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Supramolecular complexes coupling Ru(II) or Os(II) polyazine light absorbers through bridging ligands to Rh(III) or Ir(III) allow the study of factors impacting photoinitiated electron collection and multielectron water reduction to produce hydrogen. The [{(bpy)(2)Ru(dpb)}(2)IrCl(2)](PF(6))(5) system represents the first photoinitiated electron collector in a molecular system (bpy = 2,2'-bipyridine, dpb = 2,3-bis(2-pyridyl)benzoquinoxaline). The [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5) system represents the first photoinitiated electron collector that affords photochemical hydrogen production from water in the presence of an electron donor, N,N-dimethylaniline (dpp = 2,3-bis(2-pyridyl)pyrazine). The complexes [{(bpy)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5), [{(bpy)(2)Ru(dpp)}(2)RhBr(2)](PF(6))(5), [{(phen)(2)Ru(dpp)}(2)RhCl(2)](PF(6))(5), [{(bpy)(2)Os(dpp)}(2)RhCl(2)](PF(6))(5), [{(tpy)RuCl(dpp)}(2)RhCl(2)](PF(6))(3), [{(tpy)OsCl(dpp)}(2)RhCl(2)](PF(6))(3), and [{(bpy)(2)Ru(dpb)}(2)IrCl(2)](PF(6))(5) are herein evaluated with respect to their functioning as hydrogen photocatalysts (tpy = 2,2':6',2''-terpyridine, phen = 1,10-phenanthroline). With the exceptions of [{(bpy)(2)Ru(dpb)}(2)IrCl(2)](PF(6))(5) and [{(tpy)OsCl(dpp)}(2)RhCl(2)](PF(6))(3), all other complexes demonstrate photocatalytic activity. The functioning systems possess a rhodium localized lowest unoccupied molecular orbital that serves as the site of electron collection and a metal-to-ligand charge-transfer ((3)MLCT) and/or metal-to-metal charge-transfer ((3)MMCT) excited-state with sufficient driving force for excited-state reduction by the electron donor. The lack of photocatalytic activity by [{(bpy)(2)Ru(dpb)}(2)IrCl(2)](PF(6))(5), although photoinitiated electron collection occurs, establishes the significance of the rhodium center in the photocatalytic system. The lack of photocatalytic activity of [{(tpy)OsCl(dpp)}(2)RhCl(2)](PF(6))(3) is attributed to the lower-energy (3)MLCT state that does not possess sufficient driving force for excited-state reduction by the electron donor. The variation of electron donor showed the photocatalysis efficiency to decrease in the order N,N-dimethylaniline > triethylamine > triethanolamine. The general design considerations for development of supramolecular assemblies that function as water reduction photocatalysts are discussed.

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Photocatalytic generation of hydrogen from water is an integral part of the next generation clean fuel technologies. The conversion of solar energy into useful chemical energy is of great interest in contemporary investigations. The splitting of water is a multi-electron process involving the breaking and making of chemical bonds which requires multi-component photocatalytic systems. Supramolecular complexes [{(TL) 2 Ru(BL)} 2 RhX 2 ](Y) 5 (where TL ¼ terminal ligand, BL ¼ bridging ligand, X ¼ Cl À or Br À , and Y ¼ PF 6À or Br À ) have been synthesized and studied for their light absorbing, electrochemical and photocatalytic properties. The supramolecular complexes in this investigation are multi-component systems comprised of two ruthenium based light absorbers connected through bridging ligands to a central rhodium, which acts as an electron collecting center upon excitation. These complexes absorb light throughout the ultraviolet and visible regions of the solar spectrum. The supramolecular complexes possess ruthenium based highest occupied molecular orbitals (HOMO) and a rhodium based lowest unoccupied molecular orbital (LUMO). These molecular devices have been investigated and shown to function as photoinitiated electron collectors at the reactive rhodium metal center, and explored as photocatalysts to generate hydrogen from water in an aqueous solution in the presence of an electron donor.