Recent Advances in Understanding Triplet States in Metal Nanoclusters: Their Formation, Energy Transfer, and Applications in Photon Upconversion

Recent experimental findings, rapidly accumulating over the past few years, has revealed that in the electronic excited states of metal nanoclusters (MNCs) composed of noble metal atoms (e.g., Cu, Ag, or Au), triplet states are generated with remarkably high efficiency, exerting a pivotal influence over the photophysical properties of the MNCs, notably their photoluminescence characteristics. As a result, MNCs are increasingly recognized as promising luminescent nanomaterials that exhibit room-temperature phosphorescence and thermally activated delayed fluorescence. Furthermore, the significance of triplet-state-mediated energy transfer and charge transfer in intermolecular photophysical processes is gaining increasing recognition, particularly in the applications of MNCs as photosensitizers for singlet oxygen and organic molecular triplets. This Perspective focuses on recent advances in understanding of the formation and photophysics of triplet states in MNCs. Additionally, a brief overview is provided of a series of studies exploring the use of MNCs as triplet sensitizers for photon upconversion via triplet-triplet annihilation, and future prospects for this emerging application are discussed.