Controlled Dimerization of Rhodium(I) Isocyanides Enables Photophysical Properties Beyond Mononuclear Complexes.

Recent developments in photoactive transition metal complexes have largely centered on mononuclear systems. Polynuclear architectures that are capable of metal-metal interactions and cooperative effects, such as enhanced luminescence or multielectron photoreactivity, have received less attention, however. A key challenge in advancing such systems lies in achieving structural precision to prevent the formation of complex mixtures, including ill-defined oligomers of variable lengths. Here, we report the controlled assembly of discrete dimers composed of stacked square-planar complexes, notably without the use of bridging ligands, but also with them. Using new tridentate pincer-type isocyanide ligands with varying backbones and coordination bite angles, we obtain rhodium-(I) complexes that can be further modulated at the fourth coordination site. Pincer bite angle, the auxiliary ligand at the fourth coordination site, and solvent polarity control the distinct aggregation behavior of the complexes. The resulting dimers exhibit metal-metal interactions that produce near-infrared fluorescence and substantially longer triplet excited-state lifetimes than the nonemissive monomers. This work demonstrates how molecular design and synthetic control over cooperative interactions between individual metal complexes can give rise to emergent photophysical properties. It establishes design principles for precise supramolecular assembly of photoactive coordination units, highlighting new opportunities for photonic applications beyond mononuclear systems.