Mechanistic Insight into para-Substituent Control of Thermal Half-Lives in Arylazopyrazole Photoswitches

Arylazopyrazoles are versatile photoswitches with excellent photochromic properties and tunable thermal half-lives, yet the mechanistic role of substituents in controlling thermal stability remains poorly understood. Here, we synthesized an extensive library of arylazo-1,3,5-trimethylpyrazole photoswitches and rationalized the influence of para-substituents on the thermal half-lives, finding excellent agreement between calculated and measured trends. Calculations show that the electron-donating and electron-withdrawing nature of the substituents modulates the back-isomerization process through at least two distinct mechanisms. Strong electron-donating groups enhance delocalization at the azo moiety and thus favour a nonadiabatic out-of-plane rotational pathway via the lowest triplet state. In contrast, strong electron-withdrawing groups reduce delocalization and promote a conventional ground state in-plane inversion mechanism. Intermediate substituents exhibit a gradual shift that combines both major mechanisms. These findings provide prospects for rational design of responsive photoswitches with controllable thermal stability, essential for in vivo applications.