Rhodium complexes photosensitivity

The kinetic and thermodynamic studies on the octahedral rhodium(III) complexes7 suddenly gained momentum. This field was later to give rise to the important discovery of the photosensitivity of rhodium complexes.8... 

Covalent attachment has also been exploited for protein incorporation of non-native redox active cofactors. A photosensitive rhodium complex has been covalently attached to a cysteine near the heme of cytochrome c (67). The heme of these cytochrome c bioconjugates was photoreducible, which makes it possible for these artificial proteins to be potentially useful in electronic devices. The covalent anchoring, via a disulfide bond, of a redox active ferrocene cofactor has been demonstrated in the protein azurin (68). Not only did conjugation to the protein provide the cofactor with increased water stability and solubility, but it also provided, by means of mutagenesis, a means of tuning the reduction potential of the cofactor. The protein-aided transition of organometallic species into aqueous solution via increased solubility, stability and tuning are important benefits to the construction of artificial metalloproteins. 

In the example discussed above, the heterotriad consists of a photosensitizer and an electron donor. In the following example, a ruthenium polypyridyl sensitizer is combined with an electron acceptor, in this case a rhodium(lll) polypyridyl center [15]. The structure of this dyad is shown in Figure 6.21 above. The absorption characteristics of the dyad are such that only the ruthenium moiety absorbs in the visible part of the spectrum. Irradiation of a solution containing this ruthenium complex with visible light results in selective excitation of the Ru(ll) center and in an emission with a A.max of 620 nm. This emission occurs from the ruthenium-polypyridyl-based triplet MLCT level, the lifetime of which is about 30 ns. This lifetime is very short when compared with the value of 700 ns obtained for the model compound [Ru(dcbpy)2dmbpy)], which does not contain a rhodium center. Detailed solution studies have shown that this rather short lifetime can be explained by fast oxidative quenching by the Rh center as shown in the following equation ... 

There are several photocatalysts mimicking hydrogenase activity that are not based on metalloporphyrin systems. Among them there are mixed-valence complexes of rhodium or iridium, [41] as well as complex systems encompassing photosensitizers (eg ruthenium complexes) attached to a catalytic bimetallic centre [43], The design of more sophisticated systems approaches that of photosynthetic processes [44],... 

A number of rhodium(III) complexes can be used effectively in place of viologens as relays. Thus photolysis of a solution containing Ru(bpy)32+ as the photosensitizer, ascorbate as the electron donor and [Rh(dpm)3Cl]3 (dpm = diphenylphosphinobenzene-m-sulfonate) as the electron relay leads to nett formation of hydrido-rhodium species via a reductive quenching cycle. The hydrido-rhodium product acts a two-electron carrier for the reduction of NAD-i- to NADH. In place of NADH, synthetic nicotinamide analogues such as N-benzyl nicotinamide or N-alkylnicotinamides can be similarly reduced in the photosystem [68]. The sequence of cyclic redox reactions can be extended by the addition of an enzyme. In the presence of... 

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