Photoinduced coupled proton and electron transfers. 2. 7-hydroxyquinolinium ion

The hydroxyl group of 7-hydroxyquinoline in acidic media (i.e., of 7-hydroxyquinolinium cation, 7-HQ(C)) undergoes photoinduced deprotonation even in strongly acidic media. It occurs from dilute perchloric acid solutions up to 8 mol L-1 concentration, which is evidence of an outstanding photoacidity. Excited-state proton ejection was shown to be reversible at acid concentrations greater than 1 mol L-1. Below this value, the proton ejection was not balanced by the proton recombination. Concerning the rates of the excited-state reactions, (i) the proton ejection rate constants k(1) increased upon acid dilution, attaining at high dilution the limiting value of (k(1))(0) = 5.5 x 10(10) s(-1). k(1) was shown to vary with the water activity according to k(1) = (k(1))(0)(a(H2O))(4). The number 4 is then a key number for the ability of water to accept a proton from 7-HQ(C*). (ii) The proton recombination rate constant varies from 3 x 10(9) to 4 x 10(10) s(-1) when the acid concentration ranges from 1 to 5 mol L-1. This observation can tentatively be ascribed to a nondiffusional recombination process occurring with the ejected proton in concentrated acid solutions. The results as a whole are consistent with the coupling of an intramolecular electron transfer with the proton transfer, due to the withdrawing effect of the quinolinium function greater than or equal to NH+ on the -O- group generated by the deprotonation. The photoproduct is then most likely to be the ketonic tautomeric form of 7-hydroxyquinoline rather than a zwitterion. Thus, the excited-state behavior is fully accounted for by the photoinduced synergy of the two functional groups.