Intermolecular chiral recognition processes probed by chiroptical luminescence

Enantiopreferential energy transfer processes between dissymmetric lanthanide and transition metal complexes dissolved in acetonitrile are studied using chiroptical luminescence techniques. The energy donors (luminophores) in this study are a racemic mixture of Ln(dpa)3 (3-) complexes (where Ln = Eu3+ or Tb3+ and dpa = 2,6-pyridinedicarboxylate), and the energy acceptors (quenchers) are an enantiomerically-resolved population of Co(R,R-chxn)3 3+ (where R,R-chxn = trans-1R,2R-diaminocyclohexane) complexes. The luminophores are dissolved in acetonitrile as (NEt4)3[Ln(dpa)3] (where NEt(4) = tetraethlylammonium) and (NBu4)[Ln(dpa)3] (where NBu4 = tetrabutylammonium) salts. The unquenched luminescence lifetimes are reported for both Eu(dpa)3 (3-) and Tb(dpa)3 (3-) in acetonitrile over the range 263-333 K, and these results are compared to luminescence lifetimes in aqueous solution. Time-resolved chiroptical luminescence measurements of enantiopreferential quenching kinetics are reported for samples with Eu(dpa)3 (3-) and Co(R,R-chxn)3 3+ in acetonitrile over 263-333 K range. These results are analyzed using a phenomenological quenching kinetics model, and the results are compared to results in aqueous solution. These comparisons show that the overall Eu-Co luminescence quenching efficiency is reduced in acetonitrile vs. aqueous samples, because the salts of (NX4)3[Eu(dpa)3] are not completely dissociated in acetonitrile. However, the enantiopreference exhibited is identical in acetonitrile vs. aqueous solution.