TOWARD THE PHOTOSTABILITY MECHANISM OF INTRAMOLECULAR HYDROGEN-BOND SYSTEMS - THE PHOTOPHYSICS OF 1'-HYDROXY-2'-ACETONAPHTHONE

The photophysical behavior of 1'-hydroxy-2'-acetonaphthone in dry cyclohexane, methanol, and dimethyl sulfoxide was studied. The compound was found to exhibit a single fluorescence band (both at room temperature and at 77 K) that was scarcely dependent on the solvent used as well as an anomalously small Stokes shift (ca. 6000 cm-1) for an excited-state intramolecular proton-transfer mechanism and a fluorescence spectrum similar to the mirror image of its absorption spectrum. The compound was also found to exhibit a single fluorescence band both in gas phase at 368 K and in crystal phase at 298 K. Despite the fact that the compound seemingly undergoes a proton phototransfer, it is no more photostable than 2-(2'-hydroxy-5'-methylphenyl)benzotriazole itself, which is regarded as the archetype of photostability. This finding sheds some light on the mechanism whereby substances containing an intramolecular hydrogen bond acquire their photostability.