Electronic spectroscopy of a solvatochromic dye in water: comparison of static cluster/implicit and dynamical/explicit solvent models on structures and energies

In this study, we analyze the photophysics of the N-methyl-6-oxyquinolinium betaine (MQ) solvatochromic dye in aqueous solution, focussing on the important structural rearrangement of its first solvation shell following the electronic excitation, which is characterized by a strong charge transfer. To this aim, we compare the results provided by ground- and excited-state ab-initio molecular dynamics with that of full QM calculations on clusters including a small number of solvent molecules (from 1 to 4), taking into account bulk solvent effects by the Polarizable Continuum Model. The two methods agree in predicting that while in the ground electronic state between three and four water molecules are strongly coordinated to the oxygen atom of MQ, in the excited state two water molecules are present in the first solvation layer of the MQ oxygen. Vertical excitation and emission energies computed on the structures provided by the two approaches allow for estimates of the Stokes shift consistent with the experimental results. On the ground of the present results, some general considerations on the advantages and limitations of the dynamical and static approaches in describing a photoactivated process in solution are proposed.