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 () state, a ππ* state located on the guanine moiety and a nπ* state on the thymine moiety, are identified. Their spectral features are detected in the transient absorption spectra () 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 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 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 state is longer and the corresponding quantum yield higher.