The electronic and steric effects of aryl substituents and the influence of hydrogen bonding in Z-isomer stability of phenylazopyrazole derivatives have been investigated. In this regard, 38 substituted phenylazopyrazole derivatives and 6 N-methyl phenylazopyrazoles (with meta substitutions) have been synthesized. Their photoswitching behavior, photostationary states (PSS), and kinetics of thermal reverse isomerization were evaluated experimentally using UV-vis and NMR spectroscopic techniques. Furthermore, density functional theory (DFT) computations have been performed for more detailed insights. Despite the presence of substantial substituent effects inferred through Taft and Hammett relationships, the concentration dependency in controlling the isomerization rates has also been observed. Kinetics studies at different concentrations, solvent effects, and computations have confirmed the decisive role of hydrogen bonding and solvent-assisted tautomerism in this regard. Through this study, a complex interplay of steric, electronic effects and hydrogen bonding as factors in dictating the stability of Z-isomers in arylazo-1 H-3,5-dimethylpyrazoles has been demonstrated.

Reversibly photoswitchable phenylazo‐3,5‐dimethylisoxazole and 37 aryl‐substituted derivatives were synthesized. Excellent photoswitching ability of these compounds in solution and the solid state was demonstrated. Through kinetics studies by means of NMR spectroscopy, high Z‐isomer stability was demonstrated. Interestingly, the majority of the derivatives showed light‐induced contrasting color changes in solution and the solid state. Besides, many of the derivatives exhibit partial phase transition upon UV irradiation. The highlight of this class of photoswitches is the reversible light‐induced phase transition between solid and liquid phases in the parent compound, which can be used in patterned crystallization. These results show that this new class of azoheteroarene based photoswitches has opportunities to be useful in various domains.

Reversibly photoswitchable discotic liquid crystals (DLCs) with no change in columnar assembly.