Exploring the Mechanism of a Chiral N ‐Alkyl Imine‐Based Light‐Driven Molecular Rotary Motor at MS‐CASPT2//CASSCF and MS‐CASPT2//(TD) DFT Levels

The working mechanism including the photoisomerization and thermal isomerization steps of a chiral N ‐alkyl imine‐based motor synthesized by Lehn et al. are revealed by MS‐CASPT2//CASSCF and MS‐CASPT2//(TD‐)DFT methods. For the photoisomerization process of the imine‐based motor, it involves both the bright (π,π*) state and the dark ( n ,π*) state. In addition, the MECI has similar geometry and energy to the minimum of the S 1 state, which shows that the process is barrierless and keeps the unidirectionality of rotation well; the result confirms the imine‐based motor is a good candidate for a light‐driven molecular rotary motor. For the thermal isomerization process of the imine‐based motor, there are two even isomerization paths: one with the mechanism of the in‐plane N inversion, the energy barriers of which are 29.6 kcal mol −1 at MS3‐CASPT2//CAM‐B3LYP level and 29.2 kcal mol −1 at MS3‐CASPT2//CASSCF level; the other with the mechanism of ring inversion of the cycloheptatriene moiety, with energy barriers of 28.1 kcal mol −1 at MS3‐CASPT2//CAM‐B3LYP level and 18.1 kcal mol −1 at MS3‐CASPT2//CASSCF level. According to the structural feature of the stator moiety, the imine molecule can be used as a two‐step or a four‐step light‐driven rotary motor.