Vis-NIR-Electrochromic Interconversion of Dithienylmethylium-Type Cyanine Dyes with a σ-Bonded Dimer or Isolable Neutral Radical

Abstract

Abstract 4-Methoxyphenyl-substituted di(2-thienyl)methyliums were designed and synthesized as a new class of cationic dyes exhibiting absorption in the visible (Vis) and near-infrared (NIR) region. Their cyanine-type π-conjugation was confirmed by alternation of positive charges along the polymethine chain as well as by a significant disappearance of bond alternation. Upon one-electron reduction of these cations, neutral radicals were generated, which could be isolated as stable entities when an anthryl-type substituent was attached to the central methine carbon while the secondary radical without any substituent quantitatively converted to a σ-bonded dimer. Since both the stable radical and the dimer regenerated the starting cyanine dyes upon oxidation, the present redox pairs can serve as electrochromic materials that exhibit a change in absorption in the Vis-NIR region.