Synthesis and Electronic, Optical, and Electrochemical Properties of a Series of Tetracyanobutadiene-Substituted Carbazoles

A series of tetracyanobutadiene (TCBD)-substituted carbazole chromophores were synthesized by formal [2+2] cycloaddition between tetracyanoethylene (TCNE) and carbazole derivatives containing an acetylenic moiety, followed by retro-electrocyclization. The reactivity of the carbazole varied depending on the conjugation connectivity between the carbazole and acetylenic moieties, the number of the carbazole moieties, and the length of the acetylenic moieties. All of the products feature intramolecular charge-transfer bands in the visible region. TCBD substitution at the 1-position of the carbazole gave rise to small HOMO–LUMO gaps compared to that at the 2- and 3-positions. The relationship between the TCBD substitution pattern of the carbazole moiety and the effectiveness of donor–acceptor π-conjugation was evaluated, and it was established that the π-conjugation was effective in order of positions 1<2<3.