Synthesis of new U-shaped azobenzene liquid crystals for photoswitching properties

Article abstract of DOI:10.1039/c5ra12705j

A new series of liquid crystalline compounds comprising a U-shaped unit as central core incorporating azobenzene in the side arms and having terminal alkene functional groups, is synthesized and characterized by differential scanning calorimetry (DSC), polarized-light optical microscopy (POM), X-ray diffraction analysis and UV-vis spectroscopy. In the U-shaped series, all compounds showed stable enantiotropic monolayer SmA phases independent on the chain length and chain parity. These U-shaped molecules exhibit strong photoisomerisation behaviour in solutions and in solid state. The photoswitching properties of compounds showed trans to cis isomerization in about 15 seconds, whereas reverse process required much longer times ranging from 235-380 min in solutions. In case of solid film of 4a, E-Z photoisomerization takes around 4 s and the reverse transformation to original Z-E state takes about 74 min. The kinetic study using UV light irradiation shows that all compounds (4a-e) behave first order rate law throughout the relaxation time in solution. The effect of alkyl chain length for trans to cis is negligible whereas, cis to trans is substantial on the odd-even chain length. The photoisomerization study with variable light intensities shows that the photosaturation and thermal back relaxation times are decreased when intensity of the irradiated light is increased. Compounds did not degrade with light illumination at 10 mW cm^-2. The reversible isomerization did not significantly decay after multiple cycles indicating that the photo-responsive properties are stable and repeatable. Thus, the photoswitching behaviour of these materials may be suitably exploited in the field of optical data storage device and in molecular switches for suitable switching times.

Full text of DOI:10.1039/c5ra12705j

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Synthesis of new U-shaped azobenzene liquid
crystals for photoswitching properties†
Cite this: RSC Adv., 2015, 5, 87019
a
Md Lutfor Rahman, Shaheen M. Sarkar,a Mashitah M. Yusoff,a Sandeep Kumarb
*
and Carsten Tschierskec
A new series of liquid crystalline compounds comprising a U-shaped unit as central core incorporating
azobenzene in the side arms and having terminal alkene functional groups, is synthesized and
characterized by differential scanning calorimetry (DSC), polarized-light optical microscopy (POM), X-ray
diffraction analysis and UV-vis spectroscopy. In the U-shaped series, all compounds showed stable
enantiotropic monolayer SmA phases independent on the chain length and chain parity. These U-shaped
molecules exhibit strong photoisomerisation behaviour in solutions and in solid state. The
photoswitching properties of compounds showed trans to cis isomerization in about 15 seconds,
whereas reverse process required much longer times ranging from 235–380 min in solutions. In case of
solid film of 4a, E–Z photoisomerization takes around 4 s and the reverse transformation to original Z–E
state takes about 74 min. The kinetic study using UV light irradiation shows that all compounds (4a–e)
behave first order rate law throughout the relaxation time in solution. The effect of alkyl chain length for
trans to cis is negligible whereas, cis to trans is substantial on the odd–even chain length. The
photoisomerization study with variable light intensities shows that the photosaturation and thermal back
relaxation times are decreased when intensity of the irradiated light is increased. Compounds did not
degrade with light illumination at 10 mW cmꢀ2. The reversible isomerization did not significantly decay
after multiple cycles indicating that the photo-responsive properties are stable and repeatable. Thus, the
photoswitching behaviour of these materials may be suitably exploited in the field of optical data storage
device and in molecular switches for suitable switching times.
Received 30th June 2015
Accepted 8th October 2015
DOI: 10.1039/c5ra12705j
www.rsc.org/advances
Vorlander and Apel9,10 realized the synthesis of the rst U-
shaped or bent-shaped molecules as early as in 1929 using the
1. Introduction
disubstituted benzene ring substituted either at 1,2 or 1,3-
positions which deviates signicantly from classical rod-shaped
molecules. Yelamaggad et al.1 reported bent core V-shaped
mesogens consisting of salicylaldimine mesogenic segments
derived from 1,2-substitution of benzene ring and other re-
ported compounds having 1,2-substitution of benzene ring,
also termed as bent core V-shaped molecules.10–12 Literature
survey reveal that some works are reported12–15 for the U-shaped
molecular architectures using the 1,2-substituted phenylene
compounds which are very relevant to this work. Therefore, we
wish to designate the term U-shaped molecules in this paper.
The U-shaped molecules exhibit mesophases which are analo-
gous to classical calamitic LCs, whereas banana-shaped meso-
gens exhibit new type of smectic phases, which are not identical
to the phases formed by calamitics.1 U-shaped molecule, viz.
1,2-phenylene bis[4-(ethoxyphenylazoxy) benzoate] was rst re-
ported by Vorlander and Apel10 and this compound exhibit
a nematic phase which was identied by Pelzl et al.11 Three
series of compounds were reported by Kuboshita et al.12 and
these compounds showed nematic, smectic A, and smectic B
phases. The bent-core compounds are found as fused twins13 or
Anisometric molecules (mesogens), either rod-shaped or disc-
shaped, are used to design and synthesize conventional ther-
motropic calamitic or discotic liquid crystals.1–4 Generally liquid
crystals (LCs) produced with rod-like molecules display nematic
and/or smectic mesophases whereas LCs with at disc-shaped
molecules display nematic and/or columnar mesophases.1,5
Comparatively a new class of LCs whose molecular shape is
distorted away from the classical rod or disc shape is designated
as non-conventional LCs, for instance oligomeric LCs, bent-core
or banana LCs, polycatenars, and dendrimers.1,5–8 Certainly, the
non-classical molecular architectures may exhibit mesophases
even though their molecular geometry deviate signicantly
from the classical rod or disc-shapes.1
aFaculty of Industrial Sciences & Technology, Universiti Malaysia Pahang, 26300
Gambang, Kuantan, Pahang, Malaysia. E-mail: lutfor73@gmail.com; Fax: +60-9-
5492766; Tel: +60-9-5492785
bRaman Research Institute, Raman Avenue, Sadashivanagar, Bangalore 560080, India
cInstitute of Organic Chemistry, Martin-Luther-University Halle-Wittenberg, Kurt-
Mothes Str. 2, Halle D-06120, Germany
† Electronic supplementary information (ESI) available: Materials, representative
DSC graphs, POM and UV/vis absorption spectra. See DOI: 10.1039/c5ra12705j
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U-shaped molecules14 are also reported. A homologous series of shaped dimeric mesogens. In this paper, we have synthesized
U-shaped dimeric liquid crystals in which two mesogenic a series of new molecules in which photoswitchable azo moie-
groups were linked to catechol was reported by Yoshizawa and ties are attached to the 1,2-phenylene as a core via ether linkage.
co-workers.15 The odd–even effect is found on the phase These compounds exhibit smectic A phase irrespective of chain
sequences in which the even members favour smectic C phases, length and parity. Further, we report some E/Z isomerization
whereas the odd members favour smectic A phases; however, studied on the unconventional azobenzene compounds for
both members exhibits the nematic phase.15 Catechol based U- possible applications in the optical data storage and molecular
shaped dimeric liquid crystals incorporating to 3 to 6 methylene switches.
units with terminal aliphatic chain lengths varying from 1 to 12
units were prepared by Attard et al.16 In the two homologous
series, compounds with odd number of methylene units form
nematic and smectic phases as a function of terminal chain
2. Experimental
2.1 Synthesis of intermediate compounds
All intermediate compounds 1, 2a–e and 3a–e were synthesized
length whereas compounds with even number of methylene
units are smectogenic. X-ray diffraction studies conrm that
these smectic phases are composed of molecules arranged in
bilayers.16 Recent study show that two rod-shaped azobenzene
moieties, each carrying a short electron withdrawing acetyl
group at the terminals when attached to a catechol unit via
methylene spacers exhibits nematic and smectic A phases
irrespective of chain length.17 Matsuzaki and Matsunaga18 re-
ported different series of V-shaped molecules and these
compounds were also shows the N, SmA and SmB phases. A
partial bilayer structure was proposed based on X-ray diffraction
studies for the SmA phase. Kato et al.19 reveal the nematic phase
of V-shaped molecules which is formed from the hydrogen
bonding between phthalic acid and stilbazole. Tanaka and
Yoshizawa20 reported a homologous series of catechol deriva-
tives having 2,3-diuoro-1,4-diphenylbenzene including one of
the biphenyl derivatives. The phase transition behavior of each
U-shaped compound doped with a chiral dopant reveal that
some compounds show the blue phases and investigation per-
formed in terms of helical twisting power, elasticity and
molecular biaxiality, which enable to informed about the effects
of the U-shapes on stabilization of blue phases.20 Some V-
shaped tetradentate ligands were prepared by condensing 1,2-
phenylenediamine with benzaldehyde compounds which
exhibit mesomorphic behavior.21
Alongside with the design and synthesis of liquid crystals
molecules, a eld of research namely the photoinduced
phenomenon such as the incident light promoting the molec-
ular ordering/disordering of the liquid-crystalline system
gained a lot of credence.22–25 In photonics, where light can be
controlled as a stimulus, is offering as the future technology for
high-speed information processing. In such systems the
reversible photoinduced shape transformation of the molecules
containing the photochromic azobenzene groups take
place.26–28 Thus, liquid crystals with an azo-linkage have
received attention due to their unique photoswitchable prop-
erties induced by light.29–33 Upon absorption of UV light (ꢁ365
nm), the energetically more stable E conguration (trans)
converts to the Z conguration (cis). The reverse transformation
according to our earlier paper.22–24
2.2 1,2-Phenylene bis[4-{[4-(but-3-en-1-yloxy)phenyl]
diazenyl} benzoate] (4a)
Compound 3a (0.1060 g, 0.358 mmol), catechol (0.0196 g, 0.179
mmol), 50 ml of dry dichloromethane, DMAP (4.8 mg, 0.04
mmol) and DCC (0.0820 g, 0.40 mmol) were stirred for 48 h.
Esterication and purications were carried out according to
our previous paper.22,25 Yield of 4a: 0.0410 g, 33%. IR, nmax/cmꢀ1
3070 (]CH2), 2929 (CH2), 2857 (CH2), 1736 (C]O, ester), 1639
(C]C, vinyl), 1605, 1496 (C]C, aromatic), 1254, 1130, 1062 (C–
O), 830 (C–H). dH(500 MHz; CDCl3; Me4Si) 8.24 (d, 4H, J ¼ 8.6
Hz, Ph), 7.92 (d, 4H, J ¼ 8.9 Hz, Ph), 7.85 (d, 4H, J ¼ 8.5 Hz, Ph),
7.47 (m, 2H, Ph), 7.42 (m, 2H, Ph), 7.02 (d, 4H, J ¼ 8.9 Hz, Ph),
5.91–5.86 (m, 2H), 5.05 (dd, 2H, J ¼ 13.6 Hz), 4.96 (dd, 2H, J ¼ 9.7
Hz), 4.05 (t, 4H, J ¼ 6.5 Hz, OCH2–), 2.10–2.04 (m, 4H, –CH2–).
dC(125 MHz; CDCl3; Me4Si) 26.85, 67.69, 109.12, 110.78, 121.75,
122.81, 124.29, 125.34, 126.12, 131.55, 136.39, 146.78, 152.56,
156.47, 162.65, 164.12.
2.3 1,2-Phenylene bis[4-{[4-(pent-4-en-1-yloxy)phenyl]
diazenyl} benzoate] (4b)
Compound 4b was synthesis from 3b according to the method
described for 4a. Yield of 4b: 0.1081 g, 35%. IR, nmax/cmꢀ1 3072
(]CH2), 2942 (CH2), 2883 (CH2), 1737 (nC]O, ester), 1638 (C]
C, vinyl), 1600, 1499 (C]C, aromatic), 1259, 1137, 1062 (C–O),
855 (C–H). dH(500 MHz; CDCl3; Me4Si) 8.22 (d, 4H, J ¼ 8.7 Hz,
Ph), 7.91 (d, 4H, J ¼ 9.0 Hz, Ph), 7.86 (d, 4H, J ¼ 8.7 Hz, Ph), 7.46
(m, 2H, Ph), 7.41 (m, 2H, Ph), 7.01 (d, 4H, J ¼ 9.0 Hz, Ph), 5.90–
5.87 (m, 2H), 5.10 (dd, 2H, J ¼ 15.5 Hz), 5.04 (dd, 2H, J ¼ 10.1
Hz), 4.06 (t, 4H, J ¼ 6.5 Hz, OCH2–), 2.27–2.24 (m, 4H, –CH2–),
1.94–1.91 (m, 4H, –CH2–). dC(125 MHz; CDCl3; Me4Si) 27.55,
30.14, 67.66, 109.85, 110.98, 121.55, 122.32, 124.21, 125.65,
126.04, 131.47, 136.57, 146.71, 152.26, 156.11, 162.44, 164.55.
2.4 1,2-Phenylene bis[4-{[4-(hex-5-en-1-yloxy)phenyl]
diazenyl} benzoate] (4c)
of the Z isomer into the E isomer can be shied by irradiation Compound 4c was synthesis from 3c according to the method
with visible light (in the range of 400 to 500 nm) the process is described for 4a. Yield of 4c: 0.0630 g, 37%. IR, nmax/cmꢀ1 3071
known as thermal back relaxation which occurs in the dark or (]CH2), 2924 (CH2), 2854 (CH2), 1730 (C]O, ester), 1640 (C]
thermally or photochemically with visible light.34–37
C, vinyl), 1604, 1497 (C]C, aromatic), 1258, 1141, 1081 (C–O),
So far, 1,2-phenylene bis[4-{[4-(alkyloxy)phenyl]diazenyl} 800 (C–H). dH(500 MHz; CDCl3; Me4Si) 8.24 (d, 4H, J ¼ 8.5 Hz,
benzoate] moieties have not been employed to perceive such U- Ph), 7.92 (d, 4H, J ¼ 8.7 Hz, Ph), 7.85 (d, 4H, J ¼ 8.6 Hz, Ph), 7.45
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(m, 2H, Ph), 7.41 (m, 2H, Ph), 7.01 (d, 4H, J ¼ 8.89 Hz, Ph), 5.87– phase, sealed and held on a heater. X-ray diffraction was carried
5.82 (m, 2H), 5.04 (dd, 2H, J ¼ 13.6 Hz), 4.95 (dd, 2H, J ¼ 9.6 Hz), out in the mesophase obtained on cooling the isotropic phase.
4.04 (t, 4H, J ¼ 6.5 Hz, OCH2–), 2.10–2.06 (m, 4H, –CH2–), 1.86– Though a magnetic eld of about 5k Gauss was used to align the
1.81 (m, 4H, –CH2–), 1.73–1.68 (m, 4H, –CH2–). dC(125 MHz; samples, the diffraction patterns indicate that the sample was
CDCl3; Me4Si) 26.92, 28.91, 30.23, 67.69, 109.25, 110.47, 121.57, not aligned perfectly and, therefore, should be considered as
122.45, 124.51, 125.46, 126.13, 131.45, 136.54, 146.65, 152.36, unaligned sample. Absorption spectra for photochromic study
156.23, 162.34, 164.33.
were recorded using an Ocean Optics HR2000 + miniature UV-
Vis spectrophotometer. All the solutions were prepared and
measured under air in the dark at room temperature (21 ꢃ 1 ꢂC)
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 lmax ¼ 365
nm along with heat lter to avoid any extra heat radiation to the
sample. The UV light illumination is performed using Omni-
cure S2000 UV source with variable intensities (5–10 mW cmꢀ2).
2.5 1,2-Phenylene bis[4-{[4-(hept-6-en-1-yloxy)phenyl]
diazenyl} benzoate] (4d)
Compound 4d was synthesis from 3d according to the method
described for 4a. Yield of 4d: 0.0475 g, 30%. IR, nmax/cmꢀ1 3066
(]CH2), 2928 (CH2), 2852 (CH2), 1732 (C]O, ester), 1644 (C]
C, vinyl), 1602, 1489 (C]C, aromatic), 1252, 1139, 1076 (C–O),
836 (C–H). dH(500 MHz; CDCl3; Me4Si) 8.23 (d, 4H, J ¼ 8.7 Hz,
Ph), 7.92 (d, 4H, J ¼ 8.7 Hz, Ph), 7.86 (d, 4H, J ¼ 8.5 Hz, Ph), 7.46
(m, 2H, Ph), 7.40 (m, 2H, Ph), 7.02 (d, 4H, J ¼ 8.7 Hz, Ph), 5.85–
5.80 (m, 2H), 5.04 (dd, 2H, J ¼ 13.8 Hz), 4.92 (dd, 2H, J ¼ 9.6 Hz),
4.04 (t, 4H, J ¼ 6.5 Hz, OCH2–), 2.11–2.05 (m, 4H, –CH2–), 1.86–
1.80 (m, 4H, –CH2–), 1.82–1.77 (m, 4H, –CH2–), 1.74–1.67 (m,
4H, –CH2–). dC(125 MHz; CDCl3; Me4Si) 25.04, 26.35, 27.45,
30.32, 67.67, 109.66, 110.89, 121.35, 122.38, 124.29, 125.55,
126.23, 131.55, 136.47, 146.63, 152.34, 156.21, 162.14, 164.45.
3. Results and discussion
3.1 Synthesis
The synthesis of compounds 4a–e was performed as depicted in
Scheme 1. The azobenzene containing rod-like side arms were
synthesized by a literature procedure.22,24 The diazonuim salt
was prepared with sodium nitrite and subsequent coupling with
phenol to afforded the ethyl 4-[(4-hydroxyphenyl)diazenyl]
benzoate 1 which is puried by crystallization and recrystalli-
zation from methanol in about 51% yield. Compound 1 was
alkylated with 4-bromo-1-butene to give the ethyl 4-{[4-(but-3-
en-1-yloxy)phenyl]diazenyl}benzoate 2a, which was puried by
column chromatography on silica followed by crystallization
from methanol/chloroform in about 60% yield. Other
compounds (2b–e) were also synthesized from the correspond-
ing bromoalkene with the same method of compound 2a in
good overall yield up to 77% for compound 2c. Compounds 2a–
e were hydrolysed under basic conditions to yield the 4-{[4-
(alkyl-en-1-yloxy)phenyl]diazenyl} benzoic acid 3a–e. Hence,
compound 4-{(E)-2-[4-(but-3-en-1-yloxy)phenyl]diazen-1-yl}ben-
zoic acid 3a was given a nice crystals from ethanol/chloroform
mixture. For a series of U-shaped compounds, acids 3a–e were
coupled with 1,2-dihydroxybenzene by DCC and DMAP to ach-
ieved the corresponding desired molecules 4a–e (Scheme 1), all
compounds were puried by column chromatography on silica
gel and recrystallization. All compounds were characterized
using 1H, 13C NMR and elemental analysis. Analytical data were
found to be in good agreement with the structures (see detailed
synthetic procedures and analytical data in ESI†).
2.6 1,2-Phenylene bis[4-{[4-(oct-7-en-1-yloxy)phenyl]
diazenyl} benzoate] (4e)
Comp. 4e was synthesis from 3e according to the method
described for 4a. Yield of 4e: 0.1251 g, 35%. IR, nmax/cmꢀ1 3076
(]CH2), 2925 (CH2), 2855 (CH2), 1735 (C]O, ester), 1640 (C]
C, vinyl), 1602, 1500 (C]C, aromatic), 1261, 1141, 1060 (C–O),
841 (C–H). dH(500 MHz; CDCl3; Me4Si) d: 8.24 (d, 4H, J ¼ 8.7 Hz,
Ph), 7.92 (d, 4H, J ¼ 8.9 Hz, Ph), 7.84 (d, 4H, J ¼ 8.6 Hz, Ph), 7.45
(m, 2H, Ph), 7.41 (m, 1H, Ph), 7.01 (d, 4H, J ¼ 8.6 Hz, Ph), 5.86–
5.80 (m, 2H), 5.05 (dd, 2H, J ¼ 13.7 Hz), 4.94 (dd, 2H, J ¼ 9.6 Hz),
4.04 (t, 4H, J ¼ 6.5 Hz, OCH2–), 2.08–2.02 (m, 4H, –CH2–), 1.86–
1.81 (m, 4H, –CH2–), 1.74–1.66 (m, 8H, –CH2–). dC(125 MHz;
CDCl3; Me4Si) 24.94, 26.24, 30.78, 32.34, 33.40, 68.19, 114.84,
114.89, 122.63, 122.70, 125.12, 125.37, 127.62, 131.46, 138.40,
146.69, 151.90, 156.71, 162.50, 164.67.
2.7 Instruments
The structure of the compounds was conrmed by spectro-
scopic method. IR spectra were recorded with a Perkin Elmer
FTIR spectrometer (670). 1H NMR (500 MHz) and 13C NMR (125
MHz) spectra were recorded with a Bruker (DMX500) spec-
trometer. The transition temperatures and their enthalpies
were measured by differential scanning calorimetry (Perkin DSC
3.2 Mesomorphic properties
7) with heating and cooling rates were 10 ꢂC minꢀ1. Optical The liquid crystalline properties were studied by polarized-light
textures were obtained by using Olympus BX51 polarizing optical microscopy (POM), differential scanning calorimetry
optical microscope attached with Olympus DP26 digital camera (DSC) and X-ray diffraction analysis. DSC studies conrmed the
equipped with a Mettler Toledo FP82HT hot stage and a FP90 phase transition temperatures (T/ꢂC) observed by polarizing
central processor unit. X-ray diffraction measurements were microscopy and gave the enthalpy changes [DH/J gꢀ1] associated
˚
carried out using Cu-Ka radiation (l ¼ 1.54 A) generated from with these phase transitions as shown in Table 1.
a 4 kW rotating anode generator (Rigaku Ultrax-18) equipped
3.2.1 DSC-studies. For the series of U-shaped compounds
with a graphite crystal monochromator. Sample was lled in 4a–e, having even and odd number of alkyl carbons the enan-
Hampton research capillaries (0.5 mm diameter) from isotropic tiotropic phase sequence Cr–SmA–I is observed for all
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Scheme 1 Synthesis of compounds 4a–e. Reagents and conditions: (i) NaNO2, 3 equivalent HCl, 2 ꢂC; (ii) phenol, NaOH, pH 9, 2 ꢂC; (iii) K2CO3,
KI, BrCnH2nꢀ1 (n ¼ 2–6), reflux; (iv) KOH, MeOH, reflux; (v) catechol, DCC, DMAP.
Table 1 Phase transition temperature (T/ꢂC) and associated transition
smectic phases. Upon shearing homeotropic alignment is
immediately achieved; these homeotropically aligned regions
are completely isotropic down to the crystallization tempera-
enthalpy values [DH/J gꢀ1] observed for the second heating and
cooling DSC scans of 4a–ea
ture, conrming the presence of a uniaxial SmA phases for these
Compound
n
Heating
Cooling
compounds. The typical textures of 4a and 4b are shown in
Fig. 2, additional textures for 4b, 4c and 4d are presented in the
ESI (Fig. S2†). All the transition temperatures observed under
POM were matching with DSC data. The phase structures were
further characterized by X-ray diffraction analysis for a selected
example of each phase type.
4a
4b
4c
4d
4e
2
3
4
5
6
Cr 101 [30] SmA 160 [14] I I 155 [14] SmA 90 [24] Cr
Cr 98 [30] SmA 156 [14] I
Cr 93 [23] SmA 154 [12] I
I 151 [14] SmA 85 [27] Cr
I 150 [12] SmA 80 [22] Cr
Cr 111 [30] SmA 152 [12] I I 148 [12] SmA 97 [25] Cr
Cr 81 [17] SmA 146 [8] I I 144 [8] SmA 74 [16] Cr
a
Peak temperatures from DSC (rate 10 ꢂC minꢀ1); abbreviation Cr ¼
3.2.3 X-ray diffraction studies. The X-ray diffraction studies
conrm the phase assignment. X-ray diffraction was carried out
in the mesophases obtained on cooling from the isotropic
phases. The intensity versus 2q plot derived from the diffraction
pattern of the compounds 4a–e (n ¼ 2–6) are shown in Fig. 3.
The diffraction pattern of all compound 4a–e exhibits a sharp
reection in the small angle region, corresponding to d ¼
crystal, SmA ¼ smectic A phase, I ¼ isotropic phase.
compounds, independent on the chain parity. It was observed
that the melting points are reduced from compound 4a to 4e,
those exhibits the enantiotropic SmA phases. On cooling cycle,
the transition temperatures are lower than those of the lower
homologues compound, for example 4a. Also the SmA–I tran-
sition enthalpy values decrease with growing chain length as
presented in Table 1. The DSC curves for all compounds 4a–e
are shown in Fig. 1 and DSC graphs with full details are pre-
sented in the ESI (Fig. S1†).
3.2.2 Polarizing optical microscopy (POM) studies. All the
U-shaped compounds (4a–e) derived from 1,2-dihydrox-
ybenzene show fan shaped textures, typical for smectic phases,
as presented in Fig. 2. In all cases the extinction crosses are
parallel to polarizer and analyzer which indicate nontilted
˚
21.12–26.60 A, which is nearly equal to the corresponding
˚
molecular length of 4a–e (ranging from 21.10–26.49 A) as esti-
mated from top to bottom of a U-shaped conformer shown in
Fig. 4a and estimated lengths are presented in Table 2. The
˚
weak small reection at d ¼ 10.41–13.27 A is the second order of
the layer reex. In the wide-angle region a diffuse peak at d z
˚
4.22–4.66 A conrms a uid smectic phase without in-plane
order. Therefore, we anticipate that compound 4a–e exhibits
a smectic A type phase. This is most likely due to the V-shape of
the molecules 4 which favours a parallel organization of the
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Fig. 2 Polarized optical microphotographs of texture: (a) SmA phase
at 124 ꢂC of 4a and (b) SmA phase at 120 ꢂC of 4b.
Fig. 1 DSC scans of second heating (
) and cooling (
.
) curves of
compound 4a–e (n ¼ 2–6) at 10 ꢂC minꢀ1
adjacent rod-like azobenzene units (thus adopting a U-shaped
conformation).
However, the alignment of the rod-like cores cannot be fully
parallel, so that some wedge-shape (U-shaped conformation) of
the molecules is retained which requires an antiparallel packing
of the molecules with intercalated (mixed) aliphatic chains and
azobenzene cores to optimize the space lling; the catechol
groups from their own layers and separate the layers formed by
the rod-like units and terminal chains, as shown in Fig. 4b. A
different packing pattern is proposed by X-ray diffraction
analysis from the smectic A phase formed by U-shaped
Fig. 3 Intensity versus 2q graph derived from the X-ray diffraction for
the SmA phase of compound 4a–e at 124, 120, 118, 114 and 110 ꢂC,
respectively.
˚
compounds. A layer spacing of d ¼ 53.4 A was observed
whereas the length of the molecule (l) including alkyl chains in
˚
the all trans conformation was estimated to be z31 A. The ratio
d/l z 1.72 suggests that in this smectic A phase the molecules
are ordered into bilayers.16 However, similar packing patterns
were observed for some compounds.15 The X-ray diffraction
suggested that the layer spacing is about the same as the length
of the molecule in a U-shaped. The XRD result suggests that the
smectic A phase is a monolayer structure in which molecules
can exist in a U-shaped.15 Thus, depending on bent unit (1,2-
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Fig. 4 (a) Major conformation of compound 4a–e and (b) organiza-
tion of these molecules in the SmA phase.
Fig. 5 Spectra shows the absorbance behavior of the compound 4a
when UV light is shined on the sample. It is evident that within ꢁ15
seconds of illumination, photo saturation occurs. Data were taken
before shining UV light (no UV) and with subsequent time intervals
(light intensity at 5 mW cmꢀ2).
Table 2 X-ray Diffraction data for compound 4a–e
qꢂ
d-spacing, A (hk)
˚
˚
Compound
l, A
4a
21.10
22.36
23.72
25.06
26.49
2.09
21.12 (01)
10.41 (02)
4.66 (halo)
22.42 (01)
11.14 (02)
4.61 (halo)
23.78 (01)
11.72 (02)
4.57 (halo)
25.10 (01)
12.51 (02)
4.54 (halo)
26.60 (01)
13.27 (02)
4.22 (halo)
4.25
9.50
1.96
3.97
9.58
1.85
3.77
9.85
1.75
3.41
9.75
1.65
3.33
10.51
photoisomerization due to E isomer transformed to Z isomer.
Aer ꢁ14 s illumination, there is no much change in absorption
spectrum conrms the saturation of E/Z isomerization process.
Fig. 6 shows the E–Z absorption of compound 4a–e as
a function of exposure time. Data is extracted from Fig. 5 (and
Fig. S3 in the ESI†). The wavelength is xed at 364 nm and
absorption values were recorded as a function of exposure time
with UV intensity being xed at 5 mW cmꢀ2. Curve shows that
photosaturation occurs within 15 s for 4a and photosaturation
time for other compounds 4b–e shows ranging from 14–15 s,
which is faster as compared with nematic to isotropic phase
involved photoisomerization.36
4b
4c
4d
4e
Fig. 7 shows the thermal back relaxation process where the
solution is shined continuously for 20 seconds (photo stationery
state) and kept in the dark and then at subsequent time inter-
vals, spectral data were recorded. Thermal back relaxation for
the compound 4a showed in Fig. 7 (Fig. S4 for 4b–e in the ESI†).
dihydroxybenzene) and the parity of chain length three distinct
types of LC phases were observed for these non-linear azo-
benzene based mesogens. The distinct phase types seem to
result from distinct molecular conformations to which the
molecules adopt in the self-assembly process in order to opti-
mize their packing.
3.3 Photo-switching study
The photo-switching studies were initially carried out on solu-
tions and then on liquid crystal cells. Consequently the mate-
rials behavior with respect to UV light is obtained and also these
results are indispensable for creating optical storage devices.
All U-shaped molecules also showed similar absorption
spectra due to their similar molecular structure with variable
alkyl chain (n ¼ 2–6). Fig. 5 depicts the absorption spectra of 4a
(n ¼ 2) before and aer UV illumination (see spectra of 4b–e in
Fig. S3 in the ESI†). The absorption spectra of compound 4a
show absorbance maxima at 364 nm. The absorption spectra of
compound was carried out also in chloroform solution having
same concentration C ¼ 1.2 ꢄ 10ꢀ5 mol Lꢀ1. Compound 4a was
illuminated with UV light with 365 nm lter at different time
intervals and promptly absorption spectra were recorded. The
Fig. 6 Photoisomerization curve of 4a–e as a function of UV illumi-
nation time showing trans to cis behaviour (light intensity at 5 mW
absorption maximum at 364 nm decreases due to E/Z cmꢀ2).
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temperature for all ve compounds according to the eqn (1)
reported by Lutfor et al.25
AN ꢀ At
ln
¼ ꢀkt
(1)
AN ꢀ A0
Where At, A0 and AN, is the absorbance at 365 nm of time t,
time zero and innite time, respectively. A typical rst order plot
using eqn (1) at room temperature (25 ꢂC) for all ve
compounds are shown Fig. 9. A typical rst order behaviour for
4a–e shows throughout the relaxation time. All compounds
shows similar behaviour at room temperature irrespective of
their terminal alkyl chains length. Apparently, all compounds
showed rst order exponential decay in solutions. The rate
constants were observed for the Z–E isomerization of 7.92 ꢄ
10ꢀ3, 5.60 ꢄ 10ꢀ3, 6.91 ꢄ 10ꢀ3, 7.30 ꢄ 10ꢀ3 and 5.28 ꢄ 10ꢀ3 sꢀ1
for 4a–e, respectively. The rate constants are not substantial
variation with respect to the alkyl chain length (n ¼ 2–6).
3.3.2 Effect of alkyl chain length. For U-shaped molecular
architectures, we used alkyl chains as n ¼ 2–6, we found little
odd-even effect on the photoisomerization of UV light with
intensity at 5 mW cmꢀ2 for compounds 4a–e which is shown in
Fig. S5.† The UV on dynamics to the U-shaped molecules
showed small variation such as 4a ¼ 15.1, 4b ¼ 14.0, 4c ¼ 15.2,
4d ¼ 14.5 and 4e ¼ 14.6 s. Thus, a little odd–even effect is seen
on the U-shaped compounds. The odd–even effect on the pho-
toisomerization of back light for compounds 4a–e is also shown
in Fig. S6.† The UV off dynamics to the U-shaped showed
substantial variation for 4a ¼ 235, 4b ¼ 362, 4c ¼ 333, 4d ¼ 380
and 4e ¼ 350 minutes. For U-shaped compounds all exhibit
high thermal back relaxation. Presented study is a strong
evidence for odd–even effect when no light is shined on them.
3.3.3 Effect of light intensity. The photoisomerization of
compound 4b was studied with variable intensities (5, 10 and
15 mW cmꢀ2). Fig. 10 shows the photoisomerization behaviour
of 4b using UV light with intensity at 10 mW cmꢀ2. The pho-
tosaturation occurs faster 9 s when intensity of UV light is
increased (Fig. 10a) and the thermal back relaxation was also
Fig. 7 Thermal back relaxation process for the compound 4a shows
that to relax from cis to trans takes around 235 min. Solution is shined
for 20 seconds (photo stationery state) and then at subsequent
intervals data were recorded (light intensity at 5 mW cmꢀ2).
Fig. 8 shows the time dependence of the Z–E absorption of
compound 4a–e. Peak wavelength at 364 nm as obtained from
Fig. 7 (and Fig. S4 in the ESI†) is plotted as a function of recovery
time. The thermal back relaxation occurs 235, 362, 322, 380, 350
minutes were time taken to relax back to their original state for
the compounds 4a–e, respectively, within this time, it is
reasonably fast as compared with nematic to isotropic phase
involved thermal back relaxation.36
Prasad et al.36 reported that the faster thermal back relaxa-
tion is due to their layered structure since changes are conned
to in-plane rotation of the molecules as compared with nematic
to isotropic phase involve transition. This hypothesis is well-
established by the fact that a similar feature was observed in
another case wherein the two phases involved have a layer
structure.37
3.3.1 Kinetic study. We also calculated the rate constant
(kt) for the cis–trans isomerization behaviour at room
Fig. 8 Photoisomerization curve of Z isomer (4a–e) as a function of
recovery time when UV light is illuminated until it reaches photo- Fig. 9 First order plots for the compounds 4a–e for cis–trans isom-
saturation and followed by measuring back relaxation time (light erization. Conditions: c ¼ 1.2 ꢄ 10ꢀ5 mol Lꢀ1 in chloroform at room
intensity at 5 mW cmꢀ2).
temperature (25 ꢂC) (light intensity at 5 mW cmꢀ2).
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Fig. 11 Reversibility of the photoisomerization process of the azo-
benzene chromophore of 4a and 4b in chloroform with the
Fig. 10 (a) Absorption spectra of 4b UV light with intensity at 10 mW
cmꢀ2 and (b) thermal back relaxation process for 4b.
concentration of 1.1 ꢄ 10ꢀ5 mol Lꢀ1
.
mol Lꢀ1. The UV light of intensity 10 mW cmꢀ2 was illuminated
on the solution continuously, until photo-stationary state is
reach. Immediately, visible light of 10 mW cmꢀ2 (450 nm
wavelength) was irradiated on the solution for back relaxation
process aer photosaturation state. This phenomenon was
continued 4 times to evaluate the photo-stability of the
compound. Thus, compound is stable towards light and does
not degrade via light illumination. The extent of reversible
isomerization did not signicantly decay aer 4 cycles, indi-
cating that the photo-responsive properties of both compounds
were stable and repeatable. Therefore, these U-shaped liquid
crystals are suitable in optical storage device industries, due to
the excellent light induced characteristics.
fast 184 min as shown in Fig. 10b. In addition, we have studied
the photoisomerization behaviour of 4b with more intensity at
15 mW cmꢀ2. The photosaturation occurs very fast within 7 s
only and the thermal back relaxation was also very fast 144 min
as shown in Fig. S7 at ESI.† Thus, the time intervals of photo-
saturation and thermal back relaxation are decreased when
intensity of the irradiated light is increased. Therefore, the
intensity is inversely proportional to the time of photo-
saturation and thermal back relaxation (Table 3).
3.3.4 Photo-stability test. Multiple cycles of photo-
isomerization is considered here, irrespective of time of trans–
cis and cis–trans isomerization. Fig. 11 shows the photo-stability
of the light sensitive compound 4a and 4b. Both compounds are
used the chloroform solvent with the concentration 1.1 ꢄ 10ꢀ5
Spectral investigation on solid lms of 4a was used as
a representative compound and data were also recorded as
a function of UV illumination. Here guest–host effect is
employed where 5CB, room temperature nematic liquid crystal
is act as a host and bent-shaped liquid crystals are act as guest
systems. Previously prepared polyimide coated, unidirectionally
rubbed sandwiched cells were lled with the guest–host mixture
Table 3 Photoisomerization of 4b with variable intensities
Intensity (mW cmꢀ2
)
Photosaturation (s)
Back relaxation (min)
5.0
10.0
15.0
14
9
7
362
184
144
ꢂ
at isotropic temperature of the mixture (ꢁ50 C). UV/Vis spec-
tral data were recorded using spectrophotometer.
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Fig. 12a shows the absorbance versus wavelength graph,
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subjected to before and aer shining UV radiation of wave-
length 365 nm. It is observed that, within 4 seconds of illumi-
nation, system attains photosaturation state. Whereas in
Fig. 12b, thermal back relaxation for the solid cells were
captured aer illuminating the cells for photosaturation state
(around 4 seconds) and then at subsequent intervals spectral
data were recorded. To be comparison, no UV point is also
given. So it is clear from the graph that, system takes around 70
minutes of time to relax back to the original conguration.
The dynamics of UV on and UV off (thermal back relaxation)
process for the compound 4a is shown in Fig. 13. Peak wave-
length is 364 nm and data is generated from peak absorbance at
364 nm as a function of exposure time (Fig. 13a). It was clearly
observed that E–Z conversion takes around 4 seconds whereas
recovery to the original state i.e., thermal back relaxation takes
around 74 minutes. In Fig. 13b, No UV point is given to make it
clear that system already reached its original value. As compare
to liquid, solid behaviour is fast may be due to the tightly
packed molecules in cells whereas in solutions, molecules are
having more freedom to move around. The isothermal phase
Fig. 13 Photo-isomerization curve of E isomer (4a) showing UV on
process (a) and UV off process (b) as a function of time. One can see
that conversion from E/Z occurs at around 4 seconds whereas Z/E
occurs around 74 minutes.
transition of LCs in the LC cell was created with the increase of
the population of the cis-azo-LCs.38 Certainly the unstable cis-
azo-LCs can be recovered back to the stable trans-azo-LCs by
illumination with green light and the transformation rate from
cis- to trans-isomers is much higher than that of dark relaxa-
tion.39 Kundu et al.40 demonstrated in situ homeotropic align-
ment by photochromic trans- to cis- isomerization of the azo-dye
doped in a nematic host. The augmented dipole moment of
a cis-isomer formed under UV-irradiation expedites a molecular
assembly into crystalline aggregates. Subsequent deposition of
the aggregates creates roughened surface and induces
anchoring transition from the initial planar to homeotropic
alignment of LCs. The alignment is unwavering against
temperature, light and chemical treatment which practically
permanent for a device implementation.
To demonstrate the potential of the materials stated here, an
optical storage device (Fig. 14) that is realized by using the above
mentioned method. The mixture is capillary lled into the
commercially available cell (Instec) ITO + polyimide coated,
unidirectional rubbed, sandwiched cell at isotropic temperature
(ꢁ70 ꢂC). Qualities of the cells were observed under optical
polarizing microscope. The guest–host mixture was illuminated
Fig. 12 (a) Absorption spectra of 4a in solid with different exposure
time of UV light. No UV corresponds to the 0 seconds UV light illu-
mination. (b) Thermal back relaxation process for the compound 4a
after illuminating the material for 5 seconds (photo stationery state).
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Acknowledgements
This research was supported by PRGS research grant (RDU
130803). Thank Mrs. K. N. Vasudha for supporting this work.
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