Columnar self-assembly, gelation and electrochemical behavior of cone-shaped luminescent supramolecular calix[4]arene LCs based on oxadiazole and thiadiazole derivatives

Abstract

A new class of blue light-emitting supramolecular liquid crystalline cone or bowl-shaped compounds were synthesized from substituted 1,3,4-oxadiazoles and 1,3,4-thiadiazoles inbuilt with ester groups containing substituted alkoxy side chains (–OC₄H₉, –OC₈H₁₇) at the lower rim of calix[4]arene. These supramolecular compounds were investigated by polarizing optical microscopy (POM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), high-temperature X-ray diffraction (XRD), cyclic voltammetry (CV) and photophysical behavior studies. It was observed that the 1,3,4-thiadizole calix[4]arene-based derivatives are more capable of stabilizing the hexagonal columnar phase over a broad thermal range compared to the 1,3,4-oxadiazole calix[4]arene derivatives. The cone-shaped calixarene-based LCs with terminally substituted alkyl chains are packed into columns with enhanced intermolecular interactions. Moreover, the 1,3,4-oxadiazole calix[4]arene derivatives show deeper blue emissions compared to the 1,3,4-thidiazole calix[4]arene derivatives in solution as well as in thin films. The thermal and photophysical behaviors of the synthesized bowl-shaped supramolecular derivatives are extremely dependent on the terminal substituted alkoxy chains on the lower rim of calix[4]arene. Due to their hexagonal columnar phases and the emissive nature of their supramolecular liquid crystal and xerogel states, these molecules are favourable for application in emissive electronic displays. The 1,3,4-thiadiazole calix[4]arene derivatives display lower band gaps than the 1,3,4-oxadiazole calix[4]arene derivatives and are considered to be promising entities for applications in organic light emitting diodes (OLEDs).