Chiral discrimination in electronic energy-transfer processes in solution. Effects of temperature and solution properties on chirality-dependent rate parameters Original Research Article

Time-resolved chiroptical luminescence (TR-CL) measurements are used to study the kinetics of chilality-dependent excited-state quenching processes in solution. Solution samples contain a racemic mixture of chiral luminophore molecules (L) and a small, optically-resolved concentration of chiral quencher molecules (Q) in an achiral solvent (H2O, D2O, or an H2O-ethylene glycol mixture). The luminophores are excited with a pulse of unpolarized light to create an initially racemic excited-state population of ΔL∗ and ΛL∗ enantiomers, and TR-CL measurements are then used to monitor the differential decay kinetics of the ΔL∗ and ΛL∗ subpopulations. Observed differences between the ΔL∗ and ΛL∗ decay kinetics differential rate processes and efficiencies for ΔL∗-Q versus ΛL∗-Q quenching actions, and they are diagnostic of chiral discriminatory interactions between the luminophore and quencher molecules. In each of the systems examined here, quenching occurs via electronic energy-transfer processes in transient (ΔL∗-Q) and (ΛL∗-Q) encounter complexes, and the chiral discriminatory rate parameters reflect the relative stabilities and lifetimes of these complexes as well as their structures and internal (electronic and nuclear) dynamics. Both quenching rates and enantioselectivity are observed to increase with increasing temperature over the temperature ranges represented in this study (253–353 K for H2O-ethylene glycol solutions and 273–353 K for H2O and D2O solutions), and differences between the free-energies of activation for ΔL∗-Q versus ΛL∗-Q quenching processes increase from ≈ 2.5 to 4.8 kJ/mol over the 273–353 K temperature range. Both the luminophore and quencher molecules examined in this study have three-bladed propeller-like structures with very similar shapes and sizes, and differential steric constraints in (ΔL∗-Q) versus (ΛL∗-Q) contact pairs are small. However, at room temperature and above, the relative efficiencies of ΔL∗-Q versus ΛL∗-Q quenching processes differ by factors > 3.2, indicating an extraordinary degree of chiral discrimination in the underlying interaction processes.