Oxidation State Effects on Decoherence in Isovalent 4f7 Complexes

The results of pulsed EPR measurements at W-band (94 GHz) on a series of isovalent 4f7 ions (Eu2+, Gd3+, Tb4+) are presented, constituting the first example to date of pulsed EPR on molecular Tb4+. An inverse correlation is found between the 5 K phase memory relaxation times (Tm) and the degree of zero-field splitting (ZFS) in these complexes. This observation reveals the role of ZFS as an effective reporter not only of structural distortions away from ideal cubic symmetry, but also of the overall electronic structure changes wrought by chemical oxidation, which undermine the seemingly advantageous increases in excited state energies and metal-ligand covalency across the series. Building upon previous studies of the physical properties of high oxidation state lanthanide complexes, these results further highlight the necessity for detailed electronic structure-property relationship studies to fully map the electronic consequences of chemical oxidation in the lanthanides.