The importance of micro segregation for mesophase formation: Thermotropic columnar mesophases of tetrahedral and other low-aspect-ratio organic materials

Article abstract of DOI:10.1002/(SICI)1521-3765(19990503)5:5<1643::AID-CHEM1643>3.0.CO;2-T

Several low-aspect-ratio organic molecules [tetrahedral pentaerythritol derivatives, peracylated polyhydroxy compounds and aminoalcohols, a tetraphenylmethane derivative, a tetraphenylstannane, and a tetrahedral zinc bis(1,3-diketonate) all carrying long aliphatic chains] have been synthesized. These compounds were investigated by polarizing optical microscopy and differential scanning calorimetry, and some of them by X-ray diffraction. Most compounds show columnar liquid-crystalline mesophases. Their mesogenic properties are neither caused by a specific anisometric shape of these molecules nor by a strong amphiphilicity as known from conventional liquid crystals. Instead their mesogenity is mainly driven by micro segregation of the incompatible molecular parts (polar Central regions and lipophilic alkyl chains) into well-organized different microdomains. It is shown that, in analogy to block copolymers, the mesophase stability rises: on enlarging the number of repeat units connected with each other and on increasing the degree of incompatibility between the incompatible segments. During the process of self-organization the average conformation of the molecules is changed in such a way that it allows a most efficient packing of the molecules. Consequently, rigid molecules with a fixed tetrahedral geometry are not mesogenic. The molecules described herein can be regarded as the most simple star-shaped low-aspect-ratio block molecules that form liquid-crystalline phases. They bridge the gap between classical amphiphilic mesogens, several nonconventional dendritic and oligomeric liquid crystals, and mesomorphic block copolymers.