Mechanical Bond-Assisted Full-Spectrum Investigation of Radical Interactions

Molecular recognition, based on noncovalent bonding interactions, plays a central role in directing supramolecular phenomena in both chemical and biological environments. The identification and investigation of weakly associated recognition motifs, however, remains a major challenge, especially when the motifs are interlinked with and obscured by other robust binding modes in complicated systems. For example, although the host–guest recognition between the radical cations of both cyclobis(paraquat-p-phenylene) (CBPQT) and 4,4′-bipyridinium (BIPY) salts has been thoroughly investigated, the question of whether other binding modes exist between these two positively charged entities is the subject of some debate because of the complexity and dynamic nature of this supramolecular system. In order to address this conundrum, we have synthesized a [2]catenane─formed by mechanical interlocking between CBPQT and another BIPY-containing ring─which enhances the weak interactions between components and reduces significantly the complexity of the system for easier characterization. By employing this [2]catenane as a model compound, we have performed a full-spectrum investigation of radical interactions and revealed unambiguously a total of three possible binding modes between CBPQT and BIPY─to be specific, a bisradical tetracationic, a trisradical tricationic, and a bisradical dicationic association─as demonstrated by various methods of characterization including UV/vis/NIR, EPR, and NMR spectroscopies, electrochemical measurements and X-ray crystallography. The two newly discovered bisradical binding modes have potential applications in the construction of self-assembled materials and in mediating supramolecular catalysis. The mechanical bond-assisted approach used in this research is broadly applicable to investigating noncovalent bonding interactions.