Captodative Approach to Stable Nitrogen-Centered Radicals, Anions, and Cations Exhibiting Near-Infrared Electrochromism

Abstract

Redox interconversion of the oxidation state of nitrogen via hydrogenation and dehydrogenation represents a powerful strategy for designing stimuli-responsive materials. In sharp contrast, the interconversion of nitrogen centers via electron transfer has been underexplored due to the high reactivity of aminyl radicals (R₂N^•), amide anions (R₂N^–), and nitrenium cations (R₂N⁺). Herein, we demonstrate that a captodative approach, i.e., the dual incorporation of electron-donating and electron-accepting units, is effective to stabilize these three classes of nitrogen-centered species within the same molecular scaffold. We synthesized a 9,10-dihydroacridine derivative with nitrogen-doping at the 9-position and imide-substitution at the 2,3- and 6,7-positions. This molecule afforded an aminyl radical upon hydrogen abstraction with PbO₂. The injection or removal of an electron of the aminyl radical furnished the corresponding amide anion and nitrenium cation, respectively. The aminyl radical, amide anion, and nitrenium cation exhibit significant stability under ambient conditions. Redox interconversion between the amide anion and nitrenium cation results in a drastic change in near-infrared (NIR) absorption due to switching of the local aromaticity of the central six-membered ring. These attractive properties lead to electrochromism in the NIR region (up to 1050 nm) between the closed-shell species.