Quadruple stacking of macrocyclic viologen radical-cations

The solid state structures of the di-cationic (1a) and radical mono-cationic (1b) redox forms of a 4,4'-bipyridinium (viologen) containing macrocycle have been established by single crystal X-ray analysis. Formation of the radical cation has a dramatic effect on the molecular structure of the macrocycle. Addition of an electron to 1a both decreases the diameter of the ring cavity (from 8.5 to 7.8 angstrom) and drastically flattens the torsion angle between the two pyridinium residues in the viologen, reducing it from 48 degrees to either 3 degrees or 9 degrees (two independent molecules). Moreover, a close association of these near-planar radical cation units drives the formation of a tetrameric supramolecular structure in the solid state.