Electrochemical, EPR and AM1 studies of acetylenic and ethylenic carotenoids

Abstract

It has been shown, by cyclic voltammetry (CV) and simultaneous electrochemical and electron paramagnetic resonance (SEEPR) measurements of β-carotene (I) and 15,15′-didehydro-β-carotene (II) in dichloromethane solution, that the central triple bond increases the oxidation potentials by 237 ± 5 mV and decreases the solution EPR peak-to-peak linewidth (ΔHpp) by 1.7 G. Two synthetic carotenoids, 7,7′-diphenyl-7,7′-diapocarotene (III) and 7,7′-diphenyl-15,15′-didehydro-7,7′-diapocarotene (IV), close analogues of I and II, were also studied by CV and SEEPR methods. In contrast to compound I, two separate oxidation waves are observed for III, a feature previously found only for carotenoids containing electron-withdrawing substituents. The increase in oxidation potential of IV compared to III was similar to that of IIvs.I, but the decrease in ΔHpp(2.7 G) was considerably larger. Because the radical cation of III is adsorbed strongly on the platinum electrode in dichloromethane, complete analysis using the DigiSim® program was not possible; however, the oxidation potentials could be estimated. Structural parameters of neutral I–IV and their dications were calculated using the AM1 method, and the bond lengths and bond angles of I and II are in good agreement with crystallographic data. The magnitude of the EPR parameters, ΔHpp and the g factors, for the carotenoid radical cations of I–IV indicate a polyene π-radical cation structure with little unpaired electron density occurring near the terminal groups. The radical cation and dication deprotonate by loss of a proton from the chain methyl groups, with the former forming a polyene π-radical.