Synthesis and Properties of Mono-, Bis-, Tris-, and Tetrakis[1,1,4,4-tetracyano-2-(1-azulenyl)-1,3-butadien-3-yl] Chromophores Connected to a Benzene Ring by Phenylethynyl- and 2-Thienylethynyl Spacers

Mono-, bis-, tris-, and tetrakis[(1-azulenylethynyl)phenylethynyl- and (1-azulenylethynyl)-2-thienylethynyl]benzenes have been prepared by Pd-catalyzed alkynylation of iodobenzene derivatives or 2-iodothiophene derivatives substituted by 1-azulenylethynyl group with polyethynylbenzenes under Sonogashira-Hagihara cross-coupling conditions. These compounds reacted with tetracyanoethylene (TCNE) in a formal [2 + 2] cycloaddition-cycloreversion reaction to afford the corresponding 1,1,4,4-tetracyano-2-(5-isopropyl-3-methoxycarbonyl-l-azulenyl)-1,3-butadien-3-yl chromophores in excellent yields. The redox behavior of the novel azulene-substituted tetracyanobutadiene (TCBD) derivatives was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their multistep electrochemical reduction properties. Moreover, a significant color change was observed by visible spectroscopy under the electrochemical reduction conditions.