Orbitals of Molecular Ligands Control the Luminescent Excited States of Gold Clusters

Au clusters are between bulk materials and molecules, and their energy levels are partially discrete in nature, lying between band and discrete molecular structures. Therefore, degeneracy between the energy levels progresses, and multiple excited states form with energetically closed energy levels. The unique luminescence properties of pseudo-degenerate states give rise to unique luminescence properties such as thermally activated delayed fluorescence and multiemission, which have attracted attention for applications such as imaging and sensing. However, a method of controlling pseudo-degeneracy has not yet been established. In this study, Au clusters (Au36) were fabricated with different types of ligands (cyclopentanethiol, benzenethiol, and 2-naphthalenethiol) and different pseudo-degenerate states. In the Au36 protected by 2-naphthalenethiol ligands (Au36(NP)24), a charge transfer state was formed by the naphthalene orbital, which was energetically lower than the lowest unoccupied molecular orbital of Au36. The independent luminescence states generated by the naphthalene orbitals exhibited temperature-dependent emission changes. This "cluster’s ligand effect" represents a novel design strategy for metal clusters as luminescent functional materials and highlights the potential of utilizing pseudo-degenerate states in photochemistry.