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

Abstract 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.