PHOTOACTIVATED FERRIC-CHLORIDE OXIDATION OF CAROTENOIDS BY NEAR-UV TOVISIBLE-LIGHT

When canthaxanthin (I) and beta-carotene (II) dichloromethane solution s are treated with small amounts of ferric chloride (less than or equa l to 0.26 mol equiv), extensive photodegradation of the remaining unre acted neutral carotenoid occurs upon subsequent irradiation with near- UV to visible light. The rate of this photodegradation is independent of the neutral carotenoid concentration at a given initial FeCl3 conce ntration and first-order in initial FeCl3 (k(1) = 1.43 and 3.30 min(-1 ) for I and II, respectively). The data are consistent with a mechanis m in which Fe(II) is photochemically converted in the presence of CH2C l2 to Fe(III), which then oxidizes unreacted neutral carotenoids. We a lso report that canthaxanthin radical cations in dichloromethane form cation dehydrodimers at low temperature or upon irradiation with near- UV to visible light, which have an absorption maximum near 770 nm and a reduction peak at 20 mV in cyclic voltammetry. It is proposed that r adical cations associate with parent molecules and then undergo two su bsequent steps to form the final cation dehydrodimers. In the presence of supporting electrolytes, canthaxanthin radical cations form ion pa irs with the anions PF6-, resulting in a 10 nm blue shift of the absor ption maximum of the radical cations.