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J Phys Chem A. 2009 Oct 29;113(43):11809-16. doi: 10.1021/jp902944a.

Intersystem crossing and characterization of dark states in the pyrimidine nucleobases uracil, thymine, and 1-methylthymine.

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  • 1Institute of Theoretical and Computational Chemistry, Heinrich Heine University Düsseldorf, Düsseldorf, Germany.


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.

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