Proton Conduction in Tröger's Base-Linked Poly(crown ether)s

Exactly 50 years ago, the ground-breaking discovery of dibenzo[18]crown-6 (DB18C6) by Charles Pedersen led to the use of DB18C6 as a receptor in supramolecular chemistry and a host in host-guest chemistry. We have demonstrated proton conductivity in Tröger's base-linked polymers through hydrogen-bonded networks formed from adsorbed water molecules on the oxygen atoms of DB18C6 under humid conditions. Tröger's base-linked polymers - poly(TBL-DB18C6)-t and poly(TBL-DB18C6)-c - synthesized by the in situ alkylation and cyclization of either trans- or cis-di(aminobenzo) [18]crown-6 at room temperature have been isolated as high-molecular-weight polymers. The macromolecular structures of the isomeric poly(TBL-DB18C6)s have been established by spectroscopic techniques and size-exclusion chromatography. The excellent solubility of these polymers in chloroform allows the formation of freestanding membranes, which are thermally stable and also show stability under aqueous conditions. The hydrophilic nature of the DB18C6 building blocks in the polymer facilitates retention of water as confirmed by water vapor adsorption isotherms, which show a 23 wt % water uptake. The adsorbed water is retained even after reducing the relative humidity to 25%. The proton conductivity of poly(TBL-DB18C6)-t, which is found to be 1.4 × 10^-4 mS cm^-1 in a humid environment, arises from the hydrogen bonding and the associated proton-hopping mechanism, as supported by a modeling study. In addition to proton conductivity, the Tröger's base-linked polymers reported here promise a wide range of applications where the sub-nanometer-sized cavities of the crown ethers and the robust film-forming ability are the governing factors in dictating their properties.