ULTRAVIOLET ABSORBERS OF THE 2-(2-HYDROXYARYL)-1,3,5-TRIAZINE CLASS AND THEIR METHOXY DERIVATIVES - FLUORESCENCE SPECTROSCOPY AND X-RAY STRUCTURE-ANALYSIS

A close to planar conformation is established for 2-(4-methoxyphenyl)- 4,6-diphenyl-1,3,5-triazine (M-H-P) by crystal structure determination . For hexoxy-2-methoxyphenyl)-4,6-dipheny-1,3,5-triazine (H-MeO-P), hy drophobic interaction between the long hexoxy groups enforces rotation of the dialkoxyphenyl ring at C6 and the C2 phenyl ring relative to t he plane of the triazine ring. Time-resolved fluorescence measurements demonstrate the existence of a short-lived Franck-Condon (FC) and a l onger-lived TICT (twisted intramolecular charge transfer) state, emitt ing at longer wavelengths, for the O-methyl derivative xy-2-methoxyphe nyl)-3,6-bis(2,4,6-trimethylphenyl) -1,3,5-triazine (H-MeO-Ms). The de cay of the proton-transferred (PT) fluorescence for crystalline powder s of hydroxyphenyl)-4-(dimethylamino)-6-1,3,5-triazines is shown, by t he same method, to be retarded within the series phenyl --> tolyI --> xylyl, well in line with the gradation of quantum yields determined by steady-state methods. The high quantum yields of the PT fluorescence for triazines with only one 2-hydroxyaryl moiety on the triazine ring are reduced by 2 orders of magnitude each upon introduction of a secon d and a third aryl group. Most of the 2-hydroxyaryl-1,3,5 triazines wi th additional aryl moieties at C4 and C6 show PT fluorescence, as long as the 2-hydroxyaryl and the triazine ring are more or less coplanar and the intramolecular hydrogen bond remains intact. In some cases, ho wever, this fluorescence is very weak, or can be detected only in copo lymers, in mixtures of the stabilizer with the polymer or in crystalli ne powders.