Dynamic chiral quenching of europium and terbium excited states

The nature of dynamic quenching of the europium or terbum excited states for both Δ and Λ stereoisomeric cationic complexes by S and R Trolox has been studied by total emission and CPL spectroscopy and with the aid of DFT computations. In parallel, investigations of the stereoselectivity of complex association with the model protein, human serum albumin have been undertaken providing an example where binding to the chiral protein leads to preferred population of a twisted square-antiprismatic conformer for the Δ isomer only. Non-linear Stern–Volmer quenching kinetics were observed and the stereoselective behaviour in Trolox binding was interpreted in terms of an exciplex model, in which the binding constant for exciplex formation, K_ex, and the rate constant for exciplex decay, k₃, determine overall quenching efficiency. Dynamic quenching was most efficient with R Trolox for the Δ Eu and Tb complexes in three different examples. Ground state DFT calculations revealed that the Δ charge transfer complex with S Trolox was 10 kJ mol⁻¹ lower in energy than with R Trolox in the square antiprismatic conformer. However, the opposite energy order was found for the less stable twisted-square-antiprismatic conformer, suggesting that the origins of observed chiral quenching behaviour are not associated with relative free energies of each exciplex, but with their relative rates of decay.