Prototropism in aminophenols and anisidines: A reinvestigation

The absorption and fluorescence spectra obtained in various solvents indicate that aminophenols and anisidines act as proton donors in ether and acetonitrile but as proton acceptors in methanol and water in the S₀ state and as proton donors to all the solvents in the S₁ state. Stretched sigmoid curves are observed for the equilibrium between the monocation and the neutral species for all the compounds, except p-aminophenol (pAMP) and p-anisidine, giving both ground state and excited state pK_a values. For pAMP and p-anisidine, proton-induced fluorescence quenching of the neutral species is observed at moderate hydrogen ion concentrations. The values of the quenching constant are 3.3×10⁹ dm³ mol⁻¹ s⁻¹ and 2.0×10⁹ dm³ mol⁻¹ s⁻¹ for pAMP and p-anisidine respectively. The proton transfer reaction studied in methanol-water systems has indicated that, in low dielectric constant solvents, the excited state equilibrium is not established and thus the ground state pK_a value is observed from fluorimetric titration curves.