Synthesis of a bistable [3]rotaxane and its pH-controlled intramolecular charge-transfer behavior

A [3]rotaxane 1 involving two naphtho-21-crown-7 (N21C7) rings and a dumbbell-shaped component 4 was synthesized. The dumbbell-shape molecule 4 contains one viologen nucleus, two secondary alkyl ammonium sites and two phenyl stoppers. After threading the N21C7 ring with the thread-like ammonium guest 3, the copper(I)-catalyzed Huisgen alkyne-azide 1,3-dipolar cycloaddition (CuAAC “click” reaction), was performed to connect the pseudorotaxanes with viologen unit 2 and generate 1. Through treating the [3]rotaxane by the base and acid circularly, the two N21C7 rings can make shuttling motion along the axle. Meanwhile the distance between the electron-deficient viologen unit and the electron-rich naphthol group can be adjusted precisely along with a remarkable intramolecular charge-transfer (CT) behavior.