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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.

The spectroscopic and magnetic properties of a quadruply bonded dimolybdenum core intercalated into the layered metal phosphates (LMPs) of vanadium and niobium have been examined. Layer modification of the LMPs was achieved by redox intercalation of the Mo2,+ core solvated by acetonitrile and by its ion-exchange reaction following reduction of the phosphate host layers with NaBH,. Regardless of the reaction pathway, a similar product is obtained. The reacted layers display a contraction of the d-spacing with concomitant insertion of up to 0.2 molybdenum atoms per layer unit cell. Structural characterization and absorption spectroscopy of these materials reveal that the bimetallic core residing in the gallery interlayer region is the mixed-valence Moz5+ center in a transverse orientation. In this arrangement the ubiquitous D4* coordination environment of multiply bonded *Mo2O8" complexes is achieved with the dimolybdenum core keyed into tetragonal oxygen cavities of the layer. Magnetic susceptibility and EPR studies reveal that electrons in the layers are antiferromagnetically coupled and that the exchange interaction is in a weak coupling regime. This exchange interaction is enhanced by the presence of the paramagnetic Mo?+ center within the interlayer region of the LMP. IntroductionA common design of many light-to-energy conversion photochemical schemes is predicated on the electron transfer chemistry of the photosynthetic reaction center. In photosynthesis, photon absorption by the light harvesting complex promotes efficient transmembrane electron/hole separation of the pair, which is ultimately manifested in the multielectron chemistry of water oxidation at the oxygen-evolving complex and proton reduction in Photosystem 1 . l -l 2 Accordingly, the design of biomimetic assemblies has relied on the synthesis of charge separating networks that propagate efficient electron/hole separation on different time scales. The basic tenet of the approach involves the transport of charge separated electrons and holes away from a light harvesting center by covalently attaching donors and acceptors to it. Perhaps the most straightforward donor/acceptor system, in both concept and structure, are coordination compounds possessing a lowest energy excited state that involves the promotion of an electron from a metal-based orbital to a ligand-based Ensuing recombination of the ligand-centered electron and metal-centered hole can be slow, which is a prerequisite for efficient charge separation. The donor/acceptor strategy has been further elaborated with the development of porphyrin-based