INTRAMOLECULAR PROTON-TRANSFER CYCLE OF 2,4-DIMETHOXY-6-(1-HYDROXY-2-NAPHTHYL)-S-TRIAZINE STUDIED BY LASER PHOTOLYSIS

The intramolecular proton-transfer cycle of 2,4-dimethoxy-6-( 1-hydrox y-2-naphthyl)-s-triazine (NTR) was studied by laser photolysis and flu orescence measurements with the aid of MO calculations. A moderately l arge Stokes-shifted fluorescence (Delta (v) over bar similar to 6400 c m(-1)) was observed for NTR in methylcyclohexane, which was attributab le to the fluorescence from proton-transferred keto tautomer. Transien t absorption measurements of NTR in various solvents showed the presen ce of a long-lived transient which was not quenched by dissolved oxyge n in the solution. Remarkable viscosity effects were found for the rel ative formation yield of the long-lived transient and both the fluores cence lifetime and quantum yield of the excited keto form, indicating the involvement of cis-trans isomerization after the excited-state int ramolecular proton transfer. The isomerization was also supported by t he MO calculations. The kinetics of the ground-state reverse proton tr ansfer of the trans-keto form was strongly affected by the presence of protic solvents, showing solvent-catalyzed recovery processes through mutual hydrogen exchange. (C) 1998 Elsevier Science B.V. All rights r eserved.