Structural insights into the aggregation-induced emission mechanism of naphthalene diimide solids

Abstract Naphthalene diimides (NDIs) are an interesting class of electron-poor organic dyes but with weak emissions that limit their applications. Recently several examples have demonstrated that self-assembly of such weak emissive NDIs into some special aggregates could enhance their emissions. However, a detailed emission mechanism remains unexplored due to the lack of structural evidences. In order to go deep into the emissive mechanism and in turn guide material design, it is necessary to learn the molecular arrangement in the aggregates. For this purpose, in this paper we used a crystal engineering strategy to explore the structure-property relationships between the molecular packing and emissive properties of NDI derivatives in crystalline state, which revealed that the small slipped cofacial π-π stacking of NDI cores in the solid states answered for their enhanced emission behaviors. This observation has an extensive applicability and practical significance for design of NDI based aggregation-induced emission materials.