Photoinduced Coupled Proton and Electron Transfers. 2. 7-Hydroxyquinolinium Ion

Abstract

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 k1 increased upon acid dilution, attaining at high dilution the limiting value of (k1)0 = 5.5 × 1010 s-1. k1 was shown to vary with the water activity according to k1 = (k1)0(aH2O)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 × 109 to 4 × 1010 s-1 when the acid concentration ranges from 1 to 5 mol L-1. This observation can tenta...