Columnar self-assembly of luminescent bent-shaped hexacatenars with a central pyridine core connected with substituted 1,3,4-oxadiazole and thiadiazoles

Abstract

Bent-shaped molecules with a central pyridine core flanked with substituted 1,3,4-oxadiazole and thiadiazole derivatives with a variation in the number and length of terminal tails were synthesized. Thiadiazole based compounds exhibited a wider mesophase range in comparison to oxadiazole derivatives, while the oxadiazole derivatives exhibited a higher gelation tendency. All hexacatenars exhibited supergelation in hydrocarbon solvents along with an ability to form self-standing, moldable gel at higher concentration. Thiadiazole based compounds exhibited bathochromic absorption and emission in comparison to oxadiazole derivatives but a lower quantum yield. Two of the gelators investigated exhibited aggregation induced enhanced emission in gel and thin film state. This study shows that in addition to π–π interactions, nanosegregation of incompatible molecular subunits like flexible tails plays a major role in gelation and liquid crystalline self-assembly. Microscopic studies and X-ray diffraction studies revealed a fibrillar network of several micrometers in length with long range molecular self-assembly. They showed the ability to sense acids with an emission quenching/shifting mechanism, which makes it possible to detect the acids by naked eye. Considering the dearth of solid-state organic blue light emitters that are pivotal to realize the white light emission, these polycatenars are promising due to their wide-range Col phase and aggregation induced blue emission. Further the introduction of the pyridine central unit enhanced the mesophase stability and the acid sensing functionality in comparison to simple benzene-based bent-shaped polycatenars.