Photochemistry of "super" photoacids. 3: Excited-state proton transfer from perfluroalkylsulfonyl-substitutued 2-naphthols

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.