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

The Cr^IIIN₆ 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^IIIN₆ inert "complex-as-ligand" that can be included into polymetallic architectures. As a proof of concept, these CrN₆ building blocks were reacted in solution with Zn^II or Fe^II to 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 to 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.