The pH-dependent electronic and electrochemical properties of a series of triangular mu(3)-oxo-centered trinuclear clusters of the type [Ru3O(Ac)(6)(py)(2)(OHx)](n) (Ac = acetate; py = pyridine; x = 0, 1, or 2; where the formal oxidation states on the metal ions range from +II to +IV) have been investigated with emphasis on the elucidation of the successive proton-coupled redox processes spanning the interconvertible aqua- (x = 2), hydroxo- (x = 1), and oxo- (x = 0) forms, from which the complete Pourbaix diagram has been proposed. From the spontaneous slow coupling reaction of the monomeric cluster in its deprotonated (oxo) form at high pH, the p-oxo-bridged cluster dimer (i.e. {(py)(2)(Ac)(6)ORu3}-O-{Ru3O(Ac)6(PY)2}) has been isolated and fully characterized by spectroelectrochemistry as well. A further understanding of the underlying electronic interactions across the various oxidation and protonation states, and the role of such phenomena in the activation of the Ru-O bonding system in oxygen-transfer catalysis, have also been explored. ((c) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

We synthesized a novel series of compounds based on a ruthenium(II) dicarboxybipyridine (dcbpy) complex containing chloro and trans-1,4-bis[2-(4-pyridyl)ethenyl]benzene (BPEB) ligands. The binuclear species Na6[{[RuII(dcbpy)2Cl}2(BPEB)] exhibits an electrochromic effect when reduced, which is assigned to the radical BPEB0/˙, similarly as in viologen-based compounds. The carboxylate groups on the 4,4′ positions of bipyridine allow a strong attachment to the TiO2 surface, contributing to an efficient and reversible electron transfer from the oxide to the chromophoric ligand, coloring the oxide film blue. The coloration efficiency CE(λ) at 633 nm was 109 cm2 C−1, which is high compared to TiO2 itself. The complex also exhibits a high photon-to-electron conversion efficiency (IPCE > 70%) when applied as a photoanode in a dye sensitized solar cell (DSSC).

The spectroscopic and electrochemical properties of two isomeric forms of the supramolecular species [μ-(H2TPyP){Ru(bpy)2Cl}4]4+ (H2TPyP = 5,10,15,20-tetra(3- or 4-pyridyl)porphyrin, bpy = 2,2′-bipyridine) have been compared and consistently interpreted with the aid of molecular orbital calculations. In these complexes, the HOMO and LUMO levels are predominantly localized in the ruthenium complexes and porphyrin ring, respectively. There is an extensive mixing of the wave functions of both components in other MOs, however, and their contributions are reflected in the spectroelectrochemical and spectroscopic behavior. For example, the electronic mixing is enough to allow the energy-transfer from the peripheral complexes to the porphyrin ring, as well as the appearance of a RuII(dπ) → H4P(pπ*) charge-transfer band at 700 nm in the bis-protonated [μ-(H4TPyP){Ru(bpy)2Cl}4]4+ species, showing the strong stabilization of the porphyrin LUMO levels.

The properties of the trinuclear cluster [Ru3OAc6(pic)2(NO)]PF6 (pic=4-methyl pyridine, Ac=acetate ion) and the photochemical behavior of the corresponding molecular films are reported in this paper. In this compound, the unpaired π* electron from NO and the unpaired electron from the π-orbitals of the Ru3O unity are strongly coupled; as a consequence, the changes in electronic distribution associated with the several successive redox states promote dramatic effects in the spectroscopic and electrochemical properties of the nitric oxide ligand and the entire complex. NO release has been observed by light irradiation (ϕ=0.038 at 365 nm and ϕ=0.019 at 468 nm, in acetonitrile solution), changing the original violet color into deep blue. The same behavior has been observed in solid state and in PVA films incorporating this compound, revealing its potential usefulness as NO photoreleaser, as well as for the monitoration of light exposure intensities.

A novel supramolecular species [MnTPyP{Ru(bipy)2H2O}4](PF6)9 containing four cis-bis(bipyridine)(aqua)ruthenium(II) complexes attached to the peripheral pyridyl groups of meso-tetra(4-pyridyl)porphyrin manganese(III) (MnTPyP) has been synthesized, and employed as the precursor of a rather promising multinuclear oxo-transfer catalyst. This complex forms molecular films by dip coating. Such films are soluble in water, but can be stabilized by exchanging the PF−6 anions by [Fe(CN)6]4− or [Zn–TPPS]4−. They exhibit remarkable electrochemical and electrocatalytic activity, as demonstrated in the oxidation of nitrite ions. In this case, experimental evidence supports the involvement of electrochemically generated [ORuIV]2+ groups in the oxidation reaction, reflecting the characteristic reactivity of high-valence metal–oxo complexes.

Modification of wide band gap semiconductor surfaces by a new generation of supramolecular sensitizers combining porphyrin and ruthenium-polypyridyl complexes leads to versatile molecular interfaces, allowing the exploitation of photoinduced charge transfer in photoelectrochemical devices.