Effects of polyene chain length and acceptor substituents on the stability of carotenoid radical cations

The stability of radical cations of three series of carotenoids substituted with terminal ester, aldehyde, and cyano groups and with different numbers of backbone double bonds was studied by electrochemical and optical methods. The ethyl esters are 8'-apo-beta-caroten-8'-oate (I), 6'-apo-beta-caroten-6'-oate (II), and 4'-apo-beta-caroten-4'-oate (III); the aldehydes are 8'-apo-beta-caroten-8'-al (IV), 6'-apo-beta-caroten-6'-al (V), and 6'-apo-beta-carton-4'-al (VI); and the cyano compounds are 8'-apo-beta-caroten-8'-nitrile (VII), 6'-apo-beta-caroten-6'-nitrile (VIII), and 4'-apo-beta-caroten-4'-nitrile (IX). Cyclic voltammetry (CV) and Osteryoung Square wave voltammetry (OSWV) results indicate that the stability of carotenoid radical cations depends on the number of conjugated chain double bonds. For the esters, the longer the olefin chain, the more unstable the radical cations. In contrast, for the aldehydes and the nitriles, the stability of the radical cations is similar or varies slightly with backbone chain length. The half-lives, determined by stop-flow, and the decay rate of optical absorption of the radical cations generated by reaction with ferric chloride are: I, 202; II, 125; III, 2.35; IV, 149; V, 167; VI, 257; VII, 227; VIII, 158; and IX, 133 (s). AM1 molecular orbital calculations predict a large decrease in the dipole moments between radical cations and the neutral aldehydes and nitriles, but an increase for the esters as the number of chain double bonds increases. Radical cations with larger dipole moments have shorter lifetimes. It is likely that stronger interactions of the radical cation dipoles with the solvent dipoles result in enhanced decay of the radical cations. For esters, aldehydes, and nitriles, the shorter the olefin chain, the more difficult is the oxidation. The UV-vis optical absorption spectra of the carotenoids containing aldehyde groups in solvents of different polarity exhibit intramolecular charge transfer (ICT) phenomena, and their optical spectra are sensitive to the polarity of the solvents. In contrast, the esters do not show this behavior.