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 cycl ohexane, methanol, and dimethyl sulfoxide was studied. The compound wa s found to exhibit a single fluorescence band (both at room temperatur e 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-s tate intramolecular proton-transfer mechanism and a fluorescence spect rum similar to the mirror image of its absorption spectrum. The compou nd was also found to exhibit a single fluorescence band both in gas ph ase 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 pho tostable than 2-(2'-hydroxy-5'-methylphenyl)benzotriazole itself, whic h is regarded as the archetype of photostability. This finding sheds s ome light on the mechanism whereby substances containing an intramolec ular hydrogen bond acquire their photostability.