Ligand-to-ligand charge transfer state in lanthanide complexes containing π-bonded antenna ligands

To design luminescent lanthanide complexes containing both π- and σ-bonded antenna ligands in the coordination sphere, we synthesized 2,2′-bipyridine complexes of Nd, Tb and Gd with tri- and tetraphenyl substituted cyclopentadienyl ligands: [Cp^Ph₃LnCl₂(bipy)(THF)] (Cp^Ph₃ = 1,2,4-triphenylcyclopenta- dienyl, bipy = 2,2′-bipyridine) and [Cp^Ph₄LnCl₂(bipy)(THF)] (Cp^Ph₄ = tetraphenylcyclopentadienyl). Their crystal structures were determined by X-ray diffraction analysis. Optical spectroscopic and crystallographic data indicate the presence of a ligand-to-ligand charge transfer state.