Probing Co-Assembly of Supramolecular Photocatalysts and Polyelectrolytes Using Isothermal Titration Calorimetry.

The creation of renewable fuels to replace dwindling fossil energy resources is one of the greatest challenges facing the scientific community. Generating H2 fuel from water is a carbon-neutral strategy that demonstrates great promise. Photocatalysts of the molecular architecture [{(TL)2Ru(BL)}2RhX2]5+ (BL = bridging ligand, TL = terminal ligand, X = halide) catalyze the formation of H2 in deoxygenated organic solvents but are limited by poor performance in air-saturated aqueous solutions. Addition of the water-soluble polyelectrolyte poly(sodium 4-styrenesulfonate) (PSS) was recently shown as being a promising new strategy to increase efficiency and stability of H2 evolving photocatalysts in air-saturated aqueous solutions. Herein we investigate intermolecular interactions between Ru,Rh,Ru photocatalysts and water-soluble polyelectrolytes using isothermal titration calorimetry (ITC). ITC studies provide insight into the thermodynamic forces that drive assembly of PSS-photocatalyst aggregates and give new evidence for the intermolecular forces that lead to increased photocatalytic efficiency.

A new hydrophilic supramolecular photocatalyst for the production of H2 in aerobic aqueous solutions.

Addition of sulfonated terminal ligands into a Ru,Rh,Ru photocatalyst has a significant impact on the excited-state properties of the complex. The hydrophilic photocatalyst demonstrates increased solubility and H2 production in aqueous solutions. H2 production is observed under aerobic conditions for the new complex, a stark contrast to the hydrophobic analog in organic solvents.

Enhancement of Solar Fuel Production Schemes by Using a Ru,Rh,Ru Supramolecular Photocatalyst Containing Hydroxide Labile Ligands.

Polyazine-bridged Ru(II)Rh(III)Ru(II) complexes with two halide ligands, Cl(-) or Br(-), bound to the catalytically active Rh center are efficient single-component photocatalysts for H2O reduction to H2 fuel, with the coordination environment on Rh impacting photocatalysis. Herein reported is a new, halide-free Ru(II)Rh(III)Ru(II) photocatalyst with OH(-) ligands bound to Rh, further enhancing the photocatalytic reactivity of the structural motif. H2 production experiments using the photocatalyst bearing OH(-) ligands at Rh relative to the analogues bearing halides at Rh in solvents of varying polarity (DMF, CH3CN, and H2O) suggest that ion pairing with halides deactivates photocatalyst function, representing an exciting phenomenon to exploit in the development of catalysts for solar H2 production schemes.

Increased Water Reduction Efficiency of Polyelectrolyte-Bound Trimetallic [Ru,Rh,Ru] Photocatalysts in Air-Saturated Aqueous Solutions.

The groundbreaking use of polyelectrolytes to increase the efficiency of supramolecular photocatalysts in solar H2 production schemes under aqueous aerobic conditions is reported. Supramolecular photocatalysts of the architecture [{(TL)2 Ru(BL)}2 RhX2 ](5+) (BL=bridging ligand, TL=terminal ligand, X=halide) demonstrate high efficiencies in deoxygenated organic solvents but do not function in air-saturated aqueous solution because of the quenching of the metal-to-ligand charge-transfer (MLCT) excited state under these conditions. The new photocatalytic system incorporates poly(4-styrenesulfonate) (PSS) into aqueous solutions containing [{(bpy)2 Ru(dpp)}2 RhCl2 ](5+) (bpy=2,2'-bipyridine, dpp=2,3-bis(2-pyridyl)pyrazine). PSS has a profound impact on the photocatalyst efficiency, increasing H2 production over three times that of deoxygenated aqueous solutions alone. H2 photocatalysis proceeds even under aerobic conditions for PSS-containing solutions, an exciting consequence for solar hydrogen-production research.

Nonchromophoric halide ligand variation in polyazine-bridged Ru(II),Rh(III) bimetallic supramolecules offering new insight into photocatalytic hydrogen production from water.

The new bimetallic complex [(Ph2phen)2Ru(dpp)RhBr2(Ph2phen)](PF6)3 (1) (Ph2phen = 4,7-diphenyl-1,10-phenanthroline; dpp = 2,3-bis(2-pyridyl)pyrazine) was synthesized and characterized to compare with the Cl(-) analogue [(Ph2phen)2Ru(dpp)RhCl2(Ph2phen)](PF6)3 (2) in an effort to better understand the role of halide coordination at the Rh metal center in solar H2 production schemes. Electrochemical properties of complex 1 display a reversible Ru(II/III) oxidation, and cathodic scans indicate multiple electrochemical mechanisms exist to reduce Rh(III) by two electrons to Rh(I) followed by a quasi-reversible dpp(0/-) ligand reduction. The weaker σ-donating ability of Br(-) vs Cl(-) impacts the cathodic electrochemistry and provides insight into photocatalytic function by these bimetallic supramolecules. Complexes 1 and 2 exhibit identical light-absorbing properties with UV absorption dominated by intraligand (IL) π → π* transitions and visible absorption by metal-to-ligand charge transfer (MLCT) transitions to include a lowest energy Ru(dπ) → dpp(π*) (1)MLCT transition (λ(abs) = 514 nm; ε = 16 000 M(-1) cm(-1)). The relatively short-lived, weakly emissive Ru(dπ) → dpp(π*) (3)MLCT excited state (τ = 46 ns) for both bimetallic complexes is attributed to intramolecular electron transfer from the (3)MLCT excited state to populate a low-energy Ru(dπ) → Rh(dσ*) triplet metal-to-metal charge transfer ((3)MMCT) excited state that allows photoinitiated electron collection. Complex 1 outperforms the related Cl(-) bimetallic analogue 2 as a H2 photocatalyst despite identical light-absorbing and excited-state properties. Additional H2 experiments with added halide suggest ion pairing plays a role in catalyst deactivation and provides new insight into observed differences in H2 production upon halide variation in Ru(II),Rh(III) supramolecular architectures.

Electrochemical, spectroscopic, and photophysical properties of structurally diverse polyazine-bridged Ru(II),Pt(II) and Os(II),Ru(II),Pt(II) supramolecular motifs.

Five new tetrametallic supramolecules of the motif [{(TL)(2)M(dpp)}(2)Ru(BL)PtCl(2)](6+) and three new trimetallic light absorbers [{(TL)(2)M(dpp)}(2)Ru(BL)](6+) (TL = bpy = 2,2'-bipyridine or phen = 1,10-phenanthroline; M = Ru(II) or Os(II); BL = dpp = 2,3-bis(2-pyridyl)pyrazine, dpq = 2,3-bis(2-pyridyl)quinoxaline, or bpm = 2,2'-bipyrimidine) were synthesized and their redox, spectroscopic, and photophysical properties investigated. The tetrametallic complexes couple a Pt(II)-based reactive metal center to Ru and/or Os light absorbers through two different polyazine BL to provide structural diversity and interesting resultant properties. The redox potential of the M(II/III) couple is modulated by M variation, with the terminal Ru(II/III) occurring at 1.58-1.61 V and terminal Os(II/III) couples at 1.07-1.18 V versus Ag/AgCl. [{(TL)(2)M(dpp)}(2)Ru(BL)](PF(6))(6) display terminal M(dπ)-based highest occupied molecular orbitals (HOMOs) with the dpp(π*)-based lowest unoccupied molecular orbital (LUMO) energy relatively unaffected by the nature of BL. The coupling of Pt to the BL results in orbital inversion with localization of the LUMO on the remote BL in the tetrametallic complexes, providing a lowest energy charge separated (CS) state with an oxidized terminal Ru or Os and spatially separated reduced BL. The complexes [{(TL)(2)M(dpp)}(2)Ru(BL)](6+) and [{(TL)(2)M(dpp)}(2)Ru(BL)PtCl(2)](6+) efficiently absorb light throughout the UV and visible regions with intense metal-to-ligand charge transfer (MLCT) transitions in the visible at about 540 nm (M = Ru) and 560 nm (M = Os) (ε ≈ 33,000-42,000 M(-1) cm(-1)) and direct excitation to the spin-forbidden (3)MLCT excited state in the Os complexes about 720 nm. All the trimetallic and tetrametallic Ru-based supramolecular systems emit from the terminal Ru(dπ)→dpp(π*) (3)MLCT state, λ(max)(em) ≈ 750 nm. The tetrametallic systems display complex excited state dynamics with quenching of the (3)MLCT emission at room temperature to populate the lowest-lying (3)CS state population of the emissive (3)MLCT state.

High turnover in a photocatalytic system for water reduction to produce hydrogen using a Ru, Rh, Ru photoinitiated electron collector.

Covalent coupling of Ru(II) light absorbers to a Rh(III) electron collecting site through polyazine bridging ligands affords photocatalytic production of H(2) in the presence of visible light and a sacrificial electron donor. A robust photocatalytic system displaying a high turnover of the photocatalyst has been developed using the photoinitiated electron collector [{(bpy)(2)Ru(dpp)}(2)RhBr(2)](5+) (bpy=2,2'-bipyridine; dpp=2,3-bis(2-pyridyl)pyrazine) and N,N-dimethylaniline in DMF/H(2)O. Studies have shown that increased [DMA], the headspace volume, and the use of DMF solvent improves the systems performance and stability providing mechanistic insight into the deactivation routes of the photocatalytic system. Photolysis of the system at 460 nm generates 20 mL of H(2) in 19.5 h with a maximum Φ=0.023 based on H(2) produced and an overall Φ=0.014 and 280 turnovers of the photocatalyst. The photocatalytic system also displays long-term photostability with 30 mL of H(2) generated and 420 turnovers in 50 h under the same conditions. Prolonged photolysis provides 820 mol H(2) per mole of catalyst.

A new structural motif for photoinitiated electron collection: Ru,Rh bimetallics providing insight into H2 production via photocatalysis of water reduction by Ru,Rh,Ru supramolecules.

Ru,Rh,Ru complexes are photocatalysts for the reduction of H(2)O to H(2)via a Rh(I) intermediate. The herein reported Ru,Rh bimetallics undergo PEC but do not catalyze the reduction of H(2)O, establishing intact supramolecules are photoactive in the Ru,Rh,Ru systems. The Ru,Rh(I) photoproduct dimerizes via Rh-Rh bond formation, deactivating the Rh(I) center sterically prohibited in the Ru,Rh,Ru trimetallic systems.

A structurally diverse Ru(II),Pt(II) tetrametallic motif for photoinitiated electron collection and photocatalytic hydrogen production.

Coupling a reactive metal to light absorbers affords molecular devices for photoinitiated electron collection and photocatalytic conversion of substrates to fuels. A new Ru(II),Pt(II) tetrametallic supramolecule, [{(phen)(2)Ru(dpp)}(2)Ru(dpq)PtCl(2)](PF(6))(6), and the trimetallic precursors, [{(phen)(2)Ru(dpp)}(2)RuCl(2)](PF(6))(4) and [{(phen)(2)Ru(dpp)}(2)Ru(dpq)](PF(6))(6), have been synthesized, and their redox, spectroscopic, spectroelectrochemical, photophysical and photocatalytic properties studied. They efficiently absorb UV and visible light. The electrochemistry of [{(phen)(2)Ru(dpp)}(2)Ru(dpq)PtCl(2)](PF(6))(6) suggests a lowest-lying terminal Ru→dpq charge-separated state that quenches the emission of the parent complex with non-unity population of the emissive (3)MLCT excited state. Photolysis of [{(phen)(2)Ru(dpp)}(2)Ru(dpq)PtCl(2)](6+) at 470 nm with DMA gives multielectron reduction, storing electrons in a new manner on the central (dpp)(2)Ru(II)(dpq) moiety. Addition of H(2)O to the photolysis system produces 21 µmol of H(2) in 5 h, with 115 turnovers of the tetrametallic photocatalyst.

A Series of Supramolecular Complexes for Solar Energy Conversion via Water Reduction to Produce Hydrogen: An Excited State Kinetic Analysis of Ru(II),Rh(III),Ru(II) Photoinitiated Electron Collectors.

Mixed-metal supramolecular complexes have been designed that photochemically absorb solar light, undergo photoinitiated electron collection and reduce water to produce hydrogen fuel using low energy visible light. This manuscript describes these systems with an analysis of the photophysics of a series of six supramolecular complexes, [{(TL)2Ru(dpp)}2RhX2](PF6)5 with TL = bpy, phen or Ph2phen with X = Cl or Br. The process of light conversion to a fuel requires a system to perform a number of complicated steps including the absorption of light, the generation of charge separation on a molecular level, the reduction by one and then two electrons and the interaction with the water substrate to produce hydrogen. The manuscript explores the rate of intramolecular electron transfer, rate of quenching of the supramolecules by the DMA electron donor, rate of reduction of the complex by DMA from the ³MLCT excited state, as well as overall rate of reduction of the complex via visible light excitation. Probing a series of complexes in detail exploring the variation of rates of important reactions as a function of sub-unit modification provides insight into the role of each process in the overall efficiency of water reduction to produce hydrogen. The kinetic analysis shows that the complexes display different rates of excited state reactions that vary with TL and halide. The role of the MLCT excited state is elucidated by this kinetic study which shows that the ³MLCT state and not the ³MMCT is likely that key contributor to the photoreduction of these complexes. The kinetic analysis of the excited state dynamics and reactions of the complexes are important as this class of supramolecules behaves as photoinitiated electron collectors and photocatalysts for the reduction of water to hydrogen.

Emission Spectroscopy as a Probe into Photoinduced Intramolecular Electron Transfer in Polyazine Bridged Ru(II),Rh(III) Supramolecular Complexes.

Steady-state and time-resolved emission spectroscopy are valuable tools to probe photochemical processes of metal-ligand, coordination complexes. Ru(II) polyazine light absorbers are efficient light harvesters absorbing in the UV and visible with emissive ³MLCT excited states known to undergo excited state energy and electron transfer. Changes in emission intensity, energy or band-shape, as well as excited state lifetime, provide insight into excited state dynamics. Photophysical processes such as intramolecular electron transfer between electron donor and electron acceptor sub-units may be investigated using these methods. This review investigates the use of steady-state and time-resolved emission spectroscopy to measure excited state intramolecular electron transfer in polyazine bridged Ru(II),Rh(III) supramolecular complexes. Intramolecular electron transfer in these systems provides for conversion of the emissive ³MLCT (metal-to-ligand charge transfer) excited state to a non-emissive, but potentially photoreactive, ³MMCT (metal-to-metal charge transfer) excited state. The details of the photophysics of Ru(II),Rh(III) and Ru(II),Rh(III),Ru(II) systems as probed by steady-state and time-resolved emission spectroscopy will be highlighted.

Design considerations for a system for photocatalytic hydrogen production from water employing mixed-metal photochemical molecular devices for photoinitiated electron collection.

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.

Synthesis and study of the spectroscopic and redox properties of Ru(II),Pt(II) mixed-metal complexes bridged by 2,3,5,6-tetrakis(2-pyridyl)pyrazine.

The mixed-metal supramolecular complexes [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 (tpy = 2,2':6',2''-terpyridine and tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine) were synthesized and characterized. These complexes contain ruthenium bridged by tppz to platinum centers to form stereochemically defined linear assemblies. X-ray crystallographic determinations of the two complexes confirm the identity of the metal complexes and reveal intermolecular interactions of the Pt sites in the solid state for [(tpy)Ru(tppz)PtCl](PF6)3 with a Pt...Pt distance of 3.3218(5) A. The (1)H NMR spectra show the expected splitting patterns characteristic of stereochemically defined mixed-metal systems and are assigned with the use of (1)H-(1)H COSY and NOESY. Electronic absorption spectroscopy displays intense ligand-based pi --> pi* transitions in the UV and MLCT transitions in the visible. Electrochemically [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4 display reversible Ru (II/III) couples at 1.63 and 1.83 V versus Ag/AgCl, respectively. The complexes display very low potential tppz (0/-) and tppz(-/2-) couples, relative to their monometallic synthons, [(tpy)Ru(tppz)](PF6)2 and [Ru(tppz)2](PF6)2, consistent with the bridging coordination of the tppz ligand. The Ru(dpi) --> tppz(pi*) MLCT transitions are also red-shifted relative to the monometallic synthons occurring in the visible centered at 530 and 538 nm in CH3CN for [(tpy)Ru(tppz)PtCl](PF6)3 and [ClPt(tppz)Ru(tppz)PtCl](PF6)4, respectively. The complex [(tpy)Ru(tppz)PtCl](PF6)3 displays a barely detectable emission from the Ru(dpi) --> tppz(pi*) (3)MLCT in CH 3CN solution at RT. In contrast, [ClPt(tppz)Ru(tppz)PtCl](PF6)4 displays an intense emission from the Ru(dpi) --> tppz(pi*) (3)MLCT state at RT with lambda max(em) = 754 nm and tau = 80 ns.