OPTICAL-ABSORPTION SPECTRA OF DICATIONS OF CAROTENOIDS

Quantitative optical absorption spectra of the cation radicals and the dications of canthaxanthin (I), beta-carotene (II), 7'-cyano-7'-ethox ycarbonyl-7'-apo-beta-carotene (III), and 7',7'-dimethyl-7'-apo-beta-c arotene (IV) in dichloromethane solution are reported. Exclusive forma tion of dications occurs when the carotenoids are oxidized with ferric chloride. Addition of neutral carotenoid to the dications results in equilibrium formation of cation radicals. Oxidation with iodine in dic hloromethane affords only cation radicals; electrochemical oxidation u nder suitable conditions yields both dications and cation radicals. Va lues of the optical parameters depend on the nature of the oxidative m edium. The oscillator strengths calculated for gas phase cation radica ls and dications of I-IV using the INDO/S method show the same trend a s the experimental values.