Bis(azuleno)pentalenes with Six Consecutive Odd-Membered Rings.

Abstract

Non-alternant, non-benzenoid polycyclic hydrocarbons (PHs) exhibit unique optical and electronic properties distinct from their well-studied benzenoid π-conjugated counterparts. However, the synthesis of such π-systems, particularly those composed exclusively of odd-membered carbocycles, remains a significant challenge. Herein, we report the first synthesis of three diazuleno[a,e]pentalene derivatives (AP-1, AP-2, and AP-3), featuring fused five- and seven-membered rings, via a palladium-catalyzed homoannulation of o-alkynylazulenyliodide. These compounds possess a 24 π-electron system with six consecutively fused odd-membered rings in a 7-5-5-5-5-7 topology. Comprehensive characterization by X-ray crystallography, NMR spectroscopy, and theoretical calculations (ACID, NICS) reveals an electronic structure comprising two aromatic azulene units fused to a central antiaromatic pentalene core. The resulting molecules display a small HOMO-LUMO gap and amphoteric redox behavior, arising from the interplay between the aromatic and antiaromatic subunits. Notably, AP-1 and AP-2 can undergo reversible protonation and deprotonation processes upon exposure to acid and base. Furthermore, solution-processed organic field-effect transistors based on AP-2 show p-type semiconducting behavior with a maximum hole mobility of 0.72 cm2 V-1 s-1, demonstrating the potential of non-alternant, non-benzenoid PHs for applications in organic electronics.