Roles Of Conical Intersection Of Potential Energy Surfaces In Several Important Photochemical Reactions

Photochemical reactions play an important role in nature and in human daily life,and at the same time,chemists also apply photochemical reactions to the important fields such as biology and material.Therefore,understanding the mechanisms of photochemical reactions and studying the photophysical and photochemical processes of molecules are necessary.The conical intersection of potential energy surfaces（PES）provides an important channel for the nonradiative decay process of the excited state.In this thesis,the density functional theory（DFT）,complete active space self-consistent field method（CASSCF）and multiconfigurational second-order perturbation approach（CASPT2）combined with quantum mechanics/molecular mechanics（QM/MM）method and nonadiabatic molecular dynamics simulations were employed to study the photophysical processes and photochemical reactions of two aggregation induced emission（AIE）molecules and one acyl azide molecule.The main contents include:（1）We proposed a new model for explaining the AIE of dimethyl tetraphenylsilole（DMTPS）:the restricted access to conical intersection（RACI）model.The computational results indicate that in solution,the reaction path from ground state minimum to S₁/S₀conical intersection is energetically available,because the energy of S₁/S₀is 0.7 e V lower than the vertical excitation energy at ground state minimum.However,in crystal,the S₁/S₀is 1.26 e V higher than the vertical excitation energy at ground state minimum.The reaction coordinates that lead to the S₁/S₀ involve mainly a twist of the silole ring and the flapping motion of phenyl substituents.Our results can well explain the AIE phenomenon of DMTPS.In solution,because the S₁/S₀ is accessible energetically,the excited state can return to the ground state by nonradiative decay,so molecule is non-emission.However in aggregation,because of the high energy of S₁/S₀,the motions to S₁/S₀ are hindered,and thus the system is fluorescent.Using the RACI model,we not only find the reasons why DMTPS exhibits AIE phenomena,but also provide a new sight for the design of new fluorescent molecules.（2）Then,nonradiative decay process of AIE-active 10,10’,11,11’-tetrahydro-5,5’-bidibenzo[a,d][7]annulenylidene（THBA）in solution was studied by calculating the PES and performing the molecular dynamics simulations.The calculated static PES of THBA in solution indicates that after excited to S₁ state,THBA can successfully reach to S₁/S₀.This S₁/S₀ has a cyclization configuration with two adjacent phenyl groups forming a new ring.The nonadiabatic molecular dynamics simulations indicate that the S₁→S₀hopping points have the cyclization structures similar to S₁/S₀.By analyzing the dynamic trajectories,we found that after hopping to S₀ state,THBA has the 33%possibility to form the stable cyclization structures,and has the 67%possibility to form the structures similar to FC structure.Both the static PES and the kinetic simulations show that the excited state of THBA can nonadiative decay through a cyclization conical intersection and this leads to the non-emission of THBA in solution.（3）We have also studied the mechanisms of the Curtius rearrangement of chlorodifluoroacetyl azide（F₂Cl CC（O）N₃）.The calculated PES in ground state shows that thermal rearrangement of F₂Cl CC（O）N₃ prefers to occur a concerted reaction and does not produce the nitrene intermediate.However,the excited state PES indicates that the photo-induced rearrangement of F₂Cl CC（O）N₃ will undergo the stepwise reactions.In detail,the excited F₂Cl CC（O）N₃ firstly eliminates one molecule of nitrogen and produces a nitrene intermediate in S₁ state.The generated S₁ state nitrene is easily accessible to S₁/S₀ conical intersectin between the S₁ and S₀ states.After the unstable nitrene returns to the ground state by through S₁/S₀,there are two possible decay paths:one is back to the ground state of nitrene,and the other is direct rearrangement to the final product isocyanate.These results are consistent with the experimental observations.This study reveals the mechanism of Curttius rearrangement and also shows the important role of the conical intersection in photochemistry.In this thesis,all the studies on the photophysical and photochemical properties of target molecules indicate that the conical intersection of PES plays an important role in the nonradiative decay process of excited state and in photochemistry.