Effects of Electronic Structures on the Excited-State Intramolecular Proton Transfer of 1-Hydroxy-2-acetonaphthone and Related Compounds

Effects of electronic structures on the excited-state intramolecular proton transfer (ESIPT) of 1-hydroxy-2-acetonaphthone (1H2AN) and its related compounds [1-hydroxy-2-naphthaldehyde (1H2NA) and methyl 1-hydroxy-2-naphthoate (1H2MN)] were studied by means of the laser photolysis method, time-resolved thermal lensing technique, and fluorometry. Both 1H2AN and 1H2NA showed relatively large Stokes-shifted fluorescence (Δν̄ = 6120 and 6170 cm-1, respectively) and strongly temperature-dependent fluorescence observed usually for ESIPT systems. In contrast, the fluorescence properties of 1H2MN gave no indication of the occurrence of ESIPT. A 355 nm laser photolysis of 1H2AN or 1H2NA in cyclohexane produced a ground-state transient. The decay rate of the transient was markedly enhanced by addition of alcohols or triethylamine in nonpolar aprotic solvents. Furthermore, a bimolecular decay process probably due to mutual hydrogen exchange in a hydrogen-bonded complex was found for the transient from laser intensity dependence of the decay profiles. These observations could be reasonably explained by the occurrence of ESIPT in 1H2AN and 1H2NA to form a long-lived keto-tautomer. The distinct relaxation properties of excited 1H2AN and 1H2NA from that of 1H2MN were attributable to differences in relative stabilities between parent enol- and tautomeric keto-forms in the lowest excited singlet state, which are strongly affected by the electronic effects of the substituent on the carbonyl group.