THERMODYNAMICS OF THE ENANTIOSELECTIVE QUENCHING OF TB(2,6-PYRIDINEDICARBOXYLATE)3(3-) LUMINESCENCE BY RESOLVED RU(1, 10-PHENANTHROLINE)3(2+)

The temperature dependence of the enantioselective quenching of racemic (DELTA,LAMBDA)-Tb(DPA)3(3-) (DPA = 2,6-pyridinedicarboxylate) by DELTA-Ru(1,10-phenanthroline)3(2+) in H2O,D2O, and methanol is reported. Rate constants for the two diastereomeric quenching reactions may be determined from a nonlinear least-squares fit of the decay of the total luminescence from excited Tb(DPA)3(3-). Analysis of the biexponential decay data in water (5-80-degrees-C) indicates that the kinetics of the quenching reactions can be accurately described as a preequilibrium between isolated donor and quencher species and an associated ion pair, followed by energy transfer to the acceptor. In methanol a wider temperature range (-85 to 50-degrees-C) can be studied, and it is demonstrated that, dependent on solution ionic strength, at high temperatures the preequilibrium limit is appropriate, but at the lowest temperatures studied, the quenching reactions become entirely diffusion controlled, and the enantioselectivity vanishes. In all three solvents at the higher temperatures the overall quenching reactions are associated with negative activation energies. All of the quenching reactions may be analyzed within activated complex theory to yield activation energies, enthalpies, and free energies for the diastereomeric reaction schemes in both solvents. In addition, the temperature dependence of the equilibrium constant for ion-pair association has been determined. From all of these data, it is concluded that, based on enthalpy considerations, the quenching of like enantiomers (DELTA-DELTA, homochiral quenching) is larger than the quenching of unlike enantiomers (LAMBDA-DELTA, heterochiral quenching) but that entropy considerations favor the reverse preference. These generalizations are used to explain the observed differences in enantioselectivity observed in methanol and water.