Structural diversity and luminescent properties of coordination complexes obtained from trivalent lanthanide ions with the ligands: tris((1H-benzo[d]imidazol-2-yl)methyl)amine and 2,6-bis(1H-benzo[d]imidazol-2-yl)pyridine

Abstract

Herein we discuss the contribution of the NTB and BBP ligands in the coordination of trivalent lanthanide ions. There is a very versatile chemistry in these two N-donor ligands, which gives rise to great structural diversity. Ligands promote a wide range of molecular geometries and interactions, since the central atoms can reach different coordination numbers, going from heptacoordinated to decacoordinated. This review summarizes the synthesis procedures to obtain the free ligands, as well as a structural summary of their complexes with lanthanide(III) ions, together with an analysis of their luminescent properties. In the same way, this review focuses on the design of highly luminescent complexes and discusses the most relevant aspects of their sensitization, such as coordination number, molecular geometry, surrounding media, ligand functionalization, use of ancillary aromatic ligands, as well as the management of bidentate and monodentate coordinating anions. The luminescence of the complexes comes from the visible light emitting lanthanides and sensitization is promoted by ligands that act as an antenna. Obtaining an efficient emitter requires careful design of the coordination compound environment, including the use of suitable chromophores, which serve as an antenna and protect the coordination shell. The symmetry of the complex is also an important parameter, since the presence or absence of an inversion centre will influence the orbital mixing. Therefore, the coordination number is not the determining effect of luminescence, but an indicative of the symmetry of the compounds. We hope that from this review, weak points in the design of luminescent materials can be identified and a potential interest in the use of benzimidazole ligands as efficient sensitizers can be generated.