Self-Assembled Tetrahedral [Cr^III₄L₆]¹²⁺ Cage Displaying Near-Infrared Spin-Flip Photoluminescence

The thermodynamically controlled self-assembly of bis-bidentate quaterpyridine ligand, L = 2,2':5',5″:2″,2‴-quaterpyridine, with Cr^II and subsequent oxidation to Cr^III yields the first photoluminescent tetrahedral [Cr^III₄L₆]¹²⁺ molecular cage. Single-crystal X-ray diffraction reveals the presence of two homochiral cages (ΛΛΛΛ and ΔΔΔΔ) in the unit cell that crystallize as a racemic mixture. Additionally, a PF₆ anion is observed inside the cavity, in line with isostructural cages built with Ni^II or Fe^II. Each corner of the polyhedron is occupied by weakly antiferromagnetically coupled {Cr(bipy)₃}³⁺ (bipy = 2,2'-bipyridine) patterns, as revealed by magnetometry. Upon light excitation in the UV-vis region, spin-flip luminescence from the ²E/²T₁ excited states with a maximum at 727 nm (13755 cm^-1) was detected at room temperature. The measured excited state lifetime of 183 μs is longer than the 102 μs recorded for the mononuclear [Cr(bipy)₃]³⁺ complex under anaerobic conditions, whereas the luminescence quantum yields are in the same order of magnitude and amount to 10^-2 %. The photoluminescence brightness, <i>B</i>, calculated using the maxima of the absorption spectra for both species, goes from 14 M^-1·cm^-1 for the mononuclear compound to 90 M^-1·cm^-1 for the tetrahedral cage. This 6-fold improvement is observed across the entire excitation wavelength range, and it is due to the incorporation of four light-harvester units in the molecular cage.