Intramolecular proton-transfer cycle of 2,4-dimethoxy-6-(1-hydroxy-2-naphthyl)- s-triazine studied by laser photolysis

Abstract

The intramolecular proton-transfer cycle of 2,4-dimethoxy-6-(1-hydroxy-2-naphthyl)- s-triazine (NTR) was studied by laser photolysis and fluorescence measurements with the aid of MO calculations. A moderately large Stokes-shifted fluorescence ( Δ ν ̄ ≈6400 cm −1) was observed for NTR in methylcyclohexane, which was attributable to the fluorescence from proton-transferred keto tautomer. Transient absorption measurements of NTR in various solvents showed the presence of a long-lived transient which was not quenched by dissolved oxygen in the solution. Remarkable viscosity effects were found for the relative formation yield of the long-lived transient and both the fluorescence lifetime and quantum yield of the excited keto form, indicating the involvement of cis–trans isomerization after the excited-state intramolecular proton transfer. The isomerization was also supported by the MO calculations. The kinetics of the ground-state reverse proton transfer of the trans-keto form was strongly affected by the presence of protic solvents, showing solvent-catalyzed recovery processes through mutual hydrogen exchange.