Rhodium visible spectra

The absorption spectra of tris-polypyridyl Rhodium(III) complexes are characterised by several intense Ligand Centered (LC) absorption bands in the UV. Neither MC absorption bands, nor CT bands are observed in the visible region of the spectrum in contrast to their Ruthenium analogues. This makes tris(polypyridyl)Rh(III) complexes formed with bpy and phen practically colorless [1]. 

Fig. 6. Expansion of the inverse detected 13C/103Rh HMQC NMR spectra of [Rh6(CO)15(P 4 p cgjj4j3)] one-dimensional projections showing the region of the bridging carbonyls. The spectra were obtained with defocusing delays of 17.9 ms 1/(2 JRh.c) a) and 7.14 ms (r 1/(5 /Rh,c), b), respectively. The expected correlations at the true 103Rh chemical shifts are clearly visible in the bottom spectrum, but are very weak or entirely absent in the top spectrum which displays correlations due to three-rhodium spin coherences as the dominant signals. Reproduced from Ref. 33 by permission of The Royal Chemical Society.

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 ... 

The dpp-bridged Ru(II)-Rh(III) complex displays typical absorptions for ruthenium and rhodium polyazine complexes in the UV and visible regions. The UV region in the electronic absorption spectrum of [(bpy)2Ru(dpp)Rh(bpy)2] in acetonitrile reveals two bands at = 312... 

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