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

The enol-keto proton-transfer tautomerism of 7-hydroxyquinoline (7HQ) media ted 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 abso rption, emission, and theoretical approaches. Specific hydrogen-bonding sit es in the complex were determined by applying various derivatives of 7HQ in corporated with guest molecules possessing only either a proton-donating or -accepting site. The result can be qualitatively rationalized by a correla tion of the hydrogen-bonding strength with respect to the donor's acidity a nd/or acceptor's basicity. In benzene, the 7HQ cyclic dimer undergoes a fas t excited-state double proton-transfer reaction, resulting in a unique keto -tautomer emission. Surprisingly, however, the 1:1 7HQ(enol)/acetic acid co mplex possessing only a single hydrogen bond undergoes an excited-state dou ble proton-transfer reaction, forming a keto/acid complex. As a result, a p roton-transfer mechanism incorporating the rotational diffusion dynamics of guest molecules, i.e., acetic acid, is proposed.