|Timme, A.; Kress, R.; Albuquerque, R.Q.; Schmidt, H.-W.: Phase Behavior and Mesophase Structures of 1,3,5-Benzene- and 1,3,5-Cyclohexanetricarboxamides: Towards an Understanding of the Losing Order at the Transition into the Isotropic Phase, Chemistry - A European Journal, 18(27), 8329-8339 (2012) -- DOI: 10.1002/chem.201103216|
One of the simplest and most-versatile motifs in supramolecular chemistry is based on 1,3,5-benzenetricarboxamides. Variation of the core structure and subtle changes in the structures of the lateral substituents govern the self-assembly and determine the phase behavior. Herein, we provide a comprehensive comparison between the phase behavior and mesophase structure of a series of 1,3,5-benzene- and 1,3,5-cyclohexanetricarboxamides that contain linear and branched alkyl substituents. Depending on the substituent, different crystalline, plastic crystalline, and liquid crystalline phases were formed. The relatively rare columnar nematic (NC) phase was only observed in cyclohexane-based trisamides that contained linear alkyl substituents. Of fundamental interest in liquid crystalline supramolecular systems is the transition from the mesomorphic state into the isotropic state and, in particular, the question of how the order decreases. Temperature-dependent IR spectroscopy and XRD measurements revealed that columnar H-bonded aggregates were still present in the isotropic phase. At the clearing transition, mainly the lateral order was lost, whilst shorter columnar aggregates still remained. A thorough understanding of the phase behavior and the mesophase structure is relevant for selecting processing conditions that use supramolecular structures in devices or as fibrillar nanomaterials.