Synthesis, stabilities, and redox behavior of mono-, di-, and tetracations composed of di(1-azulenyl)methylium units connected to a benzene ring by phenyl- and 2-thienylacetylene spacers. A concept of a cyanine-cyanine hybrid as a stabilized electrochromic system

This paper describes the preparation of two tetracations 4a(4+) and 4b(4+) composed of di(1-azulenyl)methylium units based on a new structural principle of a cyanine-cyanine hybrid for the design of electrochromic materials with two color changes. Di- and monocations 5a(2+), 5b(2+) and 6a+, 6b+ composed of di(1-azulenyl)methylium units were also prepared for the purpose of comparison. The pKR+ values of the tetracations are rather high despite their tetracationic structure, although the stability of these cations decreases with the increase of the number of the existing cation units. The cyclic voltammetry (CV) of these cations revealed the presumed multielectron redox properties. However, the tetracations did not exhibit the idealized electrochemical behavior, in which subsequent two-electron reduction was presumed as the cyanine-cyanine hybrid, probably due to the less effective electrochemical interaction among the positive charges. The scope of the creation of the novel polyelectrochromic materials taking the new structural principle is demonstrated by these examples.