Self-Assembled Tetrahedral [CrL] 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 and subsequent oxidation to Cr yields the first photoluminescent tetrahedral [CrL] 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 PF anion is observed inside the cavity, in line with isostructural cages built with Ni or Fe. Each corner of the polyhedron is occupied by weakly antiferromagnetically coupled {Cr(bipy)} (bipy = 2,2'-bipyridine) patterns, as revealed by magnetometry. Upon light excitation in the UV-vis region, spin-flip luminescence from the E/T excited states with a maximum at 727 nm (13755 cm) 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)] complex under anaerobic conditions, whereas the luminescence quantum yields are in the same order of magnitude and amount to 10 %. The photoluminescence brightness, , calculated using the maxima of the absorption spectra for both species, goes from 14 M·cm for the mononuclear compound to 90 M·cm 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.