ULTRAVIOLET STABILIZERS OF THE 2-(2'-HYDROXYPHENYL)-1,3,5-TRIAZINE CLASS - STRUCTURAL AND SPECTROSCOPIC CHARACTERIZATION

X-ray crystal structure determination of 2,4-diphenyl-6-(2'-hydroxy-4'-methoxyphenyl)-1,3,5-triazine (M-OH-P, Chart 2) reveals a stronger intramolecular hydrogen bond than that in 2-(2'-hydroxy-5'-methylphenyl)benzotriazole (Tinuvin P). The O-H distance appears elongated in M-OH-P (0.898 Angstrom), and the H...N distance shortened (1.733 Angstrom), with a definely more linear hydrogen bond bridge (angle O-H...N = 159.6 degrees). IR and H-1 NMR data confirm this finding (delta(OH...N) = 13.5 ppm). Both H-1 and C-13 NMR spectra demonstrate the OH bridge to switch from N1 to N5 within the NMR scale, via rotation around the resorcinyl-triazine bond. Derivatives with one aryl group only, e.g. 2-(2'-hydroxyphenyl)-4,6-dimethoxy-1,3,5-triazine (DMH, Chart 1), exhibit a proton-transferred fluorescence with both large Stokes shift (9890 cm(-1)) and high quantum yield (eta(F)' = 0.24, lambda(max) = 510 nm, 77 K, methylcyclohexane/2-methylbutane). Introduction of another aryl substituent into the triazine system weakens the proton-transferred fluorescence (eta(F)' approximate to 9 X 10(-4) for 2-(2'-hydroxyphenyl)-4-(dimethylamino)-6-(4''-methylphenyl)-1,3,5-triazine (TN, Chart 1), 77 K). For compounds with three aryl groups, this fluorescence is quenched completely. M-OH-P and 2-(2',4-dimethoxyphenyl)-4,6-diphenyl-1,3,5-triazine (M-MeO-P, Chart 2; with the bridging hydrogen substituted by a methyl group) display phosphorescence behavior (Table 3) analogous to that 2,4,6-triphenyl-1,3,5-triazine (TPT, Chart 2). Additionally, a new fluorescence with an exceptionally large and strongly solvent-dependent Stokes shift is observed when the hydrogen of the intramolecular hydrogen bond is replaced by CH3 (e.g. 2,4-diphenyl-6-(4'-(dodecyloxy)-2'-methoxyphenyl)-1,3,5-triazine (D-MeO-P, Chart 2; Delta nu(Stokes) = 9340 cm(-1), eta(F) approximate to 2.5 x 10(-2), AcN, 293 K). For the six OCH3 derivatives included in this study, the change of dipole moment Delta mu upon excitation has been determined from Lippert-Mataga plots where (nu(abs) - nu(flu)) is plotted vs the solvent orientation parameter Delta f. Time-resolved emission and viscosity-dependent fluorescence data may be interpreted in terms of a twist between the subunits of D-MeO-P upon excitation.