Aggregation-induced emission, multiple chromisms and self-organization of N-substituted-1,8-naphthalimides

Abstract

Most aggregation-induced emission (AIE) luminogens possess propeller-like aromatic stator-rotor structures, such as silole, tetraphenylethylene (TPE) and triphenylamine (TPA), to restrict intra-molecular motions (RIM) and avoid aggregation-caused quenching (ACQ) effects. In our work, two simple N-substituted naphthalimides were designed and synthesized by the amide condensation to create novel AIE systems. Except for the same aromatic stator (naphthalimide), N-phenyl-1,8-naphthalimide (PhNI) contained an aromatic rotor (phenyl group) while N-cyclohexyl-1,8-naphthalimide (CyNI) contained an aliphatic rotor (cyclohexyl group). The PhNI samples obtained from different preparation processes showed multiple chromisms (MC) effects and varied levels of luminescence. Despite of the same intra-molecular structure, the crystal of PhNI had staggered parallel inter-molecular conformation with weak emission but the precipitation of PhNI had cross one with strong emission. Although without MC effects, CyNI gave an excellent fluorescence quantum yield (Φf = 0.55). The experimental results and theoretical analyses suggested that the AIE effects were decided by intra- and inter-molecular structure simultaneously and the construction and destruction of inter-molecular π-π stacking between aromatic rotors in PhNI resulted in the MC effects.