## 2 Light-Induced Response of Azobenzene Chromophores
Based on photoswitchable \(T_{\text{g}}\) and \"photomelting\" behavior of azobenzene monomer, [PERSON] et al. studied light-induced mechanical response in CLCPs. The photoresponsive moiety used by the authors is _meta_-methylazobenzene (M-azo), it was covalently crosslinked with dodecyl glyceryl itaconate (DGI) to construct CLCP films. The trans-to-cis photoisomerization of azo groups that CLCP network results in a photoswitchable \(T_{\text{g}}\) and performs reversible photomechanical motions upon exposure to UV/visible light [47]. As shown in Figure 6, the solid M-azo compound changes to liquid state. To trigger the transition, the solid M-azo compound was irradiated with UV light of wavelength 365 nm at an intensity of 125 mW cm\({}^{-2}\) for just 6 s. As the \(T_{\text{g}}\) of the polymer is closely associated with the mechanical properties of the polymers, the high \(T_{\text{g}}\) and storage modulus (140 MPa) was obtained for DGI/M-azo films before UV treatment. The lower Young's modulus of 65.6 MPa was noticed in DGI/M-azo films after UV irradiation. This is due to the creation of _cis_-form in polymer films, which will reduce the \(T_{\text{g}}\) of the polymer. This photoswitchable \(T_{\text{g}}\) is exceptionally to achieve conspicuous photomechanical actuations of DGI/M-azo films under UV illumination. The authors evidently confirmed that bending and restoration of the films to original position were successfully controlled. This color-changing behavior of the films from orange to yellow was easily recognized (See Figure 7, Section: Photomechanical response of the CLCP film). This photoabsorption process is time-dependent. The irradiated light intensity plays a crucial role in deciding the degree of change in the \(T_{\text{g}}\) as well as response speed. By applying different intensity of irradiated light (3.28 to 45 mW cm\({}^{-2}\)), authors can control the bending speed of the film (0.4 to 2 mm s\({}^{-1}\)) at ambient temperature [47]. As discussed earlier (in Figure 1), based on the properties of siloxane CLCPs [5; 66] and acrylic CLCPs, listed (in Figure 7) azobenzene chromophore containing LC materials will be utilized to fabricate CLCP films from one-step or two-step protocols. The fabricated CLCP films find their major application in the construction of photo-driven micro/macroactuators. Some of the prominent azobenzene-containing liquid crystalline (LC) materials (monomers and crosslinkers) utilized to prepare LCPs with polysiloxane and polyacrylate backbone are shown in Figure 8.  
Figure 8: Partial list of chemical structures of azobenzene chromophore containing liquid crystalline (LC) materials (monomers and crosslinkers) utilized to prepare liquid crystalline polymers (LCPs) with polysiloxane and polyacrylate backbone [60; 67; 68; 69; 70; 71]. Figure 7: Schematic illustration, snapshots of photomelting process of _meta_-methylazobenzene (M-azo) compound and photomechanical response of the crosslinked liquid crystalline polymer (CLCP) film developed by [PERSON] et al., Adapted with permission from [47], Springer Nature, 2018. [PERSON] et al. [72] studied the effect of azo crosslinkers on photo-actuation abilities of LCE systems in comparison to non-azo crosslinkers. For the construction of the LCE system, authors utilized varying quantities of azo crosslinkers, non-azo crosslinkers, and azo monomer (For chemical structures, see Figure 9). Since the phase transition of mesogens involved in the construction of LCE system is influenced by variation of applied temperature, the temperature dependency on the photo-actuation of LCE systems was studied. It was reported that an extraordinary robust photo-actuation of up to 34% was achieved using azo-crosslinkers at 70 \({}^{\circ}\)C. However weak photo-actuation (change in length) compared to thermal actuation was achieved using non-azo-crosslinkers. Irrespective of azo moieties, higher crosslinking agents are necessary to provide mechanical stability to the LCE systems, which eventually utilized in the mechanical actuation device applications.