Dual‐State Emission Naphthalimide Dyes with 7‐Azabicyclo[2.2.1]Heptane as Auxochrome

Abstract Enhancing photoluminescence efficiency remains a perpetual objective for chemists. In this study, a molecular design concept of inhibited pyramidal inversion for the amino auxochrome is proposed. Dyes 3a‐3d are designed and synthesized by introducing 7‐azabicyclo[2.2.1]heptane (pyramidal inversion energy barrier 14.1 kcal mol −1 ) as auxochrome into the 1,8‐naphthalimide fluorophore framework. Dye 3a exhibits a high fluorescence quantum yield (Φ) of 0.963 in 1,4‐dioxane, maintains 0.746 in DMSO, and still achieves 0.284 in H 2 O, and dyes 3b‐3d demonstrate analogous solvent‐dependent behavior. Density functional theory (DFT) calculations reveal that the configuration of 3a remains nearly unchanged in both the ground and excited states. Remarkably, dyes 3a‐3d display bright solid‐state fluorescence with photoluminescence quantum yields (PLQY) of 0.674, 0.596, 0.813, and 0.924, respectively. Single‐crystal X‐ray analysis of 3a shows parallel molecular packing with an interplanar distance of 3.628 Å and an intramolecular auxochrome‐fluorophore dihedral angle of 83.35°, which collectively mitigate π–π stacking interactions. When dispersed in ethylene‐vinyl acetate (EVA) matrix, dyes 3a‐3d demonstrate PLQY values exceeding 0.947. These results show that incorporating 7‐azabicyclo[2.2.1]heptane as auxochrome in 1,8‐naphthalimide‐based fluorophores effectively enhances the luminescent properties of the dyes in solution, solid, and EVA‐film states.