Synthesis of Redox‐Active, Intramolecular Charge‐Transfer Chromophores by the [2+2] Cycloaddition of Ethynylated 2 H ‐Cyclohepta[ b ]furan‐2‐ones with Tetracyanoethylene

Ethynylated 2 H ‐cyclohepta[ b ]furan‐2‐ones 5 – 15 have been prepared by Pd‐catalyzed alkynylation of 3‐iodo‐5‐isopropyl‐2 H ‐cyclohepta[ b ]furan‐2‐one ( 2 ) with the corresponding ethynylarenes or the reaction of 2‐iodothiophene with 3‐ethynyl‐5‐isopropyl‐2 H ‐cyclohepta[ b ]furan‐2‐one ( 4 ) under Sonogashira–Hagihara conditions. Compounds 5 – 15 reacted with tetracyanoethylene in a formal [2+2] cycloaddition reaction, followed by ring opening of the initially formed [2+2] cycloadducts, cyclobutenes, to afford the corresponding 1,1,4,4‐tetracyanobutadienyl (TCBD) chromophores 16 – 26 in excellent yields. The intramolecular charge‐transfer interactions between the 2 H ‐cyclohepta[ b ]furan‐2‐one ring and TCBD acceptor moiety were investigated by UV/Vis spectroscopy and theoretical calculations. The redox behavior of the novel TCBD derivatives 16 – 26 was examined by cyclic voltammetry and differential pulse voltammetry, which revealed multistep electrochemical reduction properties, depending on the number of TCBD units in the molecule. Moreover, a significant color change was observed by UV/Vis spectroscopy under electrochemical reduction conditions.