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Photoionization of Tetraphenylporphyrin in Mesoporous SiMCM-48, AlMCM-48, and TiMCM-48 Molecular Sieves Zhixiang Chang and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received September 10, 2001. In Final Form: November 20, 2001 Electron spin resonance spectroscopy has been used to detect the photoionization yield of mesotetraphenylporphyrin (H2TPP) in SiMCM-48, AlMCM-48, and TiMCM-48 mesoporous molecular sieves. MCM-48 materials are shown to be effective heterogeneous hosts to accomplish long-lived photoinduced electron transfer of bulky porphyrin molecules at room temperature. TiMCM-48 and AlMCM-48 have higher photoyields of H2TPP+ radicals than does SiMCM-48, indicating that the incorporation of Al and Ti into MCM-48 frameworks enhances the electron-accepting ability of the framework. H2TPP+ radicals are more stable in TiMCM-48 than in SiMCM-48 and AlMCM-48.
Introduction The importance and complexity of electron transfer reactions in nature have led many researchers to look for ways to study the fundamental chemistry of these processes. A significant part of this effort has been devoted to the study of photoinduced charge separation reactions as a means of capturing and storing solar energy. The goal of this research is to develop an understanding of photoinitiated electron-transfer reactions that is sufficiently advanced to enable one to design artificial systems for the conversion of solar energy into chemical potential energy. A vital part of this research is the design and synthesis of complex molecular systems which are comprised of electron donors and acceptors that mimic the charge separation function of photosynthetic proteins.1,2 Porphyrin derivatives2-6 have been used as photosensitive electron donors due to their structural and functional similarities to chlorophylls and their absorption of visible light. Many different host systems have been studied to improve the efficiency of energy storage by preventing rapid back electron transfer.7,8 The net photoionization efficiency in organic assemblies is typically higher than that in homogeneous solution, but the photoinduced radicals are not typically stable at room temperature. In some cases, the photoyields in organic assemblies such as micelles and vesicles are limited by the polarity and solubility of the photoactive molecules.9-11 However, photoionization studies in porous inorganic materials such as molecular sieves12-14 and silica gels 15,16 have shown (1) Dutta, P. K.; Ledney, M. In Molecular Level Artificial Photosynthetic Materials; Meyer, G. J., Ed.; Wiley: New York, 1997; pp 209272. (2) Wasielewski, M. R. Chem. Rev. 1991, 92, 435. (3) Gouterman, M.; Khalil, G. E. J. Mol. Spectrosc. 1974, 53, 88. (4) Kalyanasundaram, K.; Vlachopoulous, N.; Krishnan, V.; Monnier, A.; Gratzel, M. J. Phys. Chem. 1987, 91, 2342. (5) Harriman, A.; Porter, G.; Richoux, M. C. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1175. (6) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic: New York, 1987; Chapter 3. (7) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 8232. (8) Vermeulen, L. A.; Thompson, M. E. Nature 1992, 358, 656. (9) McManus, H. J.; Kang, Y. S.; Kevan, L. J. Phys. Chem. 1992, 96, 5622. (10) Kang, Y. S.; McManus, H. J.; Kevan, L. J. Phys. Chem. 1993, 97, 2027. (11) Sung-Suh, H. M.; Kevan, L. J. Phys. Chem. 1997, 101, 1414. (12) Stamires, D. N.; Turkevich, J. J. Am. Chem. Soc. 1964, 86, 749.
that their pores and cages provide an appropriate microenvironment to retard back electron transfer and increase the lifetime of the photogenerated radical ions, which can even be stable at room temperature. Aromatic molecules adsorbed on thermally activated zeolite Y12 or ZSM-517 are readily oxidized to their cation radicals, indicating that a zeolite framework can act as an electron acceptor. The pore size of microporous zeolite Y is about 0.8 nm.18 This cage size is too small for bulky porphyrins such as meso-tetraphenylporphyrin (H2TPP) to enter, since H2TPP has a square-planar structure of molecular dimensions 1.5 nm × 1.5 nm.19 Amorphous silica gel has an irregular pore structure with no long-range order and a wide pore size distribution compared to molecular sieves.20 Therefore, the development of ordered mesoporous silica molecular sieve (M41S) materials containing some transition metal ions in the pore walls has opened new possibilities for the use of mesoporous materials in the fields of photochemical solar energy conversion and of catalysis.21-24 This makes it possible to modify the M41S frameworks to enhance the photoionization efficiency from incorporated molecules such as porphyrins. M41S materials have large channels with regular pores which can be varied from 2 to 10 nm or more and which are ordered in hexagonal (MCM-41), cubic (MCM-48), or lamellar (MCM-50) arrays. These materials are characterized by quite narrow pore size distributions in the mesoporous region, long range order, high surface area, and thermal stablility after removal of (13) Chang, Z.; Krishna, M. R.; Xu, J.; Koodali, T. J.; Kevan, L. Phys. Chem. Chem. Phys. 2001, 3, 1699. (14) Krishna, M. R.; Chang, Z.; Hosun, C.; Koodali, T. J.; Kevan, L. Phys. Chem. Chem. Phys. 2000, 2, 3335. (15) Slama-Schwok, A.; Ottolenghi, M.; Avnir, D. Nature 1992, 355, 240. (16) Xiang, B.; Kevan, L. Langmuir 1995, 11, 860. (17) Kurita, Y.; Sonoda, T.; Sato, M. J. Catal. 1970, 19, 82. (18) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974; p 49. (19) Hamor, M. J.; Hamor, T. A.; Horad, J. L. J. Am. Chem. Soc. 1964, 86, 1938. (20) Scott, R. P. W. Silica Gel and Bonded Phases; Wiley: Chichester, 1993; Chapters 4 and 6. (21) Zhu, Z.; Chang, Z.; Kevan, L. J Phys. Chem B 1999, 103, 2680. (22) Luan, Z.; Xu, J.; He, Y.; Klinowski, J.; Kevan, L. J. Phys. Chem. 1996, 100, 19595. (23) Voort, V. D. P., Baltes, M.; Vansant, F. E. J. Phys. Chem. B 1999, 103, 10102. (24) Zhao, D.; Goldfarb, D. J. Chem. Soc., Chem. Commun. 1995, 875.
10.1021/la011414o CCC: $22.00 © 2002 American Chemical Society Published on Web 01/10/2002
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the organic template typically used in synthesis. Less research has been carried out on MCM-48, probably due to the difficulty of its reliable synthesis compared with the extensively studied MCM-41. In this research, a series of mesoporous SiMCM-48, AiMCM-48, and TiMCM-48 molecular sieves have been synthesized and used as heterogeneous hosts for photoinduced electron transfer from bulky H2TPP. The photoionization of H2TPP in these mesoporous materials is achieved with photoirradiation at 350 nm at room temperature, and photoinduced H2TPP+ cation radical is characterized by electron spin resonance (ESR). The associated photoproduced electron is presumably trapped in the framework of siliceous MCM-48 but is not separately detectable by ESR. Incorporation of Ti(IV) and Al(III) into the MCM-48 framework enhances the electron-accepting ability of the framework. Photoinduced H2TPP+ cation radicals are stable at room temperature and decay more slowly in TiMCM-48 compared to SiMCM-48 and AlMCM48. Experimental Section Materials. The following chemicals were used without further purification: cetyltrimethylammonium bromide (CTAB, Aldrich), aluminum sulfate hexadecahydrate (Fluka), tetraethyl orthosilicate (TEOS, Acros), 35 wt % aqueous solution of tetraethylammonium hydroxide (TEAOH, Alfa), titanium(IV) isopropoxide (Alfa), and titanium(IV) oxide (Alfa). Deionized water was used throughout the syntheses, and 125 cm3 Teflon beakers were used for crystallization. Synthesis of SiMCM-48, AlMCM-48, and TiMCM-48. MCM-48 and AlMCM-48 were hydrothermally synthesized following literature procedures.13 The product was filtered, thoroughly washed with deionized water, and dried at 353 K overnight. The as-synthesized material was calcined in flowing nitrogen while raising the temperature slowly to 823 K and then in flowing oxygen for 10 h. These calcined samples are labeled AlMCM-48(n), where n indicates the Si/Al ratio. The calcined material was cooled to 573 K in a vacuum to prevent water adsorption and kept in a desiccator before impregnation with H2TPP. The synthetic procedure for TiMCM-48 is as follows. Ten milliliters of 2 M TEAOH solution was combined with 87 mL of 10 wt % CTAB solution with stirring, and then 10 mL of tetraethyl orthosilicate was added and vigorously stirred. When the above solution becomes cloudy, different amounts of titanium isopropoxide are added. The timing is crucial to ensure simultaneous coprecipitation. The molar composition of the final gel was 1 Si:(0.01-0.07) Ti:0.46 TEAOH:120 H2O:0.55 CTAB. The assynthesized samples were dried and calcined as described above. These calcined samples are termed TiMCM-48(n), where n indicates the Si/Ti ratio. Characterization. X-ray powder diffraction patterns were recorded on a Philips PW1840 diffractometer using Cu KR radiation (40 kV, 25 mA) with a 0.025° step size and 1 s step time over the range 1.5° < 2q < 10°. The samples were prepared as thin layers on aluminum slides. The elemental composition of the samples was analyzed by a JEOL JXA-8600 electron microprobe with a beam diameter of 20 nm. The resolution is 2.5 eV. Five or more randomly selected spots on the samples were averaged to obtain the bulk composition. Diffuse reflectance (DR) UV-vis spectra of TiMCM-48 were recorded using a PerkinElmer 330 spectrometer with an integrating sphere accessory. The sample powder was put into a cylindrical quartz sample cell (19 mm diameter × 1 mm path length). N2 adsorption isotherms were measured at 77 K using a Micromeritics Gemini 2375 analyzer. Prior to adsorption, samples were dehydrated at 623 K for 5 h. The specific area, ABET, was determined from the linear part of a BET plot (P/P0 ) 0.050.30). The pore size distribution was calculated using the adsorption branches of the N2 adsorption isotherms and the Barret-Joyer-Halenda (BJH) formula.25 Samples for Photoirradiation and Radical Analysis. meso-Tetraphenylporphyrin (H2TPP) was incorporated into
Chang and Kevan Table 1. Elemental Composition, Si/Ti Ratio, Si/Al Ratio, Surface Area, and Pore Size of Calcined SiMCM-48, AlMCM-48, and TiMCM-48 sample SiMCM-48 AlMCM-48(60) AlMCM-48(100) TiMCM-48(28) TiMCM-48(133) TiMCM-48(476)
Si/Ti ratio
Si/Al ratio 60 100
28 133 476
area, m2/g
pore size, nm
1502 1548 1600 1501 1612 1540
2.2 2.3 2.4 2.3 2.2 2.3
MCM-48 samples by impregnation. For impregnation, 0.1 g of the sample was immersed in 1 mL of 10-2 M H2TPP/C6H6 solution overnight, and the benzene solvent was then removed by flowing nitrogen through the samples for about 40 min. The loading of H2TPP into MCM-48 was 10 µmol/g of MCM-48. For ESR measurement the sample powder was filled into 2 mm i.d. × 3 mm o.d. Suprasil quartz tubes to about 20 mm in height. The tube was connected to a vacuum line and evacuated to about 1 × 10-4 Torr. Then the samples were sealed in the tube under vacuum. All the samples were handled in the dark to minimize exposure to visible light. The evacuated H2TPP/SiMCM-48, H2TPP/AlMCM-48, and H2TPP/TiMCM-48 samples were irradiated with a 300 W Cermax xenon lamp (ILC-LX 300 UV) at room temperature. The light was passed through a 10 cm water filter and a Corning No. 7-51 glass filter to give 350 nm irradiation. Every sample was irradiated in a quartz Dewar that was rotated at 4 rpm to ensure even irradiation of the samples. The photoproduced PCn+ radicals were identified by ESR. The ESR spectra were recorded at room temperature at X band frequency using a Bruker ESP 300 spectrometer with 100 kHz field modulation and microwave power low enough to avoid saturation. The photoproduced H2TPP+ radical yields were determined by subtracting the ESR intensity of any dark reaction from that of photoirradiated samples by double integration of the ESR spectra using the ESP 300 software.
Results Synthesis and Elemental Analysis. In most of the reported syntheses of pure siliceous MCM-48 materials and their derivatives, NaOH was introduced into the synthesis gel.26-30 It is known, however, that there is a detrimental effect of alkali metal ions on the catalytic properties of titanium silicate molecular sieves.31,32 It is possible to synthesize high-quality MCM-48 samples in the absence of alkali metal ions.13 So we chose TEAOH as the hydroxide source and tetraethyl orthosilicate as the silicon source. The cubic phase of MCM-48 easily transforms into a lamellar or amorphous phase at 373 K after 3 days.28 So we limited the crystallization time to 3 days. Electron microprobe analysis shows that titanium and aluminum in TiMCM-48 and AlMCM-48 are homogeneously distributed throughout the MCM-48 solid particles. Table 1 lists the BET surface area, Si/Ti ratio, Si/Al ratio, and pore size of MCM-48, AlMCM-48, and TiMCM48. X-ray Diffraction. The samples give a well-defined powder X-ray diffraction (XRD) pattern typical of MCM48 mesoporous molecular sieves which can be indexed on (25) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Mosco, L.; Pierott, R. A.; Rouquerol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603. (26) Zhang, W.; Pinnavaia, J. T. Catal. Lett. 1996, 38, 261. (27) Xu, J.; Luan, Z.; Hartmann, M.; Kevan L. Chem. Mater. 1999, 11, 2928. (28) Xu, J.; Luan, Z.; He, H,; Zhou, W.; Kevan, L. Chem. Mater. 1998, 10, 3690. (29) Khouw, B. C.; Davis, E. M. J. Catal. 1995, 151, 77. (30) Morey, M.; Davidson, A.; Stucky, G. Microporous Mater. 1996, 6, 99. (31) Corma, A.; Kan, Q.; Rey, F. J. Chem. Soc., Chem. Commun. 1998, 579. (32) Camblor, A. M.; Corma, A.; Perez-Pariente, J. Zeolites 1993, 13, 82.
Molecular Sieves Used in Photoinduced Electron Transfer
Figure 1. XRD patterns of as-synthesized SiMCM-48 and AlMCM-48.
Figure 2. XRD patterns of as-synthesized TiMCM-48.
a cubic lattice (Figures 1 and 2).33 After calcination, the XRD reflections increase in intensity and shift to higher angles. This corresponds to further silicon condensation and contraction of the MCM-48 framework during calcination. N2 Adsorption Isotherms. Low-temperature nitrogen adsorption isotherms enable the calculation of the surface area, pore volume, and mesopore size distribution. All calcined samples show typical reversible type IV adsorption isotherms as defined by IUPAC.34 These isotherms (33) Beck, J. S.; Vartuli, J. C.; Roth, W. J.; Leonowicz, M. E.; Kresge, C. T.; Schmitt, 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.
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Figure 3. Diffuse reflectance UV-vis spectra of (A) TiMCM48(28), (B) TiMCM-48(133), (C) TiMCM-48(476), and (D) TiO2.
show a sharp inflection between relative pressure P/P0 ) 0.25 and 0.35, indicating capillary condensation with uniform mesopores. The fact that the initial region can be extrapolated back to the origin confirms the absence of any detectable micropore filling at low relative pressure. The pore size distribution determined by using a BJH analysis shows a very narrow peak and a mean pore diameter of ∼2.3 nm. The BET surface areas of all the calcined samples are larger than 1400 m2/g. Therefore, these MCM-48 samples possess a uniform pore structure, high surface area, and high crystallinity. UV-vis Spectra. The DR UV-vis spectra of calcined TiMCM-48 samples are shown in Figure 3. All the Ti ions in calcined TiMCM-48 are in the IV oxidation state (electronic configuration 3d0) as revealed by the absence of any d-d transition in the UV-vis spectra. Compared with bulk anatase (TiO2 particles of 50 Å diameter), the absorption edges of all TiMCM-48 samples are blue-shifted by more than 50 nm. DR UV-vis spectra of TiMCM-48 materials show a maximum near 210 nm, which can be assigned to ligand-to-metal charge-transfer transitions between the oxygen ligand and tetracoordinated Ti(IV) ions.35,36 For Ti zeolite materials, a similar absorption has been associated with Ti ions substituting for silicon in framework positions.37-39 This indicates that titanium is incorporated into the MCM-48 framework. The absence of significant absorption at 300-350 nm indicates that a segregated crystalline TiO2 phase like anatase is absent in the TiMCM-48 materials. (34) Brunauer, S.; Deming, L. S.; Deming, W. S.; Teller, E. J. Am. Chem. Soc. 1940, 62, 1723. (35) De Castro-Martins, S.; Tuel, A.; Ben Taarit, Y. Zeolites 1994, 14, 130. (36) Notari, B. Adv. Catal. 1996, 41, 258. (37) Geobaldo, F.; Bordiga, S.; Zecchina, A.; Giamello, E.; Leofanti, G.; Petrini, G. Catal. Lett. 1992, 16, 109. (38) Liu, Z.; Davis, R. J. J. Phys. Chem. 1994, 98, 1253. (39) Chang, Z.; Koodali, R.; Krishna, M. R.; Kevan, L. J. Phys. Chem. B 2000, 104, 5579.
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Figure 5. Increase of ESR intensities of H2TPP+ radical at room temperature vs 350 nm irradiation for SiMCM-48, AlMCM-48(60), and TiMCM-48(28). Figure 4. ESR spectra of H2TPP impregnated SiMCM-48, AlMCM-48(60), and TiMCM-48(28) at room temperature before irradiation (a-c) and after 50 min of room temperature irradiation at 350 nm (A-C).
H2TPP+ Analysis by ESR. The ESR spectra of H2TPP-impregnated SiMCM-48, AlMCM-48(60), and TiMCM-48(33) samples at room temperature before irradiation (a-c) and after 50 min of room-temperature irradiation at 350 nm (A-C) are shown in Figure 4. The MCM-48 pore diameter of 2.3 nm is large enough to incorporate H2TPP with 1.5 nm diameter within the MCM-48 pores. The ESR signal obtained from each photoirradiated H2TPP/MCM-48 material shows a singlet at g ) 2.004, which is assigned to H2TPP+ cation radical.40-42 This shows that some stable H2TPP+ radicals are produced by 350 nm irradiation at room temperature and confirms the photoionization of H2TPP into H2TPP+ in MCM-48 materials. If the pore structure of MCM-48 is assmumed to be cylindrical, which is the simplest model to describe a porous solid, the specific surface area (S) can be expressed as S ) Σsi ) ΣπD0li, where si is the surface area of a pore, D0 is the pore diameter, and 1i is the length of a pore. If 1i is assumed to be the same as D0, then si and the number of pores can be estimated from the given S for the MCM48 samples. Therefore, the average number of H2TPP in a pore can be calculated. One of only 15 pores is occupied by a H2TPP. It seems reasonable to assume a uniform distribution of H2TPP with negligible intermolecular interaction between two neighboring H2TPP molecules. Diffuse reflectance UV-vis spectra of the samples show no evidence of agglomeration of H2TPP. The contact area of each H2TPP molecule is about 2.25 nm2 calculated from the dimensions of a square-planar H2TPP molecule (1.5 nm × 1.5 nm).19 The surface coverage by H2TPP in MCM-48 samples is estimated from the amount of H2TPP loading and the surface area of each support in Table 1. The 0.01 M loadings cover about 3% of the surface area of the MCM-48 materials. Calcined SiMCM-48, AlMCM-48, and TiMCM-48 samples show no ESR signal at room temperature or 77 K. However, MCM48 materials impregnated with H2TPP show a weak (40) Fajer, J.; Davis, M. In The Porphyrins; Dolphin, D., Ed.; Academic: New York, 1979; Vol. 4, Chapter 4. (41) Monchida, I.; Tsuji, K.; Suetsugu, K.; Fujitsu, H.; Takeshida, K. J. Am. Chem. Soc. 1980, 84, 3159. (42) Sung-Suh, M. H.; Luan, Z.; Kevan, L. J. Phys. Chem. B 1997, 101, 10455.
Figure 6. H2TPP+ photoyields in SiMCM-48, AlMCM-48(60), and TiMCM-48(28) after 50 min of irradiation with 350 nm at room temperature.
ESR signal of H2TPP+ cation radicals at g ) 2.004 (Figure 4a-c), indicating that some H2TPP+ cation radicals are generated during the sample preparation process. Photoirradiation by 350 nm light at room temperature for 50 min greatly increases the ESR signals as shown by Figure 4A-C. This shows that a substantial number of stable radicals are produced by photoirradiation. The ESR spectra observed before photoirradiation have the same shape as those obtained after photoirradiation. Figure 5 shows an increase in the ESR intensities of SiMCM-48, AlMCM-48(60), and TiMCM-48(33) impregnated with H2TPP versus irradiation time. The ESR signal increases rapidly during the first 10 min and reaches a plateau in about 50 min. An irradiation time of 50 min was selected for comparative photoyield and stability studies. H2TPP/AlMCM-48 samples give higher photoyields than H2TPP/SiMCM-48 and lower photoyields in comparison with H2TPP/TiMCM-48 (see Figure 6). The photoyield for AlMCM-48 increases with an increase of Al content as shown in Figure 7. Figure 8 shows that the photoyield in TiMCM-48 increases with increasing Ti content. In TiMCM-48 the initial quantum yield is estimated to be of order 10%, thus the overall photoyield is quite high. After 50 min of photoirradiation, H2TPP radicals decay 28% for SiMCM-48, 29% for AlMCM-48, and 20% for TiMCM-48 after 24 h at room temperature.
Molecular Sieves Used in Photoinduced Electron Transfer
Figure 7. H2TPP+ photoyields in AlMCM-48 samples versus Al content after 50 min of irradiation with 350 nm at room temperature.
Figure 8. H2TPP+ photoyields in TiMCM-48 samples versus Ti content after 50 min of irradiation with 350 nm at room temperature.
Figure 9. Mesotetraphenylporphyrin structure.
Discussion The meso-tetraphenylporphyrin structure is shown in Figure 9. The photoionization of this porphyrin usually occurs by a two-photon process to form its cation radical.43 The ionization potential of H2TPP is about 5 eV from photoionization threshold data.40 The energy of visible
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light is 1.6-3.1 eV. The energy of one photon of visible light is not enough to ionize this porphyrin. Through a (π,π*) transition by visible light excitation, porphyrins are electronically excited to a singlet state, which converts to a long-lived triplet state with high quantum yields (∼0.8) by intersystem crossing. The lifetime of the porphyrin triplet state is long enough to absorb the second photon, and this provides sufficient energy for ionization to generate porphyrin π-cation radicals.44 The ESR signal of H2TPP+ in Figure 4 shows a singlet at g ) 2.004, which is consistent with the g value for tetraphenylporphyrin cation radical derivatives.40-42 This confirms the photooxidation of H2TPP molecules into H2TPP+ cation radicals in MCM-48, AlMCM-48, and TiMCM-48 by 350 nm irradiation at room temperature. Methylphenothiazine cation radical (PC1+) in mesoporous molecular sieve TiMCM-41 (pore size 3.3 nm)45 and TiSBA-15 (pore size 6.2 nm)46 decay after photoirradiation by about 50% after 4 h, but PC1+ decays only about 25% in microporous TAPO-5 after 9 days. The photoionization efficiency of methylphenothiazine (PC1) in TiMCM-41 and TiSBA-15 is about three times lower than that seen in TAPO-5, probably because the small cation radical PC1+ has higher mobility in mesoporous TiMCM-41 and TiSBA15 compared to microporous TAPO-5. Mesoporous molecular sieves seem to be good materials for the photoionization of large photoactive molecules such as aromatic amines and porphyrins. In previous studies of the photoionization of alkylporphyrins in vesicle solutions such as dipalmitoylphosphatidylchlorine and hexadecyl phosphate vesicles, tetraphenylporphyrin cation radicals are not stabilized at room temperature.43,47,48 The solubilization limit of alkyltetraphenylporphyrins into these vesicle solutions is about 0.4 mmol of porphyrin/1 mL of vesicle solution. A tetraphenylporphyrin with no pendant alkyl chain like H2TPP is not solubilized into such vesicle solutions.11 The photoyield of tetraphenylporphyrin cation radicals is thus limited by the porphyrin solubility in such vesicle solutions. But in mesoporous TiMCM-48 and TiMCM-41 materials,49 adsorbed tetraphenylporphyrins with or without pendant alkyl chains can form stable photoinduced porphyrin cation radicals at room temperature. The photoyield of H2TPP+ cation radicals in TiMCM-48 is about twice as great as that obtained in TiMCM-41 with a similar pore size, although the stability of H2TPP+ is similar in TiMCM-48 and TiMCM-41. Therefore, mesoporous TiMCM-48 molecular sieves are promising hosts to accomplish long-lived photoinduced charge separation of tetraphenylporphyrins at room temperature. The H2TPP+ photoyield increases in the order MCM-48 < AlMCM-48 < TiMCM-48 as shown in Figure 5. The surface hydroxy groups on silica gel were suggested to serve as electron acceptors to generate trapped hydrogen atoms whose ESR signal intensity decreases with a decrease of pore size of the silica gel.50 SiMCM-48 and AlMCM-48 apparently have some electron acceptors which may involve surface hydroxy groups.45,50 Since MCM-48 (43) Chastenet de Castaing, E.; Kevan, L. J. Phys. Chem. 1991, 95, 10178. (44) Gouterman, M.; Holten, D. Photochem. Photobiol. 1982, 19, 209. (45) Krishna, R. M.; Prakash A. M.; Kevan, L. J. Phys. Chem. B 2000, 104, 1796. (46) Luan, Z.; Bae, Y. J.; Kevan, L. Microporous Mesoporous Mater. 2001, 48, 189. (47) Kang, Y. S.; Kevan, L. J. Phys. Chem. 1994, 98, 4398. (48) Lanot, M. P.; Kevan, L. J. Phys. Chem. 1991, 95, 10178. (49) Sung-Suh, H. M.; Luan, Z.; Kevan, L. J. Phys. Chem. 1997, 101, 10455. (50) Xiang B.; Kevan, L. Langmuir 1994, 10, 2688.
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has a small pore size (∼2.3 nm), the lack of trapped hydrogen atoms is consistent with earlier studies on silica gel.50 SiMCM-48 and AlMCM-48 have similar amounts of surface hydroxyl sites.51 Isomorphous substitution of Al for Si in the framework of MCM-48 creates new acidic sites and causes the AlMCM-48 framework to become negatively charged.51-53 In the early days of zeolite chemistry, it was calculated that their internal electric fields could reach 6.3 V Å-1 at a distance of a few angstroms from a cation site in zeolite Y.1 Electric fields of this magnitude can readily polarize molecules in close proximity and alter reactivity.54 Although the acid sites in AlMCM-48 are weaker than those in zeolites, they may still play some role in enhancing the photoionization of H2TPP. This is supported by the increase in the H2TPP+ photoyield in AlMCM-48 with increasing Al content. This suggests that acid sites in AlMCM-48 are better electron acceptors than are terminal Si-OH hydroxy groups. The electrostatic field of the AlMCM-48 framework appears to influence the reactivity of encapsulated H2TPP to promote photoreaction. Figure 5 shows that the initial slopes for H2TPP+ intensity increase with photoirradiation time in the order SiMCM-48 (0.20 s-1) < AlMCM-48 (0.21 s-1) < TiMCM48 (0.38 s-1). The higher H2TPP photoyield and slower decay in TiMCM-48 compared to SiMCM-48 and AlMCM48 suggest that TiMCM-48 is a better electron acceptor than are SiMCM-48 and AlMCM-48. This is presumably due to reduction of Ti(IV) in the framework to Ti(III). It has been shown for catalytic oxidation of some aromatics, alkenes, and alcohols in titanium-substituted molecular sieves such as TS-135 that these molecules are selectively absorbed on Ti(IV) sites.55 Early studies showed that surface Ti(IV) centers in TiMCM-41 immobilize vanadium species and promote oxidation of VO2+ to V(V) probably due to the reducibility of Ti(IV) to Ti(III) in TiMCM-41.56 It also has been shown that surface titanium centers in the TiMCM-41 framework have a strong interaction with chromium species and stabilize high oxidization states of (51) Romero, A. A.; Alba, M. D.; Klinowski, J. J. Phys. Chem. B 1998, 102, 123. (52) Corma, A. Chem. Rev. 1997, 97, 2373. (53) Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. (54) Rabo, A. J.; Angell, L. C.; Kasai, H. P.; Schoemaker, V. Discuss. Faraday Soc. 1966, 41, 328. (55) Huybrechts, D. R. C.; Buskens, P. L.; Jacobs, P. A. J. Mol. Catal. 1985, 31, 355. (56) Luan, Z.; Kevan, L. J. Phys. Chem. B 1997, 101, 2020.
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Cr(V) and Cr(VI).57 Similarly, for photoionization of H2TPP in TiMCM-48, it seems that H2TPP molecules are preferentially adsorbed at surface Ti(IV) sites. This results in a higher H2TPP+ photoyield and slower radical decay in TiMCM-48 compared with SiMCM-48 and AlMCM-48. 29Si magic angle spinning NMR spectra of SiMCM-48 and TiMCM-48 show that Ti substitution causes an increase of Q4-Si content and reduces the Q2-Si and Q3-Si contents compared with SiMCM-48. Therefore, TiMCM-48 contains fewer surface hydroxyl sites than does SiMCM-48.2,6 So it is suggested that the Ti(IV) sites in TiMCM-48 act as electron acceptors.25,35,58,59 One may expect to observe an ESR signal of Ti(III) generated by photoelectron transfer from H2TPP, but this is not detected at 77 K. Although the high photoyield and stability of H2TPP+ in TiMCM-48 clearly indicate that Ti(IV) sites in the TiMCM-48 framework are the probable electron acceptors, this is not yet unambiguously proved. Conclusions The photoionization of bulky H2TPP in MCM-48 mesoporous molecular sieves at room temperature increases in the order TiMCM-48 > AlMCM-48 > SiMCM-48, indicating that the photoionization efficiency via electron transfer depends on the type of metal ion. AlMCM-48 has a higher H2TPP+ photoyield than does SiMCM-48 probably because the negatively charged AlMCM-48 framework enhances the reactivity of H2TPP and acidic sites in AlMCM-48 are better electron acceptors than the terminal Si-OH hydroxy groups in SiMCM-48. It seems that the Ti(IV) sites in TiMCM-48 framework most enhance the electron-accepting ability of the framework probably by reduction to Ti(III). Mesoporous MCM-48 molecular sieves are shown to be promising heterogeneous hosts for longlived photoinduced charge separation of incorporated meso-tetraphenylporphyrin. Acknowledgment. This research is supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Science, U.S. Department of Energy, the Texas Advanced Research Program, the Environmental Institute of Houston and the Robert A. Welch Foundation. LA011414O (57) Zhu, Z.; Hartmann, M.; Maes, M. E.; Czernuszewicz, S. R.; Kevan, L. J. Phys. Chem. B 2000, 104, 4690. (58) Topsoe, N. Y. Science 1994, 265, 1217. (59) Tuel, A.; Diab, J.; Gelin, P.; Dufaux, M.; Dutel, J. F.; Ben Taarit, Y. J. Mol. Catal. 1991, 68, 45.
