▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...

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Proton-Coupled Electron Transfer

A critical review of light-driven interfacial charge-transfer reactions of transition-metal compounds anchored to mesoporous, nanocrystalline TiO2 (anatase) thin films is described. The review highlights molecular insights into metal-to-ligand charge transfer (MLCT) excited states, mechanisms of interfacial charge separation, inter- and intra-molecular electron transfer, and interfacial charge-recombination processes that have been garnered through various spectroscopic and electrochemical techniques. The relevance of these processes to optimization of solar-energy-conversion efficiencies is discussed (483 references).

Photodriven heterogeneous charge transfer with transition-metal compounds anchored to TiO2 semiconductor surfaces

The goal of artificial photosynthesis is to use the energy of the sun to make high-energy chemicals for energy production. One approach, described here, is to use light absorption and excited-state electron transfer to create oxidative and reductive equivalents for driving relevant fuel-forming half-reactions such as the oxidation of water to O2 and its reduction to H2. In this "integrated modular assembly" approach, separate components for light absorption, energy transfer, and long-range electron transfer by use of free-energy gradients are integrated with oxidative and reductive catalysts into single molecular assemblies or on separate electrodes in photelectrochemical cells. Derivatized porphyrins and metalloporphyrins and metal polypyridyl complexes have been most commonly used in these assemblies, with the latter the focus of the current account. The underlying physical principles--light absorption, energy transfer, radiative and nonradiative excited-state decay, electron transfer, proton-coupled electron transfer, and catalysis--are outlined with an eye toward their roles in molecular assemblies for energy conversion. Synthetic approaches based on sequential covalent bond formation, derivatization of preformed polymers, and stepwise polypeptide synthesis have been used to prepare molecular assemblies. A higher level hierarchial "assembly of assemblies" strategy is required for a working device, and progress has been made for metal polypyridyl complex assemblies based on sol-gels, electropolymerized thin films, and chemical adsorption to thin films of metal oxide nanoparticles.

Chemical approaches to artificial photosynthesis. 2.

Ruthenium compounds, particularly Ru(II) polypyridine complexes, are the class of transition metal complexes which has been most deeply investigated from a photochemical viewpoint. The reason for such great interest stems from a unique combination of chemical stability, redox properties, excited-state reactivity, luminescence emission, and excited-state lifetime. Ruthenium polypyridine complexes are indeed good visible light absorbers, feature relatively intense and long-lived luminescence, and can undergo reversible redox processes in both the ground and excited states. This chapter presents some general concepts on the photochemical properties of Ru(II) polypyridine complexes and gives an overview of various research topics involving ruthenium photochemistry which have emerged in the last 15 years. In particular, aspects connected to supramolecular photochemistry and photophysics are discussed, such as multicomponent systems for light harvesting and photoinduced charge separation, systems for photoinduced multielectron/hole storage, and photocatalytic processes based on supramolecular Ru(II) polypyridine species. Interaction with biological systems and dye-sensitized photoelectrochemical cells are also briefly discussed.

Photochemistry and Photophysics of Coordination Compounds: Ruthenium

Two series of mononuclear ruthenium(II) complexes involving polypyridine-type ligands have been prepared, and their ability to act as catalysts for water oxidation has been examined. One series is of the type [Ru(tpy)(NN)Cl](PF6) (tpy = 2,2′; 6,2′′-terpyridine), where NN is one of 12 different bidentate ligands, and the other series includes various combinations of 4-picoline, 2,2′-bipyridine (bpy), and tpy as well as the tetradentate 2,9-dipyrid-2′-yl-1,10-phenanthroline (dpp). The electronic absorption and redox data for these compounds have been measured and reported. The long-wavelength metal-to-ligand charge-transfer absorption and the first oxidation and reduction potentials are found to be consistent with the structure of the complex. Of the 23 complexes, 14 catalyze water oxidation and all of these contain a tpy or dpp. Kinetic measurements indicate a first-order reaction and together with a catalyst recovery experiment argue against the involvement of RuO2. A tentative mechanism is proposed that ...

Mononuclear ruthenium(II) complexes that catalyze water oxidation.

It is known that the relaxed excited state of [Ru(bpy)3]2+ is best described as a metal to ligand charge transfer (MLCT) state having one formally reduced bipyridine and two neutral. Previous reports have suggested [Malone, R. et al. J. Chem. Phys. 1991, 95, 8970] that the electron “hops” from ligand to ligand in the MLCT state with a time constant of about 50 ps in acetonitrile. However, we have done transient absorption anisotropy measurements indicating that already after one picosecond the molecule has no memory of which bipyridine was initially photoselected, which suggests an ultrafast interligand randomization of the MLCT state.

Femtosecond Transient Absorption Anisotropy Study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. Ultrafast Interligand Randomization of the MLCT State

Two pyrazoline- and one pyrrolidine-bridged Ru(II)bipyridine-[60]fullerene dyads have been prepared and studied by ultrafast time-resolved spectroscopy. A silver-assisted synthesis route, in which Ag(I) removes the chlorides from the precursor complex Ru(bpy) 2Cl 2 facilitates successful coordination of the [60]fullerene-substituted third ligand. Upon light excitation of the ruthenium moiety, the emission was strongly quenched by the fullerene. The main quenching mechanism is an exceptionally fast direct energy transfer ( k obs > 1 x 10 (12) s (-1) in the pyrazoline-bridged dyads), resulting in population of the lowest excited triplet state of fullerene. No evidence for electron transfer was found, despite the extraordinarily short donor-acceptor distance that could kinetically favor that process. The observations have implications on the ongoing development of devices built from Ru-polypyridyl complexes and nanostructured carbon, such as C 60 or nanotubes.

Judit Modin

Papers

Femtosecond Transient Absorption Anisotropy Study on [Ru(bpy)3]2+ and [Ru(bpy)(py)4]2+. Ultrafast Interligand Randomization of the MLCT State

How close can you get? Studies of ultrafast light-induced processes in ruthenium-[60] fullerene dyads with short pyrazolino and pyrrolidino links.

New pyrazolino- and pyrrolidino[60]fullerenes with transition-metal chelating pyridine substitutents: synthesis and complexation to Ru(II).