A Theoretical Multi-level Approach for the Study of Optical Properties of Organic Dyes in Solution.

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