Cristais líquidos iônicos diméricos com propriedades luminescentes e gelificantes

Abstract

In this master’s work, 5 new dimeric ionic molecules containing the 2,5-disubstituted 1,3,4-oxadiazole heterocycle were synthesized, purified and characterized by structural, thermal and photophysical analyses. Structural analyzes were investigated by 1H NMR spectroscopy. Thermal and liquid-crystalline analyzes were performed later using POM, DSC and TGA techniques. In parallel, all compounds had their photophysical properties analyzed in solution using the UV-vis absorption and fluorescence emission technique. All 5 target molecules of the project showed liquid-crystalline properties, with the formation and stabilization of smectic A mesophase in the heating and cooling of the samples. The phase transition temperatures of the compounds were high, thus causing thermal decomposition of the materials during POM and DSC analyses. A procedure adopted to overcome this problem was the increase of the spacer between the mesogenic units of the dimers and the change of counterion of the final molecules, from bromide (Br-) to the tetrafluoroborate anion (BF4-), being the mesomorphic properties of the molecules with bromide anion not investigated in this work. Changing the counter-ion to a more voluminous one did not generate satisfactory results, but the increase in the spacer caused a gradual decrease in the isotropization temperatures of the materials, facilitating the performance of thermal analyzes and guaranteeing their reproducibility. On the other hand, photophysical analyzes were carried out on molecules containing the bromide counter-ion and then with the tetrafluoroborate counter-ion, in order to investigate the influence that the counter-ion would have on absorption and fluorescence emission regions of the materials. In addition, optical studies of intermediate molecules were carried out in order to better understand the complex spectroscopic properties of the final compounds. Thus, it was verified that the final molecules present a double photophysical behavior, which combines the optical properties of the two mesogenic units that give rise to them. Furthermore, all molecules exhibit the phenomenon of Förster resonant energy transfer (FRET), with the materials absorbing in the regions of 282 and 379 nm, and emitting at 357 and 548 nm, thus causing an overlap between one of the absorption bands with an emission band. In addition to the formation of liquid-crystalline mesophases for all materials, it was also observed the formation of gels within a few minutes when the materials were dissolved in DMSO and in chloroform, for molecules with bromide and tetrafluoroborate counter-ion.