title: Intersystem crossing and characterization of dark states in the pyrimidine nucleobases uracil, thymine, and 1-methylthymine.
abstract: 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 S(2)((1)pi-->pi*)-->T(2)((3)n-->pi*) process cannot compete with the subpicosecond decay of the S(2) population due to spin-allowed nonradiative transitions, whereas the T(1)((3)pi-->pi*) state is populated from the intermediate S(1)((1)n-->pi*) state on a subnanosecond time scale. Hence, it is very unlikely that the S(1)((1)n-->pi*) state corresponds to the long-lived dark state observed in the gas phase.
authors: Institute of Theoretical and Computational Chemistry, Heinrich Heine University Düsseldorf, Düsseldorf, Germany.
authors: Mihajlo
authors: M
authors: Etinski
authors: Timo
authors: T
authors: Fleig
authors: Christel M
authors: CM
authors: Marian
mesh_terms: D004583:Electrons; D007700:Kinetics; D008956:Models, Chemical; D011743:Pyrimidines / Q000737:chemistry*; D011789:Quantum Theory; D013816:Thermodynamics; D013941:Thymine / Q000031:analogs & derivatives / Q000737:chemistry; D014498:Uracil / Q000737:chemistry
chemical_list: D011743:Pyrimidines; C030630:1-methylthymine; D014498:Uracil; D013941:Thymine