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Abstract Attainment of quantum‐confined materials with remarkable stoichiometric, geometric, and structural control has been made possible by advances in colloidal nanoparticle synthesis. The quantum states of these systems can be tailored by selective spatial confinement in one, two, or three dimensions. As a result, a multitude of prospects for controlling nanoscale energy transfer have emerged. An understanding of the electronic relaxation dynamics for quantum states of specific nanostructures is required to develop predictive models for controlling energy on the nanoscale. Variable‐temperature, variable‐magnetic field ( ) optical methods have emerged as powerful tools for characterizing transient excited states. For example, magnetic circular photoluminescence (MCPL) spectroscopy can be used to calculate electronic g factors, assign spectroscopic term symbols for transitions within metal nanoclusters, and quantify the energy gaps separating electronic fine‐structure states. spectroscopic methods are effective for isolating the carrier dynamics of specific quantum fine‐structure states, enabling determination of electronic relaxation mechanisms such as electron‐phonon scattering and energy transfer between assembled nanoclusters. In particular ‐MCPL is especially effective for studying electronic spin‐state dynamics and properties. This Review highlights specific examples that emphasize insights obtainable from these methods and discusses prospects for future research directions. 

The magneto-optical signatures of colloidal noble metal nanostructures, spanning both discrete nanoclusters (<2 nm) and plasmonic nanoparticles (>2 nm), exhibit rich structure-property correlations, impacting applications including photonic integrated circuits, light modulation, applied spectroscopy, and more. For nanoclusters, electron doping and single-atom substitution modify both the intensity of the magneto-optical response and the degree of transient spin polarization. Nanoparticle size and morphology also modulate the magnitude and polarity of plasmon-mediated magneto-optical signals. This intimate interplay between nanostructure and magneto-optical properties becomes especially apparent in magnetic circular dichroism (MCD) and magnetic circular photoluminescence (MCPL) spectroscopic data. Whereas MCD spectroscopy informs on a metal nanostructure's steady-state extinction properties, its MCPL counterpart is sensitive to electronic spin and orbital angular momenta of transiently excited states. This review describes the size- and structure-dependent magneto-optical properties of nanoscale metals, emphasizing the increasingly important role of MCPL in understanding transient spin properties and dynamics.