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 greater than or equal to 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.