The Viologen Cation Radical Pimer: A Case of Dispersion‐Driven Bonding

Abstract

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.

An “interaction-mediating” strategy towards enhanced solubility and redox properties of organics for aqueous flow batteries

“…After the first electron transition (stage "B"), a characteristic signal assigned to MV +• radical was observed, confirming the involvement of the MV 2+ → MV +• reaction. Near the end of the second charge plateau (stage "C"), an EPR signal with multiple peaks was observed, which implies the intermolecular interactions among the viologen units [54][55][56]. When discharging, the intensity of the multiple signal decreased (stage "D", with MV +• species dominant) and finally vanished at stage "E" (MV 2+ ).…”

Exploring Curious Covalent Bonding: Raman Identification and Thermodynamics of Perpendicular and Parallel Pancake Bonding (Pimers) of Ethyl Viologen Radical Cation Dimers

“…While the two forms have not yet been distinctly detected in solution, DFT calculations show that the parallel and perpendicular forms of viologen radical cation dimers are similar in energy. Previously, the perpendicular form was predicted to be approximately 1.5 kcal/mol higher in energy than the parallel form . At room temperature, it is therefore expected that viologen dimers will rotate around their dihedral angle.…”

“…Solution phase viologens have been extensively characterized by UV–vis − and resonance Raman spectroscopies , due to their blue color and by electron paramagnetic resonance (EPR) due to their radical nature. ,, The dimer has an absorption in the near-infrared and is EPR silent which makes it easily distinguishable from the monomer form. A recent study by Geraskina et al has identified by DFT that viologen dimers may exist as three rotamers in solution: parallel, perpendicular, and canted with intermolecular dihedral angles of approximately 0, 90, and 45°, respectively. The solid phase analogues of these structures have also been detected by X-ray crystallography and feature dihedral angles of ∼90, ∼0, or 33.5°. − In the solid state, viologen radical cations do not technically form dimers but rather higher order aggregates of V •+ interacting with each other in various motifs and surrounded by counter-anions.…”

Viologen‐Immobilized 2D Polymer Film Enabling Highly Efficient Electrochromic Device for Solar‐Powered Smart Window

“…[ 5a ] In this regard, much effort has been devoted to exploring the sophisticated viologen‐based EC materials through post‐modification of viologens, polymeric viologens, and the construction of composites with inorganic materials. [ 5a,b,9 ] However, the practical use of the viologen‐based materials for ECDs is still hampered by their long switching time (>3 s), and low CE (<500 cm 2 C −1 ) due to the sluggish ionic diffusion and limited charge transport through the random and chaotic pathways during the switching process. A promising scheme to address these challenges relies on developing viologen‐based EC films with ordered channels for fast ion diffusion.…”