Fluorescence spectra of organic dyes in solution: a time dependent multilevel approach

Classical all-atom molecular dynamics (MD) simulations and quantum mechanical (QM) time-dependent density functional theory (TD-DFT) calculations are employed to study the conformational and photophysical properties of the first emitter excited state of tetramethylrhodamine iso-thiocyanate fluorophore in aqueous solution. For this purpose, a specific and accurate force field has been parameterised from QM data to model the fluorophore's first bright excited state. During the MD simulations, the consequences of the pi -> pi* electronic transition on the structure and microsolvation sphere of the dye has been analysed in some detail and compared to the ground state behaviour. Thereafter, fluorescence has been calculated at the TD-DFT level on configurations sampled from the simulated MD trajectories, allowing us to include time dependent solvent effects in the computed emission spectrum. The latter, when compared with the absorption spectrum, reproduces well the experimental Stokes shift, further validating the proposed multilevel computational procedure.