New ultraviolet stabilizers: 3- and 5-(2′-hydroxyphenyl)pyrazoles

Article abstract of DOI:10.1021/ja00039a014

A new class of ultraviolet stabilizers, the C-(2?-hydroxyphenyl)pyrazoles, is described. The combined use of X-ray crystallography [3(5)-(2′-hydroxyphenyl)pyrazole (5), l-methyl-3-(2′-hydroxyphenyl)pyrazole (7), and 1-methyl-5-(2?-hydroxyphenyl)pyrazole (8)], NMR (¹ and ¹³C), and UV spectroscopies allows the determination of the major tautomers, the coplanarity of both rings if present, and the existence of hydrogen bonds. Compounds 5 and 7, in which there is an intramolecular hydrogen bond (IMHB), do not fluoresce in cyclohexane. Solvent and temperature experiments prove that for these compounds in cyclohexane proton transfer took place in the excited singlet state but not in the triplet state (phosphorescence) and that the latter one is of higher energy than the former. The fact that the absorptions of planar 1-methyl-3-(2′-methoxyphenyl)pyrazole (9) and nonplanar 1-methyl-5-(2′-methoxyphenyl)pyrazole (10) are quite different whereas their emissions are very similar suggests that 10 might behave as a laser dye in the UV region (from an excited planar form to a ground state nonplanar form). Finally, the photostability of (2'-hydroxyphenyl)- and (2′-methoxyphenyl)pyrazoles was determined, compound 7 being even more stable than Tinuvin P.