Nonadiabatic dynamics and photoisomerization of biomimetic photoswitches

N-alkylated indanylidene pyrroline Schiff bases (NAIPs) are of great interest because their photoisomerization mimics the primary photoreactions of the retinal chromophore in rhodopsin. The nonadiabatic dynamics of two NAIP models (OMe-NAIP and dMe-OMe-NAIP) are investigated by trajectory surface-hopping method at the semi-empirical OM2/MRCI level. Both molecules show ultrafast isomerization governed by nonadiabatic dynamics via the S₀/S₁ conical intersections (CIs). Two CIs, CI_α and CI_β, play important roles in the E → Z and Z → E isomerization dynamics, respectively, and are mainly distinguished by different twisting statuses of the central C1′-C4 double bond. Although both compounds show ultrafast nonadiabatic dynamics, the S₁ lifetime of dMe-OMe-NAIP is much longer than that of OMe-NAIP. By removing the methyl group, dMe-OMe-NAIP allows less twist at the C1′-C4 double bond than OMe-NAIP. This result in the slower excited-state decay of dMe-OMe-NAIP, providing solid evidence to support a similar hypothesis proposed in a previous study.