More light on the low energy electronic spectra of hydroxy azo pyrazolopyridine dyes

The visible electronic spectra of two hydroxy azo pyrazolopyridine derivati ves, namely 3[(2,4-dihydroxyphenyl)azo]-4,6-dimethylpyrazolo[3,4-b]pyridine (DHAPP) and 3[(2-hydroxynaphthyl)azo]-4,6-dimethylpyrazolo[3,4-b]pyridine (HNAPP) in a number of organic solvents of different characteristics have b een examined. It was concluded that the dye exists in solutions of basic so lvetns (based on their autoprotolysis constant, pK(s)) in an azo double lef t right arrow hydrazone tautomeric equilibrium, while in acetone, acetonitr ile and CCl4 It exists mainly in the azo form. In DMF solution the dye exhi bits an acid-base equilibrium of the type HA + S double left right arrow A( -) + HS+. The proportional concentration of the conjugate base form A(-) is largely dependent upon the concentration of the dye, the basicity of the s olvent and the tendency of the ionised form to be stabilized by H-bonding i nteraction with the solvent molecules. The bands displayed by the different forms of the compounds have been assigned to the possible electronic trans itions. The effect of temperature on the longer wavelength visible bands ha ve been also investigated.