Ionic strength dependence of the enantioselective quenching of tris(2,6-pyridinedicarboxylate)terbium(3-) luminescence by resolved tris(1,10-phenanthroline)ruthenium(2+)

Abstract

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.

Chiral Recognition of Amino Acids Based on Cyclodextrin‐Capped Quantum Dots

“…Much attention has been paid recently to the development of synthetic molecular receptors with the ability for chiral recognition 3. Receptors with fluorescence signaling modes are preferred owing to the advantages of the technique, such as practical convenience, high sensitivity, low cost, and wide applicability along with the multisignaling modes associated with the technique 4…”

Lanthanide complexes as chiral probes exploiting circularly polarized luminescence

“…[112][113][114][115] In addition, the effect of sample variables such as solvent, temperature, pressure and ionic strength has also been assessed. [116][117][118][119][120] Results observed in all of these studies have been analysed on the basis that quenching occurs via an energy transfer process, typically assumed to involve diastereomeric Ln * -Q encounter complexes, which have non-identical relative stabilities and energy transfer efficiencies.…”

Intermolecular chiral recognition processes probed by chiroptical luminescence

“…The influence of solvent on enantiopreferential quenching kinetics has been characterized for Ln(dpa) 3À 3 -Co(R,R-chxn) 3þ 3 systems in several amphiprotic solvents. [5][6][7][8]10 The results from previous studies combined with the results obtained in the present study provide an opportunity to compare the chirality-dependent quenching kinetics of Ln(dpa) The primary focus of the work presented in this article is to determine the sensitivity of quenching rate parameters (and enantioselectivity) to a change in solvent type (amphiprotic vs. aprotic solvent), and to changes in the cation of the Ln(dpa) ). Time-resolved chiroptical luminescence (TR-CL) measurement techniques are used to observe the Eu(dpa) 3À 3 -Co(R,R-chxn) 3þ 3 and Tb(dpa) 3À 3 -Co(R,R-chxn) 3þ 3 quenching processes, and the results of these studies are analyzed using a phenomenological model of the enantiopreferential quenching kinetics.…”