UV-vis-NIR magnetic linear dichroism: a powerful complement to MCD for f-block electronic structure

The ability to synthesize next-generation lanthanide and actinide molecular materials with designer photophysical properties rests squarely on our ability to predict, control, and measure their electronic structure. This is especially true of the crystal field (CF) interactions of the metal, which are the only interactions that can be appreciably tuned by ligand design. Herein we present ultraviolet-visible-near infrared magnetic linear dichroism (MLD) spectroscopy as an underutilized magneto-optical technique that holds immense promise in the elucidation of f-block electronic structure. We use a Pr^III polyoxometalate complex with pseudo-D _4d symmetry, [n-Bu₄N]₃[Pr{Mo₅O₁₃(OMe)₄(NO)}₂] (1·Pr), to demonstrate that acquisition of both magnetic circular dichroism (MCD) and MLD spectra allows definitive assignment of the observed CF levels through the complementary selection rules of these techniques. We provide general MCD and MLD sign patterns that can be applied to any (pseudo)-D _4d Pr^III complex to facilitate the assignment of fine structure. Our assignments for 1·Pr allow us to fit its transitions with a phenomenological Hamiltonian, providing insight into its CF splitting and solution geometry along with entirely experimentally-derived wavefunctions for its states without use of density functional theory or multireference computational techniques.