Siloxanes and carbosilanes as new building blocks for T-shaped bolaamphiphilic LC molecules

Abstract

T-shaped bolaamphiphiles composed of a 4,4′-disubstituted biphenyl core and terminal diol groups with laterally attached oligo(siloxane) and carbosilane units were synthesized and investigated with respect to their self-assembly into liquid crystalline superstructures (thermotropic mesophases). Two different types of bolaamphiphiles were investigated: one has two identical glycerol groups at both ends of the biphenyl core, which are connected by ether linkages and in the other type one of the ether linkages is replaced by an amide group. The position of the lateral groups on the biphenyl core was also varied. In addition to these silylated compounds two different types of n-alkyl substituted compounds were investigated and compared with the silylated bolaamphiphiles. A hexagonal cylinder phase and three distinct types of lamellar phases were observed and a phase sequence Col_hex–Lam_Sm–Lam_N–Lam_Iso was found by increasing the size of the lateral substituent. The silyl units organize into distinct compartments either within columns or between layers. The increased flexibility of the silyl units compared to linear alkyl chains and perfluorinated chains leads to materials with phase transitions in convenient temperature ranges. For one of the carbosilanes an unusually broad region of the Lam_N phase was achieved. It turned out that an additional amide group leads to a significant mesophase destabilization which is attributed to a competition between intramolecular and intermolecular hydrogen bonding. The position of the lateral chain is also important, as it influences hydrogen bonding as well as the conformation of the biphenyl unit.