Towards the understanding of the excited state dynamics of nucelic acids: Solvent and stacking effects on the photophysical behaviour of nucleobases,

This contribution describes the results of some recent studies concerning the excited state behavior of nucleic acid bases, where solvent and stacking effect are included. Our computational approach is based on PBE0 and TD-PBE0 geometry optimizations, while bulk solvent effects are taken into account by the Polarizable Continuum Model, with the possible inclusion of the solvent molecules of the first solvation shell. This approach provides accurate absorption and emission spectra both for pyrimidine and purine bases and is able to explain solvent effect on the excited state lifetimes of uracil-like molecules. Solvent indeed modulates the accessibility of an extra decay channel for the bright excited state, involving an underlying dark state. The effect of base stacking, investigated on 9-methyl-adenine stacked dimers and trimers, on the absorption and emission spectra is also fully reproduced by our calculations. Although light absorption leads to a state (SB) delocalized over different adenine molecules, excited state geometry optimization indicates that afterward it evolves into a state where the excitation is localized on a single base. Analysis of the excited state potential energy surfaces shows that SB can easily decay into the lowest energy excited state (SCT). SCT is a dark excimer produced by inter-monomer charge transfer between two stacked bases