Synthesis and characterization of liquid crystalline azobenzene chromophores with fluorobenzene terminal

Article abstract of DOI:10.1016/j.jfluchem.2013.10.004

Two series of fluorine-substituted benzoate ester type rod-shaped liquid crystals incorporating the azobenzene as side arm linked with terminal double bonds as polymerizable functional groups were synthesized and characterized by polarized-light optical m

Full text of DOI:10.1016/j.jfluchem.2013.10.004

Journal of Fluorine Chemistry 156 (2013) 230–235
Contents lists available at ScienceDirect
Journal of Fluorine Chemistry
journal homepage: www.elsevier.com/locate/fluor
Synthesis and characterization of liquid crystalline azobenzene
chromophores with fluorobenzene terminal
a
a
a
Md Lutfor Rahman a, , Gurumurthy Hegde , Mahrokh Azazpour , Mashitah Mohd Yusoff ,
*
Sandeep Kumar b
a Faculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300 Gambang, Kuantan, Malaysia
b Raman Research Institute, C.V. Raman Avenue, Sadashivanagar, Bangalore 560080, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Two series of fluorine-substituted benzoate ester type rod-shaped liquid crystals incorporating the
azobenzene as side arm linked with terminal double bonds as polymerizable functional groups were
synthesized and characterized by polarized-light optical microscopy (POM), differential scanning
calorimetry (DSC) and UV–visible spectroscopy investigations. Thus, rod-shaped monomers, namely 4a
and 4b having odd and even number of carbon in the terminal group exhibited nematic phase and SmA
type phase was found at lower temperature. Compound 5 showed nematic phase whereas compound 6
showed SmA phase. These rod-shaped molecules exhibit strong photoisomerization behaviour in
solution. The photoswitching properties of the compounds showed trans to cis isomerization in about
10 s, whereas the reverse process takes place about 120 min in solutions. Proposed materials may have
potential to use it in optical storage devices.
Received 12 July 2013
Received in revised form 14 September 2013
Accepted 1 October 2013
Available online 11 October 2013
Keywords:
Azobenzene chromophores
Fluorine-substituted benzene
Nematic phase
Smectic phase
ß 2013 Elsevier B.V. All rights reserved.
Photo-switching property
1. Introduction
4-fluorobenzene and 3,4-difluorobenzene derivatives are used as
STN and TFT liquid crystalline materials [10,13,14].
Liquid crystals (LCs) are widely used as anisotropic materials
having self-organizing properties. They offer great advantages over
isotropic systems [1]. Due to their unique properties, liquid
crystals found widespread applications in display technology and
in other areas such as optical storage device [2], solar cells [3], ion
conductors [4,5] and templates for nanoparticles [6,7]. The
introduction of a fluorine atom or fluorinated group into liquid
crystalline systems changes physical characters of the LC
materials, which could be of intensive interest in display and
non-display applications [8]. The liquid crystals with fluorinated
substituents alter many physical parameters such as, broader
nematic phase range, lower melting point, high dielectric
anisotropy, and good voltage holding ratios [9].
Some fluorinated liquid crystals (FLCs) of fused ring systems for
TFT-LCD are currently commercially available [10]. However, FLCs
are less reported materials in spite of their potential applications
[11,12]. Tao et al. [1] synthesized pyridinium-based liquid crystals
with an extended fluorinated pyrrolidine unit and studied the
structure–properties relationship. There are many reports where
A field of research that is growing steadily is that of
photoinduced phenomenon, in which incident light itself brings
about molecular ordering/disordering of the liquid crystalline
system [15]. The photoinduced dichroism and optical anisotropy is
being proposed as future potential technology for optical storage
devices [16,17]. The azobenzene moieties are well known
molecules which exhibit reversible isomerization behaviour upon
irradiations with UV and visible light [18]. The energetically more
stable trans configuration convert into cis configuration upon
absorption of UV light (ꢀ365 nm). Photoinduced effects are
adopted by the liquid crystals in which the azobenzene group is
either chemically attached to the molecule of interest or used as a
dopant in a liquid crystal host material [19–22]. Liquid crystalline
materials in which the phase transitions take place by the
isomerization of the photoactive molecules are of significant
interest. The time required for trans to cis isomerization is much
lower compared to thermal back relaxation time. The UV
irradiation is done in the nematic phase for the material exhibit
a nematic–isotropic (N–I) transition and the lowering of the
transition temperature, TNI, could induce an isothermal N–I
transition. Thus, the photochemically induced transition is
promising for the optical image-storing systems.
In this work, we report the synthesis, characterization, and
properties of liquid crystalline azobenzene chromophores with
*
Corresponding author. Tel.: +60 169399011; fax: +60 95492766.
E-mail address: lutfor73@gmail.com (M.L. Rahman).
0022-1139/$ – see front matter ß 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jfluchem.2013.10.004

M.L. Rahman et al. / Journal of Fluorine Chemistry 156 (2013) 230–235
231
O
i
O
O
H3C
O
N
N
OH
H3C
N
N
n
O
1
Br
n
2a - b : n = 3 - 4
HO
O
ii
iii
O
N
N
n
F
F
F
K
OH
3a - b : n = 3 - 4
F
OH
F
F
F
F
O
N
O
N
n
F
F
O
4a - b : n = 3 - 4
Scheme 1. Synthesis of compounds 4a–b. Reagents and conditions: (i) K2CO3, KI; (ii) KOH, MeOH; (iii) DCC, DMAP.
terminal fluorobenzene moiety. It is been well known proven fact
that, longer the thermal back relaxation, better is the device and
with this aspect, presented compounds may be suitable for the
creation of storage devices.
microscopy and gave the enthalpy changes (D
H/J gÀ1) associated
with these phase transitions. Compound 4a shows three transition
peaks on heating which correspond to the Cr–SmA, SmA–N and N–I
transitions. On cooling, the isotropic–nematic, nematic–smectic
and smectic-crystal transition appears for compound 4a (Table 1).
Compound 4b displayed also three peaks on heating and on
cooling, again three transition peaks were observed, which shows
nematic and smectic phase (Table 1). Both compounds showed
stable enantiotropic mesophases. In compounds 5 and 6 only two
transitions peaks were obtained which correspondence to nematic
phase for 5 and SmA phase for 6, respectively. Compound 4a having
odd numbered alkyl chain (C5 including vinyl group) shows
nematic to isotropic transition is at 145.2 8C whereas compound 4b
having even number of alkyl chain (C6 including vinyl group)
shows nematic to isotropic transition at 158.2 8C. On the other
hand compound 5 and 6 also having odd numbered alkyl chain (C5
including vinyl group), but fluorine atom is different from
compound 4a. Compound 5 possesses only one fluorine atom
whereas compound 6a contains two fluorine atoms. Compound 5
shows the nematic–isotropic transition at 158.9 8C while com-
pound 6 exhibits the SmA–isotropic transition at 157.6 8C. Thus, as
the number of fluorine atom increases, transition temperatures
decreases [23].
2. Results and discussion
2.1. Synthesis
The synthesis and purification of compounds 4a and 4b were
performed as depicted in Scheme 1. Compounds 1 was alkylated
with 5-bromo-1-pentene in the presence of potassium carbonate
as base to give ethyl 4-{2-[4-(pent-4-enyloxy)phenyl]diazenyl}-
benzoate 2a. Compound 2b was prepared with the same method of
compound 2a. Then the ester compounds 2a–b were hydrolyzed
under basic conditions to yield the benzoic acid 4-{2-[4-(pent-4-
enyloxy) phenyl]diazenyl}benzoic acid 3a. Compound 3b was
prepared with the same method of compound 3a. In the final step,
two equivalents of the acids 3a–b were esterified with one
equivalent of pentafluorophenol by DCC and DMAP to achieve the
target molecules 4a–b (Scheme 1). The synthesis of compounds 5
and 6 were performed as depicted in Scheme 2. About one
equivalent of the acids 3a was esterified with one equivalent of 2-
fluorophenol or 3,6-difluorophenol by DCC and DMAP to achieve
the target molecules 5 and 6 (Scheme 2).
2.2.1. Polarizing optical microscopy (POM) studies
Under the polarizing microscope, a schlieren texture as typical
for nematic phase for compound 4a was observed upon cooling
from the isotropic phase. Optical texture was taken for compound
4a at 124 8C (Fig. 1a) and on further cooling a broken fan shaped
texture as typical for smectic A phase was found at lower
2.2. Mesomorphic properties
Differential Scanning Calorimetry (DSC) studies confirmed the
phase transition temperatures (T/8C) observed by polarizing
HO
N
O
N
O
3a : n = 5
F
OH
HO
iv
v
F
F
F
O
O
O
N
N
O
O
N
N
F
O
F
6
5
Scheme 2. Synthesis of compounds 5a and 6a. Reagents and conditions: (iv) DCC, DMAP; (v) DCC, DMAP.

232
M.L. Rahman et al. / Journal of Fluorine Chemistry 156 (2013) 230–235
Table 1
crystallization. All the transition temperatures observed under
POM were matching with DSC data.
Phase transition temperature (T/8C) and associated transition enthalpy values (DH/
J gÀ1) in parentheses given for the second heat and cooling of DSC scans for
compounds 4a–b, 5 and 6.
2.2.2. Photo-switching study
Compound
Scan
Phase transitions
Our preliminary investigation showed that all rod-shaped
molecules exhibit similar absorption spectra due to their similar
molecular structure, the only minor difference is the alkyl chain
(n = 3–4) and fluorine atom which does not alter the electronic
transitions. Consequently, compounds 4a and 4b were considered
for photoisomerization study. Photoswitching studies were
performed on solutions with appropriate solvents give an idea
of the materials behaviour with respect to UV light and also these
results are indispensable for creating optical storage devices.
Figs. 2 and 3 depict the absorption spectra of 4a and 4b,
respectively before and after UV illumination with 365 nm filter.
Heat filter is used to prevent any heat effects arising to the sample.
The absorption spectra of compound 4a and 4b show absorbance
maxima at 364 nm. The absorption spectra of compound was
carried out in dimethyl formamide (DMF) solution having
concentration C = 1.1 Â 10À5 mol LÀ1. The strong absorbance in
4a
Heat
Cool
Heat
Cool
Heat
Cool
Heat
Cool
Cr 110.5 (69.4) SmA 117.7 (1.3) N 145.2 (1.1) I
I 142.5 (0.91) N 110.1 (1.0) SmA 70.8 (52.0) Cr
Cr 105.7 (59.2) SmA 116.8 (1.2) N 158.2 (1.4) I
I 147.4 (0.9) N 113.7 (1.2) SmA 80.8 (50.0) Cr
Cr 111.1 (57.0) N 158.9 (1.0) I
4b
5
I 149.4 (1.5) N 83.5 (34.6) Cr
6
Cr 131.8 (62.8) SmA 157.6 (12.0) I
I 148.0 (11.8) SmA 112.1 (58.4) Cr
Abbreviations: Cr = crystal, N = nematic, SmA = smectic A phase, I =isotropic phase.
temperature (Fig. 1b). For compound 4b, a schlieren texture was
also observed for nematic phase upon cooling from the isotropic
phase. On further cooling a fan shaped texture which is smectic A
phase was observed as shown in Fig. 1d. Compound 5 also exhibits
schlieren texture (Fig. 1e) of typically nematic phase upon cooling
from the isotropic phase, however, on further cooling no phase
was observed. In the case of compound 6, a broken fan shaped
texture, typical for smectic A phase, was also observed as shown in
Fig. 1f. On further cooling no other phase was observed except
the UV region at 364 nm corresponds to
isomer (trans isomer) and a very weak absorbance in the visible
region around 450 nm represents to n– * transition of Z isomer
(cis isomer). After ꢀ10 s illumination, there is no change in
p–p* transition of the E
p
Fig. 1. Polarized optical micrographs obtained from cooling of isotropic phases of (a) nematic phase of 4a at 124 8C, (b) SmA phase of 4a at 105 8C, (c) nematic phase of 4b at
142 8C, (d) SmA phase of 4b at 110 8C, (e) nematic phase of 5a at 125 8C, (f) SmA phase of 6a at 130 8C.

M.L. Rahman et al. / Journal of Fluorine Chemistry 156 (2013) 230–235
233
NO UV
0.2 sec
0.4 sec
0.9 sec
1.3 sec
1.8 sec
2.3 sec
3.1 sec
4.1 sec
5.6 sec
7.6 sec
8.6 sec
10.2 sec
4a: UV ON
4a: UV OFF
0 min
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
2.0 min
4.0 min
6.0 min
9.0 min
14.0 min
19.0 min
24.0 min
29.0 min
40 min
trans
cis
60 min
80 min
100 min
120 min
300
350
400
(nm)
450
500
550
300
350
400
450
500
550
λ
λ
(nm)
Fig. 2. Absorption spectra of 4a with different exposure time of UV light with
365 nm filter. No UV corresponds to the 0 s of UV light illumination (absence of UV
light).
Fig. 5. Thermal back relaxation process for the compound 4a shows that to relax
from cis to trans takes around 120 min. 0 min corresponds to the UV off, after
illuminating the material for 20 s (photo stationery state).
Before UV
No UV
4b:UV Off
125 min
4b:UV ON
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.2 sec
0.6 sec
0.8 sec
1.2 sec
1.4 sec
1.7 sec
2.0 sec
2.4 sec
2.8 sec
3.7 sec
4.2 sec
6.4 sec
7.4 sec
9.4 sec
115 min
110 min
100 min
cis
0.8
0.6
0.4
0.2
0.0
90 min
85 min
80 min
75 min
70 min
60 min
50 min
40 min
30 min
20 min
10 min
5 min
Trans
300
350
400
450
500
550
300
350
400
450
500
550
λ
(nm)
λ
(nm)
Fig. 6. Thermal back relaxation process for the compound 4b after illuminating the
material for 20 s (photo stationery state) shows that to relax from cis to trans takes
around 125 min. Before UV corresponds to the state before illuminating the
materials.
Fig. 3. Absorption spectra of 4b with different exposure time of UV light. No UV
corresponds to the 0 s of UV light illumination (absence of UV light).
absorption spectrum confirms the photo saturation of E–Z
isomerization process.
The time dependence of the Z–E absorption of compound 4a
and 4b is shown in Fig. 7. Data were obtained from Figs. 5 and 6 at
fixed wavelength of 364 nm as a function of recovery time. The
curve shows that thermal back relaxation occurs within 125 min
which is reasonably fast as compared with nematic to isotropic
phase involved thermal back relaxation [24]. Presented studies
may lead to the creation of optical storage devices due to their
impressive thermal back relaxation time.
A possible reason for observing faster thermal back relaxation
as well as UV ON process could be that the phases involved on both
sides of transition possess a layer structure (Smectic phase) and
that the change that occurs is confined to in-plane rotation of the
molecules. The argument is supported by the fact that a similar
feature was observed in another case wherein the two phases
involved have a layer structure [24].
The E–Z absorption of compound 4a and 4b as a function of
exposure time is shown in Fig. 4. Data is extracted from Figs. 2 and
3 at fixed wavelength of 364 nm and absorption values at different
exposure time were recorded. The time dependent curve shows
that photosaturation occurs within 10 s of illumination which is
very fast as compared with nematic to isotropic phase involved
photoisomerization [24]. The reverse transformation from Z to E
can be brought by two methods, one by keeping the solution in
dark and other by shining white light of higher wavelength. The
earlier process is well known as thermal back relaxation [2].
The thermal back relaxation process of compound 4a and 4b is
shown in Figs. 5 and 6, respectively where the solution is shined
continuously for 20 s (photo stationery state) and kept in the dark
and then at subsequent time intervals, spectral data were recorded.
UV ON dynamics
1.0
1.0
0.8
0.6
0.4
0.2
4a
4b
UV OFF dynamics
0.8
4a
4b
0.6
0.4
0.2
0
3
6
9
12
0
20
40
60
80
100 120
Time (S)
Time (min)
Fig. 4. Time dependence photoisomerization curve of E isomer (4a–4b) showing
Fig. 7. Time dependence photoisomerization curve of Z isomer (4a–b) showing
effect of UV illumination.
thermal back relaxation time.

234
M.L. Rahman et al. / Journal of Fluorine Chemistry 156 (2013) 230–235
3. Conclusion
1H, J = 17.2 Hz, 55CH2), 5.01 (dd, 1H, J = 8.6 Hz, 55CH2), 4.08 (t,
2H, J = 6.5 Hz, OCH2–), 2.27 (m, 2H, –CH2–), 1.95 (m, 2H, –CH2),
1.54 (m, 2H, –CH2). 13C NMR (CDCl3)
: 28.28, 30.04, 34.34,
67.61, 114.89, 115.48, 120.20, 122.79, 125.49, 127.61, 130.58,
131.73, 132.56, 137.56, 138.56, 146.89, 156.44, 161.02, 162.59,
164.98.
The fluorine-substituted benzoate ester linked to rod-shaped
azobenzene liquid crystals with terminal double bonds as
polymerizable functional groups were synthesized and character-
ized. The double bonds can be used for preparation of polymers or
silyl functionalized mesogens, whereas the presence of the azo
linkage in these liquid crystals monomer is suitable for photo-
chromism studies and trans-cis-trans isomerizations cycles under
UV irradiation. Two compounds (4a and 4b) exhibited nematic
phase and on further cooing SmA phase, in addition another two
compounds with less fluorine atoms show only one type
mesophase with little higher temperature. These rod-shaped
molecules exhibit strong photoisomerization behaviour in solu-
tions. The photoswitching properties of compounds shows trans to
cis isomerization ranging 10–11 s, whereas reverse process takes
about 125 min in solutions. Presented studies may lead to the
creation of optical storage devices due to their impressive thermal
back relaxation time.
d
4.4. 2-Flourophenyl 4-{[4-(pent-4-en-1-
yloxy)phenyl]diazenyl}benzoate (5)
Compound 3a (0.075 g, 0.2419 mmol), 2-fluorophenol
(0.027 g, 0.2419 mmol), 30 ml of dry dichloromethane, DMAP
(3.6 mg, 0.030 mmol) and DCC (0.0610 g, 0.30 mmol) were
stirred for 48 h. Esterification was carried out as synthesis of
4a. Yield of 5: 36%. IR, n
max/cmÀ1 3122 (55CH2), 2924 (CH2), 2850
(CH2), 1755 (C55O, ester), 1620 (C55C, vinyl), 1601, 1522, 1459
(C55C, aromatic), 1242, 1132, 1050 (C–O), 842 (C–H). 1H NMR
(CDCl3) d: 8.31 (d, 2H, J = 8.4 Hz), 8.00 (d, 2H, J = 7.1 Hz), 7.95 (d,
2H, J = 8.6 Hz), 7.76 (d, 2H, J = 8.7 Hz), 7.22 (s, 1H), 7.20 (s, 1H),
7.02 (d, 2H, J = 8.6 Hz), 5.83 (m, 1H, CH55), 5.06 (dd, 1H,
J = 17.3 Hz, 55CH2), 5.04 (dd, 1H, J = 8.6 Hz, 55CH2), 4.05 (t, 2H,
J = 6.4 Hz, OCH2–), 2.16 (m, 2H, –CH2–), 1.87 (m, 2H, –CH2). 13C
4. Experimental
4.1. Materials synthesis
NMR (CDCl3) d: 25.45, 27.95, 69.43, 115.28, 118.81, 120.14,
121.95, 122.47, 125.37, 130.66, 131.82, 132.46, 138.66, 145.67,
147.34, 155.55, 161.45, 164.65.
Ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate (1) was synthe-
sized according to our earlier paper [25]. Other intermediate
compounds such as ethyl 4-{2-[4-(pent-4-enyloxy)phenyl]diaze-
nyl}benzoate (2a), ethyl 4-{2-[4-(hex-5-enyloxy)phenyl]diazenyl}
benzoate (2b), 4-{2-[4-(pent-4-enyloxy) phenyl]diazenyl}benzoic
acid (3a) and 4-{2-[4-(hex-5-enyloxy)phenyl]diazenyl}benzoic
acid (3b) were synthesized according to the earlier paper [2,26].
4.5. 3,5-Diflourophenyl 4-{[4-(pent-4-en-1-
yloxy)phenyl]diazenyl}benzoate (6)
Compound 3a (0.075 g, 0.2419 mmol), 2-fluorophenol
(0.0314 g, 0.2419 mmol), 30 ml of dry dichloromethane, DMAP
(3.6 mg, 0.030 mmol) and DCC (0.0610 g, 0.30 mmol) were
stirred for 48 h. Esterification was carried out as synthesis of
4.2. 2,3,4,5,6-Pentaflourophenyl 4-{[4-(pent-4-en-1-
yloxy)phenyl]diazenyl}benzoate (4a)
4a. Yield of 6: 39%. IR, n
max/cmÀ1 3127 (55CH2), 2928 (CH2), 2851
(CH2), 1757 (C55O, ester), 1622 (C55C, vinyl), 1600, 1521, 1452
(C55C, aromatic), 1249, 1133, 1057 (C–O), 840 (C–H). 1H NMR
(CDCl3) d: 8.32 (d, 2H, J = 8.6 Hz), 8.00 (d, 2H, J = 7.2 Hz), 7.96 (d,
Compound 3a (0.311 g, 1.00 mmol), pentafluorophenol
(0.184 g, 1.00 mmol), 85 ml of dry dichloromethane, DMAP
(14.6 mg, 0.012 mmol) and DCC (0.2480 g, 1.20 mmol) were stirred
for 48 h. The mixture was filtered and the filtrate was washed with
acetic acid and water. The solvent was removed under reduced
pressure and the 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).
2H, J = 8.4 Hz), 7.76 (d, 2H, J = 8.6 Hz), 7.21 (s, 1H), 7.03 (d, 2H,
J = 8.7 Hz), 5.84 (m, 1H, CH55), 5.08 (dd, 1H, J = 17.6 Hz, 55CH2),
5.06 (dd, 1H, J = 8.6 Hz, 55CH2), 4.04 (t, 2H, J = 6.4 Hz, OCH2–),
2.18 (m, 2H,–CH2–), 1.86 (m, 2H, –CH2). 13C NMR (CDCl3)
d:
25.88, 27.87, 69.39, 115.29, 118.71, 120.27, 121.87, 122.42,
125.37, 130.46, 131.55, 132.33, 138.56, 145.72, 147.32, 155.54,
161.34, 164.67.
Yield of 4a: 35%. IR, n
max/cmÀ1 3124 (55CH2), 2922 (CH2), 2852
(CH2), 1764 (C55O, ester), 1626 (C55C, vinyl), 1600, 1519, 1456
(C55C, aromatic), 1240, 1138, 1051 (C–O), 844 (C–H). 1H NMR
4.6. Instruments
(CDCl3) d: 8.32 (d, 2H, J = 8.5 Hz), 8.00 (d, 2H, J = 7.0 Hz), 7.96 (d, 2H,
J = 8.4 Hz), 7.04 (d, 2H, J = 8.8 Hz), 5.86 (m, 1H, CH55), 5.05 (dd, 1H,
J = 17.6 Hz, 55CH2), 5.02 (dd, 1H, J = 8.8 Hz, 55CH2), 4.08 (t, 2H,
J = 6.4 Hz, OCH2–), 2.16 (m, 2H, –CH2–), 1.88 (m, 2H, –CH2). 13C
NMR (CDCl3) d: 26.39, 28.35, 69.39, 115.22, 118.77, 120.20, 121.90,
122.55, 125.47, 130.58, 131.73, 132.56, 138.56, 145.23, 147.46,
155.62, 161.68, 164.87.
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 temperatures and their
enthalpies were measured by differential scanning calorimetry
(Perkin DSC 7) with heating and cooling rates were 10 8C minÀ1
and melting point of the intermediate compounds were
determined by DSC. Optical textures were obtained by using
Olympus BX51 polarizing optical microscope equipped with a
MettlerToledo FP82HT hot stage and a FP90 central processor
unit. UV/vis absorption spectra were recorded using UV-Visible
spectrophotometer obtained from Ocean Optics (HR2000+). For
photo-switching studies in solutions, fluorine azobenzene liquid
crystalline monomers were dissolved in dimethylformamide at
suitable concentrations. Photo-switching behaviour of the
azobenzene containing fluorine compounds investigated by
illuminating with OMNICURE S2000 UV source equipped with
365 nm filter.
4.3. 2,3,4,5,6-Pentaflourophenyl 4-{[4-(hex-5-en-1-
yloxy)phenyl]diazenyl}benzoate (4b)
Compound 3b (0.325 g, 1.00 mmol), pentafluorophenol
(0.184 g, 1.00 mmol), 85 ml of dry dichloromethane, DMAP
(14.6 mg, 0.012 mmol) and DCC (0.2480 g, 1.20 mmol) were
stirred for 48 h. Esterification was carried out as synthesis of 4a.
Yield of 4b: 40%. IR, n
max/cmÀ1 3116 (55CH2), 2924 (CH2), 2852
(CH2), 1755 (C55O, ester), 1622 (C55C, vinyl), 1600, 1518, 1456
(C55C, aromatic), 1261, 1057 (C–O), 858 (C–H). 1H NMR (CDCl3)
d
: 8.33 (d, 2H, J = 8.6 Hz), 8.00 (d, 2H, J = 7.5 Hz), 7.95 (d, 2H,
J = 8.5 Hz), 7.03 (d, 2H, J = 8.7 Hz), 5.82 (m, 1H, CH55), 5.09 (dd,

M.L. Rahman et al. / Journal of Fluorine Chemistry 156 (2013) 230–235
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This research was supported by UMP Research Grant (Nos: RDU
100338 and RDU 130619). A special thank goes to Mrs. K.N.
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