Organic intercalation material: Reversible change in interlayer distances by guest release and insertion in sandwich-type inclusion crystals of cholic acid

Cholic acid (CA) forms inclusion crystals that have a sandwich-type lamellar structure constructed by the alternative stacking of host bilayers and guest layers. Five disubstituted benzenes, o-toluidine, m-fluoroaniline, o-chlorotoluene, o-bromotoluene, and indene, are accommodated in the two-dimensional void space between the host bilayers at 1:2 host-guest stoichiometries. Thermal gravimetric analysis of the inclusion crystals revealed that all the guest molecules, except o-toluidine, are released in two separate steps, indicating the formation of intermediate crystals after the first guest release. Adequate heat treatment of the four inclusion crystals induces release of half or three quarters of the guest molecules. X-ray diffraction patterns of the intermediate crystals revealed that the crystals have a bilayer structure the same as those of the common CA inclusion crystals. They have one-dimensional cavities, in which the guest molecules are included at a 1:1 or 2:1 host-guest stoichiometry. These facts indicate that the host bilayers move 1.6-4.5 angstrom perpendicular to the layer direction by desorption of the guest molecules. Furthermore, a reverse structural change is also achieved by absorption of the guest molecules to regenerate the starting sandwich-type inclusion crystals. This reversible change in the host bilayer by the guest sorption and desorption is a novel example of organic intercalation materials.