Fluorescent properties of thiochrome in solvents of different polarity

Spectral and fluorescent properties of thiochrome in solvents of different polarity were studied. It was found that the pKa value of the transition between the cationic and neutral forms of thiochrome in aqueous solutions increased from ∼5.5 to 9.7 upon photoexcitation. It is supposed that protonation takes place in the excited state of the molecule resulting in fluorescence quenching of the thiochrome neutral form in aqueous solutions at neutral pH values. The fluorescence quantum yield of thiochrome increased by ∼2.2 times upon the transition from aqueous solutions to alcohols or polar aprotic solvents. It was found that an increase of the solvent polarity led to an increase in the Stokes shift from 3200 to 4200 cm-1 for the thiochrome neutral form emission. The change in the dipole moment upon excitation into the S1-state was estimated to be less than 3D. (authors)