Selective access to either a doubly boron-doped tetrabenzopentacene or an oxadiborepin from the same precursor

The well-known red emitter tetrabenzo[de,hi,op,st]pentacene (TBPA) has been transformed into a bright blue emitter (B(2)-TBPA; λ(em) = 472 nm; c-hexane) via substitutional doping with two boron atoms. In contrast to the electron-rich TBPA, which forms endo-peroxides with O(2) under daylight, the benchtop-stable B(2)-TBPA is a good electron acceptor and undergoes reversible reduction at a moderate half-wave potential of E(1/2) = –1.73 V (vs. FcH/FcH(+); THF). Although the size of B(2)-TBPA falls within the nanoscale, the helically twisted compound readily dissolves in c-hexane and does not require solubilizing substituents. The synthesis of B(2)-TBPA is based on the nickel-mediated Yamamoto-type dehalogenation of tetrabrominated 9,10-di(naphth-1-yl)-9,10-dihydro-9,10-diboraanthracene. This intramolecular C–C heterocoupling reaction shows a remarkable solvent dependence: B(2)-TBPA forms only in pyridine (79% yield), whereas an oxadiborepin is obtained from THF solutions (ODBE, 81%; the reaction mixture is quenched with air in both cases). Insight into the corresponding reaction mechanism was gained from the isolation of intermediates and an investigation of their chemical properties. ODBE is an interesting blue emitter in its own right. Furthermore, it can be ring-opened with excess BBr(3) at the B–O–B moiety to afford a dimeric borabenzo[de]anthracene.