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

Abstract

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 III 4 L 6 ] 12+ molecular cage. Singlecrystal 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 6 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 2 E/ 2 T 1 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.