Low-temperature nematic phase in azo functionalised reactive hockey stick mesogens possessing lateral methyl group

Article abstract of DOI:10.1016/j.dyepig.2018.12.028

We report, the design, synthesis and mesomorphic behaviour of new hockey stick mesogens containing photochromic azo group and a reactive double bond. The structure of the compounds resemble the hockey stick due to unequal distribution of the phenyl rings

Full text of DOI:10.1016/j.dyepig.2018.12.028

Accepted Manuscript
Low-temperature nematic phase in azo functionalised reactive hockey stick
mesogens possessing lateral methyl group
Manoj Kumar Paul, Sandip Kumar Saha, Gayatri Kalita, Barnali Bhattacharya, Utpal
Sarkar
PII:
S0143-7208(18)32109-0
https://doi.org/10.1016/j.dyepig.2018.12.028
DYPI 7233
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 22 September 2018
Revised Date: 14 December 2018
Accepted Date: 16 December 2018
Please cite this article as: Paul MK, Saha SK, Kalita G, Bhattacharya B, Sarkar U, Low-temperature
nematic phase in azo functionalised reactive hockey stick mesogens possessing lateral methyl group,
Dyes and Pigments (2019), doi: https://doi.org/10.1016/j.dyepig.2018.12.028.
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Low-temperature nematic phase in azo functionalised reactive hockey stick
mesogens possessing lateral methyl group
Manoj Kumar Paul*1, Sandip Kumar Saha1, Gayatri Kalita1, Barnali Bhattacharya2 and Utpal
Sarkar2
1Department of Chemistry, Assam University, Silchar-788011 (India)
2Department of Physics, Assam University, Silchar-788011 (India)
Email: paulmanojaus@gmail.com
Abstract
We report, the design, synthesis and mesomorphic behaviour of new hockey stick mesogens
containing photochromic azo group and a reactive double bond. The structure of the compounds
resemble the hockey stick due to unequal distribution of the phenyl rings in the two arms of the
molecule and possess a reactive 4-n-undecenyloxy chain attached at short arm of the bent-core
molecule, while the elongated arm consists of a small methoxy group and photochromic azo
linkage between two phenyl rings. All the compounds exhibit a wide-range of enantiotropic
nematic phase at low temperature. The photo-responsive behaviour of nematic phase was
characterized by polarizing optical microscopy with in situ white light irradiation. Upon
irradiation, the nematic phase almost completely disappeared in about 20s. We have also
investigated the effect of the position of methyl group substitution at the outer phenyl ring of the
elongated arm of the molecule on the nematic phase of azo functionalised compounds. Our
theoretical calculations predict that different position of methyl group does not bring any
significant change in reactivity profile, electronic properties and spectroscopic properties.
Irrespective of the position of a methyl group, trans conformation has slightly greater stability as
well as less reactivity than cis conformation.
Keywords: Azo-functionalised mesogen; bent-core mesogens; reactive mesogens; a nematic
phase

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1. Introduction
Azo-functionalised materials are one of the attractive classes of the functional materials due to
their interesting photoswitching behaviour and have diverse potential applications viz., optical
data storage, display devices, molecular switches, nonlinear optical devices, photomechanical
systems [1-8]. In recent years, azo functionalised mesogenic materials have generated a lot of
interest due to the great demand for new multifaceted functional and smart materials. Azo
functionalised mesogenic materials have the dual properties of azobenzene (viz., photoswitching
behaviour) and mesogens (viz., anisotropy and fluidity) which in turn generate unique photo-
induced anisotropy and photo-chemical phase transitions [9-14]. In order to understand their rich
mesomorphic behaviour and their multifaceted properties, a large number of the azo
functionalised bent-core mesogens (BCMs) have been designed, synthesised, their mesomorphic
behaviours were characterized and structure-property relationships were recently studied by
several research groups [15-19] and are well documented in excellent review published recently
[20]. Mesogens containing olefinic chain can generate new functional mesogens because of their
low viscosity, low melting point, high clearing points and good low-temperature stability [21].
In addition, the double bond in an olefinic chain is reactive and can be used to prepare mesogenic
polymers [22-24] and elastomers [25-26]. The photochromic azobenzene mesogens containing a
reactive double bond were reported in the past decade [27-29]. Since the discovery of
macroscopic polar order and chiral superstructure in the mesophase of achiral bent-core
mesogens (BCMs) is a well-established subclass of thermotropic mesogens [30-35]. The
mesophases (viz., B1-B8) displayed by the BCMs are different from the rod-shaped mesogens in
term of their physical properties and their fascinating optical textures (B1-B7). However, some of
the BCMs exhibited similar phases as displayed by the rod-like mesogens. Interestingly of these,
nematic phase exhibited by the bent-core mesogens drags special attention and one of the active
topics of debate due to their unusual physical properties like huge flexoelectricity[36], unusual
dielectric relaxation [37], unusual electro-convection pattern [38], magnetic field induced
birefringence [39] and interestingly ferro-nematics [37,40]. The unusual physical properties of
the nematic phase exhibited by the bent-core mesogens which are expected due to the presence
of nanometer-sized polar smectic clusters in the nematic phase so-called cybotactic nematic
phase [41-46]. The formation of the nanometer-sized polar smectic cluster in the nematic phase
of bent-core systems may be due to kink geometry of the molecules which restricts the free

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rotation and core-core interactions between the molecules promotes the layer formation due to
the segregation between extended aromatic moieties and end alkyl chains along the orientational
direction of the long axes. Moreover, over four decades back, theoretically, it was predicted that
the shape anisotropy of the low molar mass bent-core molecule is an ideal criterion for the
biaxiality in the nematic phase. The biaxial nematic phase has an advantage over uniaxial
nematic phase due to fast switching response and has potential application in the new generation
of ultrafast and wide-angle display devices. Since then, researchers have made several attempts
to design and synthesized new BCMs exhibiting nematic phase and claimed biaxiality in the
nematic phase and result ended without success [20, 47-48]. In the recent years, four ring bent-
core mesogens so-called hockey stick shaped mesogens have attracted considerable attention due
to their rich mesomorphism [49-52]. Interestingly, it is noted from the literature that most of the
hockey stick molecules containing azo linkage exhibit nematic phase [53-57]. Hockey stick
shaped mesogens containing functionalised azo-benzene moiety are sensitive to light [58] due to
photo-isomerization i.e., cis (Z-isomer) to trans (E-isomer) isomer which change the geometry of
the molecule further lead to a concomitant change in chemical and physical properties of the
molecules. Therefore, in order to understand how such structural modifications impact the
mesomorphic behaviour, we designed four ring hockey stick shaped molecule consist of three
linking groups (-N=N-, -COO- and -CH=N-) connecting the phenyl rings and lateral methyl
group as a substituent at the elongated arm of the molecule. In particular, the position of the
methyl group would yield further reduced nematic onset temperatures without exclusion of
mesomorphism [59]. We report here the synthesis and phase characterization of azo
functionalised reactive hockey stick mesogens and study the influence of lateral methyl
substitution on meosmorphic behaviour, photoswitching and their reactivity.
Experimental
The chemicals used for the synthesis of the hockey stick compounds were procured from M/s
Alfa Aesar or Aldrich or Tokyo Kasei Kogyo Co. Ltd. The solvents and reagents are of
analytical reagent grade and were distilled and dried before use. Elemental analysis was
performed on Carlo-Erba 1106 elemental analyzer. FT-IR spectra were recorded on a Shimadzu
IR Prestige-21 over the 400-4000 cm-1 spectral range in KBr pellets. The 1H NMR spectra were
recorded in CDCl3 solution either on JEOL AL300 FTNMR multinuclear spectrometer or Bruker

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Avance III-400 spectrometer (chemical shift δ in parts per million relative to tetramethylsilane
(TMS) as internal standard).
UV-visible absorption spectra of the compounds in
dichloromethane were recorded on Perkin-Elmer Lambda 35 spectrophotometer. Trans-cis
photo-isomerization was performed in situ in an experimental setup assembled with the white
halogen lamp. The phase transition temperatures, associated enthalpies and entropies were
recorded at a heating and cooling rate of 5 oCmin-1 using differential scanning calorimetry (DSC)
(Perkin-Elmer Pyris1 system). The DSC was calibrated with indium (156.6 oC, 28.4 Jg-1) and tin
o
(232.1 C, 60.5 Jg-1). The liquid crystalline properties of the compounds were characterized
using a polarizing optical microscope (POM) (Nikon optiphot-2-pol attached with hot and cold
stage HCS302, with STC200 temperature controller configured for HCS302 from INSTEC Inc.
USA).
The detailed synthesis of the intermediate compounds viz., 4-n-undecenyloxy-2-
hydroxybenzaldehyde (1), 3-amino-2-methyl benzoic acid (2), 2-Methyl-3-N-(4-n-
undecenyloxysalicylidene) aminobenzoic acid (3) was prepared following the earlier reported
procedures [55].
Synthesis of 4-hydroxy-4/- methoxy azobenzene (4a)
4-Hydroxy-4/-methoxy azobenzene was prepared following the general procedure reported
earlier [55, 60-61]. 4-methoxyaniline (2.46g, 20 mmol) were added 10 mL of distilled water
containing hydrochloric acid (12M, 2.5 mL, 30 mmol) and the mixture was stir and warm to
dissolve completely. The solution was then cooled down to 0oC ice bath. To this ice-cold
solution, a freshly prepared sodium nitrite solution (2.34g, 33 mmol) was added with stirring
keeping the temperature in between 0-5 oC. The resulting diazonium chloride was consecutively
coupled with an alkaline solution of phenol (1.88g, 20mmol) with constant stirring. The reaction
mixture was neutralized (pH= 7) by an aqueous solution of Na2CO3 and was allowed to attain
room temperature. The azo dye which formed immediately as a yellow precipitate was filtered,
washed several times with distilled water and dissolved in diethyl ether, and the resulting organic
solution dried over anhydrous sodium sulphate. The crude product obtained after removal of the
solvent under reduced pressure was purified by recrystallization from cold hexane. The
precipitate was filtered and washed with water and methanol and dried in vacuum to yield yellow
solid. m.p.: 139oC, Yield = 3.2g (70 %). FT-IR νmax in cm-1: 3421 (νO-H), 1467 (νN=N, azo). 1H
NMR (CDCl3, 300MHz): (δ in ppm) 7.87 (d, 2H, J = 8.7Hz, ArH), 7.82 (d, 2H, J = 8.4Hz, ArH),

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6.92 (d, 2H, J = 7.8Hz, ArH), 6.70 (d, 2H, J = 8.4Hz, ArH), 5.44 (s, 1H, -OH), 3.87 (s, 3H,-
OCH3). Elemental analysis calculated for C13H12N2O2: C, 68.41; H, 5.30; N, 12.27%. Found: C,
68.02; H, 5.23; N, 12.31 %.
Synthesis of 4-hydroxy-2-methyl-4/- methoxy azobenzene (4b)
2-Methyl-4-hydroxy-4/-methoxy azobenzene was synthesized following the same procedure
described for the synthesis of 4a. Starting material 4-n-methoxyaniline (2.46g, 20mmol),
hydrochloric acid (12M, 2.5mL, 30mmol), sodium nitrite (2.34g, 33mmol), o-cresol (2.16g,
20mmol), the crude product obtained after removal of the solvent under reduced pressure was
purified by recrystallization from cold hexane. The precipitate was filtered and washed with
water and methanol and dried in vacuum to yield yellow solid. m.p.: 147 oC, Yield = 3.1g (64.04
%). FT-IR νmax in cm-1: 3163 (νO-H), 1462 (νN=N, azo). 1H NMR (CDCl3, 300MHz): (δ in ppm):
(d, 2H, J = 9.0Hz, ArH), 7.66 (d, 2H, J = 8.7Hz, ArH), 6.98 (d, 2H, J = 9.0Hz, ArH), 6.86 (d,
1H, J = 8.4Hz, ArH), 5.25 (s, 1H, -OH), 3.86 (s, 3H,-OCH3), 2.31 (s, 3H, Ar-CH3). Elemental
analysis calculated for C14H14N2O2: C, 69.41; H, 5.82; N, 11.56 % Found: C, 69.43; H; 5.81; N,
11.67 %.
Synthesis of 4-hydroxy-3-methyl-4/- methoxy azobenzene (4c)
3-Methyl-4-hydroxy-4/-methoxy azobenzene was synthesized following the same procedure
described for the synthesis of 4a. Starting material 4-n-methoxyaniline (2.46g, 20mmol),
hydrochloric acid (12 M, 2.5mL, 30mmol), sodium nitrite (2.34g, 33mmol), m-cresol (2.16g,
20mmol), the crude product obtained after removal of the solvent under reduced pressure was
purified by recrystallization from cold hexane, precipitate was filtered and washed with water
and methanol and dried in vacuum to yield yellow solid. m.p.: 131 oC, Yield = 3.4g (70.39 %).
FT-IR νmax in cm-1: 3143 (νO-H), 1433 (νN=N, azo). 1H NMR (CDCl3, 300MHz): (δ in ppm) (d,
2H, J = 9.0Hz, ArH), 7.68 (d, 2H, J = 8.1Hz, ArH), 7.66 (d, 2H, J = 8.4Hz, ArH), 6.84 (d, 1H, J
= 8.4Hz, ArH), 5.15 (s, 1H, -OH), 3.86 (s, 3H,-OCH3), 2.34 (s, 3H, Ar-CH3). Elemental
analysis calculated for C14H14N2O2: C, 69.41; H, 5.82; N, 11.56%. Found: C, 69.52%; H, 5.80%;
N,11.67 %.
General procedure for the synthesis of azo-functionalised reactive hockey stick shaped
compounds
Appropriate quantity of substituted 4-hydroxy-4/- methoxy azobenzene (viz., 4-hydroxy-4/-
methoxy azobenzene (4a), 0.46g; 4-hydroxy-2-methyl-4/- methoxy azobenzene (4b) 0.48g; 4-

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hydroxy-3-methyl-4/- methoxy azobenzene (4c) 0.48g, (2.0 mmol) and 2-methyl-3-N-(4-n-
undecenyloxy-2-hydroxybenzylidene) aminobenzoic acid (3) (0.65 g, 2.0 mmol) and was
dissolved in dry dichloromethane (DCM) (50mL) and catalytic amount of 4-
dimethylaminopyridine (DMAP) (4 mg, 0.02 mmol) was added to the solution. A solution of N,
N/-dicyclohexylcarbodiimide (DCC) (0.49 g, 2.4 mmol) was added to the reaction mixture and
the mixture was stirred for 48 h under the nitrogen atmosphere at room temperature. The
precipitate of N, N/- dicyclohexylurea was removed by filtration and the solvent DCM was
evaporated to get crude product. The crude product was purified by column chromatography
using silica gel (60-120 mesh) with hexane/ chloroform (9:1) as eluent. The yellow solid was
recrystallized several times from absolute ethanol to get the pure product.
(4/-methoxy
hydroxybenzylidene)amino]-2-methyl benzoate (5a).
Yellow solid, Yield: 0.71 g (73 %). FT-IR υmax in cm-1: 3192 (υO-H H-bonded); 1741 (υ
phenyl
azo)
phenyl-4-yl
3-[N-(4/-n-undecenyloxy-2-
,
C=O
1
ester); 1610 (υC=N, imine). H NMR (400 MHz, CDCl3): (δ in ppm) 13.44 (s, 1H, -OH); 8.38 (s,
1H, - CH=N-); 7.93 (d, 2H, J = 6.8 Hz, ArH); 7.86 (d, 2H, J = 8.8 Hz, ArH); 7.30 (d, 2H, J =
8.4 Hz, ArH); 7.19-7.24(m, 4H, ArH); 6.89 (d, 2H, J = 8.4 Hz, ArH); 6.45 (s, 2H, ArH); 5.71-
5.75(m, 1H, -CH=); 4.85-4.95 (m, 2H, =CH2); 3.94 (t, 2H, J = 6.4 Hz,- OCH2-); 3.83 (s, 3H, -
OCH3); 2.61 (s, 3H, -CH3); 1.96-1.98 (m, 2H, -OCH2-CH2-); 1.71-1.75 (m, 2H, -OCH2-CH2-
CH2-); 1.24-1.39 (m, 12H, 6x–CH2-). Elemental analysis calculated for C39H43N3O5: C, 73.91;
H, 6.84; N, 6.63 %. Found: C, 73.87; H, 6.86; N, 6.61%.
(4/-methoxy
hydroxybenzylidene)amino]-2-methyl benzoate (5b).
Yellow solid, Yield: 0.74 g (75 %). FT-IR υmax in cm-1: 3192 (υO-H H-bonded); 1741 (υ
phenyl
azo)
phenyl-4-yl
3-[N-(4/-n-undecenyloxy-2-
,
C=O
1
ester); 1610 (υC=N, imine). H NMR (400 MHz, CDCl3): (δ in ppm) 13.51 (s, 1H, -OH); 8.45 (s,
1H, - CH=N-); 8.03 (d, 1H, J = 7.6 Hz, ArH); 7.92 (d, 2H, J = 8.8 Hz, ArH); 7.82 (d, 2H, J =
9.2 Hz, ArH); 7.26-7.41 (m, 4H, ArH); 7.02 (d, 2H, J = 8.4 Hz, ArH); 6.52 (s, 2H, ArH); 5.78-
5.85(m, 1H, -CH=); 4.92-5.02 (m, 2H, =CH2); 4.01 (t, 2H, J = 6.2 Hz,- OCH2-); 3.90 (s, 3H, -
OCH3); 2.69 (s, 3H, -CH3); 2.36 (s, 3H, -CH3); 2.02-2.07 (m, 2H, -OCH2-CH2-); 1.77-1.84 (m,
2H, -OCH2-CH2-CH2-); 1.25-1.46 (m, 12H, 6x -CH2-). Elemental Analysis calculated for
C40H45N3O5: C, 74.16; H,7.00; N,6.49%. Found: C, 74.27; H, 6.95; N, 6.43%.

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(4/-methoxy
phenyl
azo)
phenyl-4-yl
3-[N-(4/-n-undecenyloxy-2-
hydroxybenzylidene)amino]-2-methyl benzoate (5c).
Yellow solid, Yield: 0.76 g (77 %). FT-IR υmax in cm-1: 3192 (υO-H H-bonded); 1741 (υ
,
C=O
1
ester); 1610 (υC=N, imine). H NMR (400 MHz, CDCl3): (δ in ppm) 13.52 (s, 1H, -OH); 8.44 (s,
1H, - CH=N-); 7.98 (d, 1H, J = 7.6 Hz, ArH); 7.93 (d, 2H, J = 8.8 Hz, ArH); 7.73 (d, 2H, J = 8.4
Hz, ArH); 7.21-7.39(m, 4H, ArH); 7.02 (d, 2H, J = 8.4 Hz, ArH); 6.51 (s, 2H, ArH); 5.78-
5.85(m, 1H, -CH=); 4.92-5.02 (m, 2H, =CH2); 4.01 (t, 2H, J = 6.6 Hz,- OCH2-); 3.90 (s, 3H, -
OCH3); 2.74 (s, 3H, -CH3); 2.65 (s, 3H, -CH3); 2.02-2.07 (m, 2H, -OCH2-CH2-); 1.77-1.83 (m,
2H, -OCH2-CH2-CH2-); 1.31-1.46 (m, 12H, 6x –CH2-). Elemental Analysis calculated for
C40H45N3O5: C, 74.16; H, 7.00; N, 6.49%. Found: C, 74.18; H, 6.97; N, 6.56%.
Results and Discussion
Design and Synthesis
New hockey stick shaped molecules possess two arms of a different number of phenyl rings.
One arm consists of two phenyl rings and other have only one phenyl ring. The phenyl rings in
the hockey stick shaped molecules are connected by three linkages viz., imine linkage (-C=N-),
an ester linkage (-COO-) and photochromic azo linkage (-N=N-). The hockey stick shaped
molecule is considered as a subclass of unsymmetrical bent-core molecules with a bending angle
is in the borderline between a typical bent shaped molecule and a conventional rod-shaped
molecule. The short arm of the molecule possesses reactive 4-n-undecenyloxy moiety as a
terminal chain while small methoxy moieties at the elongated end of the molecule. Further,
asymmetry in a bent-core molecule is induced by the lateral methyl group at the outer phenyl
ring of the elongated arm of the molecule. The position of the lateral methyl group is either in
ortho position or meta position with respect to ester linking moieties. The presence of ortho
hydroxyl group in benzylidene moiety enhances the stability of the imine through intramolecular
H-bonding also enhances the transverse dipole-moment. The mesophase transition (nematic to
isotropic transition) of these materials possessing a photochromic azo group can be induced by
the trans-to-cis isomerization under the influence of UV irradiation [62]. The synthetic details of
the azo-functionalised hockey stick mesogens compounds as described in scheme 1 was carried
out following the procedure described in the experimental section.
The Williamson
etherification of 2,4-dihydroxybenzaldehyde with 4-n-undecenylbromide preferentially takes

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place at the p-hydroxyl group rather than the o-hydroxyl group which is involved in
intramolecular hydrogen bonding with carbonyl oxygen atom [63-64], to form 4-n-
undecenyloxy-2-hydroxy benzaldehyde (1). The 4-n-undecenyloxy-2-hydroxy benzaldehyde
was then condensed with 3-amino-2-methyl benzoic acid (2) to yield 3-(4-n-undecenyloxy-2-
hydroxybenzylidene)-2-methyl benzoic acid (3). The compound (3) was further esterified with
4-hydroxy-4/-methoxy azobenzene (4a) or substituted 4-hydroxy-4/-methoxy azobenzene (4b or
4c) by DCC-DMAP reaction [65] to obtain the azofunctionalised hockey stick reactive mesogens
(5a-5c). Elemental analyses of all the compounds were consistent with the proposed molecular
structure and confirm the purity of the compounds. All the compounds were characterized by
1
FT-IR and H NMR studies. The infrared spectra of all the compounds exhibited characteristic
peaks stretching vibration ca.1604-1612 cm-1 indicates the formation of the Schiff base (-CH=N-
) and the sharp peak ~1735 indicates the carbonyl (-C=O) stretching of the ester group. The
weak broad peak ~3200-3250 indicates the -O-H stretching of the hydroxyl group involved in H-
bonding. The phenolic C-O stretching vibration was observed around 1290 cm-1. 1H NMR
spectra of the compounds showed a singlet appeared in the range of δ = 8.38-8.45 ppm for imine
(-HC=N-) proton of the compound, which confirmed the formation of the Schiff base. The
lateral hydroxyl group appeared as a singlet in the range of δ =13.44 -13.52 ppm. All other
aromatic protons appeared in the range of δ = 6.45-8.03 ppm. The multiplates appeared for the
terminal vinyl group the in δ = 4.85-5.02 ppm as for =CH2 protons and δ = 5.71-5.85 ppm for -
CH= proton. The NMR spectra of the target molecules are presented in ESI (Fig. S1(a)-(c)).
Mesomorphic behaviour
The mesomorphic behaviour of the new hockey stick shaped compounds is characterised by
polarising optical microscopy (POM) and further confirmed by differential scanning calorimetry
(DSC). The phase transition temperatures, transition enthalpy and entropy of the compounds are
summarised in Table 1. It is observed from Table 1 that all the azo functionalised hockey stick
shaped compounds (5a-5c) exhibit a wide range of enantiotropic nematic phase. There is a
distinct decrease in melting and clearing temperature on substitution at the outer phenyl ring of
the elongated arm of the molecule. The temperature range of the nematic phase in a heating
process decreases upon substitution with a methyl group at the elongated arm of the molecule
whereas the nematic phase temperature window becomes broaden on cooling the sample.
Therefore, the nematic phase becomes supercooled upon substituted with a methyl group at the

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outer phenyl ring of the compounds. In the heating cycle, the compound (5a) exhibits two
endothermic peaks that correspond to the two transitions such as Cr-N and N-Iso. The large
change in enthalpy (41.88 kJmol-1) and entropy (105.7 Jmol-1K-1) value at 122.4 oC suggests the
melting temperature of the compound and indicated the first order transition of order crystals to
o
disorder nematic transition. On further heating, the sample at 161.8 C, a small change in
enthalpy (0.33 kJmol-1) and entropy (0.76 Jmol-1K-1) during the transition indicating the weak
first-order transition of nematic to an isotropic phase. In the cooling cycle, also two exothermic
transition peaks were observed at 158.8 °C with a small change in enthalpy (∆H = 0.27 kJmol-1)
indicated Iso-N transition. On further cooling, a second transition appeared at 78.4 °C with a
large enthalpy change of 22.74 kJmol-1 indicating N-Cr transition presented in Fig. 1. The
optical texture of the mesophase was analyzed by sandwiched the sample in between the
untreated glass plates and coverslip. The compound 5a, on cooling from the isotropic liquids,
exhibited schlieren textures with two brush defects (s=±1/2) and four brush defects (s=±1), a
characteristic of the nematic phase as presented in Fig. 2(a). It is interesting to note that the
optical texture of the nematic phase consists of a large number of the two brush defects which is
quite unusual in the rod-shaped mesogens [66-67]. Similar behaviour of the nematic phase
reported in the four ring bent-core mesogens [69]. Further, the presence of the homeotropic
domains in the optical texture of the nematic phase and low enthalpy change during Iso-N
transition indicating the formation of the cybotactic cluster which is commonly observed in
nematic phases exhibited by bent-core compounds [37, 52, 69]. The sample, on further cooling,
two brush schlieren texture with homeotropic domains of compound (5a) transform to a
o
crystalline phase at 78.4 C as presented in Fig. 2(b). Similar phase behaviour observed in a
methyl substituted hockey stick molecules (5b and 5c). In an ortho-methyl substituted
compound (5b) exhibits enantiotropic nematic phase. In the heating cycle, the compound 5b
also exhibits two endothermic peaks corresponds to Cr-N and N-Iso transition. The Cr-N
o
transition occurs at 117.4 C corresponds to the large change in enthalpy (∆H=42.69 kJmol-1)
and entropy (∆S=109.34 Jmol-1K-1) which indicate the first order transition whereas peak
o
observed at 128.2 C corresponds to N-Iso transition with a small change in enthalpy (∆H=0.14
kJmol-1)
,
and entropy (∆S=0.35 Jmol-1K-1) during the transition. In the cooling cycle, one peak is
o
observed at 126.9 C with the small change in enthalpy (∆H=0.36 kJmol-1)
,
and entropy
(∆S=1.39 Jmol-1K-1) corresponds to Iso-N transition. The lower value of entropy during the

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isotropic-nematic transition for these hockey stick molecules can be accounted for enhanced
molecular biaxiality [70]. The N-Cr transition was not detected in the cooling cycle (see Fig.3).
On slow cooling, the compound (5b) from an isotropic liquid, schlieren textures with two and
four brush defects observed (see Fig. 4a) which on further cooling transforms to homeotropic
texture (see Fig. 4b) indicates the formation of the cybotactic cluster in the nematic phase. On
further cooling, the thread like texture appeared from dark homeotropic texture (see Fig. 4c-d)
o
and finally crystallization occurs at 54.9 C (see Fig. 4e) which was not observed by DSC.
Similar behaviour was also observed for the compound (5c). The optical texture and the DSC
thermogram is presented in the ESI Fig. S2 and Fig. S3. The substitution of the lateral bulky
methyl group in a hockey stick shaped molecule decreases the melting and isotropic temperature
without changing the mesophase. Further, it is interesting to note that the position of the lateral
bulky methyl group plays an important role in the thermal stability of the compound. The
substitution of the methyl group in the meta-position of the ester linkage decrease the nematic
phase to room temperature whereas, the substitution of a methyl group at ortho-position of the
ester linkage decrease the nematic phase to low temperature as compared to non-substituted
compound (5a).
Photo-isomerization study
In solution
The azobenzene hockey stick compounds (5a-5c) are interesting dynamic functionalised
materials because of their trans-cis or cis-trans isomerisation processes due to external stimuli
viz., irradiation by UV light, mechanical stress, electrostatic stimulation or recovering the
thermodynamic stability. The photoswitching properties of all the hockey stick compounds were
performed in chloroform (c = 1x10-5 M) using UV-visible absorption spectrometer. The UV-
visible absorption spectra of compounds (5a-5c) are presented in Fig. 5 and Table 2. It shows
that strong absorption in the UV region ~346 nm with a large molar extinction coefficient (ε =
27038-28711 Lmol-1cm-1), which reflected the symmetry allowed π-π* transition of trans isomer
of the azobenzene chromophore. However, a weak absorption in the visible region ~ 438 nm (ε
~1346-1574 Lmol-1cm-1) corresponding to symmetry forbidden n-π* transition of cis isomer. It
is interesting to note that the substitution of the methyl group at the different position with
respect to azo and /or ester group in the bent-core mesogens did not change the electronic
structure of the molecule. Therefore, the UV-visible absorption spectra of the compounds are

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almost similar. The UV-visible absorption spectra of representative compound 5c shown in Fig.
6 were obtained in 1x10-5 M solution in CHCl3 at three different conditions viz., (a) virgin
sample solution, (b) solution after exposure to UV light (366 nm) for 30 mins and (c) solution
kept in dark for 24 h. It is noted that there is a change in the molar extinction coefficient (ε) of
the absorption band and peak positions of each compound from virgin sample solution to UV
light irradiated solution and solution kept in dark for 24h. The absorption bands at 346 nm (ε
=27349 Lmol-1cm-1) and ~438 nm (ε =1346 Lmol-1cm-1) are due to the π-π* and n-π* electronic
transitions, respectively. The freshly prepared dilute solution of the compounds when irradiated
with UV light (366 nm) for 30 minutes, the resulting UV-visible spectra illustrated that the π-π*
transition band is blue shifted (~10-12 nm) with a decrease in molar extinction coefficient (ε) of
the band for the compound (Fig. 6(b)). This is due to the fact that before the irradiation with the
UV light, the hockey stick shaped molecules are in trans (E-isomer) conformation with respect
to the azo (-N=N-) group and upon absorption of photon by the irradiation with the UV light, the
trans conformation flips to cis conformation (Z-isomer) with respect to the azo (-N=N-) group.
This, in turn, leads to the breaking down of the symmetry C2 for trans (E-isomer) conformation
to C2h for the cis (Z-isomer) conformation, indicating a less conjugation. Therefore, after UV
irradiation π–π* band shift towards the lower wavelength. However, the opposite phenomenon
occurred in the case of n–π* transition band. The molar extinction coefficient (ε) of the n-π*
band of the freshly prepared solution, i.e., the virgin solution is less than solution irradiated with
UV light. The increase in the molar extinction coefficient (ε) of the n-π* absorption band is the
transformation of the symmetry of the molecule from trans to cis isomer. When the solution was
kept in dark for 24h, the position of the π-π* and n-π* bands were gradually shifted and the UV-
visible absorption spectra appeared closer to the UV-visible absorption spectra of the virgin
solution which confirms reversible photo-switching behaviour (Fig. 6(c)). The similar behavior
is observed for the compounds 5a and 5b as presented in ESI Fig. S4 and Fig. S5, respectively.
In Liquid Crystalline thin film
The effect of trans-to-cis photoisomerization caused by the UV irradiation on the nematic phase
exhibited by the compounds was investigated. The nematic phase exhibited by the hockey stick
mesogens can respond to the UV irradiation immediately by changing their textures to isotropic
liquid and nematic phase reappeared after removing the UV source. This indicated that nematic
phase of the bent-core mesogens showed trans to cis photoisomerization under UV irradiation

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and cis to trans thermal isomerization under visible light irradiation. The azo-functionalised
bent-core compounds (5a-5c) exhibit the identical photoinduced effect. Of these, the compound
(5c) is considered as the representative example. The compound 5c was cooled from the
isotropic liquid state to 119.5 °C in the range of the nematic phase (Fig. 7a). The sample display
nematic phase was exposed to UV irradiation at 366 nm (10 mW/sec2), the schlieren textures of
the nematic phase is slowly started disappearing and finally replaced by dark isotropic liquid at
20 s (Fig 7b). After removal of the UV lamp, the schlieren texture of the nematic phase
reappeared (Fig.7). It seems that the trans-to-cis and cis-to-trans photoisomerization was
observed on the nematic phase of the compounds. Therefore, photoisomerization in a nematic
phase of the bent-core mesogens could be useful for light-sensitive displays or data storage
applications. It is interesting to note that the absorption maxima for the bent-core compounds
(5a-5c) fall outside the well-known spectral region of the halogen lamp (~600-950 nm) which is
the light source in our POM. The effect of halogen light on the nematic phase of these
compounds was studied. The sample slides were observed under the POM which is attached
with a hot stage, at varying time intervals under different light conditions. The observations
made for the sample (5c) are shown in Fig. 8. The black texture, (see Fig. 8a) is the isotropic
o
phase at a temperature of 119.5 C and the halogen light was kept dim. Now, we started
exposing the sample with a halogen lamp on increasing the intensity of the light, and Fig. (8b-d)
show the disappearance of the nematic phase as the sample was exposed to light at different time
intervals. At this stage, we kept the temperature of the sample constant (117oC) and went on
exposing the sample to bright light at different time intervals. Fig. 8e show the disappearance of
the nematic phase as the sample absorbs more and more light and it goes completely into an
isotropic state after 20 s without increasing the temperature. Again, the isotropic dark textures
become nematic schlierien texture when the intensity of the halogen light decreases (Fig. 8f).
DFT studies
Theoretical calculations (structural relaxation, spectroscopic properties and reactivity parameter)
have been done using density functional theory (DFT) method as employed in the Gaussian 09
program package [71]. M06, the hybrid functional [72] and 6-311g basis set [73-74] are
employed for the calculation with polarizable continuum solvation model (PCM) and chloroform
as a solvent. No geometry constraint has been imposed throughout the calculation. The positive
values of vibrational wavenumbers confirm that the optimized molecular structure is stable. The

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UV-visible absorption spectra have been simulated using time-dependent density functional
theory (TD-DFT). The lowest energy configurations of hockey stick shaped mesogens (both
trans and cis conformation) considering the different position of methyl group are presented in
Fig. 9. In order to see how electronic property changes with different positions of a methyl
group, we have illustrated in Fig. 10 highest occupied molecular orbital (HOMO) and lowest
unoccupied molecular orbital (LUMO) which dictate the stability, reactivity and other properties
of the compounds. As the presence of solvent does not introduce any notable change in the
position of HOMO and LUMO, we have presented it only for solvent phase. It can be seen from
the Fig. 10 that, both HOMO and LUMO are mainly located on the left arm of the compound
irrespective of the position of methyl group and conformation (cis or trans). The electron
density is extending over the entire left arm with contributions from the azo, ester group and
methoxybenzene ring. However, HOMO-1 (see ESI Fig. S6) is located at core region, imine (-
N=CH-) group and phenyl ring of right arm with a negligible contribution from the
methoxybenzene of the left arm without depending on the position of a methyl group. Whereas,
the core region, -N=C-H group and phenyl ring of the right arm, as well as ester and an azo
group of the left arm, contribute to LUMO+1 (see ESI Fig. S6) irrespective of the position of the
methyl group. Thus, frontier orbitals predict that position of methyl group does not bring any
substantial change in its electronic properties.
The molecular electrostatic potential distribution plot of these hockey stick compounds (solvent
phase) [Fig.11] also support these findings. It is evident that electrostatic molecular electrostatic
potential describes the electrostatic potential and charge distribution in a compound which gives
the primary idea about the reactive site. Here, the depletion in electronic charge (higher
potential), excess in electronic charge (lower potential) and average electrostatic potential are
displayed by blue, red and green colour respectively. All oxygen atoms and the nitrogen atom of
azo group basically contribute the region with lower electrostatic potential energy due to the
presence of negative charge. Out of these lower electrostatic potential regions (higher
electronegativity value), the ester group shows the lowest potential in case of trans
conformation. Whereas for cis conformation along with oxygen atoms, the nitrogen atom of the
azo group (-N═N-) also exhibits lower potential (higher electronegativity value) among which
region near the azo group (-N═N-) has the lowest potential. Hence, these regions with lower
potential are the reactive sites and would have a very strong attraction on positively charged

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particles. The other region of the compound has average electrostatic potential. The different
position of methyl group does not play any vital role on electrostatic potential distribution.
To support the experimental UV-visible absorption spectra, we have compared the
theoretically obtained UV-visible absorption spectra (both gaseous and solvent phase) with the
experimental one (Fig. 12) and the corresponding electronic excitations related to strong peak
with highest oscillator strength are listed in Table 3. The theoretically obtained strong peak
calculated in both gas and solvent phase are red-shifted compared to the experimental one.
Theoretically obtained spectra of trans conformation contain two peaks for the different position
of the methyl group. The first peak appears more or less at the same position as an experimental
one but a small shift has been observed for the strong peak. The strong peak of (5a) is located at
369.075 nm which is red shifted compared to the experimental one by ~23 nm. However, the
substitution of methyl group in the ortho position of ester linkage (5b) shifted the strong peak at
359.216 nm which is slightly blue shifted compared to compound (5a) while substitution of the
methyl group in the meta position (5c) shifted the strong peak at 376.712 nm which is slightly
red shifted compared to 5a compound. The blue/red shift of the absorption peak may be
explained by the effect of the electron donating bulky methyl group. The presence of the bulky
methyl group at the ortho position of the ester linkage (5b) increases the twisting angle
(~77.721o) as compared to the non-methylated analogue (~56.578o). Therefore, charge
conjugation is slightly restricted as a result of which the absorption peak is shifted to shorter
wavelength. It is further noted that, the presence of the electron donating methyl group at the
meta position of the ester group (5c) have not greatly change the twisting angle (~59.483 o) but
the strong electron donating nature of the methyl group may help in the charge conjugation.
Therefore, the absorption peak is shift toward longer wavelength. For both substitution sites, the
shift of strong peak is less (~10 for (5b); ~7 nm for (5c)) with respect to (5a) compound and this
shift gets reduced for (5b) compound due to the inclusion of solvent. To cross-check our result,
we have also calculated the UV-visible spectra of trans conformation (solvent phase) with higher
basis set (6-311+g(d,p)) and found that the substitution by methyl group in ortho or meta
position slightly shifted the strong peak with respect to (5a) compound also (see ESI Figure S7).
The first peak is mainly characterized by the excitation transitions (HOMO−1) →
LUMO, (HOMO-1) → (LUMO+1), whereas the strong peak is mainly originated by the

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excitation transitions HOMO → LUMO, and (HOMO-1) → LUMO. The shape of the spectra for
cis conformation resembles the experimental one. For cis conformation, the spectra contain
three peaks. Same as trans configuration, the first peak of UV-vis spectra (cis conformation)
appears at same wavelength irrespective of the substitution site of the methyl group. This peak
originates from a lot of weak electronic transitions. The most intense peak of 5a in the gas phase
is found at 329.804 nm with oscillator strength, f=0.654 which mainly appears due to the
transition between (HOMO-1) → (LUMO+1), (HOMO-1) → LUMO and (HOMO-4) →
(LUMO+1). The substitution of the methyl group in ortho or meta position slightly with respect
to ester linkage shifts the strong peak in the gas phase. But in the solvent phase, the strong peak
of (5a), (5b) and (5c) compounds are found more or less at the same wavelength as an
experimental one. For (5a) compound, besides this strong peak, a weak peak is also observed
around 555.530 nm in the gas phase (542.193 nm in solvent phase), and this peak arises due to
electronic transitions mainly between HOMO → LUMO and (HOMO-3) → LUMO. The location
of this peak changes with methyl substitution site and follow the trend (5a) > (5b) > (5c).
Reactivity parameters are quite successful to describe of reactivity trend and reaction
pathway in molecules [75-80]. The excited state phenomena [81-83] and toxicity [84-86] of a
molecule can be predicted with the help of these parameters. To shed insight on the chemical
reactivity of hockey stick shaped mesogens, we have calculated the ionization potential (IP) and
electron affinity (EA) of these compounds and tabulated in Table 4. The IP value signifies a
tendency of the molecule to remove electrons whereas EA value indicates electrons accepting
tendency of a compound. With respect to the gas phase, the IP value decreases in the solvent
phase, while the electron affinity (EA) increases. By knowing ionization potential (IP) and
electron affinity (EA) value other chemical reactivity parameters namely chemical potential (µ),
chemical hardness (η) and electrophilicity index (ω) can be obtained from the following equation
ꢀ = − ꢁꢂꢃꢄꢅ
(1)
(2)
ꢆ
ꢇ = ꢈꢉ − ꢊꢋ
ꢎ
ꢌ = ꢍꢆꢏ
(3)
Chemical potential value can be considered as a measurement of electron escaping tendency
from the system. We see that chemical potential value is decreased for cis conformation than

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trans conformation indicating the high electron-attracting power of cis conformation. Chemical
hardness indicates the stability of a compound. In accordance with the maximum hardness
principle (MHP) [87], a chemical system tends to arrange itself to achieve maximum hardness.
The larger value of ꢇ implies the less reactivity and more stability of a compound. Our
calculation (Table 4) shows that trans conformation has larger chemical hardness than cis
conformation indicating their greater stability. Moreover, the inclusion of solvent increases the
hardness (stability) compared to the gas phase. Thus in presence of the solvent, the compounds
become more inert or less reactive. Electrophilicity index measures the tendency of the
compound to gain electrons and contains the kinetic and thermodynamic information [88] of the
system. It is obvious from Table 4 that compounds having larger hardness exhibits less
electrophilicity i.e., trans conformation has low electrophilicity index than cis conformation, and
hence cis conformation is more reactive than trans conformation.
The reactivity parameter slightly depends on the substitution site of the methyl group. For both
trans and cis conformation, 5a compound is most stable. After methyl group substitution, for
trans conformation, a meta-substituted compound is more stable than ortho substituted
compound, while for cis conformation, ortho substitute compound is more stable.
Conclusion
We report design and synthesis of new azo functionalised reactive unsymmetrical hockey stick
mesogens possessing lateral methyl group. The compounds exhibited a wide range of
enantiotropic nematic phase at the ambient temperature. The room temperature nematic phase
observed on methyl substitution at the elongated arm of the molecule. Reversible photo-
switching behaviour in solution and liquid crystalline state is noted in azo functionalised reactive
hockey stick mesogens possessing lateral methyl group. Density functional theory (DFT)
calculations show that different position of methyl group does not play any vital role in
electronic properties i.e. location of HOMO, HOMO-1, LUMO, LUMO+1, electrostatic potential
distribution, reactivity profile and spectroscopic properties although a small shift in strong peak
is observed in UV-visible absorption spectra due to different position of methyl group. The trans
conformation has slightly greater stability and less reactivity than cis conformation irrespective
of methyl group substitution site.

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Acknowledgement
This research work was supported by Department of Science and Technology, New Delhi, India
(No.SR/S2/CMP-70/2007-I). GK and SKS thanks, University Grants Commission India for
University fellowship. Miss Barnali Bhattacharya likes to thanks Council of Scientific and
Industrial Research (CSIR), Govt. of India for providing fellowship.
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List of Tables
Table 1. Phase transition temperatures (oC) and nematic phase thermal range of the compounds
o
recorded at 5 C min-1 from differential scanning calorimetry (DSC) and confirmed by polarised
optical microscopy (POM). The enthalpies (∆H in kJmol-1) and entropies (∆S in Jmol-1K-1),
respectively, are presented in parentheses. ∆TN = TN-I -TCr–N, is in cooling cycle, where TN-I is
nematic-isotropic transition temperature and TCr-N is crystal-nematic transition temperatures.
Table 2. UV-visible absorption of compounds (5a-5c) in chloroform solvent at c = 1x10-5M.
Table 3. Calculated electronic transitions for maximum peak in UV-visible absorption spectra in
of hockey stick mesogens (5a-5c) in gas medium and solvent phase.
Table 4. Calculated values of chemical potential (µ), chemical hardness (η) and electrophilicity
index (ω) of hockey stick shaped mesogens (5a-5c) in gas medium and solvent.
37

ACCEPTED MANUSCRIPT
Table 1. Phase transition temperatures (oC) and nematic phase thermal range of the compounds
o
recorded at 5 C min-1 from differential scanning calorimetry (DSC) and confirmed by polarised
optical microscopy (POM). The enthalpies (∆H in kJmol-1) and entropies (∆S in Jmol-1K-1),
respectively, are presented in parentheses. ∆TN = TN-I -TCr–N, is in cooling cycle, where TN-I is
nematic-isotropic transition temperature and TCr-N is crystal-nematic transition temperatures.
∆TN (oC)
Compound
X
Y
Phase Transitions in oC (enthalpy and entropy)
H
H
Cr 122.4 (41.88, 105.7 ) N 161.0(0.33, 0.76) Iso
Cr 78.4(22.74, 102.07 ) N 158.8(0.27, 0.63 ) Iso
Cr 117.4 (42.69, 109.34) N 128.2 (0.14, 0.35) Iso
Iso 126.9 (0.36, 1.39) N 54.9† Cr
38.6
80.4
10.8
72
5a
CH3
H
H
5b
5c
CH3
Cr 99.4 (45.9, 123.3) N 120.0 (0.3, 0.8) Iso
Iso118.1(0.4, 0.9 )N r.t.
20.6
88.1
† indicated obtained by polarizing optical microscope.
Table 2. UV-visible absorption of compounds (5a-5c) in chloroform solvent at c = 1x10-5M
Compound
Conditions
Absorption Molar extinction coefficient Assignment
(λmax/nm)
346
438
332
438
348
444
346
438
334
441
346
438
346
438
332
438
346
338
(ε/Lmol-1cm-1)
28711
1443
Virgin sample
π-π*
n-π*
π-π*
n-π*
π-π*
n-π*
π-π*
n-π*
π-π*
n-π*
π-π*
n-π*
π-π*
n-π*
π-π*
n-π*
π-π*
n-π*
5a
Exposed with 366
nm light for 30min
After keeping 24 h
in dark
20896
2177
35621
1248
27038
1574
20681
1965
27563
740
27349
1346
22612
3020
26102
1346
Virgin sample
5b
Exposed with 366
nm light for 30min
After keeping 24 h
in dark
Virgin sample
Exposed with 366
nm light for 30min
After keeping 24 h
in dark
5c
38

ACCEPTED MANUSCRIPT
Table 3. Calculated electronic transitions for maximum peak in UV-visible absorption spectra of
hockey stick mesogens (5a-5c) in gas medium and solvent phase.
Medium
Compound
Maximum peak
at wavelength
Highest oscillator
strength (f)
Transitions
(
ꢀꢁꢂꢃ)
Trans (peak A)
283.682
0.126
Gas phase
5a
HOMO-4 → LUMO (15%),
HOMO-4
→
LUMO+1 (24%),
HOMO-1
→
LUMO+2 (10%),
HOMO
HOMO-4
HOMO-3
HOMO-11
HOMO-4
→
LUMO+2 (21%)
282.435
282.893
285.045
283.196
284.751
0.224
0.223
0.256
0.347
0.214
5b
5c
5a
5b
5c
→ LUMO+1 (30%),
→
→
LUMO (41%)
LUMO+1 (2%)
LUMO (22%),
→
HOMO-4
HOMO-3
→
→
LUMO+1 (36%),
LUMO+1 (11%)
Solvent phase
HOMO-4
→
LUMO (19%),
LUMO+1 (42%),
LUMO+1 (14%)
LUMO+1 (51%),
LUMO+1 (16%)
HOMO-4
HOMO-3
HOMO-4
HOMO-3
→
→
→
→
HOMO-11
HOMO-4
→
LUMO+1 (2%)
→
LUMO (12%),
HOMO-4
HOMO-3
→
→
LUMO+1 (31%),
LUMO+1 (11%)
Trans (peak B)
369.075
359.216
376.712
1.587
1.396
1.450
Gas phase
5a
5b
5c
HOMO
HOMO
HOMO
→
→
→
LUMO (96%)
LUMO (95%)
LUMO (93%)
HOMO-1
HOMO
HOMO-1
→
→
→
LUMO (4%)
LUMO (94%)
LUMO (4%)
379.978
1.409
Solvent phase
5a
376.346
388.358
1.348
1.325
5b
5c
HOMO
HOMO
→
→
LUMO (97%)
LUMO (97%)
Cis (peak B)
329.804
332.003
330.349
335.190
0.654
0.664
0.639
0.725
HOMO-1
HOMO-4
HOMO-1
HOMO-1
HOMO-1
HOMO-4
HOMO-1
HOMO-4
HOMO-1
→
→
LUMO+1 (73%)
LUMO+1 (7%),
Gas phase
5a
5b
5c
5a
→
LUMO (8%),
LUMO (10%),
→LUMO+1 (73%)
→
→
LUMO+1 (5%)
→LUMO+1 (79%)
→
→
→
LUMO+1 (8%),
LUMO (4%),
LUMO+1 (69%)
Solvent phase
HOMO-1
39