On the intrinsic population of the lowest triplet state of uracil

From CASPT2 parallel to CASSCF quantum-chemical computations it is determined that the lowest triplet state of uracil can be efficiently populated from the initially activated singlet manifold through respective singlet-triplet crossings of the singlet state with the low-lying (3)n pi* state at 4.6 eV and with the lowest (3)pi pi* state at 4.2 eV located along the minimum energy path of the low-lying (1)pi pi* state. Large spin-orbit coupling elements predict, in particular for the former case, efficient intersystem crossing processes. The wavelength dependence measured for the triplet quantum yield can be explained by the location of the singlet-triplet crossing regions. (c) 2007 Elsevier B.V. All rights reserved.