Intermolecular Interactions as Actors in Energy-Transfer Processes in Lanthanide Complexes with 2,2'-Bipyridine

A series of lanthanide complexes [LnCl(x)(bpy)(y)(H2O)(z)]-Cl-3-(x)(H2O)(n)(EtOH)(m) (Ln = Eu, Gd, Tb; x = 1, 2; y = 1, 2; z = 2-4) with different numbers of 2,2'-bipyridine (bpy), chloride ions, and water molecules in the inner coordination sphere were synthesized and investigated with the aim of relating their molecular geometry and crystal packing to the efficiency of ligand-to-metal energy transfer. In conjunction to the rotation of the pyridine rings upon coordination to the Ln ion, the high flexibility displayed by bpy ligands leads to rather unexpected bending of these rings with respect to the central bond, owing to intermolecular interactions such as Cl center dot center dot center dot pi and pi-stacking ones. Deciphering the luminescence properties of the Eu and Tb complexes needs to take into account both the composition of the inner coordination sphere and the peculiarities of the crystal packing. For instance, in addition to the classical ligand -> Eu charge-transfer state (LMCT), another charge-transfer state induced by pi-stacking interactions (SICT) could be identified. These two states, located between the singlet and triplet states of the bpy ligand(s), provide relays facilitating the energy migration from the singlet to the triplet states and eventually to the excited Eu states, improving the overall ligand-to-Eu energy transfer. Another point is the involvement of the inner-sphere water molecules in H-bonding with chloride ions, which considerably lowers their luminescence quenching ability, so that the adducts remain highly luminescent. For instance, the terbium chloride with two bpy ligands is an efficient near-UV to green light converter, with an overall quantum yield equal to 37% despite the coordinated water molecules. The interpretations given are substantiated by DFT and TD-DFr theoretical calculations of the complexes and ligand assemblies.