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

The excited-state intramolecular proton transfer (ESIPT) reactions of 2,4-dimethoxy-6-(1-hydroxy-2-naphthyl)-s-triazine (NTR) and 2,4-dimethoxy-6-(1-hydroxy-2-naphthyl)-1,3-pyrimidine (NPR) were studied by laser photolyses and fluorescence measurements with the aid of MO calculations. Large Stokes-shifted fluorescences (Δν̄= 6400 and 7300 cm−1) originating from ESIPT were observed for NTR and NPR. Laser photolyses of NTR and NPR gave long-lived transients which could be attributed to their trans-keto tautomers (1Ktr) produced by cis–trans isomerization of the proton-transferred cis-keto form (1K*cis) in the S1 state. Temperature effects on the fluorescence lifetime and fluorescence quantum yield of 1K*cis and relative formation yield of 1Ktr showed that another temperature-dependent nonradiative deactivation process competing with the cis–trans isomerization was involved in the relaxation processes of 1K*cis. A lower apparent activation energy (12 kJ mol−1) for the total nonradiative deactivation rate 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 1K*cis of NTR and NPR at 293 K, indicating that the main deactivation pathway from 1K*cis was the cis–trans isomerization 1K*cis→ 1Ktr. The results of MO calculations on the ground and excited states of NTR and NPR supported a relaxation scheme including ESIPT followed by cis–trans isomerization.