Excitation energy funnelling in NIR-emissive [YbPrYb] heterometallic complexes

Abstract

Energy transfer (ET) among different lanthanide ions is a promising avenue to enhance the numerous applications of this photophysical phenomenon, as has been demonstrated in various solid-state materials. It is far less common to observe these phenomena in discrete species containing different lanthanide ions at predetermined locations within their molecular structures. The advantages of this are many, such as the ability to investigate these phenomena with ultimate atomic control, even in solution. A site-selective molecular scaffold combining the ligands H₂LA and H₂LB has been developed to enable the targeted positioning of lanthanide ions based on differences in ionic radii. This architecture allows a Pr³⁺ ion to be stabilised in the central coordination cavity, while smaller Yb³⁺ ions preferentially occupy peripheral sites, giving rise to a [YbPrYb] topology within a discrete heterometallic complex. The resulting compound, [Yb₂Pr(LA)₂(LB)₂(H₂O)₂(py)]NO₃, has been structurally characterised and used to investigate intramolecular ET between Pr³⁺ and Yb³⁺ ions. Two structural analogues, [YbLaYb] and [LuPrLu], were employed as luminescence controls to independently probe the donor and acceptor roles of each metal ion. Photophysical studies in solution and in the solid state revealed two distinct ET processes: a highly efficient Pr³⁺ → Yb³⁺ transfer, resulting in near-infrared (NIR) emission at 980 nm, and a competitive Yb³⁺ → Pr³⁺ back-transfer, leading to non-radiative deactivation. Lifetime analyses confirmed that both transfer pathways occur with near-quantitative efficiency. These findings represent the first evidence of bidirectional Pr³⁺ ⇄ Yb³⁺ energy transfer within a molecular complex and highlight the power of ligand architecture in enabling and modulating lanthanide photophysical behaviour.