Synthesis and self-assembly of aroylhydrazone based polycatenars: A structure-property correlation

Article abstract of DOI:10.1016/j.molliq.2019.04.003

Two series of aroylhydrazone based polycatenars which differ from each other in terms of the linking groups (semi flexible ester or flexible ether linkage) were synthesized. These hexacatenars were mesogenic and also showed gelation in organic solvents at

Full text of DOI:10.1016/j.molliq.2019.04.003

Accepted Manuscript
Synthesis and self-assembly of aroylhydrazone based
polycatenars: A structure-property correlation
Hemant Kumar Singh, Ravindra Kumar Gupta, Sachin Kumar
Singh, D.S. Shankar Rao, S. Krishna Prasad, Ammathnadu S.
Achalkumar, Bachcha Singh
PII:
S0167-7322(18)32904-0
DOI:
https://doi.org/10.1016/j.molliq.2019.04.003
MOLLIQ 10726
Reference:
To appear in:
Journal of Molecular Liquids
Received date:
Revised date:
Accepted date:
4 June 2018
29 March 2019
1 April 2019
Please cite this article as: H.K. Singh, R.K. Gupta, S.K. Singh, et al., Synthesis and self-
assembly of aroylhydrazone based polycatenars: A structure-property correlation, Journal
of Molecular Liquids, https://doi.org/10.1016/j.molliq.2019.04.003
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ACCEPTED MANUSCRIPT
Synthesis and self-assembly of aroylhydrazone based polycatenars: a
structure-property correlation
a,b
b
a,b
c
Hemant Kumar Singh , Ravindra Kumar Gupta , Sachin Kumar Singh , D. S. Shankar Rao , S.
c
*b
*a
Krishna Prasad , Ammathnadu S. Achalkumar and Bachcha Singh
a
Department of Chemistry, Institute of Science, Banaras Hindu University, Varanasi-221005, India.
b
Department of Chemistry, Indian Institute of Technology Guwahati, Guwahati-781039, India.
c
Centre for Nano and Soft Matter Sciences, Jalahalli, Bangalore-560013, India.
Email: bsinghbhu@rediffmail.com
Abstract. Two series of aroylhydrazone based polycatenars which differ from each other in
terms of the linking groups (semi flexible ester or flexible ether linkage) were synthesized.
These hexacatenars were mesogenic and also showed gelation in organic solvents at very low
concentration, which is assisted by the intermolecular hydrogen bonding. Both the series of
hexacatenars were prepared by varying the peripheral chain length from n-hexyloxy to n-
hexadecyloxy chain. In the case of ester based series on moving from n-decyloxy to n-
dodecyloxy chain, a transition from columnar rectangular phase to columnar hexagonal phase
was noticed. Such behavior was not seen in the case of ether-based hexacatenars, irrespective
of the chain length. Representative hexacatenars from each series exhibited gelation at low
concentration, but the critical gelation concentration of ester-based hexacatenar was almost
half of that observed for ether-based hexacatenar. From these studies we can understand the
importance of the linking group, which constitutes very small part of the molecule, but has a
significant effect on the type of the self-assembly in bulk as well as in presence of solvents.
The presence of semi-flexible ester group provides a bent conformation to the molecule,
while the presence of flexible ether group provides a linear shape to the molecule, as evident
from the molecular modeling studies. Further the presence of the ester group enhances the
extent of intermolecular hydrogen bonding with the solvent that leads to a lowercritical
gelation concentration in comparison to the ether-based linking group. Such H-bonded liquid
crystals have potential in the preparation of liquid crystalline physical gels and related
applications.

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1
. Introduction
Aroylhydrazones containing --CO-NH-N=C-- moiety have been identified as one of the
versatile class of compounds containing lateral intermolecular hydrogen bonding (H-
bonding) [1]. The aroylhydrazone moiety provides certain unique features to the molecule
like (i) a trinuclear azomethine group (-C=N-NH-) which provides opportunities to obtain the
molecules of diverse structures, (ii) the possibility of configurational isomerisation arising
from the inherent nature of the C=N double bond and (iii) amide linkage (-C=O-NH-) that
provides the intermolecular hydrogen bonding (C=O---H-N) leading to the stabilization of
supramolecular assembly. Due to these structural features, aroylhydrazone based compounds
show different physicochemical properties which lead to many applications in various fields
ranging from organic synthesis, biological, bactericidal, fungicidal, analytical, industrial,
pharmacological, medicinal chemistry and supramolecular chemistry [2-10]. There are a
number of reports on aroylhydrazone based compounds and their metal complexes, but
research on aroylhydrazone derivatives exhibiting mesomorphic behavior is still in its initial
stages except few reports on such mesogens [11-26]. This is largely attributed to the
extensive hydrogen bonding between amide linkage in these types of ligands and the high
melting point of the corresponding metal complexes. In order to obtain mesogenic
aroylhydrazone derivatives, the commonly used approach is the diminution of hydrogen
bonding between neighboring layers via (i) coordination of aroylhydrazone derivatives with
the metal ions through oxygen and nitrogen donor atoms, so that amidic bond could not be
available for intermolecular hydrogen bonding or (ii) using the polycatenar substitution
pattern of the alkoxy chains attached to aromatic core or (iii) both approaches in a single
molecule [11-15]. The polycatenars, or phasmid-like LCs, are nonconventional LCs with a
dumbbell shape, including a central long rod-like aromatic unit connected to two terminal
hemi-discs with multiple flexible chains [27-29]. These molecules self-assemble to form
various liquid crystalline (LC) phases due to the nanophase segregation of incompatible
molecular subunits like central rigidaromatic core and terminal flexible chains. They are very
sensitive to small structural variations resulting in the changes of macroscopic self-assembly
and hence their mesogenic properties [30-32]. The length of the flexible chains present in the
polycatenars and the extent of intermolecular interactions decide the type of the mesophase
exhibited by the polycatenars. This hybrid molecular design with the features of both rod-like
and disc-like LCs, make them capable of exhibiting mesophases common to both these
classes by a systematic structural tuning. Usually the hexacatenars stabilize columnar phase
with hexagonal (Col phase), rectangular (Col phase) or an oblique (Col ) symmetry.
h
r
ob

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Supramolecular gels form a unique class of materials. Such gelators possess specific
structural attributes that lead to the immobilization of large volume of solvents during
gelation. In spite of the low concentration of organogelators in comparison to the amount of
solvent, the organogels are very sensitive to the molecular structure of the gelator [33-36].
There are subtle interactions of gelator-gelator and gelator-solvent molecules in stabilizing
the organogels which can be altered by the molecular structure. The organogelation is further
assisted by several weak forces like H-bonding, π-π interactions, hydrophobic or hydrophilic
interactions or van der Waals interactions. This class of materials is promising in a wide
range of applications in the areas of optoelectronics, inorganic nanostructure templates,
sensors and controlled drug delivery systems [37-47]. Merging of these two different fields
has given rise to a new area of ‘liquid crystalline physical gels’. This type of gels exhibit
enhanced tunable electrooptical, photoresponsive and electronic properties due to the
combination of these two different entities which form phase-separated structures [47]. Our
recent results on aroylhydrazone based polycatenar LCs, which stabilize gelation at very low
concentration (2 wt %), encouraged us to investigate more into structure-property relationship
that direct these delicate interactions which stabilize the LC and organogel self-assembly
[
26]. Thus, we have synthesized hexacatenars containing aroylhydrazone moiety with an
overall reduction in the molecular length in comparison to the earlier reported derivatives
26]. We envisaged that such molecules may have potential in the preparation of liquid
[
crystalline physical gels in future.
Two series of hexacatenars were prepared by extending the aroylhydrazone core unit
through semi-flexible ester (-C(=O)O-) and flexible ether (-O-CH ) linking groups. The
2
mesophase behavior of the ester and ether based compounds was investigated with respect to
a variation in chain length and linking group. Mesophase crossover from Col phase to Col
r
h
phase was found in these derivatives, which was studied in detail with the help of polarizing
optical microscopy (POM), differential scanning calorimetry (DSC) and powder X-ray
diffraction (XRD) studies.
2
2
. Results and discussion
.1. Synthesis and characterization
The synthesis of ester and ether based hexacatenars is provided in Scheme 1. O-alkylation of
methyl 3,4,5-trihydroxybenzoate with 1-bromoalkane in 2-butanone using anhydrous
potassium carbonate leads to the formation of methyl 3,4,5-trialkoxybenzoate. 3,4,5-
Trialkoxybenzohydrazide was prepared by reaction of methyl 3,4,5-trialkoxybenzoate with

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hydrazine monohydrate in refluxing ethanol. 4-(3,4,5-Tris(tetradecyloxy)benzoyloxy)
benzaldehyde and 4-(3,4,5-tris(hexadecyloxy)benzyloxy)benzaldehyde were synthesized
according to reported methods [26]. The aroylhydrazone based compounds were obtained by
the condensation reaction of 3,4,5-trialkoxybenzohydrazide with appropriate aldehyde in
refluxing ethanol. The structures of all the precursors and final compounds were confirmed
1
13
using H-NMR, C-NMR, IR spectroscopy and CHN analyses (see the Electronic Supporting
Information for details).
Scheme 1.Reactions and reagents: (a) RBr (n equiv.), K CO (4 × n equiv.), KI (catalyst), refluxing in 2-
2
3
butanone, 24-72 h, 70-85% (b) KOH (3 equiv.), refluxing in EtOH, 4 h, 80-90% (c) 4-hydroxybenzaldehyde (1
equiv.), DMAP (1.1 equiv.), DCC (1 equiv.), stirring in dry DCM, 4-6 h, RT, 60-80% (d) LAH (1.2 equiv.),
°
stirring in dry THF at 0°C, 4 h, 88-95% (e) SOCl (3 equiv.), stirring in dry DCM at 0 C, DMF (cat.), 1 h, 75-
2
8
5% (f) 4-hydroxybenzaldehyde (1 equiv.), K CO (3 × n equiv.), refluxing in dry DMF, 6-10 h, 60-70% (g)
2 3
NH NH .H O(1.5 equiv.), refluxing in absolute EtOH, 12 h, 85-90%. (h) Acetic acid (cat.), refluxing in EtOH,
2
2
2
1
2 h, 70–80%.
2
.2. Mesomorphic behavior
The thermal behavior of the hexacatenars was studied by the use of thermo gravimetric
analyses (TGA), DSC, POM and XRD studies (in selected cases). Transition temperatures
and their corresponding enthalpy and entropy changes are tabulated in Table 1 and

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graphically represented in Fig. 1. The mesomorphic behavior of these hexacatenars
containing ester and ether linkages are described in the following sections.
2
.2.1.Compounds with ester linkage
From TGA analyses, the degradation temperature of the compound 1 was found to be above
00 °C (ESI). Except the shorter homologues with n-hexyloxy chains (1, n=6), all the
3
hexacatenars (1, n = 8, 10, 12, 14, 16) exhibited the Col phase.
Fig. 1. Bar graph showing the thermal behavior of the compounds (1/n6→1/n16) with ester linkage (in cooling
cycle).
Table 1.Phasetransitions and corresponding thermodynamic parameters
T, °C [∆H, kJ/mol; ∆S, J/(mol K)]
Compound
Heating
Cr 130.9 (96.1; 237.8) I
Cr 103.7 (61.5; 163.2) Col
Cr 65.0 (104.5; 309.1) Col
Cooling
I 122.3 (-78.4; -198.2) Cr
1
/n6
1
/n8
r
122.3 (2.1; 5.3) I I 120.1 (-1.4; -3.5) Col
r
r
52.7 (-11.1; -34.0) Cr
24.4 (-40.3; -135.4) Cr
1
/n10
r
117.2 (2.7; 6.9) I
I 113.2 (-2.3; -5.9) Col
7.2 (-14.0; -48.2) Cr'
I 104.7 (-1.8; -4.8) Col
106.7 (2.6; 6.9) I I 102.2 (-2.0; -5.4) Col
104.9 (2.0; 5.2) I I 98.6 (-1.8; -4.8) Col
I 120.4 (-2.7; -6.9) Col
1
1
1
1
/n12
/n14
/n16
Cr 63.4 (88.6; 263.3) Col
h
108.7 (2.8; 7.4) I
h
h
13.5 (-31.1; -108.5) Cr
30.3 (-33.8; -111.4) Cr
Cr 65.3 (118.0; 348.5) Col
h
Cr 82.4 (60.4; 169.8) Col
Cr 58.7 (42.1; 127.0) Col
8.1 (-15.0; -53.4) Cr
h
h
2
/n6
/n8
r
125.9 (3.9; 9.8) I
r
38.7 (-30.9; -99.1) Cr
24.0 (-29.5; -97.5) Cr
2
Cr 47.0 (38.0; 118.8) Cr' 60.9 (8.6; 25.8) Colr I 116.4 (-2.0; -5.2) Col
19.4 (2.3; 5.9) I
r
1
2
/n10
Cr 43.9 (47.2; 148.9) Col
Cr 48.5 (64.6; 200.8) Col
r
111.6 (1.9; 5.1) I
105.6 (2.0; 5.2) I
I 108.3 (-1.7; -4.5) Col
I 102.3 (-1.7; -4.5) Col
r
r
29.2 (-36.4; -120.3) Cr
29.5 (-43.6; -144.2) Cr
2
2
2
/n12
/n14
/n16
r
Cr 50.1 (32.3; 99.9) Col
104.1 (6.7; 17.8) I
101.6 (2.2; 5.9) I
I 102.9 (-5.3; -14.2) Col
30.2 (-44.2; -145.6) Cr
r
r
Cr 53.2 (52.4; 160.7) Col
I 97.9 (-2.1; -5.7) Col 32.7 (-38.9; -127.2) Cr
r
r
Abbreviations: Cr, Cr' = crystalline state, Col = rectangular columnarmesophase, Col = hexagonal columnar
r
h
mesophase, I = isotropic liquid.

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All the hexacatenars with n-octyloxy to n-hexadecyloxy (n= 8–16) carbon chain length
exhibited enantiotropic Col phase. Compound 1/n10 showed a mesophase range of 52 °C.
The crystalline compound melts at 65 °C (ΔH = 104.5 kJ/mol) to enter a mesophase, before
converting into an isotropic liquid at 117.2 °C (Fig. 2b). On cooling the isotropic liquid,
mesophase appears at 113.2 °C with a typical fan-shaped texture characteristic of Col phase
(Fig. 2a). This remains as such up to 24.4 °C with the same texture without any signs of
crystallization, as evidenced by the DSC thermogram (Fig. 2b).
Fig. 2. (a) POM image for the Col phase of compound 1/n10 at 103 °C; (b) DSC traces obtained for the first
r
cooling and second heating cycles of compound 1/n10; (c) XRD profiles depicting the intensity against the 2θ
obtained for the Col phase of compound 1/n10 at 26 °C.
r
To confirm the mesophase structure and supramolecular organization of the compound 1/n10,
XRD studies were carried out (Fig. 2c). X-ray diffraction patterns for the compound 1/n10
were recorded in the LC phase at different temperature intervals while cooling the isotropic
liquid. Indexation of the reflections obtained in the XRD profiles of the compound 1/n10 is
tabulated in Table 2. The X-ray profile of compound 1/n10 at 26 °C showed four reflections
with d-spacings of 31.81, 22.16, 15.97 and 12.12 Å in the small-angle region, corresponding
to the Miller indices (10), (12), (20) and (05), respectively. In the wide angle region, two
diffuse peaks are found at 4.33 and 4.16 Å, corresponding to the packing of the alkoxy tails
and stacking of the molecular cores within the column, respectively. The second diffused
peak is stronger than the first indicating the ordered packing of hexacatenars within the
column. On indexing d-spacings obtained in the small angle region, the diffraction profile
showed a two-dimensional rectangular lattice with lattice constants a = 31.8 Å and b = 61.8
Å (Table 2). Thus, this phase was assigned as a Col phase.
r
This can be schematically represented as below. It is known that two or more
hexacatenars combine together to form a disc and such discs self-assemble to form Col
phases. From the XRD studies it is evident that in the Col phase of the compound 1/n10,
r
three molecules form a columnar slice, which in turn pack themselves into a column, and

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these columns self-organize in a rectangular lattice to stabilize Col phase (p2mm symmetry).
r
The packing of three molecules together is facilitated by the bent structure provided by the
ester linking group as shown in the Fig. 3.
Table 2. X-Ray diffraction data of the compounds 1/n10, 1/n12, 2/n8 & 2/n10
2
Comp Mesophase T/°C d-spacing/Å d-spacing/Å
Miller
lattice
Lattice area S (Å ),
ds.
observed
calculated
indices/hk spacing/Å
molecular volume V
3
(
Å )
1
/n10
Col
r
26
31.81
31.81
22.06
15.93
12.35
4.33
10
12
20
05
--
a=31.8
b=61.8
S=1964.8, V=8168.1,
2
1
1
2.16
5.97
2.12
Z=3.4
4
4
.33
.16
4.16
--
1
/n12
Col
h
26
26
32.15
32.08
16.08
12.15
4.44
30.44
21.21
15.29
13.87
11.93
4.36
10
20
21
--
11
02
22
31
23
--
a=37.1
S=1193.8,
V=5296.0, Z=2.1
1
1
6.15
2.20
4
.44
2
/n8
Col
r
30.58
a=43.9
b=42.6
S=1871, V=7764,
2
1
1
1
1.31
5.39
3.77
1.86
Z=2.9
4
4
.36
.14
4.14
--
2
/n10
Col
r
50
29.96
29.96
28.57
17.02
4.46
11
02
21
--
a=35.2
b=56.9
S=2005.7,
V=8937.8, Z=3.5
2
1
8.45
7.17
4
.46
Fig. 3. Schematic showing the self-assembly of the compound1n/10 in Col phase
r
Interestingly, the compound 1 with longer side chains, n=12, 14 and 16, showed a typical
Col phase. For the compound 1/n12 transitions Cr-Col -I in heating cycle, and I-Col -Cr in
h
h
h
cooling cycle was observed. From DSC thermograms a typical larger enthalpy for the Cr-Colh
transition at temperature 63.4 °C and a lower enthalpy for the Col -I transition at temperature
h

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1
1
08.7 °C in heating cycle were observed (Fig. 4b). Under the POM, compound 1 (n=12, 14,
6) showed characteristic birefringent fluid phases that are typical of Col phases. The
formation of broken fan textures with homeotropic domains has been observed (Fig. 4a). The
textures are mobile down to room temperature without any further change in the texture.
Fig. 4. (a) POM image for the Col phase of the compound 1/n12 at 100 °C; (b) DSC traces obtained for the first
h
cooling and second heating cycles of the compound 1/n12; (c) XRD profiles depicting the intensity against the
2
θ obtained for the Col phase of the compound 1/n12 at 26 °C.
r
Variable temperature powder XRD measurements were performed to determine the
symmetry of the columnar mesophase formed by the compound 1/n12 at 26 °C (Table 2).
The intensity vs 2θ plot obtained from the XRD pattern of the Col phase at 26 °C (Fig. 4c)
showed a strong reflection corresponding to a d-spacing of 32.15 Å in the low angle region,
followed by reflections with d-spacings of 16.15 and 12.20 Å. These reflections were
corresponding to the Miller indices of (10), (20), and (21) in the ratio 1: 1/√4: 1/√7 and
therefore, these values confirm the Col phase. In the wide-angle region, a relatively diffused
h
peak with a d-spacing of 4.44 Å was observed. The calculated lattice parameter a was 37.1 Å.
This is approximately 30% less than the diameter obtained from the DFT analysis. It may be
due to the folding of flexible chains or the possible interdigitation of flexible chains in the
neighboring columns. Polycatenar molecules due to their dumbbell-shape are not big enough
to fill the columnar cross-section. Therefore, two or more molecules have to aggregate side
by side to generate a columnar slice (disc), which in turn stacks to form the columns. Such
molecular association leads to association of the rigid aromatic parts of the molecules in the
center, and the periphery with flexible alkoxy chains. The force responsible for this kind of
arrangement is driven by the nanophase segregation, due to the incompatibility of these
molecular sub units as well as the tendency of the system to minimize the free space during
packing [48-54]. There are three and two molecules in columnar cross-section of the column
of 1/n10 and 1/n12, respectively. The mesophase crossover from Col to Col arrangements
r
h
with increasing terminal alkoxy chain length was observed for the hexacatenars on moving

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from 1/n10 to 1/n12; i.e. with the increasing carbon number the Col phase arrangement
h
predominates over the Col phase. The mesophase crossover from Col to Col was generally
r
r
h
attributed to the requirement of greater core-core interaction required for the stabilization of
Col phases [55-57]. It is understood that increase in length of terminal alkoxy chains
r
weakens the core-core interaction, that is assisted by the intermolecular H-bond interaction (-
…
C=O H-N-) and thus leads to the formation of Col phase from Col phase in such systems.
h
r
Thus in the case of compound 1/n12 with six n-dodecyl chains, only two molecules associate
to form a disc, as evident from XRD studies (Fig. 5, Table 2). This kind of loose packing
leads to the stabilization of Col phase. This is also complimented by the reduced core-core
h
interaction with the increase in the chain length. Such behavior is commonly observed in the
case of discotic Col phases, where the lower homologues exhibit Col phase, while the higher
r
homologues exhibit Col phase [14].
h
a = 37.1 Å
37.52 Å
1n/12
2
molecules
h
Col phase (p6mm)
form a disc
Fig. 5. Schematic showing the self-assembly of the compound 1n/12 in Col phase
h
From the bargraph shown in Fig. 1, it is evident that in the case of mesogenic ester
compounds, with the increase in chain length (n = 8→16), a significant decrease in melting
temperature was observed, while the decrease in the clearing point was very low. This results
increase in the thermal range of mesophase in the case of hexacatenars (1/n8→1/n12). This
trend of increase in the mesophase range is found to be inconsistent for the compound with
the highest chain length (1/n16) and a slight decrease was found in the case of 1/n14. Thus
we can infer that for this type of polycatenars, the appropriate peripheral chain length is a
significant aspect in deciding the thermal range of the mesophase. Moderate length soft
alkoxy tails may be assumed as most appropriate to gain good mesophase stability.
2
.2.2. Compounds with ether linkage
The hexacatenars of this series with ether linkage and of varying chain length (2, n= 6, 8, 10,
2, 14, 16), exhibited the Col phases and were characterized by DSC, polarizing optical
1

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microscope and XRD analysis (for n= 8, 10). From TGA analysis, it was found that the
hexacatenars 2 were stable at least up to 300 °C. The phase transition temperatures and
enthalpies of the compounds (2, n= 6→16) are given in Table 1. The compounds with n-
hexyloxy to n-hexadecyloxy (n= 6–16) carbon chain length exhibited enantiotropic columnar
phase. For example, in the second heating scan of the compound 2/n8, three sharp peaks
appear at 47, 69.4 and 119.4 °C which are attributed to crystal to crystal, crystal to
mesophase and mesophase to isotropic transition, respectively (Fig. 6b). The DSC
thermogram recorded in the cooling scan exhibits two distinct peaks at 116.4 and 24 °C
corresponding to isotropic to mesophase and mesophase to crystal phase transitions,
respectively. POM observations reveal that on cooling from the isotropic state to 80 °C, fan
shaped textures of the columnar mesophase emerged (Fig. 6a). This was also confirmed by
XRD studies carried out at 26 °C in cooling cycle, with a characteristic pattern commonly
observed for the Col phase (Fig. 6c). Similar type of textural pattern is also shown by the
r
compound 2/n6.
Fig. 6. (a) POM image for the Col phase of the compound 2/n8 at 90 °C; (b) DSC traces obtained for the first
h
cooling and second heating cycles of the compound 2/n8; (c) XRD profiles depicting the intensity against the 2θ
obtained for the Col phase of the compound 2/n8 at 26 °C.
r
The supramolecular organization of the compound (2/n8) in the Col phase was studied by
XRD technique. The results of indexing of diffraction pattern obtained from the powder 2D
pattern (Fig. 6c) of hexacatenar 2/n8 are presented in Table 2. The small angle region showed
reflections that can be assigned to a two-dimensional rectangular lattice with lattice constants
a = 43.9 Å and b = 42.6 Å (Table 2) and thus the Col phase can be assigned as a Col phase.
r
DSC thermogram of the compound 2/n10 shows two endothermic and two exothermic peaks.
Two endothermic transitions were seen in the DSC that are attributed to crystal to mesophase
and mesophase to isotropic phase transitions, while exothermic transitions are isotropic to
mesophase and mesophase to crystal phase (Fig. 7b). As we move from lower to higher
alkoxy chain length (n=8 to 10), we found broken fan textures of the Col phase on cooling in
the POM (Fig. 7a). The textures are mobile down to room temperature without further change

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in the texture. Similar type of textural patterns is also shown by the compounds 1 (n=12, 14
and 16). In the case of discotic columnar LCs, it is usual to predict a transition from a Col to
r
a Col phase with an increase in peripheral chain length, but such a tendency was not
h
observed in this class of polycatenars [30, 48].
Fig. 7. (a) POM image for the Col phase of the compound 2/n10 at 50 °C; (b) DSC traces obtained for the first
h
cooling and second heating cycles of the compound 2/n10; (c) XRD profiles depicting the intensity against the
2
θ obtained for the Col phase of the compound 2/n10 at 50 °C.
r
In order to identify the type of Col phase, X-ray diffraction pattern (Fig. 7c) of the compound
/n10 has been obtained. The diffractogram of the compound 2/n10 at 50 °C showed three
2
reflections at a d-spacing of 29.96, 28.45 and 17.17 Å in the low angle region (Table 2).
These reflections, along with the diffused halo at wide angles, can be indexed to a Col phase
r
with Miller indices of (11), (02) and (21). Diffused peak is observed at d-spacing of 4.46 Å,
corresponding to the packing of alkoxy chains. For the compounds under study (2/n8 &
2
/n10) there are three hexacatenars that form a columnar slice. The thermal behavior of the
compounds 2 (n=6→16) in cooling cycleis summarized in bar graph (Fig. 8). From the bar
graph, we found the significant decrease in melting temperature with the increase in chain
length (n= 8→16). The mesophase stability is almost similar for the compounds 2 (n=6, 8)
and slightly decreases from n=10 to 16.
Thus the two classes of molecules show differences in their thermal behavior based
on the linking group. The ester based hexacatenars (1) exhibited a cross-over from Col phase
r
to Col phase with an increase in chain length, while the ether based hexacatenars (2)
h
exhibited solely Col phase irrespective of their chain length. The bent structure provided by
r
the ester linking group allows the molecules to pack with certain restrictions, and due to this,
the intermolecular H-bonding interactions are fragile and get perturbed with the increase in
the terminal chain length from 1n/10 to 1n/12. However, in the ether series, the methylene
linking group is flexible and can orient in any direction, without allowing the dilution of

ACCEPTED MANUSCRIPT
intermolecular H-bonding interactions, thus the entire series exhibit Col phase irrespective of
r
the chain length (Fig. 9a).
Fig. 8. Bar graph showing the thermal behavior of the compounds (2/n6→2/n16) with ether linkage in cooling
cycle.
(a)
(b)
37.71 Å
53.1 Å
1n/10
2n/10
Fig. 9. Chemical structure showing the bond rotation of ester and ether based hexacatenars (a); Energy
minimized molecular models of hexacatenars 1n/10 and 2n/10 from a molecular mechanics (MM2) method.
2
.3. Photophysical properties
To see the effect of solvent polarity and structural changes on the optical properties of the
ester and ether based polycatenars, we have measured the absorption and fluorescence
spectra. Due to the little effect on the optical properties by the length of the alkoxy tails, we
have chosen compounds 1/n10 and 2/n10 as representatives. The absorption and fluorescence
spectra of the compounds 1/n10 and 2/n10 were recorded in micromolar chloroform and
butanol solutions. The absorption spectra of the compound 1/n10 showed a single absorption
maximum at 308 nm in both the butanol and chloroform solutions, where as the absorption
spectra of the compound 2/n10 showed absorption maximum at 317 nm in chloroform
solution and 319 nm in butanol (Fig. 10a, c). The ester based polycatenar 1/n10 in chloroform
and butanol showed an emission band centered at 397 nm with a Stokes shift of 89 nm. In
contrary, the compound 2/n10 exhibited a red shifted emission maximum at 383 nm with a

ACCEPTED MANUSCRIPT
Stokes shift of 66 nm (Fig. 10b, d). This may be due to the electron donating nature of ether
linkage in comparison to the ester linkage.
Fig. 10. Absorption spectra of the compounds 1/n10(a) and 2/n10(b); Emission spectra of the compounds
1
/n10(c) and 2/n10(d) (red trace: in butanol; black trace: in chloroform).
2
.4. Gelation studies
The gelation property of the compounds 1/n10 and 2/n10 were examined in different organic
solvents at different concentrations, the results are listed in Table 3. The organic solvent
containing 1/n10 and 2/n10 were heated in a sample vial until the solid was completely
dissolved. Then the solution was cooled at room temperature. Gel formation was observed
during the cooling process, which was confirmed by the inversion of the glass vial (Fig. 11a).
As shown in Table 3, 1/n10 and 2/n10 show good gelation properties in n-butanol. Among
these compounds, compound 1/n10 showed critical gelation concentration (CGC) of 0.8 wt%
in n-butanol while 2/n10 showed CGC of 2.1 wt% in n-butanol. Gelators which form stable
gels at less than 1 wt% concentration are termed as supergelators [58-61]. Strong
intermolecular interactions like H-bonding are required for such type of stable gel [62]. 2/n10
showed higher CGC possibly due to the flexible ether (-O-CH ) linking group in comparison
2
to the semi-flexible ester (-C(=O)O-) group in 1/n10. This is interesting because the ester
group in 1n/10 provides an additional site for intermolecular hydrogen bonding with the

ACCEPTED MANUSCRIPT
solvent (1-butanol), while the same is lacking in the case of 2n/10. Further, the ether-based
polycatenars exhibit a linear structure in comparison to the ester-based polycatenar (Fig. 9b).
Because of this the CGC observed for 2n/10 is almost double than that of 1/n10.
Table 3.Gelation behavior of 1/n10 and 2/n10
Entry
Solvent
1/n10
2/n10
Properties CGC (wt.%) T (°C) Properties CGC (wt.%)
T (°C)
gel
gel
1
2
3
4
1-Butanol
Decane
Dodecane
Hexadecane
G(O)
0.8
---
---
---
48
---
---
---
G(O)
2.1
---
---
---
40
---
---
---
S
S
S
S
S
S
5
6
7
8
9
Toluene
m-Xylene
DCM
Chloroform
Cyclohexane
DMSO
S
S
S
S
S
P
P
----
----
----
----
----
---
---
---
---
---
---
---
---
S
S
S
S
S
P
P
----
----
----
----
----
---
---
---
---
---
---
---
---
1
1
0
1
Methanol
----
----
G = stable gel; P = precipitate; O = opaque; S =Soluble; CGC= Critical gelation concentration (wt. %) is
the minimum concentration necessary for gelation. Tgel (°C) is the thermal stability of the gels at CGC.
Fig. 11. Solution and gel forms of the compounds 1/n10 and 2/n10 (a); FESEM image of the xerogel of the
compounds1/n10(b) and 2/n10(c).
In order to obtain a visual insight into the morphologies of the molecular aggregation mode,
the microstructures of the gel of 1/n10 and 2/n10 were examined with field emission
scanning electron microscopy (FESEM), as shown in Fig. 11b, c. The compounds were made
soluble in n-butanol at their CGC. A small drop of this solution was placed on a glass slide
and kept aside to form a very thin layer of stable gel. Later these films were dried in vacuum
which kept the structure intact. Complete evaporation of the solvent led to the formation of
xerogels, which were characterized by FESEM. The images of the 1/n10 showed the highly
entangled network nanofibers of several micrometers length in comparison to the 2/n10 (Fig.
1
1b, c).

ACCEPTED MANUSCRIPT
3
. Conclusion
Mesomorphic properties of two new classes of mono substituted aroylhydrazones based
hexacatenars, which differ from each other with the type of linking unit (ester and ether) have
been studied. The length of the alkoxy chain was varied from n-hexyloxy to n-hexadecyloxy.
All the hexacatenars stabilized columnar self-assembly. For the ester-based hexacatenars, the
lower homologues stabilized a columnar phase with a rectangular symmetry, while the higher
homologues exhibited a columnar phase with hexagonal symmetry. Interestingly, one of the
ester-based hexacatenar exhibited a room temperature columnar rectangular phase. This
mesophase crossover phenomenon with the increase in length of terminal alkoxy chains may
be due to the weakened core-core interaction, that is assisted by the intermolecular H-bond
…
interaction (-C=O H-N-), which eventually leads to the formation of Col phase from Col
h
r
phase in such systems. The bent structure provided by the ester linking group allows the
molecules to pack with certain restrictions, and due to this, the intermolecular H-bonding
interactions are fragile and get upset with the increase in the terminal chain length. In
contrast, ether based hexacatenars solely exhibited a columnar rectangular phase regardless of
the chain length. This may be due to the flexibility provided by the methylene linking group
that can orient in any direction, without allowing the dilution of intermolecular H-bonding
interactions, irrespective of the chain length. The effect of the molecular structure is also
witnessed from the critical gelation concentration required for the organogels formed by these
two classes. The representative ester based hexacatenar formed a stable gel at lower critical
gelation concentration due to the ester linkage, which can take part in hydrogen bonding and
a bent structure. The representative ether based hexacatenar formed a stable gel at almost
double critical gelation concentration in comparison to the ester based hexacatenar, due to the
lack of extra hydrogen bonding site and linear molecular structure. This study thus reveals the
minute interplay of the structural characteristics that affect the liquid crystalline and
organogels supramolecular self-assemblies. Such H-bonded liquid crystals have potential in
the preparation of liquid crystalline physical gels and related applications.
Acknowledgements
Hemant Kumar Singh is grateful to DST, SERB for providing fellowship under N-PDF
(PDF/2016/001706) scheme. BS is grateful to DRDO, India (Project No.
ERIP/ER/1203053/M/01/1542) for providing financial assistance.

ACCEPTED MANUSCRIPT
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Highlights:
Two series of aroylhydrazone based polycatenars were synthesized and their self-assembly and
organogelation are reported.
In ester based series on moving from n-decyloxy to n-dodecyloxy chain, a transition from Col to


r
Col was noticed.
h

The critical gelation concentration of ester-based hexacatenar was almost half of that observed for
ether-based hexacatenars.
From these studies we can understand the importance of the linking group on self-assembly in bulk
as well as in presence of solvents.
