The excited states of pi-stacked 9-methyladenine oligomers: a TD-DFT study in aqueous solution.

An extensive PCM/TD-DFT study of the excited states of pi-stacked 9-methyladenine oligomers (from the dimer up to the pentamer), adopting the B-DNA conformation, is reported. Linear response PCM/TD-PBE0 calculations provide a reliable description of the stacking effect on the bright excited states but PBE0, when compared to CAM-B3LYP, LC-omegaPBE and M052X functionals, overestimates the stability of the electronic states with inter-monomer charge transfer (CT) character. On the other hand, state specific PCM/TD-DFT calculations confirm that the lowest energy excited state of the dimer corresponds to a dark excimer produced by inter-monomer CT between two stacked bases. Confirming the experimental indications, the spectroscopic states of the oligomers are delocalized over multiple bases.