Chiral Molecular Ruby [Cr(dqp)2]3+ with Long-Lived Circularly Polarized Luminescence

The chiral resolution of a kinetically inert molecular ruby [Cr(dqp)2]3+ (1, dqp =
2,6-di(quinolin-8-yl)pyridine) displaying strong dual light emission at room temperature has been
achieved. The wrapped arrangement of the six-membered dqp chelating ligands around the Cr(III)
provided nonplanar helical conformations leading to the diastereoselective assembly of chiral bistridentate
monometallic Cr(III)-helix. The PP-(+)-[Cr(dqp)₂]³⁺ and MM-(−)-[Cr(dqp)2]³⁺
enantiomers could be separated and isolated by using cation-exchange chromatography and
subsequent salt-metathesis with KPF6. X-ray crystallographic analysis based on Flack parameters
assigned the absolute configurations of the two enantiomers. Circularly polarized luminescence
(CPL) spectra showed two polarized emission bands within the NIR region corresponding to the
characteristic metal-centered spin-flip Cr(²E → ⁴A₂) and Cr(²T₁ → ⁴A₂) transitions with
exceptionally high dissymmetry factors, |glum|, of 0.2 and 0.1, respectively, which are comparable to
those reported for rare-earth chiral complexes. Photophysical properties also revealed an extremely
long excited-state lifetime of 1.2 ms and a high quantum yield of 5.2% at room temperature in water.
These properties make [Cr(dqp)₂]³⁺ an ideal sensitizer for the preparation of enantiopure luminescent supramolecular energyconverting
devices and also open up the possibility of using chiral Cr(III) chromophores for the construction of NIR-CPL
materials and polarized photonic devices based on earth-abundant metals.