Effective tuning of optical storage devices using photosensitive bent-core liquid crystals

Journal Pre-proof Effective tuning of optical storage devices using photosensitive bent-core liquid crystals

B.N. Sunil, M.K. Srinatha, G. Shanker, Gurumurthy Hegde, M. Alaasar, C. Tschierske PII:

S0167-7322(19)36377-9

DOI:

https://doi.org/10.1016/j.molliq.2020.112719

Reference:

MOLLIQ 112719

To appear in:

Journal of Molecular Liquids

Received date:

19 November 2019

Revised date:

29 January 2020

Accepted date:

13 February 2020

Please cite this article as: B.N. Sunil, M.K. Srinatha, G. Shanker, et al., Effective tuning of optical storage devices using photosensitive bent-core liquid crystals, Journal of Molecular Liquids(2020), https://doi.org/10.1016/j.molliq.2020.112719

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© 2020 Published by Elsevier.

Journal Pre-proof

Effective tuning of optical storage devices using photosensitive bent-core liquid crystals Sunil B Na,b, Srinatha M Kc, Shanker Gc*, Gurumurthy Hegdea**, Alaasar Md,e, Tschierske Ce a

Center for Nano-materials and Displays, B.M.S. College of Engineering, Bangalore, India 560019 b

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f

Department of Chemistry, B.M.S. College of Engineering, Bangalore, India 560019

c

Department of Chemistry, Jnana Bharati Campus, Bangalore University, Bangalore, India

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d

Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt

e-

e

Institute of Chemistry, Martin Luther University Halle-Wittenberg, Kurt-Mothes-Str.2,

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06120 Halle (Germany)

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Corresponding authors: *[email protected], **[email protected]

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Abstract

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We discuss on the three series of substituted resorcinol based bent-core liquid crystals with azobenzene wings, substituents consisting of methyl (-CH3) and halogens (-F, -Cl, -Br) on the resorcinol core connected to azobenzene wings with variable alkoxy chains. The effect of electron-donating

(-CH3)

and

electron-withdrawing

(halogens)

group

on

the

photoisomerization and thermal back relaxation studied using UV-Vis spectroscopy. 4methyl resorcinol bent-core (BC) exhibits the longest thermal back relaxation of about 16.33 hour; whereas, the fluoro substituted BCs show the shortest relaxation time of 4.0 hour. In the case of fluoro/ bromo lateral substituted BCs at terminal azobenzene phenyl ring in 4-methyl resorcinol exhibits longer thermal back relaxation time of ~ 16.16 hour, which is noteworthy. These results envisage the effect of substituents on the central unit as well as the lateral and /

1

Journal Pre-proof or terminal position of azobenzene wings on the photoisomerization. The reversible trans (E) - cis (Z) isomerization occurs in 60 sec and 4-16 hours, respectively. The prolonged thermal back relaxation is attributed to the diazo linkage and the polar substituents. A prototype of optical storage device tested under crossed polarisers at λ = 365 nm, the change from order to disorder state upon illumination with the bright and dark regions was visible and representing ideal candidates for optical storage devices. Compiled results show powerful tuning of

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structure-property effects using light as external stimuli.

Keywords: Resorcinol, azobenzene, bent-core liquid crystals, photoisomerization, thermal

e-

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back relaxation, optical storage devices

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1 Introduction

The intermediate phase between solid and liquid termed as liquid crystals (LCs), and

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they are fascinating as a functionalized material for various material science applications in

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display, photovoltaics, biosensors, optical imaging, testing-strip in batteries, thermal-sensors

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and many more to add on [1-3]. These LCs are hierarchical molecular architectures that are the framework of self-organized molecular structures with their unique physical, chemical, and mechanical properties. Such intriguing properties accomplished through organic molecules with optimized design, syntheses, and characterization, especially the organic photo chromes, attracted attention over a period in a variety of optical devices using the photoisomerization process [3]. The bent-core liquid crystals (BCLCs) are the new class of shape-dependent LC material that exhibits polar order in their mesophase though being achiral as molecules [4, 5a]. BCLCs with azo-linker is one the most promising functional group in LCs with interesting properties [5b], although in the initial stage of the discovery period, the azo group considered to be ineffective in LC applications. But the photosensitive azo group in BCLCs and its isomerization phenomenon is the base for new applications in 2

Journal Pre-proof optical storage devices [6, 7], molecular switches [8-10], non-linear optics and holography [11, 12]. Light as external stimuli have been made use in such a photoswitchable azo linked bent-core molecules. Long optical/thermal back relaxations are pre-requisite criteria for optical storage devices; many studies performed focusing on fundamental to technologydriven solutions [13]. The bent-core molecules are always interesting due to their bent angle, and terminally substituted long chains account for the long molecular axis. Also, the kind of substituents on the central core and terminal phenyl rings are always important in inducing

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remarkable properties. Molecular design and functional group(s) are the key parameters in the fabrications of the storage device by UV illumination.

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In this paper, we report the photoisomerization process of bent-core molecules with

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diazo side arms, the presence of such a functional diazo linker in a bent-core will be first of

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its kind in photo-switching studies. The BCLCs studied are shown in scheme I and II were synthesized and characterized as reported before [14-18]. The effect of halogens, methyl

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systematically.

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substituents on the central resorcinol unit, and the length of terminal alkoxy chains studied

1. Synthesis and molecular structural characterization Six series of diazo substituted BCLCs as shown in scheme I and II used in the photoisomerization investigation, the same synthesized and characterized bent-core molecules were used as reported in the literature [14-18].

3

Journal Pre-proof F O O

O O

O O

NN

NN

NN

RO

OR

OR BCCl-6: R = C8H17 ; BCCl-7: R = C10H21 BCCl-8: R = C12H25 ; BCCl-9: R = C16H33

O

Br O O

O

NN

RO

BCF-1: R = C8H17 ; BCF-2: R = C10H21 BCF-3: R = C12H25 ; BCF-4: R = C14H29 BCF-5: R = C16H33

O

O

NN

Cl O O

CH3 O O

NN

NN

NN

RO

RO

OR

OR BCM-14: R = C8H17 ; BCM-15: R = C10H21 BCM-16: R = C12H25 ; BCM-17: R = C14H29 BCM-18: R = C16H33

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f

BCBr-10: R = C8H17 ; BCBr-11: R = C10H21 BCBr-12: R = C14H29 ; BCBr-13: R = C16H33

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Scheme I. Bent-core molecules with halogens and methyl substituent on the resorcinol core and alkoxy chains on terminal phenyl rings.

e-

In scheme II, the dichloro and methyl substituents varied at resorcinol central unit

Cl O O

Cl O O

X

NN

NN

OR

al

RO

Pr

with additional lateral halogen substituents at terminal phenyl rings. O NN

CH3 O O NN

RO

X OR

BCMX-24: X= F, R = C12H25 ; BCMX-25: X= F, R = C14H29 BCMX-26: X= F, R = C20H41; BCMX-27: X= Br, R = C14H29

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BCCl2-19: R = C8H17 ; BCCl2-20: R = C10H21 BCCl2-21: R = C12H25 ; BCCl2-22: R = C14H29 BCCl2-23: R = C16H33

O

Scheme II. Bent-core molecules with dichloro and methyl substituent on central phenyl ring, lateral (-F, -Br) and alkoxy chains on terminal phenyl rings.

2 Experimental

The photoiosmerization and thermal back relaxation of all the BCLCs compounds performed

using

an

Ocean

Optics

HR-2000+

UV-vis

spectrophotometer.

The

photoisomerization studies of all compounds were carried out in dark at room temperature (27 ± 1 °C) using 1 cm quartz cuvette at the concentrations of 1× 10-5 mol L-1. BCLCs irradiated with Omni cure series 2000 UV light source equipped with λ = 365 nm and heat filter to avoid heat radiation from the source. The intensity of UV light measured using UV meter (UV513AB) with irrdiation light was 1 mW/cm2. Before and after UV irradiation,

4

Journal Pre-proof absorption spectra of all the BCLCs recorded at zero time and at different time intervals until photosaturation time has attained, then the thermal back relaxation time was measured by keeping samples in dark. The changes in absorption spectra during UV illumination and thermal back relaxation was investigated with respect to illumination and recovery time to trans isomer, respectively and the first order plot vs time measured at room temperature for thermal isomerization cis (Z) - trans (E) process.

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The photoisomerization behaviour of all the BCLCs has been examined in chloroform

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solution using 1 cm quartz cuvette. Before the UV illumination, all bent-core compounds

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exhibited two absorption bands λ ~ 365 nm and λ ~ 450 nm and the intense peak corresponds to symmetrical allowed π―π* transition, while the weak band at visible region signature of

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forbidden n―π* transition [3,19-21]. Under UV light, azo derivatives undergo trans-cis

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isomerization and after reaching photostationary state, reverse cis-trans isomerization process

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thermal back relaxation [22].

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happens only by illuminating visible light or placing the sample in dark, which defines the

Whereas a representative sample from the series of BCLCs endeavoured in device

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fabrication using ITO coated glass substrates. Previously cleaned ITO coated glass substrates coated with polyimide and unidirectionally rubbed, uniform thickness obtained by spraying glass spacers of 5 m thickness and entire fabrication carried in class 1000 clean room. The mixture prepared by physical mixing 5 % of BCMX26 in 95 % of commercial liquid crystal E7 and then filled the cell using capillary action. 3

Results and Discussions

3.1

Photoisomerization in solution

3.1.1 Halogens and methyl substituted resorcinol bent-core with azo wings.

5

Journal Pre-proof Upon UV illumination, the absorption peak λ ~ 365 nm correspond to π―π* transition decreases and gradually increases the intensity of peak wavelength at ~ 450 nm. Namely, the stable trans isomer converts into thermodynamically unstable cis isomer at photoequilibrium state during the isomerization reaction [23,24]. Fig. 1 represents the time dependent absorption spectra of compound BCF-1 during UV illumination and thermal back relaxation, respectively. Fig.1a shows the conversion of trans to the cis isomer as a function of irradiation time and after reaching photoequilibrium state, cis-trans conversion recorded as

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f

a function of recovery time (Fig.1b). In the further investigation, the effect of terminal alkoxy chains, substituents on central ring including the lateral halogens on terminal phenyl ring on

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the photoisomerization were studied. Analogous results seen for the remaining BCLCs, see

b

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a

Pr

e-

ESI for their absorption spectra.

Figure 1: Representive time dependent absorption spectra of compound BCF-1, (a) represents absorption spectra of trans-cis isomeization upon UV illumination and (b) represents absorption spectra of cis-trans isomerization during thermal back relaxation.

From table 1, E-Z isomerization time of fluoro substituted bent-cores is ~ 62 sec, chloro ~ 58 sec and bromo ~ 54 sec which correlates to increase in the electronegativity of substituents along with increases in photosaturation time. As shown in Fig. 2c, 2e, 2g & 2i, the photoequilibrium state achieved during isomerization, the time remain same irrespective of alkoxy chain length but the photosaturation time depends on substituent at central unit of 6

Journal Pre-proof BCLCs. Attaining photoequilibrium state, there is no significant changes in trans-cis isomerization time after 70 sec UV irradiation. Consider in case of compound BCF-1, 83.72 % of trans isomer converted to cis isomer at photoequilibrium state with 16.28 % trans isomer after 62 sec UV illumination. The photoconversion efficiency of trans isomer at equilibrium state of isomerization was determined by eqn. 1 [25]. (1) is absorbance before and after UV

f

&

oo

Where CE is conversion efficiency,

pr

illumination respectively.

The effect of electron donating (-CH3), electron withdrawing (-F/-Br) group along with

e-

terminal alkoxy variations on photoequilibrium, trans-cis isomerization, photoconversion

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efficiency (CE) including thermal back relaxation are summarized in table 1. The time dependent absorption spectra of all the compounds which correspond to UV illumination and

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thermal back relaxation are given in ESI (Fig S1-S6). In the case of -F substituted resorcinol

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bent core, absorption peak observed at λ ~365-366 nm with no further notable changes in the

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E-Z isomerization time. All the compounds took ~62 sec to reach its photoequilibrium state due to λ ~365 nm UV light used to illuminate E-Z isomerization. This result suggests, as length of the alkoxy chain varies, photosaturation time of trans isomer is unaltered [20, 21]. Similar results observed in chloro and bromo substituted resorcinol bent-core, photosaturation time achived around 58 sec and 54 sec, respectively. There are no significant changes in photosaturation time of these molecules with variation in the lenght of alkoxy chain. Identical absorption spectar obserevd for all the BCLCs due to the absorption peak at λ~ 364-367 nm, which is very close to the illuminated light at λ~ 365 nm. .

7

Journal Pre-proof Table 1. Summarized data of trans-cis isomerization time, thermal back relaxation time and photoconversion efficiency of trans isomer at photoequilibrium state. X

O O N

O

O

N

N

X= F, Cl, Br, CH

N

3

H2 n+1 CnO

OCnH2 n+1

Thermal back relaxation in hours

Photoconversion efficiency (CE) %

62 sec

14.0 h

83.72 %

10

62 sec

9.16 h

88.02 %

F

12

62 sec

9.16 h

86.25 %

BCF-4

F

14

62 sec

BCF-5

F

16

62 sec

BCCl-6

Cl

8

58 sec

BCCl-7

Cl

10

BCCl-8

Cl

12

BCCl-9

Cl

16

BCBr-10

Br

8

BCBr-11

Br

10

BCBr-12

Br

14

BCBr-13

Br

16

BCM-14

CH3

BCM-15

CH3

BCM-16

CH3

BCM-17

CH3

n

BCF-1

F

8

BCF-2

F

BCF-3

5.0 h

85.06 %

10.16 h

89.49 %

58 sec

10.83 h

89.49 %

58 sec

13.16 h

86.96 %

58 sec

14.0 h

84.48 %

54 sec

15.80 h

85.68 %

54 sec

15.0 h

83.64 %

54 sec

14.33 h

83.69 %

54 sec

13.66 h

86.22 %

10

58 sec

12.0 h

89.58 %

12

58 sec

13.0 h

89.68 %

16

58 sec

12. 0 h

89.14 %

18

58 sec

16.33 h

86.71 %

CH3

58 sec

15.66 h

87.51 %

22

Pr

e-

pr

oo

85.46 %

rn

4.0 h

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BCM-18

trans-cis isomerization in seconds

f

X

al

Compound code

The effect of structural modification on these BCLCs observed during thermal back relaxation, in general irrespective of different structural variants there is a prolonged thermal back relaxation. Summarized data from table 1 indicates the influence of alkoxy chain length on the cis-trans isomerization, among the substituents (X = -F, -Cl, -Br and -Me) at central unit of bent-core, the -F and -Br substituted BCLCs shows identical behavior with increase in terminal alkoxy chain length there is decreases in the time of thermal back relaxation. In case of BCBr-10 compound exhibit longest back relaxation of 15.8 h and this behavior could be due the electron affinity and the size of bromine atom [19, 26]. The chloro and methyl 8

Journal Pre-proof substituted BCLCs behave in a similar fashion, as the terminal alkoxy chain length increases, the thermal back relaxation also delayed. Overall, in the series of compounds from the table1, BCBr-10 has longest thermal back relaxation of 15.8 h in comparison to fluoro and chloro may be due to the order of electron affinity (Cl > F > Br) and electron negativity (F > Cl > Br) with increases in the atomic size (F < Cl < Br). The positive inductive effect from the methyl group in BCM-18 also matches for longer thermal back relaxation of 15.66 h in the

f

series of BCLCs compounds as shown in table 1.

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The spectra from fig. 2d, 2f, 2h & 2j represents the peak absorbance plots as a function of

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recovery time during thermal back relaxation, which is plotted by extracting the absorbance valves at their peak wavelength from absorption spectra of respective compounds. As shown

e-

in fig. 2d and 2h, the slowest thermal back relaxation time was observed at about ~ 14 h and

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15.8 h, which is corresponds to the compounds BCF-1 and BCBr-10 respectively. Therefore, in fluoro and bromo substituted resorcinol bent core, thermal back realxation time has been

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decreases with length of alkoxy chain increases. Fig. 2f shows the thermal back relaxation

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time at about ~ 14 h, corresponds to chloro substituted resorcinol bent-core which is having

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higher alkoxy chain. From fig. 2j, compound BCM-17 shows thermal back relaxation time is about 16.33 h. Thus, thermal back relaxation time has been increases as the length of alkoxy chain increases which is observed in chloro and methyl substituted resorcinol bent-cores. As the electron withdrawing character decreases, slower the thermal back relaxation was observed. This suggest, electron donating and withdrawing groups play a major role in the ZE isomerization and also alkoxy chain length influences the thermal back realxation to a greater extent.

9

BCF-1 BCF-2 BCF-3 BCF-4 BCF-5

c 0.9

0.6

0.3

0

20

40

60

Peak absorbance (a.u)

Peak absorbance (a.u)

Journal Pre-proof

d

0.9

0.6 BCF-1 BCF-2 BCF-3 BCF-4 BCF-5

0.3

0

80

200

400

60

80

BCBr-10 BCBr-11 BCBr-12 BCBr-13

al

0.6

20

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rn

0.3

0

f oo 0

40

60

200

400

0.9

BCBr-10 BCBr-11 BCBr-12 BCBr-13

0.3

0

200

400

0.3

20

40

60

600

800

1000

Time (min) BCM-10 BCM-12 BCM-16 BCM-18 BCM-22

0.6

0

800

0.6

80

80

Peak absorbance (a.u)

Peak absorbance (a.u)

i

600

Time (min)

Time (sec)

0.9

BCCl-6 BCCl-7 BCCl-8 BCCl-9

h

Pr

g

0.9

0.3

e-

40

0.6

pr

0.3

Time (sec)

Peak absorbance (a.u)

Peak absorbance (a.u)

0.6

20

1000

f

0.9

Peak absorbance (a.u)

Peak absorbance (a.u)

BCCl-6 BCCl-7 BCCl-8 BCCl-9

e

0

800

Time (min)

Time (sec) 0.9

600

j

0.9

0.6 BCM-10 BCM-12 BCM-16 BCM-18 BCM-22

0.3

0

200

400

600

800

1000

Time (sec)

Time (min) Figure 2: Peak absorbance plot as a function of time, plotted by extracting absorbance valves at peak wavelength from the absorption spectra recorded during UV illumination and thermal back relaxation of respective compounds, graph c, e, g & i represents a peak absorbance with respect to UV illumination time of trans-cis isomerization and graph d, f, h & j represents a peak absorbance with respect to recovery time of cistrans isomerization.

10

Journal Pre-proof 3.1.2 Dichloro substituents on the resorcinol bent-core with azo wings. The influence of chloro addition to the 4-chloro resorcinol bent-core on photoisomerization was studied, addition of another chloro group tend to increase the transcis isomerization time. The trans-cis isomerization time of 4,6-dichloro resorcinol bent-core was about ~ 60 - 64 sec; whereas its variant, 4-chloro resorcinol was took ~ 50 sec to reach its photostationary state. The presence of additional chloro group enhances the electron

f

affinity and electro negativity, further its interaction with oxygen of carbonyl stabilizes the

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molecule and prefers to be in trans conformer during illumination. There are no significant

pr

changes in photosaturation time of these compounds with respect to chloro and alkoxy chains. The time dependent absorption spectra of BCCl2-19 to BCCl2-23 correspond to the

e-

UV illumination and thermal back relaxation are furnished in ESI.

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Table 2. Summarized data of trans-cis isomerization time, thermal back relaxation time and photoconversion efficiency Cl

al

O

O

H2 n+1 Cn O

BCCl2-19 BCCl2-20 BCCl2-21 BCCl2-22 BCCl2-23

O

O N

N OC nH2 n+1

trans-cis isomerization in seconds

Thermal back relaxation in hours

Photoconversion efficiency (CE) %

8

60 sec

8.83 h

89.75 %

10

60 sec

9.66 h

86.97 %

12

62 sec

9.66 h

91.67 %

14

62 sec

8.83 h

89.67 %

16

64 sec

10.83 h

91.14 %

n

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Compound code

N

rn

N

Cl

11

Journal Pre-proof In the series of compounds as summarized in table 2, BCCl2-23 exhibits longest back relaxation time at about 10.83 h, whereas, the lower alkoxy chain (n=8) BCCl2-19 relaxes faster compared to its highest homologue (n=16). Table 2 further explains the trans-cis isomerization time, thermal back relaxation time and photoconversion efficiency of these compounds. On comparing the mono and dichloro substituents in BCLCs, the thermal back relaxation from cis isomer of BCCl2-23 took ~ 10.83 h; whereas, the BCCl-9 mono chloro compound took ~ 14 h to reach its original state of isomerization reaction. The nature of

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f

chloro group and steric interaction [26] influences the back-relaxation time among mono- and

BCCl2-21 BCCl2-22

Pr

BCCl2-23

0.6

al

0.3

0

20

0.9

e-

Peak absorbance (a.u)

k

0.9

40

rn

Peak absorbance (a.u)

BCCl2-19 BCCl2-20

pr

dichloro variant of BCLCs.

60

0.6 BCCl2-19 BCCl2-20

0.3

BCCl2-21 BCCl2-22 BCCl2-23

0

200

400

600

Time (min)

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Time (sec)

80

l

Figure 3: Peak absorbance plot as a function of time, plotted by extracting absorbance valves at their peak wavelength from the absorption spectra of compounds BCCl2-19 to BCCl2-23, graph k & l represents a peak absorbance of UV illumination and thermal back realxation process, respectively.

3.1.3 Central 4-methyl resorcinol based bent-core with lateral substituted azo wings The lateral substituents in 4-methyl resorcinol bent-cores, the effect is not appreciable in the trans-cis isomerization but have affected on the thermal back relaxation compared to its unsubstituted lateral variant. The highest alkoxy chain with n-eicosane in BCMX-26, took time at about 66 sec to isomerize from its stable conformer, subsequent to 70 sec irradiation UV light, there is no changes in photoequilibrium time of isomerization with conversion efficiency of 90.98 % from trans to cis isomer at photostationary state. In case of lower 12

Journal Pre-proof homologues (n = 12, 14) trans-cis isomerization time is ~ 62 sec and the reverse isomerization was occurred around 15 h. The effect of lateral and central substitutions is clearly visible from Table 3, with lateral substituents being ideal for long thermal back relaxation. Fig. 4m & 4n represents a peak absorbance vs time, which is plotted by extracting absorbance valves at their peak wavelength from the absorption spectra of respective compounds. The time dependent absorption spectra of BCMX-24 to BCMX-27 which is

f

corresponds to UV illumination and thermal back relaxation are given in ESI.

pr

C H3 O

O O N

N

O

e-

X

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Table 3. Summarized data of trans-cis isomerization time, thermal back relaxation time and photoconversion efficiency

H 2n +1 CnO

N

N

X OC nH 2n +1

X

n

trans-cis isomerization in seconds

Thermal back relaxation in hours

Photoconversion efficiency (CE) %

BCMX-24

F

12

62 sec

14.0 h

87.66 %

BCMX-25

F

14

62 sec

16.16 h

88.66 %

BCMX-26

F

20

66 sec

15.33 h

90.98 %

BCMX-27

Br

14

62 sec

16.0 h

86.35 %

m

0.9

0.6

BCMX-24 BCMX-25 BCMX-26 BCMX-27

0.3

0

20

40

60

Time (sec)

80

Peak absorbance (a.u)

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Peak absorbance (a.u)

rn

al

Pr

Compound code

BCMX-24 BCMX-25 BCMX-26 BCMX-27

n

0.9

0.6

0.3

0

200

400

600

800

Time (min)

Figure 4: Peak absorbance plot as a function of time, plotted by extracting absorbance valves at their peak wavelength from absorption spectra of compounds BCMX-24 to BCMX-27, graph m & n represents a peak absorbance of UV illumination and thermal back relaxation, respectively.

13

Journal Pre-proof 3.2 Kinetic studies It is necessary to evaluate the first-order kinetics of cis-trans isomerisation reaction [27] to understand the effect of time and temperature which influences the thermal back relaxation.

0

p

-1

0

BCF-1 BCF-2 BCF-3 BCF-4 BCF-5

-1

-kc-tt

-3

-2 -3

f

-kc-tt

-2

oo

-4 -4

-5 -5

200

400

600

0

800

-kc-tt

-3

al

-4

rn

-5

200

400

600

800

Jo u

0

600

700

BCM-10 BCM-12 BCM-16 BCM-18 BCM-22

s

-2 -3 -4 -5

1000

0

200

400

600

800

Time (min) 0

BCCl2-19

t

-2

500

Time (min)

Time (min)

-1

400

-1

Pr

-kc-tt

-2

-6

300

0

BCBr-10 BCBr-11 BCBr-12 BCBr-13

r

-1

200

e-

Time (min) 0

100

pr

0

w

BCCl2-20 BCCl2-21

-1

BCCl2-22

BCMX-24 BCMX-25 BCMX-26 BCMX-27

BCCl2-23

-3

-kc-tt

-kc-tt

BCCl-6 BCCl-7 BCCl-8 BCCl-9

q

-4 -5

-2

-3

-6 0

100

200

300

400

500

Time (min)

600

0

200

400

600

800

Time (min)

Figure 5: First-order plot for cis-trans (Z-E) thermal isomerization. Graph p, q, r, s, t & w represents a firstorder plot of respective reported compounds measured at room temperature.

14

Journal Pre-proof Fig. 5 shows unimolecular thermal cis-trans isomerisation of first-oder plot which obeys eqn 2. The experiment is carried out in thermally by using UV–vis spectroscopy at room temperature ~ 27 ± 1 oC and chloroform as a solvent.

-Kc-tt

𝑙𝑛

𝐴

𝐴

𝐴

−𝐴

(2)

where At, A0 & A∞ are the peak absorbance at time t, time zero & infinite time, respectively. t

oo

f

is the relaxation time of the corresponding cis isomer. Fig. 5 shows the fitting eqn. 2 to the experimental data of absorbance versus time and

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relaxation time derived from first-order profile. The time region in fig. 5p suggest the first

e-

order reaction in fluoro substituted resorcinol bent-core, whereas chloro and bromo are the second order reaction after certain time of interval (fig. 5q & 5r). The deviation from first

Pr

order kinetics to second order is due to the long thermal back relaxation. Temperature plays

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major role in deviating reaction from first order [28]. In fig. 5t, di-substituted resorcinol bent-

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core reaction was first order at certain time of interval and deviated to second order, similar

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results obtained from laterally substituted bent-core azo wing is shown in fig. 5w.

3.3 Reason behind this phenomena It always fascinates to study the bent core azo compounds due to their unique shape and their ability to change the conformer with UV light. The influence of UV light on resorcinol based bent core materials with diazobenzene wings is always fascinating. Although exact reason is difficult to predict but we speculate the following based on the structural changes. Photoisomerization of diazo-bent-cores with UV illumination, E-isomer diazo bent-core at room temperature (27 °C) converts to Z isomer. Upon illumination of UV light at 365 nm, thermodynamically stable E-configuration converts into the meta-stable Z-configuration as depicted in Fig. 6. The influence of illumination on azobenzene moiety changes their shape, 15

Journal Pre-proof the stable E isomer have disubstituent on either side of diazo group with parallel arrangement due to electron cloud repulsion [26]. Upon illuminating at λ = 365 nm, the E isomer converts into Z with the substituents at either end of azo group in an antiparallel arrangement with overall shape of diazo cis isomer pertaining to W shaped molecule. When liquid crystal molecules turn to isotropic state along with azobenzene moieties, then it is not easy for the randomly oriented liquid crystals to return back to the least ordered nematic phase due to the steric hindrance generated in the W shaped azo moieties. As a result, the prolonged thermal

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f

back relaxation is attributed to the presence of diazo linkage and polar substituents; this process holds the liquid crystals in cis conformer for long time and this could be utilized in

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e-

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making an effective storage device.

Thermal back relaxation

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λ = 365 nm

Figure 6: Schematic representation of photoisomerization of diazo-bent-cores with UV illumination, Eisomerised diazo bent-core at room temperature (27 °C) converts to Z isomer. Upon illumination of UV light at 365 nm on the bent-cores, thermodynamically stable E-configuration converts into the meta-stable Zconfiguration.

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Journal Pre-proof 3.4 Device study Fig. 7 shows the time dependent absorption spectra of the trans-cis-trans isomerisation in solid cell, intensity of 1 mW/cm2 used for achieving photosaturation with heat filter. The photosaturation was reached at ~ 80 sec and complete thermal back relaxation was observed at ~ 3 h.

0

363 nm

50

100

150

Time (sec)

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0.2

350

400

450

BCMX-26

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0.28

0.4

0.24

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Time (min)

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trans

0.2

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400

 (nm)

00min 02min 05min 10min 15min 20min 30min 40min 60min 80min 100min 120min 150min 180min 210min 240min

450

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 (nm)

Peak absorbance (a.u)

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0.32

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BCMX-26

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Bef uv 0.2sec 0.4sec 0.6sec 0.8sec 01sec 1.5sec 02sec 03sec 05sec 10sec 15sec 20sec 30sec 50sec 80sec 120sec 150sec

Absorbance (a.u)

Absorbance (a.u)

0.6

BCMX-26

e-

Peak absorbance (a.u)

BCMX-26

0.40

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Figure 7: Photoisomerization of diazo-bent-cores with UV illumination in solid cell device. Intensity used was 1 mW/cm2. Guest-host effect was employed where BCMX-26 acted like guest and E7 was acted like host.

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Optical storage ability of the material (BCMX-26) given in the Fig. 8, upon

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illumination at 365 nm UV light, energetically stable trans converts to cis-conformer. Here in this device, bright region corresponds to trans-configuration which remains in nematic state whereas dark region corresponds to cis-configuration depicted in isotropic state of the cell. The material transform from ordered to disordered state with the illumination of light enhanced with high contrast between bright and dark states. The remarkable constrast state emphasis as potential material for optical image storage applications.

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Masked area (bright)

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Illuminated area (dark)

Conclusions

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Figure 8: Demonstration of an optical storage device based on the materials described here. Prototype was observed under the crossed polarisers of BCMX-26. The system changes from ordered to disordered state with the illumination of UV light of wavelength 365 nm along with heat filter.

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We report on the photo switching ability of resorcinol based bent-core materials with azobenzene segments. The 4-methyl resorcinol bent-core exhibits the longest thermal back relaxation of about 16.33 h; whereas, the fluoro substituted compounds show the shortest relaxation time of 4 h. In the case of fluoro/ bromo substituted at 3-position of terminal azobenzene phenyl ring in 4-methyl resorcinol bent-core exhibits longer thermal back relaxation time about ~ 16.16 h, which is noteworthy. These results envisage the effect of substituents on angular phenyl ring and lateral and or terminal position of azobenzene wings on the photoisomerization. The trans (E) - cis (Z) isomerisation occurs in around 60 sec and reversible cis (Z) to trans (E) 4-16 h respectively, prolonged thermal back relaxation is attributed to the diazo linkage and the polar substituents. Prototype from this bent-core with 18

Journal Pre-proof high contrast ratio observed which switches between order and disordered state representing ideal candidates for optical storage devices. Accumulated results show the prevailing tuning of structure-property effects using light as external stimuli.

Conflicts of interest There are no conflicts to declare.

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Acknowledgement This research work was supported by DST-SERB (Department of Science and Technology-

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Science and Engineering Research Board) Govt of India under ECR grant (File No.

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ECR/2015/000190).

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References

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Declaration of interests

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√ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Title: Effective tuning of optical storage devices using photosensitive bent-core liquid crystals Ref: MOLLIQ_2019_6154

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GRAPHICAL ABSTRACT Influences of substitutions on resorcinol based bent-core with diazobenzene wings for the application in optical storage devices.

CH3 O

O O

N

O

N

N

N

X

f

X

OCnH2n+1

Highlights

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Pr

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pr

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H2n+1 CnO



A prototype of optical storage device tested under crossed polarisers at λ = 365 nm.



Liquid crystalline phase change observed from order to disorder upon illumination.



High contrast ratio (1:1670) could be seen, representing ideal candidates for devices.



Two diazo groups in bent-core molecule makes synergetic, long thermal back relaxation in photo isomerization are remarkable.



Compiled results show powerful tuning of structure-property effects using light as external stimuli.

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