Photophysical behaviour of 2,6-diaminoanthraquinone in different solvents and at various pH

The absorption and fluorescence spectral properties of 2,6-diaminoanthraquinone (DAAQ) have been investigated in a series of organic solvents of different polarity and in aqueous solutions with Ho/pH/H_ in the range −10 to 17. The Stokes shift of DAAQ is correlated with various polarity scales. The study reveals the fluorescence of DAAQ dication is red shifted on protonation. The abnormal fluorescence of DAAQ dication is found to be due to large solvent relaxation in polar medium. The acidity constants for various prototropic reactions in So and S1 states are determined and discussed.