| Chiroptical luminescence spectroscopic techniques are used to investigate solvent effects on enantio-preferential energy-transfer processes in solution. These processes are studied via luminescence quenching experiments carried out on samples which contain a racemic mixture of chiral energy-donor, or "luminophore", species in solution with a small, resolved concentration of a chiral energy acceptor, or "quencher", species. The luminophores are excited with a pulse of linearly polarized laser light, producing a racemic excited-state population. This racemic excited-state population may evolve into a non-racemic population in the presence of chiral discriminatory non-radiative energy transfer interactions between the chiral luminophore and quencher molecules. This nonracemic excited-state population will then emit light with a net circular polarization. By monitoring the time dependence of the emitted light intensity, as well as the degree and sense of circular polarization, chiral discriminatory interactions in the energy-transfer processes may be detected and characterized.; The luminophores used in this study are chiral lanthanide complexes, consisting of a terbium (III) or europium (III) ion surrounded by three dipicolinate dianion (≡pyridine-2,6-dicarboxylate) ligands. Chiral transition metal complexes, consisting of a central metal ion surrounded by three organic ligands, namely Co(R,R-dach)33+ where R,R-dach represents trans-1(R),2(R)-diaminocyclohexane, and Os(phen)3 2+, where phen represents 1,10-phenanthroline, are used as energy acceptors or "quenchers". A series of four alcohols (methanol, ethanol, 1-propanol, and 2-propanol) are investigated as solvents to determine how solvent properties influence the energy-transfer processes.; The enantio-preferential energy-transfer processes of interest here are found to be strongly dependent on both solvent and temperature. Different trends in the temperature dependence of quenching interactions are observed between the system using Co(R,R-dach)33+ as the quencher (or energy acceptor), and the system using Os(phen)3 2+ as the quencher. Additionally, differences are observed in the strength and temperature dependence of quenching interactions across the different solvents. As a result of these studies, conclusions can be drawn about how the strength and preference of enantiodifferentiation are related to solvent properties. |
