Research And Theoretical Regulation Of Photochemical Reaction Mechanism Of Imine Molecular Switches

The goal of this thesis is to provide mechanistic understanding on the nonadiabatic photochemical reaction of imine systems as well as to shed light on the mechanistic tuning of imine-based molecular switches,which may inspire new experimental developments and applications in this field.In the first part,we carried out density function theory(DFT),time-dependent DFT(TDDFT),multireference complete active space self-consistent field(CASSCF)and second-order perturbation theory(CASPT2)studies on reaction mechanism of an experimentally synthesized ketoimine(k-Imine)and a model alkene-imine(a-Imine)propused in this study.In the second part,we carried out mechanistic studies on the cis-trans photoisomerization around the C=N bond and the excited-state intramolecular proton transfer(ESIPT)processes of salicylideneaniline(SA);on top of that,we explored the electric-field effects on the above-mentioned processes,to provide insights on the mechanistic tuning of molecular switches.The important fingdings are summarized as follows.1.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 condition(light irradiation with 365 and 455 nm for E and Z,respectively),k-Imine is excited to the S1:(nN,?*)state and then decays towards 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?,?*)and S2:(no,?*)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 photo switch.The photoisomerization of a-Imine obeys a similar but fully barrierless pattern,while requires higher excitation energy to reach the(nN,?*)state.The good directionality of thermal isomerization towards E(a-Imine),plus the barrierless photoisomerization,allow for the design of a thermal and photo-operated switch.For both imines,a MECI located at the perpendicular region,with low relative energy and close neighborhood to the rotary path,insures the directionality of C=N bond rotation,and confirms imines as optimal candidates for photo switches and motors.2.We carried out DFT and TDDFT study on the excited-state reaction mechanism of salicylideneaniline(SA)and its theoretical tunning by electric-field,and mainly focused on the photoisomerization around the C=N bond and excited-state intramolecular proton transfer(ESIPT)processes.The results show that the ground-state proton transfer is easier than the thermal isomerization around the C=N bond,and confirmed the in-plane inversion mechanism for the latter process.In excited state,both the C=N rotation and ESIPT processes are barrierless,except for a mild barrier(?2.6 kcal/mol)being observed in C=N rotaty path before the molecule reaches the S1/S0-CI to nonradiatively relax to the ground state.The competation between the above-mentioned processes may be an obstacle for SA to be used as a monomode molecular switch.Therefore,we further studied the electric-field effects on these processes,to provide insights on the mechansitc tuning of molecular switches.The results show that the electric field applied along the C?N direction can inspire the ESIPT,meanwhile suppress the C=N rotation that may cause internal conversion,thus demonstrates electric-field tuning to be an effective way to improve the selectivity of optical molecular switches.