Excimer-Induced Efficient Luminescence by Discrete Intermolecular π−π Stacking of Naphthalimide-Based Dimer

π−π stacking interactions are generally thought to reduce the luminescence of materials. Here, a systematic investigation is conducted using a π−π stacking dimer with varying steric hindrance substituents as a model to illustrate how π−π stacking structure affects the luminescence efficiency of materials. Four naphthalimide (NI) derivative molecules were designed and synthesized by incorporating sterically hindered unilateral groups to achieve NIPH, NIP1C, NIP2C, and NIP3C. It was figured out that side group modification did affect their crystal packing structures and luminescent properties. On the one hand, the excimer state formed by strongly interacted π−π NI-based dimer (NIPH and NIP3C) enhances luminescence efficiency compared to the monomer state based on weakly interacted π−π NI-based dimers (NIP1C and NIP2C). On the other hand, the discrete stacking of NI-based dimers (NIP3C) further promotes luminescence efficiency compared to the nondiscrete stacking of NI-based dimers (NIPH). Among these four compounds, NIP3C exhibits discrete stacking of π−π NI-based dimer due to the large steric hindrance generated by propyl benzene, resulting in the highest luminescence efficiency of the NIP3C crystal. This work will provide further insight into the underlying mechanisms behind the high luminescence efficiency induced by π−π dimer stacking.