Toward a reliable computatinal support to the spectroscopic characterization of excited state intramolecular proton transfer: [2,2'-bipyridine]-3,3'-diol as a test case.

Time-dependent density functional theory is applied to calculate vertical absorption and emission energies of different tautomers of [2,2'-bipyridine]-3,3'-diol both in vacuo and in solution. The relative stabilities of diketo and dienol tautomers predicted at MP2 and B3LYP levels are close and significantly different from the HF results. Solvent effects further stabilize the diketo form, which, in aqueous solution becomes just 3 kcal/mol less stable than its dienol counterpart. Time-dependent density functional theory absorption energies are in good agreement with experimental data, but different levels of geometry optimizations lead to significant differences. Also the Stokes shift of the fluorescence spectrum is correctly reproduced. These results show the potentialities of the combined density functional theory/polarizable continuum model approach for the study of structural, thermodynamic, and spectroscopic properties of large molecules in solution.