Synthesis and characterization of naphthalene-based banana-shaped liquid crystals for photoswitching properties

Article abstract of DOI:10.1002/jccs.201300191

A series of banana-shaped monomers containing naphthalene as central units, azobenzene in side arms with terminal alkenes were synthesized and characterized. Polarizing optical microscopy, DSC and X-ray diffraction measurements reveal that one compound processes a nematic phase while other four compounds exhibit B₆ phase. The absorption spectrum of trans-azobenzene displays high-intensity π-π* transition at 365 and low-intensity n-π* transition at 450 nm. These molecules exhibit strong photoisomerisation behaviour in solutions in which trans to cis isomerisation takes 55 seconds whereas reverse process takes about 32 hours. Such a long thermal back relaxation is useful for creation of optical image storage devices. Banana-shaped monomers containing naphthalene as central units linked with azobenzene in side arms were processes mesophase properties. These molecules exhibit E/Z isomers and under UV irradiation trans (left) to cis (right) isomerisation takes 55 seconds whereas reverse process takes about 32 hours. Such a long thermal back relaxation is useful for creation of optical image storage at extremely right side.

Full text of DOI:10.1002/jccs.201300191

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Synthesis and Characterization of Naphthalene-Based Banana-Shaped Liquid Crystals
for Photoswitching Properties
Md Lutfor Rahman,a* Mashitah Mohd Yusoff,a Gurumurthy Hegde,a
Muhammad Nor Fazli Abdul Malek,a Nurlin Abu Samah,a H. T. Srinivasab and Sandeep Kumarb
aFaculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Gambang, Kuantan, Malaysia
bRaman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore 560080, India
(Received: Apr. 15, 2013; Accepted: Jan. 22, 2014; Published Online: Feb. 24, 2014; DOI: 10.1002/jccs.201300191)
A series of banana-shaped monomers containing naphthalene as central units, azobenzene in side arms
with terminal alkenes were synthesized and characterized. Polarizing optical microscopy, DSC and X-ray
diffraction measurements reveal that one compound processes a nematic phase while other four com-
pounds exhibit B6 phase. The absorption spectrum of trans-azobenzene displays high-intensity p-p* tran-
sition at 365 and low-intensity n-p* transition at 450 nm. These molecules exhibit strong photoisomerisa-
tion behaviour in solutions in which trans to cis isomerisation takes 55 seconds whereas reverse process
takes about 32 hours. Such a long thermal back relaxation is useful for creation of optical image storage
devices.
Keywords: Naphthalene; Azobenzene; Isomerisation; Photoswitching; Optical image storage.
INTRODUCTION
Banana phases comprises of bent core mesogenic
cule. When azobenzene moiety is inserted between these
two phenyl rings, then banana shaped liquid crystals be-
comes very sensitive to light. Lutfor et. al.8 reported bent
shaped azobenzene monomers and their photoisomeriza-
tion properties when they are used as guest host system
along with liquid crystals. Synthesis of new azobenzene-
containing bent-shaped monomers and their photochemi-
cal properties with calamitic liquid crystals in solutions
were reported.8,9 The photoinduced thermal back relax-
ation was found to be increased which is useful for creation
of permanent optical storage devices.8
units are very attractive and interesting not only for the fun-
damental aspects but also due to their unique electro-opti-
cal properties.1,2 In particular, the electro-optic responses
of some mesophases are examined by using electro-optic
devices such as a flat-panel display, light scattering elec-
tro-optical switches and electrically switchable color-tune-
able reflectors.3 Azobenzene moiety is not only attractive
photoactive groups but also efficient for many optical stor-
age devices.4 Due to their unique cis-trans isomerization
properties, azobenzene moieties play important role when
they are mixed with anisotropic liquid crystals (for exam-
ple bent core liquid crystals, BCLCs) as guest host system.
BCLCs are one of the new supramolecular, functional and
smart materials which have received much attention due to
their interesting mesomorphism. They exhibit typical ne-
matic, lamellar (like smectic) and columnar phase and
these phases are somewhat different from the one exhibited
by conventional liquid crystals (rod-like & disc-like mole-
cules).5-7
On the other hand, a field of research that is growing
steadily is the photoinduced phenomenon in which the inci-
dent light brings about the molecular ordering/disordering
of the liquid-crystalline system.10-13 This particular aspect
of photonics, in which molecular geometry can be con-
trolled by light, is considered as the future technology for
optical storage devices.14-16 The heart of the phenomenon
in such system is the reversible photo-induced shape trans-
formation of the molecules containing the photo-chromic
azo groups.17 Upon UV irradiation (around 365 nm, corre-
sponding to the p–p* excitation of the azo group), the ener-
getically 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
by illumination with visible light (in the range 400–500
Bent core mesogens usually comprises of five or six
member phenyl rings connected by linking groups such as
azobenzene, azomethine, Schiff base, ester groups etc. In
most of the cases, bend of the molecular long axis results
from 1,3-phenylene connection to the centre of the mole-
* Corresponding author. E-mail: lutfor73@gmail.com
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nm, corresponding to the n–p* 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 photochromism and photoisomerization.18
compounds (2b-e) were synthesized with the same method
of compound 2a. These intermediate compounds 2a-e,
having rod-shaped azobenzene units with double bond at
the terminal position were found to be liquid crystalline
and showed smectic A phase. Then compound 2a-e having
ester groups were hydrolyzed under basic conditions to
yield the acids 3a-e. All the compounds, except 3a, showed
nematic phase. The liquid crystallinity in such compounds
is induced due to hydrogen-bonded dimer formation, a phe-
nomenon well documented in literature.9,21-26 The carbox-
ylic acid groups are associated to form the H-bonded cyclic
dimers either in crystalline and liquid crystalline phases
and most of the dimers exhibited enantiotropic liquid crys-
talline behaviour. In the final step, two equivalents of the
acid 3a-e were coupled with one equivalent of 2,7-dihy-
droxynaphthalene using DCC and DMAP to achieve our
desired banana-shaped molecules 4a-e (Scheme 1).
Mesomorphic properties
This paper presents synthesis and characterization of
naphthalene based liquid crystalline bent shaped mono-
mers with azobenzene units connected via ester linkage to
the core. In addition, we have studied the polarizing optical
microscopic textures along with differential scanning calo-
rimetry which gives evidences for the mesophases. We also
investigated UV induced absorption studies which gives
powerful insight of the light influence in the new mole-
cules.
RESULTS AND DISCUSSION
Synthesis
The synthesis and purification of compounds were
performed as depicted in Scheme 1. The azobenzene con-
taining rod-like side arms were prepared from ethyl 4-ami-
nobenzoate. The diazonium salt of ethyl 4-aminobenzoate
was prepared with sodium nitrite in the presence aqueous
hydrochloric acid, which was coupled with phenol to yield
ethyl 4-[(4-hydroxyphenyl)diazenyl] benzoate 1. The reac-
tion and purification by crystallization was performed ac-
cording to our earlier paper.20
Thermotropic behavior of the synthesized compounds
was studied by polarizing optical microscopy (POM), dif-
ferential scanning calorimetry (DSC) and X-ray diffraction
measurements at the mesophase temperature. The DSC
curves were obtained during two successive heating-cool-
ing scans at a heating/cooling rate of 5 °C min-1. The POM
and X-ray measurements were performed for the confirma-
tion of phase structure of the expected compounds. The
thermodynamic parameters such as the phase transition
temperatures (T/°C) and enthalpy changes (DH/Jg-1) asso-
ciated with these phase transitions of the banana shaped
monomers are listed in Table 1.
Scheme 1 Synthesis of compounds 4a-e
Compound 4a shows enantiotropic phase sequences
of Cr–N-I transitions which is confirmed by DSC and POM
textural observation. On cooling, the isotropic-nematic
transition appears which crystallizes at 133.1 °C (Table 1).
The compound 4b-e showed two transition peaks on heat-
ing which 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.
DSC and POM studies reveal that compounds 4b-e
exhibited the enantiotropic phase sequences of Cr-B6-I. A
representative compound 4e was used to confirm the phase
structure by X-ray measurement. The thermal behaviour of
all compounds 4a-e is in accordance with literature.9,20,27-29
Hence, increasing the terminal chain length (4a vs. 4e), as
expected, decreases the transition temperature (Table 1). In
addition, the odd-even effect on the isotropic temperature
Reagent and conditions: (i) NaNO2, 3 equiv HCl, 2 °C; (ii)
NaOH, pH 9, 2 °C; (iii) K2CO3, KI, BrCnH2n-1 (n = 2-6), reflux;
(iv) KOH, MeOH; (v) DCC, DMAP.
Compound 1 was alkylated with 4-bromo-1-butene in
the presence of potassium carbonate as base to give ethyl 4-
{2-[4-(but-3-enyloxy)phenyl]diazenyl}benzoate 2a. Other
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Naphthalene-Based Liquid Crystals for Photoswitching Properties
terminal chains have been reported.34,41 These compounds
were synthesized using the long chains, but only smectic
phases were observed for the ester based bent-core mole-
cules. The fluorinated banana-shaped compounds were
prepared with 2,7-dihydroxynaphthalene central unit,33
which showed SmCPA phases, B1-type rectangular co-
lumnar phases (Colr) and B6-type intercalated smectic
phases (Smintercal). Here, the overall mesophase stability of
the reported compounds,33,38 is quite similar to our com-
pounds 4a-e. These comparisons indicate the stabilizing
effect of azobenzene units on the nematic and smectic
phases of the naphthalene based banana shaped com-
pounds.
Table 1. Phase transition temperature (T/°C) and associated
enthalpy values (DH/J g-1) in parentheses given for the
second heat and cooling of DSC scans for compounds
4a-e
Comp
n
2
Scan
Phase transitions
4a
heat
cool
heat
cool
heat
cool
heat
cool
heat
cool
Cr 140.5 (20.0) N 168.7 (2.7) I
I 165.0 (3.0) N 133.1 (17.7) Cr
Cr 138.6 (23.4) B6 161.1 (9.5) I
I 159.7 (10.5) B6 94.1 (22.8) Cr
Cr 120.1 (27.7) B6 164.9 (14.1) I
I 162.3 (10.6) B6 86.6 (32.1) Cr
Cr 137.9 (20.4) B6 157.9 (8.3) I
I 153.5 (7.7) B6 104.6 (23.2) Cr
Cr 110.8 (24.0) B6 156.6 (10.7) I
I 154.4 (11.8) B6 89.82 (31.4) Cr
4b
4c
4d
4e
3
4
5
6
Abbreviation Cr = crystal, N = nematic, B6 = smectic A phase,
I = isotropic phase.
Under the polarizing microscope, a schlieren texture
as typical for nematic phase for compound 4a was observed
upon cooling from the isotropic phase. Optical textures
taken for compound 4a (n = 2) at 155 °C is shown in Fig. 1,
no other phase transition on further cooling before crystal-
lization was observed. On cooling from the isotropic phase,
compound 4d (n = 5) showed fan shaped texture and com-
pounds 4b, 4c and 4e showed broken fan shaped texture
which is a typical smectic A phase corresponding to the B6
phase showed by 4b-e, there was no other phase transitions
observed. All the transition temperatures observed under
POM were matching with DSC data.
was seen to compound 4a-e where even number of alkyl
carbon has higher transition temperature than odd number
of alkyl carbon chain.20,27-29 The enthalpy changes deter-
mined by DSC are in agreement with the values usually ob-
served for typical crystalline-nematic, nematic-isotropic,
crystalline-smectic, smectic-isotropic and smectic-crystal-
line transitions which confirm the presence of a smectic
and a nematic mesophases.30,31 The solid-liquid crystal or
liquid crystal-solid transitions involved much more energy
than the liquid crystal-isotropic liquid or liquid crystal-liq-
uid crystal transitions.32 Similar odd-even effects have
been reported in other materials.33 Increasing terminal
chain length decreases the transition temperature as usual.
A large number of bent-core liquid crystalline materials de-
rived from 2,7-dihydroxynaphthalene unit have already
been reported.33-39 Similar to widely studied resorcinol de-
rivatives,7,8,18,40 the 2,7-dihydroxynaphthalene derivatives
also possess bent angle of about 120°. Depending upon pe-
ripheral and lateral substitutions, these materials exhibit a
variety of mesophases such as nematic, smectic and colum-
nar phases. The structure-property relationship in bent-
core liquid crystals derived from 2,7-dihydroxynaphtha-
lene core has been extensively discussed by several re-
searchers.33-39
For further phase structure conformation, X-ray dif-
Compounds 4a-e are comparable with other bent-
core mesogens incorporating 2,7-dihydroxynaphthalene
units connected with rod-like wings containing Schiff’s
base units.38 Several series of bent-shaped mesogens based
on substituted naphthalene-2,7-diol having phenylben-
zoate wings and a double bond at one or at both end of the
Fig. 1. Optical micrograph of (a) compound 4a taken
at 155 °C and (b) compound 4b taken at 150 °C,
(c) compound 4c at 152 °C, (d) compound 4d at
147 °C, (e) compound 4e at 142 °C, cooling cy-
cle.
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fraction was carried out in the mesophase obtained upon
cooling of the isotropic phase of the compound 4e (n = 6).
The intensity versus 2q plot derived from the diffraction
pattern as shown in Fig. 2. The compound 4e showed one
sharp reflection at d = 26.26 Å in the lower angle region
and in addition, a diffused peak in the wide-angle region
with d-spacing of 4.29 Å at 140 °C is due to a liquid like
in-plane ordering of molecules in the layers. The XRD pat-
tern observed as typical of a B6 intercalated mesophase due
to sharp reflection is the half of the molecular length (53.77
Å).
4 shows the spectral changes by irradiation at 365 nm. The
absorbance at 365 nm decreases and after 55 seconds 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 photo-
chemical back reaction by irradiation at 436 nm is shown
in Fig. 5.
The thermal back reaction (cis ® trans) is shown in
Fig. 6. After irradiation at 365 nm to photostationary state,
the solution was kept in the dark and the back reaction was
measured at l = 365 nm after over 32 hours. The presence
of the two isobestic points indicates the absence of side re-
actions. A linear correlation of ln (E¥ - Et) as a function of
time indicates a reaction of the first order, see Fig. 7. After
1680 min the conversion degree from cis to trans was 88%.
Similar photochemical switching behaviour 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.42
UV-absorption study: Solutions with concentrations
of 2.25 × 10-5 molL-1 of 4a-e were prepared in chloroform
for UV-visible absorption studies. The spectrum of 4a and
4e shows three absorptions peaks with maximum absorb-
ance at 260, 364 and 452 nm and 4d shows also three ab-
sorptions peaks at 261, 365 and 450 nm (Fig. 3). Com-
pound 4b and 4c shows also three absorptions peaks with
maximum absorbance at 263, 368 and 450 nm (Fig. 3). The
azobenzene monomers in the trans form all showed a
strong band in the UV-region (~365 nm) which is attributed
to the p-p* transition, and a weak band in the visible region
(~450 nm) due to the n-p* transition.
Photoswitching study
A solution concentration of 5 ´ 10-5 mol L-1 of 4c was
prepared in chloroform. The spectrum shows three ab-
sorption peaks with maximum absorbance at 261 nm, 365
nm (e = 34.940 L.mol-1.cm-1) and 445 nm. Since peak
around 261 nm is non photoactive and it is not of our inter-
est, so we did not focus our discussions on this peak. Fig.
Fig. 3. UV/visible absorption spectra of 4a-e in chlo-
roform at conc. 2.25 ´ 10-5 mol L-1.
Fig. 2. The intensity versus 2q graph derived from
X-ray diffraction for SmAintercal phase of com-
pound 4e at 140 °C.
Fig. 4. UV/Vis absorption spectra of 4c in chloroform,
c = 5 ´ 10-5 mol L-1, irradiation at 365 nm.
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CHEMICAL SOCIETY
Naphthalene-Based Liquid Crystals for Photoswitching Properties
Using the materials described in this article, an opti-
cal pattern storage device is fabricated as shown in Fig. 8.
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 which per se not
contributing in the isomerization phenomena. Pattern is il-
luminated with 365 nm UV light at room temperature
through photo mask to create the pattern shown in the Fig.
8. One can see the good clarity between bright and dark
states which enable to optical pattern storage devices.
containing 2,7-dihydroxynaphthalene as central bent core
unit are studied. One of them showed nematic mesophase
behavior and other four show B6 phase. The double bonds
can be used for preparation of polymers or silyl function-
alized bent-core mesogens, whereas the presence of the azo
linkage in these liquid crystals monomers is suitable for the
photochromism application and trans-cis-trans isomeriza-
tions cycles under UV irradiation. Optical storage device
using present molecules is presented which shows the ca-
pability of the discussed materials for device applications.
The detail photochemical cis-trans isomerizations study of
CONCLUSIONS
Five new bent-core mesogens with azobenzene units
Fig. 7. Thermal back reaction (cis ® trans) of 4c in
chloroform, Plot of ln (E¥ - Et) at l = 365 nm as
a function of time.
Fig. 5. UV/Vis absorption spectra of 4c in chloroform,
c = 5 ´ 10-5 mol L-1, irradiation at 436 nm (the
sample was irradiated before at 365 nm).
Fig. 8. Demonstration of optical pattern storage capa-
bility of the device based on the principle de-
scribed 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 iso-
tropic state caused by illuminating UV and the
bright region marked as 2 is where the radiation
is masked.
Fig. 6. Thermal back reaction (cis ® trans) of 4c in
chloroform (the solution was irradiated before
at 365 nm).
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other compounds and all compounds at solid film is now in
progress and will be reported in due course.
=CH2), 5.03 (d, 2H, J = 9.8 Hz, =CH2), 4.10 (t, 4H, J = 6.7 Hz,
OCH2-), 2.18 (m, 4H, -CH2), 1.89 (m, 4H, -CH2). 2,7-Naphtha-
lene bis[4-{2-[4-(hex-5-enyloxy)phenyl] diazenyl}benzoate]
(4c): Compound 4c was synthesis from 3c according to our re-
ported paper. Yield of 4c: 0.020 g (37%). IR (KBr, nmax, cm-1):
3076 (=CH2), 2924 (CH2), 2854 (CH2), 1730 (C=O, ester), 1647
(C=C, vinyl), 1600, 1456 (C=C, aromatic), 1251, 1138, 1078
(C-O), 839 (C-H). 1H NMR (CDCl3) d: 8.41 (d, 4H, J = 8.5 Hz),
8.02 (d, 4H, J = 6.8 Hz), 8.00 (d, 4H, J = 6.8 Hz), 7.98 (d, partially
merged with 8.00 d, 2H), 7.44 (d, 2H, J = 8.6 Hz), 7.42 (d, 2H, J =
5.5 Hz), 7.05 (d, 4H, J = 8.5 Hz), 5.89 (m, 2H), 5.13 (d, 2H, J =
16.1 Hz), 5.05 (d, 2H, J = 9.4 Hz), 4.11 (t, 4H, J = 6.8 Hz, OCH2-),
2.31 (m, 4H, -CH2), 1.97 (m, 4H, -CH2-), 1.60 (m, 4H, -CH2-).
2,7-Naphthalene bis[4-{2-[4-(hept-6-enyloxy)phenyl]diazen-
yl}benzoate] (4d): Compound 4d was synthesis from 3d accord-
ing to our reported paper.Yield of 4c: 0.016 g (32%).IR (KBr,
nmax, cm-1): 3064 (=CH2), 2924 (CH2), 2854 (CH2), 1730 (C=O,
ester), 1653 (C=C, vinyl), 1600, 1456 (C=C, aromatic), 1249,
1138, 1078 (C-O), 839 (C-H). 1H NMR (CDCl3) d: 8.41 (d, 4H, J
= 8.6 Hz), 8.01 (d, 4H, J = 6.9Hz), 8.00 (d, 4H, J = 6.9 Hz), 7.98
(d, partially merged with 8.00 d, 2H), 7.76 (d, 2H, J = 8.6 Hz),
7.41 (d, 2H, J = 5.9 Hz), 7.05 (d, 4H, J = 8.6 Hz), 5.85 (m, 2H),
5.07 (d, 2H, J = 16.1 Hz), 4.99 (d, 2H, J = 9.2 Hz), 4.09 (t, 4H, J =
6.9 Hz, OCH2-), 2.14 (m, 4H, -CH2), 1.86 (m, 4H, -CH2-), 1.53
(m, 8H, -CH2-). 2,7-Naphthalene bis[4-{2-[4-(oct-7-enyl-
oxy)phenyl]diazenyl}benzoate] (4e): Compound 4e was synthe-
sis from 3e according to our reported paper.Yield of 4c: 0.017 g
(31.5%). IR (KBr, nmax, 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)
d: 8.20 (d, 4H, J = 8.6 Hz), 7.97 (d, 4H, J = 8.1 Hz), 7.93 (d, 4H, J
= 6.9 Hz), 7.92 (d, partially merged with 7.93 d, 2H), 7.76 (d, 2H,
J = 8.5 Hz), 7.41 (d, 2H, J = 6.2 Hz), 7.03 (d, 4H, J = 8.6 Hz), 5.85
(m, 2H), 5.84 (d, 2H, J = 16.1 Hz), 5.05 (d, 2H, J = 9.2 Hz), 4.06
(t, 4H, J = 6.8 Hz, OCH2-), 2.08 (m, 4H, -CH2), 1.85 (m, 4H,
-CH2-), 1.60 (m, 4H, -CH2), 1.44 (m, 8H, -CH2CH2-).
EXPERIMENTAL
Materials synthesis: Ethyl 4-[(4-hydroxyphenyl)diazen-
yl]benzoate (1) was synthesized according to our earlier paper.19
Other intermediate compounds such as ethyl 4-{2-[4-(but-3-enyl-
oxy)phenyl]diazenyl}benzoate (2a), ethyl 4-{2-[4-(pent-4-enyl-
oxy)phenyl]diazenyl}benzoate (2b), ethyl 4-{2-[4-(hex-5-enyl-
oxy)phenyl]diazenyl}benzoate (2c), ethyl 4-{2-[4-(hept-6-enyl-
oxy)phenyl]diazenyl}benzoate (2d), ethyl 4-{2-[4-(oct-7-enyl-
oxy)phenyl]diazenyl}benzoate (2e), 4-{2-[4-(but-3-enyloxy)-
phenyl]diazenyl}benzoic acid (3a), 4-{2-[4-(pent-4-enyloxy)-
phenyl]diazenyl}benzoic acid (3b), 4-{2-[4-(hex-5-enyloxy)-
phenyl]diazenyl}benzoic acid (3c), 4-{2-[4-(hept-6-enyloxy)-
phenyl]diazenyl}benzoic acid (3d) and 4-{2-[4-(oct-7-enyl-
oxy)phenyl]diazenyl}benzoic acid (3e) were synthesized accord-
ing to the little modification of our earlier paper,20 and see de-
tailed synthetic procedures and analytical data in ESI. 2,7-Naph-
thalene bis[4-{2-[4-(but-3-enyloxy)phenyl]diazenyl}benzo-
ate] (4a): Compound 3a (0.040 g, 0.135 mmol), 25 mL of dry di-
chloromethane, DMAP (4.88 mg, 0.04 mmol), 2,7-dihydroxy-
naphthalene (10.81 mg, 0.067 mmol), DCC (44 mg, 0.20 mmol)
was added and the mixture was stirred for 24 h. Work-up proce-
dure was followed according to earlier reported paper.20 Yield of
4a: 0.015 g (31%). IR (KBr, nmax, cm-1): 3072 (=CH2), 2924
(CH2), 2852 (CH2), 1730 (C=O, ester), 1644 (C=C, vinyl), 1600,
1
1456 (C=C, aromatic), 1249, 1138, 1078 (C-O), 839 (C-H). H
NMR (CDCl3) d: 8.42 (d, 4H, J = 8.9 Hz), 8.02 (d, 4H, J = 8.4 Hz),
7.99 (d, 4H, J = 8.8 Hz), 7.98 (d, partially merged with 7.99 d,
2H), 7.76 (d, 2H, J = 2.1 Hz), 7.44 (dd, 2H, J = 8.2 Hz), 7.08 (d,
4H, J = 8.9 Hz), 5.97 (m, 2H, CH=), 5.25 (d, 2H, J = 12.5 Hz,
=CH2), 5.18 (d, 2H, J = 9.4 Hz, =CH2), 4.15 (t, 4H, J = 6.8 Hz,
OCH2-), 2.62 (m, 4H, -CH2). 13C NMR (CDCl3) d: 29.50, 69.60,
114.44, 115.21, 118.28, 118.77, 119.41, 121.33, 122.39, 124.45,
125.55, 129.23, 130.51, 131.56, 132.65, 134.51, 147.34, 149.23,
155.12, 161.22, 164.77. 2,7-Naphthalene bis[4-{2-[4-(pent-4-
enyloxy)phenyl]diazenyl}benzoate] (4b): Compound 4b was
synthesis from 3b according to our reported paper. Yield of 4b:
0.018 g (35%). IR (KBr, nmax, cm-1): 3078 (=CH2), 2924 (CH2),
2852 (CH2), 1730 (nC=O, ester), 1647 (C=C, vinyl), 1600, 1456
Characterization: The structures of the intermediates and
product were confirmed by spectroscopic methods: IR spectra
were recorded with a Perkin Elmer (670) FTIR spectrometer. 1H
NMR (500 MHz) and 13C NMR (125 MHz) spectra were recorded
with a Bruker (DMX500) spectrometer. The transition tempera-
tures and their enthalpies were measured by differential scanning
calorimetry (Perkin DSC 7) with heating and cooling rates were
10 °C min-1 and melting point of the intermediate compounds
were determined by DSC. Optical textures were obtained by us-
ing Olympus BX51 polarizing optical microscope equipped with
1
(C=C, aromatic), 1249, 1138, 1078 (C-O), 839 (C-H). H NMR
(CDCl3) d: 8.40 (d, 4H, J = 8.9 Hz), 8.02 (d, 4H, J= 8.6 Hz), 7.98
(d, 2H, J = 8.9 Hz), 7.92 (d, 2H, J = 8.9 Hz), 7.75 (dd, 2H, J = 8.2
Hz), 7.71 (d, 2H, J = 6.1 Hz), 7.43 (dd, 2H, J = 8.6 Hz), 7.06 (t,
2H, J = 8.5 Hz), 5.88 (m, 2H, CH=), 5.10 (d, 2H, J = 16.2 Hz,
576
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J. Chin. Chem. Soc. 2014, 61, 571-577

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Naphthalene-Based Liquid Crystals for Photoswitching Properties
Appl. Phys. 2005, 97, 093105.
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).
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Absorption spectra for photochromic study were recorded
using a Shimazdu 3101 PC UV-Vis spectrometer. All the solu-
tions were prepared and measured under air in the dark at room at
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 irradi-
ated at exc. = 254 nm, 365 and 436 nm respective, using a 200
watt high pressure Hg-lamp HBO 200 (NARVABerlin, Germany)
and filters IF 254, HgMon 365, HgMon 436 (Zeiss, Jena, Ger-
many) generating monochromatic light as excitation source. Addi-
tional protection glass filters Code-No. 601 were used for irradia-
tion at 365 nm and 254 nm and Code-No. 805 (both Schott, Jena,
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ACKNOWLEDGMENTS
This research was supported by Universiti Malaysia
Pahang (RDU-100338, 120367). A special thank goes to
Mrs. K. N. Vasudha for supporting many aspect regarding
this work carry out.
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