The effect of methylation on the excited state dynamics of 2-thiouracil

Thiouracils, where an oxygen atom in uracil is replaced by sulfur, have been observed to undergo fast and efficient intersystem crossing (ISC) from the lowest singlet excited state, S1(nπ*), to the triplet manifold along a sulfur-out-of-plane (op-S) coordinate. Our recent time-resolved photoelectron spectroscopy (TRPES) studies on a series of thiouracils have shown that minor changes such as the position and degree of thionation profoundly alter the subsequent dynamics on the lowest triplet state, T1. These differences are attributed to intricate details of the potential energy topography, specifically the presence and relative energies of two minima with boat-like (T1 boat) and sulfur-out-of-plane (T1 op-S) geometries. In the case of 2-thiouracil, the excited state population equilibrates between both minima. ISC back to the ground state (GS) depends on spin-orbit couplings (SOC) and accessibility of the T1/GS crossing points at boat-like and op-S geometries with T1 op-S/GS clearly favored. The TRPES study presented here interrogates the equilibration and ISC dynamics of the lowest triplet state of 2-thiothymine. Methylation provides a sensitive probe of barriers along a pathway that involves displacements of the methyl substituent and will manifest itself in slower triplet dynamics. As such, it provides insight into the role of the boat-like minimum in the excited state deactivation mechanism.