Supramolecular Assembly of a Macrocyclic Rhodium(I) Isocyanide Complex with Long-Lived Near-Infrared Luminescence

Strategic ligand engineering is essential for the development of new photoactive transition metal complexes. One such application is the control of supramolecular, one-dimensional stacking of square-planar d⁸-organometallic systems. Here, we report the synthesis of a macrocyclic, tetradentate isocyanide ligand, CN⁴, and facile coordination to rhodium(I) yielding [Rh(CN⁴)][BAr^F₄]. Single-crystal X-ray diffraction upon acetone-solution grown crystals reveals formation of a tetrameric Rh^I stack, constructed of two dimeric forms of monomers. UV-visible absorption spectroscopy shows two prominent absorption bands, assigned to dimer (525 nm) and tetramer (840 nm), confirming stacked species. These assignments are further supported by DOSY measurements. Solvent control over this equilibrium is achieved first with acetonitrile, favoring only tetrameric aggregation, and also with dichloromethane, disabling aggregation completely. Upon excitation, the monomeric form returns to its electronic ground state in less than 10 ns. The near-infrared-absorbing tetramer exhibits an excited state with a lifetime of 150 ns in deaerated acetonitrile and NIR-II emission at 1040 nm. This discovery provides new opportunities for innovation in photophysics and photochemistry through polynuclear architectures that exhibit emergent properties compared to traditional mononuclear transition metal complexes.