Characterization of Fe-MCM-41 Molecular Sieves with Incorporated Carotenoids by Multifrequency Electron Paramagnetic Resonance Tatyana A. Konovalova, Yunlong Gao, and Lowell D. Kispert* Department of Chemistry, Box 870336, UniVersity of Alabama, Tuscaloosa, Alabama 35487 Johan van Tol and Louis-Claude Brunel Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Florida State UniVersity, Tallahassee, Florida 32310 ReceiVed: July 8, 2002; In Final Form: NoVember 25, 2002 Multifrequency electron paramagnetic resonance (EPR) spectroscopy was applied to study Fe(III)-MCM-41 mesoporous molecular sieves with incorporated carotenoids. It was demonstrated that high-frequency/high- field EPR is a promising technique to increase spectral resolution for proper assignment of different Fe 3+ sites, which cannot be resolved by the X-band experiments. The broad unresolved at 9 GHz EPR line in the g ) 2 region is due to overlapping signals from Fe 3+ sites with different zero field parameters. The peak with g ) 2.45 is tentatively assigned to aggregated Fe 3+ . The signal with g ) 2.07 can be attributed to Fe 3+ coordinated to oxygen atoms on the surface of the pore. A narrow line with g x ) g y ) 2.003, g z ) 1.99, and E/D ) 0.3 was attributed to a single Fe 3+ site. The X-band and 94 GHz EPR measurements indicated that extraframework iron species at the surface of the mesopores are mostly responsible for carotenoid oxidation in molecular sieves. ENDOR measurements revealed the orientation of 7′-apo-7′,7′-dicyano--carotene and canthaxanthin within Fe-MCM-41. Introduction Incorporation of transition metal ions into the framework of molecular sieves has received considerable attention over the past few years due to the new catalytic properties of the modified materials. Although unique catalytic activities of iron-containing zeolites have been widely discussed, 1-3 catalytic properties of Fe-modified MCM-41 molecular sieves are poorly characterized. MCM-41 materials belong to the family of mesoporous silicas exhibiting a hexagonal arrangement of pores with diameters from 15 to 100 Å. 4,5 Large pore sizes of MCM-41 permit reactions involving bulky molecules that are not capable of entering the channels of microporous zeolites. Incorporating metal ions into siliceous MCM-41 enhances electron-transfer efficiency between embedded molecules and the MCM-41 framework. 6-8 In this work we report oxidation of carotenoids embedded into Fe(III)-MCM-41 molecular sieves. Carotenoids are natu- rally occurring polyenes with long chains of conjugated double bonds. Carotenoids along with chlorophylls and cytochromes participate in the electron-transfer pathway in photosystem II (PSII). 9,10 They play an essential role as intermediate electron carriers in the reduction of the primary electron donor P860 + by the Cyt b 559 and Chl Z . 11-14 The Fe atoms of the cytochromes undergo oxidation and reduction during this process, cycling between the ferrous (Fe 2+ ) and ferric (Fe 3+ ) oxidation states. Examining the electron-transfer reactions of carotenoids within iron-modified MCM-41 molecular sieves is important for understanding the electron-transfer reactions in PSII. Fe(III)- substituted MCM-41 sieves were also used because chemical oxidation of carotenoids by Fe 3+ ions in organic solvents forms the carotenoid radical cations. 15 It has been shown that electron paramagnetic resonance (EPR) spectroscopy is a useful technique for characterizing the iron sites in both the low-spin (S ) 1 / 2 ) and high-spin (S ) 5 / 2 ) electronic configurations. Usually in zeolites and molecular sieves the weak field of possible ligands (water, hydroxyl ions, framework oxygen) results in the high-spin ferric ion state that is also characteristic for some iron proteins. 16-18 The spin Hamiltonian for high-spin iron is given by 19,20 In this case we can neglect other terms because the symmetry is very close to cubic. The g tensor exhibits extremely small anisotropy and the spectral characteristics are determined by the zero field splitting (ZFS) parameters D (axial) and E (rhombic). When the symmetry is axial, D * 0 and E ) 0. In the case of rhombic symmetry, E/D ) 1/3. Most of high spin d 5 systems do not belong to one of the special cases. Several different symmetries at the Fe 3+ site contribute to multicom- ponent EPR spectra with overlapping signals. Such complex spectra arising from more than one center can be analyzed at different microwave frequencies. For high-spin Fe 3+ in proteins and zeolites the electron Zeeman interaction (gB 0 S) is much smaller at the X-band frequency than the ZFS interaction. 21,22 This makes interpretation of the EPR spectra difficult due to inhomogeneous broadening arising from the ZFS and overlap- ping signals. Use of higher microwave frequency is particularly advantageous in this case. The 9-287 GHz EPR studies were carried out to characterize the Fe 3+ sites in Fe-MCM-41 molecular sieves. Multifrequency * To whom correspondence should be addressed. E-mail: lkispert@ bama.ua.edu. Fax: (205) 348 9104. H S ) gBS + D(S Z 2 - 1 / 3 S 2 ) + E(S X 2 - S Y 2 ) + other terms (1) 1006 J. Phys. Chem. B 2003, 107, 1006-1011 10.1021/jp021565f CCC: $25.00 © 2003 American Chemical Society Published on Web 12/31/2002