On the intrinsic population of the lowest triplet state of thymine |
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Authors: | Serrano-Pérez Juan José Gonzalez-Luque Remedios Merchan Manuela Serrano-Andrés Luis |
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Institution: | Instituto de Ciencia Molecular, Universitat de València, Apartado 22085, ES-46071, Valencia, Spain. |
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Abstract: | The population of the lowest triplet state of thymine after near-UV irradiation has been established, on the basis of CASPT2//CASSCF quantum chemical calculations, to take place via three distinct intersystem crossing mechanisms from the initially populated singlet bright 1pipi* state. Two singlet-triplet crossings have been found along the minimum-energy path for ultrafast decay of the singlet state at 4.8 and 4.0 eV, involving the lowest 3npi* and 3pipi* states, respectively. Large spin-orbit coupling elements predict efficient intersystem crossing processes in both cases. Another mechanism involving energy transfer from the lowest 1npi* state with much larger spin-orbit coupling terms can also be proposed. The wavelength dependence measured for the triplet quantum yield of pyrimidine nucleobases is explained by the location and accessibility of the singlet-triplet crossing regions. |
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