Langmuir 1991, 7, 1215-1221
1215
Fluorescence of Methylviologen Intercalated into Montmorillonite and Hectorite Aqueous Suspensions G. Villemuret and C. Detellier’ Ottawa-Carleton Institute, University of Ottawa Campus, Ottawa, Ontario K1N 9B4, Canada
A. G. Szabo* Institute for Biological Sciences, National Research Council, Ottawa, Ontario K1 A OR6, Canada Received March 20,1990. In Final Form: October 25, 1990 Methylviologen (MV2+)when intercalated into montmorilloniteand hectorite aqueous suspensionshas been investigated by using steady-state and time resolved fluorescence spectroscopy. As the [MV2+]/ [clay] ratio increased, different fluorescence behavior was observed for the two clays. In hectorite the fluorescence intensity decreased with an increase in ratio, the curve being biphasic. In montmorillonite there was an initial increasein fluorescenceintensity and then the fluorescencedecreasedin an approximately linear fashion. In hectorite the fluorescencedecay of MV2+was fit to two exponential decay terms, with lifetimes depending on the ratio. The longer decay time ranged from 3.8 to 4.4 ns and the shorter decay time ranged from 1.0 to 1.4 ns. The fractional fluorescences were constant over the entire ratio range studied. In the case of montmorillonitethe fluorescencedecay of MV2+fit three exponential components, which had the same values at all ratios. These decay times were 0.93,0.37, and 0.083 ns. The fractional fluorescence values were independent of the ratio of [MV2+]/[clay]. The most consistent rationalization of these results was to propose a model wherein the intercalated MV2+ molecules were involved in selfquenching processes. The data may suggest a distribution of fluorescence decay times reflecting the heterogeneity of the clay structures.
Introduction Many systems have been designed recently in order to produce hydrogen from the photocleavage of water, using visible light.’ The eventual reduction of a proton to hydrogen involves in a first step absorption of a photon by a photosensitizer such as tris( 2,2’-bipyridyl)ruthenium(11)(Ru(bpy)a2+),which, in the excited state, can transfer an electron to an electron relay, such as methylviologen (MV2+)(l,l’-dimethyl-4,4’-dipyridinium).The MV’+ cation radical produced in this manner can reduce a proton to hydrogen, in the presence of a suitable catalyst.2 The dichloride salt of MV2+ (MV), also called paraquat, has been widely used as an herbicide: and its interaction with soils, particularly swelling clay minerals, is important.4 Some clay minerals are capable of swelling upon incorporation of neutral or cationic species. The most common group of clays showing this behavior is the smectite family. Smectites are characterized by layers of two tetrahedral Si04 sheets with their vertices pointing inward, between which cations, mainly Al(II1) or Mg(II), are octahedrally
* To whom correspondence may be addressed. + Current address: Department of Chemistry, University of New Brunswick, Fredericton, NB, Canada. (1) (a) Harriman, A., West, M. A,, Eda. Photogeneration ofHydrogen; Academic Press: New York, 1982. (b) Gratzel, M., Ed.Energy Resources Through Photochemistry and Catalysis; Academic Press: New York, 1983. (2) (a) Lehn, J. M.; Sauvage, J. P. Nouu.J . Chim. 1977,1,449-451. (b) Moradpour, A.; Amouyal, E.; Keller, P.; Kagan, H. B. Nouu.J. Chim. 1978,2, 547-549. (c) Kalyanasundaram, K.; Kiwi, J.; Gratzel, M. Helu. Chim. Acta 1978,61,272&2730. (d) Kiwi, J.; Gratzel, M. Nature 1979, 281,657-658. (e) Okura, 1.; Kim-Thuan, N. J. Mol. Catal. 1979,5,311314. (0Ballardini, R.; Juris, A.; Varani, G.; Balzani, V. N o w . J. Chim. 1980,4,563-564. (g) Gratzel, M. Acc. Chem. Res. 1981,14,376-384. (h) Delcourt, M. 0.;Keghouche, N. Nouu.J. Chim. 1985,9, 235-240. (i) Okura, I.; Kaji, N.; Aono, S.; Kita, T.; Yamada, A. Znorg. Chem. 1985,24, 453-454. (3) Akhavein, A. A,; Linscott, D. L. Residue Rev. 1968,23,97-145. (4) Hayes, M. H. B.; Pick, M. E.; Toms, B. A. Residue Reu. 1976,57, 1-25.
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co~rdinated.~ These layers are negatively charged, due to isomorphous replacements of some cations by ones of lower charge. The negative charge is balanced by aquated cations (Na+and Ca2+principally) occupyingthe interlayer spaces. Montmorillonite and hectorite belong to this group. They are characterized by having Al(II1) and Mg(II), respectively, in their octahedral sheets. In the case of hectorite, all octahedral sites are filled (trioctahedral2:l smectite) while in montmorillonite only two-thirds of the octahedral sites are filled (dioctahedral 2:l smectite). Recently, clays have acquired an importance aa catalyst supports or as catalysts themselves in a number of reactions. For example, they have been used as Brsnsted acid catalysts in organic reactions: as supports for organometallic catalysts in hydrogenation or Fischer-Tropsch synthesis: or as supports of oxidants.8 In addition, they have been used as supports of catalysts involved in hydrogen photogeneratione and coating material for photoelectrodes.1° We have shown that a system consisting of Ru(bpy)32+,MV2+,and a sacrificial donor, triethanol(5) Van Olphen, H. An Introduction to Clay Colloid Chemistry; 2nd ed.; Wiley-Interscience: New York, 1977. (6) (a) Adams, J. M.; Davies, S. E.; Graham,S. H.; Thomas, J. M. J. Catal. 1982,78,197-208. (b) Thomas, J. M. In Intercalation Chemiutry; Whittingham, M. S., Jacobson, A. J., Eds.; Academic Prese: New York, 1982; Chapter 3, pp 55-101. (c) Adams, J. M.; Martin, K.; McCabe, R. W.; Murram, S. Clays Clay Min. 1986,34,597-603. (d) Adama, J. M.; Martin, K.; McCabe, R. W. J. Inclusion Phenom. 1987,5,663-674. (7) (a) Pinnavaia, T. J.; Raythata, R.; Lee,J. G.S.; Halloran, L. J.; Hoffman, J. R. J. Am. Chem. SOC.1979,101,6891+397. (b) Pinnavaia, T. J. Science 1983,220,365-371. (c) Pinnavaia,T.J.;Tzou,M. S.;Landau, S. D. J . Am. Chem. SOC.1986,107,4783-4785. (d) Giannelis, E. P.;Pinnavaia, T. J. Znorg. Chem. 1986, 24, 2115-2118. (e) Giannelie, E. P.; Rightor, E. G.;Pinnavaia, T. J. J. Am. Chem. SOC.1988,110,3880-3886. (8) (a) Cornelis, A.; Laszlo, P.; Pennetreau, P. Clay Miner. 198S, 18, 437-445. (b) Laszlo, P. Acc. Chem. Res. 1986,19,121-127. (c) Suib, S. L.; Tanguay, J. F.; Ocelli, M. L. J. Am. Chem. SOC.1986,108,69124977. (d) Laszlo, P. Science 1987,235, 1473-1477. (9) (a) Nijs, H.; Van Damme, H.; Bergaya, F.; Habti, A.; Fripiat, J. J. J. Mol. Catal. 1983,21,223-232. (b) Nijs, H.; Cruz,M. I.; Fripiat, J. J.; Van Damme, H. Nouu. J. Chim. 1982,6,551-557. (c) Nijs, H.; Fripiat, J. J.; Van Damme, H. J. Phys. Chem. 1983,87,1279-1282.
0 1991 American Chemical Society
Villemure et al.
1216 Langmuir, Vol. 7,No. 6, 1991 amine (TEOA),illuminated by visible light in the presence of montmorillonite or hectorite, produces hydrogen gas." Recently we have also reported that fluorescence of MV2+ could be observed when it was incorporated into the lamellae of colloidal swelling clay suspensions.12 The quantum yields (4) were 0.070 and 0.014 in the cases of hectorite and montmorillonite, respectively.12 In aqueous solutions, the quantum yield of the fluorescence of MV2+has been even though the tetshown to be very low (4 < 10-3),12 ramethyl and hexamethyl derivatives do fluoresce with significant intensity.13 The fluorescence decay kinetics of MV2+in montmorillonite were earlier shown to obey double exponential decay kinetics. Hence montmorillonite and hectorite play a crucial role in the origin of the observed MV2+fluorescence. An increase of the quantum yield of luminescence has been reported when Ru(bpy)a2+ was absorbed on some clays, such as hectorite or l a p ~ n i t e , ~ whereas ~ ~ J ~ l a~ decrease of its luminescence intensity was observed after intercalation into montmorillonite.1620 There are differences between the binding states of enantiomeric and racemic forms of poly(pyridy1)-ruthenium(I1) complexes intercalated into Na hectorite clay, as shown by steadystate and dynamic luminescence spectra.21 Further, the luminescence decay kinetics of Ru(bpy)g2+ and Ru(phen)a2+ have been shown to be double exponentia1.14s172021 In this paper, we report an extended study of MV2+ fluorescence intensities and decay times as a function of the degree of occupancy of the clay adsorption sites in both montmorillonite and hectorite. It is shown that the data are most consistent with the intercalation of MV2+ into the interior lamellae of the clay. The fluorescence behavior with increasing [MV2+]/[clay] ratio is most consistent with an excited-state self-quenching distribution model.
Experimental Section Methylviologen dichloride was obtained from Aldrich and used without further purification. Twice Recrystallized MV from methanol gave similar results. The sample of montmorillonite was a fraction of a bentonite (Clay Spur, WY).llC The nontronite was from Garfield (Washington). These two samples were provided to us as a
