Progress in nanotechnology is facilitated by the development of precise synthesis methods and the detailed characterization of fine and sophisticated nanomaterials. Ligand-protected metal clusters, a representative group of nanomaterials, have been the focus of numerous studies, resulting in the establishment of precise synthesis methods and the elucidation of their properties through synergistic experimental and theoretical investigations. Among these, ligand-protected metal clusters composed of noble metal elements, such as gold (Au), silver (Ag), and copper (Cu), exhibit unique properties according to the number of constituent atoms and dopant elements, including photoluminescence (PL), magnetic properties, and catalytic activity. These characteristics have attracted significant attention in both fundamental research and practical applications. Recently, studies have also been conducted on clusters with complex structures arising from combinations of noble metal elements and main group elements. Among these, Agsulfur (Ag-S) based clusters are metal clusters with cavities capable of encapsulating various anions, and recent efforts have focused on altering the encapsulated anionic species to modulate the physicochemical properties of these clusters. Ligand-protected metal clusters often exhibit PL. Recent experimental studies have shown that phosphorescence originates in the excited triplet state. Therefore, enhancing the population of the excited triplet state of metal clusters is crucial for their application as room-temperature phosphorescent materials and triplet sensitizers. Generally, transition from the excited singlet state (Sn) to the excited triplet state (Tm) is spin-forbidden, resulting in a zero transition-moment.