The purification of individual rare earth metals, which is one of the most energy-intensive and waste-producing challenges in chemical processing, arises because of their similar chemical and physical properties. This project uses visible light to introduce chemical differences that facilitate the selective separation of rare earth metals. Mechanistic investigations into how visible electromagnetic radiation induces non-equilibrium processes and selective de-mixing of metal cation mixtures are sought. For instance, molecular photoswitches, which are light-responsive compounds that reversibly change structure and are hypothesized to give rise to proximate metal-binding behavior upon irradiation, are expected to lead to selective differences in reaction and transport rates that enable kinetically controlled separations. Expected outcomes include determination of how rare earth metals influence photo-switching behavior, identification of the excited-state energy transfer processes that govern selective responses to light, and elucidation of conditions where visible light drives selective partitioning of metal complexes across liquid interfaces. By linking photochemistry, metal ion coordination, and interfacial transport, fundamental principles for controlling separations using electromagnetic radiation as an external energy input are expected. The resulting knowledge could enable new separation strategies for critical metals while centrally advancing the fundamental mission of the Basic Energy Sciences Separation Science program.