MLCT absorption

In search of new sensitizers that absorb strongly in the visible region of the spectrum, Arakawa and co-workers have developed several sensitizers based on 1,10-phenanthroline ligands. Among these new compounds, (29) and (30) are noteworthy they show an intense and broad MLCT absorption band at 525 nm in ethanol. The energy levels of the LUMO and HOMO for (29) were estimated to be —1.02 and 0.89 eV vs. SCE, respectively, which are slightly more positive than those of the sensitizer (22). These sensitizers, when anchored onto a Ti02 surface, yield more than 85% photon to electron injection efficiencies.8,52,53... 

The kinetics of this reaction were followed by tracking the appearance of Cu(dmp)J at 455 nm, the A,max of the metal to ligand charge transfer (MLCT) absorption band. At a fixed pH, the kinetics in aqueous solution followed the rate law... 

The complex has an intense MLCT absorption in the visible region. 

Reactions of c -[Ru(bpy)2Cl2] with ligands (86) or (87) (X = CH2) in EtOH(aq) lead to [Ru(bpy)2(86)] + and [Ru(bpy)2(87, X = CH2)] respectively. When X = 0 in ligand (87), the product is the pyridine carboxylate complex [Ru(bpy)2(pyC02)], the structure of which is confirmed by X-ray crystallography. Complexes of the type [Ru(bpy)2L] " in which L represents a series of mono- and dihydrazones have been prepared and characterized by spectroscopic methods (including variable temperature H NMR) and a structure determination for L = biacetyl di(phenylhydrazone). When L is 2-acetylpyridine hydrazone or 2-acetylpyridine phenylhydrazone, [Ru(bpy)2L] + shows an emission, but none is observed for the dihydrazone complexes. The pyrazoline complex [Ru(bpy)2L] (L = 5-(4-nitrophenyl)-l-phenyl-3-(2-pyridyl)-2-pyrazoline) can be isolated in two diastereoisomeric forms. At 298 K, these exhibit similar MLCT absorptions, but at 77 K, their emission maxima and lifetimes are significantly different. ... 

The preparation and characterization of K3[0s(CN)5(NH3)] 2H20 have been described. This complex is a useful starting material for the synthesis of other [Os (CN)5L] species (e.g., L = py, pyz) and [0s(CN)5(H20)] can be obtained by the controlled aquation of [Os(Cf 5(NH3)f The kinetics of these ligand displacements have been investigated and mechanisms for the processes have been discussed. The UV-vis spectrum of each complex in the series [Os"(CN)5L] in which L is py, pyz, Mepz, or derivatives thereof, exhibits an intense, asymmetric MLCT absorption, split by spin-orbit coupling, and the effects of the electronic properties of L on the spectra have been examined. Redox properties of the complexes and the kinetics of the dissociation of pyz from [Os(CN)s(pyz)] have also been reported. ... 

Other important differences relevant to our discussion are (1) Os(Il) complexes are oxidized at potentials considerably less positive than Ru(II) complexes (2) the MLCT absorption and luminescence bands lie at lower energies for the Os(II) complexes than for the Ru(II) ones (3) the eneigy of the LUMO of the (mono-coordinated) ligands decreases in the series bpy > 2,3-dpp > 2,5-dR) > biq as a consequence, the lowest (luminescent) ML(TT level involves the lowest ligand of the above series which is present in the complex and (4) the electron donor power decreases in the ligand series bpy > biq > 2,3-dpp - 2,5-dpp. 

In an entirely different experimental approach the unsymmetrical mixed-valence ion shown in equation (76) was subjected to laser flash photolysis.100 Excitation was carried out into the MLCT absorption band of the Ru11 -> 7t (pz) chromophore. Following excitation, one of the deactivation channels leads to the unstable mixed-valence isomer and its subsequent relaxation to the final, stable oxidation state distribution was observed directly using picosecond laser techniques. 

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