Photophysical Properties of 2,2′- and 4,4′-Biphenyldiols

The photophysical properties of 2,2′- and 4,4′-biphenyldiols were investigated in aqueous and various organic solvents using optical absorption, steady-state and time-resolved fluorescence measurements. The investigated properties were correlated with the prototropic equilibria between the neutral, monoanionic and dianionic forms of the biphenyldiols. The monoanionic form of 2,2′-biphenyldiol is found to be exceptionally stable and shows interesting solvatochromism in polar protic and aprotic solvents. In solvents having appreciable proton-accepting power, 2,2′-biphenyldiol undergoes deprotonation, causing the coexistence of both the neutral and monoanionic forms in solutions. In presence of a strong proton acceptor, e.g. triethylamine, 2,2′-biphenyldiol undergoes complete deprotonation in a polar solvent like acetonitrile. In a nonpolar solvent, like cyclohexane, however, 2,2′-biphenyldiol forms ground-state intermolecularly hydrogen bonded complex with triethylamine. The photophysical properties of 2,2′-biphenyldiol have been found to be drastically different from those of 4,4′-biphenyldiol. The differences are explained on the basis of the presence and absence of intramolecular hydrogen bonding in the two compounds.