CHIRAL DYNAMICS IN THE EXCITED-STATE OF A STEREOCHEMICALLY LABILE METAL-COMPLEX - ENANTIOMER INTERCONVERSION KINETICS, ENANTIOSELECTIVE QUENCHING, AND CHIROPTICAL ACTIVITY OF EU(CDA)36-IN H2O AND D2O1

Time-resolved chiroptical luminescence measurements are used to characterize the excited-state chiroptical activity and racemization kinetics of Eu(cda)36- (cda ≡ chelidamate) in H2O and D2O solutions at temperatures between 293 and 353 K. Racemic Eu(cda)36- is excited with circularly polarized light to create an enantiomeric excess of one optical (configurational) isomer in an excited electronic state, and then comparisons between time-resolved total luminescence and circularly polarized luminescence spectra are used to monitor the time dependence of the enantiomeric excess. Decay of the enantiomeric excess is related to interconversion of optical isomers (i.e., racemization) within the excited-state population of complexes, and rate constants are determined for the excited-state racemization of Eu(cda)36- in both H2O and D2O over a 60°C temperature range. Arrhenius parameters and thermodynamic activation parameters are derived from the temperature-dependent rate data, and the results obtained in H2O and D2O are compared and discussed. The racemization lifetimes (the reciprocal of the racemization rate constants) determined for 293 K solutions (16.9 and 79.5 ms in H2O and D2O, respectively) are long compared to the emission lifetimes (1.16 and 2.26 ms, respectively). The racemization process is interpreted in terms of an intramolecular mechanism without any (complete or partial) ligand dissociation. The complex passes through an achiral transition state of either D3h or C2v symmetry during interconversions between the two D3 enantiomers. Circularly polarized luminescence spectra are presented for the 7F0,1,2 ← 5D0 transition regions of europium(III) in Eu(cda)36-, and circularly polarized excitation spectra are reported for the 7F0,1 → 5D1 transition regions. The latter are analogous to circular dichroism spectra one would obtain from resolved (nonracemic) samples of Eu(cda)36- in solution. In our experiments, they are the consequence of chiral photoselection in the excitation of a racemic mixture with circularly polarized light. Time-resolved chiroptical luminescence measurements are also used to investigate enantioselective quenching of Eu(cda)36- excited-state populations by the chiral transition-metal complex Λ-Co(en)33+. It is shown that one excited-state enantiomer of Eu(cda)36- is quenched at twice the rate of the opposite enantiomer. The quenching is attributed to electronic-energy-transfer processes within Eu(cda)36--Co(en)33+ collisional complexes, and enantioselectivity in the quenching reflects chiral discriminatory contributions to intermolecular interactions between Λ-Co(en)33+ and the two enantiomers of Eu(cda)36-. © 1990 American Chemical Society.