Absorption spectra of radical ions of polyenes of biological interest

Abstract

The radical cations and anions of a number of carotenoid pigments have been generated and their absorptions characterised using pulse radiolysis techniques. For all the radical ions studied there is a broad absorption peak at longer wavelengths than the first absorption band of the parent molecule. The wavelength maxima of the absorption bands are linearly dependent on the number of conjugated double bonds. For most polyenes the radical anion absorbs at a slightly shorter wavelength than the radical cation derived from the same parent molecule, although the differences in energy of excitation are quite small (∼10 kJ mol^–1). The radical ion absorptions are shifted hypsochromically on moving from hexane to methanol solutions. Theoretical calculations (by Huckel and PPP methods) are in broad agreement with the experimental findings.