Quantitative optical absorption spectra of the cation radicals and the dications of canthaxanthin (I), -carotene (II), 7′-cyano-7′-ethoxycarbonyl-7′-apo--carotene (III), and 7′,7′-dimethyl-7′-apo--carotene (IV) in dichloromethane solution are reported. Exclusive formation 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 dichloromethane affords only cation radicals; electrochemical oxidation under suitable conditions yields both dications and cation radicals. Values of the optical parameters depend on the nature of the oxidative medium. The oscillator strengths calculated for gas phase cation radicals and dications of I-IV using the INDO/S method show the same trend as the experimental values.

Electrochemical oxidation of all-trans-canthaxanthin and β-carotene in dichloromethane leads to significant trans-to-cis isomerization, with cis isomers accounting for about 40% of the products formed. The electrochemically generated isomers were separated by reverse-phase high-performance liquid chromatography and identified as 9-cis, 13-cis, 15-cis, and 9,13-di-cis isomers of the carotenoids by 1H-NMR spectroscopy and optical spectroscopy (Q ratio). The results of simultaneous bulk electrolysis and optical absorption spectroscopy indicate the following isomerization mechanism:  the all-trans cation radicals and/or dications formed by electrochemical oxidation of all-trans-carotenoids can easily undergo geometrical isomerization to form cis cation radicals and/or dications. The latter are converted by the comproportionation equilibrium to cation radicals which are then transformed to neutral cis-carotenoids by exchanging one electron with neutral carotenoids. AM1 molecular orbital calculations, which show that the energy barriers of configurational transformation from trans to cis are much lower in the cation radical and dication species than in the neutral molecule, strongly support the first step of this mechanism.

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