The blue dimer, cis, cis-[(bpy)2(H2O)Ru(III)ORu(III)(H2O)(bpy)2](4+), is the first designed, well-defined molecule known to function as a catalyst for water oxidation. It meets the stoichiometric requirements for water oxidation, 2H2O --> -4e(-), -4H(+) O-O, by utilizing proton-coupled electron-transfer (PCET) reactions in which both electrons and protons are transferred. This avoids charge buildup, allowing for the accumulation of multiple oxidative equivalents at the Ru-O-Ru core. PCET and pathways involving coupled electron-proton transfer (EPT) are also used to avoid high-energy intermediates. Application of density functional theory calculations to molecular and electronic structure supports the proposal of strong electronic coupling across the micro-oxo bridge. The results of this analysis provide explanations for important details of the descriptive chemistry. Stepwise e(-)/H(+) loss leads to the higher oxidation states [(bpy)2(O)Ru(V)ORu(IV)(O)(bpy)2] (3+) (Ru(V)ORu(IV)) and [(bpy)2(O)Ru(V)ORu(V)(O)(bpy)2](4+) (Ru(V)ORu(V)). Both oxidize water, Ru(V)ORu(IV) stoichiometrically and Ru(V)ORu(V) catalytically. In strongly acidic solutions (HNO3, HClO4, and HSO3CF3) with excess Ce(IV), the catalytic mechanism involves O---O coupling following oxidation to Ru(V)ORu(V), which does not build up as a detectable intermediate. Direct evidence has been found for intervention of a peroxidic intermediate. Oxidation of water by Ru(V)ORu(IV) is far slower. It plays a role late in the catalytic cycle when Ce(IV) is depleted and is one origin of anated intermediates such as [(bpy)2(HO)Ru(IV)ORu(IV)(NO3)(bpy)2](4+), which are deleterious in tying up active components in the catalytic cycle. These intermediates slowly return to [(bpy)2(H2O)Ru(IV)ORu(III)(OH2)(bpy)2](5+) with anion release followed by water oxidation. The results of a recent analysis of water oxidation in the oxygen-evolving complex (OEC) of photosystem II reveal similarities in the mechanism with the blue dimer and significant differences. The OEC resides in the thylakoid membrane in the chloroplasts of green plants, and careful attention is paid in the structure to PCET, EPT, and long-range proton transfer by sequential local proton transfers. The active site for water oxidation is a CaMn 4 cluster, which includes an appended Mn site, Mn(4), where O---O coupling is thought to occur. Photochemical electron transfer results in oxidation of tyrosine Y Z to Y Z (.), which is approximately 7 A from Mn(4). It subsequently oxidizes the OEC through the stepwise stages of the Kok cycle. O---O coupling appears to occur through an initial peroxidic intermediate formed by redox nucleophilic attack of coordinated OH(-) in Ca-OH(-) on Mn (IV)=O.

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The interaction between CdSe nanocrystals (NCs) passivated with trioctylphosphine oxide (TOPO) ligands and a series of Ru-polypyridine complexes-[Ru(bpy)(3)](PF(6))(2) (1), [Ru(bpy)(2)(mcb)](PF(6))(2) (2), [Ru(bpy)(mcb)(2)](BarF)(2) (3), and [Ru(tpby)(2)(dcb)](PF(6))(2) (4) (where bpy = 2,2'-bipyridine, mcb = 4-carboxy-4'-methyl-2,2'-bipyridine, tbpy = 4,4'-di-tert-butyl-2,2'-bipyridine; dcb = 4,4'-dicarboxy-2,2'-bipyridine, and BarF = tetrakis[3,5-bis(trifluoromethyl)phenyl]borate)-was studied by attenuated total reflectance FTIR (ATR-FTIR) and modeled using density functional theory (DFT). ATR-FTIR studies reveal that when the solid film of NCs is exposed to an acetonitrile solution of 2, 3, or 4, the complexes chemically bind to the NC surface through their carboxylic acid groups, replacing TOPO ligands. The corresponding spectral changes are observed on a time scale of minutes. In the case of 2, the FTIR spectral changes clearly show that the complex adsorption is associated with a loss of proton from the carboxylic acid group. In the case of 3 and 4, deprotonation of the anchoring group is also detected, while the second, "spectrator" carboxylic acid group remains protonated. The observed energy difference between the symmetric, ν(s), and asymmetric, ν(as), stretch of the deprotonated carboxylic acid group suggests that the complexes are bound to the NC surface via a bridging mode. The results of DFT modeling are consistent with the experiment, showing that for the deprotonated carboxylic acid group the coupling to two Cd atoms via a bridging mode is the energetically most favorable mode of attachment for all nonequivalent NC surface sites and that the attachment of the protonated carboxylic acid is thermodynamically significantly less favorable.

The series of platinum acetylide oligomers (PAOs) with the general structure trans,trans-[(RO)3Ph-C[triple bond]C-Pt(PMe3)2-C[triple bond]C-(Ar)-C[triple bond]C-Pt(PMe3)2-C[triple bond]C-Ph(OR)3], where Ar = 1,4-phenylene, 2,5-thienylene, or bis-2,5-(S-2-methylbutoxy)-1,4-phenylene and R = n-C12H25 gel hydrocarbon solvents at concentrations above 1 mM. Gelation is thermally reversible (T(gel-sol) approximately 40-50 degrees C), and it occurs due to aggregation of the PAOs resulting in the formation of a fibrous network that is observed for dried gels imaged by TEM. The influence of aggregation/gelation on the photophysical properties of the PAOs is explored in detail. Aggregation induces a significant blue shift in the oligomers' absorption spectra, and the shift is attributed to exciton interactions arising from H-aggregation of the chromophores. Strong circular dichroism (CD) is observed for gelled solutions of a PAO substituted with homochiral S-2-methylbutoxy side chains on the central phenylene unit. The CD is attributed to formation of a chiral supramolecular aggregate structure. The PAOs are phosphorescent at ambient temperature in solution and in the aggregate/gel state. The phosphorescence band is blue-shifted ca. 20 nm in the aggregate/gel, and the shift is assigned to emission from an unrelaxed conformation of the triplet excited state. Phosphorescence spectroscopy of mixed aggregate/gels consisting of a triplet donor/host oligomer (Ar = 1,4-phenylene) doped with low concentrations of an acceptor/trap oligomer (Ar = 2,5-thienylene) indicates that energy transfer occurs efficiently in the aggregates. Triplet energy transfer involves exciton diffusion among the host chromophores followed by Dexter exchange energy transfer to the trap chromophore.

The effect of interchain interaction on the triplet excited state is explored in two Pt-acetylide polymers of the type [-trans-Pt(PBu(3))(2)-C triple bond C-Ar-C triple bond C-](n), where Ar is either 1,4-phenylene or is based on the pentiptycene unit (polymers 2 and 3, respectively). To explore the effect of interchain interaction in Pt-acetylide materials, the optical properties of parent polymer 2 are compared with those of polymer 3 in which interchain interaction is precluded by the sterically bulky pentiptycene moiety. Insight into the effect of the pentiptycene unit on packing in the solid state comes from the X-ray structure of monomer 1b, Ph-C triple bond C-[trans-Pt(PBu(3))(2)]-C triple bond C-Ar-C triple bond C-[trans-Pt(PBu(3))(2)]-C triple bond C-Ph. Spectroscopic studies indicate that weak phosphorescence emission from an interchain aggregate is observed from parent polymer 2, both in solution and in the solid state. By contrast, the photophysics of 3 is dominated by the intrachain triplet exciton. Interestingly, the phosphorescence emission of polymer 3 in the solid state is nearly superimposable with that of a single crystal of monomer 1b, suggesting that the solid polymer experiences an environment that is similar to that of the monomer in the crystal.

Chemical and electronic interactions between CdSe nanocrystal quantum dots (NQDs) and Ru‐polypyridine complexes are studied in solution. It is shown that photoluminescence (PL) can be used to effectively monitor the formation of NQD‐complex assemblies in real time. It is also shown that with the aid of Langmuir isotherm modeling, the PL studies can be used to quantitatively characterize the composition of the assemblies and the strength of electronic interactions between their components. The approach demonstrated here is general and can be applied to other systems that combine semiconductor NQDs and appropriately functionalized organometallic or organic molecules interacting with NQDs via energy transfer, charge transfer, or other mechanisms leading to quenching of NQD emission.

The ion radicals of two series of platinum acetylide oligomers have been subjected to study by electrochemical and pulse radiolysis/transient absorption methods. One series of oligomers, Ptn, has the general structure Ph-C[triple bond]C-[Pt(PBu3)2-C[triple bond]C-(1,4-Ph)-C[triple bond]C-]n-Pt(PBu3)2-C[triple bond]C-Ph (where x=0-4, Ph=phenyl and 1,4-Ph=1,4-phenylene). The second series of oligomers, Pt4Tn, contain a thiophene oligomer core, -C[triple bond]C-(2,5-Th)n-C[triple bond]C- (where n=1-3 and 2,5-Th=2,5-thienylene), capped on both ends with -Pt(PBu3)2-C[triple bond]C-(1,4-Ph)-C[triple bond]C-Pt(PBu3)2-C[triple bond]C-Ph segments. Electrochemical studies reveal that all of the oligomers feature reversible or quasi-reversible one-electron oxidation at potentials less than 1 V versus SCE. These oxidations are assigned to the formation of radical cations on the platinum acetylide chains. For the longer oligomers multiple, reversible one-electron waves are observed at potentials less than 1 V, indicating that multiple positive polarons can be produced on the oligomers. Pulse-radiolysis/transient absorption spectroscopy has been used to study the spectra and dynamics of the cation and anion radical states of the oligomers in dichloroethane and tetrahydrofuran solutions, respectively. All of the ion radicals exhibit two allowed absorption bands: one in the visible region and the second in the near-infrared region. The ion radical spectra shift with oligomer length, suggesting that the polarons are delocalized to some extent on the platinum acetylide chains. Analysis of the electrochemical and pulse radiolysis data combined with the density functional theory calculations on model ion radicals provides insight into the electronic structure of the positive and negative ion radical states of the oligomers. A key conclusion of the work is that the polaron states are concentrated on relatively short oligomer segments.