Study On The Photoisomerization Of Di - Stilbene Based On The Method Of Trajectory Crossing Between Pre - Construction Potential Energy Surface

This thesis has two main parts. The first part of landscapes of four-enantiomer conical intersections for photoisomerization of stilbene:Complete active space self consistent field (CASSCF) calculation. The second part of constrained trajectory surface-hopping (TSH) molecular dynamics simulation of the photoisomerization of stilbene.In the first chapter we introduce features and applications of stilbene, and then briefly describe the background and methodology of TSH method.In the second chapter, we calculate the main four conical intersections (CI) of the photoisomerization of stilbene. We find that photoisomerization of cis- and trans- stilbene takes place around the C=C double bond. The process can be described by the rotation of the C-C=C=C and H-C=C-H dihedral angles. When the two dihedral angles equal to zero, there appears mirror symmetry. There exist two types of Haul-Twist-CI enantiomers and one-bond-flip-CI enantiomers, respectively. The CASSCF method is utilized to construct the potential energy surface according to the calculation of each enantiomer along the mirror rotation coordinates. The results confirm the fact that left-hand enantiomer and right-hand enantiomer can easily transfer each other through photoisomerization.In the third chapter, we introduce the improvement of TSH method with constraint molecular dynamics in details. The new TSH method can describe reaction with potential energy surface constructed with main reaction coordinates instead of full-dimension potential energy surface. During simulation, classical trajectories are evolving along Cartesian coordinates, while potential energy is calculated with internal coordinates that have fixed coordinates. Improved Zhu-Nakamura formula can be used to calculate nonadiabatic switching probability along seam line in internal coordinates. In order to test the applicability of the method, we study photoisomerization of stilbene around one-bond-flip conical intersection. We constructed analytical potential surfaces of ground So and first excited S1 states in terms of C-C=C-C and H-C=C-H dihedral angles with the other freedoms are fixed. The branching ratio of trans-to-cis isomerization is 48:52 at conical intersection, while experimental value is 50:50. The analytical potential surfaces explicitly present that trans-area and cis-area can be separated by a seam line. From the typical trajectories, we can conclude that the torsion angle rotates monotonically no matter what the initial configuration is. Quantum yield of trans-to-cis isomerization is 49%, which coincides with experimental measurement 55%. Quantum yield of cis-to-trans isomerization is 47%, while experimental value is 35%.