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

Pseudo-octahedral Cr III N 6 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( 4 I 13/2 → 4 I 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 × 10 5 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.