Electrochemical, optical, photochemical, and chromatographic studies of carotenoid cation radicals and dications: Adsorption, polymerization, and isomerization properties.

摘要

Polymeric products, which are formed by reaction of the dications of 7E,{dollar}7spprime Z{dollar}-diphenyl-{dollar}7,7spprime{dollar}-diapocarotene (III) generated by electrochemical oxidation in dichloromethane with the neutral carotenoid, are adsorbed on various electrode surfaces. An apparent average molar mass of 5400 g/mol electron was calculated from simultaneous electrochemical quartz crystal microbalance (EQCM) measurements. The green, fiberlike structure observed by scanning electron and optical microscopy confirms the formation of polymers. X-ray microanalysis of the surface composed of an uneven, layered structure indicates that electrolyte counter anions {dollar}rm PFsb6sp-{dollar} are associated with the deposited material. Cathodic stripping voltammetry indicates that the film thickness ranges from 0.16 to 0.84 {dollar}mu{dollar}m as the charge increases from 10.0 to 51.1 {dollar}mu{dollar}C. Cation radicals of III show no adsorption behavior nor do the dications of carotenoids terminally substituted with one cyclohexene ring (V) or containing a triple bond at C15 (IV). Apparently, a diphenyl substituted carotenoid containing only double bonds in the backbone is required to observe this unusual behavior.; Electrochemical oxidation of all-trans canthaxanthin (II) and {dollar}beta{dollar}-carotene (I) 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-dicis isomers of the carotenoids by {dollar}rmsp1H{dollar}-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.; When canthaxanthin (II) and {dollar}beta{dollar}-carotene (I) dichloromethane solutions are treated with small amounts of ferric chloride {dollar}({lcub}le{rcub} 0.26{dollar} mol equiv.), extensive photodegradation of additional 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 {dollar}rm FeClsb3{dollar} concentration and first-order in initial {dollar}{lcub}rm FeClsb3{rcub} (ksb1 = 1.43{dollar} and 3.30 {dollar}rm minsp{lcub}-1{rcub}{dollar} for II and I, respectively). The data are consistent with a mechanism in which Fe(II) is photochemically converted in the presence of {dollar}rm CHsb2Clsb2{dollar} to Fe(III), which then oxidizes unreacted neutral carotenoids.; This dissertation also presents that canthaxanthin cation radicals 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 cation radicals associate with parent molecules and then undergo two subsequent steps to form the final cation dehydrodimers. In the presence of supporting electrolytes, canthaxanthin cation radicals form ion-pairs with the anions {dollar}rm PFsb6sp-,{dollar} resulting in a 10 nm blue-shift of the absorption maximum of the catio