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 racemi c (DELTA,LAMBDA)-Tb(DPA)3(3-) (DPA = 2,6-pyridinedicarboxylate) by DEL TA-Ru(1,10-phenanthroline)3(2+) in H2O,D2O, and methanol is reported. Rate constants for the two diastereomeric quenching reactions may be d etermined 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 t he quenching reactions can be accurately described as a preequilibrium between isolated donor and quencher species and an associated ion pai r, followed by energy transfer to the acceptor. In methanol a wider te mperature range (-85 to 50-degrees-C) can be studied, and it is demons trated that, dependent on solution ionic strength, at high temperature s the preequilibrium limit is appropriate, but at the lowest temperatu res studied, the quenching reactions become entirely diffusion control led, and the enantioselectivity vanishes. In all three solvents at the higher temperatures the overall quenching reactions are associated wi th 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 scheme s in both solvents. In addition, the temperature dependence of the equ ilibrium constant for ion-pair association has been determined. From a ll of these data, it is concluded that, based on enthalpy consideratio ns, the quenching of like enantiomers (DELTA-DELTA, homochiral quenchi ng) is larger than the quenching of unlike enantiomers (LAMBDA-DELTA, heterochiral quenching) but that entropy considerations favor the reve rse preference. These generalizations are used to explain the observed differences in enantioselectivity observed in methanol and water.