Structural Understanding, Photoswitchability and Supergelation of a

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Structural Understanding, Photoswitchability and Supergelation of a New Class of Four-Ring Based Bent Shaped Liquid Crystal Nazma Begum, SUPREET KAUR, Golam Mohiuddin, Rajib Nandi, Santosh Prasad Gupta, Nandiraju V. S. Rao, and Santanu Kumar Pal J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.9b01456 • Publication Date (Web): 01 May 2019 Downloaded from http://pubs.acs.org on May 2, 2019

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The Journal of Physical Chemistry

Structural Understanding, Photoswitchability

and Supergelation of a New Class of Four-

Ring Based Bent Shaped Liquid Crystal

Nazma Begum,a,b Supreet Kaur,b Golam Mohiuddin,*b Rajib Nandi,b Santosh Prasad Gupta,c Nandiraju V S Rao,*a Santanu Kumar Pal *b aDepartment

bDepartment

of Chemistry, Assam University, Silchar, Assam-788011, India.

of Chemical Sciences, Indian Institute of Science Education and Research

(IISER) Mohali, Sector-81, Knowledge City, Manauli 140306 (India). cDepartment

of Physics, Patna University, Patna-800005.

ABSTRACT

Herein, we report a new type of azobenzene based unsymmetrical bent-core

molecules exhibiting photoswitchability in liquid crystalline state, solid state, solution

state and in mixture upon UV irradiation and intense visible light. The compounds

exhibited solid state photochromism upon exposure of UV light whereas in liquid

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crystalline state reversible phase transitions were observed via both UV irradiation

and intense visible light exposure. Crystal structure analysis reveals the basic

structural understanding such as non-planar bent molecular shape, antiparallel

arrangement of the polar bent molecules, intra- and intermolecular hydrogen

bonding, different * * interactions and interdigitation of long alkyl chains.

The

compounds are also found to act as supergelator towards various organic solvents.

Hence, this is an excellent example of such potential bent shaped liquid crystals that

promise an immense perspective for device applications like optical storage,

molecular switches etc.

INTRODUCTION Photoswitchable organic materials are of great importance not only for fundamental

scientific understanding but also for the development of miscellaneous devices like

molecular photonic switches, light tunable gratings, photoswitchable nanodevices, photo-electronic devices etc.1-5 Amongst photoswitchable materials, azobenzenes

(N=N) are the classical example of the light induced photoisomerizable entity that

find enormous applications in different fields viz. optical processing, memory and 2 ACS Paragon Plus Environment

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data storage,6 holography and nonlinear optics,7 optical phase conjugation,8 etc. In

recent years, photoswitchable liquid crystalline (LC) materials have emerged as

enticing systems due to their ability to reorient the mesophase structure isothermally upon exposure of actinic light.1,9-15 Hence, light has been effectively used as an

actuator to control the phase transformation of photoresponsive LC materials that are termed as phototropic LCs.16 Majority of these photoswitchable LC materials

incorporate azobenzene (N=N) unit in their molecular architecture substantially responsible for exhibiting photoresponsive behaviour.17

In the field of liquid crystal research, bent-core LCs (BLCs) are placed in the front line due to their unusual physical properties viz., macroscopic chirality, polar order,

and richest mesomorphic behaviour as compared to rodlike and disklike molecules.18-24 In recent years BLCs with reduced number of four phenyl rings

exhibited polarization splay modulated layer undulated phase variants of B7, B1, B1Rev/Tilted mesophases.25-30 Although banana mesophases (B1 to B8) exhibited unparalleled and richest variety phase structures in the family of LC materials but

from application perspective in contemporary devices nematic phase is the ideal.



Hence via small structural modification in these four-ring based bent-core molecules

the achieved nematic phase have also shown some fascinating characteristics of

negative bend-splay anisotropy for stabilizing broad-range blue phases, positive

dielectric anisotropy, positive birefringence, comparatively low viscosity, ferroelectric nematic phase and an abnormally large Kerr constant in BPIII etc.31-34

Organogels are soft solid of low molar mass formed by self-assembly of small

molecules via various non-covalent interactions with 3D entangled network

entrapping the liquid organic phase and have attracted a great deal of interest due to its diverse applications.35-38 Organogels can act as stimuli responsive systems and

promise accessibility for designing and constructing new functional materials, such

as sensors, actuators, molecular devices, etc. Apart from thermo-reversible external

stimuli, organogels based on photoresponsive chromophores exhibiting mesogenic

property are of great interest as their properties can be modulated using light as an external trigger.39 In recent years considerable number of organogels formed from

LC materials have been reported and majority of them were constructed by discotic


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or pseudodiscotic molecules. Bent shaped LC molecules forming supergelation are

very limited.

In this article we have reported a new series of four-ring based bent shaped liquid crystals (Figure 1) exhibiting photo-induced phase transition in multiple states viz.,

LC state, solid state, solution state and in mixture with 4-Cyano-4'-pentylbiphenyl i.e.

5CB (0.5% wt/wt) upon irradiation with UV light. Intense visible light also actuated

the reversible nematic to isotropic phase transition. Crystal structure of the

compound 2-12 revealed the fundamental structural understanding unveiling various

intra- and intermolecular interaction, antiparallel arrangement, 3D molecular packing,

differently shaped * * interactions etc. Moreover the compounds of 2-n series are

found to act as supergelator in organic solvents and thereby qualifying these low

molecular weight bent-core LC materials for multifaceted application in photonic,

electronic and optoelectronic devices.

EXPERIMENTAL SECTION Synthesis and Characterization



Four unsymmetrical bent-core molecules have been designed, synthesised (via Scheme 1, ESI) and characterized via different analytical techniques such as FT-IR, 1H

NMR,

13C

NMR etc. given in ESI (Figure S1-S2(a-h)). The bent-core molecules

contain azo-ester-imine linking moieties with lateral and terminal chloro (Cl)

substituent at the long arm and methyl (CH3) at the kink/bay position to achieve nematic mesophase by destabilising the compact molecular packing of bent-core

molecules (Figure 1). By varying the position of the lateral Cl moiety (X and Y in

Figure 1) situated around the azo (-N=N-) linkage we also intended to understand

the effect of substitution in the materialistic property of these bent-core molecules.

Single crystal structural details are given in Supporting Information (Table S1, Figure

S7-S9) along with details of density functional theory (DFT) calculations.


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compounds exhibited nematic phase, except 2-12 that exhibited short range

monotropic smectic A (SmA) phase along with nematic phase (Figure S4-S6).

Table 1: Phase transition temperature recorded for second heating (first row) and second cooling (second row) cycles at 5 °C/min from DSC and confirmed by polarized optical microscopy. The enthalpies $45 in kJ/mol) and entropies $4 in J/mol/K) respectively are presented in parentheses. Compound

Phase transition temperatures in °C (enthalpy, entropy)

1-4

Cr 154.7 (57.1, 133.6) Iso Cr 79.7 (19.1,54.2) N 147.9 (0.33,0.80) Iso

1-12

Cr 96.5 (39.2, 106.2) N 111.2 (0.17, 0.44) Iso Cr 86.0 (43.4, 120.8) N 110.6 (0.18, 0.47) Iso

2-4

Cr 127.9 (39.0, 97.4) N 141.7 (0.28, 0.69) Iso Cr 97.0 (36.2, 97.9) N 140.3 (0.30, 0.73) Iso

2-12

Cr 118.9 (53.1, 135.4) Iso Cr 83.4 (19.0, 53.4) SmA 95.4 (0.40, 1.10) 113.6 N (0.37, 0.96) Iso

Single crystal X-ray analysis

In order to understand the fundamentals of molecular structural parameters,

interactions, molecular packing it is worthy to have a quality crystal structure

analysis. But for LCs, which carries long alkyl chains, it is challenging to develop a


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good quality crystal. Hence, after several attempts we were able to develop suitable

crystal of compound 2-12 for X-ray diffraction analysis. Compound 2-12 crystallizes

in triclinic system with space group P1. All the crystallographic parameters are described in details in Table S1. The unit cell of the crystal is composed of two

formula units of the molecule (Figure S8) and the molecules are arranged in an anti-

parallel fashion (Figure 2c). The bent angle calculated from the crystal structure was

found to be 145° (Figure 2a) while the torsion angles were ~ 178°, 177° and 160°

between four different phenyl rings (Figure S7a). The molecule 2-12 forms an intra-

molecular six membered ring with hydrogen bond (O3–H N3) of bond length 1.84 Å

and angle 148° (Figure 2a). As the molecule contains a polar chloro moiety at one

terminal of the bent molecule hence to minimize the net dipole moment of the overall

bulk material they are arranged in an antiparallel fashion having different weak intermolecular interactions (Figure 2c). The net dipole moment calculated (via DFT

method) for the monomeric unit was 6.3 Debye whereas it reduced to ~0.01 Debye

after forming an antiparallel arrangement in the dimeric form (Figure S7b,c). The

phenyl rings stack upon each other by virtue of weak intermolecular * * interactions

by centroid-to-centroid distances ranging from ~3.68-4.43 Å with different 9 ACS Paragon Plus Environment


geometries, parallel displaced stacked geometry and distorted edge-to-face Tshaped stacked geometry (Figure 2b, Figure S9).40 Interestingly a complete

interdigitation of the long alkyl chains was clearly observed in addition to antiparallel molecular arrangement (Figure 2d, e).41-42

Photoswitchability in LC state

Azobenzene (-N=N-) moiety is an excellent photoswitchable entity and when placed

in a LC environment it plays a crucial role for modification of phase structure. It has

been observed that azo containing bent-core LCs showed transformation of B2 phase to isotropic upon exposure to bright light.43 A phototropic bent-core nematic

material exhibits flexoelectric domains whose periodicity and DC threshold voltage

has been tuned by the photoisomerisation process upon illumination with light of different wavelength and intensity useful for diffraction gratings.3 Moreover, systems

like azobenzene LCs with gold nanoparticles and some U-shaped azo based bent-

core LCs have been studied for optical storage devices due to their photoswitching behaviour.44,45


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Since the bent-core molecules studied here contain azo (-N=N-) moiety they can

exhibit photo-induced LC phase transition. With this aim we have studied the

photoresponsive properties of the compounds in their LC state upon UV irradiation at J = 365 nm, intensity = 10 mW/cm2 which were in situ observed by POM (details of

UV light is given in ESI, page S16). The sample, 2-12, was placed between a glass

plate and a coverslip and heated to the




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*–* stacking interactions (c) antiparallel arrangement (d) different intermolecular interactions (e) 3D molecular framework exhibiting total interdigitation of long alkyl chains. isotropic state (118.9 °C). During the experiment a red filter was placed on the POM to remove the UV component of the visible light that evolves from the in situ tungsten

halogen incandescent lamp of the POM. Upon cooling the sample from isotropic liquid (at 3 °C min-1), the Schlieren texture characteristic of nematic phase appeared

at 113.6 °C. The nematic texture (Figure 3a) disappeared quickly (within 2 sec) upon

UV light exposure leaving a dark/black image (Figure 3b) indicating the occurrence

of photoinduced ordered to disordered transition from nematic to isotropic phase

isothermally at 113 °C. The initial Schlieren texture was recovered within 6 sec after

removal of the UV light (Figure 3c). The phase transition is the result of photoisomerization of the -N=N- bond from trans (bent) to cis (zig-zag) conformers

(Figure S15a,b). This observation of disappearance (under UV light) and re-

appearance (after removal of UV light) of the nematic texture continued for 3-4 °C in

the cooling cycle. Upon further cooling, at 95.4 °C, the compound exhibited SmA

phase (Figure S5f, S6b). Subsequently when the sample was exposed to UV light



(for 5 sec) at 94.9 °C, the ordered SmA phase (Figure 3d) changed to disordered

nematic phase (Figure 3e,f) which was consistent after removal of UV light as

crystallisation was observed in next 1 min at the same temperature. This is due to

the monotropic nature of the sample. For other compounds similar phenomena also

has been observed (Figure S10a-l).

Thereafter the compound (2-12) was filled into polyimide (PI) and indium-tin-oxide

(ITO) coated standard planar cells with thickness of d = 4.8 K% (purchased from INSTEC) and heated to the isotropic state (118.9 °C). Upon cooling (rate 3 °C min-1)

the cell, a planar texture of nematic phase was observed at 113.6 °C (under red

filter) which disappeared within 4 sec upon exposure of UV light. After switching off

the UV light, the planar texture of nematic phase reappeared within 9 sec. This

observation of disappearance (when UV light on) and re-appearance (when UV light

off) of the nematic texture continued until crystallization occurred (Figure S11a-i). In comparison to the previous observation via normal glass slide-cover slip where this

phase transition phenomena only last for 3-4 °C below isotropic-nematic transition

temperature, here for planar 4.8 K% cell the phenomena continued for the entire


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nematic phase range until crystallization occurred. This observation suggests that

the isothermal photo-induced phase transition depends on alignment and uniform

thickness of the LC films.

(a)

(b)

(c)

(d)

(e)

(f)

Figure 3. Photoinduced phase transition of 2-12 observed under POM with red filter: (a) nematic phase before UV exposure during cooling at 113 °C (b) transforming to isotropic phase after (~2s) UV exposure 1J = 365 nm, intensity = 10 mW/cm2) at 113 °C (c) reappearance of nematic phase after removing of UV light (~6s) (d) SmA phase at 94.9 °C (e) co-existence of SmA-N phase upon UV exposure at 94.9 °C (f) transformed to nematic phase at 94.9 °C.

The effect of visible light (arose from in situ tungsten halogen incandescent lamp of the polarizing optical microscope) on the isothermal phase transition has also been studied in the same planar cell (cell gap ~4.8µm). The wavelength range of the



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visible light was J = 200-3000 nm and the intensity falling upon the cell was 1300 lux. During cooling the sample at 110 °C the planar birefringent nematic texture converted to totally black appearance upon exposure of intense visible light for about 30 sec (by removing the red filter from the microscope) which subsequently reappeared slowly again (~20 sec) after placing the red filter on the POM light source (Figure 4a-h, video S1). It is noticeable that when the sample inserted into the planar cell (here cell gap ~4.8µm) the effect of light, UV light or visible light, on the isothermal phase transition is prominent rather than the sample sandwiched between normal glass slide and cover slip, probably due to the alignment and uniform thickness of the LC films.

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

Figure 4. Transformation of the planar nematic phase to isotropic phase in compound 2-12 (a,b,c,d) upon exposure of intense visible light 1J = 200-3000 nm, intensity = 1300 lux) of the POM i.e. when red filter removed from POM (e,f,g,h) reappearance of the nematic phase from isotropic state when red filter placed upon POM at T = 110 °C.


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Since the azobenzene-based LC materials have the potential to be used in optical switching and image storage techniques,6,44,45 we have also observed optical textures after the UV illumination via masked or covered region isothermally at 110 °C of the representative compound 2-12 inserted in planar cell (Figure 5a-c). The cell was irradiated with UV light 1J = 365 nm, intensity = 10 mW/cm2) and the unmasked region became completely dark/transparent within 6 sec as shown in Figure 5b. After removal of the mask we observed that the ‘masked area’ retained birefringent nematic arrangement (Figure 5b). Switching off the UV light the displayed reappearance of nematic arrangements in the dark region (within 10 sec) and took ~1 min for full growth (Figure 5c, video S2, S3 ESI).

(a)

(b)

Unmasked area

UV exposed area

(c)

Figure 5. Demonstration of optical pattern storage capability observed via POM after the UV irradiation. The planar cell containing sample 2-12 was illuminated with UV radiation through a mask at 110 °C and observed (a) before UV irradiation, the black region separated by red dotted line indicate the masked area or covered area, (b) after irradiation with UV 365 nm light intensity = 10 mW/cm2 for 6 sec and simultaneous removal of mask shows the unmasked area become dark/transparent separated by blue dotted line marked as ‘UV exposed area’ where the molecules were in isotropic state due to exposure of UV and the bright region marked as



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‘masked area’ is where the UV radiation was masked and nematic arrangement retains (c) after switching off the UV light the nematic arrangement reappears within 10 sec and full growth after ~1 min.

Solid-state photoswitchability

Solid-state photoswitchability of the representative compound 2-12 was carried out

via UV-vis spectroscopic studies through reflectance mode in KBr medium and presented in Figure 6a. A prominent change in both the * *P and

*P band was

observed upon irradiation of J = 365 nm, intensity = 10 mW/cm2 light that clearly

depicted the solid-state isomerization. In the virgin sample (of 2-12) the * *P band

was observed at J = 351 nm which upon UV irradiation shifted to lower wavelength

of J = 320 nm. For the

*P band the comparative intensity of the peak increases

upon irradiation. The solution state photoswitching studies were carried out on

representative compounds and are elaborated in the ESI (Figure S12, S13). It was

found that the photostationary state (PSS) in solution was 25 sec for compound 1-4

and 35 sec for compound 2-4.


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Subsequently to reinforce the solid state photochromism the compound (2-12) was

dissolved in CHCl3 and passed UV light (for ~4 hrs) followed by evaporation of solvent and drying under vacuum. The compound changed its color from native

yellowish-orange solid to red solid (Figure 6e).

To further confirm the solid state phase transition upon UV irradiation we have

performed X-ray diffraction studies in the virgin (yellowish-orange colored) sample

and UV irradiated (red colored) sample. The XRD pattern in the virgin solid phase

exhibits many narrow peaks in the small as well as in the wide angle regime (Figure

6d). The peaks could be indexed on columnar rectangular (Colr) lattice and parameters are found to be a = 29.80 Å, b = 9.73 Å (Table S2). The hc peak with spacing 3.56 Å set the value of the lattice parameter c. The phase is denoted as

crystalline

columnar

rectangular

(Cr-Colr)

phase.

The

correlation

length

corresponding to (10) peak is found to be about 271 Å. In contrast, compound after

UV irradiation showed many peaks in the small and wide angle region. But the peaks

are found to be broader in comparison to that of virgin solid phase (Figure 6b,c). The

peaks could be also indexed as Colr lattice with lattice parameter a = 58.66 Å, b =



59.10 Å (Table S3). Here, hc peak i. e. the lattice parameter c is found to be at spacing of about 3.68 Å. The phase is again Cr-Colr phase. However, the correlation length corresponding to (11) peak is found to be about 80 Å, almost one fourth of the

value that was found in previous case. This result indicates the loss in crystalline

nature of the compound after UV irradiation which is quite obvious due to the change in molecular shape from trans-to-cis isomer. Further, the average dimension of the molecule in the virgin solid is about 35x(4-5) Å2 and in the UV irradiated solid is about 34x(7-8) Å2. The average number of molecules in the cross section of a

column is estimated to be about 1 and 12 in the virgin solid and UV irradiated solid,

respectively. Based on optical textures and XRD analysis, possible models have

been proposed for the molecular organisation in the Cr-Colr phases of this compound in the virgin solid and UV irradiated solid phase shown in Figure 6f, g

respectively.


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comparison spectra are observed in the relative intensity. The bands at 1343, 1397 and 1457 cm-1 are associated with -N=N- stretching vibration. The intensity ratio of the bands at 1343 and 1397 cm-1 can be used to monitor the isomerization of azobenzene.46-48 When the sample was illuminated with UV light, LC molecules underwent trans-to-cis photoisomerization and intensity ratio I(1343 cm-1)/I(1397 cm1)

decreased significantly. The decrease in intensity of the band (at 1343 cm-1)

associated with -N=N- bond can be attributed to loss of conjugation in the molecule due to conversion of more symmetric trans-isomer to less symmetric cis-isomer and

the consequential reduced polarizability with Raman intensity. We have also

observed the changes in the relative intensity of -C=C- stretching and -C-H in-plane

bending of the rings which indicate the changes in relative orientation between the phenyl rings due to trans-to-cis isomerization upon UV light irradiation as evident

from the DFT study (Figure S15).

Raman study of isothermal nematic-to-isotropic phase transition upon UV light

irradiation on the thin film has also been investigated and presented in Figure 7b.

Similar type of behaviour of the sensitive Raman bands has been observed differing


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in the respective intensity ratio I(1343 cm-1)/I(1397 cm-1) of the trans-to-cis

photoisomerization (Figure S14) as observed in UV irradiated solid sample (Figure

7a). This photoisomerization effect is fully reversible and the transformation is

recovered when UV light is switched off. This study confirms that the photoswitching of -N=N- bonds, present in the LC molecule, from the trans-to-cis conformation

disrupt the orientational alignment of the nematic phase to transform into isotropic

phase.

Photoswitching property in mixture

Further compound 1-12 was doped (0.5% wt/wt) in 5CB (commercially available LC)

and injected in a planar cell (gap 4.8 K%3 and the effect of UV light was examined

isothermally around room temperature under POM. Upon cooling from the isotropic

liquid, typical planar textures of nematic phase were observed (Figure S16a),

indicating absence of phase segregation. Upon UV irradiation the planar texture

gradually changes and became totally dark phase indicating isotropisation of the

sample (Figure S16b-f). This is due to the fact that the doped BLC molecules undergo trans-to-cis photoisomerization as illustrated in Figure 7c. Upon removal of




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Gelation for compound 2-4 was carried out in a variety of solvents and found that a

stable organogel was formed in a mixture of polar solvents viz., DCM:MeOH (ratio

1:4) (Table S5). The gel formation was confirmed by the appearance of thick non-

flowing orangish-yellow mass even when turned upside down. The combination of

dichloromethane (DCM) and different alcohols (methanol, n-butanol, propanol, t-

butanol) yielded gels and the most stable opaque gel was formed in DCM:MeOH

mixture whereas DCM:t-BuOH yielded a transparent gel (Figure 8a, b). The critical

gelation concentration (CGC) of compound 2-4 was 0.7 wt% and qualified it in the

category of supergelator. The self-assembly of the superstructures of organogel was

investigated using FE-SEM, AFM and POM studies. The SEM micrographs are

presented in Figure 8f, g (Figure S17-S20) which show elongated thick bundle of

fibres arranged in an entangled fashion. Figure 8f clearly shows the zoomed thick

entangled nanofibres network formed by the xerogel with the diameter in the range

of 30-50 nm approximately. The morphology of the xerogel investigated by AFM

depicts the fibrillar aggregates entangled together to form a three dimensional

network (Figure 8c) wherefrom the height of the fibre was found to be ~65 nm

(Figure 8d). The POM observation shows birefringent texture of the gel (Figure 8e) 25 ACS Paragon Plus Environment


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indicating towards the lyotropic LC nature. Similar observation was also found with 2-

12 but compounds of 1-n series do not exhibit gelation property plausibly due to the

hindrance of lateral Cl group adjacent to the ester group in the compound.

Conclusions In summary, we have successfully designed and synthesized a series of azo-based

polar bent shaped LCs that exhibited photo induced phase transition in different

states. In solid state, the compounds exhibited photochromism by changing its colour

from

yellowish-orange

to

red

significant

for

technological

applications

in

optoelectronics, imaging and sensing. In LC state the compound exhibited reversible

smectic-nematic-isotropic phase transition upon UV light and intense visible light

exposure that are suitable for light modulators, smart window and diffraction grating

applications. Especially the fast thermal back relaxation property of the BLC molecules in LC state endorses for dynamic photonic switches.49 The compounds

also shown to exhibit optical pattern storage capability in LC state. In mixture, when

doped with 5CB, the compound was able to reversibly change the mesomorphic

property of the host upon UV irradiation validating its practical device application in


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interdigitation of alkyl chain due to aliphatic-aliphatic interactions, antiparallel

molecular arrangement etc. Supergelation ability of the 2-n series compounds also

enhances the periphery of application of these materials. Hence, we envisage that

this class of potential photoresponsive bent shaped LC materials are suitable for

different applications.

ASSOCIATED CONTENT

Supporting Information The details of Synthesis, Characterization, Single crystal data, Polarizing optical

microscopy, Differential Scanning Calorimetry, X-ray diffraction study, UV-visible

study, Raman Study, Density Functional Theory study, gelation studies and Videos

S1, S2, S3 are given.

AUTHOR INFORMATION Corresponding Author

Email: [email protected], [email protected], [email protected],

[email protected]

ACKNOWLEDGEMENTS


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NB acknowledges DST, India for Women Scientist-A (SR-WOS-A-CS-26-2017)

award. SK acknowledges IISER Mohali for PhD fellowship. GM and RN

acknowledge

DST-SERB,

India

for

NPDF

(NPDF/2016/000560)

and

(NPDF/2016/001036) respectively. IISER Mohali has been acknowledged for

Raman, NMR, FE-SEM, SAXS-WAXS facility and all other central and departmental

instrumentation facilities. Authors acknowledge Dr. Angshuman Roy Choudhury, Dr.

Dhiraj Das and Mayank Joshi for helpful discussion regarding single crystal

experiment. SKP acknowledges CSIR Project File No. 02(0311)/17/EMR-II.

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Structural Understanding, Photoswitchability and Supergelation of a

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