A Density Functional Theory (DFT) study of the absorbance and fluorescence emission characteristics of the cyanine thiazole orange (TO) in solution and when intercalated in DNA was carried out in combination with spectrophotometric and spectrofluorometric experiments under different conditions (temperature, concentration, solvent viscosity). T-jump relaxation kinetics of the TO monomer-dimer conversion enabled the thermodynamic parameters of this process to be evaluated. The overall data collected provided information on the features of the "light-switch" by the fluorescent TO and the comparison between experimental and calculated photo-physical properties allowed us to explain and rationalize both shifts and quenching/enhancing effects on fluorescence due to solvation, dimerisation and intercalation in the DNA.
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