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 unti l an equilibrium value is reached (phosphorescence evolution). TIN P phosph oresces even at the very beginning of irradiation, in contrast to the HPTs, such as M-OH-P (2-(2-hydroxy-4-methsxyphenyl)-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 (M PTs) of some HPTs, where the H atom of the intramolecular hydrogen bond (IM HB) is replaced by a methyl group, produce intense plzosphorescence indepen dent of irradiation time. Considerable relaxation is found for HPTs after d ark periods less than or equal to 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 only at e levated temperatures. Phosphorescence evolution is due to open conformers o f the molecules, i.e., with intermolecular rather than intramolecular hydro gen bonds, which are formed in polar solvents under the influence of UV rad iation. Relaxation, i.e., re-formation of the IMHB of open-form molecules, is faster for the investigated HPTs than for TIN P.