Effect of A-DNA and B-DNA Conformation on the Interplay between Local Excitations and Charge-Transfer States in the Ultrafast Decay of Guanine-Cytosine Stacked Dimers: A Quantum Dynamical Investigation.

We here simulate in the gas phase the population dynamics of guanine/cytosine (GC) and cytosine/guanine (CG) stacked dimers in B-DNA and A-DNA arrangement, following excitation in the lowest-energy band, and considering the four lowest-energy ππ* bright excited states, the three lowest-energy nπ* states, and the G → C charge-transfer (CT) state. We resort to a generalized Linear Vibronic Coupling (LVC) model parametrized with time-dependent density functional theory (TD-DFT) computations, exploiting a fragment-based diabatization and we run nonadiabatic quantum dynamical simulations with the multilayer version of the Multiconfigurational Time-Dependent Hartree (ML-MCTDH) approach. G → C CT results in a major decay process for GC in B-DNA but less in A-DNA arrangement, where also the population transfer to the lowest-energy excited state localized on C is an important intermonomer process. In CG arrangements, mostly intramonomeric decays take place. We simulate the dynamics of several other GC structures whose arrangement is intermediate between B-DNA and A-DNA, obtaining further insights on the effect that the sequence and, especially, the stacking geometry have on the population transfer to the G → C CT.