Maximizing\nNanoscale Downshifting Energy Transfer\nin a Metallosupramolecular Cr(III)–Er(III) Assembly

Pseudo-octahedral Cr<sup>III</sup>N<sub>6</sub> chromophores\nhold\na unique appeal for low-energy sensitization of NIR lanthanide luminescence\ndue to their exceptionally long-lived spin-flip excited states. This\nallure persists despite the obstacles and complexities involved in\nintegrating both elements into a metallosupramolecular assembly. In\nthis work, we have designed a structurally optimized heteroleptic\nCr<sup>III</sup> building block capable of binding rare earths. Following\na complex-as-ligand synthetic strategy, two heterometallic supramolecular\nassemblies, in which three peripherical Cr<sup>III</sup> sensitizers\ncoordinated through a molecular wire to a central Er<sup>III</sup> or Y<sup>III</sup>, have been prepared. Upon excitation of the Cr<sup>III</sup> spin-flip states, the downshifted Er(<sup>4</sup>I<sub>13/2</sub> → <sup>4</sup>I<sub>15/2</sub>) emission at 1550\nnm was induced through intramolecular energy transfer. Time-resolved\nexperiments at room temperature reveal a Cr<sup>III</sup> →\nEr<sup>III</sup> energy transfer of 62–73% efficiencies with\nrate constants of about 8.5 × 10<sup>5</sup> s<sup>–1</sup> despite the long donor–acceptor distance (circa 14 Å).\nThis efficient directional intermetallic energy transfer can be rationalized\nusing the Dexter formalism, which is promoted by a rigid linear electron-rich\nalkyne bridge that acts as a molecular wire connecting the Cr<sup>III</sup> and Er<sup>III</sup> ions.