Theoretical multilevel approach for studying the photophysical properties of organic dyes in solution.

Classical all-atom molecular dynamics (MD) simulations and quantum mechanical time-dependent density functional theory (TD-DFT) calculations are employed to study the conformational and photophysical properties of tetramethylrhodamine iso-thiocyanate (TRITC) in solution. The potential energy surface (PES) is explored and the minimum energy structure is identified both in water and ethanol. An accurate force-field is parameterized on the computed quantum mechanical data and used in the classical dynamics to take into account solute vibrations and solvent effects. Several configurations, extracted from the MD trajectories, are employed to investigate absorbance spectra in a time dependent approach, considering solvation models of increasing complexity. Explicit- and implicit-solvent approaches, as well as combinations of them are used to predict and explain the absorption properties and the electronic structure of the dye. The defined theoretical methodology succeeds in reproducing correctly the available experimental data.