Creation of Stabilized Electrochromic Materials by Taking Advantage of Azulene Skeletons

The preparative scope of the new structural principle of utilization of azulene skeletons as redox-active chromophores for the production of stabilized electrochromic materials is demonstrated in several examples. The structural principle takes advantage both of the cyanine-type polymethine dyes and of the violene-type redox systems. Appropriate combination of cyanine and/or violene substructures will produce at least one cyanine-type structure in the reduced or oxidized form through an overall two-electron transfer, although some will produce a cross-conjugated cyanine-type structure. Hunig's violene-cyanine hybrid structures, utilizing the bis(enediyne) system, afford electrochromic materials that exhibit strong absorptions in the near-infrared region in the dianionic state. Cyanine-cyanine hybrid structures, consisting of pairs of cyanine units, show two-step color changes based on single electron transfer in each step. Combinations of the cyanine and/or the violene substructures permit the design of novel stabilized electrochromic systems, including polyelectrochromic systems. The flexibility of the structural principle is demonstrated by the violene-violene hybrid, violene-cyanine-cyanine hybrid, violene-cyanine-violene hybrid, and violene-violene-violene hybrid structures. New examples of the violene-like redox system with inverse electron demand are provided by 6-aminoazulene derivatives. The liquid-crystalline properties of the redox-active columnar mesogen are revealed by several azulene-substituted electrochromic materials. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)