## 2 Light-Induced Response of Azobenzene Chromophores  
On exposure to suitable wavelength of light, azobenzene chromophores show rapid response behavior. Due to this characteristic feature, azobenzene derivatives are widely used as photosensitive materials for a broad range of technological applications. In brief, two benzene rings link together with azo (-N = N-) functional group to form azobenzene chromophore. Functionalization of benzene ring can be done to prepare a variety of azobenzene derivatives by tuning electron-donating and withdrawing properties to achieve push-pull type azobenzene structures [61]. The fine-tuning of rigidity and flexibility of azobenzene molecules indorsed to unveil liquid crystalline (LC) behavior, which makes them extremely useful in designing photoresponsive materials. Under photo-excitation, the two isomeric configurations of azobenzene (a thermally stable _trans_ state and a meta-stable _cis_ form) undergo conversion from _trans_ to _cis_ state. During conversion, the molecule undergoes systematic geometrical deformation, which will end up with different molecular sizes for _trans_ and _cis_ forms (As shown in Figure 3d, the distance between reference carbon atoms of aromatic rings decrease from 9.0 A to 5.5 A, as the molecules transform from _trans_ to _cis_ state) [62]. One of the important parameters to consider in technical applications of a polymer is glass transition temperature (\(T_{\mathrm{g}}\)). For a quick glance, \(T_{\mathrm{g}}\) is a characteristic property of polymer and is related to the temperature where the molecular chains begin to move. The polymers containing azobenzene moieties can have switchable \(T_{\mathrm{g}}\) and can induce reversible _cis-trans_ photoisomerization. This photoswitching of \(T_{\mathrm{g}}\) is a topic of scientific curiosity and technical importance. The first report involving \(T_{\mathrm{g}}\) of azobenzene-containing polymers reported by [PERSON] et al. [63] proposed light-induced reversible solid-to-liquid transitions of the polymers. This strategy can be applied to heal the cracks in high-\(T_{\mathrm{g}}\) polymers without harmful plasticizers. The authors also demonstrated that light can switch the \(T_{\mathrm{g}}\) of the polymer, which eventually controls the mechanical properties of the polymers [63]. The polymers with switchable \(T_{\mathrm{g}}\) developed by [PERSON] et al. are displayed in Figure 6. Since the ultraviolet (UV) irradiation of _trans_ polymer powder samples not generated adequate amount of _cis_ form, authors dissolved the trans polymer in dichloromethane (CH\({}_{2}\)Cl\({}_{2}\)) and stirred it under the illumination of UV light to achieve respective _cis_ form. The authors also prepared two control samples to prove the liquefaction was not due to the residual solvents. Figure 6: Schematic illustration and solid-to-liquid transitions of azobenzene-containing polymers developed by [PERSON] et al. [63]. The differential scanning calorimetry (DSC) studies demonstrated that _trans_ form showed a \(T_{\text{g}}\) of 48.0 \({}^{\circ}\)C and in both the heating curves (first and second), we can notice the two-phase transitions at 94.0 and 108.0 \({}^{\circ}\)C. However, for _cis_ form, we can observe the \(T_{\text{g}}\) at \(-10.0\)\({}^{\circ}\)C with broad exothermic peak at around 94.0 \({}^{\circ}\)C. Consequently, the study revealed that \(T_{\text{g}}\) of the polymer decreased with the _cis_ content, serving as plasticizer [63]. Considerable attention has been focused on phase transition compounds especially where solid-to-liquid phase change involved. Under the influence of light, the molecular interaction in the solid state weakens to form a liquid state. This \"photomelting\" behavior of certain materials is of special interest in the material science field for the development of photosensitive materials. Despite diverse functional photomelting compounds are available, fast responsive materials are having superior significance to design bio-inspired robotic structures [64; 65].