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

Pseudo-octahedral CrIIIN6 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 CrIII building block capable of binding rare
earths. Following a complex-as-ligand synthetic strategy, two heterometallic supramolecular
assemblies, in which three peripherical CrIII sensitizers coordinated through a molecular wire
to a central ErIII or YIII, have been prepared. Upon excitation of the CrIII spin-flip states, the
downshifted Er(4I13/2 → 4I15/2) emission at 1550 nm was induced through intramolecular
energy transfer. Time-resolved experiments at room temperature reveal a CrIII → ErIII 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 electronrich
alkyne bridge that acts as a molecular wire connecting the CrIII and ErIII ions.