Preparation and Properties of a Polycyclic p-Quinodimethane with Two Oxygen Bridges and its Radical Cation in Comparison with the Isomeric o-Quinodimethane.

We prepared an electron-rich p-quinodimethane compound, 2,9-dibromo-7,14-dimesityl-5,12-dioxapentacene (1). The quinoidal character was clarified by bond-length alternation in the central ring. Clear vibrational structures in both absorption and emission bands and small Stokes shift indicate the rigid dioxapentacene framework. The reversible redox waves of 1 were observed at +0.14 and +0.69 V vs. Fc+ /Fc, respectively. The radical cation salt 1⋅+ ⋅SbF6 - was successfully isolated and characterized by means of X-ray structural analysis and ESR and UV/Vis-near infrared absorption spectroscopies. The molecular structure and electronic properties of the neutral and radical cation species were compared to those of the isomeric o-quinodimethane.