The π bonds between organic radicals have generated excitement as an orthogonal interaction for designing self-assembling architectures in water. A systematic investigation of the effect of the viologen cation radical structure on the strength and nature of the pimer bond is provided. A striking and unexpected feature of this π bond is that the bond strength is unchanged by substitution with electron-donating groups or withdrawing groups or with increased conjugation. Furthermore, the interaction is undiminished by sterically bulky N-alkyl groups. Theoretical modeling indicates that strong dispersion forces dominate the interaction between the radicals, rationalizing the insensitivity of the bonding interaction to substituents: The stacking of polarizable π radicals leads to attractive dispersion forces in excess of typical dispersion interactions of small molecules and helps overcome the Coulombic repulsion of bringing two cationic species into contact.

The p bonds between organic radicals have generated excitement as an orthogonal interaction for designing self-assembling architectures in water.Asystematic investigation of the effect of the viologen cation radical structure on the strength and nature of the pimer bond is provided. As triking and unexpected feature of this p bond is that the bond strength is unchanged by substitution with electron-donating groups or withdrawing groups or with increased conjugation. Furthermore,t he interaction is undiminished by sterically bulky Nalkylg roups.T heoretical modeling indicates that strong dispersion forces dominate the interaction between the radicals, rationalizing the insensitivity of the bonding interaction to substituents:T he stacking of polarizable p radicals leads to attractive dispersion forces in excess of typical dispersion interactions of small molecules and helps overcome the Coulombic repulsion of bringing two cationic species into contact.The ability of open-shell species to form weak self-association complexes has attracted attention for av ariety of applications.I np articular,s tabilized organic p radicals can associate to form van der Waals complexes, s dimers,ormulticentered pi-bonded dimers.T he p bond in particular is afascinating one for its unusual multicenter bonding pattern that brings atoms closer than the van der Waals radii but longer than aconventional s bond (> 2.8 ), while straddling the knife edge between van der Waals interactions and conventional chemical bonds in strength and properties.Viologen cation radicals are aw ell-known class of thermally stable organic radicals that can be prepared simply by chemical or electrochemical reduction of synthetically accessible bipyridinium dications. [1] Within polar solvents that can screen the intermolecular Coulombic repulsion between like ions,t hese radicals dimerize to form such pbonded dimers,r eferred to whimsically as "pimers". p dimerization has been investigated for related p radicals, such as neutral p radicals (for example,phenalenyl radicals), [2] p anion radicals (tetracyanoethylene anion radical), [3] and cation radicals (biphenylene cation radicals), [4] but there are no detailed investigations into the effect of the radical structure on the strength and nature of the viologen cation radical p bond. Theability to switch this interaction on and off using redox control make this interaction av ersatile one for Figure 1. Library of viologen dications 1-19 synthesized as precursors to the viologen cation radicals.

A polymer containing viologen radical cation monomer units is shown to reversibly switch between paramagnetic and diamagnetic states via non-covalent host-guest interactions or temperature control in water. Cycling between diamagnetic and paramagnetic forms is accompanied by changes in optical and magnetic properties.