Excited-state racemization kinetics and chiroptical activity of a labile metal complex in aqueous solution. Time-resolved circularly polarized luminescence study of tris(dipicolinato)europate (3-) in water and deuterium oxide

Abstract

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~i s 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(dpa)33" in both H20 and D20 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 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~i n solution. In our experiments, they are the consequence of chiral photoselection in the excitation of a racemic mixture with circularly polarized light.Detailed electronic and stereochemical structure information about lanthanide complexes in solution is elusive. This is particularly true for aqueous solutions wherein most lanthanide complexes exhibit both constitutive and 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-* Authors to whom correspondence should be addressed.

Survey of factors determining the circularly polarised luminescence of macrocyclic lanthanide complexes in solution

“…[1] and [3] . Examining each as the triflate salt in acetonitrile solution (or water), the greater the degree of conformational rigidity in the complex, the higher was the measured g em value Table 1.…”

“…1 To some extent this lack of knowledge has inhibited the development of emissive lanthanide complexes as structural or reactive probes to signal the nature of the local ionic or chiral environment. The reason why our understanding remains very limited, notwithstanding the promulgation of robust theories relating local lanthanide symmetry to the electronic and chiroptical features associated with f-f transitions, 2À4 is linked directly to the absence of well-defined complexes whose structure and solution exchange dynamics have been studied in detail.…”

Lanthanide complexes as chiral probes exploiting circularly polarized luminescence

“…However, it should be noted that whilst this method has been demonstrated successfully with ligands such as L 12 , recent studies involving complexes of this ligand have reported the presence of two species in solution. [89][90][91] NMR experiments have shown that these species may be thought of as regioisomers, with their existence being attributed to the fact that it is possible for the ring substituent to be located in one of two nonequivalent positions (Figure 1.20).…”

“…It is important to highlight that a number of metalloproteins and vitamin B 12 derivatives have also been found to be capable of inducing a CPL signal from [Ln(DPA) 3 ] 3-in solution via this mechanism. [122][123][124] In the case of the corrinoid vitamin B 12 and its derivatives, the observation of an enantioselective quenching effect has been attributed to energy transfer in a hydrogen-bonded encounter complex.…”

“…[122][123][124] In the case of the corrinoid vitamin B 12 and its derivatives, the observation of an enantioselective quenching effect has been attributed to energy transfer in a hydrogen-bonded encounter complex. 125 It has been demonstrated that these enantioselective quenching effects can be exploited to provide information about the accessibility of this chromophore and therefore may be used to investigate and monitor processes such as its binding to proteins.…”