Photoredox LMCT excited states

The photoredox behavior of Cu(II) complexes, similarly to those of Fe(III), is derived mostly from the reactive decay of their LMCT states. Excitation to LMCT excited states can be attainable by direct sunlight, when the ligands coordinated to Cu(II) are good enough electron donors. As a consequence of the reactive decay of the LMCT excited state by inner-sphere electron transfer, the Cu(II) central atom is reduced to Cu(I), whereas the ligand or another available electron donor is oxidized to its radical, for example ... 

A number of unimolecular photoredox reactions have been studied. Kirk and Porter [130] reported a AF value of +4.8 cm3 mol"1 for the charge-transfer photolysis of Co(NH-,)5Br24 and suggested the formation of a caged radical pair, Con(Br ), from the LMCT excited state. Dissociation of this radical pair to the reaction products was suggested to account for the increase in volume as reflected by the positive volume of activation. Charge-transfer photolysis of trans -Pt(CN)4(N3)2 results in the reductive elimination of azide to produce Pt(CN) and dinitrogen [131,132], On... 

Co(NH3)sNC are converted into Co(NH3)sNCS and Co(NH3)50NO respectively. The other product in the solution is the aquated cobalt(II) ion that arises from a photoredox reaction. These photoisomerization reactions are believed to proceed via a pathway where irradiation leads to a thermally equilibrated LMCT excited state. In this excited state, the complex dissociates to give the radical pair... 

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. 

Dramatic medium effects are also found for the photoredox reactions of [Co(NH3)3Br]a+ (Figure 2). ° Thus whereas in water or 87% phosphoric acid a constant value of Oredox is reached as the excitation energy is increased, behaviour in 80% acetonitrile, 75%- and 50%-glycerol is strikingly different. This effect clearly demonstrates the importance of the immediate solvent environment for reactions of LMCT states. Further, the authors point out that this phenomenon... 

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