Ligands state

Lastly, photochemically unstable ligands should be avoided. Re(bpy)(CO)3Cl shows a moderately efficient MLCT emission at room temperature (R. M. Ballew, unpublished results from our laboratory). However, the apparently closely related Re(dpk)(CO)3Cl (dpk = 2,2 -dipyridyl ketone) shows a benzophenone like phosphorescence at 77K indicating that the n-n excited state of the ketone in complex is the lowest state of the complex. No luminescence is seen at room temperature, and even at 77K the dpk triplet state is such a powerful hydrogen atom extractor that it removes protons from alcohol glasses as seen by the formation of the intense blue color of the keto free radical. The absence of an MLCT emission is caused by the greater difficulty of reducing dpk relative to bpy, which pushes the MLCT states above the dpk ligand states. 

In chelate and macrocyclic complexes, electronic states may exist which are of a delocalized nature they pertain to the system of metal and ligands. Such states are not simply derived from metal d—d states or from free ligand states and transitions involving delocalized states are often quite intense. 

Further systems in which the nonradiative energy transfer from ligand states to f elements has been studied in detail are the [Pt(CN)4]3 and [Au(CN)2]3 complexes. In particular, Yersin and coworkers have studied Eu2[Pt(CN)4]3-18H20 (Yersin et al., 1979), Sni2[Pt(CN)4]3-18H20 (Yersin and Stock, 1982), and Eu[Au(CN)2]3-3H20 (Yersin et al., 1998). 

Steigemann W, Weber E (1979) Structure of erythrocruorin in different ligand states refined at 1.4 A resolution. J Mol Biol 127 309-338... 

An approximate expression for the F factor appearing above is given by equation 17, where yi is the ligand state band width at half-height and A is the difference between the donor and acceptor transition energies involved in the transfer process . 

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