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 (ΔH_pp) 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 ΔH_pp(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, ΔH_pp 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.