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 p roton transfer (ESIPT) of 1-hydroxy-2-acetonaphthone (1H2AN) and its r elated compounds [1-hydroxy-2-naphthaldehyde (1H2NA) and methyl 1-hydr oxy-2-naphthoate (1H2MN)] were studied by means of the laser photolysi s method, time-resolved thermal lensing technique, and fluorometry. Bo th 1H2AN and 1H2NA showed relatively large Stokes-shifted fluorescence (Delta<(nu)over bar> = 6120 and 6170 cm(-1), respectively) and strong ly temperature-dependent fluorescence observed usually for ESIPT syste ms. In contrast, the fluorescence properties of 1H2MN gave no indicati on of the occurrence of ESIPT. A 355 nm laser photolysis of 1H2AN or 1 H2NA in cyclohexane produced a ground-state transient. The decay rate of the transient was markedly enhanced by addition of alcohols or trie thylamine in nonpolar aprotic solvents. Furthermore, a bimolecular dec ay process probably due to mutual hydrogen exchange in a hydrogen-bond ed complex was found for the transient from laser intensity dependence of the decay profiles. These observations could be reasonably explain ed 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 low est excited singlet state, which are strongly affected by the electron ic effects of the substituent on the carbonyl group.