Ab initio multiple spawning dynamics of excited butadiene: role of charge transfer

Ab initio multiple spawning simulations of the photochemical reaction dynamics of s-trans-1,3-butadiene were performed. It is found that nonadiabatic events involving two low-lying excited states begin as early as 10 fs after excitation, resulting in the population being split between the bright 1(1)B(u) state and the dark 2(1)A(g) state. The molecule subsequently twists about a terminal carbon-carbon bond regardless of whether it is on the 1(1)B(u) or 2(1)A(g) electronic state. This twisting motion leads to conical intersections between S(1) and S(0). Several regions of the intersection seam involving states of differing character are accessed. The regions of the seam involving intersection between a state of charge-transfer character and a state of covalent character dominate the quenching dynamics, but intersections between two covalent states are also accessed a small percentage of the time. The existence and relative energies of these intersections are validated by optimization at the multistate complete active space second-order perturbation level of theory (MS-CASPT2). Our results point to a new mechanism for photoisomerization of butadiene that emphasizes the role of charge-transfer states.