Bent shaped H-bonded mesogens derived from 1, 5-bis (4-hydroxyphenyl) penta-1, 4-dien-3-one: Synthesis, photophysical, mesomorphism and computational studies

Article abstract of DOI:10.1016/j.molliq.2014.05.007

The synthesis, characterization, photophysical and mesomorphism of new hydrogen bonded (H-bonded) bent shaped mesogens derived from 1, 5-bis (4-hydroxyphenyl) penta-1, 4-dien-3-one are reported. The H-bonded bent shaped mesogens are formed by the complexation of 1: 2 molar ratio of 1, 5-bis (4-hydroxyphenyl) penta-1, 4-dien-3-one as bending unit and polar nitrile terminated ligands containing different linking groups (viz., imine (CHN), azo (NN), ester (COO)) as side wing. The transition temperatures and phase characterization have been investigated by differential scanning calorimetry and polarized optical microscopy. The bent core unit is fluorescent and non-mesogenic, whereas, all the nitrile terminated polar ligands are non-fluorescent and displayed enantiotropic smectic A phase. The H-bonded complexes are fluorescent and mesogenic. Nematic phase is induced in two H-bonded complexes whereas one of the H-bonded complexes displayed unidentified B_x phase. The computation studies are performed to obtain optimized stable molecular structures of the compounds, to investigate their electronic transitions and compared with experimental results.

Full text of DOI:10.1016/j.molliq.2014.05.007

Journal of Molecular Liquids 197 (2014) 226–235
Contents lists available at ScienceDirect
Journal of Molecular Liquids
journal homepage: www.elsevier.com/locate/molliq
Bent shaped H-bonded mesogens derived from 1, 5-bis
(4-hydroxyphenyl) penta-1, 4-dien-3-one: Synthesis, photophysical,
mesomorphism and computational studies
⁎
Manoj Kr. Paul , Popita Paul, Sandip Kumar Saha, Sudip Choudhury
Department of Chemistry, Assam University, Silchar, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The synthesis, characterization, photophysical and mesomorphism of new hydrogen bonded (H-bonded) bent
shaped mesogens derived from 1, 5-bis (4-hydroxyphenyl) penta-1, 4-dien-3-one are reported. The H-bonded
bent shaped mesogens are formed by the complexation of 1: 2 molar ratio of 1, 5-bis (4-hydroxyphenyl)
penta-1, 4-dien-3-one as bending unit and polar nitrile terminated ligands containing different linking groups
(viz., imine (\CH_N\), azo (\N_N\), ester (\COO\)) as side wing. The transition temperatures and
phase characterization have been investigated by differential scanning calorimetry and polarized optical micros-
copy. The bent core unit is fluorescent and non-mesogenic, whereas, all the nitrile terminated polar ligands are
non-fluorescent and displayed enantiotropic smectic A phase. The H-bonded complexes are fluorescent and
mesogenic. Nematic phase is induced in two H-bonded complexes whereas one of the H-bonded complexes
displayed unidentified Bx phase. The computation studies are performed to obtain optimized stable molecular
structures of the compounds, to investigate their electronic transitions and compared with experimental results.
© 2014 Elsevier B.V. All rights reserved.
Received 10 December 2013
Received in revised form 30 April 2014
Accepted 7 May 2014
Available online 28 May 2014
Keywords:
Mesogens
H-bonded mesogens
Nematic phase
Fluorescence
DFT studies
1. Introduction
[4]. In the past few decades, the application of the H-bonding in the for-
mation of the mesogenic materials has been rapidly developed and
Hydrogen bonding is an important non-covalent interaction in polar
fluids [1] (viz., water), in biological systems (viz., DNA, amino acids etc.)
and in mesogen formation [2]. Recently, H-bonding is defined as “The
hydrogen bond is an attractive interaction between a hydrogen atom
from a molecule or a molecular fragment X–H in which X is more electro-
negative than H, and an atom or a group of atoms in the same or a different
molecule, in which there is evidence of bond formation” [3]. Mesogens are
considered as complex soft matter due to various molecular interactions
and forces involved in the formation of mesophases. H-bonding is one of
the typical non-covalent molecular interactions responsible for the
organization of molecules in mesogens. This molecular interaction in
mesogens could be introduced in two general ways: i) by the introduc-
tion of appropriate functional groups, e.g., hydroxyl group or amide
groups in the ortho position to an azomethine group; ii) through
the complexation of two different components, in which one plays
the role of a H-donor (e.g., a carboxylic acid) and the other acts as a
H-acceptor (e.g., pyridine derivatives). H-bonding in a liquid crystal
generates great interest for basic research to understand the interesting
functional properties (viz., chirality) and structures (viz., helicity). How-
ever, H-bonding between the mesogens and the dissolved dichroic dye
is able to enhance the performance of the liquid crystal displays (LCDs)
opened new area of “supramolecular mesogens” [5–7]. Supramolecular
mesogens are molecular complexes generated from the complexation
of the molecular species (H-atom donor and H-acceptor species)
through H-bonding interactions. The H-atom donor and acceptor spe-
cies may be either mesogenic or non-mesogenic which leads to the for-
mation of a supramolecular mesogens. The mesogenic properties and
stabilities of supramolecular mesogens can be modified by different na-
ture of proton donors and acceptors and the position of the H-bonding
unit within the supramolecular aggregates [8]. Therefore, over past
few decades, different H-donors and acceptors moieties have been
explored to generate and stabilize new mesogenic phases [9–14].
Most of the H-bonded mesogens studied are based on the rod-like inter-
molecular H-bonding [15–19]. The discovery of the polar order and
macroscopic chirality in an achiral bent core mesogens has attracted
the attention of researchers. Recently, a few reports have been pub-
lished, which described bent shaped compound possessing intra-or
inter molecular hydrogen bonding. The intra-molecular H-bond is
formed between the H-atom of the hydroxyl group and N-atom of the
imine linkage which, in turn, promotes the stability of the imine linkage
and the generation of the new mesophases [20–23]. The intermolecular
H-bonding appeared in different combination of H-donor (e.g., deriva-
tive of the carboxylic acid) with H-acceptor (e.g., derivative of
the pyridine). In 1992, Kato et al. reported first H-bonded bent core
mesogen, originated from the interaction of pthalic acid or isopthalic
⁎
Corresponding author. Tel.: +91 3842270879.
E-mail address: paulmanojaus@gmail.com (M.K. Paul).
http://dx.doi.org/10.1016/j.molliq.2014.05.007
0167-7322/© 2014 Elsevier B.V. All rights reserved.

M.K. Paul et al. / Journal of Molecular Liquids 197 (2014) 226–235
227
acids (H-donors) with stilbazole derivatives (H-acceptors) [24] as
shown in Fig. 1a. Recently, Gimeno et al. have reported new bent shaped
H-bonded complexes of stilbazole derivatives and carboxylic acids,
which act as H-acceptors and H-donors respectively shown in Fig. 1b–
d exhibiting B2 phase characterized by its tilted lamellar and polar pack-
ing [25]. Chen et al. have reported new H-bonded bent-core mesogens
bearing a flexible carbosilane chain exhibiting only SmCPA mesophase
[26] whereas branched terminal siloxane unit displaying tilt smectic
(SmCG) phases with highly ordered layer structures [27] as presented
in Fig. 1e and f. Recently, mesogenic H-bonded supramolecular com-
plexes were prepared by non-mesogenic proton donor isophthalic
acid and mesogenic proton acceptor is a pyridine-based derivative
having different terminal groups as presented in Fig. 1g. The complexes
display nematic (N) and smectic A (SmA) phases [28,29]. Lin et al. have
reported photo-luminescent hydrogen-bonded liquid crystalline
trimers constructed by the complexation of two complementary
components containing various bi-functional photoluminescent accep-
tor cores and mono-functional proton donors (in a 1:2 molar ratio)
exhibiting N, SmA, SmC and SmX phases [30]. Moreover, Barbera et al.
have reported [31] columnar liquid crystalline organizations which
arise from non-covalent interactions between melamine derivative
and banana-like molecules. Shi et al. reported that bent core H-
bonded molecules enhance the stability of the blue phase due to the
configuration and molecular structure of bent shaped H-bonded mole-
cules [32]. 1, 5-Bis- (4-hydroxyphenyl) penta-1, 4-dien-3-one is widely
used for the biological activities [33] and nonlinear optical materials
[34]. To the best of our knowledge, 1, 5-bis- (4-hydroxyphenyl) penta-
1, 4-dien-3-one has not been used as the bent core unit for the bent
core mesogens. This new bent core unit has been chosen due to its di-
pole moment along carbonyl group and extended π-conjugation. More-
over, the bent core is fluorescent, non-mesogenic and H-atom
donor. In this article, we report synthesis, characterization,
mesomorphism and computational studies of new H-bonded bent
shaped mesogens.
2. Experimental details
2.1. Materials
All the chemicals were procured from Aldrich (USA), Tokyo Kasei
(Japan), and E-Merck (Germany) and have been used without further
purification. The solvents used (viz., acetone, ethanol, chloroform, hex-
ane, pyridine, acetonitrile, dichloromethane) are purified by standard
procedure well documented in literature. Silica gel G [E-Merck]
a
b
c
d
e
f
g
Fig. 1. Few H-bonded bent shaped molecules reported in literature.

228
M.K. Paul et al. / Journal of Molecular Liquids 197 (2014) 226–235
was used for thin layer chromatography (TLC). Silica (60–120 mesh)
Spectrochem was used for column chromatographic separation and
silica gel G (E-Merck, India for TLC). Infrared spectra were recorded on
a Shimadzu spectrometer IR Prestige 21 (νmax in cm−1) using a KBr
disk. The 1H nuclear magnetic resonance spectra were recorded on
either a JEOL AL300 FT NMR multinuclear spectrometer or Bruker
DPX-400 spectrometer in DMSO-d6 or CDCl3 (chemical shift δ in parts
per million) solution with TMS as internal standard. Micro analysis of
C, H and N elements was determined on a Perkin Elmer 2400 elemental
analyzer. UV–visible absorption spectra of the compounds in methanol
at 1 × 10−5 M concentration were recorded on a Shimadzu UV-1601PC
spectrophotometer (λmax in nm). Fluorescence spectra were recorded
with a Shimadzu RF-5301PC spectrofluorimeter with a 150 W xenon
lamp as the excitation source. The phase transition temperatures, asso-
ciated enthalpies and entropies during phase transition were recorded
using differential scanning calorimetry (DSC) (Perkin-Elmer Pyris-1
system). The mesogenic properties exhibited by the ligands and H-
bonded complexes were observed and characterized by using a polariz-
ing microscope (Nikon Optiphot-2-pol) attached with hot stage
(HCS302) and with temperature controller (STC200 from INSTEC Inc.
USA).
2.3.2. Synthesis of N-(4-n hexadecyloxysalicylidene) — 4 -cyanoaniline (B)
An ethanolic solution of (4-n-hexadecyloxy)-2-hydroxybenzaldehyde
(0.724 g, 2 mmol) was added to an ethanolic solution of 4-cyanoaniline
(0.236 g, 2 mmol). The solution mixture was refluxed with a few drops
of glacial acetic acid as catalyst for 3 h to yield the yellow Schiff's base.
The precipitate was collected by filtration and recrystallized from abso-
lute ethanol. Yield: 0.75 g (78%). IR (νmax, cm−1, KBr): 3440 (νO\H, H
bonded), 2918 (νas(C\H), CH3\ and CH2\), 2850 (νsy(C\H), CH3\ and
CH2\), 2223 (νCN, nitrile), 1627(νC_N, imine), 1602 (νC_C, alkene),
1510 (νC_C, ar), and 1253 (νC\O\Ph). 1H NMR (400 MHz, CDCl3): (δH)
13.01 (s, 1H,\OH), 8.66 (s, 1H,\CH_N\), 7.73 (d, 2H, J = 8.0 Hz,
ArH), 7.65 (d, 2H, J = 8.0 Hz, ArH), 7.54 (d, 2H, J = 8.4 Hz, ArH), 6.52
(d, 1H, J = 7.8 Hz, ArH), 6.50 (d, 1H, J = 7.8 Hz, ArH), 4.06 (t, 2H, J =
7.8 Hz, \OCH2\), 1.79 −1.26 (m, 28H, \(CH2)14\), and 0.89 (t, 3H,
J = 6.6 Hz, CH3\). Elemental analysis calculated for C30H42N2O2: C,
77.88; H, 9.15; and N, 6.05. Found: C, 78.00; H, 9.21; and N, 6.23.
2.3.3. Synthesis of 4-(n dodecyloxy)phenylazobenzonitrile (C)
To a solution of 30 ml of H2O containing hydrochloric acid (6.85 ml,
4.4 M, 30 mmol), 4-aminobenzonitrile (1.18 g, 10 mmol) was added
slowly to form a clear solution. To the resulting solution, which was
stirred and cooled to 0 °C, an aqueous cold solution of NaNO2 (0.76 g,
11 mmol) was added drop wise, maintaining the temperature of the
reaction mixture at 0–5 °C, to yield the diazonium chloride. It was
subsequently coupled with phenol (0.94 g, 0.01 mmol), which was
dissolved in 11.5 ml of aqueous 2 N NaOH (0.92 g, 23 mmol) solution.
The reaction mixture was stirred for 1 h at 0–5 °C and then allowed to
warm slowly at room temperature with stirring for over 1 h. The
resulting yellow precipitate was filtered and washed with H2O several
times. The crude product was dissolved in CH2Cl2 and dried over
Na2SO4. After the removal of the solvent under reduced pressure, the
4-(4 -hydroxy)azobenzonitrile was recrystallized from ethanol to give
a single spot on thin layer chromatography. To a suspension of 4-(4 -
hydroxy)azobenzonitrile (2.23 g, 10 mmol) in acetonitrile, K2CO3 (6.9 g,
50 mmol) and catalytic amount of tetra-n-butylammonium bro-
mide (TBAB) were added and refluxed for 10 h after adding 1-
bromododecane (2.5 ml, 10 mmol). The mixture was poured into
ice-cold water. After the filtration, the crude product was purified
by column chromatography (silica gel, eluent as petroleum ether/
ethyl acetate, 97:3v/v) followed by recrystallization from ethanol to ob-
2.2. Computational details
To study details about the origin of the electronic transitions,
the core (A), one ligand (D) and H-bonded complexes (A:2D) were
computationally analyzed as a representative cases. All calculations
were performed with GAUSSIAN 03 package on BRAF supercomputing
environment. The geometries of the core, ligand (D) and H-bonded
complexes (A:2D) were fully optimized at Becke's three-parameter hy-
brid exchange functional [35] coupled with the Lee–Yang–Parr nonlocal
correlation functional (B3LYP) [36] density functional level of theory
using 6-311++g(d,p). The electronic transitions of the compounds
were studied computationally under Time-Dependent Density Func-
tional Theory (TD-DFT) using B3LYP (solvent free) functional. This
functional has been reported to provide reasonable estimate of transi-
tion energies of different types of compounds [37].
2.3. Synthesis and molecular structural characterization
tain the pure product as orange solid. Yield: 3.3 g (84%). IR (νmax, cm−1
,
KBr): 2914 (νas(C\H), CH3\ and CH2\), 2848 (νsy(C\H), CH3\ and
CH2\), 2220 (νCN, nitrile), 1600 (νC_C, alkene), 1498 (νN_N),
1581(νC_C, ar), and 1249 (νC\O\Ph). 1H NMR (400 MHz, CDCl3): (δH)
8.16 (d, 2H, J = 7.8 Hz, ArH), 7.91 (d, 2H, J = 8.0 Hz, ArH), 7.86 (d,
2H, J = 7.8 Hz, ArH), 7.21 (d, 2H, J = 8.0 Hz, ArH), 4.08 (t, 2H, J =
7.8 Hz, \OCH2\), 1.79–1.26 (m, 20H, \(CH2)10\), and 0.91 (t, 3H,
7.6 Hz, CH3\). Elemental analysis calculated for C25H33N3O: C, 76.69;
H, 8.49; and N, 10.73. Found: C, 76.86; H, 8.55; and N, 11.02.
The synthetic procedure for the bent core unit, 1, 5-bis (4-
hydroxyphenyl) penta-1, 4-dien-3-one (A), nitrile terminated ligands
N-(4-n-hexadecyloxysalicylidene)-4/cyanoaniline (B), 4-(n dodecyloxy)
phenylazobenzonitrile (C), 4-cyanophenyl-4/-(n-dodecyloxy)benzoate
(D) and their corresponding bent shaped H-bonded complexes are
presented in Scheme 1.
2.3.1. Synthesis of 1, 5-bis (4-hydroxyphenyl) penta-1, 4-dien-3-one (A)
20 ml of conc. hydrochloric acid was added to a mixture of 6.1 g
(50 mmol) of 4-hydroxybenzaldehyde and 3.09 g (50 mmol) of boric
acid (H3BO3) in a 250 ml of round bottom flask. The reaction mixture
was cooled at room temperature. 2.3 ml (30 mmol) of acetone was
added dropwise to this mixture with vigorous stirring at room temper-
ature for 48 h. Then mixture was poured into 500 ml of ice-cold water.
The product was filtered, washed with distilled water and dried at room
temperature. The resulting compound was recrystallized from metha-
nol to yield yellow compound. Yield: 3.50 g (65%). IR (KBr, cm−1):
3340, 3190 (νO\H), 1593 (νC_O), 1593 (νC_C, alkene), 1280 (νC\O\Ph),
and 1508(ν C_C, ar). 1H NMR (400 MHz, DMSO-d6): (δ H) 7.85 (d, 2H, J =
14 Hz,\CH_CH\), 7.61(d, 4H, J = 8.4 Hz, ArH), 7.06 (d, 2H, J = 14 Hz,\
CH_CH\), 6.68 (d, 4H, J = 8.0 Hz, ArH), and 5.45 (s, 2H,\OH).
Elemental analysis calculated for C17H14O3: C, 76.68; and H, 5.30.
Found: C, 76.78 and H, 5.41.
2.3.4. Synthesis of 4-cyanophenyl-4/-(n-dodecyloxy)benzoate (D)
4-n-dodecyloxybenzoic acid (3.0 g, 10 mmol) was dissolved in
thionyl chloride and refluxed for 2 h. The excess thionyl chloride was
removed under reduced pressure and the resulting compound was
dried under vacuum. The resulting 4-n-docecyloxybenzoyl chloride
and 4-hydroxybenzonitrile (1.90 g, 15 mmol) were dissolved in dichlo-
romethane (30 ml), an aqueous solution of K2CO3 (2.76 g, 20 mmol)
was added. The resulting solution was vigorously stirred for 24 h after
adding a catalytic amount of TBAB. After the stirring was complete,
the organic layer was separated, washed several times with the alkaline
solution and water and dried over anhydrous sodium sulfate. The
evaporation of the solvent gives the crude product which was purified
by column chromatography (silica gel, eluent petroleum ether/ethyl
acetate, 97:3v/v) followed by recrystallization from ethanol to obtain
the pure product as white solid. Yield: 3.1 g (79%). IR (νmax, cm−1
KBr): 2918 (νas(C\H), CH3\ and CH2\), 2850 (νsy(C\H), CH3\ and
,

M.K. Paul et al. / Journal of Molecular Liquids 197 (2014) 226–235
229
CH2\), 2223 (νCN, nitrile), 1741(νC_O, ester), 1602(νC_C, alkene),
1510(νC_C, ar), and 1253(νC\O\Ph)
1H NMR (400 MHz, CDCl3):
confirmed by thin layer chromatography, elemental analysis and FTIR
experiments.
.
(δH) 8.13(d, 2H, J = 8.4 Hz, ArH), 7.96 (d, 2H, J = 8.0 Hz, ArH),
7.56 (d, 2H, J = 7.8 Hz, ArH), 7.13 (d, 2H, J = 8.0 Hz, ArH), 4.06 (t,
2H, J = 7.8 Hz, \OCH2\), 1.78–1.23 (m, 20H, \(CH2)10\), and
0.89 (t, 3H, 7.6 Hz, CH3\). Elemental analysis calculated for
C26H33NO3: C,76.62; H, 8.16; and N, 3.44. Found: C, 76.91; H, 8.46;
and N, 3.97.
A:2B Yield: 0.56 g (94%). IR (νmax, cm−1, KBr): 3336, 3188 (νO\H, H-
bonded), 2916 (νasy(C\H), CH3\ and CH2\), 2848 (νsy(C\H)
CH3\ and CH2\), 2228 (νCN, nitrile), 1591(νC_C, aromatic and
CH_N, alkene and imine), 1510 (νC_C, ar), and 1257 (νC\O\Ph).
,
\
Elemental analysis calculated for C77H98N4O7: C, 77.61; H, 8.29;
and N, 4.70. Found: C, 78.51; H, 8.58; and N, 5.63.
A:2C Yield: 0.5 g (95%). IR (νmax, cm−1, KBr): 3331, 3190 (νO\H, H-
bonded), 2916 (νasy(C\H), CH3\ and CH2\), 2223 (νCN, nitrile),
1496 (νN_N), and 1249 (νC\O\Ph). Elemental analysis calculated
for C67H80N6O5: C, 76.68; H, 7.68; and N; 8.01. Found: C, 77.25; H,
8.03; and N; 8.89.
2.3.5. Synthesis of bent shaped H-bonded complexes
The bent shaped H-bonded complexes were prepared by dissolving
the precise 1:2 molar proportions of the proton donor 1, 5-bis (4-
hydroxyphenyl) penta-1, 4-dien-3-one (A) and the proton acceptor
ligands (B), (C), (D) in dry pyridine, followed by ultrasonication for
30 min at 50 °C. The resulting mixture was allowed for few days in a
vacuum desiccator for the solvent to evaporate slowly to yield the
desired H-bonded bent shaped complexes, (A:2B), (A:2C) and (A:2D)
respectively. The synthesis of the H-bonded bent shaped complexes
is shown in Scheme 1. The formation of H-bonded complexes was
A:2D Yield: 0.51 g (94%). IR (νmax, cm−1, KBr): 3338, 3187 (νOH, H-
bonded), 2920 (νas(C\H), CH3\ and CH2\), 2850 (νsy(C\H)
,
CH3\ and CH2\), 2231 (νCN, nitrile), 1734(νC_O, ester),
1602(νC_C, alkene), 1510 (νC_C, ar), and 1253(νC\O\Ph).
Elemental analysis calculated for C69H80N2O9: C, 76.64; H, 7.46;
and N, 2.59. Found: C, 77.34; H, 8.11; and N, 3.73.
Scheme 1. Synthetic details of compounds A, B, C, D, A:2B, A:2C and A:2D. Reagents and conditions (i) HCl, H3BO3, stirring for 48 h at room temperature; (ii) 2–3 drops glacial acetic acid,
EtOH, reflux, 3 h; (iii) aq. HCl, NaNO2, phenol, NaOH; (iv) C12H25Br, dry acetonitrile, K2CO3, TBAB (catalytic amount), reflux 10 h; (v) SOCl2, Reflux; (vi) 4-cyano phenol, DCM, aq. K2CO3,
TBAB (catalytic amount), stir for 24 h; (vii) A, Dry Pyridine, ultrasonication for 30 min, 50 °C; (viii) A, Dry Pyridine, ultrasonication for 30 min; (ix) A, Dry Pyridine, ultrasonication for
30 min.

230
M.K. Paul et al. / Journal of Molecular Liquids 197 (2014) 226–235
Fig. 2. FTIR spectra of core(A), ligand (D) and H-bonded complex (A:2D).
3. Results and discussion
3.2. FTIR studies
3.1. Synthesis and characterization
The IR spectra of bent core (A), polar ligands (B), (C), (D) and the H-
complexes, A:2B, A:2C and A:2D were recorded at room temperature in
solid KBr. The spectra of representative compounds (A), (D) and (A:2D)
are presented in Fig. 2 and the spectra of other H-bonded compounds
(A:2B and A:2C) are presented in ESI 1E. The spectra showed a sharp
peak ~2224 cm−1 for the ν(CN) stretching mode of (B), (C) and (D).
The core (A) showed a broad peak ~3340 and 3190 cm−1 indicates
the presence of H-bonded phenolic (OH) group. The infrared absorption
frequencies of hydrogen bonded complexes (A:2B), (A:2C) and (A:2D)
showed the formation of σ type intermolecular hydrogen bonding
between the phenolic (\OH) of (A) and the cyano (\CN) group of
(B), (C) and (D). On the formation of H-bonding complexes, ν(CN)
mode of (B), (C) and (D) shift to higher frequency (i.e. Blue shift) ca.
9 cm−1 whereas ν(O\H) stretching frequency of core(A) shift to
lower frequency (i.e. Red shift) is negligibly small (2 ~ 9 cm−1) [38].
The similar shift (~10 cm−1) of the ν(CN) stretching vibration reported
when the nitrogen atom of the nitrile group of benzonitrile was com-
plexed with the \OH group of the phenol and however, experimentally
no shift was observed in ν(O\H) stretching mode of phenol [39].
Therefore, such system is considered to be a complicated supramolecu-
lar system and still a “workhorse” in many laboratories across the world.
The bent core unit (A) was synthesized by aldol condensation reac-
tion of 4-hydroxybenzaldehyde with acetone in the presence of boric
acid. The formation of the (A) was characterized by FTIR. The presence
of the band at 1593 cm−1 corresponds to aliphatic alkene group (C =
C) and further the absence of the band ~1700 cm−1 due to the presence
of aldehydic group confirms the bent core unit (A). The polar nitrile
terminated ligands (B) were prepared by the condensation of (4-n-
hexadecyloxy)-2-hydroxybenzaldehyde with 4-cyanoaniline in the
presence of a few drops of glacial acetic acid. The compound (C) was
prepared by diazotization reaction of 4-cyanoaniline followed by the
coupling of phenol in basic medium leading to the formation of the
intermediate compound, 4-(4/-hydroxy) azobenzonitrile. Further, the
etherification of the intermediate compound with n-dodecylbromide
was carried out by dissolving potassium carbonate in acetonitrile as
solvent in the presence of catalytic amount of TBAB. The compound
(D) was synthesized by converting 4-n-alkoxybenzoic acid into 4-n-
dodecyloxybenzoyl chloride followed by phase transfer reaction of 4-
n-dodecyloxybenzoylchloride with 4-cyanophenol in the presence of
the TBAB. The formation of the linkage in polar terminated ligands
was also characterized by IR spectroscopy. The band at 1627 cm−1 indi-
cates the formation of the imine group, 1498 cm−1 for azo group and
1741 cm−1 for ester linkage of the ligands (B), (C) and (D) respectively.
The new hydrogen bonded complexes were prepared by dissolving ex-
actly 1:2 molar ratio of protons donor (A) with nitrile terminated polar
ligands (B),(C) and (D) in dry pyridine followed by slow evaporation of
the pyridine in a vacuum desiccator to yield solid compounds. The
ultrasonication technique was employed to mixing two components
(H-acceptor and H-donor). The solid samples melt, cleanly, without
the appearance of the biphasic region indicating the presence of the
stoichiometric H-bonded complexes that infer the resulting mixture of
the two components is homogenous. The formation of the hydrogen
bonding in homogenous complexes was investigated by IR spectrosco-
py and elemental analysis. The elemental analyses of the compounds
were consistent with the proposed molecular formula. The optical
studies of the H-bonded complexes were characterized by UV–visible
and fluorescence spectroscopy. The thermal and mesomorphic proper-
ties of the polar ligands and new H-bonded complexes were deter-
mined by polarizing optical microscopy and differential scanning
calorimetry.
1.0
A:2B
C
A:2C
0.8
0.6
0.4
0.2
0.0
D
A
B
A:2D
250
300
350
400
450
500
550
Wavelength(nm)
Fig. 3. UV–visible spectrum of core (A), polar ligands (B), (C) and (D) and H-bonded
complexes (A:2B), (A:2C), and (A:2D) in MeOH (c = 1 × 10−5 M).

M.K. Paul et al. / Journal of Molecular Liquids 197 (2014) 226–235
231
300
A:2D
A
800
600
400
200
0
A:2C
250
ex = 376 nm
ex = 356 nm
ex = 373 nm
ex = 365 nm
A:2B
A:2C
200
A:2D
150
100
50
0
A:2B
A
400
450
500
550
600
650
450
500
550
600
650
Wavelength (nm)
Wavelength(nm)
Fig. 4. Fluorescence spectra of core (A) and hydrogen bonded complexes (A:2B), (A:2C)
and (A:2D) in methanol (c = 1 × 10−5 M) at different excited wavelength.
Fig. 5. Fluorescence spectra of core (A) and hydrogen bonded complexes in solid state at
different excited wavelength.
The examples of such supramolecular system are rare and limited. The
change in ν(CN) stretching frequency of nitrile terminated compounds
(B, C and D) when complexes with core (A) strongly support the forma-
tion of σ type hydrogen bonding between triple bond of benonitrile
and phenol in the complexes (A:2B), (A:2C) and (A:2D). In the present
studies on the bent core, polar ligands and H-bonded complexes were
carried out in MeOH solution at the concentration of 1 × 10−5 M as pre-
sented in Fig. 4. A summary of the photophysical properties of com-
pounds in MeOH is presented in Table 1. The important features are the
large Stokes shifted emission (Δνss) of ∼8.26–9.68 eV that was observed
and the structure less broad emission band with maxima in the vicinity of
∼504 nm. All the neat ligands were non-fluorescent in nature (except B
which is weakly fluorescent). The core unit A shows moderate fluores-
cence behavior in solution but fluorescence emission intensity increases
as on complexation with polar ligands showing the same pattern in all
the complexes except A:2D. However, the solid thin film, we observed
a blue shift (~3–10 nm) indicating decrease in π electron delocalization
on H-bond formation which can be due to increase in the energy gap
between the highest occupied molecular orbital (HOMO) and lowest un-
occupied molecular orbital (LUMO) as presented in Fig. 5. Interestingly,
case, the complex (A:2D) showed vibrations ~3338 and 3187 cm−1
,
confirming the stretching of the phenolic \OH group due to intermo-
lecular hydrogen bonding [40] and the peak ~2231 cm−1 confirms the
stretching mode of the CN group [41].
3.3. Photophysical studies
The UV–visible absorption of H-bond donor (A) and H-bond accep-
tors (B), (C) and (D) and H-bonded complexes (A:2B), (A:2C) and
(A:2D) were studied in methanol (MeOH) solvent at 1 × 10−5
M
concentration to obtain information regarding absorption maxima as
presented in Fig. 3. The absorption maxima of the lowest optical transi-
tion were observed 376 nm for core A (molar extinction coefficient, ε =
53,200 mol L−1 cm−1), ~239–282 nm for all the polar ligands (B, C and
D) (ε = ~29,000–82,000 mol L−1 cm−1) and the H-bonded complexes
appear ~356–373 nm (ε = ~43,000–90,000 mol L−1 cm−1). These ab-
sorption bands with large molar absorption coefficients reflect the
π–π∗ transition of the highly π-conjugated system of the ligands and hy-
drogen bonded complexes. Interestingly, peak observed ~376 nm for
core (A) shifted to lower wavelength (Blue shift ~20–3 nm) ~ 356 nm
(A:2B), 365 nm (A:2C) and 373 nm (A:2D) indicated the formation of
H-bond complexes. The H-bond formation can, also, be rationalized
from increase in the absorption intensity. The pattern of H-bond com-
plexes remains the same for all the compounds except A:2D. Emission
Table 2
Mesomorphic phase transition temperatures (T °C), associated enthalpies (ΔH, kJ mol−1
and entropy (ΔS, J mol−1 K−1) for the ligands and H-bonded complexes.
)
Compounds Transition T(°C)
ΔH(kJ mol−1
)
ΔS (J mol−1 K−1
)
ΔT(°C)
-
B
Cr-SmA
SmA-Iso
Iso-SmA
SmA-Cr
Cr1-Cr2
Cr2-Cr3
Cr3-Bx
Bx-Iso
Iso-Bx
Bx-Cr
Cr-Iso
72.9
128.7
127.5
56.3
57.2
78.7
70.3
8.4
7.1
69.4
3.4
14.9
26.9
7.6
203.2
20.9
17.7
210.8
10.2
42.5
68.2
19.2
18.7
212.2
115.1
5.9
55.8
72.1
-
-
A:2B
21.5
4.8
38.8
83.5
122.3
121.6
56.3
105.8
104.2
84.8
7.4
65.3
-
69.9
43.6
2.3
13.6
23.7
-
C
Iso-SmA
SmA-Cr2
Cr2-Cr1
Cr-Iso
19.4
Table 1
38.0
66.5
0.9
A summary of the photo-physical properties of compounds in MeOH.a.
-
83.9
-
A:2C
D
105.9
105.4tm
102.8
81.6
51.8
68.8
74.0
73.0
50.9
42.5
99.7
263.1
Compounds
Abs. λmax (ε)
241 (22,000)
Fl. λem
Δνss
-
-
Iso-[N]
2.6
A
–
–
[N]-SmA
SmA-Cr
Cr1-Cr2
Cr2-SmA
SmA-Iso
Iso-SmA
SmA-Cr2
Cr2-Cr1
Cr1-Cr2
Cr2- SmC
SmC-Iso
Iso-[N]
4.7
12.5
289.6
35.4
75.1
8.9
21.2
-
376 (53,200)
274 (9700)
504
–
128 (9.68)
–
102.7
11.5
25.7
2.6
3.1
25.9
8.7
-
B
C
239 (29,672)
282 (82,679)
350–315 (4100)
269 (35,200)
243 (51,600)
283 (41,600)
356 (90,500)
249 (31,000)
365 (74,500)
265 (81,200)
373 (43,400)
17.0
5.2
22.1
–
–
9.0
D
A:2B
–
–
79.9
27.5
22.1
137.1
8.4
-
–
–
-
–
–
A:2D
58.1
68.8
7.3
46.6
506
–
150 (8.26)
–
10.7
25.5
0.3
A:2C
A:2D
94.3
5.0
13.8
-
-
502
–
137 (9.05)
–
94.0tm
93.7
[N]-SmC
SmC-Cr
3.3
8.9
40.4
-
-
503
130 (9.53)
53.3tm
a Abs. λmax and Fl. λem are reported in nm, and the extinction coefficient, ε is calculated in
L mol−1 cm−1. Stoke shifts (Δνss) are reported in nm and eV.
tm indicates POM values. - indicates the ΔH and ΔS values of the phase transition could not
be detected by DSC. [] indicates the monotropic phase appears only on cooling.

232
M.K. Paul et al. / Journal of Molecular Liquids 197 (2014) 226–235
Fig. 6. Polarizing optical microscopic textures of polar ligands (B) and (C) under crossed polarizer. (a) SmA phase of (B) at 127.3 °C, (b) circular focal conic fan texture of SmA with
homeotropic region of (C) at 86.0 °C.
the fluorescence emission maxima found to be red shifted (~35–23 nm)
from methanol solution compared to solid thin film which can be
assigned due to the stabilization of conformationally relaxed intermolec-
ular charge transfer (CRICT) state.
3.4.1. Ligands
The bent core (A) did not exhibit mesophase and melts directly to an
isotropic liquid at 165 °C. The H-acceptor polar nitrile terminated li-
gands (B), (C) and (D) showed mesomorphism. The compounds (B)
and (D) exhibited enantiotropic SmA phase whereas (C) displayed
monotropic SmA phase. On slow cooling the compound (B) from the
isotropic liquid, the mesophase is reflected in the form of batonnets at
127.5 °C which further coalesce to give rise to a focal conic fan texture
in which large pseudo-isotropic domain occurs due to homeotropic
alignment of the molecules indicating the phase as SmA as presented
in Fig. 6(a). The sample (C) on heating directly melts to isotropic liquid
105.8 °C. On slow cooling the sample (C) from the isotropic liquid, a
homeotropic circular fan-like texture appears at 104.2 °C which further
coalesces to give a focal conic fan texture with large homeotropic region
characterizing SmA phase as shown in Fig. 6(b). The sample (D) on
cooling from the isotropic liquid, the mesophase appeared at 73.0 °C
as reflected in the form focal conic fan texture with dark homeotropic
region characterizing the phase as SmA. Therefore, all hydrogen accep-
tor polar nitrile terminated ligands (B), (C) and (D) exhibited SmA
phase with large homeotropic (dark) region. The homeotropic region
3.4. Thermal behavior: differential scanning calorimetry and polarizing
optical microscopy
The polarized optical microscopy (POM) was performed to examine
the liquid crystalline phases and to confirm the observed phase transi-
tion temperatures by POM with the obtained by differential scanning
calorimetry (DSC). The liquid crystalline phases, phase transition tem-
peratures, enthalpy and entropy associated with the phase transitions
of the ligands and H-bonded complexes are summarized in Table 2.
All the compounds exhibited strong birefringence and fluidity in the
temperature range characteristic of mesomorphic behavior between
the endothermic and exothermic peaks recorded in DSC thermograms
in the heating or cooling cycle respectively. All the ligands exhibit
mesomorphism.
Fig. 7. Optical texture of H-bonded complex, A:2B under crossed polarizers. a) Growth of spiral germ or ribbon like texture from isotropic liquid at 121.4 °C; b) and c) the spiral germ or
ribbon texture coalesce to form circular fan-like texture at 120.0 and 115.0 °C respectively; d) more ribbon like textures were grow from the dark region and coalesce to form fan-like
texture.

M.K. Paul et al. / Journal of Molecular Liquids 197 (2014) 226–235
233
Fig. 8. (a) N droplet of (A:2C) at 105.4 °C, (b) focal conic fan texture of (A:2C) 93.0 °C, (c) N droplets of (A:2D) at 94.0 °C, (d) schlieren texture of (A:2D) at 93.2 °C.
appears in a mesophase due to the presence of the polar nitrile group
present at the end of the terminal position of the ligands. The polar ni-
trile group stuck to the surface and the hydrocarbon chain pointed on
average perpendicular to the substrates [42]. The observed mesophase
thermal range (ΔT) of SmA is 72.1 °C for (B), 19.4 °C for (C) and
22.1 °C for (D) respectively. The compound (B) exhibited a wide range
of the orthogonal SmA phase as compared to compounds (C) and (D).
This is due to the presence of lateral hydroxyl group which promotes
stability of phase. Moreover, thermal stability of the (B) is higher than
(C) and (D) due to the presence of the lateral hydroxyl group forms
an intramolecular H-bonding with imine linkage. The representative
DSC thermogram of compound D is presented in Fig. 9a. The DSC ther-
mogram of the compounds, B and C is presented in ESI 2E. Compound
D on heating exhibited three enantiotropic transitions at 51.8 °C with
an enthalpy change of 11.5 kJ mol−1, second transition at 68.8 °C with
large enthalpy change 25.8 kJ mol−1 indicating the crystal to mesophase
transition and third transition occurs at 74.0 °C with small enthalpy
2.6 kJ mol−1 indicating mesophase to isotropic liquid transition.
On slow cooling from isotropic liquid, the compound showed transition
at 73.0 °C with small enthalpy change of 3.1 kJ mol−1 confirming
the isotropic-SmA transition [43]. On further cooling SmA-crystal tran-
sition phase is observed at 50.9 °C with a large enthalpy change of
25.9 kJ mol−1. Finally, crystal–crystal transition observed at 42.5 °C
with enthalpy change of 8.7 kJ mol−1. The transition temperatures
obtained from polarizing optical microscope are in good agreement
with the DSC transition temperatures.
and will be reported elsewhere. The other two H-bonded complexes
(A:2C and A:2D) on cooling the sample from the isotropic liquid, a
monotropic nematic (N) phase are induced with SmA or SmC phase.
The H-bonded complex (A:2C) on slow cooling from isotropic liquid, a
a
28
Heating
24
20
Cooling
16
12
40
50
60
70
80
90
100
Temperature (oC)
35
b
30
25
20
haeating
3.4.2. H-bonded complexes
The H-bonded complex (A:2B) on slow cooling from the isotropic
melt, a spiral germ like or a ribbon like texture appears at 121.4 °C
(Fig. 7(a)) with highly homeotropic dark region and grow (Fig. 7(b))
which on further cooling ribbon like texture coalesces to form a focal
conic and fan shaped texture at 120 °C (Fig. 7c–d) with large
homeotropic region as presented in a set of microphotograph under
crossed polarizers (see Fig. 7). The similar textural behaviors were re-
ported as characteristics of B7 phase [44]. In this paper, we designated
such phase as unidentified Bx phase. The detailed phase characterization
by X-ray diffraction and electro-optical measurements is in progress
Cooling
40
50
60
70
80
90
100
Temperature (oC)
Fig. 9. DSC thermogram a) ligand (D); b) H-bonded complex (A:2D).

234
M.K. Paul et al. / Journal of Molecular Liquids 197 (2014) 226–235
Fig. 10. Optimized structure of (a) core (A); (b) Ligand (D) and (c) H-bonded complex (A: 2D).
nematic droplet texture appears at 105.4 °C as presented in Fig. 8a
which on further cooling a typical fan-like texture appears at 102.8 indi-
cating SmA phase as presented in Fig. 8b. The H-bonded complex
(A:2D) on slow cooling from isotropic liquid, a nematic droplet texture
appears at 94.0 °C as presented in Fig. 8c. On further cooling, the four-
brush schlieren texture observed at 93.7 °C indicates SmC phase in
Fig. 8d. The thermal range of N phase for (A:2C) 2.6 °C and 0.3 °C for
(A:2D) respectively, in addition to this mesophase range, the observed
SmA mesophase range is 21.2 °C for (A:2C) and SmC is 40.4 °C for
(A:2D). The representative DSC thermogram of compounds A:2D is pre-
sented in Fig. 9b. Compound A:2D on heating exhibited three transitions
at 51.8 °C with an enthalpy change of 7.3 kJ mol−1, second transition at
68.8 °C with large enthalpy change 46.6 kJ mol−1 indicated the crystal
to mesophase transition and third transition occurs at 94.3 °C with
small enthalpy 5.0 kJ mol−1 indicated mesophase to isotropic liquid
transition while on slow cooling only one transition observed at
93.7 °C with enthalpy change 3.3 kJ mol−1. The isotropic to N and
SmC to crystal transition were observed by POM but could not observe
by DSC. The DSC thermogram of the other H-bonded complexes (A:2B
and A:2C) is presented in ESI 3E. The transition temperatures obtained
from polarizing optical microscope are in good agreement with the
DSC transition temperatures.
are at about 50° angled to each other. The TD-DFT computation on the
core (A) revealed that four bonding orbitals (HOMO, HOMO-1,
HOMO-2 and HOMO-3) and two antibonding orbitals (LUMO and
LUMO + 1) within the energy band of −1 eV to −8 eV, to be associated
in all the transitions probable under UV and visible range. Six transitions
were detected within this range, out of which the HOMO → LUMO tran-
sition at 386 nm was with the highest oscillator strength followed by
HOMO-1 → LUMO + 1 at 271 nm. Other transitions were of very low
intensity and merged in within these two peaks. After complexation
with the H-bond donor moiety (D), there has been considerable
changed in the electronic structure of the system as shown in Fig. 11.
The TD-DFT study detected only one broad band at 387 nm arising
from the HOMO (−5.45 eV) → LUMO +2 (−1.89 eV) transition. The
LUMO and LUMO + 1 orbitals degenerate orbitals. Electronic transitions
from HOMO → LUMO (and other transitions) were found to be of neg-
ligible intensity. Although all the computed results were found to be in
very good agreement with the experimental results (see Table 3), the
transition corresponding to the higher energy band (266 nm) observed
in the experimental study could not be detected in the theoretical stud-
ies. The reason could be the complex supramolecular arrangement
formed by the real molecules through extensive H-bonding, which
could not be replicated in computer.
3.5. Computational studies
4. Conclusion
We performed time-dependent density functional theory (TD-DFT)
calculations on models of core (A), ligand (D) and hydrogen bonded
complex (A:2D) to elucidate the quantum mechanical origins of the
UV–visible spectra. The optimized structures of the representative com-
pounds are presented in Fig. 10. The optimized structures show that
the core compound maintains planarity in the structure but the two
aromatic rings do not maintain co-planarity in the ligand. The rings
A new bent shaped H-bonded liquid crystal has been synthesized
successfully by employing the ultra-sonication technique for mixing
two components in an accurate stoichiometry. The three different
linking groups (\CH_N\, \N_N\, \COO\) have been used in the
mesogenic polar ligand bearing a cyano group at the terminal position
which act as H-acceptor in order to understand the structure property
relationship on the mesophases. A new non-mesogenic fluorescent
Fig. 11. TD-DFT computed orbital diagram of the (a) core (A) and (b) the H-bonded complex (A:2D) depicting the orbitals found most responsible for the intense electronic transition.

M.K. Paul et al. / Journal of Molecular Liquids 197 (2014) 226–235
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Electronic transitions
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Core (A)
242
377
266
374
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386
–
HOMO-1 → LUMO + 1
HOMO → LUMO
–
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Acknowledgment
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