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Facile Reduction of Zeolite-Encapsulated Viologens with Solvated Electrons and Selective Dispersion of Inter- and Intramolecular Dimers of Propylene-Bridged Bisviologen Radical Cation Yong Soo Park,† Kyungmi Lee,‡ Chongmok Lee,*,‡ and Kyung Byung Yoon*,† Department of Chemistry, Sogang University, Seoul 121-742, Korea, and Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea Received November 1, 1999. In Final Form: February 11, 2000 Zeolite-Y (Y), zeolite-L (L), mordenite (M), and ZSM-5 exchanged with methyl viologen C1VC12+ and N′,N′′-trimethylenebis(1-methyl-4,4′-bipyridinium) (henceforth bisviologen) C1V2+-C3-VC12+ were prepared. Treatment of the dried, viologen-doped zeolites with solvated electrons in diethyl ether readily yields the corresponding viologen radical cation, i.e., C1VC1•+ and C1V•+-C3-VC1•+ within the zeolite pores. Although C1V•+-C3-VC1•+ readily folds to its intramolecular dimer C3[C1V•+]2 in solution, it remains in the open form in the four zeolites. C1VC1•+ readily dimerizes to [C1VC1•+]2 at high concentrations in L but not in other zeolites. However, C1V•+-C3-VC1•+ does not dimerize by itself in any of the four zeolites even at high concentrations. C1VC1•+ readily dimerizes in Y, L, and M when the pores are filled with water. The hydrationinduced dimerization also works well for C1V•+-C3-VC1•+ in L and M. This leads to selective formation of the unprecedented intermolecular dimer of bisviologen radical cation, i.e., [C1V•+-C3-VC1•+]2 in L and M. The intramolecular analogue C3[C1V•+]2 can also be dispersed into Y, L, and M by direct occlusion of its bisperchlorate salt from the acetonitrile solution. The broad ∼530 nm band of the viologen radical dimer splits into two when the viologen radical moieties are forced to juxtapose in the collinear conformation within the narrow and straight channels of L and M. The near-infrared band of the viologen radical dimer progressively blue shifts with decreasing the interannular distance. This paper thus demonstrates a novel use of zeolites to selectively disperse inter- and intramolecular dimers of diradical dication and to lock the viologen radical dimers into the collinear conformation which allows us to delineate the spectral variation of viologen radical dimers with changing the interannular conformation and distance.
Introduction Zeolites are widely used as catalysts, ion exchangers, sorbents, etc., in industry.1-3 For academic purposes, they have also been extensively employed as organizing media to induce long-lived charge separation in photochemistry,4-8 hosts for various semiconductor quantum dots,9 and nanoreactors for various organic transformations.10,11 The presence of repeating units of nano- to subnanometer † ‡
Sogang University. Ewha Womans University.
(1) (a) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974. (b) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves; Academic: London, 1978. (2) (a) Chen, N. Y.; Garwood, W. E.; Dwyer, F. G. Shape-Selective Catalysis in Industrial Applications; Marcel Dekker: New York, 1989. (b) Csicsery, S. M. In Zeolites Chemistry and Catalysis; Rabo, J. A., Ed.; ACS Monograph 171; American Chemical Society: Washington, DC, 1976; p 680 ff. (c) Weisz, P. B.; Frilette, V. Z. J. Phys. Chem. 1960, 64, 382. (3) Ruthven, D. M. Principles of Adsorption and Adsorption Process; Wiley: New York, 1984. (4) (a) Ramamurthy, V., Ed. Photochemistry in Organized and Constrained Media; VCH: New York, 1991. (b) Holt, S. L., Ed. Inorganic Reactions in Organized Media; ACS Symposium Series 177; American Chemical Society: Washington, DC, 1981. (5) (a) Borja, M.; Dutta, P. K. Nature 1993, 362, 43. (b) Dutta, P. K.; Incavo, J. A. J. Phys. Chem. 1987, 91, 4443. (c) Dutta, P. K.; Turbeville, W. J. Phys. Chem. 1992, 96, 9410. (d) Dutta, P. K.; Borja, M. J. Chem. Soc., Chem. Commun. 1993, 1568. (e) Ledney, M.; Dutta, P. K. J. Am. Chem. Soc. 1995, 117, 7687. (f) Dutta, P. K.; Ledney, M. Prog. Inorg. Chem. 1997, 44, 2209. (g) Sykora, M.; Kincaid, J. R. Nature 1997, 387, 162. (6) (a) 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. (b) Kim, Y. I.; Mallouk, T. E. J. Phys. Chem. 1992, 96, 2879. (c) Krueger, J. S.; Mayer, J. E.; Mallouk, T. E. J. Am. Chem. Soc. 1988, 110, 8232. (d) Persaud, L.; Bard, A. J.; Campion, A.; Fox, M. A.; Mallouk, T. E.; Webber, S. E.; White, J. M. J. Am. Chem. Soc. 1987, 109, 7309.
sized crystalline voids is one of the key features that confer zeolites such a broad applicability. Viologens V2+ have been widely applied as herbicides,12 redox indicators,13 electron mediators for electrochromic displays,14 and relays in the photochemical15 and biological systems.16 The viologen radical cations V•+ have been (7) (a) Sankararaman, S.; Yoon, K. B.; Yabe, T.; Kochi, J. K. J. Am. Chem. Soc. 1991, 113, 1419. (b) Yoon, K. B.; Hubig, S. M.; Kochi, J. K. J. Phys. Chem. 1994, 98, 3865. (8) Fukuzumi, S.; Urano, T.; Suenobu, T. Chem. Commun. 1996, 213. (9) See: Ozin, G. A.; Kuperman, A.; Stein, A. Angew. Chem., Int. Ed. Engl. 1989, 28, 359 and references therein. (10) (a) Ramamurthy, V.; Eaton, D. F.; Caspar, J. V. Acc. Chem. Res. 1992, 25, 299. (b) Ramamurthy, V.; Garcia-Garibay, M. A. In Comprehensive Supramolecular Chemistry; Bein, T., Ed.; Pergamon Press: Oxford, 1996; Vol. 7, p 693. (11) (a) Herron, N.; Corbin, D. R. Inclusion Chemistry within Zeolites: Nanoscale Materials by Design; Kluwer Academic Press: Holland, 1995. (b) Turro, N. J. In Crystallography of Supramolecular Compounds; Tsoucaris, G., Atwood, J. L., Lipkowski, J., Eds.; Kluwer Academic Press: Holland, 1996; p 429. (12) Summers, L. A. The Bipyridinium Herbicides; Academic: New York, 1980. (13) Michaelis, L. Chem. Rev. 1935, 16, 243. (14) (a) Schoot, C. J.; Ponjee, J. J.; van Dam, H. T.; van Doorn, R. A.; Bolwijn, R. T. Appl. Phys. Lett. 1973, 23, 64. (b) van Dam, H. T.; Ponjee, J. J. J. Electrochem. Soc. 1974, 121, 1555. (c) Bruinink, J.; Kregting, C. G. A.; Ponjee, J. J. J. Electrochem. Soc. 1977, 124, 1854. (d) Barna, G. G.; Fish, J. G. J. Electrochem. Soc. 1981, 128, 1290. (e) Lee, C.; Sung, Y. W.; Park, J. W. J. Electroanal. Chem. 1997, 431, 133. (15) (a) Ebbesen, T. W.; Levey, G.; Patterson, L. K. Nature 1982, 298, 545. (b) Ebbesen, T. W.; Ferraudi, G. J. Phys. Chem. 1983, 87, 3717. (c) Nakahara, A.; Wang, J. H. J. Phys. Chem. 1963, 67, 496. (d) Ebbesen, T. W.; Manring, L. E.; Peters, K. S. J. Am. Chem. Soc. 1984, 106, 7400. (e) Hoffman, M. Z.; Prasad, D. R.; Jones, II, G. Malba, V. J. Am. Chem. Soc. 1983, 105, 6360. (f) Jones, II, G.; Malba, V. Chem. Phys. Lett. 1985, 119, 105. (16) Fultz, M. L.; Durst, R. A. Anal. Chim. Acta 1982, 140, 1.
10.1021/la991430+ CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000
Zeolite-Encapsulated Viologens
known to readily dimerize in solution with increasing the polarity of the solvent17 (or by decreasing the temperature) and with enforcing the hydrophobic interaction of the radical cations by increasing the length of alkyl chains.18 The characteristic purple color of the dimeric forms [V•+]2 readily distinguish themselves from the blue monomeric forms. Unlike monomers, the dimeric forms are inefficient in mediating electrons during catalytic generation of hydrogen from water.19 Accordingly, elucidation of the behavior of dimerization and the nature of dimeric states of viologen radical cations has been a subject of extensive research since it can provide insight into the effective use of viologens as electron mediators for various electrochromic and solar energy conversion devices. For this matter, cyclodextrins and micelles have been tested as the media to selectively induce or suppress dimerization of V•+ and to organize the resulting radical dimers.18,20-22 None of the above-mentioned media, however, showed the ability to selectively entrap the inter- and intramolecular dimers of bisviologen radical cation C1V•+-C3-VC1•+ and to orient the viologen radical cations to a specific conformation in the dimeric states. Various methods have been developed to reduce the substrates encapsulated within zeolites. However, the fundamental difficulties associated with the heterogeneity of the reaction conditions limit their diversity. Nevertheless, various reduction methods have been developed by now, and they can be classified into three types depending on the source of electrons: physical, chemical, and electrochemical. The physical methods involve exposure of substrates-bearing zeolites to high-energy radiations such as vacuum-UV,23 X- or γ-rays,24 electron beams,25-27 and electron sparks,28a often at low temperatures (77 K). However, the reduced species obtained by physical methods usually disappear upon warming to room temperature. The chemical reagents often used to reduce intrazeolite substrates are hydrogen, carbon monoxide, and hot alkali metal vapors.1,27,29,30 Since the required reaction temperatures are usually high, thermally stable (17) (a) Kosower, E. M.; Cotter, J. L. J. Am. Chem. Soc. 1964, 86, 5524. (b) Wolszczak, M.; Stradowski, Cz. Radiat. Phys. Chem. 1989, 33, 355. (c) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Am. Chem. Soc. 1977, 99, 5882. (d) Evans, A. G.; Evans, J. C.; Baker, M. W. J. Chem. Soc., Perkin Trans. 2 1977, 1787. (18) (a) Park, J. W.; Choi, N. H.; Kim, J. H. J. Phys. Chem. 1996, 100, 769. (b) Diaz, A.; Quintela, P. A.; Schuette, J. M.; Kaifer, A. E. J. Phys. Chem. 1988, 92, 3537. (19) (a) Gra¨tzel, M. Acc. Chem. Res. 1981, 14, 376. (b) Sullivan, B. P.; Dressick, W. J.; Meyer, T. J. J. Phys. Chem. 1982, 86, 1473. (c) Adar, E.; Degani, Y.; Goren, Z.; Willner, I. J. Am. Chem. Soc. 1986, 108, 4696. (d) Okuno, Y.; Chiba, Y.; Yonemitsu, O. J. Chem. Soc., Chem. Commun. 1984, 1638. (e) Meisel, D.; Mulac, W. A.; Matheson, M. S. J. Phys. Chem. 1981, 85, 179. (f) Kirch, M.; Lehn, J.-M,; Sauvage, J.-P. Helv. Chim. Acta 1979, 62, 1345. (g) Fan, F.-R. F.; Reichman, B.; Bard, A. J. J. Am. Chem. Soc. 1980, 102, 1488. (20) Yasuda, A.; Kondo, H.; Itabashi, M.; Seto, J. J. Electroanal. Chem. 1986, 210, 265. (21) (a) Lee, C.; Kim, C.; Park, J. W. J. Electroanal. Chem. 1994, 374, 115. (b) Lee, C.; M. S. Moon, Park, J. W. J. Electroanal. Chem. 1996, 407, 161. (c) Lee, C.; Lee, Y. M.; Moon, M. S.; Park, S. H.; Park, J. W.; Kim, K. G.; Jeon, S.-J. J. Electroanal. Chem. 1996, 416, 139. (22) (a) Kaifer, A. E.; Bard, A. J. J. Phys. Chem. 1985, 89, 4876. (b) Park, J. W.; Paik, Y. H. J. Phys. Chem. 1987, 91, 2005. (c) Quintela, P. A.; Diaz, A.; Kaifer, A. E. Langmuir 1988, 4, 663. (23) Liu, X. S.; Thomas, J. K. J. Chem. Soc., Faraday Trans. 1995, 91, 759. (24) Kasai, P. H. J. Chem. Phys. 1965, 43, 3322. (25) (a) Iu, K.-K.; Liu, X. S.; Thomas, J. K. J. Phys. Chem. 1993, 97, 8165. (b) Liu, X. S.; Iu, K.-K.; Thomas, J. K. J. Phys. Chem. 1994, 98, 13720. (26) Edgell, M. J.; Mugford, S. C.; Castle, J. E. J. Catal. 1988, 433. (27) (a) Rabo, J. A.; Angell, C. L.; Kasai, P. H.; Schomaker, V. Discuss. Faraday Soc. 1966, 41, 328. (b) Kasai, P. H.; Bishop, Jr., R. J. J. Phys. Chem. 1973, 77, 2308. (28) (a) Park, Y. S.; Um, S. Y.; Yoon, K. B. Chem. Phys. Lett. 1996, 252, 379. (b) Yoon, K. B.; Kochi, J. K. J. Am. Chem. Soc. 1988, 110, 6586.
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metal ions or metal oxides have often been the subjects of chemical reduction. The electrochemical methods, on the other hand, have been demonstrated to be more suitable for reduction of thermally sensitive species in zeolites.31-34 The electrochemical methods also have drawbacks arising from the basic requirement that zeolite powders should be immobilized onto the electrode surface and from the need of electrolytes which often leach the positively charged electroactive species out of the zeolite pores into solution. This method is also unsuitable for large-scale preparations. In this paper, we introduce a highly efficient method to reduce zeolite-encapsulated viologens and a novel use of zeolites to selectively disperse the inter- or the intramolecular dimer of C1V•+-C3-VC1•+ within zeolites and to lock the viologen radical dimers in the collinear conformation within channel-type zeolites. Experimental Section Materials. Zeolite-Y (LZY-52, Lot No. 968087061020-S), zeolite-L (ELZ-L, Lot No. 961687041001-S), and mordenite (LZM5, Lot No. 962487061003-S) were purchased from Union Carbide. ZSM-5 with the Si/Al ratio of 13.5 was a gift from ALSI-PENTA Zeolite GmbH. For convenience, the former three are designated as Y, L, and M, respectively. The zeolites were treated with aqueous 1 M NaCl solution and then subsequently washed with distilled deionized water until the silver ion test for chloride was negative. For L, treatment with 1 M NaCl solution was repeated for five times to exchange K+ with Na+ before the final washing with distilled deionized water. Methyl viologen dichloride (C1VC12+ 2Cl-) from Hannong Chemical Co. was recrystallized repeatedly until colorless. N′,N′′Trimethylenebis(1-methyl-4,4′-bipyridinium) tetraiodide (C1V2+C3-VC12+ 4I-) was synthesized in a stepwise manner according to the following procedure.35 Into the acetonitrile solution of 4,4′bipyridine, 0.5 equiv of 1,3-diiodopropane was added in a dropwise manner over a period of 24 h while the temperature of the reaction mixture was maintained at 70 °C. This yielded N,N′-(trimethylene)bis(4,4′-bipyridinium) diiodide (V+-C3-V+ 2I-). The yellow precipitate was recrystallized over a hot water. 1H NMR (D2O): δ ) 9.05 (4H, d), 8.77 (4H, d), 8.46 (4H, d), 7.90 (4H, d), 4.50 (4H, t), 2.94 (2H, quin). The propylene-bridged bis(bipyridinium) diiodide (V+-C3-V+ 2I-) was subsequently treated with methyl iodide in N,N-dimethylformamide (DMF) at 90 °C over a period of 6 h to yield C1V2+-C3-VC12+ 4I-. The red precipitate was collected and recrystallized in cold water. 1H NMR (D2O): δ ) 9.22 (4H, d), 9.08 (4H, d), 8.61 (4H, d), 8.52 (4H, d), 5.00 (4H, t), 4.50 (6H, s), 2.95 (2H, quin). The red iodide salt of C1V2+-C3VC12+ was converted to colorless perchlorate salt by metathesis (29) (a) Harrison, M. R.; Edwards, P. P.; Klinowski, J.; Thomas, J. M.; Johnson, D. C.; Page, C. J. J. Solid State Chem. 1984, 54, 330. (b) Anderson, P. A.; Edwards, P. P. J. Am. Chem. Soc. 1992, 114, 10608. (c) Haug, K.; Srdanov, V.; Stucky, G.; Metiu, H. J. Chem. Phys. 1992, 96, 3495. (d) Srdanov, V. I.; Haug, K.; Meiu, H.; Stucky, G. D. J. Phys. Chem. 1992, 96, 9039. (e) Sun, T.; Seff, K.; Heo, N. H.; Petranovskii, V. P. Science 1993, 259, 495. (30) (a) Martens, L. R. M.; Grobet, P. J.; Jacobs, P. A. Nature 1985, 315, 568. (b) Martens, L. R. M.; Vermeiren, W. J. M.; Grobet, P. J.; Jacobs, P. A. Stud. Surf. Sci. Catal. 1987, 31, 531. (c) Xu, B.; Chen, X.; Kevan, L. J. Chem. Soc., Faraday Trans. 1991, 87, 3157. (31) (a) Rolison, D. R. Chem. Rev. 1990, 90, 867. (b) Rolison, D. R. Stud. Surf. Sci. Catal. 1994, 85, 543. (32) (a) Calzaferri, G.; Lanz, M.; Li, J. J. Chem. Soc., Chem. Commun. 1995, 1313. (b) Li, J.; Pfanner, K.; Calzaferri, G. J. Phys. Chem. 1995, 99, 2119. (c) Li, J.; Calzaferri, G. J. Electroanal. Chem. 1994, 377, 163. (d) Li, J.; Calzaferri, G. J. Chem. Soc., Chem. Commun. 1993, 1430. (33) (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. (34) (a) Gemborys, H. A.; Shaw, B. R. J. Electroanal. Chem. 1986, 208, 95. (b) Bird, C. L.; Kuhn, A. T. Chem. Soc. Rev. 1981, 10, 49. (35) (a) Furue, M.; Nozakura, S. Chem. Lett. 1980, 821. (b) Furue, M.; Nozakura, S. Bull. Chem. Soc. Jpn. 1982, 55, 513. (c) Imabayashi, S.-I.; Kitamura, N.; Tazuke, S.; Tokuda, K. J. Electroanal. Chem. 1988, 243, 143.
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with NaClO4 in water. Metallic potassium and 18-crown-6 were purchased from Aldrich. DMF was used as received (Aldrich). Ion Exchange. C1VC12+ ion was introduced into zeolites by aqueous ion exchange of Na+. The exchanged amounts of C1VC12+ into zeolites were controlled to be 0.5 per unit cell (puc) for lowly loaded zeolites.36 For highly loaded zeolites, the incorporated amounts were increased to 2 per supercage for zeolite-Y and 1 puc for channel-type zeolites. The unexchanged amounts of C1VC12+ ion in the wash were quantified by UV-vis spectrophotometry monitored at λ ) 257 nm ( ) 2.0 × 104 cm-1 M-1).12 The amounts of C1V2+-C3-VC12+ exchanged into zeolites were controlled to be ∼0.25 puc. The unexchanged amounts were quantified similarly with ) 4.4 × 104 cm-1 M-1 at 257 nm.35b All the viologen-doped zeolites were dried in the air at ambient temperatures. The air-dried zeolites were then briefly washed with methanol prior to subsequent evacuation (
