Electronic excitation and structural relaxation of the adenine dinucleotide in gas phase and solution

The excited states and potential surfaces of the adenine dinucleotide are analyzed in gas phase and in solution using a correlated ab initio methodology in a QM/MM framework. In agreement with previous studies, a rather flat S ₁ surface with a number of minima of different character is found. Specifically, our results suggest that exciplexes with remarkably short intermolecular separation down to ∼2.0 Å are formed. A detailed analysis shows that due to strong orbital interactions their character differs significantly from any states present in the Franck-Condon region. The lowest S₁ energy minimum is a ππ* exciplex with only a small amount of charge transfer. It possesses appreciable oscillator strength with a polarization almost perpendicular to the planes of the two adenine molecules.