Charge transfer ruthenium complexes

Intervalence Charge Transfer Emd Electron Exchange Studies of Dinuclear Ruthenium Complexes Robert J. Crutchley... 

Lever ABP, Gorelesky SI (2004) Ruthenium Complexes of Non-Innocent Ligands Aspects of Charge Transfer Spectroscopy 107 77-114 LiB, see He J (2005) 119 89-119... 

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. 

Metal-to-hgand charge transfer (MLCT) systems are mostly based upon complexes of ruthenium and rhenium. The simplest and best known example of a MLCT lumophore is tris(2,2 -bipyridyl)ruthenium(ii) where photon absorption leads to an excited state composed of a centre and a radical anion on one of the bipyridyl units. 

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... 

Molecular engineering of ruthenium complexes that can act as panchromatic charge-transfer sensitizers for TiOi-based solar cells presents a challenging task, as several requirements which are very difficult to be met simultaneously have... 

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-... 

In order to obtain high conversion efficiencies, optimization of the short-circuit photocurrent (z sc) and open-circuit potential (Voc) of the solar cell is essential. The conduction band of the TiO is known to have a Nernstian dependence on pH [13,18], The fully protonated sensitizer (22), upon adsorption, transfers most of its protons to the TiO surface, charging it positively. The electric field associated with the surface dipole generated in this fashion enhances the adsorption of the anionic ruthenium complex and assists electron injection from the excited state of the sensitizer in the titania conduction band, favoring high photocurrents (18-19 inA/cm ). However, the open-circuit potential (0.65 V) is lower due to the positive shift of the conduction-band edge induced by the surface protonation. 

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