J. Phys. Chem. 1980, 84, 2447-2457
2447
Photochemical and Photocatalytic Properties of Adsorbed Organometallic Compounds. 1. Luminescence Quenching of Tris(2,2’-bipyridine)ruthenium( 11) and -chromiurn(III) in Clay Membranes D. Krenske, S. Abdo, H. Van Damme,” M. Cruz, and J. J. Frlpiat Centre de Recherche sur Ins Solides Si Organlsation Cristalline Imparfaite, C.N.R.S., 45045 Orleans Cedex, France (Recsived: May 30, 1979; In Final Form: May 6, 1980)
The absorption and emission properties of tris(2,2’-bipyridine)ruthenium(II) and of tris(2,2’-bipyridine)chromium(II1)as exchangeable cations in smectite membranes were studied, in the presence or in the absence of M3+ quenchers, as a function of the amount of adsorbed water. The absorption spectrum of adsorbed R ~ ( t ) p y )displays ~ ~ + several important changes with respect to the spectrum in water. The intensity of the r r* band near 290 nm is dramatically reduced, and a new band develops near 320 nm. The intensity of the charge-transferband in the visible is unchanged, but small and reversible shifts are observed upon changing the water content. In the absence of M3+quenchers, the luminescence quantum yield, $1, of adsorbed Ru(bpy)z+ was found to be of the same order of magnitude as in water and was shown to increase by about a factor of 2 as the amount of adsorbed water increases up to monolayer coverage. These observations were taken as evidence that covalently hydrated or slightly distorted bpy ligands are formed when Ru(bpy)3z+is absorbed on the clay surface. For Cr(bpy):?+ no significant changes were observed in the absorption spectrum, but $1 was found to be extremely sensitive to water coverage. Above monolayer coverage, the $1 values were comparable to those obtained in aqueous solutions, but, as the amount of water was decreased on the surface, a nearly 100-fold increase was observed. It appears that this dramatic variation is due to the concomitant decrease of the reactive and nonradiative rate constants upon dehydration. On the other hand, in the presence of E$+, Cr3+, Fe3+,or 02, the emission of Ru(bpy),2+was quenched. The quenching rate constants were found to be similar on the clay and in solutions for Eu3+and Cr3+,but smaller by about two orders of magnitude on the clay for O2 In addition, the effect of the M3+ions was found to be stronger when they were present as substitution ions in the clay lattice than when they were exchangeable ions on the internal surface. These effects were explained in terms of average distance between the species and of mobility of the ions on the surface.
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Introduction A great deall of effort has been devoted in recent years to the understanding of the excited-state reactivity of transition-metal coordination compounds. It is tempting to apply this knowledge to complexes adsorbed on catalytically active surfaces and to make use of their photochemical activity to induce chemical reactions between absorbed species. This is the aim of the research undertaken in our laboratory, and this paper presents a first set of experimental results dealing with this subject. The high specific area solids that have been chosen are natural or synthetic layer lattice silicates (phyllosilicates) the surface of which is negatively charged. Figure 1shows the schematic structure of one sheets1 These sheets are superimposed, forming microcrystals which can be expanded by absorbing coordination cationic complexes and polar molecules, such as water, in the interlamellar By sedimentation from dilute suspensions, thin coherent artificial membranes (or films) are easily obtained in which all sheets have their C’ crystal axes oriented parallel to each other and perpendicular to the membrane surface. A typical membrane weighing -5 mg/’cm2has an internal surface of 4 m2 and consists of more than 104 superimposed layers. If offers good transmission in the spectral range from 300 to 2500 nme4 The negative electrical charge of phyllosilicates used in this study (montmorillonite and hectorite) is usually expressed by their cation exchange capacity (CEC!) which is of the order of 100 x equiv/100 g of dry material. This charge arises from isomorphic substitutions occurring mainly in the octahedral layer1 such as ill3+by Mg2+ in montniorillonite or Mg2+ by Li+ in hectorite. If one assumes that the isomorphic substitutions are randomly distributed, the statistical 0022-3654/80/2084-2447$0 1 .OO/O
distance between negative surface sites is 11.5 A. By changing the composition of the exchangeable cations, it is possible to modify the average distance between them. This distance is an important factor for reactions in which transfer of energy or of electrons may occur. The clay membranes also contain water molecules adsorbed in the interlamellar space both in and out of the coordination sphere of the interlamellar cations. This adsorbed water has a number of interesting properties which have been extensively studied by various techniques such as NMR and IR spectroscopy, tracer diffusion, neutron diffraction, and neutron scattering studied. The main observations could be summarized as follows: (i) The interlamellar water has predominantly a liquidlike rather than an icelike character, although its mobility is increasingly restricted in comparison with bulk water as the interlayer thickness decrease^.^ Typical values for the translational diffusion coefficient in montmorillonite films comparable to those used in this work are of the order of m2s-l for samples containing one to three layers of water molecules in the interlamellar space.6 This is about one to two orders of magnitude lower than the value for bulk water (2.3 lo* m2 s-l at room temperature). A more detailed analysis718shows that, in addition to this population of liquidlike molecules attributable to nonhydration-shell water, one has to take into account a fraction of relatively immobile molecules which can be identified as the molecules directly bound to the exchangeable cations. The exchange rate between the two populations has been estimated from NMR datag to be in the range 104-106s-l. As may be expected, the vibrational properties of the two types of water molecules are different, as evidenced in the IR spectra.l@-12 The non-hydration0 1980 American Chemical Society
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The Journal of Physical Chemistry, Vol. 84, No. 19, 7980
EXCHANGEABLE CATIONS
"50
3 OXYGENS
'
ALUMINIUM,IRON,MAGNESIIJM ~}SILICON,occasionally ALUMINIUM
Figure 1. Schematic structure of one sheet of phyllosilicate. The bidimensional arrangement consists of the superimposition of two inverted tetrahedral layers sharing their apical oxygen with the octahedral layer. I n montmorillonite with structural formula M,(Si,)'"(Alc,Mg)%zo(OH)4, two out of three octahedral vacancies are occupied by AI and Mg. In hectorite with structural formula M,(Si,)'"(Mg,-,Li)lVOzO(OH),, the three octahedral vacancies are occupied by Mg and Li. M is the exchangeable cation.
shell molecules are entensively hydrogen-bonded and give a spectrum similar to that of liquid water, with a broad and strong OH stretching band near 3400 cm-l and a much weaker H-0-Hbending band near 1630 cm-l. The water directly coordinated to the cations is involved in weaker hydrogen bonds and gives a stretching maximum near 3600 cm-I and a relatively strong deformation band near 1630 cm-l. Another significant point is that the interlamellar water exhibits preferential orientation with respect to the C* crystal axis of the mineral.13 (ii) As a consequence of the fact that a large number of water molecules in the interlayer space are submitted to the strong local crystalline field of the exchangeable cations, the number of dissociated molecules is far greater than in bulk water. Estimates from IR14or NMR15 data point to a dissociation constant Kw = [H+][OH-]/ [H20] of the order of compared to in bulk water. Although this property is very often described in terms of surface acidity, as evidenced for instance by the protonation of adsorbed organic weak bases,16it is physically more nearly correct to say that, whereas the hydroxyl ions remain immobilized in the coordination shell of the hydrated exchangeable cation M", the protons are highly mobile in the network of the non-directly-coordinated molecules (eq 1). [M"(HZO).] + HzO F! [M*(OH-)(H,O).-1] + H30+ (1) The proton delocalization is even more favored by the ordering of the molecules with respect to each other.g As the amount of water on the surface is decreased, the polarizing effect of the interlayer cations on the residual water molecules gets relatively larger, and, accordingly, the fraction of molecules which are in the form of H@ is also increased. On nearly dehydrated clay surfaces, this fraction may be as high as 10-2.14,15Simultaneously, however, the environment of the exchangeable cation becomes increasingly hydroxide-like.16Changing the interlayer water
Krenske et al.
content also effects the expansion of the lattice along the C* axis. This swelling, which can be measured by X-ray diffraction, modifies the surface diffusion coefficient of the interlamellar exchangeable cation.17 We wish to report here the effect of adsorption on the clay surface on the photochemistry and photophysics of tris(2,2'-bipyridine)ruthenium(II)(Ru(bpy),2+)and tris(2,2'-bipyridine)chromium(III)(Cr(bpy)?+). Ru(bpy),2+, when excited by visible light, exhibits an emission from a long-lived, charge-transfer to ligand triplet excited state whose lifetime is 0.65 X s at 18 OC in water.ls Cr(bpy):+, on the other hand, phosphoresces at 727 nm from a spin-forbidden metal-centered doublet 2E whose lifetime s a t 25 OC.19 is -70 X The spectroscopic and redox properties of poly(pyridine)ruthenium(II) complexes have been extensively studiednZ0The excitation energy of the thermally equilibrated excited-state complex *Ru(bpy)3z+is 2.10 eV,21 whereas the redox potential of the ground-state Ru(bpy)?+/Ru(bpy):+ couple is 1.26 V.z2 Assuming that the excitation energy can be considered as free energy, i.e., that the entropy differences between ground- and excited-state complexes are negligible, the potential for the Ru(bpy)33+/*R~(bpy)32+ couple has been estimated as -0.84 V.23 In view of the relatively low value of this potential, the quenching of *Ru(bpy),2+by various Ma? metal ions such as Eu,;', Cra?, and Fe,? has been investigated in fluid media.20 Since the standard redox potential for the M3+/M2+ couples are -0.41, -0.43, and 0.74 V for Eu, Cr, and Fe, respectively, the following electron-transfer reaction is possible indeed:
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*Ru(bpy)?+ + M3+
+
Ru(bpy),3+ M2+
(2)
Alternatively however, because of the low-lying excited states of Fe3+ and Cr3+,electronic energy transfer is also possible for these ions:
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*Ru(bpy)?+ + M3+
Ru(bpy),2+ + *M3+
(3)
In aqueous solutions, the quenching of *Ru(bpy),2+by Eu,q3+, Cr,? and Fe,3+ are ascribed to electron transfer for EU?? and Fe,d+ and to energy transfer for Cr, In view of the particular properties of the interyamellar space of swelling clays, it was of interest to measure the quenching rate constants in this particular environment and to compare them with those obtained in homogeneous medium. The adsorption of various tris(2,2'-bipyridine) and tris(1,lO-phenanthroline)complexes in hectorite has already been s t ~ d i e d Of . ~particular ~ ~ ~ ~ interest is the increase in the oxidation potential which was reported for the adsorbed Fe(phen)?+/2+and R~(bpy)3~+/'+ couples with respect to the values obtained in aqueous solutions. For instance, no oxidation by PbO2 or Ce(1V) was detected for R ~ ( b p y ) on ~ ~h+e ~ t o r i t e , even ' ~ though this readily occurs in solution. It was suggestedz8that these findings could be attributed to the existence of covalently hydrated species. However, no direct evidence for the existence of such species on the surface of clays has been reported yet. The present paper also aims to provide additional data on the nature of the adsorbed tris(2,2'-bipyridine)ruthenium(11) and -chromium(III) complexes. 3+.20v24125
Experimental Section A. Phyllosilicates. Samples of naturally occurring hectorite (Hector, California) and Wyoming montmorillonite were purified by repeated exchanges with 1 N NaCl followed by washing with distilled water and mechanical
Luminescence CNenching-Ru(bpy)3z+ and Cr(bpy);+
The Journal of
Physical Chemistry, Vol. 84, No. 19,
7980 2449
Luminescence spectra were obtained with a FICA 55 separation of the clay gel from the mineral sediment. The MK spectrofluorimeter equipped with an excitation and exchange capacity of the air-dried clays was 90 mequiv/100 emission correction system. The samples were irradiated g for hectorite and 100 mequiv/100 g for the montmorilwith a 450-W Xe lamp using a monochromator and aplonite. The synthetic montmorillonites were prepared by propriate filters. The bandwidth for both the excitation dissolving the appropriate quantities of Na2C03,AUNand emission monochromators was 7.5 nm. The sample 03)3.9H20,Mg(N03)2-6H20, Cr(N03)3.9H20,and ethanolic temperature was maintained at 18 "C by flowing cooling (C2H50)4Siin distilled water to yield the following stoiwater through the sample cell holder. chiometries: Nao.g[Siel[A13,1MgO.g]C)20(OH)4,Nao.g[Si81Since our samples were in the form of films less than [A12.8Cr0.3Mg0.91020(0H)4, and N%.9[Si81[A12.5Cr0.6Mg0.910.01 cm thick, with an optical density unually higher than Oz0(OH),. The solutions were acidified with HN03 to one, the standard 90" setting used for liquid samples was decompose the carbonate and then made basic by the replaced by a front-end irradiation and emission setting. addition of aqueous ammonia. The resulting gel was aged The luminescence quantum yields were calculated from for 6 h, then dried at 200 "C for 24 h, and finally calcined eq 4, where A is the optical density of the sample and C at 600 "C overnight. The solids were ground to a fine powder and placed into individual gold tubes which were area under emission spectrum then sealed into autoclaves with enough water to give 2 (4) 91 = C(1 - 10-A) kbar hydrothermal pressure at 400 "C. The reaction was allowed to continue for 10 days, and the products were is a proportionality constant depending on the geometry purified in the same manner as the natural clays. The of the system. C was determined by measuring the luchemical analysis of the two natural samples gave the minescence of a standard fluorescein solution in a specially following structural formulas: Camp Berteau montmoconstructed 0.1-cm liquid cell which could be located in Nao.88+[Si7,,5Tio,~3(Al,Fe)~,22~ [(AI,rillonite, the solid sample holder at the same place as the clay Fe)3.32Mgo.s91020(OH)4; hectorite, Naomjt[Si8lmembrane. Such a procedure is valid inasmuch as the IMg5.45,Li0.551C)2oF2. fraction of emitted light which can enter the spectrometer B. Sample Preparation. Dilute aqueous suspensions is the same for the solid sample and for the standard (
