Further Theoretical Study Of Photoisomerization Mechanism Of Push-pull Stilbene Derivatives

This thesis introduces the effect of push-pull electron groups on photoisomerization of stilbene derivatives by the combination of quantum chemistry method and trajectory surface hopping molecular dynamic simulation.The whole thesis was divided into three parts:The first chapter breifly introduced the application and development of stilbene derivatives in the field of photochemistry,it also described the methods employed in these computations.The second chapter mainly described the photoisomerization mechanisms of4-?N,N-dimethylamino?-4'-nitrostilbene?DANS?by static-dynamic-static second-order perturbation theory with size-consistent correction?SDSPT2+Q?.As was well known,DANS consists of an electron donating and a withdrawing group,combined by a carbon-carbon double bond delocalized bridge structure.The electrons transfer from the electron donating group to withdrawing group via this conjugated bridge bond,which is hence a potential candidate for nonlinear optical materials.It is practically valuable to explore photochemical processes of DANS.The third chapter described the dynamic simulation of 4-amino-4'-nitrostilbene?ANS?by the combination of static electronic structure computations and subsequent trajectory surface hopping molecular dynamic simulations and indicated the reaction pathways in the photoisomerization process.In the first chapter,the backgrounds and developments of photoisomerization reaction stilbene derivatives were briefly introduced.The calculation methods used in the following sections were also described,including the trajectory surface hopping molecular dynamic simulation method based on the improved Zhu-Nakamura formulas.The complete active space self-consistent field method?CASSCF?and the static-dynamic-static second order perturbation theory with size-consistency correction?SDSPT2+Q?were also briefly introduced.In the second chapter,the photoisomerization mechanisms of a push-pull stilbene derivative,4-?N,N-dimethylamino?-4'-nitrostilbene?DANS?upon S₁ excitation have been investigated by utilizing the static-dynamic-static second order perturbation theory with size-consistency correction?SDSPT2+Q?.The conical intersections?CIs?and intersystem crossing points?ISCs?networks,which composed of four S₁/S₀ and two T₂/T₁ CIs and three ISCs of S₁/T₂,S₁/T₁ and T₁/S₀,respectively,were obtained.Based on the optimized crossings and minima,the singlet and triplet pathways started from the cis-and trans-Franck Condon?FC?regions were proposed.After photexcited to the S₁ FC region,trans-DANS quickly convert to the quinoid form trans-S₁.The primary triplet pathway follows the relaxation steps as trans-FC?ISC-S₁/T₂-trans?CI-T₂/T₁-trans?ISC-T₁/S₀?trans-or cis-S₀.The singlet pathway also plays important roles as a fact that the relaxation from transoid FC to the CI-S₁/S₀-twist-c or CI-S₁/S₀-twist-t crossing regions are basically energetically favorable.On the other hand,started form cis-FC,the potential energy decreases steeply along the singlet pathway,cis-FC?CI-S₁/S₀-twist-c or CI-S₁/S₀-twist-t?trans-or cis-S₀,and which would be as important as the triplet pathway,cis-FC?ISC-S₁/T₂-cis?CI-T₂/T₁-cis?ISC-T₁/S₀?trans-or cis-S₀.Another minor dihydrogenphenanthrene formation pathway,cis-S₁?ISC-S₁/T₁-cis?DHP-T₁,was also observed for cis-FC.In the third chapter,we explored the mechanisms of ANS photoisomerization reaction utilizing static electronic structure computation and nonadiabatic molecular dynamic simulation at SA5-CASSCF?10,9?/6-31G?N,O=6-31G*?level.It can be found by the dynamic simulation that the photoreaction process of ANS mainly involves singlet and triplet mechanisms,and branching ratio of the single and triplet pathways is 1:2.There were three main mechanisms for this photochemical reaction process.The pathways of the triplet mechanism are?1?trans-S₁?ISC?S₁/T₂??CI4?T₂/T₁??ISC?T₁/S₀??trans-S₀?2?trans-S₁?ISC?S₁/T₂??CI5?T₂/T₁??cis-T₁.For the triplet state,the main photoproduct in the triplet state is cis-ANS and the S₀ state product is mainly trans-ANS.For the singlet mechanism,the pathway of photoisomerization is:?3?trans-S₁?CI1/2/3?S₁/S₀??cis-S₀ or trans-S₀,corresponding to the photoproduct cis-ANS or the reactant trans-ANS.