No abstract

The molybdenum(II) chloride cluster Mo 6 Cl 12 has been immobilized on 2% cross-linked poly(4-vinylpyridine) (PVP-Mo 6 Cl 12 ) by the axial ligation of two pendant pyridines of the polymer to the hexanuclear metal core. Polymers with different cluster loadings exhibit the characteristic broad band emission of Mo 6 Cl 12 at λ max ) 765 ( 3 nm, and luminescence decays are composed of short-and long-lifetime components, which are consistent with the presence of cis-and trans-disubstituted cluster species, respectively. The lifetime of the cis form is too short to permit its bimolecular reaction with oxygen. However, the long-lifetime component is quenched efficiently by O 2 , and Stern-Volmer analyses of emission lifetimes yield quenching rate constants of (2-10) × 10 5 M -1 s -1 for cluster loadings in the range (3.5-0.3) × 10 -3 g/g of PVP, respectively. The quenching rate constant increases to ∼50 × 10 5 M -1 s -1 for all polymers in MeOH regardless of cluster loading. In aqueous solutions (pH 3-6) intermediate quenching rate constants are observed. These results correlate with the swelling properties of the PVP polymer in various solvents where greater swelling is manifested in greater accessibility of oxygen to the cluster active site and hence increased quenching rate constants. Oxygenated solutions of PVP-Mo 6 Cl 12 and alkenes photoreact to yield oxidized olefins. The products and product distributions indicate an energy transfer quenching reaction between oxygen and electronically excited polymer-bound clusters to produce singlet oxygen.

C0),2, 12560-48-4; H~R u~( C O )~~. 34438-91-0; (69) Verdonck, J. J.; Jacobs, P. A.; Gent, M.; Poncelet, G.Abstract: The excited-state properties of trans-dioxorhenium(V) ions immobilized in the intracrystalline environments of three complex-layered oxides (CLOs) have been examined. The layered silicate CLOs hectorite and fluorohectorite adsorb tr~ns-ReO~(py)~+ to their negatively charged interlayers by intercalative ion exchange to produce topotactic solids with gallery heights of 6.7 and 9.2 A, respectively. A dioxorhenium CLO of complementary charge, prepared by the addition of NaOH to an aqueous solution containing MgCl2:A1CI3 (1 :2 w/w) and trans-Re02(CN),)-, incorporates the oxoanion between the positive layers of a hydrotalcite-like Mg/Al double hydroxide. Electronic absorption and vibrational spectra of these three CLO intercalates are characteristic of the dioxorhenium ions and are indicative of structurally unperturbed oxo complexes in the CLO intracrystalhe environment. Despite structurally similar intercalated trans-ReOz+ cores, steady-state and time-resolved luminescence experiments reveal that the three CLO intercalates are quite distinct: solid Re02(py)4-hectorite is highly emissive ( & m , , , = 630 nm at 25 " C ) and luminescence spectra display progressions in 900-cm-I ( v , , # ( R e O ) ) and 200-cm-' (u,ls(Re-py)) modes similar to that of the native ion; Re02(py)4-fluorohectorite exhibits broad and featureless emission ( X , , ,at 25 "C) whose intensity is attenuated by a factor of 50 relative to the hectorite compound and, ReO2(CN),-hydrota1cite does not luminesce. The luminescence decay curve of ReOz(py)4-hectorite displays multiexponential form with a major lifetime component of 13.0 ps and a minor lifetime component of 3.9 ps. In contrast, the emission decay of the fluorohectorite intercalate is uniexponential and fast (7 = 0.63 ps). The disparate excited-state properties of the three intercalates are attributed to specific guest-host interactions which mediate the reaction between the trans-ReOz+ core and water in the CLO interlayers.Photochemical processes can significantly be modified with the intercalation of reactants within intracrystalline environments of zeolites,'-4 layered phosphate^,^ and complex-layered oxides ( C L O S ) .~~O Layered silicate clays (LSCs) and layered double 0002-7863/88/1510-3885$01.50/0 0 1988 American Chemical Society Newsham et al. (1) (a) Turro, N. J. Pure Appl. Chem. 1986,58, 1219-1228. (b) Turro, N. J.; Lei, X.-G.; Cheng, C.-C.; Corbin, D. R.; Abrams, L.

9660J . Phys. Chem. 1991. 95, 9660-9666 flexible molecules. However, the resolution obtained from a single measurement can be modest (see xR2 surfaces for single files) and estimated values of the recovered parameters can be questionable. Beechem and H a d have recently discussed this problem for time-domain measurements and suggested simultaneous analysis of the donor and acceptor decay kinetics. This type of global analysis is potentially valuable and intuitively obvious, but is very difficult to realize in practice. Usually, acceptors are excited simultaneously during the excitation of the donor, and the amount of excited acceptor molecules due to energy transfer can be small compared with the whole population of excited-state acceptors. That is, the population or^ excited acceptors can be predominately the result of acceptors which are directly excited. The time-dependent decays of these acceptors do not contain any information on the distance distributions or dynamics, and this component can only result in decreased information content and/or resolution from the data. One must also separate donor and acceptor absorption and emission spectra and introduce additional parameters into the analysis to account for direct acceptor excitation and/or spectral overlap. I n contrast, the global analysis obtained by varying the donor lifetime does not introduce any additional parameters, gives an acceptable increase of the resolution for each fitted parameter, and can be used for any donor-acceptor system including nonfluorescent acceptors.The recovered initial distance distribution (Figure 9, insert, solid line) is in very good agreement with that found previously for TU2D in viscous solution32 (Figure 9, insert, dashed line). The diffusion coefficient, D = I .26 X cm2/s (I26 A2/ns), is in the range of expected values for small molecules in low-viscosity solutions. I t is interesting to compare the end-to-end diffusion coefficient with that obtained for a nonlinked system. Indoleto-dansyl intermolecular energy transfer has been investigated previously in propylene glycol and methanol.33 The data were fitted well to a Gosele et al. model" yielding a diffusion coefficient in methanol at 20 "C, D = 2.64 X cm2/s. The end-to-end diffusion coefficient obtained for TU2D in methanol is only twice smaller than that recovered from the mixture of indole and dansyl, where the polyethylene linker was not present. The data obtained for the TU2D donor-acceptor system and indole-dansyl mixture show that the polyethylene linker containing 22 carbons is highly flexible and has a moderate influence on end-to-end diffusion.The luminescence quenching of the hexanuclear cluster ions Mo,CI,,~-and W6II4'-by pyridinium and bipyridinium ions in nonaqueous solution has been investigated. Stern-Volmer (SV) plots of the emission intensity and lifetime quenching data for the reaction of M6X142-* with pyridinium acceptors are linear, and slopes of -0.48 for plots of R T In kobs vs the free energy driving force for electron transfer are in accordance...

Luminescent oxide gels were prepared by hydrolysis of silicon alkoxides in solutions containing the luminescent hexanuclear molybdenum cluster Mo6Cl12. The resulting gel exhibited emission and lifetime behavior characteristic of Mo6Cl12 thereby indicating that the excited-state properties of the metal cluster species are not significantly perturbed by the gel environment.