Design Strategies, Properties, and Applications toward Cycloarenes and Heterocycloarenes

Abstract

Cycloarenes, fully benzene-annelated macrocyclic systems with inward-facing carbon–hydrogen bonds, serve as ideal models for defects in graphene, offering great application potential in organic electronics, supramolecular chemistry, and optics. They offer an attractive combination of synthesis challenge, aesthetic appeal, fundamental problems, and potential applications. Initially, the most typical cycloarene, kekulene, was expected to provide a crucial experimental test to determine whether π-electrons are delocalized over the entire molecule or delocalized at the benzenoid rings. This question has captivated synthetic chemists for decades. After numerous failed attempts, Staab and Diederich achieved the first conclusive synthesis of kekulene in 1978. The deshielded inner protons in the ¹H NMR spectrum conclusively demonstrated that the π-electrons in cycloarenes are delocalized at individual benzenoid rings. However, owing to limited synthetic methods, complex reaction routes, and poor solubility of the final products, progress in cycloarene research has been slow. Over the next four decades, only a few contracted or expanded kekulene homologues were reported. Nevertheless, the changes in their chemical structure bring some exciting physicochemical properties. The enlargement of the central ring of kekulene induces a transition from a planar to a saddle-shaped structure, further influencing its electronic and optical properties and unlocking unexpected applications in supramolecular chemistry. Therefore, developing new rational synthetic methods to controllably synthesize structurally diverse cycloarenes is crucial. With the continuous development of synthetic science, in recent years, some functional cycloarenes and heteroatom-embedded heterocycloarenes have been reported. Owing to their unique topological structures, well-defined cavities, and large cyclic conjugated systems, these (hetero)cycloarenes have been applied in fields such as supramolecular chemistry, organic field-effect transistors, and solar cells. However, the limited understanding of the structure–property relationship in (hetero)cycloarenes poses a formidable challenge to their custom synthesis for specific functions. Herein, we review our efforts in the design, synthesis, and applications of cycloarenes and heterocycloarenes. First, we summarize four representative synthetic methods for cycloarenes. Subsequently, we present a comprehensive overview of three molecular design strategies: π-extension, heteroatom embedding, and acceptor moiety insertion, to achieve the molecular structure diversity of cycloarenes. Then, we highlight their synthetic methods, geometries, fundamental optoelectronic properties, and unique applications in ultranarrowband emission, organic transistor devices, and supramolecular chemistry. We also delve into the intrinsic correlations among structures, properties, and applications of these cycloarenes and heterocycloarenes. Finally, we envision further development of structural diversity in cycloarenes and heterocycloarenes alongside their potential applications in sensing, phase-transfer catalysis, drug delivery, and various optoelectronic devices.