Supramolecular radical polymers self-assembled from the stacking of radical cations of rod-like viologen di- and trimers

A series of π-conjugated oligomeric viologens (COVs) whose viologen segments are connected by phenyl or biphenyl units have been designed and synthesized. They exhibited interesting electrochemical and optical properties which are different from isolated viologen, as revealed by cyclic voltammetric and spectroscopic studies. The self-assembly behavior of the COV radical cations generated by treating COVs with sodium dithionite in aqueous media was studied systematically with UV-vis-NIR, electron paramagnetic resonance (EPR), dynamic light scattering (DLS), and cryo-transmission electron microscopy (cryo-TEM), which revealed that they self-assembled into linear supramolecular radical polymers driven by the dimerization of radical cations. Comprehensive DFT calculations for the COVs and their radical cations were also performed and their structure-property relationships were revealed.