CHIRAL DISCRIMINATION IN ELECTRONIC ENERGY-TRANSFER PROCESSES IN SOLUTION - EFFECTS OF TEMPERATURE AND SOLUTION PROPERTIES ON CHIRALITY-DEPENDENT RATE PARAMETERS

Time-resolved chiroptical luminescence (TR-CL) measurements are used t o study the kinetics of chirality-dependent excited-state quenching pr ocesses in solution. Solution samples contain a racemic mixture of chi ral luminophore molecules (L) and a small, optically-resolved concentr ation of chiral quencher molecules (Q) in an achiral solvent (H2O, D2O , or an H2O-ethylene glycol mixture). The luminophores are excited wit h a pulse of unpolarized light to create an initially racemic excited- state population of Delta L and Delta L* enantiomers, and TR-CL measu rements are then used to monitor the differential decay kinetics of th e Delta L and Delta L* subpopulations. Observed differences between t he Delta L and Delta L* decay kinetics reflect differential rate proc esses and efficiencies for Delta L - Q versus Delta L* - Q quenching actions, and they are diagnostic of chiral discriminatory interactions between the luminophore and quencher molecules. In each of the system s examined here, quenching occurs via electronic energy-transfer proce sses in transient (Delta L - Q) and (Delta L* - Q) encounter complexe s, and the chiral discriminatory rate parameters reflect the relative stabilities and lifetimes of these complexes as well as their structur es and internal (electronic and nuclear) dynamics. Both quenching rate s and enantioselectivity are observed to increase with increasing temp erature over the temperature ranges represented in this study (253-353 K for H2O-ethylene glycol solutions and 273-353 K for H2O and D2O sol utions), and differences between the free-energies of activation for D elta L - Q versus Delta L* - Q quenching processes increase from appr oximate to 2.5 to 4.8 kJ/mol over the 273-353 K temperature range. Bot h the luminophore and quencher molecules examined in this study have t hree-bladed propeller-like structures with very similar shapes and siz es, and differential steric constraints in (Delta L - Q) versus (Delt a L - Q) contact pairs are small. However, at room temperature and ab ove, the relative efficiencies of Delta L - Q versus Delta L* - Q que nching processes differ by factors > 3.2, indicating an extraordinary degree of chiral discrimination in the underlying interaction processe s.