Neutral copper(I) dipyrrin complexes and their use as sensitizers in dye-sensitized solar cells.

Heteroleptic neutral copper(I) dipyrrin complexes have been synthesised with the general formula [Cu(4,4'-(R)-6,6'-(CH3)-bipyridine)(dipyrrin)], R = CH3 or CO2Et, and H-dipyrrin is either 1,3,7,9-tetramethyldipyrromethene (HL1), 1,13-diphenyl-6,8-diisoindolemethene (HL2), or 1,13-diphenyl-3,11-di(trifluoromethyl)-6,8-diisoindolemethene (HL3). Improved stability was observed across the series , likely due to better steric constraints between the ligands. Visible light absorption has also been enhanced with a red-shift in absorption from 450 nm to 600 nm. Complex 3 shows photoluminescence lifetime in the order of nanoseconds suggesting singlet fluorescence which is supported by theoretical calculations. Study of the complexes as sensitisers in dye-sensitised solar cells was achieved by assembling the dye in situ on the surface of TiO2 in a series of steps (anchoring ligand followed by ancillary ligand and [Cu(CH3CN)4][BF4]. The highest efficiency achieved was 0.41% for the dye with HL3, attributed to better dye regeneration due to a more favourable oxidation potential.

Phosphorescent, liquid-crystalline complexes of platinum(II): influence of the ß-diketonate co-ligand on mesomorphism and emission properties.

Two series of rod-like ortho-platinated complexes bound to a range of ß-diketonate co-ligands are reported of the form [Pt(N^C)(O^O)]. The liquid crystal and luminescent properties are influenced through the introduction of different ß-diketonates (O^O) as well as through the presence of a fused cyclopentene ring on the N^C-coordinated 2-phenylpyridine ligand. For metal complexes, smectic phases are dominant but no mesomorphism is exhibited when the ß-diketonate is hexafluorinated acetylacetonate (1,1,1-5,5,5-hexafluoro-2,4-pentandione). The introduction of an unsymmetrical trifluoroacetyl acetonate (1,1,1-trifluoro-2,4-pentandione) ligand is particularly interesting due to the discovery of the ß-diketonate as a dynamic system that readily isomerises to a 1:1 ratio under thermal conditions, from an initial ratio of 3:1 formed during the synthesis. As expected, the presence of the 3,5-heptanedionato co-ligand decreases the transition temperatures due to the introduction of a larger lateral substituent. Unfortunately complexes based on 2-phenylpyridine ligands with a fused cyclopentene ring are, with one exception, not mesomorphic. With the exception of the hexafluoroacetyl acetonate complexes, the materials are brightly luminescent and have excited state lifetimes between 13 and 30 µs with emission quantum efficiencies exceeding 0.5, with one as high as 0.7. A time-dependent density functional theory (TD-DFT) study suggests that the lack of emission from the hexafluoroacetyl acetonate complexes may be associated with large structural distortion upon formation of the excited state, as well as to poorer overlap of orbitals leading to a lower radiative rate constant. The same analysis provides an explanation for the apparently quite different emission efficiencies of the two isomers of the trifluoroacetyl acetonate complexes.

Molecular approaches to the electrochemical reduction of carbon dioxide.

This article reviews recent progress in the exploitation of carbon dioxide as a chemical feedstock. In particular, the design and development of molecular complexes that can act as catalysts for the electrochemical reduction of CO(2) is highlighted, and compared to other biological, metal- and non-metal-based systems.

On the electronic structure of nitro-substituted bipyridines and their platinum complexes.

We report the preparation and electrochemical studies of a systematic series of mono- and di-nitro-substituted 2,2'-bipyridine (bipy) compounds [x-NO(2)-bipy (x = 3,4) and x,x'-(NO(2))(2)-bipy (x,x' = 3, 4, 5)] and their complexes with platinum(II), [Pt(x-NO(2)-bipy)Cl(2)] and [Pt(x,x'-(NO(2))(2)-bipy)Cl(2)]. The effect of the number and substitution pattern of the nitro groups on the low-lying acceptor molecular orbitals (involved in charge transfer transitions) is probed by in situ UV/Vis/NIR and EPR spectroelectrochemical methods, supported by DFT calculations. The LUMOs of x-NO(2)-bipy (x = 3-5) are largely localised on the NO(2)-pyridyl moiety; this is also true of their {PtCl(2)} complexes but with a small but significant shift of electron density from the nitro groups. The LUMOs of x,x'-(NO(2))(2)-bipy with x = 3 and 5 are delocalised over both NO(2)-pyridyl rings, but for 4,4'-(NO(2))(2)-bipy is localised on a single NO(2)-pyridyl ring. In all cases the LUMO of the [Pt(x,x'-(NO(2))(2)-bipy)Cl(2)] complexes is delocalised over both nitro-pyridyl rings. For all complexes, the 4(4') derivatives allows greatest overlap with metal valence orbitals in the LUMO.

Electrochemistry of sulfur and polysulfides in ionic liquids.

The electrochemistry of elemental sulfur (S(8)) and the polysulfides Na(2)S(4) and Na(2)S(6) has been studied for the first time in nonchloroaluminate ionic liquids. The cyclic voltammetry of S(8) in the ionic liquids is different to the behavior reported in some organic solvents, with two reductions and one oxidation peak observed. Supported by in situ UV-vis spectro-electrochemical experiments, the main reduction products of S(8) in [C(4)mim][DCA] ([C(4)mim] = 1-butyl-3-methylimidazolium; DCA = dicyanamide) have been identified as S(6)(2-) and S(4)(2-), and plausible pathways for the formation of these species are proposed. Dissociation and/or disproportionation of the polyanions S(6)(2-) and S(4)(2-) appears to be slow in the ionic liquid, with only small amounts of the blue radical species S(3)(•-) formed in the solutions at r.t., in contrast with that observed in most molecular solvents.

An in situ electrochemical cell for Q- and W-band EPR spectroscopy.

A simple design for an in situ, three-electrode spectroelectrochemical cell is reported that can be used in commercial Q- and W-band (ca. 34 and 94 GHz, respectively) electron paramagnetic resonance (EPR) spectrometers, using standard sample tubing (1.0 and 0.5 mm inner diameter, respectively) and within variable temperature cryostat systems. The use of the cell is demonstrated by the in situ generation of organic free radicals (quinones and diimines) in fluid and frozen media, transition metal ion radical anions, and on the enzyme nitric oxide synthase reductase domain (NOSrd), in which a pair of flavin radicals are generated.

Characterisation of a ruthenium bipyridyl dye showing a long-lived charge-separated state on TiO2 in the presence of I-/I3-.

The control of the loss mechanism in a dye sensitised solar cell (DSSC) via recombination of the injected electron with the oxidised dye was investigated by incorporating a redox-active ligand, 6,7-bis(methylthio)tetrathiafulvalene dithiolate (TTF(SMe)(2)), into a ruthenium bipyridyl dye. A series of dyes with general formula [Ru(4,4'-R-bpy)(2)(TTF(SMe)(2)], where R = H, CO(2)Et and CO(2)H, were synthesised and characterised using electrochemistry, absorption and emission spectroscopy, spectroelectrochemistry and hybrid-DFT calculations. In addition, the performance of the acid derivative in a DSSC was investigated using IV measurements, as well as transient absorption spectroscopy. These complexes showed significant TTF-ligand character to the HOMO orbital, as deduced by spectroelectrochemical, emission and computational studies. Upon adsorption of the acid derivative to TiO(2) a long-lived charge-separated state of 20 ms was observed via transient absorption spectroscopy. Despite this long-lived charge-separated state, the dye yielded extremely low DSSC efficiencies, attributed to the poor regeneration of the neutral dye by iodide. As a result, the complex forms a novel long-lived charge separated state that persists even under working solar cell electrolyte conditions.

Determining the site of reduction of 4-NO2-2,2'-bipyridine and [Pt(4-NO2-2,2'-bipyridine)Cl2].

In situ UV/Vis and EPR spectroelectrochemistry experiments show that the unpaired electron in [4-NO(2)-bpy](1-) sits in an orbital which is based on the 4-NO(2)-py moiety and not spread over both py rings. The unpaired electron in [Pt(4-NO(2)-bpy)Cl(2)](1-) is based on the 4-NO(2)-bpy ligand with some [bpy](1-) character. Labelling the NO(2) group of 4-NO(2)-bpy with (15)N revealed that the larger coupling constant in the EPR spectrum of [4-NO(2)-bpy](1-) can be assigned to the N atom of the NO(2) group. This coupling constant is significantly smaller in the EPR spectrum of [Pt(4-NO(2)-bpy)Cl(2)](1-).

Synthesis and properties of [Pt(4-CO(2)CH(3)-py)(2)(mnt)]: comparison of pyridyl and bipyridyl-based dyes for solar cells.

The dye complexes [Pt(4-CO(2)R-py)(2)(mnt)] (R = H (3a), CH(3) (3b)) and the precursor complexes [Pt(4-CO(2)R-py)(2)Cl(2)] (2a, 2b) (py = pyridyl) were synthesised, characterised by electrochemical, spectroscopic, spectroelectrochemical (UV-vis-nIR and in situ EPR) and hybrid DFT computational methods and attached to a TiO(2) substrate to determine charge recombination kinetics. The results were compared to the bipyridyl analogues [Pt{X,X'-(CO(2)R)-2,2'-bipyridyl}(mnt)], (X = 3 or 4). The electronic characteristics of the bis-pyridyl complex were found to be different to the bipyridyl complexes making the former harder to reduce, shifting the lowest-energy absorption band to higher energy and showing separate degenerate LUMO orbitals on the two pyridine rings. The latter point determines that the di-reduced pyridyl complex remains EPR active, unlike the bipyridyl analogue. Complex 3a attached to nanocrystalline TiO(2) shows a long charge recombination lifetime in comparison with the analogous complex with the ubiquitous 4,4'-(CO(2)H)(2)-bipyridyl ligand, suggesting that pyridyl complexes may possess some advantage over bipyridyl complexes in dye-sensitised solar cells.

Polyacidic multiloading metal extractants.

Novel polynucleating, di- and tri-acidic ligands have been designed to increase the molar and mass transport efficiencies for the recovery of base metals by solvent extraction.

Spectroscopic, electrochemical and computational study of Pt-diimine-dithiolene complexes: rationalising the properties of solar cell dyes.

A series of [Pt(ii)(diimine)(dithiolate)] complexes of general formula [Pt{X,X'-(CO(2)R)(2)-bpy}(mnt)] (where X = 3, 4 or 5; R = H or Et, bpy = 2,2'-bipyridyl and mnt = maleonitriledithiolate), have been spectroscopically, electrochemically and computationally characterised and compared with the precursors [Pt{X,X'-(CO(2)R)(2)-bpy}Cl(2)] and X,X'-(CO(2)R)(2)-bpy. The study includes cyclic voltammetry, in situ EPR spectroelectrochemical studies of fluid solution and frozen solution samples, UV/Vis/NIR spectroelectrochemistry, hyrid DFT and TD-DFT calculations. The effect of changing the position of the bpy substituents from 3,3' to 4,4' and 5,5' is discussed with reference to electronic changes seen within the different members of the family of molecules. The performance of the mnt complexes in dye-sensitised solar cells has been previously described and the superior performance of [Pt{3,3'-(CO(2)R)(2)-bpy}(mnt)] is now explained in terms of decreased electronic delocalisation through twisting of the bipyridyl ligand as supported by the EPR and computational results.

High spin d5 complexes of tris(6-hydroxymethyl-2-pyridylmethyl)amine (H3L): hepta-coordinated [Mn(H3L)]Cl2 and linear trinuclear [Fe3L2](ClO4)3.

Reaction of MnCl(2).4H(2)O with H(3)L (H(3)L = tris(6-hydroxymethyl-2-pyridylmethyl)amine) in methanol gives hepta-coordinated [Mn(H(3)L)]Cl(2) involving attachment of Mn(II) to all four nitrogens and three hydroxymethyl arms. Reaction of H(3)L with Fe(ClO(4))(2).6H(2)O in CH(3)CN in the presence of NaO(2)CC(6)H(5) in an attempt to make [Fe(III)OH(H(3)L)(O(2)CC(6)H(5))](ClO(4)), a putative model for soybean lipoxygenase-1, instead gave rise to the linear triiron(III) complex [Fe(3)L(2)](ClO(4))(3) with all three hydroxymethyl arms deprotonated and forming three alkoxide bridges between each Fe(III) centre. The central Fe(III) is hexa-coordinated to only the alkoxide bridges and flanked by two hepta-coordinated iron(III) centres analogous to the Mn(ii) complex. [Fe(3)L(2)](ClO(4))(3) exhibits two reversible 1e(-) reductions to mixed-valence [Fe(3)L(2)](2+) and [Fe(3)L(2)](+) forms. Structure data and magnetochemistry on [Fe(3)L(2)](ClO(4))(3) reveals the tightest Fe-O-Fe angle (87.4 degrees ) and shortest Fe...Fe distance (2.834 A) yet found for any weakly antiferromagnetically-coupled high spin alkoxide-bridged di- or triiron(iii) system and challenges current theories involved in correlating the extent/nature of magnetic interactions in such systems based on Fe-O(bridge) distances and Fe-O-Fe angles. The central hexa-alkoxide coordinated Fe(III) is novel and shows a remarkable resistance towards reduction to Fe(II).

Influence of the R-substituents on the properties of [Ni(R2pipdt)(dmit)] complexes and crystal structure where R = CH2C6H5.

[Ni(R2pipdt)(dmit)], (R2pipdt = 1,4-disubstituted-piperazine-3,2-dithione, R = CH2C6H5; dmit = 1,3-dithiole-2-thione-4,5-dithiolate) (1b) has been prepared and characterised and the properties compared with those of the known complex 2b belonging to the same class where R = Pr i. Cyclic voltammetry of 1b and 2b was carried out and compared with that of the respective R2pipdt ligand precursors (1a and 2a ). The nature of the R-groups of the pipdt ligand exerts an effect on the redox potentials and confirmed the position of the LUMO as mainly on the R2pipdt part of the complex. Accordingly the low frequency absorption, assigned to the HOMO-LUMO transition which has inter-ligand charge-transfer character, is found for 1b at lower frequency when compared to the corresponding transition of 2b. In situ EPR was carried out on electroreduced radical species of the R2pipdt ligand precursors (1a, 2a ) and corresponding complexes (1b, 2b ). This revealed considerable delocalisation of the unpaired electron on the R2pipdt ligand in 1b and 2b with coupling constants to N and H comparable with those of 1a and 2a . Complex 1b crystallised in the space group Pnma and shows an essentially planar complex (with out-of-plane R groups) pi stacked at a distance of 3.65(1) A. Such a one-dimensional structure is not achieved in the case of 2b, where the complex units are almost parallel and head-to-tail with each other forming dimers and this difference in solid-state packing is apparent in the diffuse reflectance spectrum of each. Plane-wave DFT calculations for 1b revealed a highly one-dimensional band structure with considerable band dispersion along the direction of greatest molecular interaction via pi-stacking.

Phosphine-substituted dithiolene complexes as ligands: communication between ruthenium(II) centers through a dimolybdenum bis(dithiolene) core.

The reaction of Mo2(SCH2CH2S)2Cp2 (1; Cp=eta-C5H5) with an excess of an alkyne in refluxing dichloromethane affords the bis(dithiolene) complexes Mo2(micro-SCR1=CR2S)2Cp2 (2a, R1=R2=CO2Me; 2b, R1=R2=Ph; 2c, R1=H, R2=CO2Me) whereas with 1 equiv of alkyne at room temperature the mixed dithiolene-dithiolate species Mo2(micro-SCR1=CR2S)(micro-SCH2CH2S)Cp2 (3a, R1=R2=CO2Me; 3b, R1=R2=Ph) are formed. The remaining dithiolate ligand in 3 can then be converted into a different dithiolene by reaction with a second alkyne. Applying this methodology, we have used bis(diphenylphosphino)acetylene to prepare the first examples of complexes containing phosphine-substituted dithiolene ligands: Mo2{micro-SC(CO2Me)=C(CO2Me)S}{micro-SC(PPh2)=C(PPh2)S}Cp2 (2g) and Mo2{micro-SC(PPh2)=C(PPh2)S}2Cp2 (2h). Tri- and tetrametallic complexes can then be assembled by coordination of these diphosphines to CpRuCl units by reaction with CpRu(PPh3)2Cl. Electrochemical studies of the Ru(II)/Ru(III) couple in Mo2{micro-SC(PPh2)=C(PPh2)S}2Cp2(RuClCp)2 (4b) reveals that the two separate ruthenium centers are oxidized electrochemically at different potentials, demonstrating communication between them through the dimolybdenum bis(dithiolene) core. Density functional theory calculations were carried out to explore the electronic structures of these species and to predict and assign their electronic spectra.

Photogeneration of titanium(III) from titanium(IV) citrate in aqueous solution.

Current interest in the biochemistry of Ti(IV) arises from its widespread use in white pigments and its potential in therapeutic agents. Citrate is known to form strong complexes with Ti(IV). We show here that Ti(III) citrate is generated in a facile manner and in good yield by the action of UV radiation on Ti(IV) citrate in aqueous solution. The Ti(III)-citrate species formed was isolated and characterised by UV-Visible spectroscopy, showing an absorption at 547 nm (epsilon=100 M(-1)cm(-1)), and by electron paramagnetic resonance (EPR) spectroscopy giving a resonance at g=1.949 (linewidth=60G) . An X-ray structure of the parent Ti(IV) complex in the form [TiNa(3)(C(6)H(6)O(7))(2)(C(6)H(5)O(7))(H(2)O)(6.8)].2H(2)O is reported along with a study of the reaction of Ti(IV)-citrate with N,N-ethylenebis(o-hydroxytoluene)glycine (EHTG), which was more rapid than those of other related Ti(IV) complexes.

Complexation of dimethylmagnesium with alpha-diimines; structural and EPR characterisation of single electron and alkyl transfer products.

Treatment of dimethylmagnesium with the alpha-diimine ligands Ar'N=C(R)C(R)=NAr' [R = naphth-1,8-diyl (1), H (2), CH3 (3); Ar' = 2,6-diisopropylphenyl] in diethyl ether provides the neutral methyl-bridged dimeric complexes [(alpha-diimine-.)Mg+(mu-CH3)]2 via single electron transfer (SET) to the coordinated diimine and elimination of a methyl radical. These biradical species have been characterised by EPR spectroscopy and, for the ligand , X-ray crystallography. In the presence of THF the reaction of ligand proceeds to the diamagnetic [(ene-1,2-diamide)Mg(THF)3] complex in which the diimine ligand has been doubly reduced to an ene-diamide by two successive SET processes. Comparison of the structural data for the free ligand with that obtained for the alpha-diimine radical anion and ene-diamide complexes shows the expected increases in C-N, and decreases in C-C, bond lengths within the N-C-C-N unit consistent with the progressive reduction of the ligand. In the case of ligand , reaction at low temperature provides the complex [Mg(mu2-Me){Ar'NC(Me)2C(Me)NAr'}]2 in which methyl transfer to a ligand imine carbon atom has occurred. This species has also been structurally characterised. This contrasts with the formation of the radical species at room temperature, and indicates the involvement of an intermediate in which the radical products of the SET process are held in close proximity by the solvent cage. Two competing processes of methyl radical escape and methyl transfer to the ligand account for the formation of the observed products at different temperatures.

Structural and EPR characterisation of single electron and alkyl transfer products from reaction of dimethyl magnesium with bulky alpha-diimine ligands.

Treatment of dimethylmagnesium with bulky alpha-diimine ligands provides either the biradical methyl-bridged complexes [(alpha-diimine-.)Mg+(mu-CH3)]2 via single electron transfer (SET), or the product of methyl transfer to an imine carbon atom depending upon conditions.

Synthesis, structure, and properties of [Pt(II)(diimine)(dithiolate)] dyes with 3,3'-, 4,4'-, and 5,5'-disubstituted bipyridyl: applications in dye-sensitized solar cells.

A family of [Pt(II)(diimine)(dithiolate)] complexes of general formula [Pt{X,X'(CO(2)R)(2)-2,2'-bipyridyl}(maleonitriledithiolate)] (where X = 3, 4, or 5 and R = H or Et) have been synthesized, spectroscopically and electrochemically characterized, and attached to a TiO(2) substrate to be tested as solar cell sensitizers. A single-crystal X-ray structure showing a large torsion angle between the bipyridyl rings was determined for [Pt{3,3'(CO(2)Et)(2)-2,2'-bipyridyl}(maleonitriledithiolate)].MeCN. The effect of changing the position of the bipyridyl substituents from 3,3' to 4,4' and 5,5' is discussed with reference to structural and electronic changes seen within the different members of the family of molecules. The first UV/vis/NIR spectroelectrochemical study of complexes of this general formula is discussed. All three complexes (where R = H) were tested as solar cell sensitizers, with the 3,3'-disubstituted bipyridyl complex giving an intermediate dye loading value but superior photovoltaic performance to those of the other two. The performance of this sensitizer is then compared with that of a well-known Ru polypyridyl sensitizer, the ditetrabutylammonium salt of [RuL(2)(NCS)(2)] (L = 2,2'-bipyridyl-4,4'-dicarboxylato), commonly called N719.

Ruthenium bipyridyl compounds with two terminal alkynyl ligands.

Compounds of the form Ru(X2bipy)(PPh3)2(-C triple bond CC6H4NO2-p)2(X2bipy = 4,4'-X(2)-2,2'-bipyridine, X = Me 3a, Br 3b, I 3c) have been synthesised from the mono-alkynyl precursors Ru(X2bipy)(PPh3)2(-C triple bond CC6H4NO2-p)Cl (X = Me 2a, Br 2b, I 2c); the former are the first ruthenium bis-alkynyl compounds that also contain a bipyridyl ligand. Spectroelectrochemical investigation of 3a shows that the metal is readily oxidised to form the ruthenium(III) compound 3a+, and will also undergo a single-electron reduction at each nitro group to form 3a2-. ESR and UV/visible spectra of these redox congeners are presented. We also report the synthesis of [Ru(Me2bipy)(PPh3)2(-C triple bond CBut)(N triple bond N)][PF6] during the attempted synthesis of Ru(Me2bipy)(PPh3)2(-C triple bond CBut)2, and report its X-ray crystal structure and IR spectrum. X-Ray crystal structures of 3b and 3c(as two different solvates) are presented, and the nature of the intermolecular interactions seen therein is discussed. Z-Scan measurements on Ru(Me2bipy)(PPh3)2(-C triple bond CR)Cl (R = C6H4NO2-p2a, But, Ph, C6H4Me) are also reported, and show that Ru(Me2bipy)(PPh3)2(-C triple bond CR)Cl (R = C6H4NO2-p2a, Ph) exhibit moderate third-order non-linearities.