MLCT band

Effects of spacer groups on the formation and properties of the mixed-valence states of conjugated ferrocene dimers have been extensively studied by both electrochemical and spectroscopic methods. It should be noted that a characteristic feature in the electronic spectra of ferrocene dimers with conjugated spacer groups is the appearance of metal-to-ligand charge transfer (MLCT) bands in the neutral form as well as IT bands in the mixed-valence state. The dimer Fc — CH=CH — Fc... 

Strong MLCT bands appear in the neutral form of azo-bridged ferrocene oligomers, as noted above. The absorption of the MLCT band at 534 nm diminishes, and a new band appears and increases at 672 nm with the oxidation to 25+. The new band can be assigned to a ligand-to-... 

Efficient operation of these cells requires, inter alia, that the M(III) species (obtained by electron injection) be reduced by 1 in solution preferentially to recombination with the electrons that were injected into the Ti02.To probe this competition, both recombination rates and iodide oxidation rates were determined. Recombination rates were investigated by monitoring the recovery of the characteristic ground-state M(II) MLCT bands in the visible region. These recovery rates were found to obey a rate law having two second-order terms, as in Eq. (22). 

Specific examples are now used to demonstrate these concepts. First, consider the group Ru(bpy)j2+ (luminescent), Os(bpy)32+ (slightly luminescent), and Fe(bpy)32+ (nonluminescent) (Table4.1). For Fe(bpy)32+, despite an exhaustive search no emission has ever been detected even at 77K we routinely use it as a nonemissive solution filter. All three iso structural eft systems are in the same oxidation state with the same electronic configuration (ft6). The Fe(II) complex has an intense MLCT band at 510 nm, and the Ru(II) complex at 450 nm the Os(II) complex has intense MLCT bands that stretch out to 700 nm. The n-n transitions are all quite similar in all three complexes with intense absorptions around 290 nm and ligand triplet states at 450 nm (inferred from the free ligand and other emissive complexes and the insensitivity of these states to coordination to different metals). 

Chemical modifications can be used to tune the state energies and enhance properties. CO ligands greatly stabilize the t levels, which results in both an increased A and a higher-energy MLCT transition. For example, the primary MLCT bands of Os(phen)32+ and [Os(phen)2Cl(CO)l+are at 430 and 365 nm, respectively. The emissions are similarly shifted from 710 to 646 nm. In keeping with the expectations of the energy gap law, the r of[Os(phen)3l2+ and [Os(phen)2Cl(CO)J+ are 74 and 234 nsec, respectively,(21)... 

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 longer degree of conjugation of the phenanthroline ligands in these complexes causes a bathochromic shift at the ti-ti band and the different nuclearity shows the 1 2 3 ratio of the extinction coefficient of the ti-ti as well as the MLCT bands. To avoid diastereomeric mixtures the authors established the first controlled synthesis of stereochemically defined multinuclear Ru(ll) complexes [59]. 

The preparation and structure of [Ru(phen)2(l,5,6,10-tetraazaphenanthrene)]Cl2 have been reported NMR spectroscopic data provide insight into the hydrophilic properties of the complex. The bpy-containing complexes [Ru(bpy)2(92)] + and [ Ru(bpy)2 2(M 92)] ((92) = dipyrido(2,3-a 2, 3"-/z)phenazine) were described earlier in the chapter.The analogous [Ru(phen)2(92)] and [ Ru(phen)2 2(M 92)]" have also been prepared and characterized, as has [ (phen)2Ru(/i-92) 3Ru] +. The electronic spectra exhibit intense MLCT bands in the visible region the electrochemical properties of the complexes have been investigated and for [ (phen)2Ru(/i-92) 3Ru], two sets of reduction waves centered on ligand (92) are separated by those assigned to phen reductions. ... 

When only polypyridine-type ligands are present, each mononuclear metal-based unit exhibits intense LC bands in the UV region and moderately intense MLCT bands in the visible. As it is shown by the electrochemical behavior vide supra), in the polymetallic species there is some interaction among the neighboring metal-based units. To a first approximation, however, each building block carries its own absorption properties in the polynuclear species so that the molar absorption coefficients exhibited by the compounds of higher nuclearity are huge, as it is clear by a cursory examination of the absorption data reported in Table 3. For example, the spectra of the decanuclear compounds lOB and lOC (Scheme 1 and Table 1)... 

That the above redox isomer is formed instead of the FenRuin one, can be demonstrated by careful analysis of the MLCT band of the product, as well as by the properties of the intervalence (IV) band. However, it is well known that the [Run(NH >)r,LV,+ ions are generally much more reactive that the [Fen(CN)5L]" analogues toward oxidation by peroxydisulfate (126,128), as required by the lower redox potential at the ruthenium center. A careful mechanistic analysis showed that, although the FeinRun isomer is the thermodynamically stable product, it is not the kinetically accessible one. Then, the reaction evolves as follows ... 

The MLCT bands of these complexes are broad and red-shifted by approximately 140 nm, compared to (1). The lowest-energy MLCT transitions within... 

The demetalation process was followed by absorption spectrophotometry (measurement of the decay, as a function of time and cyanide concentration, of the di-copper(I) complexes characteristic metal-to-ligand-charge-transfer (MLCT) bands in the visible region [111]) which gave access to the kinetic parameters, in particular to the second-order dissociation rate constants CN given in Table 1. 

The most spectacular effect resulting from the highly rigid and compact structure of Cu2(K-84)p+ is undoubtedly its extraordinary kinetic inertness in the cyanide demetalation process. Measurement of the absorbance decay of its MLCT band in the visible region (A = 524 nm) could be performed by classical absorption spectrophotometry (whereas stopped-flow techniques were required for the methylene-bridged knots) and allowed to demonstrate that its demetalation implies two rate-limiting steps, well resolved in time, as schematically represented in Figure 27. 

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