Photoinduced nonadiabatic dynamics of pyrimidine nucleobases: on-the-fly surface-hopping study with semiempirical methods.

The photoinduced relaxation dynamics of pyrimidine nucleobases (uracil, thymine, and cytosine) was studied using the surface-hopping approach at the semiempirical OM2/MRCI level of theory. The relevant potential energy surfaces were characterized by performing geometry optimizations of the energy minima of the lowest electronic states and of the most important conical intersections and by computing excitation energies at each configuration. Surface-hopping molecular dynamics simulations were performed to describe the nonadiabatic dynamics after excitation into the optically active state. In each of the molecules, the two lowest excited singlet states are involved in the dynamics, and there are competing relaxation paths. The dynamics is dominated by a two-step relaxation mechanism in uracil and thymine, while the direct decay to the ground state is most important in cytosine. For all three molecules, the simulations yield ultrafast S(2)-S(1) deexcitation within 50 fs and internal conversion to the ground state in less than 1 ps, consistent with recent experimental results from time-resolved photoelectron spectroscopy.