Theoretical insight into adenine-uracil and adenine-thymine photodeactivation mechanisms.

In this work, a thorough description of the photochemical and photophysical response of the Watson--Crick base pairs of adenine with uracil (A-U) or thymine (A-T) according to the SCS-ADC(2) results is discussed. Although widely explored, these systems lack a complete characterization of possible intra- and intermolecular relaxation channels induced by charge- or proton-transfer phenomena that may result due to the interaction of nucleobases in electronically excited states. In particular, we address the still open debate on photodeactivation via purine-ring puckering at the C2 or C6-atom position. We also consider the presence of low-lying long-living $^1\mathrm{n}\pi^\ast$ states to be a significant factor in the relaxation handicap through the EDPT process, as population of these states leads to internal conversion processes or efficient intersystem crossing to triplet manifold, whose estimated rate of $1.6\times 10^{10}\; \mathrm{s}^{-1}$, exceeds by an order of magnitude the corresponding internal conversion to the ground state. Additionally, the use of the SCS variant of the ADC(2) method is shown to provide a more balanced description of valence and charge-transfer excited states.