Azulene-Fused Polycyclic Aromatic Hydrocarbons: Synthesis, Structural, Optical, and Electrochemical Properties of...

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 ¹H 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.