Photoswitching Characteristics, Photochromism and Mechanochromism of Extended π-Conjugated Azopyrazole and Azoisoxazole Derivatives

Herein, we report extended π-conjugated arylazopyrazole and arylazoisoxazole photoswitches towards tuning and modulation of their intrinsic properties. In this regard, we systematically introduced phenyl, styryl, (triphenylvinyl)styryl, phenylethynyl groups at ortho, meta, and para positions relative to the azo unit to extend the π-conjugation and vary the electronic coupling. In addition, the heterocyclic substituents were also modified. Overall, 20 photoswitches with modular substituents have been synthesized and studied for the photoswitching ability and thermal relaxations of Z isomers in the solution phase and solid state. Besides deciphering the importance of extending conjugation in photoswitching characteristics, intriguing properties such as mechanochromism and reversible bidirectional photochromism in the solid state were also unravelled. Through extensive spectroscopic studies, we envisage the effect of electronic coupling due to π-conjugation and its impact on longer wavelength light-induced photoswitching that holds several application potentials.