Synthesis of bent-shaped azobenzene main-chain polymers for photo-switching properties

Abstract This work presents the synthesis of the new bent-core polymers with siloxane units connected to the one side of azobenzene units. The structure of siloxane-based azobenzene bent-core polymers, 7a–c, was elucidated by spectral analysis (nuclear magnetic resonance and Fourier-transform infrared spectroscopy). The results of gel permeation chromatography suggested that all polymers (7a–c) showed polydisperse (polydispersity index >1). Besides, the extent of polymerization in the following order: 7a > 7b > 7c, where the degree of polymerization values were 7, 8 and 11, respectively. Polarizing optical microscopy revealed that the bent-core liquid crystal (BCLC) monomers, 6a and 6b, displayed the smectic A phase, whereas BCLC monomer 6c and all siloxane-based main-chain polymers (MCPs) (7a–c) were crystalline in nature. The result of ultraviolet-visible spectroscopy demonstrated that all MCPs (7a–c) exhibited strong photoisomerization behavior in solution. All polymers (7a–c) showed trans to cis isomerization in about 200 s, whereas the reverse process required much longer times ranging from 400 to 520 min in solution. The photo-switching study on azobenzene containing polymers stated that the effect of alkyl chain length and type of central core units on trans to cis isomerization were negligible. In contrast, both parameters influence the cis to trans process in which the photo-switching behavior of these materials may be primarily suitably exploited in the field of photo-induced phenomenon. GRAPHICAL ABSTRACT Time dependence photoisomerization curve of a) trans isomer (7a–c) showing the effect of UV illumination at 365 nm wavelength, which take time for trans to cis isomerization in about 200s, and b) cis isomer (7a–c) showing the thermal back relaxation time, the reverse process required much longer times ranging from 400–520 min in solution


Introduction
The azobenzene unit is often introduced into the molecular structure to obtain the photo-responsive materials, owing to its promising light sensitivity.One of the important characteristics of azobenzene is trans-cis isomerization.Azobenzene has thermodynamically more stable trans configuration at room temperature and normal conditions.The trans isomer converts to cis isomer when UV light of suitable wavelength is illuminated on the system of molecules. [1]The reverse process to the original configuration (trans-form) can be achieved by irradiating the azobenzene-containing molecule with visible light (within the range 400-500 nm). [2]The latter change can also happen by keeping the azo-containing material in the dark, a process well-known as thermal back relaxation.[4] The trans-cis isomerization phenomenon in the azobenzene-containing bent-core liquid crystals (BCLCs) is the fundamental for new applications in optical storage devices, [5][6][7] molecular switches, [8,9] nonlinear optics, holography [8,10] and photoalignment. [11]enerally, the photoisomerization of azobenzene differs with varieties of structure, spacers and functional groups associated with it.14] On the other hand, the functional group also impacts the photoisomerization process of azobenzene to a great extent. [15,16]or instance, an oligosiloxane has a peculiar contribution with azobenzene moiety in light induced studies.[19][20][21] The oligosiloxane end chain has proven to be exceptionally useful as it reduces the melting temperature and generates ferroelectric switching phases with low viscosity, [17] mesophases with chiral superstructural (SmCPFE[ � ]), as well as generates different modulated and undulated smectic phases that exhibit field-induced switching of superstructural layer chirality. [22]However, it is also interesting to investigate the morphologies of photoisomerization in siloxane-substituted azobenzene derivatives because the steric hindrance resulting from voluminous silicon-substituted units shows significant impact on the N ¼ N photoisomerization process. [23]he optical storage devices are used for storing and processing optical images in recyclable operating systems via light irradiation method.The principle of image-storing involves selective controlled birefringence of the liquid crystal media prior to photoisomerization of azobenzene.Optical storage devices that display (i) high sensitivity to light such that it can be reversibly driven between two states, (ii) high rewriting-ability and switchable between states with a reasonable efficiency, (iii) high stability over long periods of time and (iv) energy and cost saving are always desirable.Owing to the fast-growing demand for photonic devices, the present optical storage technology suffers from an absence of functional organic materials and synthesizing light sensitive molecules becomes a daunting task.Azobenzene has been exploited in many fields of photonics due to their promising light sensitivity.However, their metastable state in cis form gives them a short thermal back relaxation, which is a drawback for creating the optical storage device.Thus, the main issue is their stability, which they are unable to withstand for a long time due to their photoisomerization nature.In fact, a necessary condition in creating optical storage devices is to employ materials with slow thermal back relaxation, that is, the cis isomers have lifetime long enough so that the optically generated information can be stable for the required period.However, the simple fact is that the present optical storage industry suffers from an absence of functional organic materials that can apply to the next generation optical rewriting technology.For this reason, the creation of optical storage devices is a challenging subject.
The present work focuses on the synthesis and photoisomerization behavior of a new series of siloxane-based azobenzene containing bent-core polymers derived from resorcinol and 3,4 0 -biphenyl-diol as a central unit.The structures of monomers (6a-c) and polymers (7a-c) were elucidated by spectral analysis such as nuclear magnetic resonance (NMR), Fourier-transform infrared spectroscopy (FTIR) and gel permeation chromatography (GPC).Their thermal behaviors were characterized by thermogravimetric analysis (TGA), followed by the mesophase properties were determined by differential scanning calorimetry (DSC) and polarizing optical microscopy (POM).The photo-switching studies to evaluate the photoisomerization of siloxane-substituted azobenzene containing bent-core polymers with variation in alkyl spacers and type of central core units.Long thermal back relaxation was detected in all polymers, which could provide a path for the exploration of systems to obtain long-term optical storage devices.

Compound 3 derived from hydrolysis of ester
Firstly, compounds 2a-b (11.82 mmol of each azo compound for every reaction) were dissolved in 500 mL of methanol.Sodium hydroxide (35.46 mmol for every reaction) dissolved in water (20 mL) was added and the solution was refluxed for 4 h.After that, the mixture was poured into the ice-cold water (500 mL) and acidified with conc.hydrochloric acid (15 mL) for precipitation.The precipitate was filtered off and washed with water before purifying on silica gel by column chromatography using hexane/ethyl acetate (3:1) as an eluent.After that, the solids were recrystallized from ethanol/chloroform (2:1) to give compound 3a and 3b. [2,24]

3,4 0 -Dimethoxybiphenyl (4)
4-Methoxybenzeneboronic acid (10.7 mmol) was added to a solution of 3-bromoaisole (10.7 mmol) in a mixture of ethanol (20 mL) and K 2 CO 3 solution (12 eq. in 10 mL water), followed by the addition of the metal catalyst Pd(II)@ PHA (0.05 g, 5 mol%).Then, the mixture was refluxed for 6 h.The organic layer was separated whereas the aqueous layer was extracted with CH 2 Cl 2 (50 mL).Then, the combined organic layers would be washed with brine and dried over Na 2 SO 4 .After evaporation of the solvent, the crude product was purified by column chromatography (CHCl 3 ) using hexane/ethyl acetate (3:1) as an eluant.After that, the solids were recrystallized from methanol to yield the product, which was 3,4-dimethoxybiphenyl (4). [17]Yield: 1.769 g (77.25%) as white solid.IR, n max /cm

3,4 0 -Bipheny-diol (5)
3,4 0 -Dimethoxybiphenyl 4 (7 mmol) was firstly dissolved in 150 mL of acetic acid, followed by the addition of 15 mL of 48% hydrobromic acid (HBr).After that, the reaction was refluxed for 3 h.The reaction is completed when the evolution of methyl bromide ceases.The organic layer was separated and washed with an aqueous solution of sodium hydroxide to eliminate excess of HBr.Then, it was dried over anhydrous MgSO 4 , filtered and the solvent evaporated under vacuum.The crude compound was recrystallized from methanol affording white crystals of biphenyl-3,4 0 -diol (5). [25,26]Yield: 0.844 g (64.75%).IR, n max /cm

Compound 6 derived from Steglich esterification
Compounds 3a-b (4.83 mmol of each azo compound for every reaction) and resorcinol (2.42 mmol) were dissolved in 250 mL of dry dichloromethane.
Then N,N 0 -Dicyclohexylcarbodiimide (DCC) (7.25 mmol) and 4-(Dimethylamino)pyridine (DMAP) (0.725 mmol) were added, and the mixture was stirred for 48 h at room temperature.The mixture was filtered, and the solvent was removed under reduced pressure.The product was then dissolved in chloroform and water.The organic phase was washed with diluted acetic acid, sodium carbonate and water successively.Again, the solvent was removed under reduced pressure.The compound was purified on silica gel by column chromatography using chloroform/methanol (100:1) as an eluent.The solids were recrystallized from ethanol and chloroform to get the compounds 6a-b. [2,24]The preparation of compound 6c followed the above procedure, except that the resorcinol was replaced by the biphenyl-3,4 0 -diol, the compound (5).

Synthesis of intermediate and final compounds
The synthetic procedure to the target compounds i.e., siloxane-based azobenzene containing bent-shaped MCPs (7a-c) are described and reaction paths are presented in Schemes 1 and 2. Firstly, 4-ethyl aminobenzoate was diazotized using sodium nitrite in the presence of acid medium.Then, the diazonium ions were coupled by phenol to get a compound namely ethyl 4-[(4-hydroxyphenyl)diazenyl]benzoate (1).Next, the hydroxyl group of the compound 1 was methylated with bromo-alkene (n ¼ 1 and 2) in the presence of basic medium (Scheme 1).Ethyl 4-f[4-(allyloxy)phenyl]diazenylgbenzoate (2a) and ethyl 4-f[4-(but-3-ene-1yloxy)phenyl]diazenylg benzoate (2b), the azobenzene side arm connected with the polymerizable alkene terminal chain, were synthesized by alkylating the free hydroxyl group of compound 1 with n-bromo-1-alkene (n ¼ 1 and 2) in the presence of potassium carbonate as a base.This is called Williamson reaction, which occurs in Williamson etherification by a nucleophilic substitution reaction (S N 2) in which a metal alkoxide displaces a halide ion from an alkyl halide (IR and NMR spectra at supporting information).
The third intermediate, 4-f[4-(allyloxy)phenyl]diazenylgbenzoic acid (3a) 4-f[4-(but-3-en-1-yloxy)phenyl]diazenylgbenzoic acid (3b), were obtained by hydrolyzing the ester part of the compounds 2 under a basic condition (Scheme 1).The mechanism of base-catalyzed ester hydrolysis in compounds 2 involves two steps.Firstly, a nucleophilic addition to the ester carbonyl group occurred when a strong nucleophile -OH attacked an electrophilic carbonyl to form a tetrahedral intermediate.After that, the tetrahedral intermediate was able to stabilize itself by eliminating the OCH 2 CH 3 to form 4-f[4-(but-3-en-1-yloxy)phenyl]diazenylgbenzoic acid 3.The mechanism of ester hydrolysis is shown in Scheme 1 (IR and NMR spectra at supporting information).
Monomers derived from the resorcinol central core units (6a and 6b) were prepared by the Steglich esterification of compounds 3 (n ¼ 1 and 3) and resorcinol.On the other hand, the Steglich esterification between compounds 3b and the biphenyl-3, 4 0 -diol central core unit (5) generated monomer 6c.In the Steglich esterification process, two equivalents of the acid-based compounds 3 coupled with one equivalent of the alcohol-based central core unit (resorcinol/biphenyl-3, 4 0 -diol) by using DCC as a coupling agent and DMAP as a catalyst to give monomers 6a-c, as shown in Schemes 1 and 2. The initial step of the reaction mechanism is the reaction between acid-based compound 3 and the carbodiimide DCC, most likely via an ion pair, to form the intermediate O-acylisourea.This intermediate would react with the catalytic amounts of DMAP to form an acyl pyridinium species, a compound that cannot undergo intramolecular rearrangement to form the by-product N-acylurea.This species could later react with resorcinol/biphenyl-3,4¢-diol to form the desired ester-typed monomers 6 (IR and NMR spectra at supporting information).In the case of monomer 6c, the central core unit of biphenyl-3,4 0 -diol (5) would be prepared first through the demethylation of 3,4 0 -dimethoxybiphenyl (4) before coupling with the acid-based compound 3b (Scheme 1).Lastly, the divinyl terminated bent-core mesogenic monomers, 6a and 6b, were further treated with 1,1,3,3,5,5-hexamethyltrisiloxane to produce the target polymer 7a and 7b, respectively (Scheme 1).Similar to the polymers 7a and 7b, the polymer 7c was formed by treating the monomer 6c with 1,1,3,3,5,5-hexamethyltrisiloxane in the presence of metal catalyst (Scheme 2).The FT-IR spectra 7a-c are presented as supporting information.

Synthesis of siloxane-based azobenzene containing bent-shaped MCP (7a-c)
The siloxane-based MCPs (7a-c) were prepared by the hydrosilylation polyaddition reaction of the unsaturated double bond of monomers (6a-c) with Si-H bonds of 1,1,3,3,5,5-hexamethyltrisiloxane in the presence of Pt catalyst.The solvent was evaporated, and the dark brown viscous product was dissolved in a small amount of dichloromethane.After that, the solution was poured into the cold methanol to precipitate the polymer, followed by the centrifugation at 3000 rpm at 0 � C for 15 min.This process was repeated three times and, afterward, the solvent was evaporated.The structure of polymers (7a-c) was confirmed through FTIR and NMR studies without further purification due to the practical difficulty in purifying the compounds.Based on the reading displayed on the meltingpoint apparatus, the melting point for polymers 7a-c were in the range of 250-270 � C. Number average molar mass (Mn) of polymers 7a-c prepared in THF were 3712, 2281 and 3329 g/mol, respectively.The distribution of molecular weights of polymers is often described by the polydispersity index (PDI), which is the ratio of weight average molecular weight (Mw) to the number average molecular weight (Mn).If the value is one, it indicates monodispersed or the given polymer has monomers arranged in equal chain length.On the other hand, the polymer with the PDI value larger than one (>1) has monomer units arranged in chains of different lengths. [28,29]The PDI values of three polymers are displayed in Table 1.Each of them had a large PDI value (>1.5), suggesting that they are polydisperse that contained polymer chains of unequal length.Besides, the repeating unit in each polymer was calculated by dividing the Mw of polymer by the Mw of monomer in order to know the degree of polymerization (DP).The polymer 7c had the highest repeating units (DP) whereas the polymer 7a had the lowest DP value, as demonstrated in Table 1.GPC traces for the polymers 7a-c in THF at 35 � C were presented at supporting information along with all calculations of yield, PDI and DP values for each of the polymers.

NMR spectral analysis
The NMR spectra of the desired compounds 7a-c are presented at Figures 1-3, respectively.The 1 H NMR spectra of polymers 7a-c show no peaks in the region of about 5.00-6.10ppm, clearly indicating the absence of alkene (-CH ¼ CH 2 ) protons.Moreover, the presence of the characteristic triplet at around 0.60 ppm (-CH 2 -Si-) and the -Si-(CH 3 ) 2 peak at 0.07-0.09ppm in the NMR spectrum also confirmed the existence of siloxane compound and thereby confirmed the conversion of the terminal alkene in monomers (6a-c) to siloxane moiety through hydrosilylation reaction.However, the integral value with more than 18 was detected due to the presence of the unreacted reactant, 1,1,3,3,5,5-hexamethyltrisiloxane.On the other hand, d of Si-H of 1,1,3,3,5,5-hexamethyltrisiloxane near 4.53 ppm became almost disappeared in the process of hydrosilylation, suggesting that the Si-H in 1,1,3,3,5,5-hexamethyltrisiloxane were substituted (Figures 1-3).

DSC
Both heating and cooling cycles of DSC were measured in the constant rate of 5 � C/min with mainly a second heating and second cooling cycles were considered in order to determine the phase transition temperatures (Cr-SmA-Iso) and enthalpy changes [DH/J g −1 ] associated with them are presented in Table 2.  the other side, no mesomorphic behavior was detected for monomer 6c.The melting point of monomer 6c was detected at about 190 � C. Polymers 7a-c did not show mesophase.Only melting peaks of 7a-c showed at 160.42, 156.12 and 162 � C, respectively; associated enthalpy changes are presented in Table 2. Thus, only crystallization peaks of 7ac appear at 80.68, 77.51 and 81.56 � C, respectively.

POM
The structure of the mesophase was captured by using POM, and the hot stage was used to control the temperature during the observation of mesophase.The temperature rate of 5 � C min −1 was maintained in the hot stage during the determination of liquid crystallinity of monomers 6a and 6b.Upon cooling of monomers 6a and 6b from the isotopic phase, a focal conic or fan-like texture, which is a typical for SmA phase, was observed under the POM. [2]Optical textures were observed for monomers 6a and 6b as shown in Figure 4 (left and right), respectively.There was no other phase transition on further cooling up to room temperature, except for the crystallization.In the case of monomer 6c, no image of POM was displayed due to the absence of mesophase.

FE-SEM studies
Bent-shaped compounds (6a-c) showed the flake-like multilayered clusters and growth orientation.The clusters were appeared in irregular multi-layered, and these morphological arrangements can be easily revealed from field emission scanning electron microscopy (FE-SEM) images as shown in Figure 5(a,c,e: left side top to bottom).After silylation of compound 6a-c, long-chain siloxane-based MCPs were formed termed as MCPs (7a-c).
Microscopic images of polymers (7a-c) indicate different physicochemical properties due to silylating.The corresponding compound polymers (7a-c) showed a crystalline structure with spherical shapes and an unsmooth surface with irregular spherical bead like morphology which indicates that silylating reaction occurred as shown in Figure 5(b,d,f: right side top to bottom).

Photo-switching studies
The azobenzene unit introduced functional properties into the bent-shaped MCPs, causing the possibility of photoisomerization and photochromic behavior. [30]Photo-switching studies were performed on solutions to give an idea of the materials behavior with respect to UV light and these results are indispensable for creating optical storage devices.Tacitly, azobenzene containing liquid crystal materials were characterized using ultraviolet-visible light to evaluate the photo-switching properties.
The absorption spectra of siloxane-based azobenzene containing bent-shaped MCPs (7a-c) in the presence of UV light were depicted in Figure 6.During the illumination of UV light with wavelength ¼ 365 nm, the maxima of all polymers decreased due to the trans-cis photoisomerization. [1,31,32]   After the UV irradiation of 150 s, no alteration in the absorption maxima of all polymers (7a-c) was observed, which confirmed the photosaturation of the trans-cis isomerization process (Figure 6(a-c)).Figure 6(d) displays the UV on dynamics absorption of polymers 7a-c as a function of exposure time.The data for Figure 6(d) were extracted from Figure 6(a-c), where 365 nm peak wavelength was fixed and absorption values at 365 nm at different exposure times were recorded.The absorption maxima can go to the lower level, and it even disappears most of the time at the stage of photosaturation. [33]The low level of the absorption maxima at photosaturation state corresponds to cis isomeric form. [1]he polymer (7a-c) containing solutions were illuminated continuously for another 20 s (photostationary state) and being kept in a dark condition.Then, their absorption spectra were recorded simultaneously from the point of photosaturation to the original state (trans configuration).Spectroscopic studies showed that the thermal back relaxation time of polymers for 7a-c were about 531, 400 and 520 min, respectively (Figure 7).On the other hand, the time dependence of cis to trans absorptions (UV off dynamics occurred at 365 nm) for polymers 7a, b and c were displayed in Figure 7(a-c).Data displayed in Figure 7(d) were extracted from Figure 7(a-c), where 365 nm peak wavelength was fixed and absorption values at 365 nm at different exposure times were recorded.Besides, the presence of the isosbestic points indicates no side reactions.In most circumstances, the thermal back relaxation (cis to trans isomerization) is spontaneous, and it does not need an external stimulus. [24]Nevertheless, the back relaxation is very fast when the light of 450 nm is illuminated on the azobenzene derivatives. [34]but thermal back relaxation is interesting due to the variation in the duration depending on the structure, alkylene spacers and functional group present in the molecule. [12,14]The thermal back relaxation time plays an important role in making the optical storage device because the higher thermal back relaxation time ensures that the optically generated pattern lasts longer. [35]n the other hand, the very weak absorbance at around 450 nm, which indicates n-p � transition of the Z isomer (cis isomer), also displayed noticeable changes during UV on and off process. Figure 8(a) demonstrates the time dependence of the trans to cis absorption, whereas Figure 8(b) shows the time dependence of the cis to trans absorption of polymers occurred at 450 nm.These data were plotted by fixing the peak wavelength at 450 nm, and absorption values at 450 nm at different exposure times were recorded.During UV on and off processes, the changes in peak intensity at 450 nm act in opposite way as compared to ?max at 365 nm.
Generally, the photoisomerization process of azobenzenecontaining compounds is significantly affected by the spacer (alkyl chain) and functional group associated with it.Mainly, the optical activity of the compounds depends on the presence of spacers associated with each molecule. [12,14]bviously, the different time interval of thermal back relaxation between polymers 7a and 7b was mainly caused by the variation in number of alkyl chains (CH 2 units) that linked between azobenzene side arms and siloxane moieties.For the siloxane-substituted azobenzene, increasing the alkyl chain length/spacer increases the distance between the azobenzene side arms and the bulky siloxane functional group.Consequently, the steric hindrance caused by the siloxane moiety on the N ¼ N part is reduced during the thermal back relaxation process.As a result, the thermal back relaxation decreases with increasing the number of spacers present in the molecular structure.On the other hand, the thermal back relaxation is also impacted by the type of central core unit based on the current study.In comparison to the polymer 7b, the polymer 7c that derived from the 3,4biphenyl-diol took a longer time to revert from cis to the original trans state.The occurrence of this phenomenon possibly resulted from the rigidity of the central core unit.
The conversion efficiency (CE), which is also known as the extent of the cis-trans photoisomerization, is estimated from Equation (1).

CE
where A(t 0 ) and A(t 1 ) are absorbance before UV and after UV, respectively.The extent of isomerization in polymers 7a-c is shown in Table 3.After 200 s of photosaturation, the conversion of trans to cis isomers was 74.28% for polymer 7a.In the case of polymers 7b and 7c, the extents of isomerization were 82.57% and 82.80%, respectively.

Thermal analysis of siloxane-based bent-shaped MCPs (7a-c)
The thermal stability of polymers, a parameter that is used to evaluate their working temperature limits, relies very much on their molecular structure, degree of crystallinity and molecular weight. [35]The thermal stability of all polymers was investigated by TGA, as demonstrated in Figure 9.According to the TGA curves, the thermal stability of all polymers was maintained up to 300 � C before the   occurrence of two mass-loss processes.The first stage of degradation for polymers 7a-c occurred from about 330 to 390 � C whereas the second mass-loss process occurred from about 390 to 510 � C. The high thermal degradation temperature (up to 300 � C) of all polymers was contributed by the presence of aromatic structures in the polymer � backbone. [35]The first weightloss process (330-390 � C) could be assigned to the breakage of the ethers, ester groups and alkyl chains that require less energy for the degradation.The rigidity and high thermal stability of azobenzene with central aromatic cores need high energy to degrade.Therefore, the second decomposition step occurring from about 390 to 510 � C could be attributed to the scission and decomposition of azo group with aromatic rings. [36]Since the structures of polymer 7c (derived from the biphenyl-3,4 0 -diol) and 7b (derived from resorcinol) only differ in the central core but remained the same at the periphery, the variation in the decomposition temperatures were not very high. [37]

Conclusion
Based on the results of GPC, all the synthesized polymers (7a-c) were polydisperse because each of them had the PDI values larger than one.On the other hand, the degree of polymerization increased in the following order: 7a > 7b > 7c, where the DP values were 7, 8 and 11, respectively.All polymers were accompanied by two stages of thermal decomposition, where the first mass loss could be assigned to the breakage of the ethers, ester groups and alkyl chains, whereas the second mass loss could be attributed to the decomposition of azo group with aromatic rings.
The mesomorphic behaviors of BCLC monomers were also successfully characterized by DSC and POM.Both monomers 6a and 6b displayed the SmA mesophase as the image of the fan-like texture was observed under the POM during the cooling of monomers from the isotropic phase.During the heating cycle, the monomer 6a showed the Cr-SmA and SmA-I transitions at 170.91 and 177.57� C, respectively.For monomer 6b, the Cr-SmA and SmA-I transitions were detected at 167.90 and 173.46 � C, respectively, upon the heating cycle.All the transition temperatures observed under POM agreed with DSC data.Although the liquid crystal phase was identified in both monomers 6a and 6b, no liquid crystalline property was found in their respective polymers (7a and 7b).On the other hand, both monomer 6c and polymer 7c that derived from the biphenyl-3,4diol central core displayed no mesomorphic behavior at all.Moreover, experimental photo-switching studies suggested that all polymers exhibited strong photoisomerization behavior in solution.The photo-switching properties of all polymers (7a-c) demonstrated trans to cis isomerization at about 200 s, whereas the reverse process (thermal back relaxation) took place at around 531, 400 and 520 min for polymers 7a, b and c, respectively.The extent of trans to cis photoisomerization was about 74.28%, 82.57% and 82.80% for polymers 7a, b and c, respectively.It is very interesting that the presence of spacer in the molecules plays an important role in the thermal back relaxation in photoisomerization.From the observation, we can conclude that compound with the short chain aliphatic spacers (7a) took a longer time thermal back relaxation compared to that of the longer one (7b) as the decreased alkylene spacer contributed more steric hindrance causing by bulky siloxane functional group on the N ¼ N part.The aliphatic spacers affect the thermal back relaxation process which may be largely of significance for promising applications in photochromism (photo storage) and photo-responsive molecular sensing devices.On the other hand, the variation in duration of cis to trans isomerization between polymers 7b and 7c revealed that type of central core unit has impact on the thermal back relaxation process.
For further study, investigation on the photo-switching effect of polymers 7a-c in the solid samples has to be carried out for practical applications.Besides, it is important to evaluate the photostability of these azobenzene-containing polymers after multiple cycles of prolonged UV irradiation because reusability of these compounds helps to reduce the disposal of chemical wasters by industries.

Disclosure statement
No potential conflict of interest was reported by the authors.

Figure 6 .
Figure 6.Absorption spectra of polymers for 7a (a), 7b (b) and 7c (c) with different exposure times of UV light.Before UV corresponds to the 0 s UV light illumination (absence of UV light).Time dependence photoisomerization curve of trans isomers in polymers for 7a-c showing the effect of UV illumination (365 nm wavelength) from the data obtained from (a)-(c) which is depicted in (d).

Figure 7 .
Figure 7. Thermal back relaxation process for the polymers of 7a (a), 7b (b) and 7c (c) took around 531, 400 and 520 min, respectively, to relax from cis to trans.Time dependence cis to trans photoisomerization curve in 7a-c showing thermal back relaxation time as depicted in (d).

Figure 8 .
Figure 8.Time dependence trans to cis photoisomerization curve (7a-c) occurred at 450 nm as shown in (a).Time dependence cis isomer (7a-c) occurred at 450 nm showing the thermal back relaxation time as shown in (b).

Table 1 .
Yield (%), number average molar mass (Mn), weight average molar mass (Mw), PDI and number of repeating units (DP) for the polymers