Structure and photophysical behavior of 2,2′-bipyrimidine/lanthanide ion complexes in various environments

The photophysical behavior of 2,2′-bipyrimidine has been studied alone and in the presence of several lanthanide or other metal ions. This substance, which is employed as bridging ligand in homo- and hetero-dinuclear complexes, can form stable complexes with luminescent lanthanide ions like Eu³⁺ and Tb³⁺. Complexes precipitated from common solvents are crystalline with a structure that consists of discrete, centrosymmetric dinuclear entities with a planar ligand configuration. These complexes are strongly luminescent. Luminescence is sensitized by ligand-to-metal energy transfer. However, when the ligand and metal ions are mixed in an unconventional solvent, like a poly(ethylene glycol) oligomer, all reagents stay in solution and produce a different type of complex where only an enhanced ligand-centered fluorescence can be observed. It is possible that such fluorescence is emitted by 2,2′-bipyrimidine in a non-planar configuration. This behavior has also been observed with other heterocyclic ligands that can exist in different conformers, like terpyridine, and it may explain why some ligand-lanthanide complexes sometimes fail to sensitize efficient photoluminescence.