Photoluminescent organic crystals and co-crystals

Crystal engineering is a potential strategy to design functional materials, including solid-state emitters. The design and development of organic emitters are of multi-fold significance as they are applied in fields ranging from biology to industry, defence and artificial intelligence. Herein, we discuss crystal engineering-based developments in crystalline organic solid-state emitters. We highlight the understanding and realization of emission tuning in multi-component crystalline materials and provide insights into their structure-property relationship, yielding important generalizations. Moreover, for the benefit of early researchers, a discussion on fundamental aspects such as mechanistic and crystal packing insights into solid-state emissions is provided. Subsequently, structure-property relationship studies are discussed considering the nature of pi-aggregates (H, J, X and I) and polymorphism. A comprehensive account of different classes of multi-component systems to realise emission tuning in organic crystals is provided, including recent advances such as the development of hetero-structures through epitaxial growth and hydrogen-bonded frameworks (HOFs), in addition to doping and co-crystallization. Finally, a concise summary and future outlook on the scope of crystal engineering intervention for the design of next-generation organic crystalline emissive materials are provided. This perspective highlights the significance of crystal engineering intervention in the design and development of applied optical materials.