Ultraviolet stabilizers of the 2-(hydroxyphenyl)benzotriazole class: influence of substituents on structure and spectra

X-ray crystal structure determinations are reported for 2-(3'-tert-butyl-2-hydroxy-5',6'-dimethylphenyl)benzotriazole (HM-TIN), 2-(3'-tert-butyl-2'-hydroxy-5'-methylphenyl)benzotriazole (HTBM-TIN), and 2-(2',4'-dihydroxy-6'-methylphenyl)benzotriazole (HH-TIN). In contrast to 2-(2'-hydroxy-5'-methyl)benzotriazole (R = H; trade name Tinuvin P (TIN P)) and HTBM-TIN (dihedral angle 5.1-degrees) there is no planar arrangement of the bonded benzotriazole and the substituted phenol rings within one molecule in the neat crystal (dihedral angles: HM-TIN, 89.3-degrees; HH-TIN, 59.8-degrees (crystallized at 293 K) and 89.9-degrees (crystallized at 277 K)). There is a strong influence of crystallization temperature on crystal packing for HH-TIN. UV absorption spectra of HM-TIN and HH-TIN in unpolar fluid solvents suggest the existence of a weak intramolecular hydrogen bond. Methyl and/or tert-butyl substituents in the 3' and/or 5' position and particularly a CH3O substituent in the 5 position of the benzotrialzole ring strengthen the intramolecular hydrogen bond as revealed by IR spectroscopy of the solids and LN absorption spectroscopy in solvents of different polarity. MOHTM-TIN (2-(3'-tert-butyl-2'-hydroxy-5'-methylphenyl)-5-methoxybenzotriazole), whose intramolecular hydrogen bond is stronger than that of TIN P, provides a higher yield of the proton-transferred fluorescence and-in contrast to TIN P-no phosphorescence in EPA. Time-resolved measurements of BHDTB-TIN (2-(3',5'-di-tert-butyl-2'-hydroxyphenyl)-5-(alpha,alpha'-dimethylbenzyl)benzotriazole), DHDTB-TIN (2-(3',5'-di-tert-butyl-2',6'-dihydroxyphenyl)benzotriazole), and MOHTM-TIN at 293 K show red fluorescence in hexane (lambda(em) is similar to 700 nM, tau'(F) < 5 p.3), while in DMSO blue (lambda(em) is similar to 420 nm, tau(F) is similar to 1 ns) and red emissions are observed.