Azulene‐Fused Polycyclic Aromatic Hydrocarbons: Synthesis, Structural, Optical, and Electrochemical Properties of Azuleno[1,2,3‐ cd ]Benzo[ mn ]Pyrene‐8,14‐diones and Cyclohepta[1,2]Indeno[6,5,4‐ de ]Anthracen‐9‐ones

We report a modular synthesis of anthracene‐based azulene‐fused polycyclic aromatic hydrocarbons (PAHs) from 2‐chloroazulenes via Suzuki–Miyaura coupling with 9‐anthracenyl organoboron reagents, followed by Brønsted acid‐promoted intramolecular annulation. The cyclization pathway was significantly influenced by the reaction medium: Eaton's reagent afforded doubly annulated products, whereas methanesulfonic acid preferentially gave mono‐cyclized products through the annulation accompanied by decarboxylation. In one notable case, additional Scholl‐type cyclization resulted in the formation of a more extensively fused naphtho[2,1‐ a ]aceanthrylene analogue. Spectroscopic, crystallographic, and TD‐DFT studies revealed that the appended 10‐aryl substituents adopt orientations that are almost perpendicular to the fused core. This means that they have little influence on the neutral‐state frontier electronic structure. Accordingly, most derivatives showed similar visible absorption profiles, whereas naphtho[2,1‐ a ]aceanthrylene analogue exhibited substantially red‐shifted absorption because of its expanded fused π‐system. Under acidic conditions, both series displayed pronounced halochromic and fluorochromic behavior, including long‐wavelength to near‐infrared emission, and 1 H NMR spectral analysis indicated preferential protonation at the carbonyl oxygen rather than the azulene ring. Electrochemical and spectroelectrochemical studies further demonstrated reductive electrochromism, with the naphtho[2,1‐ a ]aceanthrylene analogue showing particularly high redox reversibility and spectral recovery. These findings identify medium‐controlled annulation as a versatile platform for curved, acid‐responsive, redox‐active nonbenzenoid PAH systems.