Chiroptical spectroscopy of lanthanide complexes

Abstract

Chiroptical properties of lanthanide complexes are extraordinarily sensitive to electronic state structure, ligand coordination geometry, ligand stereochemistry, and interactions between a complex and its environment (in solution or in crystals). The chiroptical properties most frequently measured for lanthanide systems are circular dichroism (the differential absorption of left- and right-circularly polarized light), and circularly polarized luminescence (the differential spontaneous emission of left- and right circularly polarized light). All systems may exhibit circular dichroism (CD) and circularly polarized luminescence (CPL) in their absorptive and luminescent transitions when subjected to an externally applied magnetic field (aligned parallel to the direction of light propagation in a CD experiment and parallel to the direction of emission detection in a CPL experiment). However, in the absence of any externally applied fields, CD and CPL are exhibited only by systems that have net chirality in their structures or are subject to chiral perturbations by their environment. In lanthanide complexes, overall ( net) chirality may reflect chiral arrangements of ligands about the lanthanide ion and/or chiral conformations and atomic centers located within one or more coordinated ligands or chelate rings. Natural CD and CPL exhibited by the 4f-4f transitions of chiral lanthanide systems offer very sensitive probes of coordination and structure in solution media. Applications are limited to systems which possess some element of chirality, but in many cases this merely requires that at least one ligand of interest have a chiral atom ( e.g. an asymmetric carbon atom) or carry a chiral label (such as a chiral substituent group). Recent CD and CPL studies reported for chiral lanthanide complexes in crystals demonstrate the utility of chiroptical measurements for characterizing 4f N state structure and 4f-4f transition mechanisms.