Photooxidation of Carotenoids in Mesoporous MCM-41, Ni-MCM-41

Department of Chemistry, BOX 870336, UniVersity of Alabama, Tuscaloosa, Alabama ... Center for Materials for Information Technology, UniVersity of Ala...
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J. Phys. Chem. B 2001, 105, 7459-7464

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Photooxidation of Carotenoids in Mesoporous MCM-41, Ni-MCM-41 and Al-MCM-41 Molecular Sieves Tatyana A. Konovalova, Yunlong Gao, Rainer Schad,† and Lowell D. Kispert* Department of Chemistry, BOX 870336, UniVersity of Alabama, Tuscaloosa, Alabama 35487, and Center for Materials for Information Technology, UniVersity of Alabama, Tuscaloosa, Alabama 35487

Charles A. Saylor and Louis-Claude Brunel Center for Interdisciplinary Magnetic Resonance, National High Magnetic Field Laboratory, Florida State UniVersity, Tallahassee, Florida 32310 ReceiVed: March 6, 2001; In Final Form: May 19, 2001

Photooxidation of β-carotene and canthaxanthin in mesoporous MCM-41, Ni-MCM-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 Å) 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.

Introduction Use of mesoporous molecular sieves has attracted considerable attention due to their unique properties, namely, large pore size, uniform pore distribution, high surface area, and longrange ordering of the pore packing, all of which are potentially important in catalysis involving large molecules. MCM-41, the most extensively studied member of the class of mesoporous materials, exhibits a regular hexagonal arrangement of cylindrical pores embedded in a matrix of amorphous silica. MCM-41 molecular sieves are synthesized from silica gels using quaternary ammonium surfactants (CnH2n+1-(CH3)3N+) with different alkyl chain lengths, which give different pore sizes from 15 to 100 Å and a surface area of about 1000 m2/g.1-5 The unique properties of the MCM-41 materials make them attractive as potential heterogeneous hosts for catalytic reactions of bulky organic molecules. Ethylene dimerization, butene isomerization, and photoionization of a number of large molecules such as 2,4,6-triphenylpyrylium, tritylium, porphyrins, and N-alkylphenothiazines in mesoporous MCM-41 have been reported.6-10 It has been shown that the photoyield of porphyrin9 and Nalkylphenothiazine10 radical cations are strongly affected by the molecular sieve pore size. The largest photoyield has been obtained in pores whose size correlates with the molecular * To whom correspondence should be addressed. [email protected]. Fax: (205) 348 9104. † Center for Materials for Information Technology.

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diameter of the adsorbed compound. The lower photoyield in large pores is due to greater mobility and rapid decay of the photoinduced radicals. It has been proposed that the MCM-41 framework acts as an electron acceptor upon photoionization of incorporated organic compounds. However, the nature of the electron acceptor centers is still unclear. Incorporation of metal ions has been used for introducing active sites in zeolites.11 Numerous reports of MCM-41 modifications with a variety of metal elements, including Al,5,8,9 Ni,12-14 Cu,15,16 Ti,9,10,17 Fe,18 Mn,19 V,20 Cr,21,22 Ce,23 and so on, are known. Replacement of some tetrahedral Si(IV) in the MCM-41 framework by metal ions has been found to make it a better acid catalyst compared to the original siliceous MCM-41.24-26 In this study we report the photooxidation of carotenoids within MCM-41, Ni-MCM-41 and Al-MCM-41 molecular sieves. Carotenoids are important components of the lightharvesting antenna systems.27,28 They are also involved in electron-transfer reactions in the photosynthetic reaction centers. Large extinction coefficients of carotenoids in the visible spectral region 420-550 nm make them potentially attractive as photosensitive electron donors for artificial photoredox systems for solar energy conversion and storage. It is of considerable interest to enhance the efficiency of carotenoid electron-transfer reactions by minimizing back electron transfer and extending the lifetime of photoinduced charge-separation. For solving this major problem carotenoid radical cation (Car•+) formation has been studied on different solid supports such as silica gel29 or

10.1021/jp0108519 CCC: $20.00 © 2001 American Chemical Society Published on Web 07/12/2001

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silica-alumina.30 Use of mesoporous MCM-41 with pore sizes appropriate for penetration of these large molecules (25-30 Å) inside pores is expected to restrict mobility of incorporated carotenoids and increase the lifetime of the photogenerated radical cations. The pore size of MCM-41 materials was characterized by powder X-ray diffraction. Modification of MCM-41 with Ni(II) and Al(III) ions was expected to enhance the photooxidation efficiency of carotenoids. In the Al-MCM41, a portion of the framework silicon atoms was replaced by Al(III), while in the Ni-MCM-41, Ni(II) ions were incorporated in the original MCM-41 by liquid ion exchange of silanol protons or sodium ions in the cages. EPR spectroscopy was used to study photoinduced electron-transfer between carotenoids and the MCM-41 framework. Modification of MCM41 with Ni(II) was verified by high-frequency (110-220 GHz) EPR measurements. The orientation of carotenoids in Al-MCM41 channels was monitored by ENDOR spectroscopy. Experimental Section Sample Synthesis. MCM-41. Tetrabutylammonium silicate (TBAS) was prepared from tetrabutylammoium hydroxide (TEAOH, 40 wt %, Aldrich) and fumed silica (Sigma) in a 10:1 ratio. Then 12.21 g of TBAS was combined with 20.3 g of cetyltrimethylammonium chloride (CTAC, 25 wt %, Aldrich) and 5.94 g of H2O with stirring followed by the addition of 5.91 g of fumed silica. The resulting gel was placed in a Teflon bottle and heated for 6 days at 95 °C for crystallization. The product was cooled to room temperature, centrifuged, filtered, washed with deionized (d.i.) water, and dried in air. The template CTAC was removed by calcination in a stream of air at 530 °C for 18 h. Al-MCM-41. A dispersion of 0.25 g sodium aluminate (Pfaltz & Bauer) in 4 mL of d.i. water was mixed with 2.3 mL of TEAOH (40 wt %, Aldrich) and 10 g of silica sol gel (5 g of tetramethyl orthosilicate (99 wt %, Aldrich) dispersed in 5 g of d.i. water). The final gel was stirred vigorously for about 4 h at room temperature, and was then combined with 5.0 mL of cetyltrimethylammonium hydroxide solution (25 wt %, TCI). The mixture was transferred into a Teflon bottle and heated for 7 days at 105 °C. After crystallization, the solid product was centrifuged, filtered, washed several times with d.i. water, and air-dried, then calcinated in a stream of air at 530 °C for 18 h. Ni-MCM-41. These molecular sieves were prepared by liquidstate Ni(II) ion-exchange. Forty milliliters of 2 mM NiCl2 (98 wt %, Aldrich) solution was added to 0.25 g of MCM-41, the resulting mixture was stirred for 24 h at room temperature. The final product was washed with d.i. water and filtered. β-Carotene was supplied by Sigma and canthaxanthin by Fluka (Scheme 1). Purity of the carotenoids was checked by 1H NMR (360 MHz, CDCl ) and TLC analyses. They were 3 stored at -14 °C in a desiccator containing drierite. The solvent,

Konovalova et al. methylene chloride, CH2Cl2 (Aldrich, anhydrous), was stored under nitrogen in a drybox and used without further purification. MCM-41 samples were activated by heating at 260 °C for 3 h in air in a quartz EPR tube, cooled to room temperature, and then the carotenoid solution was added to the sample. Carotenoid solutions in CH2Cl2 (10-2 -10-3 M) were degassed before use by three freeze-pump-thaw cycles. The solvent was evaporated under reduced pressure, and the tube was evacuated and sealed. The samples were stored at 77 K. Measurements. X-ray powder diffraction (XRD) data were obtained from thin layers of sample, and measurements were carried out with a Philips 1840 diffractometer using Cu KR radiation (λ ) 1.541 Å) within the scattering angle 2θ range of 1.5-10°. A Xe/Hg lamp (1 kW) equipped with a Kratos monochromator was used as the light source. X-band (9 GHz) EPR and ENDOR measurements were carried out with a Bruker ESP 300E EPR spectrometer, containing a DICE ENDOR ESP 350 system and a temperature controller. The 110 to 220 GHz measurements were performed using the high-field EPR facility of the National High Magnetic Field Laboratory (Tallahassee, FL) with a spectrometer described in ref 31 with fundamental microwave frequencies of 110 ( 3 and 220 ( 3 produced by two Gunn oscillators. The fundamental frequency was measured using an EIP 578B counter. The magnet used was an Oxford Instruments teslatron consisting of a main set of superconducting coils that can be put in persistent mode in the vicinity of resonance and a smaller superconducting sweep coil operating in nonpersistent mode and allowing a field sweep of ( 0.1 T with respect to the persistent field. The spectrometer operated in a single-pass transmission mode (no resonator employed). The spectra were recorded in the first derivative mode, using 8 kHz magnetic field modulation. The typical magnetic field sweep rate was 60 G/min. Quartz EPR tubes of o.d. 8 mm were used for the 110-220 GHz frequency measurements. The sample in a sealed quartz tube fitting the oversized stainless steel waveguide (i.d. 9.6 mm) was placed within an Oxford Instruments CF 1200 continuous flow cryostat. Approximate sample volume was 500 mm3. A liquid-heliumcooled hot-electron InSb bolometer (QMC Instruments, UK) was used as a power detector to measure the mm- and submmwave absorption through the sample. EPR simulations were carried out using WINEPR SimFonia program (version 1.25, Bruker Analytische Messtechnik GmbH, December 15, 1996). Results and Discussion X-ray Diffraction. Figure 1 shows the X-ray powder diffraction spectra of MCM-41 (top) and Al-MCM-41 (bottom) after removing the template. Both siliceous and aluminosilicate samples exhibited the d100 diffraction peak as well as three other reflections indexed as d110, d200, and d210 which were less intense. This suggests the presence of a periodic hexagonal arrangement of channels. The XRD patterns of Al-MCM are broader and less resolved. The repeat distance (a0 ) 41 Å) determined from the location of the first XRD peak (d100) includes the pore wall thickness. The approximate pore size (33 Å) was estimated by subtracting the wall thickness from the value of a0. Theoretical calculations using a hexagonal model of cylindrical pores for MCM-41 give a pore wall thickness of 8 ( 1 Å.5 Although variations in this wall thickness depending on the surfactant chain length are possible, it has been shown that for the chain

Photooxidation of Carotenoids

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Figure 3. EPR (220 GHz) spectrum of Ni-MCM-41 activated at 260 °C, degassed, and measured at 5 K. Modulation frequency 81 kHz, modulation amplitude 30 mV, sweep rate 0.1 T/min. Figure 1. X-ray powder diffraction patterns of calcined MCM-41 (top) and Al-MCM-41 (bottom).

Figure 2. Dependence of canthazanthin radical cation photoyield in Al-MCM-41 on irradiation time.

lengths C10-C16 a wall thickness of 8-10 Å is reasonable.9 The pore size of 33 Å is sufficient for incorporation of carotenoid molecules with average diameter 25-30 Å inside the pore. EPR and ENDOR Studies. Incorporation of β-carotene and canthaxanthin into MCM-41 and Ni-MCM-41 channels followed by 350 nm irradiation results in formation of carotenoid radical cations whose EPR spectra exhibit a single line with g value of 2.0028 ( 0.0002. Carotenoids embedded in Al-MCM-41 produce radical cations before irradiation. After UV-irradiation the concentration of the radical cations increases. Figure 2 shows the dependence of the canthaxanthin radical cation yield in AlMCM-41 on irradiation time. Behavior of the other carotenoid is similar. Maximum photoyield is achieved after ca. 1 min of 350 nm irradiation at 77 K. Ni-MCM-41. Ni-containing samples measured at 9 GHz (77 K) showed no EPR signals consistent with Ni(II) ions. Therefore, the presence of Ni(II) in modified MCM-41 was verified by HF-EPR spectroscopy. The 220 GHz EPR spectrum (5 K)

Figure 4. EPR (110 GHz) spectrum at 5 K of Ni-MCM-41 after 350 nm irradiation (solid line), simulated spectrum (dotted line).

of activated (at 260 °C) Ni-MCM-41 exhibits a peak with g value of 2.26 (Figure 3). This is in agreement with data reported for Ni(II) ions in an octahedral environment which give rise to very broad EPR lines with g values of 2.10 to 2.33.32 This provides direct evidence of incorporation of Ni(II) ions into MCM-41. Figure 3 also shows a narrow signal around g ) 2.0, which may be due to molecular oxygen and is always observed at high frequencies. Irradiation of Ni-MCM-41 at 350 nm generates paramagnetic species stable at 77 K whose spectra are superimposed. From the 110 GHz spectrum of Ni-MCM41 (5 K) two different paramagnetic species were detected (Figure 4). Spectral simulations determined g tensors of these species. The signal with a rhombic g tensor g1 ) 2.0115, g2 ) 2.0049, g3 ) 2.00 is characteristic of O2- species generated in MCM-41 (g1 ) 2.012, g2 ) 2.003, g3 ) 2.00).14 We assigned the second rhombic g tensor (g1′ ) 2.0154, g2′ ) 2.0058, g3′ ) 1.996) to V-centers (Figure 4). So-called V-centers or trapped

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Figure 5. EPR (9 GHz) spectrum at 77 K of β-carotene in Ni-MCM41 after 350 nm irradiation.

Figure 6. EPR spectrum of canthazanthin in Al-MCM-41 measured at 77 K.

holes on the framework oxygens have been observed for metalsubstituted MCM-41 after γ-irradiation at 77 K.17,33 Similar, but less intense, signals were observed for the siliceous MCM41. Photooxidation of carotenoids in Ni-MCM-41 produces an intense EPR signal (Figure 5) with g value of 2.0028 ( 0.0002 due to the carotenoid radical cations; another, less intense, with g ) 2.09, is attributed to an isolated Ni(I) species produced as a result of electron transfer from the carotenoid molecule to Ni(II). It has been reported that Ni(I) ions prepared upon reduction of Ni(II)-MCM-41 by heating in a vacuum or in dry hydrogen exhibits an EPR spectrum with g⊥ ) 2.09 and g| ) 2.5.6 The g| component is often too weak to observe. The Ni(I) EPR signals were not detected upon 350 nm irradiation of NiMCM-41 samples before adsorption of carotenoids. Al-MCM-41. An X-band EPR spectrum of canthaxanthin radical cation produced in Al-MCM-41 and measured at 77 K is shown in Figure 6. The ENDOR spectrum of the same sample was recorded at 120 K. The low-frequency part of the spectrum (from 0.5 to 5.0 MHz) contains three matrix signals (Figure 7a). The sharp signal at 1.1 MHz is due to 14N from residual surfactant which is usually present in the final samples. The signal at 2.3 MHz is assigned to 29Si, and the signal at 3.45 MHz can be attributed to 27Al although we cannot exclude the contribution of 23Na (νNa ) 3.82 MHz). Figure 7b exhibits the powder 1H ENDOR spectrum of canthaxanthin in Al-MCM-41 (120 K) in the range of 5-25 MHz. The three pairs of proton lines with hyperfine coupling constants 2.6, 5.4, and 13.6 MHz are clearly observable. The ENDOR spectrum of β-carotene in

Figure 7. ENDOR spectra of canthazanthin in Al-MCM-41 measured at 120 K (a) the matrix ENDOR spectrum measured from 0.5 to 5.0 MHz, (b) the powder 1H ENDOR spectrum measured from 5 to 25 MHz.

TABLE 1: Proton Coupling Constants (MHz) of Canthaxanthin and β-Carotene Observed in Powder ENDOR Spectra and Determined by RHF-INDO/SP MO Calculations

protons β-carotene C5-CH3 C9-CH3 C15-HR C13-CH3 canthaxanthin C5-CH3 C9-CH3 C15-HR C13-CH3 a

silicafrozen RHFAl-MCM-41 aluminaa Aiso solutionb Aiso INDO/SPc Aiso(exptl) (simulated) (simulated) Aiso(calcd) 1.9, 1.9, 2.0 8.2, 8.2, 8.5 4.5 13.3 2.6 5.4 13.6

2.2, 2.3 8.3, 8.5 5.0 12.2, 12.5

13.0, 13.0, 13.0 1.9, 1.9, 2.0 8.2, 8.2, 8.5 13.0, 13.0, 13.0

2.2 8.3 4.5 13.0

2.1, 2.2 8.1, 8.5 5.2, 5.3 13.1, 13.4

b

Reference 30. Reference 34. c References 30, 34, and 35.

Al-MCM-41 (120 K) exhibits two pairs of proton lines with hyperfine couplings of 4.5 and 13.3 MHz. Proton couplings deduced from the powder ENDOR spectra of β-carotene and canthaxanthin radical cations prepared in Al-MCM-41 were compared with those obtained on a silica-alumina surface,30 in frozen methylene chloride solution,34 and calculated using the RHF-INDO/SP method30,34,35 (see Table 1). On the basis of our previous ENDOR data and RHF-INDO/SP molecular

Photooxidation of Carotenoids

Figure 8. The photoyield of carotenoid radical cations in MCM-41, Ni-MCM-41, and Al-MCM-41 molecular sieves.

orbital calculations30,34,35 for canthaxanthin, the hyperfine couplings of 2.6 and 13.6 MHz were assigned to the methyl group protons attached to the C5,5′ (C5,5′-CH3) and C13,13′ (C13,13′-CH3) carbon atoms, respectively (see Scheme 1). The 5.4 MHz coupling is due to the R-protons bonded to the C15 atom (C15-HR). For β-carotene the hyperfine coupling constants of 4.5 and 13.3 MHz can be attributed to the C15-HR and C13,13′-CH3 protons, respectively. It should be noticed that the proton ENDOR spectra of canthaxanthin and β-carotene within Al-MCM-41 channels differ from those obtained when they are adsorbed on a silicaalumina surface.30 Our previous ENDOR measurements of both carotenoids on silica-alumina showed that only the features due to the C5 and C9 methyl protons were well-resolved.30 Signals of protons of the central methyl groups at C13 and C13′ were scarcely observable because of line broadening and determined only by spectral simulation. The line width and spectral resolution depends on mobility of the methyl groups. It has been assumed that symmetrical carotenoids, such as β-carotene and canthaxanthin, are in contact with the silicaalumina surface through the center of the polyene chain.30 As a result, rotation of the central C13,13′-CH3 groups is restricted more than rotation of the C5,5′-CH3 and C9,9′-CH3 groups. In contrast, in Al-MCM-41 narrow and well-resolved proton lines due to the C13,13′-CH3 appeared in the ENDOR spectra. The lines from R-protons at C15, not observed on silicaalumina surface, can also be determined from the ENDOR spectra of both carotenoids embedded in Al-MCM-41. This indicates that the environment of the protons of carotenoids in the MCM-41 and on silica-alumina is different. While close proximity of the protons in the center of the carotenoid polyene chain (C13,13′-CH3) to the surface restricts their mobility on silica-alumina, in the case of MCM-41 they enable rapidly rotate. However, the ENDOR lines from the C9,9′-CH3 methyl protons are too broad to be observable in MCM-41. This demonstrates restriction of their rotation. Most likely in MCM41 the carotenoids are bound to the surface through the ends of the polyene chain near the C9,9′-CH3 groups. Figure 8 shows that modification of the MCM materials with Ni(II) and Al(III) ions changes the carotenoid oxidation efficiency compared to siliceous MCM-41. We suggest that metal ions incorporated into MCM-41 act as electron acceptor centers. The photoyield of carotenoid radical cation increases in the order MCM-41 < Ni-MCM-41 < Al-MCM-41. In Al-MCM-41 the photoyield is 4-5 times greater than that in Ni-MCM-41. This corresponds to a larger content of Al(III) ions in the A-MCM41 than that of Ni(II) ions in Ni-MCM-41. The concentration

J. Phys. Chem. B, Vol. 105, No. 31, 2001 7463 of Ni(II) should be not too large in order to avoid the formation of superparamagnetic nickel clusters Ni(0).36,37 The type of incorporated metal ion can also affect the Car•+ photoyield. Incorporation of aluminum in tetrahedral coordination into the framework of MCM-41 produces Lewis and Brønsted acid sites.38,39 It has been proposed that the Lewis acidic sites, coordinately unsaturated Al(III) ions, are responsible for the chemical formation of the Car•+ on silica-alumina support and the Brønsted acidic sites are responsible for their stabilization.30 Our previous ENDOR measurements provided strong support for the formation of carotenoid radical cations on activated alumina and silica-alumina as a result of electron transfer from carotenoids to the Lewis acid sites.40 The ENDOR spectrum exhibited a doublet centered about the 27Al Larmor frequency 3.6 ( A/2 (coupled Al) instead of a matrix 27Al peak.40 This indicates the formation of strong complexes between carotenoids and the surface Al(III) sites. Similar observations have been reported for paramagnetic complexes of quinones with Al(III) ions in HY zeolites.41 The presence of the Lewis acid sites in the Al-MCM-41 framework explains the chemical formation of a small amount of the Car•+ in Al-MCM-41 before irradiation. The ENDOR spectrum in Figure 7a shows a peak at 3.45 MHz which can be attributed to 27Al nuclei in the vicinity of the unpaired electron. A similar 27Al ENDOR peak has been observed upon photooxidation of perylene on alumina and silica-alumina.42 Conclusions EPR (9-220 GHz) and ENDOR (9 GHz) spectroscopies were used to study the formation of carotenoid radical cations photochemically generated within mesoporous MCM-41, NiMCM-41, and Al-MCM-41 molecular sieves. X-ray powder diffraction was applied to determine the MCM-41 pore size (33 Å) sufficient for embedding carotenoid molecules (25-30 Å). Mesoporous MCM-41 molecular sieves with incorporated carotenoid molecules are found to be promising hosts for longlived photoinduced charge-separation. Incorporating metal ions into siliceous MCM-41 enhances the electron-accepting ability of the Si-MCM-41 framework. The photoyield and stability of generated carotenoid radical cations increased in the order MCM < Ni-MCM < Al-MCM and depended on the concentration and the nature of the metal ion. Additional evidence for incorporation of Ni(II) ions into MCM-41 was provided by the 220 GHz EPR measurements which were sufficient to detect the Ni(II) signal with g value of 2.26. Formation of carotenoid radical cations within the modified MCM-41 is due to electron transfer between incorporated carotenoid molecules and metal ions, which act as electron acceptor sites. Detected at 9 GHz, EPR signals of an isolated Ni(I) species with g ) 2.09 provide direct evidence for the reduction of Ni(II) ions by carotenoids. The 110 GHz EPR measurements resolved overlapped signals of O2- species (g1 ) 2.0115, g2 ) 2.0049, and g3 ) 2.00) and V-centers (g1 ) 2.0154, g2 ) 2.0058, and g3 ) 1.996). ENDOR measurements indicated that the orientation of carotenoids embedded in MCM-41 and adsorbed on silicaalumina is different. β-Carotene and canthaxanthin when adsorbed on silica-alumina have maximum interaction with the surface via the center of the polyene chain. This resulted in restricting rotation of the central methyl groups and freely rotating methyl protons away from the center. In the case of Al-MCM-41, the central C13-CH3 and C13′-CH3 groups of

7464 J. Phys. Chem. B, Vol. 105, No. 31, 2001 both carotenoids are freely rotating and give well-resolved ENDOR lines, while rotation of the C9,9′-CH3 protons is restricted and they are not observed in the ENDOR spectra. This indicates that carotenoids are bound to the MCM-41 pore walls via the ends of the polyene chain in close proximity to the C9,9′-C3 groups. Acknowledgment. This work was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, the U.S. Department of Energy, Grant DEFG02-86ER13465, and the National Science Foundation-supported National High Magnetic Field Laboratory (Tallahassee, Florida). We thank Dr. E. Hand for helpful discussions. References and Notes (1) Kresge, C. T.; Lewnowitz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (2) Beck, J. S.; Vatuli, J. C.; Roth, W. J.; Lewnowitz, M. E.; Kresge, C. T.; Schmit, K. D.; Chu, C. T.-W.; Olson, D. H.; Sheppard, E. W.; McCullen, S. B.; Higgins, J. B.; Schlenker, J. L. J. Am. Chem. Soc. 1992, 114, 10834. (3) Chen, C. Y.; Li, H. X.; Davis, M. Mesoporous Mater. 1993, 2, 17. (4) Casci, J. L. In AdVanced Zeolite Science and Application Studies; Jansen, J. C., Sto¨cker, M., Karge, H. G., Weitkamp, J., Eds.; Elsevier: Amsterdam, 1994; Vol. 85, p 329. (5) Stucky, G. D.; Monnier, A.; Schuth, F.; Hou, Q.; Margolese, D.; Kumar, D.; Kridhnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B. F. Mol. Cryst. Liq. Cryst. 1994, 240, 187. (6) Hartmann, M.; Pu¨ppl, A.; Kevan, L. J. Phys. Chem. 1996, 100, 9906. (7) Corma, A.; Fornes, V.; Garcia, H.; Miranda, M. A.; Sabater, A. J. Am. Chem. Soc. 1994, 116, 9767. (8) Cano, M. L.; Cozens, F. L.; Garcia, H.; Marti, V.; Scaiano, J. C. J. Phys. Chem. 1996, 100, 18152. (9) Sung-Suh, H. M.; Luan, Z.; Kevan, L. J. Phys. Chem. B 1997, 101, 10455. (10) Krishna, R. M.; Prakash, A. M.; Kevan, L. J. Phys. Chem. B 2000, 104, 1796. (11) Szostak, R. In Molecular SieVes. Principles of Synthesis and Identification; Van Nostrand Reinhold: New York, 1989; p 211. (12) Hartmann, M.; Pu¨ppl, A.; Kevan, L. J. Phys. Chem. 1995, 99, 17494. (13) Prakash, A. M.; Kevan, L. J. Phys. Chem. 1996, 100, 19587. (14) Chang, Z.; Zhu, Z.; Kevan, L. J. Phys. Chem. B1999, 103, 9442. (15) Pu¨ppl, A.; Newhouse, M.; Kevan, L. J. Phys. Chem. 1995, 99, 10019.

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