The enantioselective excited-state quenching of r~c-Tb(2,6-pyridinedicarboxylate)~~- [A,A-Tb(DPA)33-] by A-(-)-Ru-(1,lO-phenanthroline):' [A-R~(phen)~~+] has been studied in aqueous solution (298 K) and in methanol (298 and 255 K) as a function of solution ionic strength. The ionic strength of methanol and water solutions were controlled through the addition of NaBr. The bimolecular rate constants for homochiral (A-A or A-A) and heterochiral (A-A) quenching were obtained by numerical fitting of the biexponential decay of Tb(DPA)3f luminescence in a series of time-resolved experiments.It is shown that the diastereomeric quenching reactions can be modeled by a kinetic scheme in which the luminophore and quencher form an encounter complex and then undergo a reactive step leading to energy transfer. In water, it is demonstrated that the ionic strength dependence of the quenching rates can be accurately described by the predicted ionic strength dependence of a pseudoequilibrium constant for the encounter complex. Experimental results for a wide range of ionic strengths are presented, and it is shown that the enantioselectivity of the quenching in water at room temperature does not depend on ionic strength. Results for enantioselective quenching in methanol (which are opposite in sign to those measured in water) do show variations in enantioselectivity at room temperature due to interference from racemization of the lanthanide complex, which can be suppressed at lower temperatures. The remaining ionic strength dependence is attributed to a situation in which the reaction becomes nearly diffusion controlled. A theoretical description of chiral recognition in both solvent system is developed through calculations of diffusion and dissociation rates using DebyeSmoluchowski and Eigen equations, respectively, and it is concluded that most of the observed enantioselectivity is due to chiral differences in the reactive step. The analysis also yields a charge product that is very close to the expected value of -6 for both solvent systems and a reactive distance of 1.1 nm. This latter result indicates that the observed quenching is due to very short-range interactions.
Circularly polarized luminescence is observed from racemic solutions of Tb( 2,6-pyridinedicarbo~ylate)~~ when small amounts of resolved Ru( 1 ,lO-phenanthr~line)~*+ are added. This phenomenon has been attributed to enantioselective quenching of the terbium complex by the ruthenium species, resulting in a nonracemic excited-state population. In this work, a simple kinetic scheme for this quenching process is presented, and it is demonstrated that the individual diastereomeric rate constants may be determined directly through numerical fitting of the biexponential decay of the total luminescence intensity. This is shown to give essentially the same results as numerical fitting of the time decay of the luminescence dissymmetry factor when data from both of these measurement techniques are available. It is also demonstrated that the enantioselectivity in the chiral quenching is solvent dependent. In aqueous solutions, for example, addition of A-(-)-Ru(phen)?+ results in an exam of L~-T~( D P A )~~-, Le., the A enantiomer is quenched more efficiently, whereas in methanol the opposite enantiomer is in excess.
Comparison is made between the enantioselective excited-state quenching of optically active tris-terdentate D3 complexes of Tb(II1) and Dy(II1) with 2,6-pyridinedicarboxylate by resolved Ru( l,lO-phenanthroline)3*+. It is found that the enantioselective quenching of the two species display almost identical dependence on temperature and solvent composition. The quenching is discussed within the contexts of a simple model involving encounter pair formation, reorientation, and energy transfer. From the resultsgiven, it isconcluded that theenantioselectivity in the quenching does not depend significantly upon the nature of the chiral electronic states of the donor lanthanide ions but rather has its origin in stereochemical structural aspects of the encounter pair formed between the oppositely charged chiral complexes.
The temperature dependence of the enantioselective quenching of racemic (A,A)-Tb(DPA)3j-(DPA = 2,6-pyridinedicarboxylate) by A-Ru( 1, 10-phenanthroline)32+ in HzO, D20, 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)33-. Analysis of the biexponential decay data in water (5-80 "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 "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 (A-A, homochiral quenching) is larger than the quenching of unlike enantiomers (A-A, 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.
An instrument is described for the measurement of circular and linear polarizations of the luminescence with a time resolution up to 50 ns. The spectrometer, which makes use of a 50-kHz photoelastic modulator and a differential photon-counting technique, is accurate and absolute and is capable of detecting small degrees of polarization. Making use of excitation light pulses which lie randomly in the modulator's duty cycle, the instrument can be readily interfaced with a variety of externally triggered or free running pulsed light sources.
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