Secondary Coordination Sphere‐Directed Design of Thermo‐Responsive and Bright Luminescent Manganese(II) Complexes

Luminescent Mn II complexes have attracted considerable attention as low‐cost and earth‐abundant alternatives to precious metal‐based emitters. However, their development is limited by certain challenges, including severe thermal quenching, moisture sensitivity, and a lack of efficient strategies for generating stimuli‐responsive luminescence. In this study, these issues are addressed by decorating the secondary coordination sphere of Mn II complexes, thereby enhancing their emission efficiency and stimuli‐responsive behavior. These Mn II complexes can be easily synthesized by manual mechanochemical reactions. Among them, the bulkiness‐introduced complexes [MnBr 2 {OP( o ‐Tol) 3 } 2 ] ( o ‐Tol = o ‐tolyl) and [MnBr 2 {OP( o ‐An) 3 } 2 ] ( o ‐An = o ‐anisyl) exhibit high emission quantum yields of 0.86 and 0.78, respectively. Furthermore, [MnBr 2 {OP( o ‐An) 3 } 2 ] exhibits thermochromic luminescence owing to excited‐state interactions between the Mn II center and the O atom of the methoxy group in the secondary coordination sphere. The results of this study confirm the potential of secondary coordination‐sphere engineering as an effective strategy for optimizing Mn II luminescence, thereby offering new design principles for stimuli‐responsive photofunctional materials.