Light-Induced Opening of the Intramolecular Hydrogen Bond of UV Absorbers of the 2-(2-Hydroxyphenyl)-1,3,5-triazine and the 2-(2-Hydroxyphenyl)benzotriazole Type

A number of 2-(2-hydroxyphenyl)-4,6-diaryl-1,3,5-triazines (HPTs) and TIN P (2-(2-hydroxy-5-methylphenyl)benzotriazole) show phosphorescence in polar solvents at 77 K which increases in intensity with UV-irradiation time until an equilibrium value is reached (phosphorescence evolution). TIN P phosphoresces even at the very beginning of irradiation, in contrast to the HPTs, such as M-OH-P (2-(2-hydroxy-4-methoxyphenyl)-4,6-diphenyl-1,3,5-triazine), which exhibit no such initial phosphorescence provided that they were not recently exposed to UV radiation. The corresponding methoxy derivatives (MPTs) of some HPTs, where the H atom of the intramolecular hydrogen bond (IMHB) is replaced by a methyl group, produce intense phosphorescence independent of irradiation time. Considerable relaxation is found for HPTs after dark periods ≤ 1 h at 77 K resulting in a significantly lower initial phosphorescence intensity upon renewed irradiation. TIN P, in contrast, shows much slower relaxation which becomes significant ...