Induction of thermotropic bicontinuous cubic phases in liquid-crystalline ammonium and phosphonium salts

Two series of wedge-shaped onium salts, one ammonium and the other phosphonium, having 3,4,5-tris(alkyloxy)benzyl moieties, exhibit thermotropic bicontinuous "gyroid" cubic (Cub _bi) and hexagonal columnar liquid-crystalline (LC) phases by nanosegregation between ionophilic and ionophobic parts. The alkyl chain lengths on the cationic moieties, anion species, and alkyl chain lengths on the benzyl moieties have crucial effects on their thermotropic phase behavior. For example, triethyl-[3,4,5-tris(dodecyloxy) benzyl]ammonium hexafluorophosphate forms the thermotropic Ia3̄d Cub _bi LC phase, whereas an analogous compound with trifluoromethanesulfonate anion shows no LC properties. Synchrotron small-angle diffraction intensities from the Ia3̄d Cub _bi LC materials provide electron density maps in the bulk state. The resulting maps show convincingly that the Ia3̄d Cub _bi structure is composed of three-dimensionally interconnected ion nanochannel networks surrounded by aliphatic domains. A novel differential mapping technique has been applied successfully. The map of triethyl-[3,4,5-tris(decyloxy)benzyl]ammonium tetrafluoroborate has been subtracted from that of the analogous ammonium salt with hexafluorophosphate anion in the Ia3̄d Cub _bi phases. The differential map shows that the counteranions are located in the core of the three-dimensionally interconnected nanochannel networks. Changing from trimethyl-via triethyl-to tripropylammonium cation changes the phase from columnar to Cub _bi to no mesophase, respectively. This sensitivity to the widened shape for the narrow end of the molecule is explained successfully by the previously proposed semiquantitative geometric model based on the radial distribution of volume in wedge-shaped molecules. The LC onium salts dissolve lithium tetrafluoroborate without losing the Ia3̄d Cub _bi LC phase. The Cub _bi LC materials exhibit efficient ion-transporting behavior as a result of their 3D interconnected ion nanochannel networks. The Ia3̄d Cub _bi LC material formed by triethyl-[3,4,5-tris(decyloxy) benzyl]phosphonium tetrafluoroborate shows ionic conductivities higher than the analogous Ia3̄d Cub _bi material based on ammonium salts. The present study indicates great potential of Cub _bi LC nanostructures consisting of ionic molecules for development of transportation nanochannel materials.