Triptycene-based polymers of intrinsic microporosity: Organic materials that can be tailored for gas adsorption

We report the synthesis and properties of network polymers of intrinsic microporosity (network - PIMs) derived from triptycene monomers that possess alkyl groups attached to their bridgehead positions. Gas adsorption can be controlled by the length and branching of the alkyl chains so that the apparent BET surface area of the materials can be tuned within the range 618 - 1760 m² g^{- 1}. Shorter (e.g., methyl) or branched (e.g., isopropyl) alkyl chains provide the materials of greatest microporosity, whereas longer alkyl chains appear to block the microporosity created by the rigid organic framework. The enhanced microporosity, in comparison to other PIMs, originates from the macromolecular shape of the framework, as dictated by the triptycene units, which helps to reduce intermolecular contact between the extended planar struts of the rigid framework and thus reduces the efficiency of packing within the solid. The hydrogen adsorption capacities of the triptycene-based PIMs with either methyl or isopropyl substituents are among the highest for purely organic materials at low or moderate presures (1.83% by mass at 1 bar/77K; 3.4% by mass at 18 bar/77 K). The impressive hydrogen adsorption capacity of these materials is related to a high concentration of subnanometre micropores, as verified by Horvath - Kawazoe analysis of low-pressure nitrogen adsorption data.