A Water-Soluble pH-Triggered Molecular Switch

A bistable donor–acceptor [2]­catenane, which is composed of a crown ether containing a hydroquinone unit and a 1,5-diaminonaphthalene unit, interlocked mechanically by cyclobis­(paraquat-p-phenylene) as its tetrachloride, exists as a mixture of translational isomers, both in the solid state and in aqueous solution. UV/vis and 1H NMR spectroscopies demonstrate that this isomeric mixture can be switched in water in the presence of hydrochloric acid to afford a single diprotonated derivative in which only the hydroquinone unit resides inside the cavity of the tetracationic cyclophane. Treatment with 1,4-diazabicyclo[2.2.2]­octane resets the molecular switch.