We use a combined, theoretical and experimental, approach to investigate the spectroscopic properties and electronic structure of three ruthenium polypyridyl complexes, [Ru(tpy)(2)](2+), [Ru(tpy)(bpy)(H(2)O)](2+), and [Ru(tpy)(bpy)(Cl)](+) (tpy = 2,2':6',2''-terpyridine and bpy = 2,2'-bipyridine) in acetone, dichloromethane, and water. All three complexes display strong absorption bands in the visible region corresponding to a metal-to-ligand-charge-transfer (MLCT) transition, as well as the emission bands arising from the lowest lying (3)MLCT state. [Ru(tpy)(bpy)(Cl)](+) undergoes substitution of the Cl(-) ligand by H(2)O in the presence of water. Density functional theory (DFT) calculations demonstrate that the triplet potential energy surfaces of these molecules are complicated, with several metal-centered ((3)MC) and (3)MLCT states very close in energy. Solvent effects are included in the calculations via the polarizable continuum model as well as explicitly, and it is shown that they are critical for proper characterization of the triplet excited states of these complexes.

FTIR difference spectroscopy is used to reveal changes in the internal structure and amino acid protonation states of bovine cytochrome c oxidase (CcO) that occur upon photolysis of the CO adduct of the two-electron reduced (mixed valence, MV) and four-electron reduced (fully reduced, FR) forms of the enzyme. FTIR difference spectra were obtained in D(2)O (pH 6-9.3) between the MV-CO adduct (heme a(3) and Cu(B) reduced; heme a and Cu(A) oxidized) and a photostationary state in which the MV-CO enzyme is photodissociated under constant illumination. In the photostationary state, part of the enzyme population has heme a(3) oxidized and heme a reduced. In MV-CO, the frequency of the stretch mode of CO bound to ferrous heme a(3) decreases from 1965.3 cm(-1) at pH*

A common challenge in the molecular photocatalysis of water splitting toward artificial photosynthesis [1] has been the realization of modular, multicomponent chromophore-catalyst assemblies that can meet the kinetic and thermodynamic requirements whilst successfully integrating both 1) the charge-transfer photoexcitation and accompanying stepwise transfer of a single electron to/from an acceptor/donor at the chromophoric end, and 2) the proton-coupled, multielectron redox buildup and chemical reactivity of the catalytic unit. Of particular interest to us is the potential utilization of visible sunlight energy to photochemically drive the catalytic oxidation of water into dioxygen. This reaction is highly endergonic and mechanistically complex, and involves a four-electron/ four-proton transformation that has been recognized as the bottleneck for the overall water splitting leading to H 2 and O 2 evolution. The photocatalysis of this process remains to be demonstrated in (supra)molecular chemistry.As a step toward this goal, we have designed and prepared a structurally simple dyad assembly of ruthenium complexes that is capable of catalytically performing the homogeneous visible-light photooxidation of organic compounds at ambient conditions in aqueous solution. As a proof of concept, we chose the dehydrogenation of alcohols, which is a thermodynamically uphill conversion involving a two-electron/twoproton coupled process. Besides their practical importance in organic processes, [2] such transformations are also of relevance to hydrogen-based energy technologies because the anodic liberation of protons and electrons [Eq. (1)] can be coupled with recombination on a cathode for H 2 fuel production in an integrated photoelectrochemical cell. The photocatalyst was constructed from ruthenium polypyridyl building blocks using the synthetic strategy shown in Scheme 1. A key consideration in the design of this assembly was the fact that the [Ru 2+ couple has been extensively explored [4,5] in proton-coupled electron-transfer (PCET) reactions [6] and oxidation of organic substrates upon redox activation by either electrochemistry or chemical oxidants, that is, H 2 ORuunit is a well known chromophore [7] , owing to its efficient metal-to-ligand charge transfer (MLCT) "pump", with a strong absorption in the visible region. [Ru(tpy) 2 ]2+ is a more appealing alternative to the bipyridine [Ru(bpy) 3 ] 2+ analogue because substitution at the 4-position of terpyridine can be used to afford linear, rigid structures favoring electron-transfer directionality. [7,8] Scheme 1. Synthetic strategy for the preparation of the dyad assembly and its monometallic precursors/components: A) [Ru(tpy)(dmso)Cl 2 ] (0.8 equiv) in N,N-dimethylformamide, reflux; isolation, then NH 4 PF 6 (excess) in water. B) cis-[Ru(bpy)(dmso) 2 Cl 2 ] (1.0 equiv) in methanol, reflux; then NH 4 PF 6 (excess). C) cis-[Ru(bpy)(dmso) 2 Cl 2 ] (0.7 equiv) in N,N-dimethylformamide, reflux; isolation, then NH 4 PF 6 (excess) in water. D) cis-[Ru(tpy)(dmso)Cl 2 ] (1.0...

The dinuclear complexes [(tpy)Ru(tppz)Ru(bpy)(L)](n+) (where L is Cl(-) or H(2)O, tpy and bpy are the terminal ligands 2,2':6',2''-terpyridine and 2,2'-bipyridine, and tppz is the bridging backbone 2,3,5,6-tetrakis(2-pyridyl)pyrazine) were prepared and structurally and electronically characterized. The mononuclear complexes [(tpy)Ru(tppz)](2+) and [(tppz)Ru(bpy)(L)](m+) were also prepared and studied for comparison. The proton-coupled, multi-electron photooxidation reactivity of the aquo dinuclear species was shown through the photocatalytic dehydrogenation of a series of primary and secondary alcohols. Under simulated solar irradiation and in the presence of a sacrificial electron acceptor, the photoactivated chromophore-catalyst complex (in aqueous solutions at room temperature and ambient pressure conditions) can perform the visible-light-driven conversion of aliphatic and benzylic alcohols into the corresponding carbonyl products (i.e., aldehydes or ketones) with 100% product selectivity and several tens of turnover cycles, as probed by NMR spectroscopy and gas chromatography. Moreover, for aliphatic substrates, the activity of the photocatalyst was found to be highly selective toward secondary alcohols, with no significant product formed from primary alcohols. Comparison of the activity of this tppz-bridged complex with that of the analogue containing a back-to-back terpyridine bridge (tpy-tpy, i.e., 6',6''-bis(2-pyridyl)-2,2':4',4'':2'',2'''-quaterpyridine) demonstrated that the latter is a superior photocatalyst toward the oxidation of alcohols. The much stronger electronic coupling with significant delocalization across the strongly electron-accepting tppz bridge facilitates charge trapping between the chromophore and catalyst centers and therefore is presumably responsible for the decreased catalytic performance.

Mixed valency in metal complexes such as the Creutz-Taube ion [1,2] [(NH 3 ) 5 Ru II (m-pz)Ru III (NH 3 ) 5 ] 5+ (1; pz = pyrazine) has been investigated for over forty years, [2][3][4][5][6][7] although a major remaining challenge is to develop a detailed understanding of the localized-to-delocalized transition (that is, Class II to Class III in the Robin-Day classification scheme). [4][5][6]8] A useful probe for assessing the extent of electronic delocalization has come from analysis of the intervalence-transfer (IT) absorption bands that typically appear in the near-IR spectra. [2,3,5,6] In the valence-localized limit, these IT bands arise from photoinduced intramolecular electron transfer across the ligand bridging the mixed-valence centers, that is, MA semi-classical theoretical treatment of electronic coupling and localization versus delocalization was provided by Hush.[3] This treatment was based on the average-mode approximation and assumed a single orbital interaction. However, the multiple ligand-mediated orbital interactions in transition-metal complexes, such as 1, can result in multiple IT transitions split by low symmetry and spin-orbit coupling.[5]For dp 6 -dp 5 systems, in particular, this is predicted to give rise to five low-energy transitions, three of IT origin and two of interconfigurational (IC) origin, as depicted in Figure 1.Neglecting the reorganization energy for the IC transitions, the IT band energies are related by E IT(1) = l, E IT(2) % E IT(1) + E IC(1) , and E IT(3) % E IT(1) + E IC(2) , where l is the reorganization energy for the bridge-mediated electron transfer. The IC band energies E IC(1) and E IC(2) depend on the local symmetry at M III and the magnitude of the spin-orbit coupling constant z.[5] Only the lowest-energy IT(1) absorption arises from electron transfer in the ground state-the higher-energy IT(2) and IT(3) absorptions lead to IC excited states at the donor site and are of mixed IT/IC character.The predicted spectral pattern of five low-energy bands has been observed in the near-IR/IR regions for mixedvalence Os dimers [5,9] in Class II-III.[5] Although valences (oxidation states) are localized in this intermediate class, electron transfer between the metal sites is sufficiently rapid that solvent is averaged and no longer contributes to l. As a consequence, the decreased band energies and widths result in well separated IT absorptions for Os systems, where the large spin-orbit coupling constant of Os III (z % 3000 cm À1 ) increases the energy spacings for both IT and IC transitions.Although the model is generally applicable to d 6 -d 5 systems, there is limited evidence for analogous behavior in complexes of Ru or Fe, for which the decreased magnitudes of the spin-orbit coupling (z(Ru III ) % 1000 cm À1 ; z(Fe III ) % 400-500 cm À1 ) lead to closely spaced IT bands with overlapping absorptions as well as IC bands that are shifted into the IR and have greatly reduced absorptivities (which vary as the square of the spin-orbit coupling constant [5] (2 À ; tppz = 2,3...