Tuning the photochemical isomerization in azobenzene analogues containing phenol groups and heterocyclic rings

Azobenzenes and their heterocyclic analogues, such as azopyrazoles and azoisoxazoles, are versatile photoswitches with applications in responsive materials. Fully exploiting these systems requires a comprehensive understanding on how structural modifications and environmental factors control their isomerization kinetics and the stability of the Z (cis) isomers. In this work, we synthesized and systematically investigated the photochromic properties of a series of azobenzene and azoheteroarene derivatives. We combined experimental and theoretical methods to correlate molecular structure with photoswitching behavior. The photoinduced dynamics were studied using steady-state UV-vis and ultrafast time-resolved pump-probe spectroscopy, while the underlying molecular mechanisms were investigated using density functional theory (DFT). We show that structural modifications, including phenol groups, secondary amines, alkoxy chains, and ortho-methyl substituents, strongly affect ground-state thermal relaxation. By comparing the photoswitching behavior in toluene (non-polar solvent) and methanol (polar protic solvent), we show that the thermal half-lives of the Z isomers can be tuned over six orders of magnitude, from hundreds of milliseconds to several days. For derivatives bearing phenol or secondary amine groups, protic environments strongly accelerate relaxation. Quantum chemical calculations support these observations and show that solvent-assisted proton transfer opens an energetically favorable, low-barrier zwitterionic pathway with a single nitrogen‑nitrogen bond that allows free rotation. These results provide guidelines for designing molecular switches with tailored properties.