Switching molecular recognition selectivities by temperature in a diffusion-regulatory porous material.

Over the long history of evolution, nature has developed a variety of biological systems with switchable recognition functions, such as the ion transmissibility of biological membranes, which can switch their ion selectivities in response to diverse stimuli. However, developing a method in an artificial host-guest system for switchable recognition of specific guests upon the change of external stimuli is a fundamental challenge in chemistry because the order in the host-guest affinity of a given system hardly varies along with environmental conditions. Herein, we report temperature-responsive recognition of two similar gaseous guests, CO 2 and C 2 H 2 , with selectivities switched by temperature change by a diffusion-regulatory mechanism, which is realized by a dynamic porous crystal featuring ultrasmall pore apertures with flip-flop locally-motive organic moiety. The dynamic local motion regulates the diffusion process of CO 2 and C 2 H 2 and amplifies their rate differences, allowing the crystal to selectively adsorb CO 2 at low temperatures and C 2 H 2 at high temperatures with separation factors of 498 (CO 2 /C 2 H 2 ) and 181 (C 2 H 2 /CO 2 ), respectively.