Controlling Near-Infrared Fluorescence-to-Phosphorescence Ratios and Triplet Lifetimes in Rhodium(I) Dimers via...

Rhodium(I) and other d⁸-metals form discrete dimers in which bridging ligands position two square-planar coordination units in close proximity, enabling metal-metal interactions. Although the metal-metal distance is known to modulate absorption and photoluminescence wavelengths, clear synthetic guidelines for controlling intersystem crossing and triplet excited-state lifetimes remain elusive. We show that homoleptic coordination with four identical bridging di-isocyanide ligands produces a phosphorescent rhodium(I) dimer emitting in the near-infrared (NIR)-II region. In contrast, heteroleptic complexes containing two di-isocyanide and two di-phosphine ligands yield rhodium(I) dimers that show mainly NIR-I fluorescence and NIR-II phosphorescence. In these heteroleptic systems, the fluorescence-to-phosphorescence ratio and triplet lifetime depend on the extent of metal-metal interactions, which can be tuned by over 0.2 Å through modifications at the di-isocyanide ligand periphery while preserving the primary coordination sphere. Our results are consistent with a picture in which rigidification of the central bimetallic core arises from changes to both the primary and secondary coordination environments, thereby reducing nonradiative excited-state relaxation pathways, most notably from the phosphorescent T₁ state. These findings provide guidelines for tuning fluorescence and phosphorescence relevant to imaging and phototherapy, as well as controlling singlet versus triplet photoreactivity in photocatalytic systems for synthetic chemistry and solar energy conversion.