The Effect of Excited State Energy Transfer on the Circularly Polarized Luminescence from Sol−Gels Containing Racemic Complexes of Eu(III):  Theory and Experiment

Excited state energy transfer between enantiomers in concentrated samples will diminish the differential excited state concentrations of a photoenriched racemic mixture, and this phenomenon will be reflected in a decrease in the observed circularly polarized luminescence as compared to low concentration samples where this energy transfer process can be neglected. Such a decrease is reported here for complexes of Eu(III) with 2,6-pyridine-dicarboxylate and 2,2‘-bipyridine-N,N‘-dioxide = bpyO2 ligands. This work represents the first experimental observation of this phenomenon for samples prepared in high concentration by incorporation into high-quality strain-free sol−gels. These results are fit to a theoretical model appropriate for randomly oriented and dispersed solute chromophores, and the Förster critical transfer distance is obtained. Limitations of the model and other experimental difficulties are described.