Bisphosphonium Benzene Diimides

The incorporation of cationic groups onto electron-poor compounds is a viable strategy for achieving potent electron acceptors, as evidenced by reports of air-stable radical forms of large aromatic diimides such as naphthalene and perylene diimides. These ions have also been observed to exhibit anion-π interaction tendencies of interest in molecular recognition applications. The benefits of phosphonium incorporation, however, have not yet been extended to the smallest benzene diimides. Here, we report that dibrominated pyromellitic diimide and mellophanic diimide both readily undergo substitution reactions with phosphine sources to yield bisphosphonium compounds. In the single crystalline form, these dications display anion-π interactions and, in the case of mellophanic diimide, the stabilization of a bromide-water H-bonding ring pattern. The reaction of these dications with chemical reductants readily provides the singly and doubly reduced redox states, which were characterized by UV-vis spectroscopy and found to exhibit intense absorptions extending into the near-IR region. Taken together, this work demonstrates that phosphonium incorporation onto congested aromatic diimide scaffolds is synthetically viable and produces unusual electron-poor compounds.