Solvent Effects on Ultrafast Charge Transfer Population: Insights from the Quantum Dynamics of Guanine-Cytosine in Chloroform

We study the ultrafast photoactivated dynamics of the hy-drogen bonded dimer Guanine-Cytosine in chloroform solu-tion, focusing on the population of the Guanine->Cytosinecharge transfer state (GC-CT), an important elementaryprocess for the photophysics and photochemistry of nu-cleic acids. We integrate a quantum dynamics propaga-tion scheme, based on a linear vibronic model parameter-ized through time dependent density functional theory cal-culations, with four different solvation models, either im-plicit or explicit. On average, after 50 fs, 30~40% of thebright excited state population has been transferred to GC-CT. This process is thus fast and effective, especially whentransferring from the Guanine bright excited states, in linewith the available experimental studies. Independent of theadopted solvation model, the population of GC-CT is how-ever disfavoured in solution with respect to the gas phase.We show that dynamical solvation effects are responsiblefor this puzzling result and assess the different chemical-physical effects modulating the population of CT states onthe ultrafast time-scale. We also propose some simple analy-ses to predict how solvent can affect the population transferbetween bright and CT states, showing that the effect ofthe solute/solvent electrostatic interactions on the energyof the CT state can provide a rather reliable indication ofits possible population.