Extrazeolite Electron Transfer at Zeolite-Encapsulated Polypyridyl

V. Ganesan and R. Ramaraj*. School of Chemistry, Madurai Kamaraj University, Madurai-625 021, India. Received June 20, 1997. In Final Form: December 1...
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Langmuir 1998, 14, 2497-2501

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Extrazeolite Electron Transfer at Zeolite-Encapsulated Polypyridyl Metal Complex Coated Electrodes V. Ganesan and R. Ramaraj* School of Chemistry, Madurai Kamaraj University, Madurai-625 021, India Received June 20, 1997. In Final Form: December 12, 1997 Polypyridyl metal complexes ([Ru(bpy)3]2+ and [Fe(bpy)3]2+) were synthesized inside the supercages of zeolite-Y and adsorbed on the surface of zeolite-Y (represented as Ru(II)-Y(c/s) and Fe(II)-Y(c/s), where “c” refers the supercage of zeolite-Y and “s” refers the surface of zeolite-Y). These zeolite-encapsulated metal complexes were coated on a platinum electrode, and their electrochemical properties were studied in 0.1 M Na2SO4 and in 0.05 M H2SO4. The Ru(II)-Y(c) and Fe(II)-Y(c) were electroinactive in 0.1 M Na2SO4. The same metal complexes in zeolite show electrochemical activity in the presence of H+ ions. The electroinactivity of Ru(II)-Y(c) and Fe(II)-Y(c) complexes in 0.1 M Na2SO4 shows that the intrazeolite electron transfer is not occurring. The disintegration of zeolite-Y in the presence of H+ ions leads to the random distribution of the polypyridyl metal complexes and shows that extrazeolite electron-transfer mechanism is operating as in the case of zeolite surface adsorbed metal complexes.

Introduction Researchers have recently become increasingly interested in chemically modified electrodes, because they offer the basis for new applications in electrocatalysis and photoelectronic devices.1-3 To prepare chemically modified electrodes, polyelectrolyte polymers (Nafion, polystyrenesulfonate, etc.), clays, and zeolites have been used.1-5 A key question concerning the behavior of modified electrodes is the kinetics of the “homogeneous” charge transport process within the redox molecule incorporated film. This redox process will require concurrent uptake of counterions into the film or repulsion of co-ions initially present in the film as ion pair for charge neutrality.2 Preparing structured modified electrodes which can incorporate inorganic metal complexes in zeolites and clays is a new direction in this research.6-9 Zeolites are aluminosilicates with supercages and window openings of molecular dimensions.10 Molecules can be encapsulated inside the supercages of the zeolite based on their size and shape selective properties.11 Bedioui et al.12-16 have synthesized metal Schiff base complexes inside the supercages of zeolite (ship-in-a-bottle (1) Murray, R. W., Ed; Molecular Design of Electrode Surfaces; John Wiley: New York, 1992. (2) Oyama, N.; Ohsaka, T. Prog. Polym. Sci. 1995, 20, 761. (3) (a) Kaneko, M.; Woehrle, D. Adv. Polym. Sci. 1988, 84, 141. (b) Lin, R. J.; Kaneko, M. In Molecular Electronics and Molecular Electronic Devices; Sienicki, K., Ed.; CRC Press: London, 1993. (4) (a) Rolison, D. R. Chem. Rev. 1990, 90, 867. (b) Walcarius, A. Electroanalysis 1996, 8, 971. (5) Murray, C. J.; Novak, R. J.; Rolison, D. R. J. Electroanal. Chem. 1984, 164, 205. (6) Bard, A. J.; Mallouk, T. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; John Wiley: New York, 1992; p 271. (7) Seneratne, C.; Zhang, J.; Baker, M. D.; Bessel, C. A.; Rolison, D. R. J. Phys. Chem. 1996, 100, 5849 and references citied therein. (8) Bedioui, F.; Devynck, J.; Balkus, K. J., Jr. J. Phys. Chem. 1996, 100, 8607. (9) Rolison, D. R.; Bessel, C. A.; Baker, M. D.; Seneratne, C.; Zhang, J. J. Phys. Chem. 1996, 100, 8610. (10) Breck, D. W. Zeolite Molecular Sieves; Krieger, R. E., Malabar, F. L., Eds.; 1984. (11) Ramamurthy, V.; Turro, N. J. J. Inclusion Phenom. 1995, 25, 239 and references citied therin. (12) Masfer, K.; Carre, B.; Bedioui, F.; Devynck, J. J. Mater. Chem. 1993, 3, 873. (13) Bedioui, F.; Boysson, E. De.; Devynck, J.; Balkus, K. J., Jr. J. Electroanal. Chem. 1991, 315, 313.

complexes) and studied the electron transfer mechanism and their electrocatalytic applications. They have concluded that the electroactive species were reduced or oxidized within the supercages of zeolite-Y (intrazeolite electron transfer). The electron transfer to the electroactive molecules residing in the supercages of the zeolite is important, particularly in the case of electrocatalysis using zeolite-encapsulated catalytic systems.12 Shaw and co-workers17 and others18,19a have studied the electrontransfer mechanism of methyl viologen (MV2+) exchanged zeolite-Y. Rolison and co-workers7 and Baker et al.20 showed that the electron transfer takes place at the zeolite-solution interface (extrazeolite electron transfer). Calzaferri et al.19,21,22 studied the electrochemical properties of MV2+-, Cu2+- and Ag+-exchanged zeolites and proposed intrazeolite electron transfer at the coated electrodes. Very recently, the question of intra- or extrazeolite electron-transfer process has been discussed in detail by Rolison et al.7,9 and Bedioui et al.8 During the course of the electrochemical studies on the zeoliteencapsulated molecules, Sykora and Kincaid,23a Dutta and (14) Bedioui, F.; Boysson, E. De.; Devynck, J.; Balkus, K. J. Jr. J. Chem. Soc., Faraday Trans. 1991, 87, 3831. (15) Gaillon, L.; Sajot, N.; Bedioui, F.; Devynck, J.; Balkus, K. J., Jr. J. Electroanal. Chem. 1993, 345, 157. (16) Bedioui, F.; Briot, L. R. E.; Devynck, J.; Bell, S. L.; Balkus, K. J., Jr. J. Electroanal. Chem. 1994, 373, 19. (17) (a) Gemborys, H. A.; Shaw, B. R. J. Electroanal. Chem. 1986, 208, 95. (b) Creasy, K. E.; Shaw, B. R. Electrochim. Acta 1988, 33, 551. (c) Shaw, B. R.; Creasy, K. E.; Lanczycki, C. J.; Sargeant, J. A.; Tirhado, M. J. Electrochem. Soc. 1988, 135, 869. (18) (a) Walcarius, A.; Lamberts, L.; Derouane, E. G. Electrochim. Acta 1993, 38, 2257. (b) Walcarius, A.; Lamberts, L.; Derouane, E. G. Electrochim. Acta 1993, 38, 2267. (19) (a) Calzaferri, G.; Lanz, M.; Li, J. J. Chem. Soc., Chem. Commun. 1995, 1313. (b) Li, J.; Calzaferri, G. J. Chem. Soc., Chem. Commun. 1993, 1430. (20) (a) Seneratne, C.; Baker, M. D. J. Electroanal. Chem. 1992, 332, 357. (b) Baker, M. D.; Seneratne, C.; Zhang, J. J. Phys. Chem. 1994, 98, 1688. (c) Baker, M. D.; Seneratne, C.; Zhang, J. J. Phys. Chem. 1994, 98, 13687. (d) Baker, M. D. Seneratne, C. Anal. Chem. 1992, 64, 697. (e) Baker, M. D.; Zhang, J.; McBrain, M. J. Phys. Chem. 1995, 99, 6685. (f) Baker, M. D.; Zhang, J. J. Phys. Chem. 1990, 94, 8703. (g) Baker, M. D.; Seneratne, C.; Zhang, J. J. Chem. Soc., Faraday Trans. 1992, 88, 3187. (21) Li, J.; Calzaferri, G. J. Electroanal. Chem. 1994, 377, 163. (22) Li, J.; Pfanner, K.; Calzaferri, G. J. Phys. Chem. 1995, 99, 2119. (23) (a) Sykora, M.; Kincaid, J. R. Nature 1997, 387, 162. (b) Borja, M.; Dutta, P. K. Nature 1993, 362, 43.

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co-workers,23b,24 and Calzaferri et al.25 reported the application of the intrazeolite-metal complexes and metal nanoparticles systems in the photooxidation of water. The long-range order and molecular dimensions of the supercages inside the zeolites make these materials particularly attractive hosts for a variety of photochemical reactions.11,23-25 The photophysical and photochemical properties of [Ru(bpy)3]2+ in the supercages of zeolite-Y have been studied in detail.26-31 Only a few reports appeared in the literature on the electrochemical behavior of [Ru(bpy)3]2+ on the zeolite surface32 and [Fe(bpy)3]2+ in the zeolite supercage.7,12 In this paper, we present the electrochemical studies of the zeolite-Y encapsulated [Ru(bpy)3]2+ and [Fe(bpy)3]2+ complex coated electrodes and their electron-transfer processes at the coated electrode. Experimental Section Zeolite-Y (Na-Y) (TOSOH Co.) was purified to remove the Fe2+ impurity by stirring the zeolite-Y in 1 M NaCl.30 [Ru(bpy)3]2+ (Ru(II)) and [Fe(bpy)3]2+ (Fe(II)) (where bpy ) 2,2′-bipyridine) were synthesized inside the supercages of zeolite-Y by the reported procedures33,34 with modification (represented as Ru(II)-Y(c) and Fe(II)-Y(c), respectively, where (c) refers the supercage of the zeolite-Y). The removal of the surface bound metal complexes was achieved by exposing the Ru(II)-Y and Fe(II)-Y samples to aqueous solutions of 0.5 M NaCl for 24 h with stirring.7 We found that all the surface-bound complexes could not be removed in a single exposure. The samples were repeatedly exchanged (four to five times) in 0.5 M NaCl, until the supernatant solution showed no absorbance for the corresponding metal complexes. Then the zeolite samples were washed with double distilled water, until the filtrate showed negative test for Clions, dried at 100 °C for 3 h, and stored in a desiccator. The zeolite-encapsulated Ru(II)-Y(c) and Fe(II)-Y(c) were characterized by recording absorption reflectance spectra of the solid samples. The emission spectrum of Ru(II)-Y(c) was recorded in the colloidal state (0.02% colloid in water). Surface adsorbed [Ru(bpy)3]2+ and [Fe(bpy)3]2+ were prepared by exchanging 0.5 g of free Na-Y with known concentrations of [Ru(bpy)3]2+ and [Fe(bpy)3]2+ (represented as Ru(II)-Y(s) and Fe(II)-Y(s), respectively, where (s) refers the surface of zeolite-Y). In a similar way, Ru(II)-Y(c) and Fe(II)-Y(c) were also used to prepare the metal complexes adsorbed on the surface of the zeolite (represented as Ru(II)-Y(c+s) and Fe(II)-Y(c+s)). The loading of the Ru(II) in the zeolite was calculated by disintegrating a known weight of Ru(II)-Y(c) in a known volume of H2SO4,16 and the loading of Fe(II) in the zeolite was calculated from the concentrations of Fe2+ ions before and after equilibration.35 The amounts of analytes present on the surface of the zeolite were calculated from the difference in concentration of the analytes in solution (24) Ledney, M.; Dutta, P. K. J. Am. Chem. Soc. 1995, 117, 7687. (25) (a) Calzaferri, G.; Hug, S.; Hungentobler, T.; Sulzberger, B. J. Photochem. 1984, 26, 109. (b) Calzaferri, G.; Gfeller, N.; Pfanner, K. J. Photochem., Photobiol., A 1995, 87, 81. (c) Calzaferri, G.; Spahni, W. J. Photochem. 1986, 32, 151. (26) (a) Maruszewski, K.; Strommen, D. P.; Handrich, K.; Kincaid, J. R. Inorg. Chem. 1991, 30, 4579. (b) Maruszewski, K.; Strommen, D. P.; Kincaid, J. R. J. Am. Chem. Soc. 1993, 115, 8345. (27) Incavo, J. A.; Dutta, P. K. J. Phys. Chem. 1990, 94, 3075. (28) Kim, Y. I.; Mallouk, T. E. J. Phys. Chem. 1992, 96, 2879. (29) (a) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 8232. (b) Persaud, L.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. C.; White, J. M. J. Am. Chem. Soc. 1987, 109, 7309. (30) Turbeville, W.; Robins, D. S.; Dutta, P. K. J. Phys. Chem. 1992, 96, 5024. (31) Dutta, P. K.; Turbeville, W. J. Phys. Chem. 1992, 96, 9410. (32) (a) Li, Z.; Mallouk, T. E. J. Phys. Chem. 1987, 91, 643. (b) Li, Z.; Wang, C. M.; Persaud, L.; Mallouck, T. E. J. Phys. Chem. 1988, 92, 2592. (33) DeWilde, W.; Peeters, G.; Lunsford, J. H. J. Phys. Chem. 1980, 84, 2306. (34) Quayle, W. H.; Peetrs, G.; DeRoy, G. L.; Vasant, E. F.; Lunsford, J. H. Inorg. Chem. 1982, 21, 2226. (35) Iwakura, C.; Miyazaki, S.; Yoneyama, H. J. Electroanal. Chem. 1988, 246, 63.

Figure 1. (A) Absorption reflectance spectra of Na-Y (a), [Ru(bpy)3]2+-Y(c) (b), and [Ru(bpy)3]2+-Y(s) (c). (B) Absorption reflectance spectrum of [Fe(bpy)3]2+-Y(c) (a) and [Fe(bpy)3]2+-Y(s) (b). (C) Emission spectrum of 0.02% colloidal [Ru(bpy)3]2+-Y(c) in water (excitation wavelength ) 453 nm). before and after equilibrium with the zeolite. The concentrations of Ru(bpy)32+ and Fe(bpy)32+ complexes in solution were calculated spectrophotometrically. Solutions were prepared using double distilled water. All other chemicals were of analytical grade and used as received. The modified electrodes were prepared as follows:7,19b Typically, 0.5% of the zeolite-encapsulated metal complex colloid was prepared and 0.1 mL of the colloid was transferred to 1 cm2 area of the platinum (Pt) electrode. After air-drying, 0.1 mL of 0.1% of polystyrene in tetrahydrofuran was applied on the zeolite layer and air-dried. Cyclic voltammograms were recorded using an EG&G PAR 273A potentiostat/galvanostat, equipped with a RE 0151 recorder. A three electrode cell, with zeolite-modified Pt electrode as working electrode, 1 cm2 Pt plate as counter electrode, and a saturated calomel electrode (SCE) as reference, was used for the cyclic voltammetric studies. All cyclic voltammetric studies were carried out under nitrogen atmosphere. Absorption-reflectance spectra were recorded using a Hitachi U-3410 spectrophotometer. Absorption spectra for solution samples were recorded using a JASCO 7800 UV-vis spectrophotometer. The emission spectra were recorded using Hitachi F4500 fluorescence spectrophotometer.

Results and Discussion The absorption-reflectance spectra of the Ru(II)-Y(c) and Fe(II)-Y(c) samples at a loading level of 12 and 28 µmol of Ru(II)/g and Fe(II)/g are shown in Figure 1. These loading levels correspond to approximately one Ru(II) per 39 supercages and one Fe(II) per 17 supercages of zeoliteY.30 The reflectance spectra of Ru(II)-Y(c) (Figure 1A(b)) and Fe(II)-Y(c) (Figure 1B(a)) are very similar to the

Extrazeolite Electron Transfer

Figure 2. (A) Cyclic voltammograms of [Ru(bpy)3]2+-Y(c) coated electrode dipped in 0.1 M Na2SO4 (a) and after the addition of 0.05 M H2SO4 (b) ([Ru(II)(c)] ) 12 µmol/g, scan rate ) 50 mV/s). (B) Cyclic voltammograms of [Fe(bpy)3]2+-Y(c) coated electrode dipped in 0.1 M Na2SO4 (a) and after the addition of 0.05 M H2SO4 (b) ([Fe(II)(c)] ) 28 µmol/g, scan rate ) 50 mV/s).

reported spectra of zeolite-encapsulated Ru(II)30,33 and Fe(II)34 samples. The emission spectrum of the colloidal Ru(II)-Y(c) is shown in Figure 1C, and it shows the characteristic emission maximum at 598 nm for the [Ru(bpy)3]2+ complex.33 The absorption and emission spectra clearly show the encapsulation of Ru(II) and Fe(II) complexes in the supercages of zeolite-Y. In the present work, we have used the zeoliteencapsulated polypryidyl metal complexes ([Ru(bpy)3]2+ and [Fe(bpy)3]2+) to prepare zeolite-coated electrodes. In this particular system, the polypyridyl metal complexes occupy the supercages (12-13 Å)33,34 of zeolite-Y. The metal complexes are well separated from one another and a shell of empty supercage will produce a minimum of 13 Å between the guest molecules, effectively killing the self-exchange electron-transfer process.7 The pore opening of the supercage is 7-8 Å, so the metal complexes cannot move or diffuse from the supercage. The cyclic voltammograms of these metal complexes would clearly reflect the intra-/extrazeolite electron-transfer process rather than the non-size-excluded cation-exchanged zeolites.17-22,35,36 In the case of metal ions (Fe2+, Cu2+, and Ag+) or MV2+ exchanged zeolites, the electroactive ions can freely move within the supercages of zeolite-Y or escape out of the supercages or, in some cases, deposition of metal particles on the electrode surface may also take place.7 The cyclic voltammograms recorded for Ru(II)-Y(c) and Fe(II)-Y(c) coated electrodes in 0.1 M Na2SO4 are shown in Figure 2. These electrodes did not show redox behavior in 0.1 M Na2SO4 supporting electrolyte solution (Figure 2A(a) and Figure 2B(a)). Even after continuous cycling for 1 h the redox wave was not observed. Very similar results were also observed when the cyclic voltammograms were run in other supporting electrolyte solutions (CsNO3, LiClO4, and tetraethylammonium perchlorate (TEAP)). It should be noted that the Ru(II) and Fe(II) complexes in solution showed redox responses in the above supporting electrolytes. Bedioui et al.12-16 observed cyclic voltammograms for the zeolite-encapsulated metal complexes ([Co(Salen)], [Fe(Salen)]+, [Fe(bpy)3]2+, etc.) in acetonitrile for a pressed graphite powder composite electrode containing zeolite-Y encapsulated metal complexes. In the present system, Ru(II)-Y(c) and Fe(II)-Y(c) coated electrodes (36) (a) Phani, K. L. N.; Pitchumani, S. Electrochim. Acta 1992, 37, 2411. (b) Phani, K. L. N.; Pitchumani, S.; Ravichandran, S. Langmuir 1993, 9, 2455.

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do not show redox behavior in 0.1 M Na2SO4 (Figure 2A(a) and Figure 2B(a)). The cyclic voltammograms for the Ru(II)-Y(c) and Fe(II)-Y(c) due to RuII/III and FeII/III were observed only upon the addition of H+ ions to the supporting electrolyte and are shown in Figure 2A(b) and Figure 2B(b). In the absence of any molecular wire or mediator, the electron transfer through electron hopping will not be possible.7 Electron transfer through physical motion of the redox species is also not possible, because of the restricted environment of the metal complexes in the zeolite-Y supercage. However, the appearance of the redox waves in the presence of H+ ions is to be noted. The appearance of the redox waves for Ru(II)-Y(c) and Fe(II)Y(c) in the presence of H+ ions can be explained by considering (i) the difference in the sizes of the Na+ and H+ ions and (ii) the disintegration of the zeolite framework in 0.05 M H2SO4. It is to be noted that the sizes of the Na+ and H+ ions are such that they can easily enter into zeolite supercages through the pore opening for charge compensation. However, the electrochemical response was not observed for the zeolite-encapsulated metal complexes in the presence of Na+ ions. The observed electrochemical response for Ru(II)-Y(c) and Fe(II)-Y(c) in the presence of H+ ions is not merely by the diffusion of the H+ ions into the supercages of zeolite-Y. In an earlier report, Yoneyama et al.35 reported the cyclic voltammograms of Fe2+-X in pH 1 solution and discussed that the zeolite framework was not disintegrated. However, the stability of the zeolite framework was not supported by experimental evidence. In a later report, Phani and Pitchumani36a have carried out cyclic voltammetric studies of Fe3+-Y coated electrode and reported that the zeolite-Y is not stable in pH 1 but stable in pH 2 and supported their results by powder XRD patterns of the zeolite samples treated in pH 2 solution. In the present investigation, the crystallinity of the metal complexes incorporated zeolite, both before and after treated in 0.05 M H2SO4, has been checked by recording the powder X-ray diffraction (XRD) patterns of the samples (Figure 3). The XRD patterns remain unchanged before and after encapsulation of Ru(II) complex (parts A and B of Figure 3) and this shows that the degree of crystallinity and integrity of zeolite-Y is maintained.36b However, the XRD patterns of acid-treated zeolite-encapsulated metal complexes (acidtreated zeolite sample was prepared by dispersing 0.2 g of Ru(II)-Y(c) in 10 mL of 0.05 M H2SO4, gently shaking, and allowing to stand for 20 min) (Figure 3C) shows slight shift in the characteristic XRD patterns with reduction in intensity. These XRD patterns clearly show that the crystallinity and integrity of the parent zeolite and metal complex incorporated zeolite are not maintained in the acid-treated zeolite sample.36a Thus, we conclude that the disintegration of zeolite-Y framework in 0.05 M H2SO4 leads to the release of the metal complexes from the supercages and their random distribution in the zeolite coating. In this modified situation, the metal complexes showed redox waves. In the presence of 0.05 M H2SO4, the peak currents were slowly decreased, and after 60-90 min, the complete disappearance of the redox waves was observed. After the continuous cycling of the Ru(II)-Y(c) and Fe(II)-Y(c) coated electrodes in 0.05 M H2SO4, the absorption spectra of the supporting electrolyte solutions showed the presence of Ru(II) and Fe(II) complexes in solution. At longer time the disintegration of the zeolite framework leads to the escape of the metal complexes from the zeolite supercage into solution. The cyclic voltammograms of Ru(II)-Y(s) and Fe(II)-Y(s) coated electrodes in 0.1 M Na2SO4 are shown in Figure 4.

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Figure 3. Powder XRD patterns of Na-Y (A), [Ru(bpy)3]2+Y(c) (B) and acid-treated [Ru(bpy)3]2+-Y(c) (C).

Figure 5. (A) Cyclic voltammograms of [Ru(bpy)3]2+-Y(c+s) coated electrode dipped in 0.1 M Na2SO4 (a), after continuous cycling for 1 h (b), and after the addition of 0.05 M H2SO4 (c) (scan rate ) 50 mV/s, [Ru(bpy)3]2+-Y(c+s) ) 33 µmol/g. Ru(II) complex present in the cage ) 12 µmol/g (as in Figure 2A). Ru(II) complex adsorbed on the surface ) 11 µmol/g). (B) Cyclic voltammograms of [Fe(bpy)3]2+-Y(c+s) coated electrode dipped in 0.1 M Na2SO4 (a), after continuous cycling for 1 h (b), and after the addition of 0.05 M H2SO4 (c) (Scan rate ) 50 mV/s, [Fe(bpy)3]2+-Y(c+s) ) 43 µmol/g. Fe(II) complex present in the cage ) 28 µmol/g (as in Figure 2B). Fe(II) complex adsorbed on the surface ) 15 µmol/g). Figure 4. Cyclic voltammograms of [Ru(bpy)3]2+-Y(s) (A) and [Fe(bpy)3]2+-Y(s) (B) coated electrodes dipped in 0.1 M Na2SO4 ([Ru(II)(s)] ) 54 µmol/g and [Fe(II)(s)] ) 40 µmol/g, scan rate ) 50 mV/s).

It shows the redox waves due to RuII/III and FeII/III couples. On continuous cycling, these electrodes showed a decrease in the peak currents, and after 60-90 min, the redox waves completely disappeared. The absorption spectra of the

supporting electrolyte solutions showed that the zeolite surface adsorbed Ru(II) and Fe(II) complexes desorbed into solution. In a similar experiment, the Ru(II)-Y(c+s) and Fe(II)-Y(c+s) coated electrodes showed redox waves (Figure 5A(a) and Figure 5B(a)), and on continuous cycling the peak currents decreased. The redox waves disappeared at longer time (Figure 5A(b) and Figure 5B(b)). This is due to the slow leaching of the zeolite surface bound

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(not the trapped species) Ru(II) and Fe(II) complexes into the solution at which point they are far from the electrode surface to undergo electron transfer. When all the surfacebound species were leached into the solution, only background current was observed (Figure 5A(b) and Figure 5B(b)). At that stage only the trapped Ru(II) and Fe(II) complexes were present in the zeolite cages, and these trapped species are electroinactive in 0.1 M Na2SO4 as discussed earlier. When the same electrodes were exposed to 0.05 M H2SO4, redox waves were again observed (Figure 5A(c) and Figure 5B(c)) due to the disintegration of the zeolite framework which led to the random distribution of the metal complexes in the zeolite film. The probable pathways of electron transfer at the zeolite-modified electrode was first discussed by Shaw et al.17c and later supported by others.6-9,16,19-22 The proposed two pathways are (i) intrazeolite electron transfer (eq 1) and (ii) extrazeolite electron transfer (eq 2).

E(z)m+ + ne- + nC(s)+ h E(z)(m-n)+ + nC(z)+

(1)

E(z)m+ + mC(s)+ h E(s)m+ + mC(z)+ E(s)m+ + ne- h E(s)(m-n)+

(2)

Here Em+ is the electroactive probe, (z) indicates zeolite matrix, (s) indicates solution, and C+ is the supporting electrolyte cation. In the case of intrazeolite electron transfer in a zeolite-encapsulated metal complex (eq 1), the movement of counterions within the zeolite for charge neutrality and/or the physical motion of the metal complexes themselves should occur concurrently. In a number of papers,12-16,19,21,22 the intrazeolite electron-transfer mechanism has been discussed for the zeolite-encapsulated metal complexes and metal ion exchanged zeolites. The electron transfer to the metal complexes can be achieved only if the metal complexes are present on the electrode surface or in the vicinity of the electrode surface. The complexes present in the supercages which are nearer to the electrode surface or in the broken cages should be electroactive even if any

one of the electron transfer mechanisms is operating. The failure of detection of such species by cyclic voltammetric technique can be explained as follows: The loading levels of the metal complexes are low and only 14% Fe(II)-Y(c) and 44% Ru(II)-Y(c) of the complexes are electroactive even in 0.05 M H2SO4. It may be assumed that the metal complexes in the supercages nearer to the electrode surface and the metal complexes in the broken cages are too low to detect by the cyclic voltammetric technique. Even if all the supercages are occupied by the redox active molecules, the movement of cations for charge balance becomes hindered and blocked. When the Ru(II) and Fe(II) complexes are adsorbed on the surface of zeolite-Y, the cyclic voltammetric responses were observed in 0.1 M Na2SO4. Considering these facts and the previous reports,7-9 our results of the zeolite-encapsulated metal complex coated electrodes support the extrazeolite electron-transfer mechanism.7,9 The present work also shows the fact that the zeoliteencapsulated metal complexes show extrazeolite electron transfer in the electrochemical study, but the photoinduced electron transfer can occur from the Ru(II)-Y(c) to the viologen derivatives in solution mediated by ion-exchanged N,N′-tetramethylene-2,2′-bipyridinium ion23a or N,N′trimethylene -2,2′-bipyridinium ion23b within the zeolite framework. The emission spectrum observed for the Ru(II)-Y(c) (Figure 1d) and its longer lifetime31 clearly show that the Ru(II) complex trapped in the zeolite-Y is photoactive. Earlier, Mallouk and co-workers29,37 have also showed the photoinduced electron transfer between the zeolite-L surface adsorbed Ru(II) or porphyrin and viologen exchanged into zeolite-L through direct electron transfer or mediated electron transfer by the covalently attached N,N′-dialkyl-2,2′-bipyridinium ion. Acknowledgment. The financial support from the Department of Science and Technology is gratefully acknowledged. LA970658Z (37) Yonemoto, E. H.; Kim, Y. I.; Schmenl, R. H.; Wallin, J. O.; Shoulders, B. A.; Richardson, R. B.; Haw, J. F.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 10557.