Metal-bipyridine charge-transfer

Kalyanasundaram, K. Zakeeruddin, S. M. Nazeeruddin, M. K. Ligand-to-metal charge-transfer transitions in Ru(III) and Os(III) complexes of substituted 2,2 -bipyridines. Coord. Chem. Rev. 1994, 132, 259-264. 

Electronic Stark effect studies by Oh and co-workers revealed that intervalence absorption by the CT ion is accompanied by essentially zero change in dipole moment and, therefore, zero metal-to-metal charge transfer. The observations are consistent with a description of the complex as fully delocalized in both the ground state and intervalence excited state. At the same time, the researchers found that intervalence excitation of (NH3)sRu(/Lt-4,4 -bipyridine)Ru(NH3)5 is accompanied by a large change in dipole moment and, therefore, substantial transfer of charge between metal centers, consistent with a valence-localized description. 

Cationic ferrocene complexes with one, two, and four cationic [B(R)bpy] (bpy = 2,2,-bipyridine) acceptors such as 66 show absorption at Amax = 496-540 nm with the contribution of charge transfer between the ferrocene unit and the B(R)bpy substituent(s) (165). This is confirmed by the EPR spectrum of the monoreduced neutral species, which features a line shape indicating a considerable admixture of the ligand and metal orbitals. Preparation and physical properties of the related polymer, 67, have also been reported (166). 

Fluorescent redox switches based on compounds with electron acceptors and fluorophores have been also reported. For instance, by making use of the quinone/ hydroquinone redox couple a redox-responsive fluorescence switch can be established with molecule 19 containing a ruthenium tris(bpy) (bpy = 2,2 -bipyridine) complex.29 Within molecule 19, the excited state of the ruthenium center, that is, the triplet metal-to-ligand charge transfer (MLCT) state, is effectively quenched by electron transfer to the quinone group. When the quinone is reduced to the hydroquinone either chemically or electrochemically, luminescence is emitted from the ruthenium center in molecule 19. Similarly, molecule 20, a ruthenium (II) complex withhydroquinone-functionalized 2,2 6, 2"-terpyridine (tpy) and (4 -phenylethynyl-2,2 6, 2"- terpyridine) as ligands, also works as a redox fluorescence switch.30... 

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-... 

The most widely studied and most exploited of inorganic ECL reactions, however, is that of tris(2,2 -bipyridine) ruthenium(II), [(bpy)3Ru]2+. The emissive excited state is a metal-to-ligand charge transfer state of triplet spin multiplicity the emission yield is approximately 10% and oxygen does not significantly quench the luminescence [22], The complex can exhibit ECL following sequential oxidation at the metal center and reduction of the coordinated 2,2 -... 

Low-valence transition metal complexes of a-diimine ligands are highly colored because of the presence of low-energy metal to a-diimine charge transfer (MLCT) transitions. For a series of d6-M(CO)., (a-diimine) (M=Cr,Mo,W) and d8- M (CO)3 (a-diimine) (M =Fe, Ru) complexes, we have studied the spectroscopic and photochemical properties (1-10). The a-diimine ligands used are 1,4-diaza-1,3-butadiene (R-DAB), pyridine-2-car-baldehyde-imine (PyCa), 2,2 -bipyridine (bipy) or 1,10-phenanthroline (phen) molecules. A close relationship was deduced between the photochemical behavior of these complexes and their resonance Raman (rR) spectra, obtained by excitation into the low-energy MLCT band. 

With the exception of a few highly studied states, such as, for example, the redox active lowest metal-to-ligand charge transfer excited state (MLCT) in the [Ru(bipyridine)3]2t cation, or the ligand field active 2E state of Cr(IlI), we know relatively little about these excited states. An enormous body of knowledge is waiting to be explored. 

Electron transfer rate constant Metal to ligand charge transfer Bipyridine... 

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