PHOTOINDUCED INTRAMOLECULAR PROTON-TRANSFER AS THE MECHANISM OF ULTRAVIOLET STABILIZERS - A REAPPRAISAL

Evidence based on theoretical calculations and photophysical experiments is presented to show that, contrary to general belief, the photostability of 2-(2′-hydroxy-5′-methylphenyl)benzotriazole (Tinuvin P) cannot be explained as being due to an excited-state intramolecular proton transfer (ESIPT) through the intramolecular hydrogen bond (IMHB). Support for this conclusion comes from a related study on several members of a new family of photostable compounds, namely the 1-(2′-hydroxyphenyl)pyrazoles, which were obtained by the reaction of pyrazole and of 3,5-dimethylpyrazole with benzoquinone. The structures of these pyrazole derivatives, namely 2,3-bis(3′,5′-dimethylpyrazol-1′-yl)-, 2,3-bis(pyrazol-1′-yl)-, 2-(pyrazol-1′-yl)-, and 2,5-bis(pyrazol-1′-yl)-1,4-dihydroxybenzene (1, 2, 3, and 4, respectively) were established by spectroscopic techniques and X-ray crystallography. Both derivatives 3 and 4 possess a strong intramolecular hydrogen bond and are reasonably photostable; derivatives 1 and 2 appear, however, to possess only a weak intramolecular hydrogen bond. 1 and 3 pack in helical systems, 4 does it with intercrossing stacking, and 2·H2O has a three-dimensional network involving water molecules. © 1990, American Chemical Society. All rights reserved.