Our efforts in the first year of this project focused on three important photochemical reactions listed below:

1) The photodissociation of cyclobutane to form two molecules of ethylene: This reaction

is a textbook example of an addition/elimination reaction involving the Woodward-Hoffmann rules. The reaction proceeds either directly through a transition state at the saddle point of the activation barrier or through a two-step process, with one of the C-C bonds first breaking to form tetramethylene, a diradical reaction intermediate, which then passes through a transition state, finally yielding two molecules of ethylene. Our simulation investigation, collaborated with Zheyi Wen's research group, demonstrates the formation of the tetramethylene intermediate diradical, with dissociation completed in roughly 400 femtoseconds. The dynamics simulation combined with CASSCF/MRPT2 calculations of the potential energy surfaces (for the ground state and lowest excited state of the molecule while “on the fly”) provides a clear picture of the mechanism for this reaction and reveals that the C-C-C-C torsional angle is an important internal coordinate during the reaction, but C-C-C bond bending also plays a key role in shaping the potential energy surface for the tetramethylene intermediate. This work has been performed in collaboration with Prof. Zhenyi Wen (Chongqing University of Posts and Telecommunications, China). The results of this work are published in J. Phys. Chem. A 111, 1133 (2007),  J. Appl. Surf. Sci, 253, 6400 (2007) and Int. J. Quant. Chem. (in press).

2) Dynamics of ring opening reaction of 1,3-cyclohexadiene: Semiclassical dynamics model is applied to study the ring opening reaction of 1,3-Cyclohexadiene (CHD) triggered by a femtosecond-scale laser pulse. The results clearly demonstrate that, following the excitation by the laser pulse, the ring opening occurs at ~110 fs and the molecule decays to the ground electronic state at ~210 fs due to nonadiabatic transition of electrons from LUMO to HOMO orbitals. Isomerization of the product of the ring opening reaction, 1,3,5-hexatriene (HT), to various stable isomers are also well presented by the simulations. Further investigation using the combination of semiclassical dynamics simulation technique and CASSCF/MRPT2 calculations of the potential energy surfaces is in progress. This work has been conducted in collaboration with Prof. Zhenyi Wen (Chongqing University of Posts and Telecommunications, China). The results of this work are published in Appl. Surf. Sci, 253, 6404 (2007).

3) Isomerization of azobenzene induced by ultrashort laser pulse: The photoisomerization of azobenzene (Ab) and its derivatives has been proposed for many applications, including molecular switches, image storage devices, as well as the recently designed light-driven molecular shuttle. The goal of our research for this topic is to examine the reaction mechanisms under different laser excitations using dynamics simulation technique. We performed simulation investigations of both cis-trans and trans-cis isomerization of azobenzene induced by a laser pulse with a duration from 50 to 200 fs at different photon energies. We found that the predominant reaction path is the rotation of azobenzene about N-N bond for both n--pi* and  pi--pi* excitations. Further investigation for this topic is in progress. This work has been conducted in collaboration with Prof. Zhenyi Wen (Chongqing University of Posts and Telecommunications, China).

4) Ultrafast excited-state dynamics of tetraphenylethylene: We also performed detailed simulation study on the excited-state dynamics of isomerization of cis tetraphenylethylene (TPE) following excitation by a femtosecond laser pulse. The simulation results provide a basis for understanding several spectroscopic observations at molecular levels, including ultrafast dynamic Stokes shift, multicomponent fluorescence, viscosity dependence of the fluorescence lifetime and radiationless decay from electronically excited state to the ground state along the isomerization coordinate. . This work has been performed in collaboration with Prof. Keli Han (Dalian Institute of Chemical Physics, Dalian 116023, China) and is published in J. Chem. Phys. 107, 094307 (2007).