Ruthenium complexes charge transfer transitions

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-... 

Intensily colored (near 450 nm) complexes involving Ruthenium (154) and 1,8-naphthyridine and its more basic derivative, 2,7-dimethyl-1,8-naphthyridine, have recently been described.105 The highly colored nature of these complexes has been ascribed to metal-to-ligand charge transfer transitions. 

In asymmetric complexes of the type [(bpy)2RuCl(pi-pyz)Ru-(NH3)4L]4+, studies (94) revealed that there is a solvent donor-number (DN)-dependent contribution to the Frank-Condon barrier of approximately 0.006 eV/DN, which completely overwhelms the dielectric-continuum-theory-derived (l/Dop-l/Ds) solvent dependence typically observed in symmetrical dimers. In this case, variations in MMCT Eop with solvent give linear correlations when plotted against solvent dependent AEm, the difference in potential between the two ruthenium(III/II) couples, as shown in Fig. 10. The microscopic origin of this solvent effect was described by Curtis, Sullivan, and Meyer (122) in their study of solvatochromism in the charge transfer transitions of mononuclear Ru(II) and Ru(III) ammine complexes. The dependence... 

Curtis, J.C., Sullivan, B.P., Meyer, T.J. Hydrogen-bonding-induced solvatochromatism in the charge-transfer transitions of ruthenium(II) and ruthenium(ni) complexes, fnorg. Chem. 22, 224-236 (1983)... 

Good relationships are found between thermodynamic data and optical charge transfer transitions in a series of ion pairs of [Ru(NH3)5L] where L is a pyridine derivative, with [Fe(CN)6], [Ru(CN)6] , and [Os(CN)6]. The transitions are substantially independent of the reducing metal ion and calculations suggest that these ion pairs also involve contact on the amine side of the ruthenium complex. 

Os(bipy) displays properties similar to those of Ru(bipy) " [10] however, the excited state has a smaller lifetime and the redox properties are quite different from those of Ru(bipy) " . In particular, the MLCT state is a more powerful reductant than that of its ruthenium equivalent. It is noteworthy that the redox parameters can be adjusted by introducing electron withdrawing or electron donating substituents at the appropriate positions of the diimine ligands. Copper(I) complexes are and some of them show intense charge transfer transitions in the visible [11]. In particular, using properly substituted phenanthrolines, it has been possible to obtain long lived MLCT... 

Figure 2.16 Structure of the octahedral ruthenium (II) trisbipyridyl complex. The orange colour of this complex results from metal-to-ligand charge-transfer (MLCT) transitions...

UV/Vis absorption spectra of the polymers and the model complexes show four absorption maxima in acetonitrile. The absorption maxima in the visible region (around 450 and 440 nm, respectively) are similar to those of Ru(bpy) +, and therefore correspond to the metal-to-hgand charge-transfer (MLCT) band of ruthenium(II) complex units. The high molar absorptivity can therefore be explained by the fact that the MLCT band is hkely buried under the considerably more intense hgand-centered tt-tt transition. 

The Ru(phen)2(dppz)2+-chromophore is the electron donor in this study. It has absorbance maxima in aqueous solution at 372 and 439 nm, respectively, e = 2.48 and 2.23 x 104 M 1 cm-1. The absorbance at 439 nm arises from a metal to ligand charge transfer (MLCT) transition that is common to tris(bipyridyl)-ruthenium(II) complexes, while the absorbance at 372 arises from an intraligand... 

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