Theoretical Study On Photochromic Reaction Mechanism Of Stilbene And Benzopyran Derivatives

In this thesis,the photochromic mechanisms of two stilbene and benzopyran derivatives were studied theoretically by means of static electronic structure calculation.In the first part,the photoinduced cis-trans isomerization mechanism of 4-（N,N-dimethylamine）-4’-nitrostilbene（DANS）was discussed in detail based on the multi-state N-electron valence second-order perturbation theory（MS-NEVPT2）.In the second part,the photoinduced ring opening mechanism of 2,2-dimethylbenzopyran was studied by the static-dynamic-static second-order perturbation theory（SDSPT2）.In the first chapter,the development and classification of photochromic materials,the mechanism of photochromic change and the application prospect are summarized.The research backgrounds of the photoinduced cis-trans isomerization of stilbenes and the photoinduced ring-opening reactions of benzopyran compounds are introduced.Additionally,the quantum chemical calculation methods used in this thesis are briefly introduced.In the second chapter,the theoretical study of the photoinduced cis-trans isomerization of 4-（N,N-dimethylamine）-4’-nitrostilbene（DANS）by using highly accurate quantum chemical calculation method of MS-NEVPT2 is introduced.Five conical intersecting points（CIs）of S₀/S₁ and three CIs of T₁/T₂,and three intersystem intercrossing points（ISCs）of S₁/T₂,one ISC of S₁/T₁ and one ISC of S₀/T₁ were obtained by optimizing the key stationary points during the reaction.Based on the optimized geometries of CI_S,ISC_S and minima,the reaction paths of cis-trans isomerization are analyzed and summarized.The relaxation process starting from trans-S₁-FC mainly follows the triplet path ISC-S₁/T₂-trans→CI-T₁/T₂-trans→ISC-S₀/T₁-twist→trans-S₀ or cis-S₀.The relaxation process starting from cis-S₁-FC mainly proceeds along two singlet paths:cis-S₁-FC→CI-S₀/S₁-twist-c→trans-S₀ or cis-S₀and cis-S₁-FC→CI-S₀/S₁-DHP→DHP-S₀ or cis-S₀.In the third chapter,the theoretical study of 2,2-dimethylbenzopyran photoinduced ring opening reaction by using the high precision quantum chemistry calculation method of SDSPT2 is introduced.The key stationary points in the photochemical reaction,the conical intersecting points（CIs）of S₀/S₁ and S₁/S₂ were obtained.Based on the molecular geometries of CIs and the stable structures,we summarized the photoinduced ring opening mechanism of 2,2-dimethylbenzopyrane.There is an energy barrier of 0.78 e V on the photochemical reaction path along the potential energy surface of the excited state S₁,which may lead to a long lifetime of excited states in the reaction.Starting from S₂,the light-induced ring-opening reaction passing through CF-S₂-FC→CI-S₁/S₂→CF-S₁→CI-S₀/S₁→OF-ct→OF-tt will rapidly generate the ground state products OF-ct and OF-tt.