Aggregation‐Induced Emission in Bridged (<i>E</i>,<i>E</i>)‐1,4‐Diphenyl‐1,3‐butadiene Derivatives with Six‐ and Seven‐Membered Rings

Recently, we developed a new aggregation-induced emission (AIE) luminogen (AIEgen), bridged stilbene, by incorporating a propylene group into the C=C bond of the luminescent phenyl stilbene. This bridged structure, featuring a seven-membered ring, induces a significant conformational change, causing the C=C bond to twist in the excited state, thereby enhancing non-radiative decay in solution. In this study, we introduced bridged structures with alkylene groups of varying lengths into (E,E)-1,4-diphenyl-1,3-butadiene (DPB). The variation in the bridged structures of the synthesized DPB derivatives notably influenced the environmental sensitivity of fluorescence. Whereas the compound with two six-membered ring structures exhibited emission in solution and in the polycrystalline state, derivatives with a seven-membered ring exhibited AIE properties. Specifically, BDPB[7,7], featuring two seven-membered ring structures, demonstrated AIE characteristics with solid-state luminescence originating from J-aggregates. However, the fluorescence quantum yield was low in poly(methyl methacrylate) (PMMA) dispersion films, where molecular motion was restricted. These findings open new possibilities for designing unique AIEgens that remain nonluminescent even in highly viscous or confined environments, such as PMMA films.