Quantum-mechanical and spectral studies on the Thiazole Orange (TO) fluorophore: dimerisation and DNA intercalation

Despite the unquestionable importance of fluorescent dyes, theoretical studies, aimed to an in dept understanding of the photophysical characteristics of these molecules, are still limited. In all cases, at the best of our knowledge, the effects of different environments have never been considered in a detailed fashion. In this communication we present a combination of a Time-dependent Density Functional Theory (TDDFT) study of the absorbance and fluorescence emission characteristics of the cyanine thiazole orange (TO) free in solution and when intercalated in DNA with spectro-photometric and -fluorometric experiments under different conditions (temperature, concentration, solvent viscosity). Moreover, the DH and DS values for TO dimerisation are evaluated by T-jump experiments. The comparison between experimental and calculated photo-physical properties shows that (TD)DFT approaches when combined with spectroscopic measurements represent a valid strategy not only to reproduce the experimental solvatochromic shifts, induced by both dimerisation and intercalation in the DNA, but also to rationalize quenching/enhancing of fluorescence when changing the environment conditions.