LMCT states

L denotes a hole on the anion, or better in the anion valence band. This is a LMCT state. Its energy is assumed to be A. 

There is no essential difference between quenching via a MMCT state or a LMCT state. The latter occurs, for example, in Eu(III) if the LMCT state is either at low energy or if this state shows a large offset in the configurational coordinate diagram [23, 35]. The latter occurs in glasses [123], certain cryptates [124] and lanthanum compounds [125]. 

As a matter of fact low-lying MMCT states can also influence radiative transition probabilities. The long decay time of the VO4 luminescence is considerably shortened by the presence of Bi " [27] due to a Bi(IV)-V(IV) MMCT state (see also above). Such effects are very well-known for LMCT states in case of transition-metal ions and lanthanide ions [6]. They will not be discussed here any further. 

There are other cases in which triplet-mediated energy transfer might compete with LMCT-mediated energy transfer, particularly where the metal ion is either europium or samarium. In these cases, the redox potentials of the metal ions are such that LMCT is feasible, although the LMCT state will be lower in energy than the emissive state of the metal, precluding luminescence.65... 

Such reactions are commonly found as a result of the decomposition of charge transfer excited states. For example, while excitation of the MC bands of Co(NH3)5X (X = Cl, Br, I) leads to photosolvation and the formation of Co(NH3)5(0112) and Co(NH3)4(OH2)X, shorter wavelengths yield the LMCT state which decomposes into Cc II) ions and halogen atoms. The quantum yield for the reaction is found to depend on the excitation energy, indicating a role for the initially formed radical pair (Eq. 5). This may reform the starting complex (Eq. 6) or decompose to the redox products stabilised by the solvent or some other species (Eq. 7). The Co(II) complexes eventually decomposes (Eq. 8). 

CofNHs) also photo-cleaves DNA [117], but in this case, formation of Co (II) and oxidised ligand from the LMCT state could represent an alternative pathway to a direct oxidation, leading to strand scission. 

MovO(TPPS)X X = OCjHj or NOJ IL(Soret) or LMCT states Mo,vO(TPP)... 

LMCT states. Unlike bimolecular, outer-sphere, hydrogen-atom abstraction reactions, the formation of zwitterionic products is a typical innercomplex process. 

Table V. SCF excitation energy AE and repulsion AC for two LMCT states refers to the average of 4T] U and4T2u...

LMCTegrc0 tj(x is placed at a higher energy than the lowest lying acido ligand to Co (III) excited states and photoredox reactions are, respectively, initiated in each of these excited states, a nonradiative conversion of one into the other electronic state must be sluggish or not available. The experimental observations clearly demonstrate that the photophysics of LMCT states in complexes Com(NH2R)sX2 + with R = alky is different from that with R = H and exemplifies how the properties of the excited state have control over the photochemical properties. 

This mechanism is thought to be operative in coordination compounds as well (Beeby et al., 2002c), although in some instances, a fraction of the Ybm ions is probably excited via the LMCT state. 

In the case of photolysis of acidic aqueous solutions of Eu2+ ion at 366 nm, the excited state results from 4f —> 5d transitions localized on the metal centre. These excited states have also considerable MLCT character because of strong mixing of metal 5d orbitals with ligand orbitals. In the case of the reaction of Eu3+ with H2 which occurs on photolysis at 254 nm, the photo reaction is due to the formation of an LMCT excited state. This process has been successfully used in the photochemical separation of Eu from other members of the lanthanides because Eu2+ is the only member of the lanthanide series which is at suitably low energy that an LMCT state is accessible [98]. Yb3+ and Sm3+ ions behave in a similar fashion to Eu3+ as far as their photochemical behaviour is concerned. Aqueous solutions of Sm3+ or Yb3+ containing 2-propanol on photolysis at 185 nm give hydrogen and acetone as products probably by a mechanism similar to Eu3+ ion. 

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