New pyrimidine-based photo-switchable bent-core liquid crystals

Article abstract of DOI:10.1039/c3nj00359k

The first examples of liquid crystalline pyrimidine-based photo-switchable bent-core monomers incorporating azobenzene as side arms linked with terminal double bonds as polymerizable functional groups are synthesized and characterized. Polarizing optical

Full text of DOI:10.1039/c3nj00359k

NJC
View Article Online
View Journal | View Issue
PAPER
New pyrimidine-based photo-switchable bent-core
liquid crystals†
Cite this: NewJ.Chem., 2013,
37, 2460
Md Lutfor Rahman,*a Gurumurthy Hegde,a Mashitah Mohd Yusoff,a
Muhammad Nor Fazli A. Malek,a Hosapalya T. Srinivasab and Sandeep Kumarb
The first examples of liquid crystalline pyrimidine-based photo-switchable bent-core monomers incorporating
azobenzene as side arms linked with terminal double bonds as polymerizable functional groups are
synthesized and characterized. Polarizing optical microscopy (POM), differential scanning calorimetry
(DSC) and X-ray diffraction analysis reveal that the bent-shaped lower homologue compounds are
crystalline in nature whereas higher homologue compounds display the stable enantiotropic B6 phase.
They exhibit fast photoisomerization effects in solution and relatively slow photoisomerization effects in
liquid crystal cells. In solution both trans–cis and cis–trans occur at around 3 s and 200 s, respectively,
whereas in solids they occur at around 10 s to 200 min. These are some of the first examples of azobenzene
liquid crystals which exhibit very fast switching properties in solutions.
Received (in Porto Alegre, Brazil)
5th April 2013,
Accepted 13th May 2013
DOI: 10.1039/c3nj00359k
www.rsc.org/njc
Colr (rectangular columnar)or Colob (columnar oblique lattice)
for B1 and Smintercal (intercalated smectic) for the B6 phase.5
Introduction
Pyrimidine and its derivatives have been widely used as bio-
logically active compounds.1 They have also been used in
materials science to generate calamitic and discotic liquid
crystals.2 However, to the best of our knowledge, use of pyrimidine
as a core molecule for the generation of bent-core liquid crystalline
materials has so far not been explored. Here, we report the first
examples of pyrimidine-based photo-switchable bent-core liquid
crystalline molecules.
Bent-core or banana-shaped liquid crystals comprise five,
six and seven aromatic rings, whereby aromatic rings are
connected through a wide variety of connecting groups to the
central rigid core, such as benzene and naphthalene deriva-
tives. A large number of bent shaped molecules containing
1,3/1,5-substituted benzene as a central unit have been exten-
sively investigated as ‘‘Banana’’ phases.3,4 In general the meso-
phases formed by the banana-shaped compounds are termed
as ‘‘Banana’’ (Bn) phases, designated B1–B8 phases, the B3 and
B4 phases are crystalline, while the others are mesomorphic.5
The nomenclature used is SmCP (polar tilted smectic) for B2,
The bent shaped molecules exhibit the Bn phases in addition to
normal smectic or nematic phases.6–9 There are numerous
studies accomplished on the effect of different lateral substi-
tuents on the arms of the five-ring bent-core molecules and a
lateral fluorine substituent ortho to the terminal n-alkoxy chain
induces interesting electro-optical switching properties to the
mesophases.10–12
In recent years, a field of research that has been growing
steadily is the photo-induced phenomenon in which the incident
light brings about the molecular ordering/disordering of the
liquid-crystalline system.13,14 The typical thermally accompanied
process of cis–trans transformations leads to dramatic changes
in the electronic spectra of the azobenzene molecules to favour
photochromism.15 This particular aspect of photonics, in which
molecular geometry can be controlled by light, is being proposed
as the future technology for optical storage devices.16 Molecules
containing the azobenzene moiety are well known to show
reversible isomerization transformations upon irradiation with
UV and visible light.17 Upon absorption of UV light (B365 nm)
the energetically more stable trans configuration transforms into
a cis configuration. Photoinduced effects reported in the litera-
ture are on 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.18–20 The time taken for
the phase transition to takes place following the isomerization of
the photoactive molecules is not only of significant interest from
a basic point of view, but is also of concern for applications.
a Faculty 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
b Raman Research Institute, C. V. Raman Avenue, Sadashivanagar,
Bangalore 560080, India
† Electronic supplementary information (ESI) available: Materials, detailed syn-
thetic procedure with analytical data, the crystalline phase by POM and UV/vis
absorption. See DOI: 10.1039/c3nj00359k
c
2460 New J. Chem., 2013, 37, 2460--2467
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
Paper
The azobenzene liquid crystals are among the most promising
NJC
2a: IR, nmax/cmÀ1 3076 (QCH2), 2928 (CH2), 2862 (CH2), 1728
materials for photo-switchable devices. These highly stable mole- (CQO, ester), 1642 (CQC, vinyl), 1605, 1497 (CQC, aromatic),
cules are promising materials for the photochromic applications. 1240, 1136, 1064 (C–O), 829 (C–H). dH(500 MHz; CDCl3; Me4Si) 8.18
Therefore, a number of monomeric and polymeric bent-core (2H, d, J = 8.6 Hz, Ph), 7.97 (2H, d, J = 6.8 Hz, Ph), 7.91 (2H, d, J =
molecules containing an azo (–NQN–) linkage have been reported 6.9 Hz), 7.01 (2H, d, J = 6.8 Hz), 6.05 (m, 1H, CHQ), 5.42 (d, 1H,
for the possibility of photochromism and photoisomerization.21,22 J = 16.2 Hz,QCH2), 5.31 (d, 1H, J = 10.2 Hz, QCH2), 4.61 (d, 2H,
Polymerization of the appropriate bent-core liquid crystals has J = 4.8 Hz, OCH2–), 4.18 (q, 2H, OCH2CH3), 1.43 (t, 3H, –CH2CH3).
also been received significant attention in the last several years.23–25 dC(125 MHz; CDCl3; Me4Si) 14.48, 61.34, 69.25, 115.22, 118.25,
The cross-linked liquid crystal polymers derived from acrylate 122.61, 125.29, 130.22, 131.36, 132.33, 147.22, 155.45, 161.79 and
monomers containing banana liquid crystals have also been 166.48.
reported.26,27 Bent core liquid crystalline monomers with double
4-{2-[4-(Prop-2-enyloxy)phenyl]hydrazinyl}benzoic acid (3a).
bonds at both ends were used for polymerization to obtain main Compound 2a (1.30 g, 4.19 mmol) was prepared according to
chain LC polymers.28,29 Several oligomeric/polymeric compounds the reported procedure.32b A solution of potassium hydroxide
are reported30 with vinyl-terminated side chains and a bent core (0.94 g, 16.76 mmol) in water (10 ml) was added and the solution
mesogenic unit of polysiloxane-based liquid crystals.31
was refluxed for 4 h. Yield: 0.45 g (38%) and m.p. 221 1C. A
The present investigation focuses on the synthesis and similar procedure was adopted for synthesis of other higher
photoisomerization behaviour of new bent-shaped monomers members of homologues such as 3b–f from compounds 2b–f.
derived from 2,6-pyrimidinediol as a central unit and rod
3a: IR, nmax/cmÀ1 3072 (QCH2), 2922 (CH2), 2864 (CH2), 1684
shaped azobenzene units with terminal chains having double (CQO, acid), 1644 (CQC, vinyl), 1597, 1496 (CQC, aromatic), 1249,
bonds as the side arms. In addition, we have created the optical 1136, 1064 (C–O), 829 (C–H). dH(500 MHz; CDCl3; Me4Si) 8.18 (2H,
storage device using a guest–host system where the bent-core d, J = 8.2 Hz), 7.94 (2H, d, J = 7.1 Hz), 7.93 (2H, d, J = 6.7 Hz), 7.05
azo dye (guest) is mixed with the liquid crystalline material E7 (2H, d, J = 8.9 Hz), 6.04 (m, 1H, CHQ), 5.45 (d, 1H, J = 16.6 Hz,
(room temperature liquid crystals acting like host) for measur- QCH2), 5.31 (d, 1H, J = 10.2 Hz, QCH2), 4.60 (d, 2H, J = 4.1 Hz,
ing the thermal back relaxation. In the light of this, guest–host OCH2–). dC(125 MHz; CDCl3; Me4Si) 69.13, 115.18, 118.25, 122.41,
effects in liquid crystals with azo dyes could provide a path 125.24, 130.64, 131.66, 132.73, 147.23, 155.38, 161.69 and 166.98.
for the exploration of systems for obtaining long-term optical
storage devices.
2,6-Pyrimidine bis[4-{[4-(allyl-en-1-yloxy)phenyl]diazen-yl}-
benzoate] (4a). Compound 3a (0.290 g, 1.02 mmol) and
2,6-pyrimidinediol (0.0570 g, 0.510 mmol) were dissolved in 50 ml
of dry dichloromethane. Then DCC (0.226 g, 1.10 mmol) and DMAP
(0.013 g, 0.11 mmol) were added and the mixture was stirred for
48 h. The precipitate was removed by filtration and the solvent was
Experimental
Synthetic procedures
Ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate (1). Compound removed. The product was dissolved in dichloromethane and water.
1 was synthesized according to the reported procedure32a from The organic phase was washed with diluted acetic acid and water
ethyl 4-aminobenzoate (10.18 g, 0.0616 mol), conc. hydrochloric and the solvent was removed under reduced pressure. The com-
acid (14 ml), sodium nitrite (4.257 g, 0.0616 mol) and phenol pound was purified on silica gel by column chromatography using
(5.79 g, 0.0616 mol) in 600 ml methanol. Yield: 8.01 g, 50.7% as dichloromethane/hexane as an eluent. Solid was recrystallized from
red crystals and m.p. 160 1C. IR, nmax/cmÀ1 3322 (OH), 1724 ethanol and chloroform to obtain the target compound 4a. Yield:
(CQO, ester), 1604, 1480 (CQC, aromatic), 1249, 1142 (C–O), 0.112 g, 34%. A similar procedure was adopted for synthesis of other
828 (C–H). dH(500 MHz; acetone-d6; Me4Si) 8.16 (2H, d, J = higher members of homologues with odd and even numbers of
8.0 Hz, Ph), 7.90 (2H, d, J = 6.8 Hz, Ph), 7.88 (2H, d, J = 7.8 Hz, Ph), alkyl carbons such as 4b–f from compounds 3b–f.
7.0 (2H, d, J = 8.9 Hz, Ph), 5.57 (1H, s, OH), 4.42 (2H, q, J =
4a: IR, nmax/cmÀ1 3090 (QCH2), 2923 (CH2), 2858 (CH2),
6.9 Hz, –CH2CH3), 1.47 (3H, –CH2CH3). dC(125 MHz; acetone-d6; 1749 (CQO ester), 1642 (CQC, vinyl), 1594, 1496 (CQC,
Me4Si) 14.58, 61.40, 116.10, 122.59, 125.58, 130.70, 131.62, aromatic), 1248, 1129, 1052 (C–O), 852 (CH, aromatic).
147.19, 155.41, 159.21, 166.41. Elemental analysis, found: C, dH(500 MHz; CDCl3; Me4Si) 8.30 (4H, d, J = 8.9 Hz, Ph), 8.02
66.56; H, 5.26; N, 10.27. Calc. for C15H14N2O3: C, 66.69; H, 5.22; (1H, d, Py), 7.97 (4H, d, J = 6.8 Hz, Ph), 7.92 (4H, d, J = 6.9 Hz, Ph),
N, 10.36%.
Ethyl4-{2-[4-(prop-2-enyloxy)phenyl]hydrazinyl}benzoate
7.03 (4H, d, J = 5.6 Hz, Ph), 6.95 (1H, t, Py), 6.07 (2H, m, CHQ),
5.46 (2H, dd, J = 17.2 Hz, QCH2), 5.32 (2H, dd, J = 10.3 Hz,
(2a). Compound 2a was synthesized according to the reported QCH2), 4.63 (4H, t, J = 6.5 Hz, OCH2–). dC(125 MHz; CDCl3;
procedure32b from compound 1 (2.0 g, 7.4 mmol) in dry acetone Me4Si) 69.19, 115.11, 115.21, 122.31, 122.91, 124.15, 125.48,
(60 ml), allyl bromide (1.1 g, 9.0 mmol), potassium carbonate 131.76, 132.64, 138.55, 147.17, 156.42, 161.92, 165.55 and
(1.24 g, 9.0 mmol) and a catalytic amount of potassium iodide 168.67. Elemental analysis, found: C, 67.16; H, 4.22; N, 13.02.
(20 mg) was refluxed for 24 h under an argon atmosphere. Yield Calc. for C36H28N6O6: C, 67.49; H, 4.40; N, 13.11%.
of 2a: 1.44 g (63%), m.p. 101 1C. A similar procedure was
Instrumentation
adopted for synthesis of other higher members of homologues
with odd and even numbers of alkyl carbons such as butene, The structures of the intermediates and desired products were
pentene, hexene, heptene and octene (2b–f).
confirmed by spectroscopic methods: IR spectra were recorded
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 2460--2467 2461

View Article Online
NJC
Paper
using a Perkin Elmer (670) FTIR spectrometer. 1H NMR (500 MHz)
and 13C NMR (125 MHz) spectra were recorded using a Bruker
(DMX500) spectrometer. The transition temperatures and their
enthalpies were measured using differential scanning calorimetry
(Perkin DSC 7) at heating and cooling rates of 5 1C minÀ1. Optical
textures were obtained by using an Olympus BX51 polarizing
optical microscope equipped with a Mettler Toledo FP82HT hot
stage and a FP90 central processor unit. X-ray diffraction measure-
ments were carried out using Cu-Ka radiation (l = 1.54 Å)
generated from a 4 kW rotating anode generator (Rigaku Ultrax-18)
equipped with a graphite crystal monochromator. The sample
was filled into Hampton research capillaries (0.5 mm diameter)
from the isotropic phase, sealed and held on a heater. X-ray
diffraction was carried out in the mesophase obtained on cooling
the isotropic phase and diffraction patterns were recorded.
Scheme 1 Synthesis of 4a–f. Reagents and conditions: (i) K2CO3, KI, BrCnH2nÀ1
(n = 1–6), reflux; (ii) KOH, MeOH, reflux; (iii) 2,6-pyrimidinediol, DCC, DMAP.
Sample preparation for the photo-switching study
UV/Vis absorption spectra were recorded using a UV-Visible
spectrophotometer obtained from Ocean Optics (HR2000+).
For photo-switching studies in solutions, pyrimidine-based
photoswitchable bent-core liquid crystals were dissolved in
dimethylformamide at suitable concentrations. Photo-switching
behaviour of the azobenzene containing pyrimidine moiety was
investigated by illuminating with an OMNICURE S2000 UV
source equipped with a 365 nm filter. For photoswitching
studies in solids, pyrimidine-based photoswitchable bent-
core liquid crystals were mixed with room temperature liquid
crystals E7 to make a guest–host system. Concentration used
for room temperature nematic liquid crystals to bent core
liquid crystals is around 1 : 20 ratio (5% of the bent core azo
dye is dissolved in 95% E7). Room temperature nematic liquid
crystals E7 are used as host materials along with bent core
liquid crystals as guest materials. The mixture is capillary filled
into the previously prepared ITO + polyimide coated, unidir-
ectionally rubbed, sandwiched cell at isotropic temperature
(B70 1C). Qualities of the cells were observed under an optical
polarizing microscope.
compounds (4a–f) consist of five-ring aromatic compounds
including pyrimidine as a central core and terminal chains
having double bonds as polymerizable functional groups as the
side arms both sides with odd and even numbers of carbon
atoms (1–8). Phase transition temperature and associated transi-
tion enthalpies are given in Table 1. DSC thermograms of the
compound were measured at a rate of 5 1C minÀ1. Compounds
4a and 4b did not exhibit any mesomorphism, both compounds
showed only one peak upon heating and cooling cycles.
Compounds 4c and 4e with odd numbers of carbon atoms in
the side chains showed enantiotropic phase sequences of
Cr–B6–I (Table 1). Compounds 4d and 4f having an even
number of carbons in the alkyl chain showed enantiotropic
phase sequences of Cr1–Cr2–B6–I with higher isotropic tem-
perature compared to those having odd numbers of carbon
alkyl chains (Table 1). The isotropic temperatures are slightly
increased with longer alkyl chains which is consistent with
reported paper.34
Polarized optical microscopy (POM) was used for textural
observations of the six compounds (4a–f) in order to identify
the mesophase type. Among these homologous series of com-
pounds, the shorter alkyl chains as 4a and 4b showed no
Results and discussion
Materials synthesis
Table 1 Phase transition temperature (T/1C) and associated enthalpies [DH/J gÀ1
in parentheses of DSC scan for 4a–f
]
The synthesis of the new materials is depicted in Scheme 1.
Compound 1, compounds 2a–f and 3a–f were synthesized
according to the earlier papers.32b In the final step, acid compounds
3a–f were coupled with 2,6-pyrimidinediol by DCC and DMAP to
achieve corresponding desired molecules 4a–f (Scheme 1).
Compounds were purified on silica gel by column chromato-
graphy and recrystallized from ethanol/chloroform and are
Comp.
n
Scan
Phase transitions
4a
1
Heat
Cool
Heat
Cool
Heat
Cool
Heat
Cool
Heat
Cool
Heat
Cool
Cr 98.7 [45.2] I
I 93.9 [41.1] Cr
Cr 91.1 [54.6] I
I 88.7 [51.4] Cr
Cr 71.1 [68.7] B6 96.5 [11.0] I
I 92.7 [10.7] B6 67.1 [57.8] Cr
Cr1 64.0 [20.4] Cr2 72.3 [29.4] B6 102.8[18.3] I
I 101.3 [18.0] B6 68.2 [48.8] Cr
Cr 65.7 [24.8] B6 100.4 [20.6] I
I 98.2 [9.9] B6 62.8 [25.5] Cr
4b
4c
4d
4e
4f
2
3
4
5
6
1
characterized by H, 13C NMR and elemental analysis. Analytical
data confirmed the structures and purity of all the samples (see
detailed synthetic procedures and data in ESI†).
Mesomorphic properties
Cr162.1 [8.6] Cr2 100.6 [28.8] B6 113.8 [12.9] I
I 111.2 [14.4] B6 81.8 [49.4] Cr
Thermal behaviour of all the compounds was studied using
polarized optical microscopy (POM) and differential scanning
Abbreviation: Cr = crystal, N = nematic, B6 = smectic A, I = isotropic
calorimetry (DSC). The homologous series of symmetric bent-core phase.
c
2462 New J. Chem., 2013, 37, 2460--2467
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
Paper
NJC
mesophase texture, instead showing a clear transition from
crystalline solid to isotropic liquid or isotropic to crystalline
form. This could be due to very short flexible aliphatic chains in
these molecules. Under POM, a typical fan shaped texture
which is a smectic A phase corresponding to the B6 phase
was observed for compounds 4c and 4d (Fig. 1) upon
cooling from the isotropic state, which upon further cooling
transformed into a crystalline phase. Broken fan shaped and
pseudo fan shaped textures were found for 4e and 4f, respec-
tively, as a B6 phase (Fig. 1). Compounds 4c–f showed the
enantiotropic phase which is in good agreement with DSC
data. Overall, higher homologues of the series show mesophase
behaviour (4c–f), therefore, the alkyl chain influences the meso-
phase character rather than double bonds on the terminals as
expected.
X-ray diffraction studies: X-ray diffraction was carried out in
the mesophase obtained upon cooling from the isotropic phase
of compound 4f (n = 6) for further examination of mesophase
structure. For example, the powder XRD profile of compound 4f
showed two reflections in the small angle region with spacing
d1 = 26.27 Å, d2 = 13.12 Å and a diffuse peak in the wide angle
region with spacing d3 = 4.25 Å. These small angle peaks at d1
and d2 can be indexed as (01) and (02) indicating a smectic type
ordering. In addition, a diffused peak in the wide-angle region
with d-spacing of 4.25 Å is due to a liquid like in-plane ordering
of molecules in the layers. In the small angle region, two
reflections at d1 = 26.27 Å and d2 = 13.12 Å with the ratio of
spacing as 1 : 12 indicate a smectic type ordering with a layer
spacing of 26.27 Å.33a The XRD pattern observed which is
typical of a B6 intercalated mesophase due to a sharp reflection
is half of the molecular length (50.13 Å).33b The X-ray diffraction
profile of compound 4f at 105 1C is shown in Fig. 2.
Photo-switching study
All bent-core molecules showed similar absorption spectra due
to their similar molecular structures, the only difference being
in the alkyl chain (n = 1–6), which does not alter the electronic
transitions, and the absorption spectra of compounds 4c–f are
shown in ESI† (Fig. S2). Consequently, compounds 4c and 4d
were considered for a photoisomerization study.33a–c Photo-
switching studies were initially performed on solutions and
then on liquid crystal cells. It gives an idea of the behaviour of
materials with respect to UV light and also these results are
indispensable for creating optical storage devices.
Fig. 3 depicts the absorption spectra of 4d (n = 6) before and
after UV illumination. The absorption spectra of compound 4d
show absorbance maxima at 392 nm. The absorption spectra of
compound were carried out in dimethyl formamide (DMF)
solution having concentration C = 1.2 Â 10À5 mol LÀ1. The
strong absorbance in the UV region at 392 nm corresponds to
p–p* transition of the E isomer (trans isomer) and a very weak
absorbance in the visible region at around 450 nm represents
n–p* transition of the Z isomer (cis isomer).
Fig. 1 Polarized optical micrographs obtained from cooling of isotropic phases
of (a) B6 phase of 4c at 82 1C, (b) B6 phase of 4d at 88 1C, (c) B6 phase of 4e at
84 1C, (d) B6 phase of 4f at 96 1C.
The photo-switching property of 4d was investigated in DMF UV light having a 365 nm filter at different time intervals and
using UV/visible spectroscopy in the absence and the presence immediately the absorption spectra were recorded. The absorption
of UV light illumination. Compound 4d was illuminated with maximum at 392 nm decreases due to E/Z photoisomerization,
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 2460--2467 2463

View Article Online
NJC
Paper
Fig. 5 Thermal back relaxation process for the compound 4d shows that to
relax from cis to trans takes around 200 seconds. 0 seconds corresponds to the
UV off, after illuminating the material for 20 seconds (photostationary state).
Fig. 2 The intensity versus 2y graph derived from the X-ray diffraction for the B6
phase of compound 4f at 105 1C.
shows that photosaturation occurs within 2 seconds which is
very fast as compared with nematic to isotropic phase involved
photoisomerization.35 The reverse transformation from Z to
E can be brought by two methods, one by keeping the solu-
tion in dark and other by shining white light of higher wave-
length. The earlier process is well known as thermal back
relaxation.
Fig. 5 shows the thermal back relaxation process where the
solution is illuminated continuously for 20 seconds (photo-
stationary state) and kept in the dark and then at subsequent
time intervals, spectral data were recorded.
Fig. 6 shows the time dependence of the Z–E absorption of
compound 4d. Peak wavelength at 392 nm as obtained from
Fig. 5 is plotted as a function of recovery time. The curve shows
that thermal back relaxation occurs within 200 seconds which
is reasonably fast as compared with nematic to isotropic phase
involved thermal back relaxation.35
A possible reason for observing faster thermal back relaxa-
tion as well as the UV ON process could be that the phases
involved on both sides of transition possess a layer structure
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.36
Fig. 3 Absorption spectra of 4d with different exposure time of UV light. Before
UV corresponds to the 0 seconds UV light illumination (absence of UV light).
which leads to the E isomer being transformed into the Z
isomer. After B10 s illumination, there is no change in the
absorption spectrum which confirms the saturation of the E/Z
isomerization process.
Fig. 4 shows the E–Z absorption of compound 4d as a
function of exposure time. Data were extracted from Fig. 3
and 392 nm peak wavelength was fixed and absorption values at
392 nm at different exposure times were recorded. The curve
Fig. 4 Time dependence photoisomerization curve of E isomer (4d) showing
Fig. 6 Time dependence photoisomerization curve of Z isomer (4d) showing
effect of UV illumination.
thermal back relaxation time.
c
2464 New J. Chem., 2013, 37, 2460--2467
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
Paper
NJC
Fig. 7 Time dependence photoisomerization curve of E isomer (4c) showing UV
ON process (a) and time dependence photoisomerization curve for the thermal
back relaxation to E isomer (b).
Fig. 8 Time dependence photoisomerization curve of E isomer (4d) showing UV
ON process (a) and time dependence photoisomerization curve for the thermal
back relaxation to E isomer (b).
Spectral investigations on solid films were also recorded as a
function of UV illumination. Here the guest–host effect is that E–Z conversion takes around 10 seconds whereas recovery
employed where E7, room temperature nematic liquid crystals, to the original state i.e., thermal back relaxation takes around
act as a host system and pyrimidine based bent core liquid 250 minutes.
crystals act as guest systems. Previously prepared polyimide
In Fig. 8(b), the No UV point is given to make it clear that the
coated, unidirectionally rubbed sandwiched cells were filled system has already reached its original value. As compared to
with the guest–host mixture at isotropic temperature of the the liquid state, the solid state behaviour is very slow, which
mixture (B70 1C). UV/Vis spectral data were recorded using a may be due to the tightly packed molecules in cells, whereas in
spectrophotometer.
solutions, molecules have more freedom to move around.
As a demonstration of the efficacy of the materials, an
Fig. 7(a) and (b) show the UV ON and the UV off (thermal
back relaxation) process for the compound 4c. Peak wavelength optical storage device is shown here (see Fig. 9) that is realized
is B360 nm and data are generated from peak absorbance by using the above mentioned method. The guest–host mixture
at 360 nm as a function of exposure time. One can clearly was illuminated with UV light of 10 mW cmÀ2 intensity through
observe that E–Z conversion takes around 10 seconds whereas a standard mask for 5 minutes. Bright regions are the areas
recovery to the original state i.e., thermal back relaxation takes masked with UV radiation and the dark regions are the areas
around 200 minutes. In Fig. 7(b), the No UV point is given to illuminated with UV radiation.
make it clear that the system has already reached its original
value.
As expected material transforms from the ordered to the
disordered state with the illumination of light giving high
Fig. 8(a) and (b) shows the UV ON and the UV off (thermal contrast between bright and dark states. Research is in progress
back relaxation) process for compound 4d. Peak wavelength is to stabilize these materials to use it as permanent optical
B360 nm and data are generated from peak absorbance at storage devices by incorporating polymeric chains to photo-
360 nm as a function of exposure time. One can clearly observe polymerize the structure.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 2460--2467 2465

View Article Online
NJC
Paper
substituent in the azobenzene moiety.33c The faster isomeriza-
tion process is a key factor for optical switching applications.
Conclusion
A new series of bent core monomers derived from pyrimidine as
a core and azobenzene as a side arm were synthesized. POM,
DSC and X-ray studies showed that four compounds exhibited
the B6 phase and other two were crystalline in nature. The
compounds can be used for preparation of polymers or silyl-
functionalized bent-core mesogens, whereas the presence of
the azo linkage in these liquid crystal monomers is suitable for
photochromism studies and trans–cis–trans isomerization
cycles under UV irradiation. The photoswitching properties of
compound 4d of this series show trans to cis isomerization at
around 3 s, whereas the reverse process takes place at around
200 s in solutions and in solids E–Z photoisomerization takes
around 10 s and reverse back to original Z–E takes around
200 min. Thus, the photoswitching behaviour of these materials
may be suitably exploited in the field of optical data storage
devices and in molecular switches which need fast switching as
well as for permanent optical storage devices. Indeed, so far,
hardly any azo compound has been reported which exhibits
such a fast switching property in solutions.
Fig. 9 Demonstration of optical pattern storage capability of the device based
on the principle described in this article observed under the crossed polarizers.
The sample was kept at room temperature and illuminated with UV radiation
through a photo mask. The dark regions are e the molecules are exposed to UV
radiation and the bright regions are where the radiation is masked. P and A were
the polarizer and analyser.
Comparison of mesophase and photoisomerization behaviour
Although the comparison of mesomorphic properties of the
compounds 4c–f with other compounds is difficult because the
core molecule used in this study is pyrimidine which is the first
example, at least compounds 4c–f can be compared with our
previously reported bent-core compounds derived from the
resorcinol core with variable alkyl groups and double bonds
at both terminals.32,33 Several bent-shaped monomers con-
taining azobenzene as a side arm, resorcinol as a central unit
and terminal double bonds as polymerisable functional groups
were reported as SmAintercal mesophases.33a Six new bent-
shaped monomers were reported to have substituted or
non-substituted azobenzene moiety in the periphery and sub-
stituted or non-substituted resorcinol as a central unit with
polymerisable double bonds linked at both ends of all the
molecules. Four members of the family showed an intercalated
smectic phase and two were crystalline in nature.33b
Although the pyrimidine-based bent-core is considered to be
a new molecule, this molecule is open to comparison with
other bent-shaped compounds exhibiting excellent photo-
isomerization behaviour. The photo-isomerization behaviour
in solution of compound 4d can be compared with similar five-
aromatic ring containing bent-core compounds derived from
the resorcinol core which was reported earlier.33a,b The photo-
isomerization studies reveal that compound 4d intensifies
isomerization properties in which E to Z and Z to E isomeriza-
tions take place only at 3 s and 200 s, whereas our previously
reported resorcinol based compounds were recorded at about
50 s.33a,b It was observed that photoisomerization described in a
very recent paper takes place in 30 s for E to Z transformation.33c
We have anticipated that the time taken for the photo-
isomerization depends on the core molecules though a pre-
vious report was based on the nature of the linking group
(electron withdrawing group/donating group) and size of the
Acknowledgements
This research was supported by Universiti Malaysia Pahang
Research Grant (RDU100338) and Ministry of Higher Education
Malaysia ERGS Grant 120602. Special thanks to Mrs K. N. Vasudha
for supporting many aspects of this work.
Notes and references
1 (a) I. M. Lagoja, Chemistry and Biochemistry, Verlag Helvetica
Chimica Acta Ag, Zurich, 2005, vol. 2, pp. 1–50; (b) M. R.
Ahmada, V. G. Sastrya, Y. R. Prasada, M. H. R. Khanb,
N. Banob and S. Anwarc, Chem.–Eur. J., 2012, 3, 94–98.
2 (a) V. F. Petrov, Mol. Cryst. Liq. Cryst., 2006, 457, 121–149;
(b) Y. C. Lin, C. K. Lai, Y. C. Chang and K. T. Liu, Liq. Cryst.,
2002, 29, 237–242.
3 T. Akutagawa, Y. Matsunaga and K. Yashahura, Liq. Cryst.,
1994, 17, 659–666.
4 T. Ikeda, J. Mater. Chem., 2003, 13, 2037–2057.
5 (a) R. A. Reddy and C. Tschierske, J. Mater. Chem., 2006, 16,
907–961; (b) M. B. Ros, J. L. Serrano, M. R. de la Fuente and
C. L. Folcia, J. Mater. Chem., 2005, 15, 5093–5098;
(c) H. Takezoe and Y. Takanishi, Jpn. J. Appl. Phys., Part 1,
2006, 45, 597–625; (d) C. V. Yelamaggad, I. S. Shashikala,
´
U. S. Hiremath, G. Liao, A. Jakli, D. S. Shankar Rao,
S. K. Prasad and Q. Li, Soft Matter, 2006, 2, 785–792.
6 D. Shen, S. Diele, I. Wirt and C. Tschierske, Chem. Commun.,
1998, 2573–2574.
7 (a) D. M. Walba, E. Korblova, R. Shao, J. E. Maclennan,
D. R. Link, M. A. Glaser and N. A. Clark, Science, 2000, 288,
2181–2184; (b) J. C. Rouillon, J. P. Marcerou, M. Laguerre,
c
2466 New J. Chem., 2013, 37, 2460--2467
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013

View Article Online
Paper
NJC
H. T. Nguyen and M. F. Achard, J. Mater. Chem., 2001, 11, 23 A. C. Sentman and D. L. Gin, Angew. Chem., 2003, 115,
2946–2950. 1859–1863.
8 G. Pelzl, S. Diele, A. Jakli, Ch. Lischka, I. Wirth and 24 L. H. Wu, C. S. Chu, N. Janarthanan and C. S. Hsu, J. Polym.
W. Weissflog, Liq. Cryst., 1999, 26, 135–139. Res., 2000, 7, 125–134.
9 D. Shen, A. Pegenau, S. Diele, I. Wirth and C. Tschierske, 25 S. Demel, C. Slugovc, F. Stelzer, K. Fodor-Csorba and
J. Am. Chem. Soc., 2000, 122, 1593–1601.
G. Galli, Macromol. Rapid Commun., 2003, 24, 636–641.
26 D. M. Walba, E. Korblova, R. Shao, J. E. Maclennan,
D. R. Link, M. A. Glaser and N. A. Clark, Science, 2000,
288, 2181–2184.
10 C. K. Lee and L. C. Chien, Liq. Cryst., 1999, 26, 609–612.
11 C. K. Lee and L. C. Chien, Ferroelectrics, 2000, 243, 231–237.
12 (a) J. Matraszek, J. Mieczkowski, J. Szydlowska and
E. Gorecka, Liq. Cryst., 2000, 27, 429–436; (b) I. Wirth, 27 L. H. Wu, C. S. Chu, N. Janarthanan and C. S. Hsu, J. Polym.
S. Diele, A. Eremin, G. Pelzl, S. Grande, L. Kovalenko, Res., 2000, 7, 125–134.
N. Pancenko and W. Weissflog, J. Mater. Chem., 2001, 11, 28 C. L. Folcia, I. Alonso, J. Ortega, J. Etxebarria, I. Pintre and
1642–1650. M. B. Ros, Chem. Mater., 2006, 18, 4617–4626.
13 (a) S. K. Prasad, G. Nair and H. Gurumurthy, Adv. Mater., 29 G. Dantlgraber, A. Eremin, S. Diele, A. Hauser, H. Kresse,
2005, 17, 2086–2091; (b) D. Tanaka, H. Ishiguro, Y. Shimizu
and K. Uchida, J. Mater. Chem., 2012, 22, 25065–25071;
G. Pelzl and C. Tschierske, Angew. Chem., Int. Ed., 2002, 41,
2408–2412.
(c) E. Westphal, I. H. Bechtold and H. Gallardo, Macromolecules, 30 M. G. Tamba, A. Bobrovsky, V. Shibaev, G. Pelzl, U. Baumeister
2010, 43, 1319–1328; (d) Y. Norikane, Y. Hirai and M. Yoshida,
Chem. Commun., 2011, 47, 1770–1772.
14 T. Ikeda and O. Tsutsumi, Science, 1995, 268, 1873–1875.
and W. Weissflog, Liq. Cryst., 2011, 38, 1531–1550.
31 D. Shen, S. Diele, I. Wirt and C. Tschierske, Chem. Commun.,
1998, 2573–2574.
15 D. Y. Kim, S. K. Tripathy, L. Li and J. Kumar, Appl. Phys. 32 (a) M. R. Lutfor, J. Asik, S. Kumar, S. Silong and M. Z. Ab.
Lett., 1995, 66, 1166–1668.
Rahman, Phase Transitions, 2009, 82, 228–239; (b) M. R. Lutfor,
A. Jahimin, S. Kumar and C. Tschierske, Liq. Cryst., 2008, 35,
1263–1270.
16 H. Gurumurthy, G. Nair, S. K. Prasad and C. V. Yelamaggad,
J. Appl. Phys., 2005, 97, 093105; S. K. Prasad, G. Nair and
H. Gurumurthy, Phase Transitions, 2005, 78, 443–455.
17 T. Ikeda and I. Tsutsumi, Science, 1995, 268, 1873–1875.
18 G. Nair, S. K. Prasad and G. Hegde, Phys. Rev. E: Stat.,
Nonlinear, Soft Matter Phys., 2004, 69, 021708.
19 S. K. Prasad, G. Nair and G. Hegde, J. Phys. Chem. B, 2007,
111, 345–350.
20 L. M. Blinov, M. V. Kozlovsky, M. Ozaki, K. Skar and
K. Yoshino, J. Appl. Phys., 1998, 84, 3860.
21 A. C. Sentman and D. L. Gin, Angew. Chem., Int. Ed., 2003,
42, 1815–1819.
33 (a) M. R. Lutfor, S. Kumar, C. Tschierske, G. Israel, D. Ster
and G. Hegde, Liq. Cryst., 2009, 36, 397–407; (b) M. R. Lutfor,
G. Hegde, S. Kumar, C. Tschierke and V. G. Chigrinov,
Opt. Mater., 2009, 32, 176–183; (c) M. V. Srinivasan,
P. Kannan and A. Roy, New J. Chem., 2013, 37, 1584–1590.
34 N. G. Nagaveni, A. Roy and V. Prasad, J. Mater. Chem., 2012,
22, 8948–8959.
35 G. G. Nair, S. K. Prasad and C. V. Yelamaggad, J. Appl. Phys.,
2000, 87, 2084–2089.
36 S. K. Prasad, K. L. Sandhya, D. S. Shankar Rao and
Y. S. Negi, Phys. Rev. E: Stat., Nonlinear, Soft Matter Phys.,
2003, 67, 051701.
22 S. Demel, C. Slugovc, F. Stelzer, K. Fodor-Csorba and
G. Galli, Macromol. Rapid Commun., 2003, 24, 636–641.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem., 2013, 37, 2460--2467 2467