Photochemical and Photophysical Deactivation of 2,4,6-Triaryl-1,3,5-triazines

Both UV absorption and fluorescence maxima of 2-(2-methoxyaryl)-1,3,5-triazines show a marked bathochromic shift with increasing proton concentration. Well-defined isosbestic points establish an equilibrium between protonated and nonprotonated species for the ground state. 1 H and 13 C NMR data unequivocally prove a rapid prototropic equilibrium (> 10 2 /s) between tautomers protonated at N-1, N-3, and N-5, respectively. The NMR data also show a substantial increase in charge transfer, upon protonation, from the phenyl, and even more from the alkoxy-substituted aryl rings into the triazine system already for the ground state. At higher proton concentrations, the twisted intramolecular charge transfer (TICT) fluorescence of the nonprotonated (2-methoxyaryl) triazines is gradually replaced by the much weaker fluorescence of the protonated species, which is shifted to still longer wavelengths. Because the electron-accepting capacity of triazines is enhanced in the excited state, their p K a values increase, upon photoexcitation, by 6.8−9 units; thence, the excited-state energy level of the protonated form (S 1 ‘) is calculated to be lower by 37−51 kJ/mol than that of the respective nonprotonated species (S 1 ). Protonation thus leads to static quenching of the fluorescence. Halide ions, in contrast, can act as external electron donors toward triazines only in their highly electron-affine excited state, and so effect merely dynamic fluorescence quenching, with a corresponding reduction in fluorescence quantum yield, for the (2-methoxyaryl) triazines. Photochemical stabilization by protonation, therefore, is more efficient than by electron transfer. For all systems investigated, the excited-state electron transfer is exergonic and hence may be considered as diffusion-controlled, in accordance with the Rehm−Weller equation.