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

The thermotropic liquid-crystalline behavior of the metal complex salts [(C 18H37)(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 cat ionic amphiphiles was found to result in enhancement of their thermal prope rties. Although the parent tetraalkylammonium chlorides showed no liquid-cr ystalline phase (or indistinct transition behavior), the derived tetrachlor ometalate salts exhibited a stable liquid-crystalline phase and simple phas e transition behavior. For instance, the tetrachlorocobaltate salt, [(C18H3 7)(2)(CH3)(2)N](2)[CoCl4], showed a smectic A (S-A) phase in an extremely w ide 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 alk yl segments. Furthermore, distinct polymorphism in the S-A phase was observ ed for several tetrachlorometalate salts, which differs essentially from th e S-A-S-A transition described in the usual rodlike liquid crystals.