Redox- and Protonation-Tunable Diboraheptacenes

Acenes are attractive molecular platforms with characteristic π-electron delocalization, resulting in exceptionally small HOMO-LUMO gaps, which makes them valuable for use in a variety of organic electronics. Incorporating heteroatoms into the backbones of acenes enables systematic tuning of their electronic structures and introduces well-defined sites for redox and protonation chemistry. However, the redox chemistry of boron-doped higher acenes remains poorly understood. Herein, we report the synthesis, structural characterization, and multistate redox and protonation chemistry of a series of diboraheptacenes. Starting from tetrahydrodiboraheptacene (1), the corresponding radical anion (1^•-) was generated via one-electron reduction. The fully aromatized diborataheptacene dianion (2^2-) was accessed via double deprotonation, which represents the first diboron-doped π-isostere of all-hydrocarbon heptacene. Two-electron oxidation of 2^2- gave a neutral quinodimethane (2) that is isoelectronic with the heptacene dication, demonstrating redox-interconversion between benzenoid and quinoidal structures. Dianion 2^2-, isoelectronic to heptacene, features NIR absorption at λ_abs = 951 nm, reflecting its exceptionally small HOMO-LUMO gap. In addition, 2 shows remarkable low-energy electronic transitions, displaying red fluorescence (λ_em = 682 nm). Furthermore, reactions of 2^2- with CO₂ or acrylonitrile yield cycloaddition products, while the reaction with H₂O affords a hydroxylated diboraheptacene dianion. These results establish redox- and protonation-state-dependent electronic structures, photophysical properties, and reactivity of an extended diboraacene platform.