Ru polypyridine complexes

Since the propensity to form adducts in chemistry is high and these adducts undergo a variety of reactions, the rate law (1.98) is quite common. This is particularly true in enzyme kinetics. In reality, these reaction schemes give biphasic first-order plots but because the first step is usually more rapid, for example between A and B in (1.101) we do not normally, nor do we need to, examine this step in the first instance. The value of A", in (1.107) obtained kinetically can sometimes be checked directly by examining the rapid preequilibrium before reaction to produce D occurs. In the reactions of Cu(I) proteins with excited Cr and Ru polypyridine complexes, it is considered that (a) and (b) schemes may be operating concurrently. 

Of course, there is no reason why the Ru-polypyridine complexes have to be the best photosensitizers. Studies should be performed on other families of transition metal compounds, with emphasis on complexes of inorganic ligands and on binuclear and polynuclear species. 

Importantly, it was found [80-82, 311] that interfacial electron transfer from MLCT-excited Ru polypyridine complexes to Ti02 is an ultrafast process, completed in 25-150 fs This groundbreaking discovery implies that the search for new sensitizers need not to be limited to complexes with long-lived excited states. Indeed, [Fe(4,4 -(COOH)2-bpy)2(CN)2], whose MLCT excited state lifetime is only ca 330 ps, was found [304] to act as a sensitizer in a Ti02-based solar cell. In fact, even the classical Gratzel cell [36, 77, 78] would not operate as well as it does, were the interfacial electron transfer not ultrafast, since the [Ru(4,4 -(COOH)2-bpy)2-(NCS)2] sensitizer has an inherent excited state lifetime of only 50 ns. 

Electron injection from MLCT-excited Ru-polypyridine complexes are used to investigate electron transfer along DNA strands, that is to decide whether DNA can behave as a molecular wire [358-360]. In these studies, derivatives of [Ru(phen)2(dppz)] + act as excited-state electron donors and [Rh (phi)2(bpy)] + as a ground-state electron acceptor. Both complexes are anchored at different DNA sites and the rate of Ru —> Rh photoinduced electron transfer is measured. In another study [361], a [Ru (bpy)2(im)(NH2-)] + unit attached to a terminal ribose of a DNA duplex acted as an excited-state oxidant toward a [Ru (NH3)4(py)(NH2-)] " unit attached at the other end. 

Intermolecular reactions taking place between a unit incorporated in the den-drimer structure and an external partner. This is the most commonly observed situation. Often, well-known luminescent species like Ru -polypyridine complexes [30, 31] or metallo-porphyrin species [31] are used as dendrimer cores (Figure 2a). In this case, interesting issues are how the dendritic branches can influence the photophysical properties of the core, and to which extent quencher access to the core can be prevented. 

M. Sykora, M.A. Petruska, J. Alstrum-Acevedo, I. Bezel, T.J. Meyer, and V.I. Klimov, Photoinduced charge transfer between CdSe nanocrystal quantum dots and Ru-polypyridine complexes, J. Am. Chem. Soc., 128, 9984-9985 (2006). 

Figure 10.6 Cytochrome C556 fragment Pep18 with a datively anchored diiron hexacarbonyl cluster and a photosensitizing Ru polypyridine complex [32].

Indeed, photoredox catalysis with Ru polypyridine complexes has emerged as a powerful tool for redox reactions including formation of carbon-carbon bonds based on oxidation of sp C-H bonds via single-electron-transfer (SET) processes. Results that are closely related to those shown in Schemes 33,34, and 35, where the carbon-carbon bond formation resulted from the benzyUc sp C-H oxidative activation in the presence of BuOOH, have been recently reported for the regioselective functionalization of tetrahydroisoquinolines with cyanide and a variety of nucleophiles arising from ketones, nitroalkanes, allyltrimethylsilane, silyl enol ethers, 1,3-dicarbonyl compounds under photocatalytic conditions [67-70] as illustrated in Scheme 62 [67]. Other applications of Ru(bipy)3Cl2 in photocatalytic cycUzation reactions involving carbon-carbon btmd formation have appeared [71, 72]. 

The photochemical and photophysical properties of Ru(bpy)3 and related d° polypyridine complexes have been the subject of intense recent interest (43-49). This is due to their potential in photochemical energy conversion, their intrinsically significant excited state behavior, and their attractive chemical properties. The structures of the excited states of these complexes are clearly of great interest. 

P. R. Ashton, R. Ballardini, V. Balzani, E. C. Constable, A. Credi, O. Kocian, S. J. Langford, J. A. Preece, L. Prodi, E. R. Schofield, N. Spencer, J. E Stoddart, S. Wenger, Ru(II)-Polypyridine Complexes Covalently Linked to Electron Acceptors as Wires for Light-Driven Pseudorotaxane-Type Molecular Machines , Chem. Eur. J. 1998, 4, 2411-2422. 

A. Juris, V. Balzani, F. Barigellitti, S. Campagna, P. Belser, andA. vonZelewsky, Ru(II) Polypyridine complexes Photophysics, photochemistry, electrochemistry, and chemiluminescence, Coord. Chem. Rev. 84, 85-277 (1988). 

Probably the larger class of light-harvesting dendrimers investigated up to now is constituted of dendrimers based on Ru(II) and Os(II) polypyridine complexes, and the more extensive studies within these species involve the systems containing... 

Rotaxane 316+ was specifically designed36 to achieve photoinduced ring shuttling in solution,37 but it also behaves as an electrochemically driven molecular shuttle. This compound has a modular structure its ring component is the electron donor macrocycle 2, whereas its dumbbell component is made of several covalently linked units. They are a Ru(II) polypyridine complex (P2+), ap-terpheny 1-type rigid spacer... 

Ortmans I, Moucheron C, Kirsch-De Mesmaeker A (1998) Ru(ll) polypyridine complexes with a high oxidation power. Comparison between their photoelectrochemisty with transparent SnC>2 and their photochemistry with desoxyribonucleic acids. Coord Chem Rev 168 233-271 Ozawa T, Ueda J, Flanaki A (1993) Copper(ll)-albumin complex can activate hydrogen peroxide in the presence of biological reductants first ESR evidence for the formation of hydroxyl radical. Biochem Mol Biol Int 29 247-253... 

In dichloromethane solution, the [Ru(bpy)2(l)]2+ complex (Scheme 1) exhibits an absorption band at 455 nm (emax = 10400 M em Figure 5) and an emission band at 619 nm (x = 733 ns, cf> = 0.05, Figure 5, Table 1). These bands can straightforwardly be assigned to spin-allowed and, respectively, spin-forbidden metal-to-ligand-charge-transfer (MLCT) excited states, characteristic of Ru(II) polypyridine complexes[6a,c,e]. 

An assessment of the energy gap, AE, between emissive and 3MC levels can be obtained by studying the temperature dependence of the luminescence properties, as illustrated by several systematic studies for Ru(II)-polypyridine complexes [3,97-107]. This can be done by using an Arrhenius-like expression, Eq. 7 [98]. 

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... 

Luminescent ruthenium(II) polypyridine indole complexes such as [Ru (bpy)2(bpy-indole)]2+ (37) and their indole-free counterparts have been synthesised and characterised [77]. The ruthenium(II) indole complexes display typical MLCT (djt(Ru) tt (N N)) absorption bands, and intense and long-lived orange-red 3MLCT (djt(Ru) -> Ti (bpy-indolc)) luminescence upon visible-light irradiation in fluid solutions at 298 K and in alcohol glass at 77 K. In contrast to the rhenium(I) indole complexes, the indole moiety does not quench the emission of the ruthenium(II) polypyridine complexes because the excited complexes are not sufficiently oxidising to initiate electron-transfer reactions. Emission titrations show that the luminescence intensities of the ruthenium(II) indole complexes are only increased by ca. 1.38- to... 

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). 

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