Chiroptical properties of an optically pure dicopper(I) trefoil knot and its enantioselectivity in luminescence quenching reactions

Chiroptical spectroscopy is used to investigate the properties of an optica lly pure dinuclear copper(I) trefoil knot. For the metal-to-ligand charge t ranfer (MLCT) transition in the visible region (520 nm), the electric and m agnetic transition dipole moments are determined from absorption and circul ar dichroism spectra: 2.8Debye and 0.5 Bohr magneton (mu(B)). Circular pola rization in the luminescence (CPL) of the knot is determined and this allow s the electric and magnetic transition dipole moments in emission to be cal culated: 0.02 Debye and 0.003 mu(B), The large difference between the momen ts in absorption and emission shows that the emission observed does not ori ginate directly from the (MLCT)-M-1 state, Given the low probability for ra diative decay we assign the long-lived emitting-excited state to a (MLCT)-M -3 state. The copper(I) trefoil knot is found to quench the emission from T b-III and Eu-III(dpa)(3)(3-)(dpa = pyridine-2,6-dicarboxylate) with a bimol ecular rate constant of 3.2 and 3.3x10(7)M(-1)s(-1), respectively, at room temperature in water-acetonitrile (1:1 by volume). Experimental results ind icate that the (Lambda)-knot preferentially quenches the Lambda enantiomer of the lanthanide complex with an enantioselectivity (ratio of quenching ra te constants for Delta and Lambda: kq(Lambda)/kq(Delta)) of 1.012 +/- 0.002 for Eu-III and 1.018 +/- 0.003 for Tb-III.