Synthesis of bent-shaped azobenzene liquid crystals derived from resorcinol for optical storage properties

Article abstract of DOI:10.1080/15421406.2016.1147328

The liquid crystalline and light-induced properties of resorcinol spacer based dimeric azobenzene were investigated. First, the compound was synthesized and elucidated the molecular structure by spectral analysis such as NMR, IR, and UV–VIS. polaring opti

Full text of DOI:10.1080/15421406.2016.1147328

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: http://www.tandfonline.com/loi/gmcl20
Synthesis of bent-shaped azobenzene liquid
crystals derived from resorcinol for optical storage
properties
A. R. Yuvaraj, Mohd Yusoff Mashitah & Rahman Lutfor
To cite this article: A. R. Yuvaraj, Mohd Yusoff Mashitah & Rahman Lutfor (2016) Synthesis of
bent-shaped azobenzene liquid crystals derived from resorcinol for optical storage properties,
Molecular Crystals and Liquid Crystals, 631:1, 21-30, DOI: 10.1080/15421406.2016.1147328
To link to this article: http://dx.doi.org/10.1080/15421406.2016.1147328
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Date: 13 July 2016, At: 03:05

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
, VOL. , –

http://dx.doi.org/./..
Synthesis of bent-shaped azobenzene liquid crystals derived
from resorcinol for optical storage properties
A. R. Yuvaraj, Mohd Yusoff Mashitah, and Rahman Lutfor
Faculty of Industrial Sciences and Technology, Universiti Malaysia Pahang, Gambang, Kuantan, Malaysia
ABSTRACT
KEYWORDS
The liquid crystalline and light-induced properties of resorcinol spacer
based dimeric azobenzene were investigated. First, the compound was
synthesized and elucidated the molecular structure by spectral analy-
sis such as NMR, IR, and UV–VIS. polaring optical microscope studied
showed that compound exhibit nematic and smectic A phases. Then,
the photoisomerization effect was evaluated in solution and also in the
solid state. The photosaturation occurred exactly at 18 sec whereas ther-
mal back relaxation was observed at around 11 hr. Long duration of the
thermal back relaxation is due to the presence of rigid resorcinol spacer
substituted in the bent-shaped molecules. This compound is useful in
the fabrication of optical storage device and molecular switches.
Azobenzene; liquid crystal;
optical storage;
photoisomerization;
synthesis
Introduction
The information can be optically stored via light irradiation method. This process is called
optical storage and the gadget is known as optical storage device. Many photo-sensitive
organic compounds were used to evaluate the light-induced behavior of optical data stor-
age device. Tacitly, the molecular structure and properties of illuminated light are responsible
for the performance of optical storage device [1, 2]. The photoisomerization and molecular
reorientation are depend on chromophores present in the chemical structures [3–5]. Azoben-
zene is a good candidate among other chromophores to evaluate light-induced effects, due to
its promising light sensitivity [1–4]. The liquid crystalline and photoisomerization proper-
ties of azobenzene derivatives were investigated by many research groups [1, 2–5]. The liquid
crystallinity of the bent-core molecules were first reported by Vorlander et al. [6]. However,
banana-shaped liquid crystals are also interested widely, due to its unique electro-optic and
photo-induced characteristics [7, 9]. Even though, banana-shaped liquid crystals have a sig-
nificant role in the guest–host system. It means these liquid crystals (LC) can be mixed as a
guest with commercial liquid crystal (host) to elucidate the device characteristics.
Azobenzene derivatives have delocalized electronic charge distribution between accepter
and donor atom or group of atoms substituted with it. Nevertheless, trans–cis isomerization
of azobenzene derivatives have a significant importance in the field of optics. This photo-
physical property plays the main role in the optical data storage. In particular, trans–cis iso-
merization is important especially in the research areas, such as holographic media [10], opti-
cal storage [11], and photoalignment of LC systems [12].
CONTACT Rahman Lutfor
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/gmcl.
 Taylor & Francis Group, LLC
lutfor@gmail.com, lutfor@ump.edu.my
©

2
2
A. R. YUVARAJ ET AL.
The spacer effect is significant in the symmetric bent-core LC molecules [13]. Recently, we
have reported the behavior of aliphatic and aromatic spacer incorporated in the azobenzene
dimeric system [14]. It is clear that different spacers exhibit different optical activities [14].
Hence, back relaxation of azobenzene dimers are quite interested, due to its dramatic variation
with respect to spacers present at the molecular center. trans–cis isomerization takes place in
the energetically more stable cis isomer of the azobenzene, when light of suitable wavelength
(
say 365 nm) is illuminated on the molecular system [1–4, 15]. Then, the reverse process is
spontaneous and it is called thermal back relaxation [2]. Cis–trans isomerization occurs in
two ways; one is by keeping the excited trans isomer in dark place (thermal back relaxation)
and other one is by shining 450 nm white light on the molecular system [16]. Among these,
the thermal back relaxation is considerable because of its spontaneity.
In this report, we have synthesized bent-shaped azobenzene liquid crystals having short
alkyl chains (C ), which is so far new compound. We also have investigated the mesomor-
6
phic (LC) and light-induced isomerization properties of resorcinol-based azobenzene liquid
crystals, which gave interesting photoisomerization properties. This new compound shows
suitable photochromic and optical data storage properties.
Experimental
Raw materials and instruments
Ethyl-4-aminobenzoate (Fluka), sodium nitrite (Aldrich), hydrochloric acid (Fluka), sodium
hydroxide (Fluka), methanol (Fluka), phenol (Aldrich), 1-bromo hexane (Fluka), potas-
sium carbonate (Aldrich), potassium iodide (Aldrich), acetone (Fluka), potassium hydroxide
(
(
Aldrich), 1, 3-dicyclohexylcarbodiimide (DCC) (Fluka), 4-(N, N-dimethylamino)pyridine
DMAP) (Fluka), dichloromethane (Fluka), Resorcinol (Aldrich). Acetone was dried over
phosphorus pentoxide (Fluka) and dichloromethane was dried over calcium hydride (Fluka)
and distilled out before the use. Other solvents and chemicals were used as such.
Instrumental details are as follows: IR spectra were recorded using a Perkin Elmer (670)
1
13
FTIR spectrometer. H NMR (500 MHz) and C NMR (125 MHz) were recorded in Bruker.
The photoswitching study was performed using UV–Visible spectrophotometer from Ocean
Optics (HR2000+). For photoswitching studies in solutions, the compounds were dissolved
−5
−1
in chloroform at fixed concentration of C = 1.1 × 10 mol L . Photoisomerization of
these compounds were investigated by illuminating it with an OMNICURE S2000 UV source
equipped with a 365 nm filter and heat filter was used to avoid unwanted heat from the radi-
2
ation. The intensity used for measuring E/Z is around 5.860 mW/cm measured using UV
meter. The photoswitching behavior of this compound was studied by shining UV light of
suitable wavelength. Then, mesomorphic property was investigated using polarized optical
microscopy (Nikon, Olympus BX 51 POL) along with linkam hotstage. Although both heat-
ing and cooling cycles were measured using DSC by NETZSCH DSC214 Polyma. The X-ray
˚
diffraction (XRD) was measured using Cu-Kα radiation with λ = 1.54 A and rotation anode
generator of 4,000 W (Rigaku Ultrax-18) connected with graphite crystal monochromator.
The sample was heated to obtain the mesophase in Hampton research capillaries with 0.5 mm
diameter.
Synthesis and elucidation
The resorcinol spacer substituted azobenzene dimer (V) can be synthesized by five steps. First,
4-ethyl aminobenzoate was diazotized using sodium nitrite in the presence of acid medium.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
23
Scheme . Synthesis of bent-shaped azobenzene liquid crystals.
Then, the diazonium ion (I) produced in the first step was coupled by phenol to get II. Next,
free hydroxyl group of the compound II was methylated with bromo-alkene in the presence
of mild basic medium. On the other hand, the substituted ester group was hydrolyzed using
potassium hydroxide and water. Lastly, two molecules of IV were coupled with one molecule
of resorcinol by DCC coupling reaction. The synthetic strategy is given in Sch. 1.
General procedure for diazotization reaction (II)
The solution of 4-ethyl amino benzoate (7.59 g, 46.00 mmol) was prepared by methanol
(
40 mL) and cooled to 2°C. 25% HCl was added dropwise to the solution at 2°C. The solu-
tion of sodium nitrite (3.03 g, 44.6 mmol) was prepared in water and added to the reaction
mixture dropwise at 2°C for 15 min to get I. Then, phenol (4.19 g, 44.6 mmol) was dissolved
in methanol and added to the reaction mixture at 2°C. Next, 50% NaOH solution was added
to the reaction mixture to increase the pH (>9.0) and reaction mixture was stirred for 4 hr.
Finally, the reaction mixture was poured into the beaker containing distilled water (2 L). Solu-
tion is filtered to collect the reddish brown precipitate and solid II was recrystallized from
methanol.
A reddish yellow colored solid; yield: 82%; melting point: 160.2–160.5°C; IR (KBr Pellet)
−1
γ
in cm : 3321 (OH), 1728 (CO C H ), 1602, 1484, 1248, 1140 (C=C aromartic), 829
max
2
2
5
1
(
2
1
C–H); H NMR (500 MHz, acetone-d6): δ 8.12 (d, J = 10 Hz, 2H, Ar), 7.92 (d, J = 5 Hz,
H, Ar), 7.88 (d, J = 5 Hz, 2H, Ar), 7.14 (d, J = 5 Hz, 2H, Ar), 5.54 (s, 1H, OH), 4.42 (q, J =
2.2 Hz, 2H, CH CH ), 1.44 (t, 3H, CH CH ).
2
3
2
3
General procedure for etherification reaction (III)
The solution of II (4.77 g, 18.5 mmol) and 1-bromohexane (6.12 g, 37.1 mmol) was prepared
by acetone (30 mL). The catalytic amount of potassium iodide (50 mg) and also potassium

24
A. R. YUVARAJ ET AL.
carbonate (2.55 g, 18.5 mmol) were added to the reaction portionwise and refluxed. The reac-
tion was completed after 24 hr and it was confirmed by TLC. Lastly, the reaction compound
III was dried and used it for the next step.
General procedure for hydrolysis of ester (IV)
Dried compound III from the previous step was dissolved in methanol (100 mL) and the solu-
tion of potassium hydroxide (3.48 g, 62.2 mmol) in water (10 mL) was added dropwise. The
reaction mixture was refluxed for 4 hr and the completion of the reaction was confirmed by
TLC. Then, the reaction mixture was washed with n-hexane to remove the nonpolar impuri-
ties. Dilute HCl was added to the crude solution until the pH of the solution become 3. The
compound was extracted with ethyl acetate and brine wash was given to remove ionic impu-
rities. The crude solution was dried over anhydrous sodium sulfate and solvent was removed
by rotor vapor. The crude solid obtained was recrystallized from methanol.
A dark yellow colored solid; yield: 75%; melting point is 173.2–173.6°C; IR (KBr pellet)
−1
γ
in cm : 1681 (COOH), 2942, 1421, 1600, 1289„ 1138 (C=C aromatic), 1248, 1025 (C–
max
1
O), 837 (C–H); H NMR (500 MHz, DMSO): δ 8.12 (d, J = 5 Hz, 2H, Ar), 7.93 (d, J = 5 Hz,
2
1
H, Ar), 7.91 (d, J = 5 Hz, 2H, Ar), 7.14 (d, J = 10 Hz, 2H, Ar), 4.09 (t, J = 15 Hz, 2H, OCH ),
2
.74 (q, J = 15 Hz, J = 10 Hz, 2H, CH ), 1.43 (q, J = 5 Hz, J = 10 Hz, 2H, CH ), 1.32 (m,
2
2
13
J = 5 Hz, 4H, CH ×2), 0.89 (t, J = 5 Hz, 3H, CH ); C NMR (125 MHz, DMSO): δ 166
2
3
(
COOH), 162, 154, 146, 132, 130, 125, 123, 122, 119, 114, 68 (OCH ), 30, 28, 25, 22, 13.
2
General procedure to esterification by DCC coupling reaction (V)
The solution of IV (200 mg, 0.49 mmol) and resorcinol (30 mg, 0.27 mmol) was prepared
with dichloromethane. The reaction mixture was stirred for 20 min. Then, catalytic amount
of DMAP (10 mg) and also DCC (84 mg, 0.81 mmol) was added portionwise. The reaction
mixture was stirred for 9 hr and the completion of reaction was confirmed by TLC. The reac-
tion mixture was quenched with water (10 mL) and also washed with dilute acetic acid. Then,
the compound was extracted with ethyl acetate and dried over anhydrous sodium sulfate.
Ethyl acetate was removed by rotor vapor and the crude sample was purified by column chro-
matography with the mobile phase of 80% ethyl acetate in n-hexane and finally the target
compound (V) was recrystallized from methanol.
−1
A pale yellow colored solid; yield: 35%; melting point is 173.5–173.8°C; IR (KBr) cm :
1
(
8
5
735 (C = O), 2923, 2859, 1597, 1654, 1283, 1107 (C=C aromatic), 1253, 1025 (C–O), 841
1
C–H); H NMR (500 MHz, CDCl ): δ 8.33 (d, J = 10 Hz, 4H, Ar), 7.89–7.99 (m, J = 5 Hz,
3
H, Ar), 7.67 (d, J = 5 Hz, 4H, Ar), 7.52 (s, 1H, Ar), 7.02 (q, J = 5 Hz, 4H, Ar), 6.77 (d, J =
Hz, 2H, Ar), 4.07 (t, J = 10 Hz, 4H, OCH ×2), 2.22 (q, 4H, CH ×2), 2.01 (m, 4H, CH
2
2
2
×
2), 1.35 (m, 8H, CH ×4), 0.92 (q, 6H, CH ×2).
2
3
Result and discussion
Mesophase characterization
Differential Scanning Calorimetry (DSC) provides the information about phase transition
temperature and change in enthalpy for the liquid crystal compound V (Fig. 1). Both heating
−1
and cooling cycles of DSC were measured with the constant rate of 10°C min with mainly

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
25
−
Figure . DSC heating and cooling cycles for V (°C min ).
second heating and cooling cycle was considered in order to investigate the mesomorphic
properties.
In compound V, endothermic peaks were found at 125.8°C and 158.2°C. Then, the sample
was cooled spontaneously from the isotropic phase and the exothermic peaks were observed
at 158.1°C and 152°C before crystallization (115.4°C). Therefore, the phase sequence for V is
Cr 125.8, SmA 158.2 on heating and I 158.1, N 152, SmA 115.4, Cr on cooling mode.
The structure of the mesophase was captured using polaring optical microscope (POM,
Fig. 2) and the hot stage was used to control the temperature during the observation of
−1
mesophase. The temperature rate of 5°C min was maintained in the hot stage during the
determination of liquid crystallinity of V.
The mesophase was further evaluated by X-ray diffraction method for the compound V.
The X-ray diffraction was measured using Cu-Kα radiation with λ = 1.54A° and rotation
anode generator of 4,000 W (Rigaku Ultrax-18) connected with graphite crystal monochro-
mator. The sample was heated to obtain the mesophase in Hampton research capillaries with
0
.5 mm diameter. The assignment of mesophase was confirmed by X-ray analysis and the
intensity verses 2θ was plotted using the diffraction pattern of the compound V as shown in
Fig. 3 (ii). The X-ray diffraction was studied at mesophase via cooling mode from the isotropic
Figure . MicroscopicimageswerecapturedbyPOMforandV(leftSmAat°CandrightNphaseat°C).

26
A. R. YUVARAJ ET AL.
Figure . (i) The schematic picture of V at smectic A phase and (ii) the intensity versus θ plot derived from
X-ray diffraction pattern of V at °C.
˚
˚
phase. A sharp reflection was found at 23.72 A and small reflection at 11.6 A, also a wide
˚
angled diffused scattering was observed at ˜4.45 A at 145°C as shown in Fig. 3 (ii). The min-
imum conformation of the compound V is bent shape with respect to the molecular length
˚ ˚
6.13 A (Fig. 3 (i)). The peak at 23.72 A corresponds to half of the total length of Y6c and also,
the wide angled diffused scattering at ˜4.45 A revealed the planar structured arrangement in
4
˚
the fluid phase. Therefore, V consists of the intercalated smectic A phase. The diffusion scat-
˚
tering at 11.6 A is almost one-fourth of total length of LY 6c as shown in Fig. 3 (i) and this
is due to the shrinkage of molecular order in the fluid phase. It means that there is a com-
pressed arrangement of V molecules using intercalated packing system, when they are near
to the isotropic phase via phase transition.
Photoswitching properties
The compound V is showing the absorption maxima at 356.14 nm. Here, the nature of the
spacers is responsible for the variation in photo-reactivity of the compounds [14]. Tacitly, the
nature of the spacers involves flexibility, elasticity, and free rotation parameters [2, 17–20].
Ostensibly, the photoisomerization was analyzed using solution to get the idea about the
structure–property relationship of the spacer substituted azobenzene dimers. The trans to cis
isomerization of V is given in Fig. 4. The isomerization started when the UV light of threshold
−
19
energy 5.5848 × 10
J was illuminated on the system of molecules. The solution of the
−5
−1
compound V was prepared with chloroform with the concentration of 1.1 × 10 mol L .
−2
Photoswitching properties were investigated with the fixed intensity of 5.86 mW cm and
heat filter was used to avoid the thermal trans to cis isomerization. The compound was illu-
minated with the UV light to reach the photostationary state. The photostationary state was

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
27
Figure . Trans to cis isomerization of V.
found at 18 sec for the compound V. The absorption maxima was decreased stepwise with the
irradiation of UV light [4, 21] and the absorption parameters were recorded during systematic
illumination of light.
Flexibility and rigidity of the spacers are an important factor for free rotation and molec-
ular movements [22]. In most of the cases, the thermal back relaxation is spontaneous and
it does not need an external stimuli [23–25]. Nevertheless, back relaxation is very fast when
the light of 450 nm is illuminated on the azobenzene derivatives [4, 26], but thermal back
relaxation is interesting due to the variation in the duration [14]. Absorption spectra were
recorded simultaneously from the point of photosaturation to the original state. The back
relaxation was determined finally for V with the values 10.95 hr (Fig. 5).
Tacitly, spacers play an important role in the thermal back relaxation and the factor affect-
ing the process is flexibility [27]. The flexibility of spacer mainly has an impact on free rotation
and mobility of the molecules [22]. Thermal back relaxation is longer in case of these resorci-
nol spacer substituted azobenzene compound. This is due to the rigid core of resorcinol spacer
Figure . Thermal back relaxation of V after photostationary state.

28
A. R. YUVARAJ ET AL.
Figure . Liquid crystal prototype cell image and it is constructed by V.
that disturbs the mobility [14] and free rotation [22] of molecules in the system. Usually, the
mobility of the molecules increases [2, 14] and the configuration of them also changes [28]
when sufficient amount of energy is supplied to the molecular system. Generally, light is a
form of energy and it changes the molecular configuration and increases the mobility of each
molecule [29–30]. Therefore, dimeric azodyes with resorcinol rigid spacers provide long ther-
mal back relaxation. So, the activity of the compounds can be tuned by incorporating various
spacers in the chromophoric and light sensitive compounds.
Exactly, 5% of V was mixed with commercial liquid crystal 5CB and this homogeneous
mixture was eluted into an ITO coated, unidirectionally rubbed cells (Fig. 6). Initially, absorp-
tion parameters of liquid crystal prototype cell were recorded and the UV light (intensity =
−
2
−19
5
.86 mW cm and E = 5.5848 × 10 J) was illuminated on it. Absorption spectra were
th
recorded simultaneously during the systematic irradiation of UV light. The photostationary
state was found at 14 sec and the thermal back relaxation was observed after 6.14 hr from
photosaturation. Interestingly, disorder to order state transition was observed in this case and
it is confirmed by further investigation.
Again, 9% of the compound V was mixed with commercially available liquid crystal 5CB
and coated on the previously glass substrates using spin coater. Then, the polarized images
were captured from the microscope before and after illuminating UV light for 1 min of inten-
−2
sity 20 mW cm . The mixture showed the disordered state before UV light irradiation and
interestingly the clear ordered texture was observed after UV light irradiation. Prasad et al.
proved this phenomenon earlier [31]. Even though, this disorder to order transition is not
Figure . Disordered to ordered molecular orientation of compound V.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
29
Figure . The determination of photo-stability of V.
common in light sensitive compounds and it is rarely found in nature. In Fig. 7, (i) before
illuminating UV light (disordered state) and (ii) after illuminating the UV light (ordered state).
Figure 8 shows the photo-stability of the compound V. Multiple cycles of photoisomer-
ization are considered here, irrespective of time of trans–cis and cis–trans isomerization. The
−5
−1
compound V is dissolved in chloroform with the concentration 1.1 × 10 mol L . The
UV light of intensity 10 mW cm was illuminated on the solution continuously, until photo-
−2
−2
stationary state. Immediately, visible light of 10 mW cm (450 nm wavelength) was irradiated
on the solution for the back relaxation process after photosaturation state. This phenomenon
was continued several times to evaluate the photo-stability of the compound. Thus, these com-
pounds are stable toward light and does not degrade via light illumination. Also, one can reuse
the compound several times in the photo-induced process without discording the compound
as a waste. Therefore, these siloxane substituted azodyes are suitable in optical storage device
industries, due to the excellent light-induced characteristics. Mainly, industries may reduce
the chemical wastes by the usage of these compounds.
Conclusion
The new dimeric azodye with resorcinol spacer was synthesized. It was showed nematic and
smectic A phases in the mesomorphic investigation. Next, mesophases were studied in detail
using DSC, POM, and XRD. The presence of spacer in the molecule plays an important role in
photoisomerization. This compound is suitable for photochromism studies under UV irradi-
ation. It is clear that the effect of resorcinol spacer is considerable in the study of optical prop-
erties of the compounds due to the rigid core. The photoswitching properties of compound
show E–Z isomerization at 18 sec, whereas the reverse process takes place at around 11 hr in
solution. Solid study showed E–Z photoisomerization takes around 14 sec and the reverse to
the original Z–E takes around 6 hr. Thus, the photoswitching behavior of this material may
be suitably exploited in the field of optical data storage devices and in molecular switches.
Interestingly, order to disorder state transition was found in the prototype device evaluation.
Also, the photo-stability of this compound was checked and the compound was not degraded
in many cycles of prolonged irradiation of UV light. Hence, this compound is suitable for
industrial use.

3
0
A. R. YUVARAJ ET AL.
Acknowledgments
This research was supported by FRGS research grant (RDU 130121). Thank Mrs. K. N. Vasudha for
supporting this work.
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