Time-resolved circularly polarized luminescence (TR-CPL) measurements are used to characterize the excited-state chiroptical activity and racemization kinetics of Eu (dpa) 33~(dpa= dipicolinate) in H20 and D20 solutions at temperatures between 293 and 353 K. Racemic Eu (dpa) 33~ is excited withcircularly 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 (ie, racemization) within the excited-state population of complexes, and rate constants are determined for the excited-state racemizationof Eu (dpa) 33 “in both H20 and D20 over a 60 C temperature range. Arrhenius parametersand thermodynamic activation parameters are derived from the temperature-dependent rate data, and the results obtained in H20 and D20 are compared and discussed. The racemization lifetimes (the reciprocal of the racemization rate constants) determined for 293 K solutions (31.6 and 45.5 ms in H20 and D20, respectively) are long compared to the emission lifetimes (1.60 and 3.19 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 Dih or C^, symmetry during interconversions between the two D} enantiomers. Circularly polarized luminescence spectra are presented for the 7F0i1i2<-5D0 transition regions of europium (III) in Eu (dpa) 33", and circularly polarized excitation spectra are reported for the 7F0, i-*• 5D, transition regions. The latter are analogous to circular dichroism spectra one would obtain from resolved (nonracemic) samples of Eu (dpa) 33~ in solution. In our experiments, they are the consequence of chiral photoselection in the excitation of a racemic mixture withcircularly polarized light.

Detailed electronic and stereochemical structure information about lanthanide complexes in solution is elusive. This is par-ticularly true for aqueous solutions wherein most lanthanide complexes exhibit both constitutiveand stereochemical lability. Constitutive lability most often reflects ligand-solvent molecule or bound ligand-free ligand exchange processes that produce changes in the chemical composition of the inner-coordination sphere, and stereochemical lability reflects configurational and/or conformational isomerization processes within the inner-coordi-