Photooxidation of carotenoids in mesoporous MCM-41, Ni-MCM-41 and Al-MCM-41 molecular sieves

Photooxidation of beta -carotene and canthaxanthin in mesoporous MCM-41, Ni-NICM-41, and Al-MCM-41 molecular sieves was studied by 9-220 GHz electron paramagnetic resonance (EPR) and 9 GHz electron nuclear double resonance (ENDOR). X-ray powder diffraction (XRD) measurements established that the MCM-41 pore size (33 Angstrom) was large enough to accommodate carotenoids. Mesoporous MCM-41 molecular sieves are found to be promising hosts for long-lived photoinduced charge-separation between carotenoid radical cations (Car(.+)) and the MCM-41 framework. Incorporating metal ions into siliceous MCM-41 enhances efficiency of carotenoid oxidation. The photoyield and stability of generated carotenoid radical cations increased in the order MCM < Ni-MCM < Al-MCM. Formation of carotenoid radical cations within the Me-MCM-41 is due to electron transfer between incorporated carotenoid molecules and metal ions, which act as electron acceptor sites. Detected EPR signals of Ni(I) species provide direct evidence for the reduction of Ni(II) ions by carotenoids. The presence of Ni(II) ions in Ni-MCM-41 was verified by 220 GHz EPR spectroscopy. ENDOR measurements revealed that the central C13-CH3 and C13 ' -CH3 groups of both carotenoids in Al-MCM-41 are rapidly rotating, while mobility of the C9-CH3 and C9 ' -CH3 groups is restricted. We propose that carotenoids are bound to the MCM-41 pore walls via the ends of the polyene chain in close proximity to the C9,9 ' -CH3 groups.