Maximizing Nanoscale Downshifting Energy Transfer in a Metallosupramolecular Cr(III)–Er(III) Assembly

Pseudo-octahedral Cr^IIIN₆ chromophores hold a unique appeal for low-energy sensitization of NIR lanthanide luminescence due to their exceptionally long-lived spin-flip excited states. This allure persists despite the obstacles and complexities involved in integrating both elements into a metallosupramolecular assembly. In this work, we have designed a structurally optimized heteroleptic Cr^III building block capable of binding rare earths. Following a complex-as-ligand synthetic strategy, two heterometallic supramolecular assemblies, in which three peripherical Cr^III sensitizers coordinated through a molecular wire to a central Er^III or Y^III, have been prepared. Upon excitation of the Cr^III spin-flip states, the downshifted Er(⁴I_13/2 → ⁴I_15/2) emission at 1550 nm was induced through intramolecular energy transfer. Time-resolved experiments at room temperature reveal a Cr^III → Er^III energy transfer of 62-73% efficiencies with rate constants of about 8.5 × 10⁵ s^-1 despite the long donor-acceptor distance (circa 14 Å). This efficient directional intermetallic energy transfer can be rationalized using the Dexter formalism, which is promoted by a rigid linear electron-rich alkyne bridge that acts as a molecular wire connecting the Cr^III and Er^III ions.