Photoactivated ferric chloride oxidation of carotenoids by near-UV to visible light

When canthaxanthin (I) and beta-carotene (II) dichloromethane solutions are treated with small amounts of ferric chloride (less than or equal to 0.26 mol equiv), extensive photodegradation of the remaining unreacted 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 concentration 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 mechanism in which Fe(II) is photochemically converted in the presence of CH2Cl2 to Fe(III), which then oxidizes unreacted neutral carotenoids. We also 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 radical cations associate with parent molecules and then undergo two subsequent steps to form the final cation dehydrodimers. In the presence of supporting electrolytes, canthaxanthin radical cations form ion pairs with the anions PF6-, resulting in a 10 nm blue shift of the absorption maximum of the radical cations.