posted on 2018-12-03, 00:00authored byCongzhi Zhu, Xiaozhou Ji, Di You, Teresa L. Chen, Anthony U. Mu, Kayla P. Barker, Liana M. Klivansky, Yi Liu, Lei Fang
The introduction of B ← N
coordinate bondisoelectronic
to C–C single bondinto π-systems represents a
promising strategy to impart exotic redox and electrochromic properties
into conjugated organic molecules and macromolecules. To achieve both
reductive and oxidative activities using this strategy, a cruciform
ladder-type molecular constitution was designed to accommodate oxidation-active,
reduction-active, and B ← N coordination units into a compact
structure. Two such compounds (BN-F and BN-Ph) were synthesized via highly efficient N-directed borylation. These
molecules demonstrated well-separated, two reductive and two oxidative
electron-transfer processes, corresponding to five distinct yet stable
oxidation states, including a rarely observed boron-containing radical
cation. Spectroelectrochemical measurements revealed unique optical
characteristics for each of these reduced/oxidized species, demonstrating
multicolor electrochromism with excellent recyclability. Distinct
color changes were observed between each redox state with clear isosbestic
points on the absorption spectra. The underlying redox mechanism was
elucidated by a combination of computational and experimental investigations.
Single-crystal X-ray diffraction analysis on the neutral state, the
oxidized radical cation, and the reduced dianion of BN-Ph revealed structural transformations into two distinct quinonoid
constitutions during the oxidation and reduction processes, respectively.
B ← N coordination played an important role in rendering the
robust and reversible multistage redox properties, by extending the
charge and spin delocalization, by modulating the π-electron
density, and by a newly established hyperconjugation mechanism.