The excited state properties of a series of singly bonded dirhodium compounds, consisting of Rh(0)(2), Rh(0)Rh(II)X(2), and Rh(II)(2)X(4) (X = Cl and Br) cores coordinated by three bis(difluorophosphino)methylamine ligands, have been investigated. The newly synthesized complexes with X = Br have been structurally characterized. The mixed-valence complex Rh(2)[&mgr;-CH(3)N(PF(2))(2)](3)Br(2)[(PF(2))CH(3)N(PF(2))] crystallizes in the orthorhombic space group P2(1)2(1)2(1) with a = 13.868(7) Å, b = 16.090(5) Å, c = 11.614(5) Å, V = 1591(3) Å(3), and Z = 4; the structure was refined to values of R = 0.052 and R(w) = 0.062. Orange crystals of Rh(2)[&mgr;-CH(3)N(PF(2))(2)](3)Br(4) are monoclinic with a C2/c space group: a = 14.62(6) Å, b = 12.20(2) Å, c = 14.33(1) Å; beta = 106.0(2) degrees; V = 2457(11) Å(3); Z = 4; and R = 0.058 and R(w) = 0.056. Crystalline solids and low-temperature glasses of each member of the chloride and bromide series exhibit long-lived red luminescence. Excitation profiles and temperature dependencies of the emission bandwidths and lifetimes for all complexes are characteristic of luminescence originating from a dsigma excited state. Efficient nonradiative decay is observed upon the thermal population of an excited state proximate to the lowest energy emissive excited state of these complexes. The nonradiative decay rate constant of the upper excited state is 10(2)-10(3) and 10(3)-10(4) greater than that of the emissive excited state for complexes with X = Cl and Br, respectively.

Acknowledgment.We thank the National Institutes of Health are two possible orientations for this group, with refined group occupancies of 54.8 (5)% for the "B" form and 45.2 (5)% for the "A" form.An absorption correction was applied.25 Final refinement was carried out with anisotropic thermal parameters for all non-hydrogen atoms that were not disordered. Hydrogen atoms were included at calculated positions with use ofa riding except for those bonded to the disorder& ethyl carbons. Isotropic thermal parameters for the hydrogen atoms were fixed at 1.2 times that of the bonded carbon. The largest peak in a final difference map had a value of 0.69 e A-3 and is located near the disordered ethyl peroxide.

We are currently interested in the development of chemical sensors that are based on the incorporation of reactive molecular species into polymeric thin films by low-temperature sol-gel methods. Previous work in our laboratories and in others has demonstrated the general usefulness of sol-gel derived glasses for the encapsulation of guest molecules such as inorganic clusters, porphyrins, and lanthanide cryptate complexes.'S2 Herein we report 'Departmencof Chemical and Agricultural Engineering.

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Thiol-functionalized mesostructured silica, prepared by direct assembly methods, is an effective trapping agent for the removal of arsenite from water.