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PAPER Jason B. Benedict et al. The role of atropisomers on the photo-reactivity and fatigue of diarylethene-based metal–organic frameworks

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Photo-responsive liquid crystals derived from azobenzene centered cholesterol-based tetramers† Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/

Monika Gupta, Vaibhav Pal and Santanu Kumar Pal* This paper demonstrates the design and synthesis of azobenzene centred cholesterol based tetramers. A series of eight oligomeric compounds with varying flexible alkyl spacers (n = 1-8) is reported. These compounds displayed enantiotropic and monotropic chiral nematic (N*) mesophase which has been characterized by polarized optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray scattering studies (SAXS/WAXS). We have also explored the photoisomerization behaviour of the compounds with n = 1 and n = 8. Although no photoisomerization effect was observed in the liquid crystalline (LC) state, however, it was noticed in dilute solutions and monolayers as depicted from UV-vis studies and Brewster angle microscopy (BAM) respectively.

Introduction Cholesterol based liquid crystals (LCs) have gained an 1 increasing interest over the last few years. This is mainly because of presence of chiral centres in cholesterol which results in helical supramolecular structures that imparts intriguing properties to these cholesterol based LCs. These properties are sensitive to external stimuli such as temperature, pressure, electric field etc. and thus these compounds are advantageous for applications in optical storage, colour displays and rewritable recording devices.2 Further, in literature, various dimeric and oligomeric LCs derived from cholesterol have been widely reported because of their fascinating mesomorphic properties due to restricted molecular motions.3, 4 These compounds have shown interesting mesomorphic behaviour such as chiral nematic (N*) and smectic C (SmC*) phases as well as some short-lived frustrated phases such as twist grain boundary (TGB) and blue phases (BP). These oligomeric LCs are advantageous since they represent the thermal properties of both monomeric and polymeric LCs. They can be obtained by grafting different functional units in a single molecule. The physical properties of these LC oligomers are also notably different from their constituent monomeric units and they are more capable of forming glasses where the anisotropic properties of mesophase are vitrified while preserving the ordering.5 Therefore, oligomers combine the desirable alignment properties of monomers with the long-lived glassy state of the polymers.

Mostly trimeric and tetrameric LCs have been described in 6 the literature. In case of tetrameric LCs, mostly linear 7-11 tetrameric LCs have been reported so far. For example, Yelamaggad et. al. reported for the first time LC tetramers having four different mesogenic units i.e. azobenzene, tolane, 12 biphenyl and cholesteryl units. Further LC tetramers based on dicholesteryl and two Schiff’s base moieties have also been 13 reported in the literature. However, only a very few starshaped tetramers based on cholesterol have been reported so far. For example, Yin and co-workers prepared the polypedal LCs based on tetrathiafulvalene/1, 3-dithiol-2-thione and four 14 cholesterol units. Datta and Bhattacharya synthesised hexagonal columnar mesogens composed of triazole derivative with four cholesteric units in the presence of alkali metal 15 ions. Xiong et. al. also reported the columnar LC by attaching 16 two/four cholesterol units on calixarene skeleton. They have further reported symmetric hairpin-shaped cholesterol tetramers bridged by rigid or hydrogen-bonding Schiff-base 17 spacers exhibiting columnar phases. In this report, we aimed to synthesize star-shaped tetramers derived from cholesterol and azobenzene unit (Scheme 1). The motivation behind the work was to obtain materials with the combination of properties of cholesterol and azobenzene units. The cholesterol moiety is capable of forming helical superstructures whereas the azobenzene is mostly known for its photo-responsive behaviour. LCs comprising photo-responsive moieties such as azobenzene have been well established as promising materials for applications in photomechanics, instant displays, reversible holographic storage and digital storage due to their high 18-21 resolution and sensitivity. Earlier, Zhang and co-workers have synthesized a series of symmetric gelators containing dicholesterol units linked to azobenzene with different spacer

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lengths which displayed reversible gel–sol transitions on 22 irradiation with UV and visible light. This was due to the fact that irradiating the gel with UV light leads to trans–cis photoisomerization of the azobenzene group thus breaking the van der Waals interactions and resulting in the gel–sol transition which was further recovered by the reverse cis– trans photoisomerization on exposure to visible light. They further prepared a series of one-armed cholesterol-linked azobenzene molecules that were found to induce BPs which 23 exhibited isotropization on irradiation with UV light. POM observation disclosed that the LC samples doped with these compounds displayed an increasing trend in transition temperature for photo-induced phase transition from the BP to the isotropic phase with the increasing concentration of dopant. In the present work, azobenzene was chosen as rigid discotic core attached to which are four rod-like cholesteryl units via varying flexible alkyl spacers. Therefore, azobenzene will not only act as a linker for the cholesteryl cores but can

Scheme 1 Design of the compounds synthesized in the study.

also impart interesting properties to these oligomers due to its photoresponsive behaviour. In fact, the photomerization process can also affect the helical pitch length of the supramolecular structures which can lead to changes in the physical properties of these LCs. Thus, by designing these compounds, light can be used as external stimuli to bring changes in the various properties of these mesogens.

Experimental Section Materials and reagents Chemicals and solvents were all of AR quality and were used without further purification. Cholesterol, n-bromoalkanoic acids, dicyclohexyl carbodiimide, 4-dimethylaminopyridine, 5nitroisophthalic acid, sodium hydroxide, potassium hydroxide, dextrose, hydrochloric acid, n-butanone and tetraoctyl ammonium bromide were all purchased from Sigma–Aldrich (Bangalore, India). Column chromatographic separations were performed on silica gel (60-120 & 230-400 mesh). Thin layer chromatography (TLC) was performed on aluminium sheets pre-coated with silica gel (Merck, Kieselgel 60, F254).

hours & was then cooled to room temperature. The compound was extracted with dichloromethane. The organic layer was washed with brine & dried over anhydrous sodium sulphate. The chloroform was removed by rotary evaporation and the resulting residue was purified by column chromatography over silica gel using hexane & ethyl acetate as eluent. The 1 synthesized compounds 5a-h were characterized by H NMR, 13 C NMR, IR, UV-vis and HRMS (Details of the analytical data of 5a-h are given in the ESI†). Instumental Structural characterization. Structural characterization of the compound was carried out through a combination of infrared spectroscopy (Perkin Elmer Spectrum AX3), 1H NMR and 13C NMR (Bruker Biospin Switzerland Avance-iii 400 MHz) and UV-vis-NIR spectrophotometer (Agilent Technologies UVvis-NIR Spectrophotometer). NMR spectra were recorded using deuterated chloroform (CDCl3) as solvent and tetramethylsilane (TMS) as an internal standard. HRMS-MALDI spectometry was carried out on a Waters synapt G2-S system. Differential Scanning Calorimetry. DSC measurements were performed on Perkin Elmer DSC 8000 coupled to a Controlled Liquid Nitrogen Accessory (CLN 2) with a scan rate of 5 °C/min. The apparatus was calibrated using indium as a standard. Polarized Optical Microscopy. Textural observations of the mesophase were performed with Nikon Eclipse LV100POL polarising microscope provided with a Linkam heating stage (LTS 420). All images were captured using a Q-imaging camera. X-ray Diffraction. X-ray diffraction (XRD) was carried out on powder samples using Cu Kα (λ=1.54 Å) radiation from a source (GeniX 3D, Xenocs) operating at 50 kV and 0.6 mA. The diffraction patterns were collected on a two module Pilatus detector. Surface Manometry. Ultra-thin films of compound 5a and 5h were studied using surface manometry and Brewster angle microscopy (BAM). The surface manometry experiments were carried out using a KSV NIMA 611M trough. The subphase used was the ultrapure deionized water obtained from a Millipore Milli-Q system. The stock solution of 0.227 nM concentration was prepared using chloroform (HPLC grade, Merck). After spreading it on the air–water interface, the film was left for 20 min, allowing the solvent to evaporate. The π–Am isotherms were obtained by the symmetric compression of barriers with a constant barrier speed of 2 mm per minute. The surface pressure (π) was measured using the standard Wilhelmy plate technique. BAM images were obtained using an Accurion ellipsometer.

Synthesis and characterization of oligomers 5a-h Synthesis of compounds 2 and 4 has been described in the earlier reports (Scheme 2).24-29 For the synthesis of the target compound 5, compound 4 (1 equivalent) was dissolved in aqueous KOH (4.1 equivalent). To that solution, compound 2 (6 equivalents) was added followed by the addition of tetraoctylammonium bromide in catalytic amounts. The reaction mixture was refluxed under vigorous stirring for 6-7

Results and discussion Synthesis and Characterization Cholesterol based tetraesters 5 were synthesized following the route shown in Scheme 2. The synthesis of compound 2 was carried out by DCC (N, N‘-Dicyclohexylcarbodiimide) coupling 24, 25 reaction as reported previously.

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(i) O HO

Br

n(H2C) C

O

1

2

O


O

O n O C

O N

O O

NO2

OC n O

O CO

n

O

(iii)

N 5 5a: n = 1 5b: n = 2 5c: n = 3 5d: n = 4 5e: n = 5 5f: n = 6 5g: n = 7 5h: n = 8

CO O

n

O

HOOC

(ii) HOOC

COOH

+

O

extinction of the analyzer/polarizer direction. On close examination of these spherulites, a macroscopic periodicity along the radius was also observed which indicates a helical 32 twist of crystallites within the fibers (Figure 1d). Compound 5b, on heating, displayed a transition from crystalline to isotropic phase at 174.2 °C (∆H = 48.5 kJ/mol), whereas, on cooling a nice Grandjean texture characteristic of a N* phase was observed at 154.2 °C (∆H = -4.6 kJ/mol) which further crystallized on subsequent cooling at 135.8 °C (∆H = -11.29 kJ/mol, Figure 2a). Similarly compound 5c and 5d also exhibited a crystalline to isotropic transition on heating at 140.3 °C (∆H = 14.88 kJ/mol) and 165.2 °C (∆H = 40.8 kJ/mol) respectively (SI, Figure S4). On further cooling, 5c displayed

COOH

N HOOC

3

N

4

COOH

Scheme 2 Synthesis of the target compounds 5. Reagents and conditions: (i) 5bromoalkanoic acid, dry DCM, DCC, DMAP, 12 h, R.T., 88%; (ii) NaOH, dextrose, 80 °C, HCl, 70%; (iii) KOH, H2O, TOAB, reflux, 5h, 50%.

Azobenzenetetracarboxylic acid 4 was also synthesized following the literature methods.26, 27 Oligomers 5 were obtained by refluxing compound 4 (1 equivalent) and compound 2 (6 equivalents) with a catalytic amount of tetraoctylammonium bromide in aqueous KOH (4.1 equivalents, see experimental section for details). All the synthesized compounds were characterized by 1H NMR, 13C NMR, FTIR, UV-vis and mass spectrometry (Supporting information, section-1 and Figures S1-S3). The thermal behaviour of synthesized compounds was examined by differential scanning Calorimetry (DSC) and was further investigated by polarizing optical microscopy (POM) and small/wide-angle X-ray scattering studies (SAXS/WAXS). The thermal behaviour of the synthesized oligomers 5 as observed from DSC studies is represented in Table 1. Compound 5a was crystalline at room temperature and displayed a transition from crystal to mesophase at 179.6 °C (∆H = 31.13 kJ/mol) which further cleared at 185.9 °C (∆H = 0.50 kJ/mol). On cooling from isotropic, a planar Grandjean texture was obtained at 164.6 °C (∆H = -7.2 kJ/mol) which is typical of a N* mesophase (Figure 1a).30 We also observed a fingerprint texure of N* phase for this compound at 150 °C (Figure 1b).31 On further cooling, the compound displayed a crystallization peak at 144.9 °C (∆H = -19.31 kJ/mol). Interestingly, compound 5a in crystalline state displayed formation of well-ordered spherulites which were nucleated randomly over the whole sample area (Figure 1c). This crystallization behaviour is generally observed only for lowmolecular mass LCs and is atypical for oligomeric LCs. These spherulites were spreaded over several hundred micrometers. The formation of these spherulites indicates a coherent longrange order due to high anisotropy which is further revealed by the Maltese cross where the isogyres followed the

Fig. 1 Cross-polarized optical microscopy image of compound 5a showing (a) planar Grandjean texture of N* phase at 163.2 °C, (b) fingerprint texture of N* phase at 150 °C, (c) spherulitic texture at 120 °C and (d) observation of periodic features along the spherulite radius indicating the twist of the crystallites within the fibers.

Table 1 Thermal behavior of the synthesized compounds 5 a, b Compound

Heating Scan

Cooling Scan

5a

Cr 179.6 (31.13) N* 185.9 (0.50) I

I 164.6 (-7.2) N* 144.9 (-19.31) Cr

5b

Cr 174.2 (48.5) I

I 154.2 (-4.6) N* 135.8 (-11.29) Cr

5c

Cr 140.3 (14.88) I

I 136.4 (-6.15) N* 120 (-11.84) Cr

5d

Cr 165.2 (40.8) I

I 130 (-3.47) N* 119.0 (-10.14) Cr

5e

Cr 139.6 (26.73) N* 149.5 (6.76) I

I 123 (-3.72) N* 104.6 (-4.12) Cr

5f

Cr 164.17 (35.13) I

I 138.4 (-3.6) N* 118.4 (-42.5) Cr

5g

Cr 145.2 (42.55) I

I 123.8 (-4.4) N* 107.2 (-3.1) Cr

5h

Cr 128 (6.83) N* 135.4 (36.92) I

I 116.8 (-4.2) N* 100 (-3.9) Cr

[a] Phase transition temperatures (peak) in °C and transition enthalpies in kJ mol-1 (in parentheses). [b] Phase assignments: Cr = Crystalline, N* = Chiral nematic, I = isotropic.

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Fig. 2 Cross-polarized optical microscopy of compounds (a) 5b at 148 °C showing Grandjean texture, (b) 5c at 130 °C showing fingerprint texture and focal conic texture (c) at 125 °C respectively, (d) 5d at 125 °C (e) 5e, at 120 °C (f) 5f, at 130 °C (g) 5g at 118 °C showing Grandjean textures and (h) crystalline phase of compound 5h at 90 °C showing spherulitic domains with (i) periodic features along the radius of spherulite.

fingerprint and fan texture at 136.4 °C (∆H = -6.15 kJ/mol) and 130 °C (∆H = -3.47 kJ/mol) respectively corresponding to a chiral nematic arrangement which finally crystallized at 120.0 °C (∆H = -11.84 kJ/mol) whereas 5d displayed a Grandjean texture at 125 °C which crystallized at 119.0 °C (∆H = -10.14 kJ/mol) respectively (Figure 2b-c).33 Compound 5e displayed an enantiotropic mesophase similar to that observed for compound 5a (SI, Figure S5). It melted at 139.6 °C (∆H = 26.73 kJ/mol) to form the mesophase which finally cleared at 149.5 °C (∆H = 6.76 kJ/mol). On cooling further, it displayed Grandean textures of chiral nematic phase at 123 °C (∆H = 3.72 kJ/mol) which crystallized at 104.65 °C (∆H = -4.12 kJ/mol).34 Compound 5f and 5g exhibited monotropic N* mesophase as they a displayed a transition from crystal to isotropic on heating at 164.1 °C (∆H = 35.13 kJ/mol) and 145.1 °C (∆H = 42.55 kJ/mol), respectively. On cooling, the mesophase appeared at 138.41 °C (∆H = -3.6 kJ/mol) and 123.85 °C (∆H = -4.4 kJ/mol) respectively which further crystallized at 118.4 °C (∆H = -42.5 kJ/mol) and 107.1 °C (∆H = 3.1 kJ/mol) respectively (Figure 2d-g). Compound 5h also displayed an enantiotropic mesophase as it displayed a peak from crystal to mesophase transition at 128.0 °C (∆H = 6.83 kJ/mol) on heating which isotropized at 135.4 °C (∆H = 36.92 kJ/mol). Whereas, on cooling mesophase appeared at 116.8 °C (∆H = -4.2 kJ/mol) which further crystallized at 100 °C (∆H = 3.9 kJ/mol). Interestingly, on crystallization, we observed the

formation of spherulites similar to those observed for 5a (Figure 2h-i). However, the radius of these spherulites was less as compared to those observed for compound 5a. A possible reason for the observation of these large superstructures in case of compound with lowest alkyl spacer i.e. 5a could be because of the decreased molecular interactions where the self-assembly into superstructures is regulated which in turn leads to low nucleation rates and thus formation of larger spherulites. For quantitative characterization of mesophases, we have carried out SAXS/WAXS studies on compounds 5a-5h. It was observed that all the compounds exhibited two peaks in their mesophase: a less intense diffuse peak at small-angle whereas a broad peak in the wide-angle region (Figure 3a). It indicates the absence of any positional order and thus excludes the possibility of the existence of a smectic and columnar phase structure of these oligomers. The d-spacing corresponding to peak at small-angle was found to be in the range of 16-20 Å and increases with increasing flexible alkyl spacer (Table 2). The d-spacing corresponding to the peak in the wide angle region was found to be around 4.8-4.9 Å and corresponds to molten alkyl chain correlation in the LC state. The d-value corresponding to peak at small-angle generally represents the diameter/length of the molecule. However, we found that the observed value (i.e. 16-20 Å) is much lesser than the total length of molecule which is around (40-44 Å). This can be

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Figure 3. (a) X-ray diffraction pattern of compound 5a-5h in the N* mesophase and (b) schematic representation of tetramers in the mesophase.

explained as follows. These compounds can be considered as mesogenic tetramers where four cholesteryl moieties are attached to azobenzenetetracarboxylic acid (Figure 3b). In the nematic mesophase generally there is no correlation between the mesogens and they are only orientationally ordered. Therefore, it can be considered that it is a molecularly mixed kind of mesophase similar to that observed earlier.25-27 The observed d-spacing corresponds to the average length of cholesteryl unit with flexible alkyl spacer (20-24 Å) and that of the azobenzene unit (9 Å). The length of azobenzene in all the compounds remains same, however, the length of cholesteryl unit increases with increasing flexible alkyl spacers which is also reflected in the increasing d-values corresponding to peak at small-angle region. Further, the observation of the peak at small-angle corresponding to a considerably smaller d-value as compared to total length of the oligomer also affirms the Table 2 X-ray reflections and corresponding correlation lengths in the nematic phases of compound 5a-h.

Compound

d-spacing (Å) Small angle Wide angle peak peak

5a

16.76

4.79

5b

16.95

4.89

5c

17.55

4.99

5d

18.03

4.95

5e

18.58

4.91

5f

18.88

4.85

5g

19.59

4.95

5h

19.75

4.93

compatibility of both the components (i.e. cholesteryl and azobenzene units) in the mesophase and the absence of any nanophase segregation between them. Photoisomerization Studies We have further carried out photoisomerization studies on LC compounds with shortest (5a) and longest (5h) flexible alkyl spacer. We have first investigated dilute solutions of these compounds (5 μm) which were kept in dark for 48 h before the analysis to ensure that the compounds were exclusively in the trans configuration. For studying the photostationary equilibrium capacity, we have further irradiated these solutions at a wavelength of 365 nm and measured the UV-vis absorption spectra from time to time. Both the compounds 5a and 5h displayed trans to cis photoisomerization which was confirmed from the reduction of absorbance band around 320 nm which is assigned to the trans configuration and a small increase in the absorbance band around 430 nm which is 35 related to the cis form (Figure 4a-b). We found that the photoisomerization process continues and the photostationary point was not reached even after 3 h of irradiation. Both the compounds displayed photoisomerization to nearly same extent in dilute solutions. Further, when we stopped the UV irradiation after 180 minutes for the oligomer 5a and further exposed the solution in visible light, we found that most of back relaxation took place in just 5 minutes and the complete conversion took place in 10 minutes after which no further change was observed (SI, Figure S6). We have also compared the photostationary equilibrium capacity of azobenzene tetracarboxylic acid in dilute solution to compare it with that of synthesized oligomers. We found that in case of azobenzene tetracarboxylic acid, the photostationary state was reached just after 5 minutes and no change was observed after further irradiation (SI, Figure S7). This huge difference in photoresponsive behavior is due to the fact that in case of synthesized oligomers, there is a restricted rotation of the photoresponsive core as it is attached to four bulky cholesteryl

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0 min 10 min 20 min 30 min 40 min 50 min 60 min 180 min

2.0

1.5

π - π*

1.0

0.5

b

1.0 0.8

0 min 10 min 20 min 30 min 40 min 50 min 60 min 180 min

π - π*

0.6 0.4 0.2

n - π*

n - π*

0.0 300

350

400

450

500

300

350

400

450

500

Wavelength (nm)

Wavelength (nm)

Fig. 4 UV-vis absorption spectra of compound (a) 5a and (b) 5h obtained from time to time on irradiation at 365 nm wavelength.

Fig. 5 Surface pressure vs mean molecular area isotherms of (a) compound 5a and (d) compound 5h obtained at a constant compression rate of 2 mm/min. (b) Brewster angle microscopic images of compound 5a at a surface pressure of 28 mN m-1 and (e) 5h at 13 mN m-1. (c) and (f) shows the respective BAM images obtained after irradiation at 365 nm wavelength.

units, therefore, the photostationary state was not reached 35 even after a long time irradiation with UV light. The photoisomerization rate strongly depends on the local free volume available for the conformational changes. Further, free volume required for the cis form is larger as compared to the trans form considering either a rotation or inversion mechanism of azo bond.36 In the present oligomers, since they are surrounded by four bulky cholesteryl groups, the rotation of inversion of the azo bond is not fast due to less available free volume. Therefore, the rate of photoisomerization is very slow. Further, the incomplete conversion to cis form can be most possibly assigned to the thermal back relaxation to trans form due to longer time of conversion from cis to trans form. Further, we did not observe any photoisomerization effects due to azobenzene core in the bulk material even in the LC

state. This could be ascribed to the restricted rotation of the photoresponsive azobenzene core as it is tethered to four cholesteryl units which provides more steric restriction in bulk 37 as compared to that in dilute solution. We further aimed to understand whether the photoisomerization process is retained in the Langmuir monolayers i.e. one molecule thick layers of these compounds. These studies were carried out again on compounds 5a and 5h. The surface pressure (π, mN m-1) vs area per molecule (Am, Å2) isotherm for compounds 5a and 5h is represented in Figure 5a and 5d. For compound 5a, at an Am of 450 Å2, the surface pressure starts increasing gradually and increases rapidly after the value of Am reaches around 300 Å2. The film collapses at an Am of 112.09 Å2 with a collapse pressure of 40.70 mN m-1. Similarly, for compound 5h, the collapse pressure was found to

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design and characterize the molecules for photocontrollable surfaces.

Conclusions A series of oligomeric mesogens based on azobenzene tetracarboxylic acid surrounded by four cholesteryl core have been prepared. Out of the eight oligomers synthesized (n = 18), three compounds with n = 1, 5, 8 exhibited enantiotropic N* phase, whereas the remaining compounds displayed monotropic N* phase which were observed on cooling from isotropic state. Compounds with n = 1 and 8 displayed the formation of spherulites in their crystalline state which were spreaded over several hundred micrometers in case of compound with n = 1. Further, both these compounds displayed photoisomerization in dilute solutions and Langmuir monolayers. However, due to steric restriction the photoisomerization was not observed in bulk even in the LC state.

Conflicts of interest There are no conflicts to declare.

Acknowledgements The acknowledgements come at the end of an article after the conclusions and before the notes and references. We are grateful to SAXS/WAXS facility at IISER Mohali. Dr. SK Pal is grateful for financial supports from CSIR-NET (09/947(0061/2015-EMR-1) and INSA (File No. SP/YSP/124/2015/433). M. Gupta acknowledges the receipt of a graduate fellowship from IISER Mohali. The NMR facility at IISER Mohali is acknowledged.

Notes and references 1

C. V. Yelamaggad, G. Shanker, U. S. Hiremath and S. K. Prasad, J. Mater. Chem., 2008, 18, 2927-2949. 2 V. A. Mallia and N. Tamaoki, Chem. Soc. Rev., 2004, 33, 7684. 3 C. T. Imrie, P. A. Henderson and G. Y. Yeap, Liq. Cryst., 2009, 36, 755-777. 4 C. T. Imrie and P. A. Henderson, Liq. Cryst., 2007, 36, 20962124. 5 N. Boden, R. J. Bushby, A. N. Cammidge and P. S. Martin, J. Mater. Chem., 1995, 5, 1857−1860. 6 A. S. Achalkumar, U. S. Hiremath, D. S. S. Rao and C. V. Yelamaggad, Liq. Cryst., 2011, 38, 1563-1589. 7 C. V. Yelamaggad and G. Shanker, Liq. Cryst., 2007, 34, 799809. 8 C. V. Yelamaggad, A. Srikrishna, D S. Shankar Rao and S. K. Prasad, Liq. Cryst., 1999, 26, 1547-1554. 9 C. V. Yelamaggad, S. A. Nagamani, D. S. Shankar Rao and S. K. Prasad, J. Chem. Res.-S., 2001, 2001, 493-495. 10 T. Donaldson, P. A. Henderson, M. F. Achard and C. T. Imrie, J. Mater. Chem., 2011, 21, 10935-10941. 11 C. T. Imrie, D. Stewart, C. Remy, D. W. Christie, I. W. Hemley and R. J. Harding, J. Mater. Chem., 1999, 9, 2321-2325.

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be 16.55 mN m corresponding to an Am of 236 Å . The Brewester angle microscopy (BAM) image of compound 5a at a -1 surface pressure of 28 mN m is represented in Figure 5b. Figure 5c shows the corresponding image obtained after an irradiation at a wavelength of 365 nm for 5-10 minutes. The monolayer was initially very uniform but after irradiation, it displayed formation of small bright domains although no significant change in the surface pressure value was observed. This morphology remains same and does not get affected on further increasing the exposure time. The BAM image of -1 compound 5h at a surface pressure of 13 mN m is displayed in Figure 5e which also shows the fomation of a uniform layer. Interestingly, on irradiation with UV light of wavelength 365 nm for 5-10 minutes, the morphology changed to a fibre network pattern with a significant reduction of surface -1 pressure to 11 mN m . A possible explanation for the above observation could be that in case of compound 5a, the length of flexible alkyl spacer is relatively very short due to which the azobenzene units are sterically crowded by nearby cholesteryl moieties and thus do not have the freedom to undergo cistrans isomerization due to which only a very slight change in the monolayer was observed on irradiation at 365 nm. However, in case of compound 5h, the length of spacer is relatively large and thus cholesteryl units are not very effectively crowding the azobenzene moiety. Thus, on UV light irradiation, a change in the morphology was observed. Further, since the molecules turn from expanded form in trans form to bent shape in the cis-form, the intra- and intermolecular interactions increases on switching from trans to cis form. This could be the most possible reason for the observation of fibrenetwork pattern observed in Figure 5f after irradiation at 365 nm. We also observed that these changes were irreversible on further irradiation with visible light which might be due to due to inter- and intra-molecular interactions present in the system. For the photoisomerization, a sufﬁcient free volume for the involved motion of the whole chromophore moieties is 38 required. It has been observed earlier that in the bulk state, the photoresponsiveness of the azobenzene is generally remarkably weakenend or completely suppressed due to severe H-aggregation. Thus, there is a restricted rotation of the photoresponsive core in the LC state as it is attached to four bulky cholesteryl units. However, this steric restriction is eliminated in the dilute solution. This is the reason that photoisomerization was observed in the solution and not in the LC state. Further, in the Langmuir monolayer, the steric restriction is more as compared to that in the dilute solution, however, it is considerably less than that in the bulk LC state. Therefore, we observed the changes in morphology of the monolayer on irradiation with UV light. We further found that the changes were more prominent in the oligomer with longest alkyl spacer as compared to the one with shortest spacer. This can be due to the fact that the cholesteryl units crowd the chromophore more effectively in case of shorter alkyl spacer derivative as compared to the oligomer with longer alkyl spacer. Not only that, the Langmuir Blodgett studies together with BAM observations provide a platform to



DOI: 10.1039/C7NJ05142E

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12 C. V. Yelamaggad, S. A. Magamani, U. S. Hiremath, D. S. S. Rao and S. K. Prasad, Liq. Cryst., 2002, 29, 231-236. 13 X. Zhan, X. Zing and C. Wu, Liq. Cryst., 2009, 36, 1349-1354. 14 R. Hou, K. Zhong, Z. Huang, L. Y. Jin and B. Yin, Tetrahedron, 2011, 67, 1238-1244. 15 S. Datta and S. Bhattacharya, Soft Matt., 2015, 11, 1945– 1952. 16 X. Zhang, H. Guo, F. Yang and J. Yuan, Tetrahedron Lett., 2016, 57, 905-909. 17 J. Xionga, X. Lina, H. Guoa, F. Yanga and J. Lai, Liq. Cryst., 2017, 1-8. 18 A. S. Matharu, S. Jeeva and P. S. Ramanujam, Chem. Soc. Rev., 2007, 36, 1868-1880. 19 V. Shibaev, A. Bobrovsky and N. Boiko, Prog. Polym. Sci., 2003, 28, 729-836. 20 S. K. Yesodha, C. K. S. Pillai and N. Tsutsumi, Prog. Polym. Sci., 2004, 29, 45−74. 21 (a) X. Peng, H. Gao, Y. Xiao, H. Cheng, F. Huang and X. Cheng, New J. Chem., 2017, 41, 2004-2012; (b) M. Brehmer, J. Lub and P. V. Dewitte, Mol. Cryst. Liq. Cryst., 1999, 331, 333-340; (c) H. K. Lee, K. Doi, H. Harada, O. Tsutsumi, A. Kanazawa, T. Shiono and T. Ikeda, J. Phys. Chem. B, 2000, 104, 7023–7028; (d) N. Tamaoki, Y. Aoki, M. Moriyama and Masatoshi Kidowaki, Chem. Mater., 2003, 15, 719–726; (e) V. A. Mallia and N. Tamaoki, Chem. Soc. Rev., 2004, 33, 76-84; (f) T. J. White, M. E. McConney and T. J. Bunning, J. Mater. Chem., 2010, 20, 9832-9847; (g) D. Zhao, Y. Qiu, W. Cheng, S. Bi, H. Wang, Q. Wang, Y. Liao, H. Peng and X. Xie, Langmuir, 2018, 34, 700–708; (h) Y. Kim, M. Frigoli, N. Vanthuyne and N. Tamaoki, Chem. Commun., 2017, 53, 200-203; (i) J. Vapaavuori, C. G. Bazuin and A. Priimagi, J. Mater. Chem. C, 2018, 6, 2168-2188; (j) A. Miniewicz, H. Orlikowska, A. Sobolewska and S. Bartkiewicz, Phys. Chem. Chem. Phys., 2018, 20, 2904-2913; (k) Y. Xiong, L. Zhang, P. Weis, P. Naumov and S. Wu, J. Mater. Chem. A, 2018, 6, 3361-3366. 22 Y. Wu, S. Wu, X. Tian, X. Wang, W. Wu, G. Zou and Q. Zhang, Soft Matter, 2011, 7, 716–721. 23 L. Yin, Y. Wu, J. Gao, J. Ma, Z. Hu, G. Zoua and Q. Zhang, Soft Matter, 2015, 11, 6145—6151. 24 M. Gupta and S. K. Pal, Liq. Cryst., 2015, 42, 1250–1256. 25 S. K. Gupta, S. Setia, S. Sidiq, M. Gupta, S. Kumar and S. K. Pal, RSC Adv., 2013, 3, 12060–12065. 26 M. Gupta, N. Agarwal, A. Arora, S. Kumar, B. Kumar, G. Sheet and S. K. Pal, RSC Adv., 2014, 4, 41371−41377. 27 M. Gupta, S. P. Gupta, S. S. Mohapatra, S. Dhara and S. K. Pal, Chem. Eur. J., 2017, 23, 10626 – 10631. 28 M. Gupta, I. Bala and S. K. Pal, Tetrahedron Lett., 2014, 55, 5836–5840. 29 M. Gupta and S. K. Pal, Langmuir, 2016, 32, 1120-1126. 30 I. Dierking, Symmetry, 2014, 6, 444-472. 31 S. J. Eichhorn, Soft Matt., 2011, 7, 303-315. 32 W. Pisula, M. Kastler, D. Wasserfallen, T. Pakula and K. Müllen, J. Am. Chem. Soc., 2004, 126, 8074-8075. 33 A. S. Achalkumar, D. S. S. Rao and C. V. Yelamaggad, New J. Chem., 2014, 38, 4235-4248. 34 M. P. Aldred, R. Hudson, S. P. Kitney, P. Vlachos, A. Liedtke, K. L. Woon, M. O’ Neill and S. M. Kelly, Liq. Cryst., 2008, 35, 413-427. 35 E. Westphal, I. H. Bechtold and H. Gallardo, Macromolecules, 2010, 43, 1319–1328. 36 (a) Y. S. Duo, Y. Hu, S. A. Yuan, W. F. Wu and H. Tang, Mol. Phys., 2009, 107, 181-190; (b) C. M. Stuart, R. R. Frontiera and R. A. Mathies, J. Phys. Chem. A, 2007, 111, 12072-12080; (c) A. Shaabani and M. Zahedi, J. Mol. Struct., 2000, 506, 257261. 37 M. Gupta, S. K. Gupta and S. K. Pal, J. Phys. Chem. B, 2017, 121, 8593−8602.

38 S. Pan, M. Ni, B. Mu, Q. Li, X. Y. Hu, C. Lin, D. Chen, L. Wang, Adv. Funct. Mater., 2015, 25, 3571–3580.

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ARTICLE



Page 9 of 9

New Journal of Chemistry View Article Online

Azobenzene centered cholesterol based tetramers showing spherulitic domains and photoresponsive behaviour in solution as well as Langmuir monolayers.



DOI: 10.1039/C7NJ05142E