Inefficient Crystal Packing in Chiral [Ru(phen)3](PF6)2Enables Oxygen Molecule Quenching of the Solid-State MLCT Emission

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Abstract
The molecular oxygen quenching of the solid-state emission from pure crystals of Δ-Ru(phen)3(PF6)2, Λ-Ru(phen)3(PF6)2, and racemic Ru(phen)3(PF6)2 (phen = 1,10-phenanthroline) was studied by emission spectroscopy. Crystals of the pure enantiomers exhibit significant and nearly identical emission-intensity quenching [0.36(2) and 0.33(2), respectively] in the presence of air [where the fraction quenched is (Initrogen − Iair)/Initrogen]; in comparison, the racemic compound shows a much lower value [0.05(2)]. The large difference in the quenching behavior is a result of major structural differences between the two chiral salts and the racemic salt. The chiral compounds crystallize in the space groups P41 and P43, respectively, with toluene and acetonitrile molecules in the lattice that can be partially removed to create void-space channels. These open channels allow the diffusion of oxygen molecules within the crystals and enable efficient emission quenching that is not possible in the closely packed racemic salt. Lifetime measurements, thermal gravimetric analysis, and single-crystal X-ray structure determinations support these conclusions.