Chemistry of Cyclo[18]Carbon (C18): A Review

Abstract Carbon‐based allotropes are propelling a technological revolution in communication, sensing, and computing, concurrently challenging fundamental theories of the previous century. Nevertheless, the demand for advanced carbon‐based materials remains substantial. The crux lies in the efficient and reliable engineering of novel carbon allotrope. Although C 18 has undergone theoretical and experimental investigation for an extended period, its preparation and direct observation in the condensed phase occurred only recently through STM/AFM techniques. The distinctive cyclic ring structure and the dual 18‐center π delocalization character introduce various uncommon properties to C 18 , rendering it a subject worthy of in‐depth exploration. In this context, this review delves into past developments contributing to the state‐of‐the‐art understanding of C 18 and provides insights into how future endeavours can expedite practical applications. Encompassing a broad spectrum, this review comprehensively investigates almost all facets of C 18 , including geometric characteristics, electron delocalization, bonding nature, aromaticity, reactivity, electronic excitation, UV/Vis spectrum, intermolecular interaction, response to external fields, electron affinity, ionization, and other molecular properties. Moreover, the review also outlines representative strategies for the direct synthesis and characterization of C 18 using atom manipulation techniques. Following this, C 18 ‐based complexes are summarized, and potential applications in catalysis, electrochemical devices, optoelectronics, and sensing are discussed.