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

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2-(2-Hydroxyphenyl)-4,6-diaryl-1,3,5-triazines (HPTs) bearing one or more intramolecular hydrogen bonds (IMHBs) show phosphorescence in polar solvents at 77 K which increases in intensity with UV-irradiation time (λirr = 333 nm) until an equilibrium value is reached (phosphorescence evolution). Phosphorescence emission is produced from open conformers of the molecules with intermolecular rather than intramolecular hydrogen bonds, which are formed in polar solvents under the influence of UV radiation. All IMHBs of an individual molecule must be broken to enable triplet state population. Reformation of the closed form is observed for HPTs after dark periods resulting in a lower initial phosphorescence intensity upon renewed irradiation (relaxation). The methoxy derivatives (MPTs), where the OH groups are replaced by methoxy groups, can be employed as spectroscopic models for the open form; they emit phosphorescence independent of irradiation time. For the equilibrium between open and closed form of M-OH-P (2-(2-hydroxy-4-methoxyphenyl)-4,6-diphenyl-1,3,5-triazine) under constant irradiation (λirr = 333 nm, 100 W-HBO lamp) a proportion of 5% of open form was evaluated.

Affiliations: 1: Institut für Physikalische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany; 2: Ciba Specialty Chemicals Inc., Rosental R 1059, CH-4002 Basle, Switzerland


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