Ligand-to-ligand and intraligand charge transfer and their relation to charge transfer interactions in organic zwitterions

Coordination compounds, which simultaneously contain a reducing and an oxidizing ligand are frequently characterized by low-energy LLCT transitions. In suitable cases, CT interactions between such donors and acceptors may also occur in purely organic molecules. In particular, certain zwitterions have been known for many decades to display CT bands in their absorption spectra. In the present review the close relationship between such zwitterions and metal complexes with low-energies LLCT transitions is illustrated by suitable examples. Moreover, in some cases the zwitterions themselves serve as ligands. The CT properties of these zwitterions are preserved but modified by coordination. Some examples of ILCT transitions of this type are discussed in the second part of our report.