Interfacial polymerization of molecular squares: Thin microporous membranes featuring size selective transport

Flat porphyrin molecules have been linked at right angles, using metal atoms, to create square or open-ended box structures having minimum cavity widths of about 2.4 nm and nominal volumes of ca. 14 nm(3). Chemically modified versions of the squares feature reactive hydroxyl groups at the top and bottom of each of the four walls of the box structure. These groups combine readily with acid chloride functionalized molecules to yield robust ester linkages. If the reactions are carried out at a liquid-liquid interface with squares in one layer and difunctional acid chloride linkers in the other, polymer formation occurs at the interface. As with other liquid-liquid interfacial polymerizations, the reaction is self-limiting because transport of reactants is inhibited by membrane formation. Polymer films prepared by this method have controllable thicknesses ranging from about 200 nm to 2 microns. The membranes display size-selective porosity with respect to molecules smaller than the intra-square cavity and blocking behavior with respect to larger molecules. Electrochemical and electrochemiluminescence methods are used to measure qualitative and quantitative sieving behavior by employing a conductive metal or ceramic electrode as an underlying platform. In these and related membranes, selective molecular transport is being engineered for applications such as chemical sensing, energy conversion, and chemical catalysis.