Combining the Advantages of Alkene and Azo E–Z Photoisomerizations: Mechanistic Insights into Ketoimine Photoswitches

We carried out CASPT2//(TD)­DFT and CASPT2//CASSCF studies on the working mechanism of imine switches, including a camphorquinone-derived ketoimine (shortened as k -Imine) switch designed by Lehn as well as a model camphorquinone alkene-imine ( a -Imine) proposed in this study. Under the experimental conditions (light irradiation with 455 and 365 nm for E and Z, respectively), k -Imine is excited to the S1:(n N,π*) state and then decays toward a perpendicular intermediate following the CN bond rotation coordinate. During the bond rotation, a mild energy barrier caused by the strong interaction of S1:(n N,π*) and S2:(n O,π*) states will more or less slow down the rotation speed of k -Imine. The large difference in irradiation light wavelength supports k -Imine as a two-way photoswitch. The photoisomerization of a -Imine obeys a similar but fully barrierless pattern while requiring a higher excitation energy to reach the (n N,π*) state. The good directionality of thermal isomerization toward E ( a -Imine), plus the barrierless photoisomerization, allows for the design of a thermal and photo-operated switch. For both imines, a minimal-energy crossing point (MECI) located at the perpendicular region, with low relative energy and close to the rotary path, ensures the directionality of CN bond rotation and confirms imines as optimal candidates for photoswitches and motors.