Irradiation-dependent equilibrium between open and closed form of UV absorbers of the 2-(2-hydroxyphenyl)-1,3,5-triazine type

2-(2-Hydroxyphenyl)-4,6-diaryl-1,3,5-triazines (HPTs) bearing one or more i ntramolecular hydrogen bonds (IMHBs) show phosphorescence in polar solvents at 77 K which increases in intensity with UV-irradiation time (lambda (irr ) = 333 nm) until an equilibrium value is reached (phosphorescence evolutio n). phosphorescence emission is produced from open conformers of the molecu les with intermolecular rather than intramolecular hydrogen bonds, which ar e formed in polar solvents under the influence of UV radiation. All IMHBs o f an individual molecule must be broken to enable trip]et state population. Reformation of the closed form is observed for HPTs after dark periods res ulting in a lower initial phosphorescence intensity upon renewed irradiatio n (relaxation). The methoxy derivatives (MPTs), where the OH groups are rep laced by methoxy groups, can be employed as spectroscopic models for the op en form: they emit phosphorescence independent of irradiation time. For the equilibrium between open and closed form of M-OH-P (2-(2-hydroxy-4-methoxy phenyl)-4,6-diphenyl-1,3,5-triazine) under constant irradiation (lambda (ir r) = 333 nm, 100 W-HBO lamp) a proportion of 5% of open form was evaluated.