A deuterium NMR investigation of polymorphism in benzene pizzanes

Abstract

The polymorphism of three members of the homologous series hexa(p-alkoxyphenoxymethyl) benzene (benzene pizzanes) with 5, 6 and 7 carbons in the alkoxy chains and several of their deuteriated isotopomers have been investigated by differential scanning calorimetry, polarizing optical microscopy, X-ray diffraction and deuterium NMR spectroscopy. These homologues exhibit several solid phases and a high temperature M phase, which is isomorphic in the three homologues and whose nature is discussed. In the solid phases, the benzene cores of the molecules remain rigid, but the side chains are mobile, as reflected by rapid pi-flips of the benzene rings in the side chains. It is found that there are two types of such benzene rings, differing in the rates of flips. In the M phase the molecules undergo fast overall reorientation and the side chains are even more disordered than in the solid phases. However the X-ray measurements do not provide a clear cut determination as to whether this phase is crystalline or mesomorphic. Mixing of the benzene pizzanes with p-xylene yields lyomesophases, which appear to belong to the Dho class.