Dye-sensitized solar cells (DSCs) offer the possibilities to design solar cells with a large flexibility in shape, color, and transparency. DSC research groups have been established around the worl ...

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Dye-Sensitized Solar Cells

The use of conductivity measurements in organic solvents for the characterisation of coordination compounds

J. Appl. Cryst.の発刊に際して

A fundamental aim in the field of catalysis is the development of new modes of small molecule activation. One approach toward the catalytic activation of organic molecules that has received much attention recently is visible light photoredox catalysis. In a general sense, this approach relies on the ability of metal complexes and organic dyes to engage in single-electron-transfer (SET) processes with organic substrates upon photoexcitation with visible light.

Many of the most commonly employed visible light photocatalysts are polypyridyl complexes of ruthenium and iridium, and are typified by the complex tris(2,2′-bipyridine) ruthenium(II), or Ru(bpy)32+ (Figure 1). These complexes absorb light in the visible region of the electromagnetic spectrum to give stable, long-lived photoexcited states.1,2 The lifetime of the excited species is sufficiently long (1100 ns for Ru(bpy)32+) that it may engage in bimolecular electron-transfer reactions in competition with deactivation pathways.3 Although these species are poor single-electron oxidants and reductants in the ground state, excitation of an electron affords excited states that are very potent single-electron-transfer reagents. Importantly, the conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes.






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Figure 1


Ruthenium polypyridyl complexes: versatile visible light photocatalysts.

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Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

Excitonic solar cells1—including organic, hybrid organic–inorganic and dye-sensitized cells (DSCs)—are promising devices for inexpensive, large-scale solar energy conversion. The DSC is currently the most efficient2 and stable3 excitonic photocell. Central to this device is a thick nanoparticle film that provides a large surface area for the adsorption of light-harvesting molecules. However, nanoparticle DSCs rely on trap-limited diffusion for electron transport, a slow mechanism that can limit device efficiency, especially at longer wavelengths. Here we introduce a version of the dye-sensitized cell in which the traditional nanoparticle film is replaced by a dense array of oriented, crystalline ZnO nanowires. The nanowire anode is synthesized by mild aqueous chemistry and features a surface area up to one-fifth as large as a nanoparticle cell. The direct electrical pathways provided by the nanowires ensure the rapid collection of carriers generated throughout the device, and a full Sun efficiency of 1.5% is demonstrated, limited primarily by the surface area of the nanowire array.

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Nanowire dye-sensitized solar cells

: The MLCT (Metal to Ligand Charge Transfer) excited state of the Ru(bipyrazine)3(2+) is quenched by a series of organic amines and methoxybenzenes, in acetontrile solution. Linear Stern-Volmer plots were obtained, and various rate parameters were extracted from the data. The excited state is also quenced in neutral aqueous solution by a range of metal ions and complexes. Rate constants for both oxidative and reductive quenching mechanisms were obtained. In general the rates are faster for this ion than for the corresponding Ru(bipyridine)3(2+) excited state. Similar data were also obtained for the monoprotonated complex in 2M sulfuric acid and the hexaprotonated species in concentrated sulfuric acid. Originator-supplied keywords include: Ruthenium, Bipyrazine, Quenching, Luminescence.

Luminescence quenching of the tris(2,2′-bipyrazine)ruthenium(ii) cation and its monoprotonated complex

: The electrochemistry of 9, 16, 23-TETRA(NEOPENTOXY)PHTHALOCYANATO cobalt, and some binuclear analogs, has been studied in dichlorobenzene and in dimethylformamide. The redox mechanisms and species on the electrode are discussed. Using an optically thin electrode, the electronic spectra of seven different oxidation states of the mononuclear derivative are reported. Data for a selection of oxidation states of several binuclear species are also presented.

Electrochemistry and Spectroelectrochemistry of Mononuclear and Binuclear Cobalt Phthalocyanines.

Using pentaerythritol as a framework, it has been possible to synthesize a tetranuclear phthalocyanine with a spiro type linkage. Electrochemical and spectroelectrochemical data are presented for the cobalt complex in the Co/sup III/Pc(-2), Co/sup II/Pc(-2), Co/sup I/Pc(-2), Co/sup III/Pc(-1), Co/sup II/Pc(-1), and Co/sup I/Pc(-3) oxidation levels. The Co(II) species aggregates very strongly, dimerizing with an aggregation constant in o-dichlorobenzene of 2.4 x 10/sup 5/ M/sup -1/, 2 orders of magnitude greater than for the parent Co/sup II/TNPc. Monolayers of the cobalt(II) species laid upon an ordinary pyrolytic graphite electrode are shown to electrocatalytically reduce oxygen more efficiently than previously described analogous mononuclear and binuclear phthalocyanines. The Co(II) tetranuclear species disproportionates into a 1:1 mixture of Co(I) and Co(III) species upon reaction with hydroxide ion in a nondonating or weakly donating solvent. A film containing (Co/sup I/TrNPc(-2))/sub 4//sup 4 -/ reduces nitrite ion in NaOH slowly, with oxidation to the Co(II) tetranuclear species. 60 references, 8 figures, 5 tables.

Synthesis, Aggregation, Electrocatalytic Activity, and Redox Properties of a Tetranuclear Cobalt Phthalocyanine.

Using the density functional theory, [(N2)RuIIL5]n+ species are studied in silico. The properties of the Ru-N2 bond are derived, including σ-donation, π-back donation, Ru-N and N-N bond lengths and bond orders, net charges and NN stretching frequencies, and so forth. These data are correlated using the ligand electrochemical parameter (EL) theory, whereby the availability of electrons in the [RuL5]n+ fragment is defined by its electron richness, which is the sum of the EL parameters, ΣEL(L5). The objective is to better understand the binding of the N2 ligand, leading to a molecular design whereby a specific species is constructed to have a desired property, for example, a particular bond length or charge. We supply cubic expressions linking ΣEL(L5) with these many metrics, allowing researchers to predict metric values of their own systems. The extended charge decomposition analysis is used. For the given target, N2, σ-bonding does not vary greatly with the nature of ligand L, and π-back donation is the dominant property deciding the magnitudes of the various metrics. The EL parameter provides the path to design the desired species. This contribution is devoted to dinitrogen, but the method is expected to be general for any ligand, including polydentate ligands.

Ligand Electrochemical Parameter Approach to Molecular Design. σ-Donation, π-Back Donation, and Other Metrics in Ruthenium(II) Dinitrogen Complexes.

Generic series of complexes [Ru(bpy) 3− n (LL) n ] 2+ (bpy=2,2′-bipyridine), where LL is a diimine ligand including specifically 2,2′-bipyrazine (bpz), 2,2′-azobipyridine (abpy), and o -benzoquinonediimine (bqdi), are studied with respect to their electrochemistry, optical spectroscopy and electronic structure as elucidated using Zerner's INDO/S method. Characteristics of their electrochemistry and optical spectroscopy are explained in terms of mixing between ruthenium d orbitals and diimine ligand π and π* orbitals, increasing in importance from bpy to bpz to abpy to bqdi. In this last case, these species have characteristics not unlike fully delocalized organic molecules.

A. B. P. Lever

Papers

Luminescence quenching of the tris(2,2′-bipyrazine)ruthenium(ii) cation and its monoprotonated complex

Electrochemistry and Spectroelectrochemistry of Mononuclear and Binuclear Cobalt Phthalocyanines.

Synthesis, Aggregation, Electrocatalytic Activity, and Redox Properties of a Tetranuclear Cobalt Phthalocyanine.

Ligand Electrochemical Parameter Approach to Molecular Design. σ-Donation, π-Back Donation, and Other Metrics in Ruthenium(II) Dinitrogen Complexes.

Trends in Metal—Ligand Orbital Mixing in Generic Series of Ruthenium N-Donor Ligand Complexes — Effect on Electronic Spectra and Redox Properties