Proton-transfer tautomerism of 7-hydroxyquinolines mediated by hydrogen-bonded complexes

The enol-keto proton-transfer tautomerism of 7-hydroxyquinoline (7HQ) mediated by hydrogen-bond formation has been studied in the ground as well as in the excited state. Thermodynamics of self-association and hydrogen-bonded complexes of 7HQ in various nonpolar solvents was obtained by means of absorption, emission, and theoretical approaches. Specific hydrogen-bonding sites in the complex were determined by applying various derivatives of 7HQ incorporated with guest molecules possessing only either a proton-donating or -accepting site. The result can be qualitatively rationalized by a correlation of the hydrogen-bonding strength with respect to the donor's acidity and/or acceptor's basicity. In benzene, the 7HQ cyclic dimer undergoes a fast excited-state double proton-transfer reaction, resulting in a unique keto-tautomer emission. Surprisingly, however, the 1:1 7HQ(enol)/acetic acid complex possessing only a single hydrogen bond undergoes an excited-state double proton-transfer reaction, forming a keto/acid complex. As a result, a proton-transfer mechanism incorporating the rotational diffusion dynamics of guest molecules, i.e., acetic acid, is proposed.