Toward the peri-pentacene framework

Based on modern pyrene chemistry, a new route toward the synthesis of the peri-pentacene framework was developed. Firstly, the tetraketo form as stable precursor was targeted, because the fully aromatic peri-pentacene is predicted to suffer from high instability and reactivity. Therefore, a recently developed 4,5,9,10-tetrafunctionalization of pyrene was utilized to build up a planar and a dragonfly-shaped derivative in five steps, which only differ in the two outer bonds between the peripheral benzene rings. The planar tetraketone could only be dissolved in strong acids upon protonation, which limited the use of analytical methods. In contrast, the dragonfly-shaped form showed excellent organosolubility. In-depth investigation by NMR spectroscopy revealed co-existence of atropo-isomers in solution. The optical and electronic properties were analyzed by UV/Vis spectroscopy and cyclic voltammetry. To approach aromatic conjugation, stepwise reduction in solution by using tetrabutylammonium borohydride was carried out. Reduced species were studied by ESR and UV/Vis spectroscopy, as well as by MALDI-TOF MS, confirming the formation of the radical anion.