Metal-centred excited state

This is expected to be favoured for metal-centred excited states for example, in d-d states of d or d complexes, where excitation often involves promotion of an electron from an essentially non-bonding orbital to one with appreciable sigma antibonding M-L character (e.g. in CrfNHalsCl Eq. 3). The net effect is lengthening of the M-L bond, which predisposes the complex to dissociation or associative substitution. The incoming ligand is often the solvent (e.g. as in Eq. 3) or counterion of an ion pair (Eq. 4). 

Metal centred excited states emission from lanthanides... 

The occurrence of an ET process can be unambiguously assessed if, after irradiation of the solution at the wavelength of the absorption band of FI, one observes tbe emission of the metal-centred excited state M. It has been already pointed out that the highly distorted d-d excited states are rarely emissive and undergo non-radiative decay processes. One of the few exceptions is Cr . 

Quenching of the ( CT)[Ru(bipy)3] by [Cr(bipy)]3 has been studied. This is via electron transfer to the Cr complex and a rapid back reaction. The ruthenium complex will also quench the 727 nm emission of the metal-centred doublet excited state of the chromium species, by a similar mechanism. Evidently both ligand- and metal-centred excited states can be quenched by bimolecular redox processes. A number of Ru complexes, e.g. [Ru(bipy)3] and [Ru(phen)3] also have their luminescence quenched by electron transfer to Fe or paraquat. Both the initial quenching reactions and back reactions are close to the diffusion-controlled limit. These mechanisms involve initial oxidation of Ru to Ru [equation (1)]. However, the triplet excited state is more active than the ground state towards reductants as well as... 

In order to have MLCT excited states separate in energy from metal-centred excited states and, as a consequence, to avoid fast deactivation, the best situation corresponds to or transition metals. In an octahedral d complex, if the ligand field is strong enough, the level will... 

For transition metals compounds, excited states are often extremely important, and this should be reflected in any parameterisation scheme. We have shown that one way such information can be incorporated into a given parameterisation strategy is by fitting the one-centre parameters (t/ss, f/pp, Ua, Gss, Gsd, Gm, Hsd) to experimental excitation and ionisation energies of the neutral and charged metal atom (as a starting point for future parameterisations these parameters have been reported elsewhere for... 

It can be expected that d-d states of metal complexes should lead to dissociation, since this is a metal-centred excitation in which an electron leaves a d orbital which binds a ligand. However, CT states d-7f and 7T-d will be involved in redox reactions of the metal centre such reactions often lead to the oxidative or reductive dissociation of the complex. 

As described earlier, the lifetimes and quantum yields of emissive Ln complexes vary dramatically due to the extremely sensitive nature of the 4/-centred excited states to 0-H, N-H and C-H vibrational manifolds, which can provide efficient, non-radiative deactivation pathways the efficiency of energy transfer between the antenna and lanthanide ion also determines overall quantum yields. A classical approach to maximising the emissivity of Ln complexes is to therefore inhibit the approach of water solvent to the inner coordination sphere (and where q denotes the number of coordinated solvent molecules) high denticity, metal ion encapsulating ligands with hydrophobic peripheries can achieve this very effectively, reducing q to zero [4]. 

Emissions from both a MC and LC excited state were observed at low temperature with sterically hindered ligands such as 3,3 -Me2-bpy [127] and 2,2 6, 2"-terpyridine [128], The MC emission is the dominant feature at 77 K, but the LC emission is enhanced relative to the metal centred one in fluid solution [127],... 

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. 

Figure 4.75 Schematic representation of the charge transfer in various excited states of a metal complex. M is the metal centre and L stands for a ligand. LF is a ligand field transition, CTs are the charge transfer transitions, LL is an intraligand transition, and CTTS is a charge transfer to solvent...

Figure 4.77 Potential energy diagrams of weak and strong couplings between the ground and excited states of a metal complex. (r is the distance between the metal centre and a ligand, a are the absorption and e are the emission spectra.)...

In practice, metal complexes of bpy and phen, such as [Ru(bpy)3]2+ and [Ru(phen)3]2+, exhibit long-lived phosphorescent excited states, arising from ligand-centred triplet charge transfer states (3MLCT). Lifetimes are of the order of 102—103 ns in fluid solution at room temperature. 

Perturbation of the lanthanide singlet excited state, in which quenching by electron- or charge-transfer processes may be altered by the binding of analytes to the metal centre. For example the W-alkylphenanthridinium unit in 11.32 binds selectively to the electron-rich guanine-cytosine... 

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