INTERMOLECULAR CHIRAL RECOGNITION PROBED BY ENANTIOSELECTIVE QUENCHING KINETICS - A MECHANISTIC MODEL FOR DISSYMMETRIC METAL-COMPLEXES IN SOLUTION

A mechanistic model for enantioselective excited-state quenching kinetics is described. The model is developed for systems in which quenching occurs via electronic energy-transfer processes within donor (luminophore)-acceptor (quencher) encounter complexes in solution. Applications of the model are illustrated for systems in which the luminophores (L*) and quenchers (Q) are dissymmetric metal coordination complexes and in which luminescence quenching rates are slow compared to translational and rotational diffusion rates (of L* and Q) in the solution medium. Several types of intermolecular chiral discrimination are represented in the enantioselective quenching model: (a) enantio-differential interactions in the formation and dissociation of L*-Q (transient) complexes; (b) differential geometries and stereochemical preferences in L*-Q diastereomeric structures (as dictated by stereoselective contact interactions); and (c) chiral discrimination in the electronic interactions that are directly involved in the luminophore-to-quencher energy-transfer processes. The chiral discriminatory electronic interactions are represented by chirality-dependent ''critical transfer distance'' parameters that reflect the relative chiralities of donor (L*) and acceptor (Q) state wave functions. The rate parameters derived from enantioselective quenching measurements are expressed in terms of both electronic and stereoselective contributions to intermolecular chiral discrimination, and applications are illustrated for several lanthanide (luminophore)-transition-metal (quencher) systems.