Kenneth R. Seddon

Chemical modification of a titanium (IV) oxide electrode to give stable dye sensitisation without a supersensitiser

SEMICONDUCTING electrodes stable in aqueous solution during the photoelectrochemical oxidation of water have generally been large-bandgap oxides, and their photosensitisation to visible light is of continuing interest1. In particular, n-type TiO2 (rutile, with a bandgap of 3.0 eV) is a stable photoanode for the photoelectrolysis of water by uv light2, and considerable effort has been devoted to its photosensitisation to sunlight1. One approach to photosensitisation is the use of a dye that, on excitation by visible light, transfers an electron to the conduction band of the solid and subsequently returns to its reduced ground state by the oxidation of water. The three major problems to be avoided are (1) irreversible degradative oxidation of the dye3, (2) failure of the oxidised form of the dye to oxidise water, reduction being carried out by a supersensitiser, such as hydro-quinone, that is consumed by the reaction4,5, and (3) inefficient electron transfer from the dye to the solid6. To circumvent the first two of these problems, we have selected an inorganic complex as the dye, a derivative of [Ru(BIPY)3]2+ (BIPY = 2,2′-bipyridine). [Ru(BIPY)3]3+ is known to oxidise water in suitable pH conditions evolving about 80% of the theoretical yield of oxygen7. The last problem was avoided by chemically attaching the complex to the electrode surface, a procedure recently demonstrated for organic dyes (such as rhodamine-B) that require the use of a supersensitiser4,5. We report here that a single-crystal n-type TiO2 electrode, chemically modified by the attachment of a monolayer of a derivative of [Ru(BIPY)3]2+, will produce significant anodic photocurrents when irradiated with visible light in the absence of a supersensitiser.

Preparation and characterisation of 2,2′-bipyridine-4,4′-disulphonic and -5-sulphonic acids and their ruthenium(II) complexes. Excited-state properties and excited-state electron-transfer reactions of ruthenium(II) complexes containing 2,2′-bipyridine-4,4′-disulphonic acid or 2,2′-bipyridine-4,4′ dicarboxylic acid

We report the syntheses of 2,2′-bipyridine-4,4′-disulphonic acid (H2bp-4,4′-ds) and 2,2′-bipyridine-5-sulphonic acid (Hbp-5-s), and several ruthenium(II) complexes derived therefrom, including [Ru(bp-4,4′-ds)3]4–,[Ru(bipy)(bp-4,4′-ds)2]2–(bipy = 2,2′-bipyridine), [Ru(bipy)2(bp-4,4′-ds)], and [Ru(bp-5-s)3]– and their 2,2′-bipyridine-4,4′-dicarboxylic acid (H2bpdc) analogues, viz. [Ru(bpdc)3]4–, [Ru(bipy)(bpdc)2]2–, and [Ru(bipy)2(bpdc)]. Some novel thioalkyl derivatives of 2,2′- bipyridine, including 4,4′-di(methylthio)-2,2′-bipyridine, 4,4′-di(ethylthio)-2,2′-bipyridine, and 4,4′,6,6′-tetra(methylthio)-2,2′-bipyridine, were also prepared and characterised during the course of this investigation. The luminescent states of the complexes [Ru(bp-4,4′-ds)3]4–,[Ru(bipy)(bp-4,4′ds)2]2–, [Ru(bpdc)3]4–, [Ru(bipy)(bpdc)2]2–, and [Ru(bipy)2(bpdc)] were studied using variabletemperature lifetime measurements. Studies of the quenching of {[Ru(bipy)3]2+}*, {[Ru(bipy)2(bpdc)]}*, {[Ru(bipy)(bp-4,4′-ds)2]2–}*, and {[Ru(bp-4,4′-ds)3]4–}* by 1,1′-dimethyl-4,4′-bipyridinium bromide (methyl viologen) in aqueous solution as a function of ionic strength have demonstrated that the effects of charge in these electron-transfer reactions can be understood in terms of conventional theories of ionic reactions whilst, at the same time, confirming the effective charges of the ruthenium(II) complex ions. The rate constants for the quenching of {[Ru(bp-4,4′-ds)3]4–}* and {[Ru(bipy)(bp-4,4′-ds)2]2–}* by copper(II) ions in neutral aqueous solution show unusual (non-Arrhenius) temperature dependences. A novel kinetic scheme involving parallel innerand outer-sphere quenching mechanisms has been proposed to account for the observed behaviour. The luminiscence decay of {[Ru(bipy)2(bpdc)]}* in the presence of aqueous copper(II) ions at pH 3.5 is non-exponential. This is interpreted in terms of a combination of static and dynamic quenching effects.

Sensitisation of semiconducting electrodes with ruthenium-based dyes

Practical photoelectrolysis of water by sunlight requires the development of suitable semiconductor electrodes. One approach is the photosensitisation of wide-bandgap oxide semiconductors by the chemical attachment of a suitable dye to the surface. We report on the photosensitisation of n-TiO2, n-SrTiO3 and n-SnO2 with chemically attached Ru(bipy)2(bpca) in which the 2,2′-bipyridine-4,4′ dicarboxylic acid (bpca) is chemically attached to the semiconductor surface by two ester linkages. Kinetic analysis reveals a surprisingly low quantum efficiency (ηe≈ 0.25%) for electron injection from the photoexcited dye into the semiconductor and a low rate constant for reoxidation of the dye. A suggestion is made how the dye molecule might be designed to improve the situation radically. Some deterioration in performance was observed after illumination of the sensitised electrode for many hours.

Reactions and properties of some trimethyleneplatinum(IV) complexes : I. Thermal decomposition studies

Abstract The thermal decomposition of the complexes PtX2(C3H6), PtX2(C3H6)L2 and PtX2(C3H6)(LL) [where X = Cl, Br; L = pyridine, 4-methylpyridine; LL = 2,2′-bipyridyl, ethylenediamine] has been studied by differential scanning calorimetry and thermogravimetric analysis. The first estimated PtC bond strength for platinum—trimethylene complexes is reported.

A deuterium exchange reaction of the tris-(2,2′-bipyridine)ruthenium(II) cation: evidence for the acidity of the 3,3′-protons

The first observation of deuterium exchange upon 2,2′-bipyridine (bipy) co-ordinated to ruthenium(II) is reported; the reaction of [Ru(bipy)3]2+ in (CD3)2SO with Na[OCD3] in CD3OD to yield [Ru([3,3′-2H2]-bipy)3]2+ is demonstrated to involve deprotonation of the co-ordinated bipy ligand at the slightly acidic 3,3′-positions, an acidity promoted by steric strain.

Preparation and properties of some cationic binuclear platinum(I) complexes

Abstract Reaction of [Pt2Cl2(μ-dppm)2] with ligands, L, in the presence of [PF6- gave stable cationic diplatinum(I) complexes [Pt2L2(μ-dppm)2][PF6]2 where L = PMe2Ph, PMePh2, PPh3, NH3, C5H5N. Reaction of [Pt2(NH3)2(μ-dppm)2][PF6]2 with CO gave [Pt2(CO)2(μ-dppm)2][PF6]2 and an unsymmetrical complex [Pt2(CO)(C5H5N)(μ-dppm)2][PF6]2 was also prepared. The compounds were characterized by vibrational and 1H and 31P NMR spectroscopy and the presence of direct platinumplatinum bonds is indicated.

Photophysical properties of mixed-ligand ruthenium(II) complexes [Ru(1,10-phenanthroline)n {2-(2-pyridyl)quinoline}3-n] 2+ (n = 0, 1, 2, or 3)

Abstract Electronic absorption, emission, and excitation spectra together with lumincscence lifetimes, have been measured for the mixed-ligand 1,10-phenanthroline/2-(2-pyridyl)quinoline complexes of ruthenum(II). Charge-transfer absorption and emitting states appear to be located on the individual ligands and there is evidence that energy transfer from the phenanthroline absorption state to the pyridylquinoline emitting state occurs.

The calculation of the ionization energies of CrOCl3 by configuration-interaction and ΔSCF methods

Abstract The calculation of the valence ionization energies of CrOCl3 by a configuration-interaction method is described and compared with the He(I) and He(II) photoelectron spectra of this molecule. Such a calculation yields ionization energies in good agreement with experiment and is superior to the ΔSCF method which predicts the incorrect ground state of CrOCl+3.

]Preparation and x-ray crystal structure of biss(diphenylphosphino)methanido platinum(II), [Pt(Ph2PCHPPh2)2]

Abstract [Pt(Ph 2 PCHPPh 2 ) 2 ], the first homoleptic complex containing the chelated bis(diphenylphosphino)methanide ligand, has been synthesised by reaction of K 2 [PtCl 4 ] and Ph 2 PCH 2 PPh 2 with KOH in ethanol, and characterised by 1 H and 31 P NMR spectroscopy and X-ray crystallography.

Preparation and characterisation of tris-μ-[bis(diphenylphosphino)methane]diplatinum(0)

Summary The preparations of [Pt 2 (dppm) 3 ] (dppm = bis(diphenylphosphino)methane) by three different routes are described, and a propellane-like structure is proposed for the complex on the basis of multinuclear NMR studies.