Effect of temperature on rate of a spin-forbidden transition in uracil and thymine

Abstract

The intersystem crossing rates of uracil and thymine molecules in interaction with a heat bath were studied by means of ab initio methods. The rates were calculated employing the time-dependent approach based on the correlation function. The normal modes of the singlet and triplet electronic states were related by the Duschinsky transformation. The correlation function was calculated using the Condon approximation for the spin-orbit matrix element and harmonic approximation for the nuclear motion. The excess vibrational energy in the initial singlet excited electronic state decreased the rate of triplet formation in uracil and thymine. This decrease was more pronounced for uracil. In addition, it was found that the change of the adiabatic energy gap could significantly modify the rate of triplet formation.