Structure and Rheology of Hydrogen Bond Reinforced Liquid Crystals

Phase behavior and structure in both the crystalline and liquid-crystalline states is dependent on molecular size and shape and competition between various noncovalent intermolecular interactions. Numerous guanidinium organomonosulfonate ( GS ) crystals have been shown to adopt layered architectures because of their propensity to form a two-dimensional hydrogen-bonded network consisting of the sulfonate moieties ( S ) and complementary guanidinium ( G ) ions. This characteristic has now been demonstrated for nearly all of a series of guanidinium alkylbenzenesulfonates (GCnBS), in which the alkyl chain length ( n ) ranges from 0 to 16 carbons, and some members of a series of guanidinium alkylbiphenylsulfonates (GCnBPS). Single-crystal X-ray diffraction reveals that these layered architectures include previously observed simple bilayer and continuously interdigitated layer architectures as well as a new “box bilayer” architecture. Whereas the related sodium organosulfonates, phenylalkanes, and biphenylalkanes alone do not form liquid-crystal phases upon heating, the reinforcement provided by the hydrogen-bonded GS network leads to the transformation of these crystalline phases to persistent smectic liquid-crystal phases at elevated temperatures for n ≥ q (as reported previously for some of the GCnBS compounds). This chain length corresponds to single-crystal packing motifs that signal the onset of a structure-directing role for alkane−alkane interactions. X-ray diffraction reveals that the d -spacings of the smectic phases increase monotonically with n, consistent with a common structure that is independent of the preexisting crystal architecture. This behavior can be ascribed to facile rearrangements of the hydrogen-bonding connectivity in the GS sheet during the crystal-to-smectic transition. Several GCnBS compounds also exhibit polymorphic transitions involving rearrangement of the hydrogen-bond connectivity at temperatures below the smectic transition. The general rheological behavior of the GCnBS smectic phases was consistent with that reported for other small-molecule smectics; however, the viscosity of the GCnBS smectic phases was at least 2 orders of magnitude larger. The large viscosities observed in the GCnBS smectic phases are attributed to the persistence of intermolecular hydrogen bonding in the liquid-crystal state.