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 (22–) was accessed via double deprotonation, which represents the
first diboron-doped π-isostere of all-hydrocarbon heptacene.
Two-electron oxidation of 22– gave a neutral quinodimethane (2) that is isoelectronic
with the heptacene dication, demonstrating redox-interconversion between
benzenoid and quinoidal structures. Dianion 22–, isoelectronic to heptacene, features NIR
absorption at λabs = 951 nm, reflecting its exceptionally
small HOMO–LUMO gap. In addition, 2 shows remarkable
low-energy electronic transitions, displaying red fluorescence (λem = 682 nm). Furthermore, reactions of 22– 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.