Real-time observation of sub-100-fs charge and energy transfer processes in DNA dinucleotides

Using as showcase the DNA dinucleotide 5′-dTpdG-3′, in which the thymine (T) is located at the 5′ end with respect to the guanine (G), we study the photoinduced electronic relaxation of coupled chromophores in solution with an unprecedented refinement. On the one hand, transient absorption spectra are recorded from 20 fs to 45 ps over the 330-650 nm range with a temporal resolution of 30 fs; on the other hand, quantum chemistry calculations determine the ground state geometry of the 4 possible conformers with stacked nucleobases, the associated Franck-Condon states, and map the relaxation pathways leading to excited state minima. Important spectral changes occurring before 100 fs are correlated with concomitant G⁺ → T⁻ charge transfer and T → G energy transfer processes. The lifetime of the excited charge transfer state is only 5 ps and the absorption spectrum of a long-lived nπ*T state is detected. Our experimental results match the transient spectral properties computed for the anti-syn conformer of 5′-dTpdG-3′, which is characterized by the lowest ground state energy and differs from that encountered in B-form duplexes.