"Vermellogens" and the Development of CB[8]-Based Supramolecular Switches Using pH-Responsive and Non-Toxic Viologen Analogues.

We present herein the "vermellogens", a new class of pH-responsive viologen analogues, which replace the direct linking between para -substituted pyridinium moieties within those by a hydrazone functional group. A series of such compounds have been efficiently synthesized in aqueous media by hydrazone exchange reactions, displaying a marked pH-responsivity. Furthermore, the parent N , N '-dimethylated "vermellogen": the "red thread", an analogue of the herbicide paraquat and used herein as a representative model of the series, showed anion-recognition abilities, non-reversible electrochemical behavior, and non-toxicity of the modified bis-pyridinium core. The host-guest chemistry for the "red thread" with the CB[7,8] macrocyclic receptors has been extensively studied experimentally and by dispersion corrected density functional theory methods, showing a parallel behavior to that previously described for the herbicide but, crucially, swapping the well-known redox reactive capabilities of the viologen-based inclusion complexes by acid-base supramolecular responsiveness.