Synthesis, Gelation And Liquid Crystalline Behaviors Of1,3,4-oxadiazole Deirvatives

Supramolecular gels are fascinating organized soft materials that can respond toexternal stimuli such as temperature, light, and chemicals. The gelation propertieswhich can be correlated with external factors （such as light, temperature, solvent） andinternal factors （such as molecular structure） determine the potential applications ofgels in optoelectronics, drug delivery and release, and sensors for chemicalsurroundings. We have synthesized three series1,3,4-oxadiazole derivatives:amide-containing bi-1,3,4-oxadiazole derivative （BOXDH-T12）, pyridyl-containing1,3,4-oxadiazole derivatives （4-POXD-6,4-POXD-Bn,4-POXD-T12）, andbiphenyl-containing1,3,4-oxadiazole derivative （Bp-OXD-B8）. The correlationsbetween solvent properties and gelation behaviors of4-POXD-Bn were systematicallyinvestigated, and the effects of molecular structures （lengths of the terminal chainsand rigid cores） and metal ions were discussed to understand the driving force fortheir gelation. The obtained results were outlined as follows:The self-assembling behaviors of BOXDH-T12depended on solvents that itself-assembled into H-aggregates in alcohols and slipped-packing aggregates inDMSO. FTIR,1H NMR and TGA results revealed that strong gelator-gelatorhydrogen bonding interaction induced H-aggregation of BOXDH-T12in alcohols andthe interactions between DMSO and BOXDH-T12molecules caused a slippedstacking. BOXDH-T12can gel the mixtures of DMSO and ethanol through a cooperative effect of the hydrogen bonding, van der Waals interaction and π-πstacking forces, furthermore, helical ribbons could be observed in DMSO/ethanol dueto DMSO molecule interacting.By employing a van’t Hoff analysis of solubility data, determined from simpleNMR measurements, we calculated the molar enthalpies （ΔHdiss） of4-POXD-B12and4-POXD-T12are much higher than that of4-POXD-B8and Bp-OXD-B8, indicatingthat the solvophobic force was the driving force for self-assembly. For the specificgelator （4-POXD-Bn）, we observed that the strength of the solvophobic force wasrelated to the solvent properties. Solvent effect on gelation was studied quantitativelyusing the solvent polarity parameters: polar solubility parameter （δa）, dielectricconstant （ε）, and polarity parameter （ET（30））. The results revealed that the favorable δadomain and ET（30） domain for the gelation of4-POXD-B8was in the range of8.1–12.6（cal cm-3）1/2and43.3–55.7kcal mol-1, respectively. Meanwhile, a decreaseof δaand ET（30） in the solvents is unfavorable for gelation.[Ag（4-POXD-B8）₂][NO₃] that was obtained by coordinating AgNO₃with4-POXD-B8. The transparent organogels of [Ag（4-POXD-B8）₂][NO₃] formed in theTHF/ACN, TOL/ACN and ethanol and exhibited good rheological property. Thecomplex shows liquid crystalline columnar phases. A series of supramolecularmesogenic complexes were obtained by mixing4-POXD-6and4-alkoxybenzoic acids（Cn, n=6,8,12）. Through the means of differential scanning calorimetry, polarizedoptical microscopy, and wide-angle X-ray diffraction, it was found that4-POXD-6/Cndisplay monotropic nematic and smectic A phases.