Guest-Responsive Covalent Frameworks by the Cross-Linking of Liquid-Crystalline Salts: Tuning of Lattice Flexibility by the Design of Polymerizable Units

Cross‐linked polymers prepared by the in‐situ polymerization of liquid‐crystalline salts were found to work as solid‐state hosts with a flexible framework. As a component of such hosts, four kinds of polymerizable amphiphilic carboxylic acids bearing alkyl chains with acryloyloxy (A), dienyl (D), and/or nonreactive (N) chain ends (monomeric carboxylic acids; MAAA, MANA, MDDD, and MDND) were used. The carboxylic acids were mixed with an equimolar amount of a template unit, (1R,2S)‐norephedrine (guest amine; GRS), to form the corresponding salts. Every salt exhibited a rectangular columnar LC phase at room temperature, which was successfully polymerized by 60Co γ‐ray‐induced polymerization without serious structural disordering to afford the salt of cross‐linked carboxylic acid (polymeric carboxylic acid; PAAA, PANA, PDDD, and PDND) with GRS. Owing to the noncovalency of the interactions between the polymer framework P and the template GRS, the cross‐linked polymers could reversibly release and capture a meaningful amount of GRS. In response to the desorption and adsorption of GRS, the cross‐linked polymers dramatically switched their nanoscale structural order. A systematic comparison of the polymers revealed that the choice of polymerizable groups has a significant influence on the properties of the resultant polymer frameworks as solid‐state hosts. Among these polymers, PDDD was found to be an excellent solid‐state host, in terms of guest‐releasing/capturing ability, guest‐recognition ability, durability to repetitive usage, and unique structural switching mode. Solid‐state hosts with covalent frameworks were obtained by the in situ cross‐linking of liquid‐crystalline salts composed of a polymerizable carboxylic acid and an enantiopure amino alcohol (see scheme). In response to the desorption and adsorption of the amino alcohol unit, the cross‐linked polymers reversibly changed their structures. The dynamic properties of the cross‐linked polymers could be tuned by the choice of cross‐linking units.