Key Strategy for the Rational Incorporation of Long-Lived NIR Emissive Cr(III) Chromophores into Polymetallic Architectures

The Cr III N 6 chromophores are particularly appealing for low-energy sensitization via energy transfer processes since they show extremely long excited state lifetimes reaching the millisecond range in the technologically crucial near-infrared domain. However, their properties were barely harnessed in multimetallic structures because of the lack of both monitoring methods and accessible synthetic pathways. We herein report a remedy to monitor and control the formation of Cr III -containing assemblies in solution via the design of a Cr III N 6 inert "complex-as-ligand" that can be included into polymetallic architectures. As a proof of concept, these CrN 6 building blocks were reacted in solution with Zn II or Fe II o give extended trinuclear linear Cr-M-Cr assemblies, the structure of which could be addressed by NMR spectroscopy despite the presence of two slowly relaxing Cr III paramagnetic centers. In addition to long Cr III excited state lifetimes and weak sensitivity to oxygen quenching, these polymetallic assemblies display controlled Cr III o M II energy transfers, which pave the way for use of the "complex-as-ligand" strategy for introducing photophysically active Cr III probes into light-converting polymetallic devices.