Extraordinary Redox Activities in Ladder-Type Conjugated Molecules Enabled by B ← N Coordination-Promoted Delocalization and Hyperconjugation
journal contributionposted on 2018-12-03, 00:00 authored by Congzhi 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.
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BN-Phredox stateredox propertiesisosbestic pointscoordinationreduction processesoxidative activitiesquinonoid constitutionsExtraordinary Redox ActivitiesDistinct color changesbondcruciform ladder-typeSingle-crystal X-ray diffraction analysiselectrochromic propertiesLadder-Type Conjugated Molecules Enabledoxidation statesstrategyhyperconjugation mechanismoxidative electron-transfer processescationBN-Fπ- electron densitySpectroelectrochemical measurementsmoleculeabsorption spectraredox mechanismN-directed borylation