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The dye-sensitized photoelectrosynthesis cell (DSPEC) integrates high bandgap, nanoparticle oxide semiconductors with the light-absorbing and catalytic properties of designed chromophore-catalyst assemblies. The goals are photoelectrochemical water splitting into hydrogen and oxygen and reduction of CO by water to give oxygen and carbon-based fuels. Solar-driven water oxidation occurs at a photoanode and water or CO reduction at a cathode or photocathode initiated by molecular-level light absorption. Light absorption is followed by electron or hole injection, catalyst activation, and catalytic water oxidation or water/CO reduction. The DSPEC is of recent origin but significant progress has been made. It has the potential to play an important role in our energy future.

photoelectrochemical cell | photosytem II | water splitting | solar fuels | artificial photosynthesis

Isomorphous metal-organic frameworks (MOFs) based on {M[4,4'-(HO(2)C)(2)-bpy](2)bpy}(2+) building blocks (where M = Ru or Os) were designed and synthesized to study the classic Ru to Os energy transfer process that has potential applications in light-harvesting with supramolecular assemblies. The crystalline nature of the MOFs allows precise determination of the distances between metal centers by X-ray diffraction, thereby facilitating the study of the Ru→Os energy transfer process. The mixed-metal MOFs with 0.3, 0.6, 1.4, and 2.6 mol % Os doping were also synthesized in order to study the energy transfer dynamics with a two-photon excitation at 850 nm. The Ru lifetime at 620 nm decreases from 171 ns in the pure Ru MOF to 29 ns in the sample with 2.6 mol % Os doping. In the mixed-metal samples, energy transfer was observed with an initial growth in Os emission corresponding with the rate of decay of the Ru excited state. These results demonstrate rapid, efficient energy migration and long distance transfer in isomorphous MOFs.

We report temperature-dependent excited-state lifetime measurements on [Ru(bpy)(2)dppz](2+) in both protic and aprotic solvents. These experiments yield a unifying picture of the excited-state photophysics that accounts for observations in both types of solvent. Our measurements support the notion of bpy-like and phz-like states associated with the dppz ligand and show that the ligand orbital associated with the bright state is similar in size to the corresponding orbital in the (3)MLCT state of [Ru(bpy)(3)](2+). In contrast to the current thinking, the experiments presented here indicate that the light-switch effect is not driven by a state reversal. Rather, they suggest that the dark state is always lowest in energy, even in aprotic solvents, and that the light-switch behavior is the result of a competition between energetic factors that favor the dark state and entropic factors that favor the bright (bpy) state.

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Temperature-dependent excited-state lifetime measurements have been performed on four different Ru(II)-based dppz compounds in protic and aprotic solvents. This work supports the existence of a dynamic equilibrium between two MLCT states associated with the dppz ligand:  one is a bright state with a ligand orbital similar in size to that associated with the 3MLCT state of [Ru(bpy)3]2+, and the other is a dark phz-like state. Our results are consistent with a light-switch mechanism involving a competition between energetic factors that favor the dark (phz) state and entropic factors that favor the bright (bpy) state. This paper explores the photophysics of these light-switch compounds through a systematic variation of the equilibrium energetics. This is accomplished by (1) varying the dielectric strength of the solvent and (2) making chemical substitutions on the dppz ligand. Observations obtained from all four compounds in six different solvents can be explained using this equilibrium model.

Microscale metal-organic frameworks (MOFs) were synthesized from photoactive Ru(II)-bpy building blocks with strong visible light absorption and long-lived triplet metal-to-ligand charge transfer ((3)MLCT) excited states. These MOFs underwent efficient luminescence quenching in the presence of either oxidative or reductive quenchers. Up to 98% emission quenching was achieved with either an oxidative quencher (1,4-benzoquinone) or a reductive quencher (N,N,N',N'-tetramethylbenzidine), as a result of rapid energy migration over several hundred nanometers followed by efficient electron transfer quenching at the MOF/solution interface. The photoactive MOFs act as an excellent light-harvesting system by combining intraframework energy migration and interfacial electron transfer quenching.