Microstructure and dynamics of a mesogenic diol

A number of techniques have been used to elucidate the structure and dynamics of 4,4'-bis(6-hydroxyhexyloxy)biphenyl (BHHBP) in its various phases. X-ray diffraction studies indicate that the molecules pack in a crystalline phase which melts to produce a highly ordered smectic/disordered crystal mesophase. Based on molecular models and the infrared results, the all trans conformation requires a 45°-55° tilt of the molecules in the smectic layers. Infrared spectroscopic results indicate that a predominantly trans chain conformation and hydrogen bonding of the layered crystal structure persists through the mesophase. Additionally, rotational freedom about the biphenyl linkage appears to occur only in the isotropic phase. NMR data indicate that the alkoxy chain is at or near co-planarity with the respect to the phenyl ring in the crystalline phase, with reorientational motion of the biphenyl group becoming allowed in the mesophase in the form of rapid (τ_c ～ 3 μs at 100°C) small angle liberations and, perhaps, slower (τ_c ～ 0·5 ms at 100°C) 180° ring flips. The alkyl chains exhibit a progressive increase in mobility with distance from the biphenyl core and achieve considerable mobility at the hydroxy end of the chain despite the fact that hydrogen bonding still occurs in the mesophase.