Molecular Turnstiles Regulated by Metal Ions

A family of novel molecular turnstiles 1-3 composed of two stators with pyridyl binding sites and a different-sized triptycene rotor was synthesized. The molecular turnstiles behave in an open state at room temperature in the absence of metal ions but display significantly different closed states in the presence of Ag+ and Pd2+. The Ag+-mediated turnstiles 1-3Ag exhibited closed states but unreadable bistability at ambient temperature because the Ag+-mediated macrocyclic framework is not able to restrict the rotations of the rotors; while temperature was decreased, the macrocyclic frameworks became stable enough to halt the rotations of the rotors, eventually leading to the readable closed states for 1-3Ag. In contrast, Pd2+-mediated macrocyclic frameworks are stable, giving rise to a detectable closed state of turnstiles 1-3Pd in a wide range of temperatures. These findings have also been supported by DFT calculations.