Pentaleno[1,2‑a:4,5′]diacenaphthylenes: Uniquely Stabilized Pentalene Derivatives

We demonstrate the preparation of diacenaphtho­pentalene derivatives via a palladium-catalyzed dimerization of 1-iodo-2-arylethynyl-acenaphthylenes. The resulting 7,14-diaryl­pentaleno­[1,2-a:4,5a′]­diace­naphthylenes, which contain four linearly fused five-membered rings, are benchtop stable and behave as hole-transporting or ambipolar semiconductors in organic field effect transistors. The X-ray crystal structure shows the important role of the fused naphthalene unit that enforces a formal pentalene subunit at the central five-membered rings and [5]-radialene-like structures at the proximal five-membered rings. Nucleus-independent chemical shift (NICS) calculations show the internal pentalene rings are intermediate in antiaromaticity character between known pentalene and dibenzo­pentalenes derivatives. The diace­naphtho­pentalene derivatives give high optical gap materials owing to a forbidden HOMO to LUMO transition, yet have narrow electrochemical gaps and are reduced at small negative potentials giving LUMO energy levels of −3.57 to −3.74 eV.