Crystal engineering: Design, syntheses and characterization of a family of compounds demonstrating liquid crystalline properties

Abstract

Intermolecular interactions govern molecular self-recognition and self-assembly, giving rise to 3-dimensional solids. Thus, independent intermolecular interactions should result in a predictable crystal packing. Our working hypothesis is the crystal packing of symmetrically substituted tetra-n-alkoxy-bis-indane-piperazinediones of H-shaped topography will be governed by three factors: First, assembly of molecules into "one-dimensional" tapes through establishment of reciprocal amide hydrogen bonds. Second, assembly of the tapes into "two-dimensional" grooved sheets through establishment of edge-to-face arene interactions. Third, assembly of the sheets into "three-dimensional" solids by "tongue-in-groove" interdigitation of the n-alkyl "tails" in extended conformations independent of chain length. Liquid crystal properties are anticipated for molecules with hydrocarbon chains of sufficient length. Rupture of one or more of the three structure-determining interactions may be independent of the others and observable. "Dissection" of the melting process by correlation of crystallographic data with thermochemical data would permit formulation of a mechanism for melting. Compounds of this homologous series with methoxy, ethoxy, butoxy, hexyloxy, octyloxy and nonyloxy substituents have been synthesized, crystallized, and studied by X-ray crystallography, differential scanning calorimetry (DSC) and optical microscopy. The solid-state structures for the first three in the series were determined and each engaged in intermolecular amide-to-amide hydrogen bonding interactions that established "ladder-like" parallel tapes. Intertape organization was governed by the development of arene and/or van der Waals contact interactions. The melting behaviors of the series were remarkable in that these were not only high melting solids but most also demonstrate transitions by DSC at lower temperatures and magnitude consistent with liquid crystalline behavior. Also remarkable and consistent with liquid crystalline behavior are the optical properties of the longer alkoxy substituted compounds of the series, demonstrating birefringence of cross-polarized fight. In the case of the tetra-n-hexyloxy compound a transition was observed by optical microscopy that correlates with DSC data. Design, syntheses and characterizations of this new family of compounds, which are highly ordered at the molecular level, supporting our hypothesis of liquid crystalline behavior, are discussed. Using these data a mechanism of melting is postulated and discussed.