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.

The synthesis and photophysical characterization of a series of (N,C2‘-(2-para-tolylpyridyl))2Ir(LL‘) [(tpy)2Ir(LL‘)] (LL‘ = 2,4-pentanedionato (acac), bis(pyrazolyl)borate ligands and their analogues, diphosphine chelates and tert-butylisocyanide (CN-t-Bu)) are reported. A smaller series of [(dfppy)2Ir(LL‘)] (dfppy = N,C2‘-2-(4‘,6‘-difluorophenyl)pyridyl) complexes were also examined along with two previously reported compounds, (ppy)2Ir(CN)2- and (ppy)2Ir(NCS)2- (ppy = N,C2‘-2-phenylpyridyl). The (tpy)2Ir(PPh2CH2)2BPh2 and [(tpy)2Ir(CN-t-Bu)2](CF3SO3) complexes have been structurally characterized by X-ray crystallography. The Ir−Caryl bond lengths in (tpy)2Ir(CN-t-Bu)2+ (2.047(5) and 2.072(5) A) and (tpy)2Ir(PPh2CH2)2BPh2 (2.047(9) and 2.057(9) A) are longer than their counterparts in (tpy)2Ir(acac) (1.982(6) and 1.985(7) A). Density functional theory calculations carried out on (ppy)2Ir(CN-Me)2+ show that the highest occupied molecular orbital (HOMO) consists of a mixture of phenyl-π and Ir-d orbitals...

Synthetic control of excited-state properties in cyclometalated Ir(III) complexes using ancillary ligands.

Aromatic nitrogen heterocycles as bridging ligands; a survey

Electrochemical studies of ruthenium compounds part I. Ligand oxidation levels

Strategies toward the realization of molecular control of interfacial charge transfer at nanocrystalline semiconductor interfaces are described. Light excitation of coordination compounds, based on (dpi)6 transition metals, anchored to wide band-gap semiconductors, such as TiO2, can initiate electron-transfer processes that ultimately reduce the semiconductor. Such photoinduced charge-separation processes are a key step for solar energy conversion. The thermodynamics and kinetic rate constants for three different interfacial charge separation mechanisms are discussed. Tuning the energetic position of the semiconductor conduction band relative to the molecular sensitizer has provided new insights into interfacial charge transfer. Supramolecular compounds that efficiently absorb light, promote interfacial electron transfer, and feature additional functions such as intramolecular electron transfer when bound to semiconductor surfaces have also been studied. New approaches for enhancing charge-separation lifetimes for solar energy conversion are presented.

Molecular approaches to solar energy conversion with coordination compounds anchored to semiconductor surfaces.

The synthesis, structure, and redox and photophysical properties of (Ru(bpy)/sub n/(BL)/sub 3-n/)/sup 2 +/, where n = 0-2, bpy is 2,2'-bipyridine, and BL is 2,3-bis(2-pyridyl)quinoxaline, are described. The (Ru(bpy)/sub 2/(BL))(PF/sub 6/)/sub 2/ complex crystallized in the monoclinic space group P2/sub 1//a and the cell parameters are repeated. All non-H atoms were refined anisotropically to an R value of 0.0672 for 5766 reflections with I > sigma(I). The ruthenium to nitrogen bond distances were Ru-N(pyridine) = 2.06 A (average) and Ru-N(pyrazine) = 2.096 (4) A. Absorption spectra contained bands (250-300 nm) in the ultraviolet region that were assignable to ligand ..pi.. ..-->.. ..pi..* transitions and visible bands (517-300 nm) that were assignable to d..pi.. ..-->.. ..pi..* MLCT transitions. Reduction potentials for the Ru/sup 3+/2+/ couples varied from about 1.40 to 1.70 V vs SSCE. Three reductions were observed and assigned to the one-electron reduction of each bidentate ligand. Weak luminescence was observed, and corrected emission energy maxima were located at 766 +/- 4 nm. Excited-state reduction potentials were estimated from the difference between emission energy maxima and ground-state reduction potentials. Excited-state lifetimes at room temperature in 4:1 EtOH-MeOH ranged from 167 to < 70 ns. Temperature-dependent-lifetime (90-298 K) data gave evidencemore » for ..delta..E' values of 1100 +/- 300 cm/sup -1/. The temperature dependence was attributed to either the third MLCT state according to the localized-orbital model or the presence of a fourth CT state for (Ru(BL)/sub 2/(bpy))/sup 2 +/, since little photochemistry was found for it, and to the third and/or the fourth CT state and the dd state for both (Ru(BL)/sub 2/(bpy))/sup 2 +/ and (Ru(BL)/sub 3/)/sup 2 +/. 54 references, 7 figures, 6 tables.« less

Structure and redox and photophysical properties of a series of ruthenium heterocycles based on the ligand 2,3-bis(2-pyridyl)quinoxaline

The photophysical and photochemical properties of a series of 2-(2[prime]-pyridyl)pyrimidine and bis(pyridine) polypyridyl complexes of ruthenium(II) are described The temperature dependences of the emission decay were fit to the equation I/[tau](T) = k[sub 1] + k[sub 2] exp([minus][Delta]E[sub 2]/RT) The values of k[sub 1] ranged from 71 [times] 10[sup 5] to 87 [times] 10[sup 6] s[sup [minus]1], k[sub 2] ranged from 9 [times] 10[sup 6] to 3 [times] 10[sup 17] s[sup [minus]1], and [Delta]E[sub 2] ranged from 428 to 4,776 cm[sup [minus]1] A plot of ln k[sub 2] vs [Delta]E[sub 2], known as a Barclay-Butler plot, was linear with a slope of 29 [times] 10[sup [minus]3] cm and a y intercept of 59 The study demonstrates the usefulness of excited-state parameters for assessing the photolability and/or stability of ruthenium(II) pyridyl complexes It has also led to the design of the photostable core [Ru(bpy)(bpz)][sup 2+] which may be of value in the design of molecular assemblies

L. A. Worl

Papers

Structure and redox and photophysical properties of a series of ruthenium heterocycles based on the ligand 2,3-bis(2-pyridyl)quinoxaline

MLCT-dd energy gap in pyridyl-pyrimidine and bis(pyridine) complexes of ruthenium(II)

Structure and Redox and Photophysical Properties of a Series of Ruthenium Heterocycles Based on the Ligand 2,3-Bis(2-pyridyl)quinoxaline.