Chirality-dependent interactions between molecular propeller structures in solution. Chiral recognition and discrimination processes modulated by temperature and incremental changes in structural chirality Original Research Article

Time-resolved chiroptical luminescence (TR-CL) measurements are used to study chirality-dependent intermolecular interactions in dynamic excited-state quenching processes. The measurements are carried out on solution samples that contain a racemic mixture of chiral luminophore molecules (with enantiomeric structures denoted by ΛL and ΔL) and a small, optically resolved concentration of chiral quencher (CQ) molecules. The luminophores are excited with a pulse of linearly polarized laser radiation to produce an initially racemic excited-state population of ΛL* and ΔL* enantiomers, and TR-CL measurements are then used to monitor the differential decay kinetics of the ΛL* and ΔL* subpopulations. Observed differences between the ΛL* and ΔL* decay kinetics reflect differential rate processes and efficiencies for ΛL*–CQ vs. ΔL*–CQ quenching actions, and they are diagnostic of chiral discriminatory interactions between the luminophore and quencher molecules. Twelve different luminophore–quencher systems are examined, in both H2O and D2O solutions, and in each case the quenching kinetics are measured over the 273–308 K temperature range. In all of the systems examined here, quenching occurs via electronic energy-transfer processes in transient (ΛL*–CQ) and (ΔL*–CQ) 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. All of the luminophore and quencher molecules examined in this study have three-bladed propeller-like structures that are very similar in overall shape and size. However, they exhibit small differences in the structural details of their propeller blades, and it is found that these small differences in structure can produce both qualitative and very substantial quantitative differences in their chiral recognition and discrimination properties.