Twin-rotor system created on a [4]radialene frame.

[reaction: see text] Three extended [4]radialenes with two tricyclic rings connected with exocyclic butatriene units have been synthesized. The compounds, possessing thioxanthene and dihydroanthracene moieties as the terminal substituents, show a fast rotation around the butatriene bonds at ambient temperatures (DeltaG() = 13.7 and 14.9 kcal/mol, respectively). In contrast, the fluorene-substituted analogue shows a much higher rotational barrier (DeltaG() = 17.8 kcal/mol) of the butatriene bonds due to the reduced steric repulsion between the two fluorene moieties at the ground state.