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

Abstract

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. 1H and 13C NMR data unequivocally prove a rapid prototropic equilibrium (> 102/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 pKa values increase, upon photoexcitation, by 6.8−9 units; thence, the excited-state energy level of the protonated form (S1‘) is calculated to be lower by 37−51 kJ/mol than that of the respective nonprotonated species (S1). 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.