Self-Assembled Tetrahedral [Cr<sup>III</sup><sub>4</sub>L<sub>6</sub>]<sup>12+</sup> Cage Displaying Near-Infrared Spin-Flip\nPhotoluminescence

The thermodynamically controlled\nself-assembly of bis-bidentate\nquaterpyridine ligand, L = 2,2′:5′,5″:2″,2‴-quaterpyridine,\nwith Cr<sup>II</sup> and subsequent oxidation to Cr<sup>III</sup> yields\nthe first photoluminescent tetrahedral [Cr<sup>III</sup><sub>4</sub>L<sub>6</sub>]<sup>12+</sup> molecular cage. Single-crystal X-ray\ndiffraction reveals the presence of two homochiral cages (ΛΛΛΛ\nand ΔΔΔΔ) in the unit cell that crystallize\nas a racemic mixture. Additionally, a PF<sub>6</sub> anion is observed\ninside the cavity, in line with isostructural cages built with Ni<sup>II</sup> or Fe<sup>II</sup>. Each corner of the polyhedron is occupied\nby weakly antiferromagnetically coupled {Cr(bipy)<sub>3</sub>}<sup>3+</sup> (bipy = 2,2′-bipyridine) patterns, as revealed by\nmagnetometry. Upon light excitation in the UV–vis region, spin-flip\nluminescence from the <sup>2</sup>E/<sup>2</sup>T<sub>1</sub> excited\nstates with a maximum at 727 nm (13755 cm<sup>–1</sup>) was\ndetected at room temperature. The measured excited state lifetime\nof 183 μs is longer than the 102 μs recorded for the mononuclear\n[Cr(bipy)<sub>3</sub>]<sup>3+</sup> complex under anaerobic conditions,\nwhereas the luminescence quantum yields are in the same order of magnitude\nand amount to 10<sup>–2</sup> %. The photoluminescence brightness, <i>B</i>, calculated using the maxima of the absorption spectra\nfor both species, goes from 14 M<sup>–1</sup>·cm<sup>–1</sup> for the mononuclear compound to 90 M<sup>–1</sup>·cm<sup>–1</sup> for the tetrahedral cage. This 6-fold improvement\nis observed across the entire excitation wavelength range, and it\nis due to the incorporation of four light-harvester units in the molecular\ncage.