Synthesis of banana-shaped liquid crystals for photoswitching properties

Article abstract of DOI:10.1080/15421406.2013.807204

Three banana-shaped monomers containing naphthalene as central units and azobenzene in side arms with terminal alkenes were synthesized and characterized. DSC, polarizing optical microscopy, and X-ray diffraction measurements prove that all compounds exhi

Full text of DOI:10.1080/15421406.2013.807204

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Synthesis of Banana-Shaped Liquid
Crystals for Photoswitching Properties
Md Rahman Lutfor a , Mashitah Mohd Yusoff a , Gurumurthy Hegde a
, Muhammad Nor Fazli a , Abdul Malek a , Nurlin Abu Samah a & H. T.
Srinivasa b
a Faculty of Industrial Sciences and Technology , Universiti Malaysia
Pahang , Kuantan , Pahang , Malaysia
b Raman Research Institute , Bangalore , Karnataka , India
Published online: 16 Dec 2013.
To cite this article: Md Rahman Lutfor , Mashitah Mohd Yusoff , Gurumurthy Hegde , Muhammad Nor
Fazli , Abdul Malek , Nurlin Abu Samah & H. T. Srinivasa (2013) Synthesis of Banana-Shaped Liquid
Crystals for Photoswitching Properties, Molecular Crystals and Liquid Crystals, 587:1, 41-53, DOI:
10.1080/15421406.2013.807204
To link to this article: http://dx.doi.org/10.1080/15421406.2013.807204
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Mol. Cryst. Liq. Cryst., Vol. 587: pp. 41–53, 2013
Copyright © Taylor & Francis Group, LLC
ISSN: 1542-1406 print/1563-5287 online
DOI: 10.1080/15421406.2013.807204
Synthesis of Banana-Shaped Liquid Crystals
for Photoswitching Properties
MD RAHMAN LUTFOR,1,∗ MASHITAH MOHD YUSOFF,1
GURUMURTHY HEGDE,1 MUHAMMAD NOR FAZLI,1
ABDUL MALEK,1 NURLIN ABU SAMAH,1
AND H. T. SRINIVASA2
1Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang,
Kuantan, Pahang, Malaysia
2Raman Research Institute, Bangalore, Karnataka, India
Three banana-shaped monomers containing naphthalene as central units and azoben-
zene in side arms with terminal alkenes were synthesized and characterized. DSC, polar-
izing optical microscopy, and X-ray diffraction measurements prove that all compounds
exhibit B6 phase. The trans-azobenzene showed high-intensity π–π∗ transition at about
365 nm and low-intensity n–π∗ transition at about 450 nm. The strong photoisomer-
ization behavior in solutions of the molecules displayed that trans to cis isomerization
takes 45 sec whereas reverse process takes about 28 h and 30 min. This long thermal
back relaxation is useful for creation of optical image storage.
Keywords Azobenzene; isomerization; naphthalene; optical image storage;
photoswitching
Introduction
The electro-optic responses of some liquid crystals are examined by using electro-optic
devices such as a flat-panel display, light scattering electro-optical switches, and electrically
switchable color-tunable reflectors [1]. Azobenzene moiety is not only attractive photoactive
groups but also efficient for many optical storage devices [2]. Due to their unique cis-trans
isomerization properties, azobenzene moieties play important role when they are mixed
with bent core liquid crystals (BCLCs). Bent core mesogens usually comprise of five or
six member phenyl units connected by linking groups like azomethine groups. In most of
the cases, bend of the molecular long axis results from 1,3-phenylene connection near the
center of the molecule. When azobenzene moiety is inserted between these two phenyl
rings, then banana-shaped liquid crystals become very sensitive to light. Lutfor et al. [3]
reported bent-shaped azobenzene monomers and their photoisomerization properties when
they used it as guest–host system along with liquid crystals. Synthesis of new azobenzene-
containing bent-shaped monomers and their photochemical properties when mixed with
calamitic liquid crystals in solutions were reported [3, 4]. Increase in the photo-induced
∗Address correspondence to Md Lutfor Rahman, Faculty of Industrial Sciences and Technology,
Universiti Malaysia Pahang, 26300 Gambang, Kuantan, Malaysia. Tel. 006-095492785; Fax: 006-
095492766. E-mail: lutfor@ump.edu.my
41

42
M. R. Lutfor et al.
thermal back relaxation was found, which could be useful for creation of permanent optical
storage devices [3].
A field of research that is growing steadily is that of photo-induced phenomenon,
in which the incident light brings about the molecular ordering/disordering of the liquid-
crystalline system [5–8]. This particular aspect of photonics, in which molecular geometry
can be controlled by light, is being proposed as the future technology for optical storage de-
vices [9–11]. The heart of the phenomenon in such systems is the reversible photo-induced
shape transformation of the molecules containing the photo-chromic azo groups [12]. Upon
UV irradiation (around 365 nm, corresponding to the π–π∗ excitation of the azo group),
the energetically more stable E or trans configuration, with an elongated rod-like molecular
form, changes into a bent Z or cis configuration. The reverse transformation can be brought
about by illumination with visible light (in the range 400–500 nm, corresponding to the
n–π∗ band). This latter change can also occur in the “dark” by a process known as thermal
back relaxation. Several bent-core molecules containing an azo (–N N–) linkage have
been reported for the possibility of photoisomerization [13]. In this report, we emphasize
on the synthesis and characterization of naphthalene-based liquid crystalline bent-shaped
monomers with rod-shaped azobenzene units connected to the core. In addition, an inves-
tigation on the UV-induced absorption is carried out, which gives powerful insight on the
influence of light on them.
Experimental
Materials
Ethyl 4-aminobenzoate (Fluka), sodium nitrite (BDH), phenol (Merck), potassium carbon-
ate (Fluka), 9-bromonon-1-ene (Aldrich), 10-bromodec-1-ene (Aldrich), 11-bromoundec-
1-ene (Aldrich), 2,7-dihydroxynaphthalene (Aldrich), 1,3-dicyclohexyl-carbodiimide
(DCC; Fluka), 4-(N,N-dimethylamino)pyridine (DMAP; Fluka), and silica gel-60 (Merck)
were used as received. Acetone was refluxed over phosphorus pentaoxide (Merck) and
dichloromethane was refluxed over calcium hydride and both were distilled before use.
Other solvents and chemicals were used without further purification.
Ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate (1)
Compound 1 was synthesized according to the reported procedure [14] from ethyl 4-
aminobenzoate (10.18 g, 0.0616 mol), conc. hydrochloric acid (14 mL), sodium nitrite
(4.257 g, 0.0616 mol), and phenol (5.79 g, 0.0616 mol) in 600 mL methanol. Yield: 8.01 g,
50.7% as red crystals and mp 160◦C. IR, νmax/cm−1 3322 (OH), 1724 (C O, ester), 1604,
1480 (C C, aromatic), 1249, 1142 (C–O), 828 (C–H). 1H NMR (CDCl3) δ: 8.16 (2H, d,
J = 8.0 Hz), 7.90 (2H, d, J = 6.8 Hz), 7.88 (2H, d, J = 7.8 Hz), 7.0 (2H, d, J = 8.9 Hz),
5.57 (1H, s, OH), 4.42 (2H, q, J = 6.9 Hz, –CH2CH3), 1.47 (3H, –CH2CH3). 13C NMR
(CDCl3) δ: 14.58, 61.40, 116.10, 122.59, 125.58, 130.70, 131.62, 147.19, 155.41, 159.21,
166.41. Elemental Analysis, Found: C, 66.56; H, 5.26; N, 10.27. Calc. for C15H14N2O3: C,
66.69; H, 5.22; N, 10.36%.
Ethyl 4-{2-[4-(non-8-enyloxy)phenyl]diazenyl}benzoate (2a)
Compound 1 (1.20 g, 4.44 mmol) was dissolved in dry acetone (100 mL); 9-bromonon-
1-ene (6.07 mmol), potassium carbonate (0.840 g, 6.08 mmol), and a catalytic amount
of potassium iodide (20 mg) were added and the mixture was refluxed for 24 h under

Synthesis of Banana-Shaped Liquid Crystals
43
argon atmosphere. Afterwards, it was poured into ice-cold water and acidified with dilute
hydrochloric acid (pH < 3). The precipitate was filtered off and was crystallized from
methanol/chloroform (10:2). Yield of 2a was 57%. IR, νmax/cm−1 3076 ( CH2), 2929
(CH2), 2856 (CH2), 1725 (C O, ester), 1645 (C C, vinyl), 1600, 1477 (C C, aromatic),
1250, 1137, 1056 (C–O), 825 (C–H). 1H NMR (CDCl3) δ: 8.18 (d, 2H, J = 8.5 Hz), 7.98
(d, 2H, J = 8.8 Hz), 7.91 (d, 2H, J = 8.5 Hz), 7.02 (d, 2H, J = 8.6 Hz), 5.86 (m, 1H,
CH ), 5.06 (dd, 1H, J = 13.2 Hz, CH2), 5.01 (dd, 1H, J = 9.5 Hz, CH2), 4.07 (t, 2H, J
= 6.7 Hz, OCH2–), 3.97 (s, 2H, OCH2CH3), 2.18 (m, 2H, –CH2–), 1.88 (m, 4H, –CH2–),
1.57 (m, 8H, –CH2–), 1.49 (t, 3H, –CH2CH3). 13C NMR (CDCl3) δ: 14.44, 20.44, 25.56,
26.68, 27.55, 28.55, 29.68, 30.15, 62.22, 69.18, 116.23, 118.24, 122.88, 125.32, 130.68,
131.67, 132.79, 147.25, 155.38, 161.77, 166.22.
Ethyl 4-{2-[4-(dec-9-enyloxy)phenyl]diazenyl}benzoate (2b)
Compound 1 (1.30 g, 4.81 mmol), dry acetone 100 mL, 10-bromodec-1-ene (6.64 mmol),
potassium carbonate (0.919 g, 6.65 mmol), and potassium iodide (20 mg) were refluxed for
24 h. Etherification was carried out by the same method as used for synthesis of 2a.Yield:
64.5%. IR, νmax/cm−1 3070 ( CH2), 2928 (CH2), 2856 (CH2), 1725 (C O, ester), 1644
(C C, vinyl), 1601, 1478 (C C, aromatic), 1251, 1127, 1050 (C–O), 825 (C–H). 1H NMR
(CDCl3) δ: 8.19 (d, 2H, J = 8.4 Hz), 7.98 (d, 2H, J = 8.7 Hz), 7.92 (d, 2H, J = 8.5 Hz),
7.02 (d, 2H, J = 8.5 Hz), 5.87 (m, 1H, CH ), 5.05 (dd, 1H, J = 13.3 Hz, CH2), 5.02
(dd, 1H, J = 9.3 Hz, CH2), 4.06 (t, 2H, J = 6.7 Hz, OCH2–), 3.98 (s, 2H, OCH2CH3),
2.19 (m, 2H, –CH2–), 1.87 (m, 6H, –CH2–), 1.59 (m, 8H, –CH2–), 1.47 (t, 3H, –CH2CH3).
13C NMR (CDCl3) δ: 14.43, 20.45, 25.66 26.78, 27.12, 28.51, 29.78, 30.25, 31.51, 62.22,
69.18, 116.22, 118.23, 122.78, 125.32, 130.78, 131.68, 132.77, 147.33, 155.44, 161.82,
166.32.
Ethyl 4-{2-[4-(undec-10-enyloxy)phenyl]diazenyl}benzoate (2c)
Compound 1 (1.10 g, 4.07 mmol), dry acetone 100 mL, 11-bromoundec-1-ene (5.07 mmol),
potassium carbonate (0.700 g, 5.07 mmol), and potassium iodide (20 mg) were refluxed
for 24 h. Etherification was carried out by the same method as used for synthesis of 2a.
Yield: 78.3%. IR, νmax/cm−1 3077 ( CH2), 2926 (CH2), 2862 (CH2), 1724 (C O, ester),
1642 (C C, vinyl), 1600, 1467 (C C, aromatic), 1248, 1136, 1052 (C–O), 824 (C–H).
1H NMR (CDCl3) δ: 8.18 (d, 2H, J = 8.6 Hz), 7.97 (d, 2H, J = 8.8 Hz), 7.94 (d, 2H,
J = 8.6 Hz), 7.01 (d, 2H, J = 8.5 Hz), 5.86 (m, 1H, CH ), 5.05 (dd, 1H, J = 13.5 Hz,
CH2), 5.02 (dd, 1H, J = 9.5 Hz, CH2), 4.06 (t, 2H, J = 6.7 Hz, OCH2–), 3.95 (s,
2H, OCH2CH3), 2.16 (m, 2H, –CH2–), 1.82 (m, 6H, –CH2–), 1.55 (m, 10H, –CH2–), 1.48
(t, 3H, –CH2CH3). 13C NMR (CDCl3) δ: 14.41, 20.44, 25.16 26.68, 27.32, 28.75, 29.78,
30.35, 31.77, 32.88, 62.21, 69.19, 116.33, 118.22, 122.78, 125.31, 130.78, 131.77, 132.55,
147.23, 155.24, 161.67, 166.29.
4-{2-[4-(non-8-enyloxy)phenyl]diazenyl}benzoic acid (3a)
Compound 2a (2.46 mmol) was dissolved in 150 mL of methanol. A solution of potassium
hydroxide (0.400 g, 7.13 mmol) in water (10 mL) was added and the solution was refluxed
for 4 h. The mixture was poured into ice-cold water (200 mL) and the precipitate was
acidified with conc. hydrochloric acid (10 mL). The precipitate was filtered off, washed
with water, and crystallized from ethanol/chloroform (2:1) to give compound 3a. Yield:

44
M. R. Lutfor et al.
56.4%. IR, νmax/cm−1 3080 ( CH2), 2928 (CH2), 2852 (CH2), 1684 (νC O, acid), 1624
(C C, vinyl), 1602, 1507 (C C, aromatic), 1260, 1143, 1012 (C–O), 827 (C–H). 1H NMR
(CDCl3) δ: 8.20 (d, 2H, J = 8.8 Hz), 7.97 (d, 2H, J = 8.9 Hz), 7.92 (d, 2H, J = 8.7 Hz),
7.04 (d, 2H, J = 9.0 Hz), 5.85 (m, 1H, CH ), 5.05 (dd, 1H, J = 13.6 Hz, CH2), 4.96
(dd, 1H, J = 8.2 Hz, CH2), 4.08 (t, 2H, J = 6.5 Hz, OCH2–), 2.11 (m, 2H, –CH2), 1.87
(m, 2H, –CH2–), 1.51 (m, 2H, –CH2–), 1.44 (m, 6H, –CH2–). 13C NMR (CDCl3) δ: 21.50,
25.86, 26.12, 28,67, 29.67. 29.65, 69.88, 115.66, 118.34, 122.78, 125.33, 130.68, 131.69,
132.89, 147.27, 155.48, 161.78, 167.73.
4-{2-[4-(dec-9-enyloxy)phenyl]diazenyl}benzoic acid (3b)
The hydrolysis of 2b was carried out according to the method described for 3a. Yield of
3b: 52.5%. IR, νmax/cm−1 3084 ( CH2), 2925 (CH2), 2852 (CH2), 1684 (νC O, acid),
1627 (C C, vinyl), 1601, 1503 (C C, aromatic), 1261, 1149, 1012 (C–O), 825 (C–H).
1H NMR (CDCl3) δ: 8.20 (d, 2H, J = 8.8 Hz), 7.96 (d, 2H, J = 8.6 Hz), 7.93 (d, 2H, J
= 8.6 Hz), 7.05 (d, 2H, J = 9.1 Hz), 5.86 (m, 1H, CH= ), 5.05 (dd, 1H, J = 13.1 Hz,
CH2), 4.92 (dd, 1H, J = 8.3 Hz, CH2), 4.08 (t, 2H, J = 6.5 Hz, OCH2–), 2.11 (m,
2H, –CH2), 1.87 (m, 2H, –CH2–), 1.53 (m, 2H, –CH2–), 1.44 (m, 8H, –CH2–). 13C NMR
(CDCl3) δ: 22.56, 25.46, 26.22, 28,47, 29.61. 29.65, 30.31, 69.78, 115.46, 118.54, 122.68,
125.44, 130.61, 131.64, 132.83, 147.26, 155.46, 161.72, 167.63.
4-{2-[4-(undec-10-enyloxy)phenyl]diazenyl}benzoic acid (3c)
The hydrolysis of 2c was performed in similar manner. 3c: 55.3%. IR, νmax/cm−1 3081
( CH2), 2924 (CH2), 2851 (CH2), 1684 (νC O, acid), 1624 (C C, vinyl), 1604, 1502
(C C, aromatic), 1260, 1144, 1011 (C–O), 826 (C–H). 1H NMR (CDCl3) δ: 8.20 (d, 2H,
J = 8.4 Hz), 7.98 (d, 2H, J = 8.9 Hz), 7.91 (d, 2H, J = 8.7 Hz), 7.04 (d, 2H, J = 9.0 Hz),
5.85 (m, 1H, CH ), 5.05 (dd, 1H, J = 13.2 Hz, CH2), 4.91 (dd, 1H, J = 8.4 Hz, CH2),
4.05 (t, 2H, J = 6.7 Hz, OCH2–), 2.18 (m, 2H, –CH2), 1.88 (m, 2H, –CH2–), 1.52 (m,
2H, –CH2–), 1.44 (m, 10H, –CH2–). 13C NMR (CDCl3) δ: 22.40, 25.86, 26.22, 27.32,
28.47, 29.62. 29.55, 30.44, 69.78, 115.56, 118.44, 122.73, 125.34, 130.66, 131.63, 132.79,
147.37, 155.55, 161.65, 167.79.
2,7-Naphthalene bis[4-{2-[4-(non-8-enyloxy)phenyl]diazenyl}benzoate] (4a)
Compound 3a (0.358 mmol), resorcinol (0.179 mmol), 50 mL of dry dichloromethane,
DMAP (0.04 mmol), and DCC (0.40 mmol) were stirred for 48 h. The mixture was
filtered and filtrate was washed with acetic acid and water. The solvent was removed by
reduced pressure and product was purified by column chromatography over silica gel 60
eluted with dichloromethane and hexane (10:1). Finally, product was recrystallized from
ethanol/chloroform (2:1). Yield of 4a: 35%. IR, νmax/cm−1 3080 ( CH2), 2924 (CH2),
2852 (CH2), 1722 (C O, ester), 1624 (C C, vinyl), 1602, 1456 (C C, aromatic), 1251,
1143, 1030 (C–O), 840 (C–H). 1H NMR (CDCl3) δ: 8.20 (d, 4H, J = 8.4 Hz), 7.97 (d, 4H,
J = 8.8 Hz) 7.92 (d, 4H, J = 8.4 Hz), 7.76 (d, 2H, J = 4.1 Hz), 7.44 (dd, 2H, J = 8.2 Hz),
7.04 (d, 4H, J = 8.8 Hz), 5.85 (m, 2H, CH ), 5.05 (dd, 2H, J = 15.5 Hz, =CH2), 4.96
(dd, 2H, J = 9.7 Hz, CH2), 4.07 (t, 4H, J = 6.5 Hz, OCH2–), 2.10 (m, 4H, –CH2–), 1.87
(m, 4H, –CH2–), 1.71 (m, 4H, –CH2–), 1.50 (m, 12H, –CH2). 13C NMR (CDCl3) δ: 24.93,
25.25, 27.26, 28.16, 29.99, 30.34, 68.57, 114.64, 115.29, 118.38, 118.71, 119.21, 121.23,

Synthesis of Banana-Shaped Liquid Crystals
45
122.79, 124.21, 125.35, 129.33, 130.57, 131.76, 132.65, 134.53, 147.34, 149.53, 155.72,
161.12, 164.23.
2,7-Naphthalene bis[4-{2-[4-(dec-9-enyloxy)phenyl]diazenyl}benzoate] (4b)
Compound 3b (1.00 mmol), resorcinol (0.50 mmol), 80 mL of dry dichloromethane, DMAP
(0.012 mmol), and DCC (1.20 mmol) were stirred for 48 h. Esterification was carried out
according to the method described for 4a. Yield of 4b: 40%. IR, νmax/cm−1 3077 ( CH2),
2928 (CH2), 2862 (CH2), 1732 (νC O, ester), 1642 (C C, vinyl), 1601, 1456 (C C,
1
aromatic), 1242, 1148, 1078 (C–O), 835 (C–H). H NMR (CDCl3) δ: 8.35 (d, 4H, J =
8.4 Hz), 8.01 (d, 4H, J = 5.9 Hz) 7.97 (d, 4H, J = 6.8 Hz), 7.75 (d, 2H, J = 4.3 Hz), 7.43
(dd, 2H, J = 8.1 Hz), 7.04 (d, 4H, J = 9.0 Hz), 5.90 (m, 2H, CH ), 5.23 (dd, 2H, J =
15.1 Hz, CH2), 5.10 (dd, 2H, J = 9.2 Hz, CH2), 4.10 (t, 4H, J = 6.6 Hz, OCH2–), 2.32
(m, 4H, –CH2–), 1.98 (m, 4H, –CH2–), 1.73 (m, 6H, –CH2–), 1.50 (m, 12H, –CH2). 13C
NMR (CDCl3) δ: 24.73, 25.21, 26.76, 27.36, 28.26, 29.49, 30.55, 67.59, 114.85, 115.44,
115.83, 118.45, 119.32, 122.57, 124.22, 125.33, 129.94, 130.15, 131.27, 132.51, 135.57,
146.94, 149.78, 151.46, 155.90, 162.38, 164.38.
2,7-Naphthalene bis[4-{2-[4-(undec-10-enyloxy)phenyl]diazenyl}benzoate] (4c)
Compound 3c (0.355 mmol), resorcinol (0.177 mmol), 50 mL of dry dichloromethane,
DMAP (0.04 mmol) and DCC (0.40 mmol) were stirred for 48 h. Esterification was carried
out according to the method described for 4a.Yield of 4c: 37%. IR, νmax/cm−1 3081
( CH2), 2925 (CH2), 2862 (CH2), 1728 (C O, ester), 1624 (C C, vinyl), 1604, 1457
(C C, aromatic), 1252, 1144, 1031 (C–O), 840 (C–H). 1H NMR (CDCl3) δ: 8.20 (d, 4H,
J = 8.5 Hz), 7.98 (d, 4H, J = 8.6 Hz) 7.92 (d, 4H, J = 8.6 Hz), 7.75 (d, 2H, J = 4.3 Hz),
7.43 (dd, 2H, J = 8.1 Hz), 7.04 (d, 4H, J = 8.6 Hz), 5.85 (m, 2H, CH= ), 5.03 (dd, 2H, J
= 15.1 Hz, CH2), 4.96 (dd, 2H, J = 9.3 Hz, CH2), 4.07 (t, 4H, J = 6.6 Hz, OCH2–),
2.10 (m, 4H, –CH2–), 1.88 (m, 4H, –CH2–), 1.71 (m, 8H, –CH2–), 1.51 (m, 12H, –CH2–).
13C NMR (CDCl3) δ: 24.93, 25.66, 26.71, 27.26, 28.16, 29.99, 30.32, 33.21, 68.77, 114.12,
114.78, 115.46, 111.34, 119.47, 123.17, 124.23, 125.43, 129.22, 130.25, 131.43, 132.67,
138.22, 146.62, 149.12, 151.25, 155.66, 162.12, 164.36.
Characterization
The products were confirmed by spectroscopic methods: IR spectra were recorded with a
1
Perkin Elmer (670) FTIR spectrometer. H NMR (500 MHz) and 13C NMR (125 MHz)
spectra were recorded with a Bruker (DMX500) spectrometer. The transition temperatures
and their enthalpies were measured by differential scanning calorimetry (Perkin DSC
7), heating and cooling rates were 10◦C min−1, and melting point of the intermediate
compounds was determined by DSC. Optical textures were obtained by using Olympus
BX51 polarizing optical microscope equipped with a Mettler Toledo FP82HT hot stage
and a FP90 central processor unit. Absorption spectra were recorded using a Perkin Elmer
UV/VIS Spectrometer (Lambda 25). Absorption spectra for photochromic study were
recorded using a Shimazdu 3101 PC UV-Vis spectrometer. All the solutions were prepared
and measured under air in the dark at room temperature (21 1◦C) using 1 cm quartz cells.
The cells were closed to avoid the evaporation of the solvent and the solutions were stirred
during the irradiation time. The solutions were irradiated at λ exc. = 254 nm, 365 nm, and
436 nm respectively, using a 200 watt high pressure Hg-lamp HBO 200 (NARVA, Berlin,

46
M. R. Lutfor et al.
Germany) and filters IF 254, HgMon 365, HgMon 436 (Zeiss, Jena, Germany) generating
monochromatic light as excitation source. Additional protection glass filters Code-No. 601
were used for irradiation at 365 nm and 254 nm and Code-No. 805 (both Schott, Jena,
Germany) for irradiation at 436 nm was used.
Results and Discussion
Synthesis
The azobenzene containing rod-like side arms is synthesized from ethyl 4-aminobenzoate.
The diazonuimsalt was preparedwithsodiumnitriteinthe presence of aqueous hydrochloric
acid, which was coupled with phenol to give ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate
1. The reaction and purification by crystallization was performed [15]. Compound 1 was
alkylated with 9-bromonon-1-ene in the presence of potassium carbonate as base to give
ethyl 4-{2-[non-8-enyloxy)phenyl]diazenyl}benzoate 2a. Other compounds (2b and 2c)
were prepared with the same method as compound 2a. These intermediate compounds
2a–c, were found to be liquid crystalline and showed smectic A phase. Then compounds
2a–c having ester groups were hydrolyzed under basic conditions to yield the acids 3a–c.
All the acid compounds showed nematic phase.
The liquid crystallinity in such compounds is induced due to hydrogen-bonded dimer
formation, a phenomenon well documented in literature [4, 16–21]. The carboxylic acid
groups are associated to form the H-bonded cyclic dimers either in crystalline or liquid
crystalline phases, and most of the dimers exhibited enantiotropic mesophase behavior.
In final step, two equivalents of acids 3a–c were coupling with one equivalent of 2,7-
dihydroxynaphthalene by DCC and DMAP to achieve desired molecules 4a–c (Scheme 1).
Scheme 1. Synthesis of compounds 4a–c. Reagent and conditions: (i) NaNO2, 3 equiv HCl, 2◦C;
(ii) NaOH, pH 9, 2◦C; (iii) K2CO3, KI, BrCnH2n–1 (n = 7–9), reflux; (iv) KOH, MeOH; (v) DCC,
DMAP.

Synthesis of Banana-Shaped Liquid Crystals
Mesomorphic Properties
47
The mesomorphic behavior of the compounds was studied by polarizing optical microscopy
(POM), differential scanning calorimetry (DSC), and X-ray diffraction measurements at el-
evated temperature. The DSC curves were obtained during two successive heating–cooling
scans at a heating/cooling rate of 5◦C min−1. The POM and X-ray measurements were
performed for the confirmation of phase structure of the expected compounds. The thermo-
dynamic parameters such as the phase transition temperatures (T/◦C) and enthalpy changes
(ꢀH/Jg−1) associated with these phase transitions of all the synthesized banana-shaped
monomers are listed in Table 1. Compound 4a shows enantiotropic phase sequences of
Cr–B6–I transitions, which is confirmed by DSC and POM textural observation. On cool-
ing, the isotropic-B6 transition appears that crystallizes at 78.6◦C (Table 1). The compounds
4b and 4c also showed two transition peaks on heating that are attributed to the Cr–B6 and
B6–I transitions. On cooling, again two peaks corresponding to I–B6 and B6–Cr transitions
were observed (Table 1).
Compounds 4a–c exhibit enantiotropic mesophase identified by DSC and POM stud-
ies. A representative compound 4a was confirmed by X-ray measurements. The thermal
behavior of all these compounds 4a–c is in accordance with literature [4, 15, 22–24]. Hence,
increasing the terminal chain length (4a vs. 4c), as expected, decreases the transition tem-
perature (Table 1). In addition, the odd–even effect on the isotropic temperature was seen
on compounds 4a–c where even number of alkyl carbon has higher transition temperature
than odd number of alkyl carbon chain [15, 22–24]. The enthalpy changes determined by
DSC are in agreement with the values usually observed for typical crystalline–smectic,
smectic–isotropic, and smectic–crystalline transitions that confirm the presence of smectic
mesophases [25, 26]. The solid–liquid crystal or liquid crystal–solid transitions involved
much more energy than the liquid crystal–isotropic liquid or liquid crystal–liquid crystal
transitions [27].
Similar odd–even effects have been reported in other materials [28]. Increasing terminal
chain length decreases the transition temperature as usual. A large number of bent-core
liquid crystalline materials derived from 2,7-dihydroxynaphthalene unit has already been
reported [29–34]. The structure–property relationship in bent-core liquid crystals derived
from 2,7-dihydroxynaphthalene core has been extensively discussed by several researchers
[28–34]. Several series of bent-shaped mesogens based on substituted naphthalene-2,7-diol
having phenylbenzoate wings and a double bond at one or at both ends of the terminal
Table 1. Phase transition temperature (T/◦C) and associated enthalpy values (ꢀH/J g−1) in
parentheses given for the second heat and cooling of DSC scans for compounds 4a–c
Compound
n
Scan
Phase transitions
4a
7
Heat
Cool
Heat
Cool
Heat
Cool
Cr 107.1 (22.7) B6 150.5 (18.1) I
I 147.3 (16.6) B6 78.6 (22.3) Cr
Cr 109.8 (20.2) B6 152.1 (10.9) I
I 150.4 (15.8) B6 80.8 (24.4) Cr
Cr 102.8 (21.0) B6 146.6 (11.7) I
I 144.1 (18.8) B6 77.8 (23.4) Cr
4b
4c
8
9
Note. Cr = crystal; B6 = smectic A phase; I = isotropic phase.

48
M. R. Lutfor et al.
chains have been reported [29, 35]. These compounds were synthesized using the long
chains, but only smectic phases were observed for the ester-based bent-core molecules.
These comparisons indicate the stabilizing effect of azobenzene units on the nematic and
smectic phases of the naphthalene-based banana-shaped compounds [29, 35].
Under the polarizing microscope, a fan-shaped texture as typical for smectic A phase
corresponding to the B6 phase for compound 4a was observed upon cooling from the
isotropic phase. Optical textures taken for compound 4a (n = 7) at 110◦C is shown in
Fig. 1; no other phase transition on further cooling, except crystallization, was observed.
On cooling from the isotropic phase, compound 4b (n = 8) showed also fan-shaped texture
and compound 4c showed broken fan-shaped texture, which is a typical smectic A phase
corresponding to the B6 phase for 4b and 4c; there was no other phase transitions observed.
All the transition temperatures observed under POM were matching with DSC data.
The X-ray diffraction was carried out for further phase structure conformation. The
mesophase was obtained upon cooling of the isotropic phase of the compound 4a (n = 7).
The intensity versus 2θ plot derived from the diffraction pattern is shown in Fig. 2. The
compound 4a showed one sharp reflection at d = 27.57 Å in the lower-angle region and in
addition, a diffused peak in the wide-angle region with d-spacing of 4.22 Å at 105◦C is due
to a liquid-like in-plane ordering of molecules in the layers. The XRD pattern, observed
as typical of a B6 intercalated mesophase due to sharp reflection, is half of the molecular
length (55.92 Å).
Photoswitching Study
All banana-shaped molecules showed similar absorption spectra due to their similar molec-
ular structure; the only difference is the alkyl chain (n = 7–9), which does not alter the
electronic transitions, the absorption spectra of compounds 4a–c. Consequently, compound
4a was considered for photoisomerization study [3]. A solution of concentration 5 × 10−5
mol L−1 of 4a was prepared in chloroform. The spectrum shows three absorption peaks
with maximum absorbance at 261 nm, 365 nm (ε = 34.940 L.mol−1.cm−1), and 445 nm.
Since peak around 261 nm is non-photoactive and it is not our interest, so we eliminate our
discussions regarding this peak. Fig. 3 shows the spectral changes by irradiation at 365 nm.
The absorbance at 365 nm decreases and after 45 sec the photostationary state of a cis-trans
mixture is reached. By irradiation at 436 nm, the absorbance at 365 nm increases until a
new photostationary state is reached. The photochemical back reaction by irradiation at
436 nm is shown in Fig. 4.
The thermal back reaction (cis→ trans) is shown in Fig. 5. After irradiation at 365 nm to
photostationary state, the solution was kept in the dark and the back reaction was measured
at λ = 365 nm after over 28 h and 30 min. The presence of the two isosbestic points
indicates the absence of side reactions. A linear correlation of ln (E – Et) as a function of
∞
time indicates a reaction of the first order (see Fig. 6).
After 1590 min, the conversion degree from cis to trans was 96%. Similar photochemi-
cal switching behavior has previously been reported to center-linked bent-core azobenzene
liquid crystalline polymers in which two azobenzene groups are attached to a central
resorcinol unit [36].
Using the materials described in this article, an optical pattern storage device is fab-
ricated as shown in Fig. 7. Here, experimental cell is filled with the mixture of guest
naphthalene bent-shaped molecules and the host liquid crystalline molecules (10% of guest
mixed with 90% of guest 5CB). The region 1 represents UV-illuminated area and region 2
represents masked region that per se is not contributing in the isomerization phenomena.

Synthesis of Banana-Shaped Liquid Crystals
49
Figure 1. Optical micrograph of (a) compound 4a taken at 110◦C, (b) compound 4b taken at 112◦C,
and (c) compound 4c taken at 105◦C from cooling cycle.

50
M. R. Lutfor et al.
Figure 2. The intensity versus 2θ graph derived from X-ray diffraction for SmAintercal corresponding
to B6 phase of compound 4a at 105◦C.
Figure 3. UV/Vis absorption spectra of 4a in chloroform, c = 5 × 10−5 mol L−1, irradiation at
365 nm.
Figure 4. UV/Vis absorption spectra of 4a in chloroform, c = 5 × 10−5 mol L−1, irradiation at
436 nm (the sample was irradiated before at 365 nm).

Synthesis of Banana-Shaped Liquid Crystals
51
Figure 5. Thermal back reaction (cis→ trans) of 4a in chloroform (the solution was irradiated before
at 365 nm).
Figure 6. Thermal back reaction (cis→ trans) of 4a in chloroform. Plot of ln (E – Et) at λ =
∞
365 nm as a function of time. After 1590 min (26 h and 30 min), the conversion of cis→ trans was
96%.
Figure 7. Demonstration of optical pattern storage capability of the device based on the principle
described in the article observed under the crossed polarizers (marked as P and A). The sample was
kept at room temperature and illuminated with UV radiation through a mask. The dark region marked
as 1 is where the molecules are in isotropic state caused by illuminating UV and the bright region
marked as 2 is where the radiation is masked.

52
M. R. Lutfor et al.
Pattern is illuminated with 365 nm UV light at room temperature through photomask to
create the pattern shown in the figure. One can see the good clarity between bright and dark
states.
Conclusion
Three new banana-shaped mesogens with azobenzene units containing 2,7-dihydroxyna-
phthalene as central bent unit are studied and all compounds show B6 phase. The double
bonds can be used for preparation of polymers or silyl functionalized bent-core mesogens,
whereas the presence of the azo linkage in these liquid crystal monomers is suitable for
photochromism application and trans-cis-trans isomerization cycles under UV irradiation.
Optical storage device is fabricated by using present molecule, which shows the capability
of the discussed materials for device applications. The detailed photochemical cis-trans
isomerization study of other compounds of this series and all compounds at solid film is
now in progress and will be reported in due course.
Acknowledgments
This research was supported by Universiti Malaysia Pahang (RDU-100338). A special
thank you goes to Mrs. K. N. Vasudha for supporting many aspects regarding carrying out
this work.
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