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 H₂O, 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.