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.