B₂,N₄-Doped Heptacenes: Ambipolar Charge-Transfer Compounds with Deep LUMO Levels

The B₂,N₄-doped heptacene H4 in which two N,N'-dihydrophenazine units are linked by two BMes bridges (Mes = mesityl) was synthesized via fourfold Buchwald-Hartwig coupling between 2,3,6,7-tetrachloro-9,10-dimesityl-9,10-dihydro-9,10-diboraanthracene and o-phenylenediamine (tBuXPhos-Pd-G3, DBU/NaOTf, 2-MeTHF, 50 °C, 16 h). Upon exposure to ambient air, H4 is oxidized to its N,N'-dihydro form H2; further oxidation with MnO₂ furnishes the di(phenazine) derivative H0. Stirring under a blanket of H₂ in the presence of Pd/C hydrogenates H0 back to H2 and ultimately H4. Yellow-colored H0 is a remarkably good electron acceptor with a LUMO-energy level of -3.9 eV; upon irradiation with a 405 nm LED in the presence of THF or 1,4-cyclohexadiene, H0 accepts two H atoms to furnish H2. One-electron reduction of H0 yields the isolable radical-anion salt Li[H0] (lithium naphthalenide, THF, -30 °C to rt). The ambipolar compounds H2 and H4 possess a navy blue and deep purple color, respectively, due to charge-transfer interactions from the electron-rich N,N'-dihydrophenazine donor(s) to the electron-accepting B₂C₄ core.