data were analyzed using a reconvolution
method using the software provided by Edinburgh instruments.
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VI. Summary
In summary, we have synthesized two series of polycatenars containing 1,3,4ꢀ
thiadiazole moiety in their molecular structure. The main difference between the two series is
the position at which the 1,3,4ꢀthiadiazole moiety is connected to a central benzene ring. In
the first series the thiadiazole moieties are connected to the central benzene ring at 1,4
position (pꢀsubstituted), while in the second series the thiadiazole moiety is connected at 1,3
position (mꢀsubstituted) to the central benzene ring. The thermal behavior was studied with
the help of POM, TGA, DSC and powder XRD studies. Both the series stabilized
enantiotropic columnar phase. pꢀSubstituted polycatenars stabilized Colh and/or Colob phase,
while mꢀsubstituted polycatenars exhibited Colob phase exclusively. mꢀsubstituted
polycatenars showed enhanced coreꢀcore interaction in comparison to their pꢀsubstituted
polycatenars. In both the series mesophase range is decreased with the increase in the chain
length. Incorporation of sulphur in the molecular structure enhanced the thermal range of
mesophase with respect to their oxadiazole counterparts. Polycatenar thiadiazoles exhibited
reduced mesophase range when compared to the star shaped thiadiazole derivatives.
Both the series exhibited luminescence in solution. pꢀSubstituted compounds showed
green emission, while mꢀsubstituted compounds exhibited blue emission in solution state.
Thus the substitution plays a very important role in the luminescence properties of these
molecules. Most importantly, the thin films prepared by the drop casting or by annealing
from the isotropic state showed similar visually perceivable emission. The length of the
peripheral tails did not have much effect on the absorption and emission behavior of these
compounds. Cyclic Voltammetry studies have shown that pꢀsubstituted polycatenars showed
lower bandgap than the mꢀsubstituted compounds. Both the polycatenars exhibited a lower
band gap than their star shaped analogue. Thus in this report we have shown that
regioisomerism affects significantly on the selfꢀassembly and photophysical behavior of
1,3,4ꢀthiadiazole based polycatenars.
VII. Associated Content
1
The synthesis and charcterization details, H NMR, 13C NMR spectra of all new compounds,
absoroption and emission spectra, POM photographs, DSC thermograms, XRD profiles of
LC compounds, TGA curves and Cyclic voltammograms are provided as electronic
supporting information. This material is available free of charge via the Internet.
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Acknowledgements
ASA sincerely thanks Science and Engineering Board (SERB), DST, Govt. of India and
Board of Research in Nuclear SciencesꢀDepartment of Atomic Energy (BRNSꢀDAE) for
funding this work through the project No. SB/S1/PCꢀ37/2012 and No.
2012/34/31/BRNS/1039 respectively. We thank Ministry of Human Resource Development
for Centre of Excellence in FAST (F. No. 5ꢀ7/2014ꢀTSꢀVII). ASA acknowledges Central
Instrumentation Facility, IIT Guwahati for analytical facilities. We acknowledge Dr. Chandan
Mukherjee, IITGuwahati for providing his Electrochemical workstation.
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