Supramolecularly Organized Lanthanide Complexes for Efficient Metal Excitation and Luminescence as Sensors in Organic and Biological Applications

Using molecular shape for the functional control of multicomponent systems is one of the challenges in the field of supramolecular chemistry. The well-known emission in the visible region of certain lanthanide (III) salts has found a new dimension by the controlled assembly of supramolecular architectures to enshroud and excite the metal. The emission properties of these systems find applications as luminescent probes in sensing schemes, interaction with biomolecules, or light conversion systems. Complexes of Tb(III) and Eu(III) can in some cases be highly luminescent and typically exhibit emission lifetimes in the millisecond range. These long lifetimes provide a facile means to detect biomolecules labeled with lanthanide from the short-lived background emission of biosystems. The strongly ionic nature of the bonding in solid lanthanide complexes results in coordination geometries that reveal a balance between electrostatic and steric demands. The inclusion of lanthanide guests into macrocycles, cryptands, cavitands and spherands had the purpose of shielding the metals from solvent molecules whose OH, NH or CH groups are able to heavily quench lanthanide luminescence (intensity and lifetime) by deactivation of their emissive levels by vibronic coupling. The literature on the various chemical systems, based on nitrogen heteroaromatic rings, used to sensitize lanthanides will be thoroughly reviewed including those molecules synthesized by the authors which displayed high quantum yield of lanthanide emission.