Amphotropic liquid crystals (ALC) are possible aligning media for 3D structure determinations of various molecules by NMR spectroscopy because they can be oriented within strong magnetic fields and, thereby, orient the solute molecules being investigated. Here, we report the results of a systematic investigation of liquid-crystalline methyl-tri-n-alkylphosphonium salts (PmA, where m =10, 14, and 18 is the number of carbon atoms in the long alkyl chains and A an anion such as Cl^-, Br^-, NO₃^-, PF₆-, BF₄^-, or (CF₃SO₂)₂N^-. The phase diagrams, transition temperatures, and liquid-crystalline properties of the thermotropic liquid-crystalline phases of the neat compounds, as well as the lyotropic phases formed in the presence of various concentrations of organic liquids such as methanol, butanol, hexanol, decanol, and DMSO have been investigated by NMR spectroscopy and polarizing optical microscopy. The concentration of liquid is critical to obtaining pure lyotropic liquid-crystalline phases. Changing the type and concentration of the liquid, the anion structure, and the length of alkyl chains permit the mesophase temperature ranges and the degrees of orientation in magnetic fields to be varied over a wide range. In turn, the degrees of orientations of selected solutes were also varied in this way. Aspects of the liquid crystalline phases and some preliminary results demonstrating the ability of the PmA to yield structural parameters from residual dipolar couplings will be presented.

We thank the US National Science Foundation for its support of this research and the Fulbright Foundation for a Visiting Researcher Fellowship to A. A. S.