Excited-state intramolecular proton transfer followed by cis-trans isomerization of (1-hydroxy-2-naphthyl)-s-triazine derivatives

The excited-state intramolecular proton transfer (ESIPT) reactions of 2,4-d imethoxy-6-(1-hydroxy-2-naphthyl)-s-triazine (NTR) and 2,4-dimethoxy-6-(1-h ydroxy-2-naphthyl)-1,3-pyrimidine (NPR) were studied by laser photolyses an d fluorescence measurements with the aid of MO calculations. Large Stokes-s hifted fluorescences (Delta<(nu)over bar> = 6400 and 7300 cm(-1)) originati ng from ESIPT were observed for NTR and NPR. Laser photolyses of NTR and NP R gave long-lived transients which could be attributed to their trans-keto tautomers (K-1(tr)) produced by cis-trans isomerization of the proton-trans ferred cis-keto form (K-1(cis)*) in the Si state. Temperature effects on th e fluorescence lifetime and fluorescence quantum yield of K-1(cis)* and rel ative formation yield of K-1(tr) showed that another temperature-dependent nonradiative deactivation process competing with the cis-trans isomerizatio n was involved in the relaxation processes of K-1(cis)*. A lower apparent a ctivation energy (12 kJ mol(-1)) for the total nonradiative deactivation ra te of NTR including cis-trans isomerization was obtained in comparison with that (18 kJ mol(-1)) of NPR. Remarkable viscosity effects were observed on the fluorescence lifetimes of K-1(cis)* of NTR and NPR at 293 K, indicatin g that the main deactivation pathway from K-1(cis)* was the cis-trans isome rization K-1(cis)* --> K-1(tr). The results of MO calculations on the groun d and excited states of NTR and NPR supported a relaxation scheme including ESIPT followed by cis-tuans isomerization. (C) 2000 Elsevier Science B.V. All rights reserved.