Synthesis and Properties of (3-Phenyl-1-azulenyl)tetracyanobetadienes and Tris(aryltetracyanobetadiene)s Connected with 1,3,5-Tri(1-azulenyl)benzene Core

Tris[(3-phenyl-1-azulenyl)tetracyanobutadiene]s and tris(aryltetracyanobutadiene)s connected to a 1,3,5-tri(1-azulenyl)benzene core (tris-AzTCBDs) have been prepared by the Pd-catalyzed alkynylation of tris(3-iodo-1-azulenyl)benzene derivative with the corresponding 1-ethynylazulene or ethynylbenzene derivatives under Sonogashira–Hagihara cross-coupling conditions, followed by the reaction with tetracyanoethylene (TCNE) in a formal [2 + 2] cycloaddition-retroelectrocyclization (CA–RE) reaction. The intramolecular charge-transfer (ICT) interactions of the new chromophores were investigated by the comparison with those of the model tetracyanobutadiene derivatives with azulene substituents (AzTCBDs), which were prepared by the reaction of the corresponding 1-ethynylazulene derivative with iodoarenes, followed by the CA–RE reaction with TCNE, utilizing UV/Vis spectroscopy and theoretical calculations. The redox behavior of the alkyne precursors and the tetracyanobutadiene derivatives was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their multistep electrochemical reduction properties. Moreover, significant color changes of these compounds were observed by visible spectroscopy under the electrochemical reduction conditions.