From Excited-State Dynamics to Product Selectivity: Theoretical Insights into the Photoinduced Chemistry of Aza-Diarylethenes

Abstract

Aza-Diarylethenes (aDAEs) are a novel class of photoswitches, which undergo reversible C−N bond formation upon irradiation. Previous experimental studies on aDAEs, differing in the heterocyclic aryl cores and substituents at the sp³-carbons of the cyclopentene, have revealed the formation of distinct types of photoproducts, on the one hand, DAE-typical closed ring isomers and on the other hand hitherto unknown zwitterionic structures. Here, we present a comprehensive comparative computational study of aDAE photocyclization mechanisms using nonadiabatic molecular dynamics simulations at the mixed-reference spin-flip time-dependent density functional theory level, to provide a unified mechanistic framework for understanding the divergent photoproduct formation. We show that photoexcitation drives ring closure via an isomerization conical intersection, but this crossing does not necessarily define the experimentally observed product: in one system it yields the closed isomer directly, whereas in another aDAE it accesses a ground-state pathway leading to the formation of a zwitterionic isomer.