Absorption and fluorescence spectra of uracil in the gas phase and in aqueous solution: a TD-DFT quantum mechanical study.

Here we present the first computations of fluorescence spectra in aqueous solution at an accurate quantum mechanical level. From a methodological point of view, our study shows that by only taking into account both bulk effects and explicit solvent molecules it is possible to reproduce solvent effects on the energy and the intensities of the electronic spectra, especially for what concerns pi/pi* transition. The computed absorption and fluorescence spectra are in a good agreement with the available experimental results. The energy ordering between the lowest energy n-pi* and the pi/pi* transitions in uracil strongly depends on the nature of the embedding medium. The geometry of the first solvation shell is remarkably sensitive to the specific electronic state, suggesting that solvent degrees of freedom can act as S1/S2 coupling modes.