Vertical Singlet Excitations on Adenine Dimer: A Time Dependent Density Functional Study

The condense phase, excited state dynamics of the adenylyl$(3′→5′)$adenine (ApA) dinucleotide has been previously studied using transient absorption spectroscopy with femtosecond time resolution (Crespo‐Hernández et al. Chem. Rev. 104, 1977–2019 (2004)). An ultrafast and a long‐lived component were observed with time constants of $<1 ps$ and $60±16 ps,$ respectively. Comparison of the time constants measured for the dinucleotide with that for the adenine nucleotide suggested that the fast component observed in ApA could be assigned to monomer dynamics. The long‐lived component observed in ApA was assigned to an excimer state that originates from a fraction of base stacked conformations present at the time of excitation. In this contribution, supermolecule calculations using the time dependent implementation of density functional theory is used to provide more insights on the origin of the initial Franck‐Condon excitations. Monomer‐like, localized excitations are observed for conformations having negligible base stacking interactions, whereas delocalized excitations are predicted for conformations with significant vertical base‐base overlap.