Strategies to control the geometry and symmetry around lanthanide centres for tailored luminescence and magnetism.

The luminescent and magnetic properties of trivalent lanthanides (Ln) are indispensable for many emerging technologies, but exacting fine control over these properties requires an understanding of how to purposefully engineer the coordination geometry and site symmetry of Ln centres. Here, we use the Cambridge Structural Database to extract the structures of 12,670 eight-coordinate Ln centres and use Continuous Shape Measures, Continuous Symmetry Operation Measures, and a new structural similarity-based network analysis to survey geometry and structure trends. This survey is then leveraged to deliver concrete strategies for controlling the coordination geometry of eight-coordinate Ln centres using familiar concepts like Ln metal size and ligand denticity, bite angle, flexibility, shape, symmetry, and size. Ultimately, we present a roadmap for targeting each of the six common eight-coordinate geometries - hexagonal bipyramidal, cubic, square antiprismatic, dodecahedral, bicapped trigonal prismatic, and snub disphenoid - which are each demonstrated to have unique use-cases in diverse research areas. The effects of crystal packing and non-covalent interactions are also illustrated, allowing fine-grained control over the geometry and symmetry of Ln centres. This work ultimately serves to inform the deliberate design of Ln coordination complexes and materials for applications including data-storage, quantum information processing, lighting, thermometry, and medical bioimaging.