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suitable wavelengths [9]. The more stable trans configuration of
N@N in the azodyes [10], isomerizes to form cis-isomer in presence
of 365 nm UV light. It is observed in the previously reported jour-
nals, azobenzene group is either chemically bonded to liquid crystal
molecules or physically mixed with the liquid crystals as dopants.
The phase changes are very essential factor in the photo-induced
studies for the application point of view. The order to disordered
state transition is commonly observed in light sensitive materials,
when the light of suitable wavelength is illuminated on that [11].
But, Prasad et al. reported the reverse process using same UV light
irradiation [12] where disorder to order transition is reported.
Azobenzene liquid crystals are the best materials among other
photo-sensitive compounds for the device fabrication, due to their
photochromic properties. Hence, lot of bent-core azodyes are
reported in the fields of photoisomerization and photochromism
[12,13]. Also, photo-polymerization of azobenzene based liquid
crystals are reported from last few decades [14,15]. Along with
this, some of the acrylate monomers containing banana shape also
reported due to their photo-cross-linking behaviour [16,17]. How-
ever, photo-cross-linking in bent core and oligomeric liquid crys-
tals are interesting topics in liquid crystal research [18–21].
The present investigation focuses on the synthesis and photo-
isomerization behaviour of four new, non-symmetric liquid crys-
tals incorporating different para-substituted isoflavones and
azobenzene chromophores connected via a flexible methylene
spacer. According to literature, materials derived from biological
sources such as glycolipids have gained much interest in liquid
crystal research [22]. However, isoflavone has been rarely used
as a molecular fragment in the design of new liquid crystalline
materials [23–26]. Isoflavones are water-soluble compounds found
in many plants. They comprise of a class of naturally occurring
organic compounds related to flavonoids and their derivatives
are made up of the large number of natural isoflavonoids [27].
Here in these investigations, the optical behaviour of synthe-
sized materials was studied in solution. Also, we have created
the optical storage device using guest–host system on solids. The
isoflavone dye (guest) is mixed with the liquid crystalline material
E7 (room temperature liquid crystal act like host) for measuring
the thermal back relaxation. This guest–host effects in liquid crys-
tals with azo dyes could be provides a path for the exploration of
systems for obtaining fast switching light shutters.
29.6, 25.6, 31.8, 22.7, 14.1; MS (FAB+): m/z for C18 H22N2O2, calcu-
lated: 298.38. Found: 298.16; elementary analysis: calculated
(found) %: C 62.47 (62.81), H 7.21 (7.15), Br 17.32 (17.13), N
6.07, (6.01), O 6.93 (6.58).
1-[4-(n-Bromodecyloxy)phenyl]-2-(40-hexyloxyphenyl)diazene (IIa)
Compound IIa was synthesized according to the reported
method [29]. Compound I (1.93 mmol, 1 equiv.), potassium car-
bonate anhydrous (5.79 mmol,
3 equiv.) and dibromodecane
(5.79 mmol, 3 equiv.) in acetone (20 mL) and refluxed for 6 h.
Afterwards, it was poured into ice-cold water and acidified with
dilute hydrochloric acid (pH < 5). The precipitate was filtered off
and was crystallised from methanol/chloroform (10:2).
A similar procedure was adopted for the synthesis of compound
IIb.
Bright yellow coloured solid; Rf = 0.45 (60% CHCl3–MeOH);
yield: 70%; IR (KBr Pellet) m
max in cmꢁ1: 2954, 2850 (CH2 aliphatic),
1602 (C@C), 1497 (N@N), 1246 (CAO ether). 1H NMR (400 MHz,
CDCl3): d 7.85 (d, J = 8.11 Hz, 4H, Ar), d 6.99 (d, J = 8.01 Hz 4H,
Ar), d 4.09 (t, J = 4.12 Hz, 4H, OCH2), d 3.45 (t, J = 4.01 Hz, 2H,
CH2Br), d 1.12–1.98 (m, 24H, CH2), d 0.89 (t, 3H, CH3); 13C NMR
(100 MHz, CDCl3): d 1616.6, 144.3, 114.7, 123.6, 33.7, 68.7, 32.6,
29.6, 28.0, 25.9, 28.6, 31.8, 22.7, 14.1; MS (FAB+): m/z for
C28H41BrN2O2, calculated: 517.54. Found: 516.23; elemental
analysis: calculated (found) %: C 64.98 (7.02), H 7.98 (7.86), Br 15.44
(14.99), N 5.41 (5.23), O 6.18 (6.14).
7-Hydroxy-3-(40-fluorophenyl)-4H-1-benzopyran-4-one (IIIa)
Compound IIIa was synthesized according to the reported pro-
cedures [30,31]. 4-fluorophenylacetic acid (11.7 mmol, 1 equiv.),
resorcinol (12.87 mmol, 1.1 equiv.) in BF3ꢂEt2O (30 mL) and heated
for 4 h at 70–75 °C under nitrogen atmosphere. The reaction
mixture was then cooled to room temperature. Then dry DMF
and MeSO2Cl (35.1 mmol, 3 equiv.) were added and heated at
75–80 °C for 1.5 h (distillation of DMF was done by using
anhydrous CaCl2). The reaction mixture was poured into ice cold
water. Then filtered the precipitate obtained.
A light reddish yellow coloured solid; yield = 59%. IR (KBr) mmax
in cmꢁ1: 3193 (OH), 1641 (C@O) and 1598 (C@C). 1H NMR
(400 MHz, DMSO-d6): d 10.85 (s, 1H, OH), d 8.41 (s, 1H, H) and d
6.89–7.99 (d, J = 8.11 Hz, 7H, Ar). 13C NMR (100 MHz, CDCl3): d
162.1, 158.6, 153.2, 175.3, 165.0, 123.5, 118.2, 115.4, 101.4,
128.2, 128.0; MS (FAB+): m/z for C15H9FO3, calculated: 256.23.
Found: 256.05; elemental analysis: calculated (found) %: C 70.31
(70.85), H 3.54 (3.42), F 7.41 (7.25), O 18.73 (18.64).
Experimental
Synthetic procedures
Similar procedures were adopted for the preparation of com-
pounds IIIb–IIIc by replacing compound IIIa with compounds IIIb
and IIIc, respectively.
4-[(40-Hexyloxyphenyl)diazenyl]phenol (I)
Compound I was synthesized according to the reported method
[28] from 4-hexyloxyaniline (3.35 mmol, 1 equiv.), concentrated
hydrochloric acid (2 mL), water (15 mL) and cooled to 0 °C. After
the solution was neutralized to pH = 8 with a dilute sodium hydrox-
ide solution. Sodium nitrite (4.08 mmol, 1.2 equiv.) in cold water
(12 mL) was added drop wise to the solution and the mixture was
stirred at 0 °C for 1 h. Phenol (4.02 mmol, 1.2 equiv.) in cold ethanol
(12 mL) was then added drop wise to the solution and stirred at 0 °C
for 1 h. The solution was neutralized with a diluted sodium hydrox-
ide until pH become 6–7. The mixture was stirred for 1 h at 0 °C.
Finally, the distilled water (30 mL) was added to the mixture and
the precipitate was filtered off and treated with chloroform.
Reddish yellow coloured solid; Rf = 0.4 (40% CHCl3–EtOH);
yield: 75%. IR (KBr Pellet) mmax in cmꢁ1: 3186 (OH), 2954, 2920
(CH2 aliphatic), 1600 (C@C), 1499 (N@N), 1249 (CAO ether). 1H
NMR (400 MHz, CDCl3): d 7.85 (d, J = 8.01 Hz, 4H, Ar), d 6.90
(d, J = 8.12 Hz, 4H, Ar), d 4.07 (t, J = 4.11 Hz, 2H, OCH2), d 1.10–1.90
(m, 8H, CH2), d 0.89 (t, 3H, CH3); 13C NMR (100 MHz, CDCl3):
d 161.6, 160.7, 144.3, 145.3, 114.7, 116.2, 123.6, 124.4, 116.2, 68.7,
7-[{4-[(40-Hexyloxyphenyl) diazenyl]phenoxy}decyloxy]-3-(40-
fluorophenyl)-4H-1 benzopyran-4-one (IVa)
Compound IVa was synthesized according to the method
reported in literature [32] from a mixture of compounds IIa,
(0.58 mmol, 1 equiv.), IIIa, (0.64 mmol, 1.1 equiv.), potassium
carbonate (1.16 mmol, 2.0 equiv.) and a catalytic amount of KI in
acetone (20 mL) refluxed for 18 h. Afterwards, it was poured into
ice-cold water and acidified with dilute hydrochloric acid
(pH < 5). The precipitate was filtered off and was crystallised from
methanol/chloroform (10:2).
A pale yellow coloured solid; yield = 59%. IR (KBr Pellet)
cmꢁ1: 2919, 2851 (CH2 aliphatic), 1642 (C@O), 1602 (C@C), 1447
(N@N), 1244 (CAO). 1H NMR (500 MHz, CDCl3):
8.19
mmax in
d
(d, J = 8.9 Hz, 1H, Ar,), d 7.92 (s, 1H, Ar), d 7.90 (d, J = 8.9 Hz, 2H,
Ar), d 7.85 (d, J = 8.9 Hz, 2H, Ar), d 7.55–7.52 (m, 2H, Ar), d 7.12
(t, J = 8.7 Hz, 3H, Ar), d 6.98 (d, J = 8.9 Hz, 4H, Ar), d 6.84 (d, J = 2.3 Hz,
1H, Ar), d 4.07–4.02 (m, J = 4.23 Hz, 6H, OCH2), d 1.87–1.78 (m, 6H,