Design Criteria for Ionic Liquid Crystalline Phases of Phosphonium Salts with Three Equivalent Long n-Alkyl Chains

Abstract

The factors influencing the formation, organizations, and temperature ranges of the smectic phases of a structurally diverse family of phosphonium salts have been examined. The salts consist of one short group and three long n-alkyl chains attached to a positively charged phosphorus atom and either a free or covalently attached counterion, the latter resulting in zwitterionic salts. Of the 61 salts investigated, of which 37 have not been synthesized previously, most pack in lamellae within their solid phases. Single-crystal X-ray structures of two of amidomethyl-tri-n-tetradecylphosphonium bromide (1P14CONH(2)Br) and carboxymethyl-tri-n-tetradecylphosphonium bromide (1P14CO(2)HBr) have been solved. In each, the constituent molecules are packed in stacks of bilayers in which the directors of molecules on opposite sides of the ionic planes (where the phosphonium cationic centers and anions are located) that separate the layers are antiparallel. In each molecule, two of the long n-alkyl chains are paired while the third is antiparallel to the other two and paired with an n-alkyl chain of a molecule in a neighboring bilayer. The tri-n-alkylmethylphosphonium salts (1PnX) with small anions X (where n = 6-18 is the number of carbon atoms in the three long chain and 1 is the methyl group) do not form liquid-crystalline phases as a consequence of strong alternating intra- and intermolecular P(+)-X(-) interactions within the ionic planes that separate the bilayers of long chains. Thermotropic and enantiotropic liquid-crystalline phase formation of 1PnX salts is favored by larger anions and longer n-alkyl chains, which reduce order within ionic planes while promoting order within the lipophilic layers. We conjecture that covalent attachment of a hydroxymethylene, carboxy, or amido functional group Y to the alpha-methyl group of a 1PnX salt (resulting in mPnYX salts, where m is the number of methylene units separating the phosphorus atom from the Y group on the short chain) moves the anion X farther from the P(+) ion as a result of intramolecular X(-)...H(Y) H-bonding interactions and, therefore, substantially weakens intramolecular P(+)-X(-) ionic interactions within the ionic planes. In contrast to the trends mentioned for the 1PnX salts, liquid-crystalline phases of mPnYX are found more frequently when n is shorter and X is smaller. The observation that the liquid-crystalline phases of mPnYX salts have lower clearing and onset temperatures than the corresponding 1PnX may be attributed to the greater freedom of motion at and near the ionic planes of the former as a result of their more dispersed ionic interactions. Overall, a detailed study of the dependence of phase type and phase transition temperatures on several key structural factors of phosphonium salts has been made. The correlations found provide insights into how new mesmorphic phosphonium salts can be designed and exploited for a wide range of potential applications.