Synthesis, properties, and redox behavior of tris(1-azulenyltetracyanobutadiene) and tris[1-azulenylbis(tetracyanobutadiene)] chromophores connected to a 1,3,5-tri(1-azulenyl)benzene core

Tris(1-azulenylacetylene) and tris(1-azulenylethynylarylacetylene) chromophores connected to a 1,3,5-tri(1-azulenyl)benzene core have been prepared by the Pd-catalyzed alkynylations of 1-ethynylazulene with tris(3-iodo-1-azulenyl)benzene or iodoarene derivatives substituted with a 1-azulenylethynyl group with tris(3-ethynyl-1-azulenyl)benzene under Sonogashira-Hagihara cross-coupling conditions. These compounds reacted with tetracyanoethylene in formal [2+2] cycloaddition-retroelectrocyclization reactions to afford the corresponding tris(1-azulenyltetracyanobutadiene) and tris[1-azulenylbis(tetracyanobutadiene)] chromophores in excellent yields. The redox behavior of the tetracyanobutadiene (TCBD) derivatives was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their multistep electrochemical reduction properties. Moreover, significant color changes were observed by visible spectroscopy under the electrochemical reduction conditions. Tris(1-azulenyltetracyanobutadiene) and tris[1-azulenylbis(tetracyanobutadiene)] chromophores with a C₃-symmetrical structure are prepared by the formal [2+2] cycloaddition-retroelectrocyclization reactions of tetracyanoethylene (TCNE) with the corresponding alkynes, and their redox behavior is examined by CV, differential pulse voltammetry (DPV), and spectroelectrochemical measurements.