N-Oxidation of hydroxyquinolines leads to a dramatic increase in their excited-state acidity. Time-resolved and steady-state emission characterization of 6-hydroxyquinoline-N-oxide and 2-methyl-6-hydroxyquinoline-N-oxide reveals a rich but less complex proton-transfer behavior than that of its parent hydroxyquinoline. The electronic effect of the oxidized heterocyclic nitrogen atom makes the excited state both less basic and more acidic than the parent and adds hydroxyquinoline N-oxides to the class of high-acidity excited-state proton donors in photochemistry and photobiology. Adiabatic photoinduced proton transfer is accompanied by the efficient nonreversible deoxygenation and 1-2 oxygen migration.

As a continuation of our efforts in the synthesis and investigation of novel “super” photoacids, in this article we report on the effect of fluoroalkanesulfonyl groups on the photoacidity of 2-naphthol. These groups, known to be the strongest electron-withdrawing substituents, were expected to increase the photoacidity to a greater extent as compared to previously described cyano- and methanesulfonyl groups. Indeed, we have found that 6-perfluoromethylsulfonyl-2-naphthol (6F3) is more acidic in the ground state and noticeably more acidic in the excited state than is previously synthesized 6-cyano-2-naphthol. The unusually short fluorescence lifetimes of the naphthol and the conjugated anion, which are explained by effective resonance/intramolecular charge transfer, mask the extended photoacidity of 6F3. Photochemical investigations of 6-perfluorohexylsulfonyl-2-naphthol (6F13) in protic solvents are complicated by aggregation.