UV protection and singlet-oxygen quenching activity of intramolecularly hydrogen-bonded hydroxyanthraquinone derivatives found in aloe

The UV protection and singlet-oxygen (1O2) quenching activity of intramolecularly hydrogen-bonded hydroxyanthraquinone derivatives found in aloe have been studied by means of laser spectroscopy. The UV protective activity provided by excited-state intramolecular proton-transfer (ESIPT) in these molecules correlates with their 1O2 quenching activity, and the UV protective molecules have high 1O2-quenching function. The reason for this correlation can be understood by considering ESIPT-induced distortion of ground-state potential surfaces in encounter complexes with 1O2.