Metal Complexes of Pincer Ligands: Excited States, Photochemistry, and Luminescence

Pincer ligands of the form ECE that incorporate N-heterocyclic lateral units, E, are terdentate analogues of NC-cyclometallating ligands such as 2-phenylpyridine. They are able to form a variety of highly luminescent complexes with platinum(II) and iridium(III). They can also be thought of as cyclometallating analogues of the NNN-coordinating ligand terpyridine. The introduction of the carbon atom can impart significant changes on the nature and energy of the electronic excited states, as is evident for complexes of ruthenium(II), where a shift in the absorption to low energy in the metallated systems has sparked interest in applications for dye-sensitised solar cells. Investigations in these areas over the past decade are reviewed. We also consider related complexes in which the lateral coordinating units are aliphatic N donors, phosphines, or sulphur ligands. The lack of good π-accepting units in such compounds tends to lead to weaker ligand fields and hence to low-energy metal-centred states. These states dictate much of the excited-state chemistry and compromise the efficiency of luminescence at room temperature.