CHIRAL DISCRIMINATION IN ELECTRONIC ENERGY-TRANSFER PROCESSES BETWEEN DISSYMMETRIC METAL-COMPLEXES IN SOLUTION - TIME-RESOLVED CHIROPTICAL LUMINESCENCE MEASUREMENTS OF ENANTIOSELECTIVE EXCITED-STATE QUENCHING KINETICS

Steady-state and time-resolved chiroptical luminescence measurements are used to investigate enantioselective excited-state quenching processes in solution. Theory and measurement methodology are presented for the case in which an initially racemic excited-state population of chiral luminophores evolves to a nonracemic population in the presence of chiral quencher molecules. Generation of enantiomeric excess in the luminophore excited-state population produces differential (spontaneous) emission of left and right circularly polarized light, and the time dependence of this chiroptical luminescence provides a direct measure of the differential excited-state quenching kinetics associated with homochiral versus heterochiral luminophore-quencher interactions. Experimental results are presented for a series of systems in which the luminophores are dissymmetric, tris(terdentate) lanthanide complexes and the quenchers are dissymmetric, tris(bidentate) transition-metal complexes. Each of the lanthanide and transition-metal complexes has trigonal-dihedral (D3) symmetry, but the complexes differ with respect to their electronic state structures, spectroscopic properties, and ligand sizes and shapes. These differences in structural and spectroscopic properties are reflected in the quenching rate parameters measured for different luminophore-quencher (Q) combinations, and they have a particularly strong influence on the differential rate parameters associated with enantioselective (homochiral versus heterochiral) quenching. Lanthanide excited-state (Ln⋆) quenching in these systems occurs via an electronic energy-transfer (Ln⋆ to Q) mechanism, and enantioselective quenching reflects chiral discrimination in the intermolecular interactions that govern energy-transfer probabilities. The quenching results are interpreted within the framework of a phenomenological model for energy transfer via an electron-exchange mechanism, and enantioselectivity in the quenching is attributed to (1) structural differences between the collisional complexes formed by homochiral versus heterochiral diastereomeric donor (Ln⋆)-acceptor (Q) pairs and/or (2) chiral discrimination in the purely electronic interactions directly responsible for energy transfer (i.e., chirality dependence in the electron-exchange integrals). Our results indicate that both 1 and 2 contribute to the enantioselective quenching kinetics observed for the Ln⋆-Q systems and that the respective contributions may be either additive or subtractive. © 1990, American Chemical Society. All rights reserved.