An efficient synthesis of highly π-extended carbazoles is described via an unexpected domino [5 + 3 + 1] annulation approach. The Cu(I)-/base-promoted reactions of 2-nitrocinnamaldehydes with benzyl cyanides provide diverse benzo[ b]carbazoles. The reaction is proposed to proceed via a sequential Michael addition/intramolecular addition of an enol into a nitro group, 6π- electrocyclization, and the final oxidative aromatization as supported by density functional theory calculations. Some of the synthesized carbazoles showed significant potential in fluorescence sensing of Cu2+ ions.