Anion-capped metallohost allows extremely slow guest uptake and on-demand acceleration of guest exchange.

The switching of molecular recognition selectivity is important for tuning molecular functions based on host-guest binding. While the switching processes in artificial functional molecules are usually driven by changes of the thermodynamic stabilities, non-equilibrium phenomena also play an important role in biological systems. Thus, here we designed a host-guest system utilizing a non-equilibrium kinetically trapped state for on-demand and time-programmable control of molecular functions. We synthesized a bis(saloph) macrocyclic cobalt(III) metallohost 1(OTf)2, which has anion caps at both sides of the cation-binding site. The anion caps effectively retard the guest uptake/release so that we can easily make a non-equilibrium kinetically trapped state. Indeed, we can obtain a long-lived kinetically trapped state {[1·K]3++La3+} prior to the formation of the thermodynamically more stable state {[1·La]5++K+}. The guest exchange to the more stable state from this kinetically trapped state is significantly accelerated by exchange of TfO- anion caps by AcO- in an on-demand manner.