Novel class of ionic metallomesogens without organic ligands: Thermotropic liquid-crystalline behavior of tetrachlorometalate salts and distinct polymorphic transition in smectic A phases

The thermotropic liquid-crystalline behavior of the metal complex salts [(C18H37)(CH3)(3)N](2)[MCl4] and [(C18H37)(2)(CH3)(2)N](2)[MCl4] (M = Co, Ni, Cu, Zn, Cd) was evaluated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffractometry, and electronic absorption as well as by Raman spectroscopy. Introduction of divalent metal ions into the cationic amphiphiles was found to result in enhancement of their thermal properties. Although the parent tetraalkylammonium chlorides showed no liquid-crystalline phase (or indistinct transition behavior), the derived tetrachlorometalate salts exhibited a stable liquid-crystalline phase and simple phase transition behavior. For instance, the tetrachlorocobaltate salt, [(C18H37)(2)(CH3)(2)N](2)[CoCl4], showed a smectic A (S-A) phase in an extremely wide temperature range from 45 to 155 degreesC on heating. In the S-A phase, the complexes were confirmed to form a layered self-assembly that is built up from alternating layers of tetrahedral [MCl4](2-) ions and flexible alkyl segments. Furthermore, distinct polymorphism in the S-A phase was observed for several tetrachlorometalate salts, which differs essentially from the S-A-S-A transition described in the usual rodlike liquid crystals.