Synthesis of supramolecular self-assembled benzene-1,3,5- tricarboxamides and their liquid crystalline properties

Abstract

Supramolecular self-assembly of soft matter have received many attention for various applications such as liquid crystals, sensors, catalysts, and optics due to its precise control and easily tunable characteristics. Particularly, benzene-1,3,5-tricarboxamides (BTAs) as p- stacked organic motif were recognized for formation of liquid crystals materials; however, these BTAs were only reported bearing different short alkyl or aromatic side chains in order to have liquid crystalline properties. Herein, self-assemblies of BTAs (CRNH-BTA, where R = 10, 12, 14, 16, and 18) via hydrogen bonding interactions have been demonstrated as mesophase liquid crystalline materials by simply tuning its long hydrophobic alkyl chains attached to amide. Series of BTAs were synthesized by a Schotten-Baumann reaction using 1,3,5-benzenetricarbonyl trichloride with respected alkyl amides in the presence of triethylamine in dry tetrahydrofuran (THF) under an inert condition for overnight. Nuclear magnetic resonance (NMR), matrix-assisted laser desorption/ionization time-of-flight mass (MALDI-TOF-MS) and Fourier transform infrared spectroscopy (FT–IR) have proved the successful isolation of the hydrophobic BTAs using column chromatography as powdery solids. Scanning electron microscope (SEM) measurements showed self-assembled aggregation of a tape-like morphology with different aspect to ratio lengths corresponding to different alkyl side-chains lengths. Likewise, FT–IR spectra showed vibration bands at 1540 and 1719 cm-1 due to formation of N-H and C=O bondings, respectively, corresponding to the presence of hydrogen bonding interactions for all BTAs. Moreover, polarized optical microscopy (POM) showed spehrulitic-like texture upon isotropic melts at 140 ºC monitored for 20 hours of C12NH-BTA, strongly supports the characteristics of discotic liquid crystals for BTAs. In addition, differential scanning calorimetry (DSC) thermograms showed that longer alkyl side-chains shortened mesophase ranges due to the presence of higher steric hindrance of the alkyl amide chains.