Synthesis, Mesomorphism and Gelation Properties of Triazole-Modified Triphenylene 2,3-Dicarboxylic Esters and 2,3-Dicarboxyimides

The introduction of different functional groups at the periphery chains of discotic liquid crystals is a general strategy for the design and synthesis of new promising mesomorphic materials. Starting from triphenylene 2,3-dicarboxylic acid and anhydride, two kinds of triazole-modified triphenylene 2,3-dicarboxylic esters and 2,3-dicarboxyimides were synthesized via nucleophilic substitution and subsequent Cu-catalyzed azide-alkyne click reactions. Thermogravimetric analysis (TGA) measurements indicated that the prepared precursors and desired compounds exhibit good thermal stability with the temperatures of 5% weight loss in the range of 274 similar to 389. The thermal behavior and mesomorphism of these compounds were studied by differential scanning calorimetry (DSC), polarised optical microscopy (POM) and variable temperature X-ray diffraction (XRD) experiments. With the exceptions of esters 3a and 3b, which both show a single crystalline phase with a rather high melting temperature, the key precursors (2 and 5) and two triazole-modified imides (6a and 6b) all exhibit enantiotropic hexagonal columnar phase. Besides, the prepared triazole-bridged imide dimer 10 is a room temperature liquid crystalline material and has a wider mesophase range of 173 degrees C from 8 degrees C up to 181 degrees C. XRD confirmed the existence of two columnar mesophases with different degrees of order in dimer 10, unambiguously characterized as Col(h1) and Col(h2). Due to the triazole moieties, additionally, all the triazole-modified esters and imides can form organogels in some organic solvents. Notably dimer 10 exhibits strong gelation ability in 1,2-dichloroethane or 1,4-dioxane with a very low critical gel concentration (1 mg/mL). In comparation, the precursor compounds 2 and 5, which do not possess any triazole ring in structure show no tendency to gelate, indicating that the strong dipole-dipole and pi-pi interactions between the triazole rings play an important role in the formation of the gel. Thus triazole-modified triphenylene 2,3-dicarboxyimides represent an interesting example of molecules exhibiting both liquid crystalline and gelling properties. This investigation sheds light on the potential applicability of the triazole-modified triphenylene 2,3-dicarboxyimides as promising multifunctional materials.

Automated summary

Introducing different functional groups to the peripheral chains of discotic liquid crystals is a common strategy for designing and synthesizing new promising mesomorphic materials. By modifying the synthesis method, compounds with good thermal stability and high mesophase forming ability were successfully prepared.