Synthesis of 2-azulenyl-1,1,4,4-tetracyano-3-ferrocenyl-1,3-butadienes by [2+2] cycloaddition of (ferrocenylethynyl)azulenes with tetracyanoethylene.

1-, 2-, and 6-(Ferrocenylethynyl)azulene derivatives 10-16 have been prepared by palladium-catalyzed alkynylation of ethynylferrocene with the corresponding haloazulenes under Sonogashira-Hagihara conditions. Compounds 10-16 reacted with tetracyanoethylene (TCNE) in a [2+2] cycloaddition-cycloreversion reaction to afford the corresponding 2-azulenyl-1,1,4,4,-tetracyano-3-ferrocenyl-1,3-butadiene chromophores 17-23 in excellent yields. The redox behavior of the novel azulene chromophores 17-23 was examined by using 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 electrochemical reduction conditions.