Pillar[6]arene/Paraquat Molecular Recognition in Water: High Binding Strength, pH-Responsiveness, and Application in Controllable Self-Assembly, Controlled Release, and Treatment of Paraquat Poisoning

The complexation between a water-soluble pillar[6]arene (WP6) and paraquat (G1) in water was investigated. They could form a stable 1:1 [2]pseudorotaxane with an extremely high association constant of (1.02 ± 0.10) × 10(8) M(-1) mainly driven by electrostatic interactions, hydrophobic interactions, and π-π stacking interactions. This molecular recognition has not only high binding strength but also pH-responsiveness. The threading and dethreading processes of this [2]pseudorotaxane could be reversibly controlled by changing the solution pH. This novel recognition motif was further used to control the aggregation of a complex between WP6 and an amphiphilic paraquat derivative (G2) in water. The reversible transformations between micelles based on G2 and vesicles based on WP6⊃G2 were realized by adjusting the solution pH due to the pH-responsiveness of WP6. The controlled release of water-soluble dye molecules from the vesicles could be achieved by the collapse of the vesicles into the micelles upon changing the solution pH to acidity. Additionally, the high binding affinity between WP6 and paraquat could be utilized to efficiently reduce the toxicity of paraquat. After the formation of a stable host-guest complex between WP6 and paraquat, less opportunity was available for paraquat to interact with the reducing agents in the cell, which made the generation of its radical cation more difficult, resulting in the efficient reduction of paraquat toxicity.