Ultrafast dynamics of the UV-induced electronic relaxation in DNA guanine–thymine dinucleotides: from the Franck–Condon states to the minima of the potential energy surfaces

We study the DNA dinucleotide 5'-dGpdT-3' (abbreviated as GT) as a model system for the relaxation of the electronic excited states in stacked nucleobases. Quantum chemistry calculations determine the Franck-Condon states and follow their evolution along the potential energy surfaces of the two most stable conformers. Three minima, corresponding to an excited charge transfer (¹<i>CT</i>) state, a ¹ππ* state located on the guanine moiety and a ¹nπ* state on the thymine moiety, are identified. Their spectral features are detected in the transient absorption spectra (<i>TAS</i>) recorded for buffered aqueous solutions between 330 and 650 nm with a time-resolution of 30 fs upon excitation at 266 nm. The striking difference between the <i>TAS</i> obtained for GT and an equimolar mixture of the corresponding mononucleosides indicates that the nucleobases are stacked in the majority of the dinucleotide molecules. The ¹<i>CT</i> state, in which a charge of 0.8 a.u. is transferred from the guanine to the thymine, is stabilized within 120 fs. The comparison of the GT behaviour with that of 5'-dTpdG-3', characterized by an opposite polarity and studied previously by the same methodology, reveals that, when the guanine is positioned at the 5' end, the lifetime of the G⁺ → T^{- 1}<i>CT</i> state is longer and the corresponding quantum yield higher.