SEMICONDUCTOR PHOTOCATALYSIS - PHOTODEGRADATION AND TRANS-CIS PHOTOISOMERIZATION OF CAROTENOIDS

In the presence of semiconductor CdS or ZnO particles, irradiation (>3 50 nm) of all-trans-beta-carotene (II) in dichloromethane leads to rap id degradation of the carotenoid, which is relatively stable in the ab sence of the semiconductors. Canthaxanthin (I), however, undergoes sig nificant photocatalyzed degradation only on ZnO, not on CdS. High-perf ormance liquid chromatographic studies indicate that CdS catalyzes tra ns-cis photoisomerization of both I and II. As in the photoisomerizati on in the absence of semiconductor, the major cis isomers have the 9-c is and 13-cis configuration, but, under otherwise the same condition, the ratio of cis/trans isomers has doubled. In contrast to CdS, ZnO do es not catalyze the photoisomerization of either I or II, although it enhances their rate of degradation. A photoisomerization mechanism inv olving carotenoid radicals formed by reaction with interstitial sulfur on the CdS surface is proposed.