Mechanistic Insight into <i>Para</i> ‐Substituent Control of Thermal Half‐Lives in Arylazopyrazole Photoswitches

Abstract

Abstract 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 favor 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.