Our group and others have reported selective coordination of Ln to the asymmetric redox-active ligand 1,10-phenanthroline-5,6-dione (pd, Scheme 1). The pd ligand has three different accessible oxidation states, where sequential reduction of the neutral pd ligand results in electronic structure changes at the O,Oʹ site (pd 0 , dione; pd •-, semiquinone; and pd 2-, catecholate). Conveniently, Ln tris--diketonate moieties coordinate selectively at the diimine N,Nʹ-binding site in all three oxidation states of the ligand. For this work, we chose to focus on pd •-to take advantage of the selectivity of the [Ln(Tp)2] + moiety (Tp = hydrotris(1-pyrazolyl)borate) to anionic bidentate O-donor ligands (e.g. semiquinones, -diketonates). We expected that [Ln(Tp)2] + should therefore target the O,Oʹbinding site of pd •-over the N,Nʹ site. In combination with the established N,Nʹ selectivity of tris-diketonates, this modular route allows us to construct well-defined heterobimetallic complexes, the first of which to incorporate a radical bridging ligand. We study two Ln pairings with opposite radius ratios and show how the strategy is applicable for any combination. We establish the selectivity of the binding for the two chosen lanthanide pairings in the solution state by NMR, UV-Vis-NIR and photoluminescence (PL) spectroscopies, as well as by X-ray crystallography. Our preliminary investigation into the PL of the heterobimetallic revealed complex energy transfer (ET) pathways including selectivity in and participation of the radical bridging ligand in sensitisation.