Ru bipyridine complex

Fig. 5.6 Lifetime of the photoexcited state of dendritic Ru -bipyridine complexes - in the presence and absence of atmospheric oxygen (according to Balzani, De Cola, Vogtle et a .), as compared with unsubstituted ruthenium-tris-bipy...

Table 5. Metal-to-ligand charge-transfer spectra simple Ru bipyridine complexes.

Rates of Cu+ to Ru + electron transfer also have been measured in modified mutants of spinach plastocyanin, a blue copper protein from the photosynthetic ET chain [79], Ru-bipyridine complexes were introduced at surface sites, with Cu-Ru distances ranging from 13 to 24 A. ET rate constants, measured using laser flash-quench techniques, vary from 10" to 10 s. ET in Ru-modified plastocyanin is not activationless as it is in Ru-modified azurin, suggesting a slightly greater reorganization energy for the photosynthetic protein. The distance dependence of ET in Ru-modified plastocyanin is exponential with a distance decay factor identical with that reported for Ru-modified azurin (1.1 A ). 

Ru bipyridine complexes electrostatically bound to ionomeric non-conjugated polymer... 

The second example which emphasized the electron-transfer step as a key process in polymer catalysis is the Ru(bipyridine) complex sensitized photoreduction of methylviologen, which is of interest as a means to reduce protons leading to hydrogen evolution (Scheme 3). To suppress a spontaneous backward reaction which consumes the acquired energy, a heterogeneous reaction... 

A significant body of data exists showing the validity of this analysis. Indeed, the correlation is generally excellent. For example, a very extensive early analysis of Ru bipyridine complexes revealed that the Ru tt bpy MLCT band energy follows (data in eV)... 

Tris(2,2 -bipyridine)ruthenium(II) complex (Ru(bpy)3+) has been most commonly employed as a chromophore in the studies of photoinduced ET. Electrostatic effects on the quenching of the emission from the Ru(II) complex covalently bound to polyeletrolytes have been studied by several groups [79-82]. 

Ru 2,2 -bipyridine complexes can form a large number of colored compounds upon successive reduction, with the formal Ru oxidation state from +2 to -4. In the case of highly reduced complexes, proper representation of the electrochromic reaction is actually the reduction of the hgand, not that of the metal center. 

Non-luminescent, octahedral Ru(ii) complexes bearing two 4,4 -di(/ r/-butyl)bipyridine and one bibenzimidazole ligand become luminescent upon coordination of diethylzinc to the bibenzimidazole 39, as shown in Scheme 34.85... 

S = HjO, MeCN, MeOH, or Me2CO N N = pyrazine, 4,4 -bipyridine, or trans-1,2-bis-(4-pyridyl)ethylene]. Starting with [Ru(bipy)2(N0)(N02)], and employing the reaction of Ru—NO2 complexes with acid to yield Ru—NO... 

Formal potentials of dinuclear and hexanuclear Ru(II) bipyridine complexes (40 redox processes ) are given in Ref. 45. 

Ruthenium bipyridyl complexes are suitable photosensitizers because then-excited states have a long lifetime and the oxidized Ru(III) center has a longterm chemical stability. Therefore, Ru bipyridyl complexes have been studied intensively as photosensitizers for homogeneous photocatalytic reactions and dye-sensitization systems. The Ru bipyridyl complex, bis(2,2 -bipyridine)(2,2 -bipyri-dine-4, 4,-dicarboxylate)ruthenium(II), having carboxyl groups as anchors to the semiconductor surface was synthesized and single-crystal Ti02 photoelectrodes sensitized by this Ru complex were studied in 1979 and 1980 [5,6]. 

To illustrate the tuning aspects of the MLCT transitions in ruthenium polypyridyl complexes, let us begin by considering the well-known ruthenium mT-bipyridine complex (1). Complex 1 shows strong visible band at 466 nm, due to charge-transfer transition from metal t2g (HOMO) orbitals to tt orbitals (LUMO) of the ligand. The Ru(II)/(III) oxidation potential is at 1.3 V, and the ligand-based reduction potential is at -1.5 V versus SCE [36]. From spectro chemical and electrochemical studies of polypyridyl complexes of ruthenium, it has been con-... 

Thus we can directly deduce the XANES spectrum of the product state from the measured transient signal and the reactant state XAS, if we knowXO Alternatively, we can derive fit), if we know the exact shape of the product state XAS, P(E,t). The details are given in ref. 14. The resulting spectrum for the [Ruln(bpy )(bpy)2]2+ species is shown in Fig. 4a (trace P). It contains an energetic shift of all features by 1.2 eV, together with the photoinduced appearance of the A feature, as expected. The A-B splitting (4 eV) and the B /A intensity ratio (ca. 2.3) is indeed close to the values observed for [Ru"(NH3)6]3+ complex [18,20], which has the same valency and a similar coordination symmetry (Oh versus D3) as the bipyridine complex. 

Another model compound, the tris(2,2 -bipyridine)ruthenium(II) complex, has prompted considerable interest because its water-splitting photoreactivity has been demonstrated in various types of photochemical systems (77,99,100,101). Memming and Schroppel (102) have attempted to deposit a monolayer of a surfactant Ru(II) complex on a Sn02 OTE. In aqueous solution, an anodic photocurrent attributable to water oxidation by the excited triplet Ru complex was observed. A maximum quantum efficiency of 15% was obtained in alkaline solution. 

A chromophore such as the quinone, ruthenium complex, C(,o. or viologen is covalently introduced at the terminal of the heme-propionate side chain(s) (94-97). For example, Hamachi et al. (98) appended Ru2+(bpy)3 (bpy = 2,2 -bipyridine) at one of the terminals of the heme-propionate (Fig. 26) and monitored the photoinduced electron transfer from the photoexcited ruthenium complex to the heme-iron in the protein. The reduction of the heme-iron was monitored by the formation of oxyferrous species under aerobic conditions, while the Ru(III) complex was reductively quenched by EDTA as a sacrificial reagent. In addition, when [Co(NH3)5Cl]2+ was added to the system instead of EDTA, the photoexcited ruthenium complex was oxidatively quenched by the cobalt complex, and then one electron is abstracted from the heme-iron(III) to reduce the ruthenium complex (99). As a result, the oxoferryl species was detected due to the deprotonation of the hydroxyiron(III)-porphyrin cation radical species. An extension of this work was the assembly of the Ru2+(bpy)3 complex with a catenane moiety including the cyclic bis(viologen)(100). In the supramolecular system, vectorial electron transfer was achieved with a long-lived charge separation species (f > 2 ms). 

Photosensitized generation of hydrido-metal complexes in aqueous media provides a general route for H2-evolution, hydrogenation of unsaturated substrates (i.e. olefins, acetylenes), or hydroformylation of double bonds, see Scheme 2. Co(II) complexes, i.e. Co (II)-fn s-bipyridine, Co(bpy) +, or the macrocyclic complex Co(II)-Me4[14]tetraene N4, act as homogeneous H2-evolution catalysts in photosystems composed of Ru(bpy) + (or other polypyridine (Ru(II) complexes) as photosensitizers and triethanolamine, TEOA, or ascorbic acid, HA-, as sacrificial electron donors [156,157], Reductive ET quenching of the excited photosensitizer... 

The lowest energy MLCT transition of Ru polypyridyl complexes of the type tris-[Ru(4,4/-dicarboxy-2,2/-bipyridine)3] (1), can be lowered so that it absorbs more in the red region of the visible spectrum by replacing one 4,4/-dicarboxy-2,2/-bipyridine (dcbpy) with two thiocyanate donor ligands [Ru(dcbpy)2(NCS)2] (2). In complex 2, the two 4,4/-dicarboxylic acid 2,2 -bipyridine ligands pull while the two thiocyanate donor ligands push electrons. The oxidation potential of the complex 2 is 0.85 V vs. SCE, which is cathodically shifted significantly (0.65 V vs. SCE) compared to the homoleptic type of complex 1, which shows Ru(III/II) couple at 1.5 V vs. SCE. Thus, the... 

Acid-Base Equilibria of c/s-Dithiocyanato Bis(2,2 -bipyridine-4,4 -dicarboxylate)Ru(ll) Complex(2)... 

Several systematic studies of the driving force dependence of the rate of forward and back ET in type 1 dyads (see Fig. 1) were carried out during the past decade. As might be expected, the type 1 dyads used in these investigations consist of covalently linked assemblies of metal complexes and organic quenchers used in early studies of bimolecular photoinduced ET reactions. Thus, the type 1 dyads consist of polypyridine Ru(II) complexes linked to pyridinium acceptors such as paraquat and diquat (quatemized 2,2 -bipyridine). 

Balzani V, Bergamini G, Marchioni F, Ceroni P. Ru(II)-bipyridine complexes in supramo-lecular systems, devices and machines. Coord Chem Rev 2006 250 1254-66. 

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