GROUND-STATE TRIPLE PROTON-TRANSFER IN 7-HYDROXYQUINOLINE .4. OBSERVATION IN ROOM-TEMPERATURE METHANOL AND AQUEOUS-SOLUTIONS

The stabilization of the ground-state keto tautomer of 7-hydroxyquinoline (7HQ) has been observed in room-temperature methanol and aqueous solutions. Concentration studies have shown that the 7HQ cyclic dimer is formed at high 7HQ concentrations in methanol and water, and either the 7HQ-(MeOH)(2) or the 7HQ-water complex, presumably the cyclic 7HQ-(H2O)(2) structure, is formed at low 7HQ concentrations in methanol or water solvents, respectively. These species give rise to the keto-tautomer excitation band centered at 398 nm (7HQ dimer in water), 420 nm (7HQ dimer in methanol), 377 nm (7HQ-water complex), and 387 nm (7HQ-methanol complex). The emission band for all species appears at 520 nm. This is the first report of the ground-state stabilized keto tautomer spectrum in methanol at room temperature. The spectrum in water has been investigated in the past, but the interpretation differs from that of this work.