Self-Assembled Tetrahedral [CrIII 4L6]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 CrII and subsequent oxidation to CrIII yields the first photoluminescent tetrahedral [CrIII 4L6]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 NiII or FeII. 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, B, 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.