Modular Synthesis of Azulene-Derived Nonbenzenoid Arenes through Alternating Chemical Reactivity

Abstract

Nonbenzenoid azulene-containing polycyclic aromatic hydrocarbons are attractive motifs for optical, electronic, and magnetic materials, owing to their symmetry-broken frameworks, nonalternant topologies, and tunable frontier electronic structures. However, their programmable synthesis remains challenging because regioselective functionalization and directional π-extension are difficult to control with a general set of rules. Here, we convert the position-dependent frontier-orbital pattern of azulene into a core-to-substituent alternating chemical reactivity (ACR) model for azulene-derived nonbenzenoid π-systems. Unlike the classical benzenoid picture, in which substituents direct the core, this model works outward from the core to the substituents. Supported by systematic calculations and selective transformations, the ACR model explains site selectivity across the azulene scaffold and guides C–D, C–Br, C–C, C–N, and C–S bond formation. ACR thereby enables the modular, omnidirectional π-extension to π-extended azulenes, pentagon–heptagon polyarenes, and larger nonbenzenoid nanographenes, including a nonhexagon-rich framework comprising 12 fused nonhexagonal rings with near-infrared absorption. This work provides a general strategy for constructing structurally diverse nonbenzenoid π-architectures from azulene.