Photoisomerization of Arylazopyrazole Photoswitches: Stereospecific Excited-State Relaxation

Abstract

Electronic structure calculations and nonadiabatic dynamics simulations (more than 2000 trajectories) are used to explore the Z–E photoisomerization mechanism and excited-state decay dynamics of two arylazopyrazole photoswitches. Two chiral S1/S0conical intersections with associated enantiomeric S1relaxation paths that are barrierless and efficient (timescale of ca. 50 fs) were found. For the parent arylazopyrazole (Z8) both paths contribute evenly to the S1excited-state decay, whereas for the dimethyl derivative (Z11) each of the two chiral cis minima decays almost exclusively through one specific enantiomeric S1relaxation path. To our knowledge, the Z11 arylazopyrazole is thus the first example for nearly stereospecific unidirectional excited-state relaxation.