CHIRAL DISCRIMINATION IN ELECTRONIC-ENERGY TRANSFER BETWEEN DISSYMMETRIC LANTHANIDE(III) AND COBALT(III) COMPLEXES IN SOLUTION - EFFECTS OF LIGAND SIZE, SHAPE, AND CONFIGURATION IN THE ACCEPTOR COMPLEXES

Chiral discrimination in electronic energy-transfer processes is investigated for a series of systems in which the energy donors and acceptors are dissymmetric metal complexes. The sample systems contain a racemic mixture of luminescent donor complexes in solution with an enantiomerically resolved concentration of nonluminescent acceptor complexes. Chiral discrimination in the donor-acceptor electronic energy-transfer processes produces differential quenching kinetics for the two enantiomeric structures in the donor excited-state population, and these quenching kinetics are characterized by time-resolved chiroptical luminescence (TR-CL) measurements. The rate parameters obtained from these measurements provide a basis for assessing the degree and sense of chiral recognition between the dissymmetric donor and acceptor complexes in solution. Four different donor complexes and four different acceptor complexes are examined in this study. The donor complexes are Eu(dpa)3(3-), Tb(dpa)3(3-), Eu(cda)3(6-), and Tb(cda)3(6-), where dpa = dipicolinate dianion and cda = chelidamate trianion. Among these complexes there are significant differences in electronic state structures (between the Eu3+ and Tb3+ complexes) and in net charge (between the dpa and cda complexes), but the size, shape, and stereochemical properties of these complexes are very similar. In contrast, the acceptor complexes employed in this study have identical net charges and similar electronic state structures, but they differ significantly with respect to size, shape, and stereochemical details. These complexes are optical isomers of Co(en)33+, Co(en)2(R,R-chxn)3+, Co(en)(R,R-chxn)2(3+), and Co(R,R-chxn)3(3+), Where en = ethylenediamine and R,R-chxn = trans-(1R,2R)-1,2-diaminocyclohexane. The quenching data obtained in this study show large variations among the 16 different donor-acceptor systems investigated, and these variations include changes in both the degree and sense of enantioselective quenching. The observed variations in quenching behavior are correlated with a number of donor and acceptor properties, including charge, size, shape, elements of structural chirality, and the donor and acceptor electronic state structures relevant to resonance energy-transfer processes.