B-N Fused Anthracene as Functional Linker for π-Extended Viologens: Near-IR Emission and Electrochromism

As prototypical organic electrochromic materials viologens have been extensively studied in display technologies, smart materials, and energy storage applications. Their properties can be fine-tuned by introducing different substituents on the pyridine rings, fusion with heteroatoms, or insertion of π-conjugated linkers. In this article we study the effect of B-N fused dipyridylanthracene (BDPA) as a novel linker unit in viologens on the electronic structure, optical properties, and electrochromic characteristics. Quaternization of pyridyl-functionalized BDPA (1Py) by N-methylation or complexation with B(C₆F₅)₃ as a powerful Lewis acid gives rise to two fundamentally different π-extended viologens, dicationic [1Py-Me](PF₆)₂, and the neutral complex 1Py-BCF. We investigate the effect of these different quaternization methods on the LUMO energy, band gaps, absorption and emission, and the self-sensitized reactivity toward oxygen. We also demonstrate facile electrochemical reduction to singly and multiply reduced species. Spectroelectrochemical and computational studies reveal formation of strongly colored doubly reduced species with a closed shell electronic configuration and prominent quinoidal delocalization. The corresponding radical anions give rise to absorptions in the near-IR. A prototype electrochromic device with 1Py-BCF as the redox-active material is also presented.