Isotope and substituent effects on the intramolecular proton transfer in the excited state of 6-(2-hydroxy-5-methylphenyl)-s-triazines

Isotope and substituent effects on the intramolecular proton transfer in the excited singlet state of the title compounds have been studied by means of picosecond and nanosecond time-resolved spectroscopy. The absolute rate constants k/sub PT/ for proton transfer were determined from the build up curve of the S/sub n/' ..-->.. S/sub 1/' absorption to be 1.2 x 10/sup 10/ s/sup -1/ (for (ON)/sub h/ and (ON)/sub d/) and 1.8 x 10/sup 10/ s/sup -1/ for (NN)/sub h/ at 298 K. No isotope effect on proton transfer was observed, indicating that the intramolecular proton transfer in the excited state does not proceed via quantum mechanical tunneling but via a radiationless transition S/sub 1/ ..-->.. S/sub 1/' having no potential barrier. Substitution of electron-donating groups into the s-triazinyl moiety favors proton transfer in the excited state.