The inclusion of small molecules changes the distance between layers in lamellar material

01-22-2004: A crystal has a defined structure that cannot be altered much once it is formed-or such was the belief until now. Spanish chemists have recently demonstrated this assertion to be false; they have produced a layered material whose porosity can be changed from one moment to the next. This could signal the birth of a new type of material with tailored pores that respond to external signals. Microporous materials can take up "guest" molecules, and are highly coveted for selective catalysts, ion exchangers, or storage for pharmaceuticals or other chemical agents.

The team, led by Ernesto Brunet, selected gamma-zirconium phosphate, which is known to have a layered structure, as their starting material. The surfaces of the individual layers have phosphate groups [PO4] sticking out of them, and these can easily be replaced without changing the structure of the layers. The researchers replaced some of the phosphates with short polyethylene glycol chains that had a phosphonic acid group [PO(OH)2] at each end. These "diphosphonates" forced out one phosphate on each of two adjacent layers, bridging the layers together. The chains support the space between the layers like columns in an arcade. The researchers then exchange the remaining phosphate groups for hypophosphite groups [H2PO2]. These are embedded in the surface of the layer so that their apolar PH2 side protrudes into the space between the layers. In contrast to the product of the precursor step, the apolar interactions between the layers now cause the columns to lie flat (parallel to the layers), which significantly decreases the distance between the layers. If this material is then treated with basic methylamine, it abruptly soaks it up-within a narrow pH range-and the separation between layers increases by almost 70%. How? Polar molecules like methylamine initially cannot enter into the apolar interlayer spaces. However, the anchors for the columns, the phosphonate groups, are polar. Once the pH value reaches about 4.5, the attraction between the basic methylamine and the acidic phosphonates is so strong that individual methylamine ions force their way in at the edges of the crystals. These ions are relatively large compared to the distance between the layers. Like a wedge, they drive the rigid layers apart and cause the columns to stand upright. A few wedges are enough to completely neutralize the apolar attractive forces between the layers and bring all of the columns into an upright position. The distance between the layers-and with it the porosity-increases abruptly. "Such a high sensitivity of microcrystalline porosity towards the incorporation of small molecules is thus far unique," says Brunet.