J. Phys. Chem. B 2000, 104, 7981-7986
7981
Photoinduced Charge Separation of Methylphenothiazine in Microporous Metal Silicoaluminophosphate M-SAPO-n (M ) Cr, Fe, Mn, n ) 5, 8, 11) Materials Koodali T. Ranjit, Zhixiang Chang, R. M. Krishna, A. M. Prakash, and Larry Kevan* Department of Chemistry, UniVersity of Houston, Houston, Texas, 77204-5641 ReceiVed: April 18, 2000
Metal silicoaluminophosphate, M-SAPO-n, (M ) Cr, Fe or Mn, n ) 5, 8, 11) microporous materials with incorporated methylphenothiazine are photoionized with 320 nm light at room temperature. Methylphenothiazine cation radicals are produced and characterized by electron spin resonance. Transition metal ion containing microporous M-SAPO are found to be efficient hosts for the formation and stabilization of methylphenothiazine cation radicals (PC1+). The photoyield is either negligible or not observed in the case of H-SAPO-n materials, indicating that the transition metal ions serve as suitable electron acceptors and assist in the stabilization of the photoproduced PC1+ radicals. Chromium containing SAPO materials exhibited the highest photoyield among the Cr, Fe, and Mn transition metal ions studied. The photoionization efficiency was found to depend on the nature of the metal ion, the concentration of the metal ion, and also on the pore size of the microporous SAPO material used.
Introduction The design of materials that can be potentially developed into molecular photochemical energy conversion and storage systems is an exciting area of research.1-3 The challenge lies in the design of systems in which each part of the energy conversion process can be incorporated into one material. One of the main objectives is maintaining the integrity of the charge-separated state long enough so that the free energy of the photoproduced cation can be utilized to drive a chemical reaction. The main problem in achieving such long-lived charge separation is reverse electron transfer, which is often very rapid. Recombination results in the loss of the stored energy of the photoproduced cation into heat. A variety of host systems have been examined to improve the efficiency of photoinduced energy storage by preventing rapid back electron-transfer reaction.4-6 Heterogeneous systems such as micelles, vesicles, silica gels, and molecular sieves can provide appropriate spatial organization of both the donor and acceptor molecules to retard back electron transfer. Thus, appropriate tuning of the electronic and spatial properties of the host system7-9 can prevent undesirable back electron transfer. Methylphenothiazine and other N-alkylphenothiazines with longer alkyl chains have excellent photoactive electron donating capacity with a number of electron acceptors. They can be easily oxidized by ultraviolet irradiation to form phenothiazine cation radicals, which have been characterized by electron spin resonance (ESR) and optical spectroscopic techniques.10-13 A photoactive molecule such as methylphenothiazine can be incorporated inside microheterogeneous host systems which have pore dimensions greater than its molecular dimension; such a host system can often stabilize the photoinduced radical cations with relative ease.14,15 Silicoaluminophosphates (SAPO-n) belong to a class of microporous materials which are potential candidates in the ongoing search for new efficient catalysts.16 SAPO-5, SAPO-11, and SAPO-8 silicoaluminophosphates have no aluminosilicate zeolite analogues. The SAPO-11 molecular
sieve is composed of 4-ring, 6-ring, and 10-ring straight channels, whereas in SAPO-5, 4-ring, 6-ring, and 12-ring channels are formed, while for SAPO-8, 4-ring, 6-ring, and 14ring channels are formed. This leads to channels of 6.3, 7.3, and 8.7 Å diameter in SAPO-11, SAPO-5 and SAPO-8, respectively. The photooxidation of phenothiazine and methylphenothiazine in micelles with metal ions as acceptors has been studied.17-20 In the present study we have examined the photoionization of methylphenothiazine incorporated into M-SAPO-n materials (M ) Cr, Fe, or Mn and n ) 5, 8, 11). The idea that the metal ions can act as suitable electron acceptors prompted us to incorporate metal ions into the SAPO-n material and then examine the photooxidation of methylphenothiazine by UV irradiation. The photoyield was found to depend on the nature of the metal ion, the amount of the metal ion, and the pore size of the SAPO materials used. Relatively high photoyield and excellent stability were observed, which suggests that such systems can act as potential candidates for photochemical conversion and storage devices. Experimental Section Synthesis of SAPO-n (n ) 5, 8, 11). The synthesis of SAPO-5 was achieved following a literature procedure.21 In brief, aluminum isopropoxide (23.16 g) was slurried in 35 mL of water. Phosphoric acid (85 wt %, 13 g) was diluted in 20 mL water and added to the alumina slurry and stirred for 1 h. Cyclohexylamine (4.0 g) was then added and the gel stirred for 90 min at room temperature. Silica gel (Ludox AS-40, 40 wt %, 6 g) was then added and stirred for a further 10 min. The gel was then placed in a Teflon-lined autoclave and heated to 473 K for 14 h under autogenous pressure. After the hydrothermal synthesis the autoclave was quenched to room temperatur and the product separated from the mother liquor, washed with deionized water, and dried at 353 K in air overnight. The X-ray diffraction (XRD) pattern of this assynthesized SAPO-5 is consistent with literature data.21
10.1021/jp001485g CCC: $19.00 © 2000 American Chemical Society Published on Web 07/26/2000
7982 J. Phys. Chem. B, Vol. 104, No. 33, 2000 SAPO-11 was also synthesized according to the literature.22 A reaction mixture was prepared by combining 11.56 g of 85 wt % orthophosphoric acid and 5.0 g of water with 20.42 g of aluminum isopropoxide and stirring well. This mixture was added to 6.0 g of an aqueous sol containing 30 wt % SiO2 and 0.8 g of water and stirred until homogeneous. Then, 4.6 g of di-n-propylamine was added and the mixture stirred until homogeneous. The reaction mixture was then placed in a Teflonlined autoclave and heated to 473 K for 48 h. The solid product was recovered, washed with water, and dried in air at 353 K. The X-ray diffraction pattern was the same as that reported in the literature.22 SAPO-8 was prepared by calcination of SiVPI-5 at 403 K for 12 h in air. SiVPI-5 was prepared by a procedure developed in our laboratory.23 A typical synthesis involves 6.9 g of alumina (Catapal B, Vista) mixing with 20 g of water and stirring for 30 min. Then 11.53 g of 85 wt % phosphoric acid was dissolved in 10 g of water and added to the alumina slurry. Finally 10.49 g of triisopropanolamine (Fluka) and tetramethylammonium hydroxide (0.91 g) in 12 mL of water was added and the gel aged for 2 h. The gel was placed in a Teflon-lined autoclave and heated at 408 K for 15 h under autogenous pressure. After the hydrothermal synthesis the solid was washed with deionized water and dried at 353 K in air overnight. Preparation of M-SAPO-n. Metal ions were incorporated into the SAPO materials in both extraframework and framework positions. To incorporate metal ions into extraframework positions, ion exchange of protons of calcined H-SAPO materials was done by liquid state and solid-state ion exchange. Typical liquid-state ion exchange was performed by adding 10 mL of 5 × 10-3 M Fe(NO3)3, Cr(NO3)3, or Mn acetate and 40 mL water to 2 g of H-SAPO-n and stirring the mixture overnight at room temperature. The samples were then filtered, washed with hot distilled water to remove any excess metal ions on the surface of the sample, and then dried in air to form MH-SAPO-n. The samples prepared in this manner are designated as MH-SAPO-n(l). Solid-state ion exchange was performed with ∼2 g H-SAPO-5 and 0.02 g FeCl3, CrCl3, or Mn acetate ground together well and heated to 873 K for 18 h in air. The reaction product was slowly cooled to room temperature and ground to a fine powder. The samples prepared in this manner are designated as MH-SAPO-n(s). In addition to liquid state and solid-state ion exchange, CrAPSO-5 was prepared. As-synthesized CrAPSO-5 was prepared as described for SAPO-5, except that CrCl3 was introduced into the synthesis gel. Methylphenothiazine Incorporation. Methylphenothiazine (Aldrich) was incorporated by immersing 0.1 g M-SAPO-n in 1 mL of 1 × 10-2 M methylphenothiazine in benzene for ∼12 h in the dark. The benzene was removed by flowing nitrogen gas over the sample for 1 h. For electron spin resonance (ESR) measurements, 0.1 g of the sample was transferred into Suprasil quartz tubes (2 mm i.d. × 3 mm o.d.), which were sealed at one end. The samples were then evacuated below 1 Torr for 2 ∼ 4 h and flame sealed. It was observed that the photoyield of samples subjected to evacuation to remove possible remaining traces of solvent was similar to the photoyield obtained for samples that were not evacuated. Hence, further photoionization experiments were carried out without evacuation. For diffuse reflectance spectroscopy experiments the samples were loaded into a cylindrical quartz sample cell (22 mm diameter × 2 cm path length).
Ranjit et al. Characterization. XRD powder patterns were recorded on a Siemens 5000 X-ray diffractometer using Cu KR radiation in the range 10° < 2θ < 50°. Chemical analysis was performed by electron microprobe analysis on a JEOL JXA-8600 spectrometer. The composition of the SAPO materials was determined by calibration with known standards and by averaging over several defocused areas to give the bulk composition. ESR spectra were recorded at room temperature at 9.5 GHz using a Bruker ESP 300 spectrometer with 100 kHz field modulation and low microwave power to avoid power saturation. Photoproduced phenothiazine radical cation (PC1+) yields were determined by double integration of the ESR spectra using the ESP 300 software. Each photoyield is an average of three determinations. Diffuse reflectance spectra were recorded at room temperature using a Perkin-Elmer model 330 spectrophotometer equipped with an integrating sphere. Thermal gravimetric analyses (TGA) of the samples were performed using a TGA 2050 analyzer from TA instruments in oxygen atmosphere at a heating rate of 10 °C/min. Photoirradiation. The methylphenothiazine-containing SAPO materials were irradiated using a 300 W Cermax Xenon lamp (ILC-LX 300 UV) at room temperature. The light was passed through a 10 cm water filter to prevent infrared radiation and through a Corning No. 7-54 filter to give light of ∼1 × 106 erg cm-2 s-1 intensity with a maximum at 320 ( 20 nm. This wavelength is absorbed by methylphenothiaizne and not by the SAPO materials. The samples were placed in a quartz Dewar and rotated at a speed of 4 rpm to ensure even irradiation. The photoproduced methylphenothiazine cation radicals were identified by ESR. Results The pore sizes of the SAPO materials prepared range from 6.3 to 8.7 Å, which are bigger than the smaller molecular dimension of 5 Å for methylphenothiazine. Thus it is possible to incorporate the photosensitizer molecule into these SAPO materials. Preliminary photoionization experiments were carried out with M-SAPO-5 microporous materials. The structures of as-synthesized SAPO-5, calcined H-SAPO-5, and as-synthesized CrAPSO-5 show no significant differences in their XRD patterns and agree well with the literature data for SAPO-5. The XRD patterns of SAPO-11 samples show them to be highly crystalline and in good agreement with literature reports. It was noted that the sample crystallinity was reduced by ∼20% and ∼30% after calcination and ion-exchange, respectively. SAPO-8 was obtained by the thermal transformation of SiVPI-5 in air. The X-ray pattern shows that the sample is highly crystalline. UV-vis Spectra. The spectra of calcined CrAPSO-5 show four bands at 270 nm, 350 nm, 450 nm, and a broad band centered at 650 nm. The bands at 270 and 350 nm are typically assigned to charge-transfer transitions associated with Cr(VI).24 However, we did not detect any Cr(VI) in our calcined samples. To test for extraframework Cr(VI), 1 g of CrAPSO-5 was stirred in 20 mL of 1 M Ca(NO3)2 solution overnight. The filtrate was collected, and 1 M AgNO3 was added dropwise to test for CrO42-, but no Ag2CrO4 precipitate was formed. Also, d-d optical transitions of Cr(V) occur in the same region.25 Thus, we assign the 270 and 350 nm bands to Cr(V) formed by the oxidation of Cr(III) during calcination. The four bands obtained in calcined CrAPSO-5 materials can be assigned to Cr(V) in either square-pyramidal or distorted-octahedral coordination, where the additional coordination is due to water molecules. The bands of 450 and 650 nm are typical of octahedral Cr
Photoinduced Charge Separation of Methylphenothiazine
J. Phys. Chem. B, Vol. 104, No. 33, 2000 7983
Figure 2. Room-temperature photoinduced methylphenothiazine radical cation yield measured by ESR versus irradiation time for MHSAPO-5(l)-PC1 (M ) Cr, Fe, and Mn).
Figure 1. ESR spectra of CrH-SAPO-5(l)-PC1 sample at room temperature upon UV irradiation at (a) 0 min, (b) 5 min, (c) 30 min, and (d) 60 min.
species. A similar UV-vis spectrum was obtained for CrSAPO-5(s), suggesting that the oxidation state of Cr is +5 in this sample. Thus, Cr exists in the +5 oxidation state in calcined as-synthesized samples and in solid-state ion-exchange sample. However, for liquid-state ion-exchanged samples, Cr exists in the +3 oxidation state. The Cr(V) and Cr(III) assignments in CrAPSO-5(s) and Cr-SAPO-5(l) agree well with ESR results. Samples of CrH-SAPO-5(l) impregnated with methylphenothiazine show very weak ESR signals of PC1+ based on g ) 1.9994 with incompletely resolved hyperfine lines before irradiation as shown in Figure 1. This shows that some methylphenothiazine cation radicals are produced during sample preparation. After being irradiated by 350 nm light at room temperature for 5 min, the samples showed strong ESR signals. With further increase in irradiation time to 30 min, the intensity of ESR signal increases further and then almost reaches a plateau after 60 min. The spectral widths of the ESR spectra are consistent with those of the methylphenothiazine cation radical in homogeneous solution and in micelles at room temperature.26,27 The weak ESR signals arising before irradiation have the same line shape as those observed after irradiation. The partially resolved ESR spectra of PC1+ at room temperature show that the radicals have some mobility in the SAPO-5 framework at room temperature. In addition, a visual change in the color of the SAPO-PC1 is easily discerned; the samples are light pink in color prior to irradiation but turn dark pink after irradiation, characteristic of PC1+ cation radicals.28,29 This further confirms the photoionization of PC1 into PC1+ cation radicals. Figure 2 shows the changes in the intensity of the ESR signal due to PC1+ cation radicals in liquid-state ion-exchanged MHSAPO-5(l) samples. From Figure 2 it is seen that the highest
Figure 3. Decay of photoproduced PC1+ cation radical at room temperature for FeH-SAPO-5 and MnH-SAPO-5 after 60 min irradiation.
yield is obtained for CrH-SAPO-5(l)-PC1 sample. Also, the ESR signals rapidly increase during the first 20 min of irradiation and then reach a plateau in about 60 min. An irradiation time of 60 min was hence selected for comparative photoyield and stability studies. The photoyield of CrH-SAPO5(l) is about 3-fold higher than the photoyield in H-SAPO5-PC1. The rate of formation of the PC1+ cation radicals can be evaluated from the initial slopes in Figure 2 assuming firstorder kinetics. The calculated rate constants for the formation of PC1+ cation radicals are k ) 15 × 10-4 s-1 for CrH-SAPO5(l); k ) 5.7 × 10-4 s-1 for MnH-SAPO-5(l), and k ) 1.5 × 10-4 s-1 for FeH-SAPO-5(l). This order of rate constants is consistent with the order of photoyields. The stability of the photoproduced PC1+ as monitored by ESR is also an important factor in the design of efficient photoredox systems. The decay of PC1+ can be calculated by assuming firstorder kinetics from eq 1.
M(n-1)+ + PC+• f Mn+ + PC
(1)
The rate of decay of PC1+ cation radicals for FeH-SAPO-5(l) and MnH-SAPO-5(l) was evaluated from the initial slopes in Figure 3, assuming first-order kinetics. In case of CrH-SAPO5(l), the ESR intensity was essentially the same for several hours. Hence, for CrH-SAPO-5(l), the intensity of the ESR signal was monitored every 24 h. The rate constant was
7984 J. Phys. Chem. B, Vol. 104, No. 33, 2000
Figure 4. Room-temperature photoinduced methylphenothiazine radical cation yield measured by ESR versus irradiation time for MHSAPO-5(s)-PC1 (M ) Cr, Fe, and Mn).
evaluated assuming a first-order expression for CrH-SAPO5(l). The rate constants for the decay of the PC1+ in M-SAPO5(l) were evaluated to be k ) 4.3 × 10-4 s-1 for MnH-SAPO5(l) and k ) 2.4 × 10-4 s-1 for FeH-SAPO-5(l). In the case of CrH-SAPO-5(I)-PC1 the half-life (t1/2) was estimated to be 14 days. The solid-state ion-exchange process introduces metal ions into the nonframework sites in a manner similar to the liquid ion-exchange process. To study the influence of the metal content on the photoyield, a different ratio of Si/M (M ) Cr, Fe) was prepared. It was found that the photoyield was dependent on the metal content. In the case of MnH-SAPO5(s), the ratio of Si/Mn was maintained in a manner similar to the one in liquid ion-exchange sample, and the photoyields were found to be similar. Figure 4 shows changes in the ESR intensity with time for MH-SAPO-5(s) samples. The stability of the photoproduced PC1+ radical cations was also evaluated and it was found that the CrH-SAPO-5(s)-PC1 sample exhibited the highest stability toward PC1+ radical cations; t1/2 was estimated to be 12 days. We have observed that the Cr-containing SAPO-5 materials exhibit maximum activity. To understand the influence of the Cr content on the photoyield and the stability, Cr containing as-synthesized samples designated as CrAPSO-5 were prepared with a varying Si/Cr ratio. The results obtained from the photoionization of methylphenothiazine in CrAPSO-5 are shown in Figure 5. It is evident that the photoyield increases as the concentration of Cr increases, but beyond a certain Cr content the photoyield decreases. Thus, there is an optimum chrominum concentration at which the photoyield is maximum. One of the most interesting results obtained in the present study is the remarkable stability of the CrAPSO-5 samples toward PC1+ radical cations. The half-life of the CrAPSO-5 (Si/Cr ) 105) was calculated to be as high as 100 days. The pore size of the host medium plays a significant role in the photoyield and stability of the photoproduced PC1+ radical cations.14 To examine the effect of pore size, the photoionization of methylphenothiazine was examined for M-SAPO-11(l) samples. The results obtained from photoionization are shown in Figure 6. As can be seen from Figure 6, CrH-SAPO-11(l)-PC1 exhibits the maximum photoactivity. The trends obtained in the photoactivity are similar to those observed for SAPO-5(l) samples. It is noteworthy that the photoyield is lower in the case of M-SAPO-11(l)-PC1 when compared with M-SAPO-5(l)-PC1. The stability of PC1+ radical cations in
Ranjit et al.
Figure 5. Room-temperature photoinduced methylphenothiazine radical cation yield versus Si/Cr ratio for CrAPSO-5-PC1.
Figure 6. Room-temperature photoinduced methylphenothiazine radical cation yield measured by ESR versus irradiation time for MHSAPO-11(l)-PC1 (M ) Cr, Fe, and Mn).
SAPO-11 is also found to be lower compared to SAPO-5. For example, the half-life of CrH-SAPO-5(l)-PC1 is 14 days, whereas in the case of CrH-SAPO-11(l)-PC1 it is only 6 days. SAPO-8 is a large pore SAPO material formed by the intersection of 6-ring and 4-ring pores to form 14-ring channels having dimensions of 7.9 Å × 8.7 Å. Since the Cr-containing samples exhibited the maximum photoyield, the photoionization studies were carried out for CrH-SAPO-8(l) samples. The results are depicted in Figure 7. Discussion The ESR results clearly confirm the photooxidation of methylphenothiazine molecules into methylphenothiazine cation radicals in the SAPO materials at room temperature. The increase in the intensity of the ESR signal due to PC1+ radical cations with time in the case of M-SAPO-5(l) samples (Figure 1) suggests that transition metal ions assist in photoionization of methylphenothiazine. The role of metal ions as electron acceptors is clearly evident from the fact that there is net photoionization of methylphenothiazine only in metal-containing SAPO samples, whereas in the case of the H-SAPO-5, no net photoionization was observed. The H-SAPO-5-PC1 show very weak ESR signals and almost no color indicating that the methylphenothiazine radical cations are not stabilized in it. Additional evidence for the role of the metal ion as the electron acceptor is a constant total ESR intensity versus photoirradiation time for MH-SAPO-PC1, whereas a decrease
Photoinduced Charge Separation of Methylphenothiazine
Figure 7. Room-temperature photoinduced methylphenothiazine radical cation yield measured by ESR versus irradiation time for CrHSAPO-n(l)-PC1 (n ) 5, 8, 11).
in the spin concentration of the metal ions is observed. These points clearly suggest that transition metal ions either in a framework site or in a nonframework site act as efficient electron acceptors and assist in stabilizing the methylphenothiazine cation radical. Attempts to observe reduced metal ion paramagnetic species were not successful because of overlap with the PC1+ spectrum. The remarkable stability observed for CrAPSO-5 suggests that the electron remains on the Cr ion. Since Cr4+ is ESR silent it is not observed by ESR, but the factors discussed above clearly suggest that the transition metal ion acts as an electron acceptor. Experiments with transition metal ions (Cr3+, Fe3+, Mn2+) in SAPO-PC1 show strong ESR signals at room temperature. Figure 1 shows the increase in the intensity of the ESR signal due to PC1+ with irradiation time for M-SAPO-5(l), with transition metal-ions in ion-exchange sites. As one can observe from Figure 1, the presence of Cr in the SAPO-5 material enhances the photoyield compared to either Mn or Fe. The photoionization efficiency of transition metal ions in SAPO-5 and SAPO-11 decreases in the order Cr-SAPO-PC1 > Mn-SAPO-PC1 > Fe-SAPO-PC1. Thus the photoionization efficiency is controlled by the nature of the metal ion in the ion-exchange sites. The presence of chromium in a framework position dramatically increases the photoyield and the stability of the methylphenothiazine cation radical. The photoyield increases as the Cr ratio in the SAPO sample increases, but above a certain concentration, the photoyield decreases. Diffuse reflectance spectra of the CrAPSO-5 samples containing higher Cr content, namely Si/Cr ) 58 and Si/Cr ) 13, did not reveal any Cr oxide phase. The lower activity obtained by the CrAPSO-5 samples having Si/Cr ) 58 and Si/Cr ) 13 probably suggests the formation of secondary radicals. Evidence for this comes from three facts. First, the intensity of the ESR signal of the CrAPSO-5 samples having Si/Cr ratios of 58 and 13 was found to increase in the dark by about 10% at room temperature after 60 min of photoirradiation. This might be due to some secondary oxidation leading to the formation of radicals other than PC1+. Second, visually there was a difference in the color of these two CrAPSO-5-PC1 samples after 60 min photoirradiation. The samples usually turn dark pink after irradiation, but for CrAPSO5-PC1 having Si/Cr ratios of 58 and 13, the color was found to be light brown. Third, the decays for MH-SAPO-PC1 and CrAPSO-PC1 (Si/Cr ) 242 and Si/Cr ) 105) could be fit by a simple first-order exponential decay equation. However, for
J. Phys. Chem. B, Vol. 104, No. 33, 2000 7985
Figure 8. Differential thermal analysis of SAPO-n (n ) 5, 8, 11) with incorporated methylphenothiazine.
CrAPSO-5-PC1 (Si/Cr ) 58 and Si/Cr ) 13) a first-order exponential decay did not give an appropriate fit. These factors suggest that in these two samples some secondary oxidation products are also formed in addition to PC1+. A semiquantitative estimation of the efficiency of the photoreaction for CrAPSO-5 can be obtained from the ratio of the number of PC1+ radicals produced to the number of initial PC molecules. The photoyield is thus estimated to be ∼14%, which is reasonably high. One of the remarkable results obtained from the present study lies in the stability of the photoproduced PC1+ radical cations. The half-life of PC1+ in CrAPSO-5-PC1 (Si/Cr ) 105) was estimated to be as high as 100 days. The higher photoyield and the excellent stability of the methylphenothiazine cation radicals obtained can be accounted for on the basis of the higher electron affinity of Cr5+ (Eo Cr5+/Cr4+ ) 1.340 V) as compared to Fe3+ (Eo Fe3+/Fe2+ ) 0.771 V).30 Apparently the reduction potential of the Mn2+ (Eo Mn2+/Mn1+ ) -3.0 V)31 is too negative, making reaction 2 energetically unfavorable.
Mn+ + PC f M(n-1)++ PC+
(2)
The fact that CrAPSO-5-PC1 exhibits remarkable stability toward PC1+ suggests that the standard potential of the Mn+/ M(n-1)+ redox couple may be used as a guide for the efficiency of the process. The pore size is a critical parameter affecting the yield of the photoproducts. This is probably related to the control of the spatial separation between the electron donor and the acceptor by the pore size. Thermal gravimetric analysis suggests that SAPO materials can accommodate molecules such as methylphenothiazine inside their channels and cages. The mobility of the molecule is restricted based on the channel geometrical dimensions. Figure 8 shows the results obtained from the incorporation of methylphenothiazine into the SAPO samples. All the three curves typically show three weight losses, the first near 100 °C attributed to water desorption, the second near 200 °C attributed to methylphenothiazine desorption, and the third broad peak centered around 470 °C assigned to the decomposition of methylphenothiazine in an oxygen atmosphere. This peak is however not observed when the experiments are carried out in a nitrogen atmosphere. Thus we can conclude from the TGA results that methylphenothiazine penetrates completely into the channels of SAPO-8, to a lesser degree into SAPO-5, and only partially into SAPO-11. This is consistent with the results obtained from the photoionization experiments
7986 J. Phys. Chem. B, Vol. 104, No. 33, 2000 which show a decrease in the order MH-SAPO-8 > MHSAPO-5 > MH-SAPO-11. Conclusions Microporous transition metal ion containing SAPO-n (n)5, 8, 11) materials are potential candidates for stable photoinduced charge separation of methylphenothiazine molecules. The PC1+ cation radical photoyield depends on the incorporation of metal ions into ion-exchange sites of SAPO materials. New physical insights are shown by the dependence of the photoyield on the metal type and on the pore size of the SAPO material. The results clearly indicate that metal-containing SAPO materials provide appropriate steric and electrostatic environment to retard back electron transfer and increase the lifetime of the photogenerated radical ions for many days or even months at room temperature. Acknowledgment. This research was supported by the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy; the Texas Advanced Research Program, and the Environmental Institute of Houston. References and Notes (1) Kalyanasundaram, K. Photochemistry in Microheterogeneous Systems; Academic Press: New York, 1987. (2) Ramamurthy, V. Photochemistry in Organized and Constrained Media; VCH Publishers: New York, 1991. (3) Scaiano, J. C.; Garcia, H. Acc. Chem. Res. 1999, 32, 783. (4) Slama-Schwok, A.; Avnir, D.; Ottolenghi, M. Photochem. Photobiol. 1991, 54, 525. (5) Yonemoto, E. H.; Kim, Y. I.; Schmehl, R. H.; Wallin, J. O.; Shoulders, B. A.; Richardson, B. R.; Haw, J. F.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 10557. (6) Vermeulen, L. A.; Thompson, M. E. Nature, 1992, 358, 656.
Ranjit et al. (7) Kalyanasundaram, K. Coord. Chem. ReV. 1982, 46, 159. (8) Kuchi, V.; Oliver, A. M.; Paddon-Row, M. N.; Howe, R. F. Chem. Commun. 1999, 1149. (9) Brigham, E. S.; Snowden, P. T.; Kim, Y. I.; Mallouk, T. E. J. Phys. Chem. 1993, 97, 8650. (10) Forbes, W. F.; Sullivan, P. D. J. Am. Chem. Soc. 1966, 88, 2862. (11) Clarke, D.; Gilbert, B. C.; Hanson, P. J. Chem. Soc., Perkin Trans. 2 1975, 1078. (12) Clarke, D.; Gilbert, B. C.; Hanson, P. J. Chem. Soc., Perkin Trans. 2 1978, 1103. (13) Fujihara, H.; Fuke, S.; Yoshihara, M.; Maeshima, T. Chem. Lett. 1981, 1271. (14) Kurshev, V.; Prakash, A. M.; Krishna, R. M.; Kevan, L. Microporous Mesoporous Mater 2000, 34, 9. (15) Krishna, R. M.; Prakash, A. M.; Kurshev, V.; Kevan, L. Phys. Chem. Chem. Phys. 1999, 1, 4119. (16) Wilson, S. T.; Lok, B. M.; Messina, C. A.; Cannon, T. R.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. (17) Alkaitis, S. A.; Beck, G.; Graetzel, M. J. Am. Chem. Soc. 1975, 97, 7, 5723. (18) Moroi, Y.; Braun, A. M.; Graetzel, M. J. Am. Chem. Soc. 1979, 101, 567. (19) Kang, Y. S.; Baglioni, P.; McManus, H. J. D.; Kevan, L. J. Phys. Chem. 1991, 95, 7944. (20) Kang, Y. S.; McManus, H. J. D.; Kevan, L. J. Phys. Chem. 1993, 97, 2027. (21) Young, D.; Davis, M. E. Zeolites 1991, 11, 277. (22) Wilson, S. T.; Lok, B. M.; Flanigen, E. M.; U.S. Patent 4,310,440, 1982. (23) Hartmann, M.; Kevan, L. Faraday Trans. 1996, 92, 3661. (24) Weckhuysen, B. M.; Schoonheydt, R. A. Zeolites 1994, 14, 360. (25) Garner, C. D.; Kendrick, J.; Lambert, P.; Mabbs, F. E.; Hillier, I. H. Inorg. Chem. 1976, 15, 1287. (26) Shine, H. J.; Thompson, D. R.; Venziani, C. J. Heterocyl. Chem. 1967, 4, 517. (27) Hovey, M. C. J. Am. Chem. Soc. 1982, 104, 4196. (28) Xiang, B.; Kevan, L. Langmuir 1994, 10, 2688. (29) Xiang, B.; Kevan, L. J. Phys. Chem. 1994, 98, 5120. (30) Lide, D. R., Ed. Handbook of Chemistry and Physics, 80th ed.; CRC: Boca Raton, FL, 1999; pp 8-22. (31) Baxendale, J. H.; Dixon, R. S. Z. Phys. Chem. 1969, 43, 161.
