Light induced generation of stable blue phase in photoresponsive diphenylbutadiene based mesogen

Article abstract of DOI:10.1039/b926524d

Light induced isothermal phase transition leading from smectic A* phase to a stable blue phase was achieved via photoisomerization of a chiral diphenylbutadiene based mesogen.

Full text of DOI:10.1039/b926524d

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Light induced generation of stable blue phase in photoresponsive
diphenylbutadiene based mesogenw
Ratheesh K. Vijayaraghavan,a Shibu Abraham,a D. S. Shankara Rao,b S. Krishna Prasadb
and Suresh Das*a
Received (in Cambridge, UK) 16th December 2009, Accepted 5th February 2010
First published as an Advance Article on the web 1st March 2010
DOI: 10.1039/b926524d
Light induced isothermal phase transition leading from smectic
A* phase to a stable blue phase was achieved via photo-
isomerization of a chiral diphenylbutadiene based mesogen.
stable blue phase. To the best of our knowledge such
photoinduced generation of the blue phase has not been
reported before in single component LCs. Films drawn from
inherently photoactive LCs can exhibit much faster switching
times and enhanced stability compared to doped films.6,8 The
blue phase which could be generated from the SmA* phase by
irradiation with low intensity 360 nm light (0.6 mW cmÀ2), had
a significantly wide temperature range and its Bragg reflection
could be tuned by varying the irradiation time.
Blue phases (BP) are an interesting class of chiral liquid
crystals possessing a fluid lattice which self-assemble into
three-dimensional cubic defect structures.1 The periodicity of
these defect structures is of the order of the wavelength of
visible light, resulting in interesting optical properties such as
selective reflection of circularly polarized light.2 These properties
combined with the ability to reorientate the molecules in blue
phases using external electric fields makes them very attractive
for applications such as fast light modulators and tunable
photonic crystals.3 In view of this there have been several
efforts in designing materials exhibiting thermally stable blue
phases, including the use of polymeric and hydrogen bonded
systems.4 Since a clear structure property relationship for this
class of materials is yet to be established, methods for generation
of the rarely observed blue phase is attracting significant
attention. An interesting method for generation of frustrated
liquid crystalline phases is to design photoresponsive liquid
crystalline materials from molecules which can undergo
structural transformation following absorption of light.
Although there have been several reports on the generation
of chiral nematic phases, including twist grain boundary
phases using such photoresponsive liquid crystals,5 reports
on the photogeneration of the blue phase are limited.6
The mesomorphic properties of the chiral butadienes
were investigated by polarized optical microscopy (POM),
differential scanning calorimetry (DSC) and temperature
dependent small angle X-ray diffraction (SAXRD). The phase
transition properties of these compounds are summarized in
Table 1.
The diphenylbutadiene derivative (1), exhibited enantio-
tropic LC phases, with a smectic A* phase (SmA*) characterized
by focal conic texture appearing at 120 1C (Fig. 1a) in the
heating cycle. XRD analysis of the sample at this temperature
indicated formation of a bilayered smectic phase (Fig. S2w).
On increasing the temperature to 126 1C the SmA* phase
transformed into a N* phase characterized by its Grandjean
texture (Fig. 1b), which was further confirmed using a wedge
cell in which Cano lines were clearly visible (Fig. S4w). The
characteristic reflection spectra of the N* phase could be tuned
from 560 nm to 740 nm by varying the temperature from 131
to 125 1C (Fig. 2) in the cooling cycle. Further heating of the
sample resulted in isotropization at 132 1C. In the cooling
cycle a twist grain boundary A* (TGBA*) phase was observed
at 124 1C (Fig. 1c) in addition to the other phases (Table 1).
Phase transition temperatures were further confirmed by DSC
(Fig. S1w).
In a series of earlier studies we have shown that diphenyl-
butadiene forms a versatile chromophore for the design of
liquid crystals capable of undergoing photoinduced isothermal
phase transitions.7 The photoisomers formed from these
systems are thermally stable, and hence the design of chiral
photoresponsive diphenylbutadiene based mesogens would be
an interesting method of introducing defect structures leading
to the formation of frustrated chiral liquid crystalline phases.
With this objective we synthesised a series of chiral diphenyl-
butadiene derivatives (Chart 1). None of the derivatives showed
a blue phase in heating or cooling cycles. Photoirradiation of
the smectic phase of 1 however, resulted in the formation of a
POM studies of 2 on the other hand indicated the formation
of a monotropic chiral nematic phase (Table 1). Cooling its
isotropic phase to 151 1C resulted in the appearance of an oily
a Photosciences and Photonics Section, Chemical Sciences and
Technology Division, National Institute for Interdisciplinary Science
and Technology (CSIR), Trivandrum, 695 019, Kerala, India.
E-mail: sureshdas55@gmail.com; Fax: +91-471-2490186
b Centre for Liquid Crystal Research, Jalahalli, 560 013, Bangalore,
India
w Electronic supplementary information (ESI) available: Experimental
details. See DOI: 10.1039/b926524d
Chart 1 Structures of the chiral diphenylbutadienes investigated.
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Table 1 Phase transition temperatures of the chiral diphenyl butadienes
Heating cycle, 1C
Cooling cycle, 1C
1
2
3
Cr 120 SmA* 126 N* 132 I
Cr 162 Iso
Cr 135 SmA* 141 Iso
Iso 131 N* 124 TGBA* 123 SmA* 110 Cr
Iso 151 N* 143 Cr
Iso 141 SmA* 122 Cr
The linearly shaped trans isomers of diphenylbutadiene
derivatives are known to exhibit stable LC phases whereas
their corresponding bent cis isomers do not. Thus photo-
irradiation of the diphenylbutadiene mesogens can lead to
destabilization of their LC phases.9 Photoinduced isothermal
phase transitions of LC phases of 1 were investigated by
irradiating its films held at different temperatures with
360 nm light. Photoirradiation of a film of 1 cooled to 118 1C
from its isotropic phase (Fig. 3a) for 50 s, resulted in
disappearance of the focal conic texture of the SmA* phase
and concomitant appearance of an oily streak texture
characteristic of the cholesteric phase (Fig. 3b), which was
confirmed by the observation of Cano lines in a wedge cell. On
further irradiation (200 s) complete conversion of the smectic
phase to N* phase was observed (Fig. 3d).
Fig. 1 Textures of compound 1 at: (a) SmA* (120 1C), (b) N*
(128 1C) and (c) TGBA* (123.5 1C), in the cooling cycle.
Subsequent irradiation did not lead to any further textural
changes, indicating that the photostationary state had been
attained. HPLC analysis of the film irradiated for 200 s at
118 1C indicated that the photostationary state contained 6%
of the cis isomers. Photoirradiation of the SmA* film held at a
higher temperature (124 1C), for 100 s, resulted in its
transformation to a phase with a characteristic classical blue
phase texture shown in Fig. 4 (also see Fig. S3w).
Additionally the characteristic sharp reflection bands
(Fig. 5) compared to the rather broad reflection bands
observed for the chiral nematic phase confirmed the formation
of the blue phase. When the material held in the SmA* LC
state at 124 1C in the heating cycle was irradiated with 360 nm
light for 100 s, photoinduced formation of the cis isomer
Fig. 2 Dependence of N* reflection band, of a 10 mm thin film of 1 on
temperature in the cooling cycle.
streak texture with selective reflection of light, characteristic of
the chiral nematic (N*) phase. The derivative 3, which is
structurally similar to 1 (Chart 1), did not exhibit any chiral
nematic phases, with only a SmA* phase being observed in its
heating and cooling cycles.
Cyano substituted diphenylbutadiene derivatives have been
reported to undergo photoisomerization in solution, liquid
crystalline phases and in solid films to form thermally stable
cis isomers.9 Photolysis of 1 in acetonitrile using 360 nm light
resulted in a substantial decrease in the intensity of its major
absorption band centered at 360 nm, with a concomitant
increase in intensity of the short wavelength absorption band
around 266 nm (Fig. S5w). These changes could be attributed
to the trans–cis photoisomerization of the butadiene
chromophore.9 The cis isomers (EZ and ZE) formed on
photoirradiation were isolated using HPLC and characterised
by 1H NMR spectroscopy. On photoirradiation, as the
concentration of the cis isomers increased, some of the light
was absorbed by them resulting in competing forward and
backward photoisomerization reactions until a photostationary
state was reached, where the concentration of each species was
determined by the relative efficiencies of each of these
processes. HPLC analysis indicated a ratio of 52/40/8 for the
EE/EZ/ZE isomers at the photostationary state of 1 in
1
acetonitrile. H NMR spectral analysis clearly indicated that
the peripheral double bond in the alkoxy chain had not
undergone isomerization upon 360 nm light illumination.
Fig. 3 Photoinduced isothermal phase transition of 1 at 118 1C on
irradiation with 360 nm light.
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temperature. Photoisomerization of the trans isomers of
polyenes is known to be very sensitive to viscosity, decreasing
drastically with increasing viscosity.10 Further irradiation led
to isotropization of the sample where the cis content was 24%.
In conclusion, chiral mesophases could be generated by
covalently attaching alkyl chains containing asymmetric
carbons to the diphenylbutadiene moiety, whose nature
and stability depended upon the nature of the alkyl chain.
Photochemically generated cis isomers of the diphenyl-
butadiene derivative 1 resulted in isothermal phase transitions
from smectic A* to stable chiral phases such as the chiral
nematic and blue phases depending upon the extent of photo-
isomerization. Further investigations are in progress on
increasing the stability of the photochemically induced blue
phases.
Fig. 4 Photoinduced isothermal phase transition of compound 1
from SmA* to blue phase at 124 1C on irradiation with 360 nm light.
resulted in the formation of the blue phase and the reflection
band was centred at 510 nm. Subsequent irradiation led to the
blue shift in the reflection band. The blue phase thus formed
was thermodynamically stable and could be maintained at this
state for several hours. On heating, this phase underwent
isotropization at 128 1C.
The texture displayed in Fig. 4 clearly exhibits the mosaic
platelet pattern, ruling out the photoinduced BP being of the
BPIII (fog phase) type and indicating a cubic structure BP i.e.,
either BPI or BPII. It is known that the temperature dependence
of the selective reflection wavelength lmax decreases signi-
ficantly with increasing temperature in the BPI phase,2,3 but
has a negligible or at best a weak positive variation in the BPII
phase. Since the appearance of the BP in the present experi-
ments is temporal i.e., time elapsed since the beginning of
trans–cis isomerization, it is akin to increase in temperature.
The data in Fig. 5, where lmax exhibits strong time dependence
suggests that the photoinduced phase is BPI.
R. K. Vijayaraghavan and S. Abraham acknowledge CSIR,
Government of India for the fellowship. Research grant from
the Council of Scientific and Industrial Research Network
Program NWP 0023 is gratefully acknowledged. This is
contribution No. PPG-296 from NIIST, Trivandrum.
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Fig. 5 Changes in the transmittance spectra in the blue phases
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This journal is The Royal Society of Chemistry 2010
2798 | Chem. Commun., 2010, 46, 2796–2798