EXCITED-STATE ENANTIOMER INTERCONVERSION KINETICS PROBED BY TIME-RESOLVED CHIROPTICAL LUMINESCENCE SPECTROSCOPY - THE SOLVENT AND TEMPERATURE-DEPENDENCE OF U(DPA)(3)(3-)REVERSIBLE-ARROW-DELTA-EU(DPA)(3)(3-) ENANTIOMER INTERCONVERSION RATES IN SOLUTION

Trisdipicolinate coordination complexes of europium(m), Eu(dpa)(3)(3-) , have chiral, three-bladed propeller-like structures with either left -handed or right-handed structural helicity about a S-fold symmetry ax is. The left- and right-handed forms of these structures, labeled here as Lambda and Delta, respectively, are structural enantiomers, and th ey exhibit characteristic circular dichroic absorption and emission pr operties in their optical excitation and luminescence spectra. In solu tion media that contain no other chiral species, the Eu(dpa)(3)(3-) co mplexes exist as a racemic mixture of interconverting Lambda and Delta enantiomeric structures (optical isomers). This racemic mixture of La mbda- and Delta-Eu(dpa)(3)(3-) complexes exhibits no chiroptical prope rties; but if it is excited with a pulse of circularly polarized light , partially resolved ground- and excited-state populations of Lambda a nd Delta enantiomers are created, and these populations exhibit chirop tical properties reflective of their enantiomeric compositions (define d in terms of enantiomeric excess quantities). In this study, time-res olved chiroptical luminescence (TR-CL) measurements are used to monito r changes in excited-state enantiomeric excess as a function of time a fter circularly polarized excitation, and the rate parameters derived from these measurements are related to the rate constants of Lambda re versible arrow Delta processes. Measurements are performed over a rang e of sample temperatures, and the temperature-dependent rate data are analyzed in terms of Arrhenius and Eyring activation models. Results a re reported for solution samples prepared in six different solvent med ia: H2O, D2O, an ethylene glycol-H2O mixture, CH3OH, C2H5OH, and CH3CN . These results show a significant solvent dependence both in the rate s, of h Lambda reversible arrow Delta enantiomer interconversion proce sses and in the thermal activation energies determined for these proce sses.