EPR and ENDOR studies of carotenoid-solid Lewis acid interactions

EPR and ENDOR results provide strong support for the formation of carotenoid radical cations on activated alumina and silica-alumina as a consequence of electron transfer from the adsorbed carotenoid molecules to the Lewis acid sites on these surfaces. This assignment is confirmed by the correlation between the Al-111 Lewis acid site content on the activated surfaces and the maximum concentration of the generated carotenoid radical cations. The results are compared with those obtained for a solution of carotenoid and AlCl3, which serves as a model system. The ENDOR line at ca. 6.5 MHz obtained for the carotenoids adsorbed on solid supports and carotenoid-AlCl3 solution, is attributed to the high-frequency feature of a hyperfine doublet, centred about the Al-27 Larmor frequency. ENDOR detection of the hyperfine doublet, instead of the single line at the Larmor frequency, indicates the formation of strong complexes between carotenoid molecules and Lewis acid sites on the surface.