Intersystem Crossing and Characterization of Dark States in the Pyrimidine Nucleobases Uracil, Thymine, and 1-Methylthymine

The ground and low-lying excited states of the pyrimidine nucleo bases uracil, thymine, and 1-methylthymine have been characterized using ab initio coupled-cluster with approximate doubles (CC2) and a combination of density functional theory (DFT) and semiempirical multireference configuration interaction (MRCI) methods. Intersystem crossing rate constants have been determined perturbationally by employing a nonempirical one-center mean-field approximation to the Breit−Pauli spin−orbit operator for the computation of electronic coupling matrix elements. Our results clearly indicate that the S2(1π→π*) ⇝ T2(3n→π*) process cannot compete with the subpicosecond decay of the S2 population due to spin-allowed nonradiative transitions, whereas the T1(3π→π*) state is populated from the intermediate S1(1n→π*) state on a subnanosecond time scale. Hence, it is very unlikely that the S1(1n→π*) state corresponds to the long-lived dark state observed in the gas phase.