Star-shaped Triphenylene-triazine Multi-stimuli Responsive Discotic Liquid Crystals: Synthesis, Properties and Applications

The electronic and optical properties of p-conjugated materials are determined by their molecular structures and supramolecular architectures. Discotic liquid crystals consist of p-conjugated aromatic core and peripheral alkyl chains, they self-organize into two-dimensional discotic columnar mesophase, which can be applied as charge carrier transport channel in optical and electronic thin-film devices. Order and dynamics are two unique features of discotic liquid crystalline semiconductors. Three star-shaped 1,3,5-triazine-triphenylene discotic liquid crystals TPPTn (n=6, 8, 12) were synthesized by Suzuki-Miyaura cross-coupling reaction and characterized. Thermogravimetric analysis (TGA) shows that TPPTn have good thermal stability, and their thermal decomposition temperatures were higher than 350.. The ultraviolet-visible (UV-Vis) absorption spectra and fluorescence spectra showed that they have outstanding fluorescence emissions in solution, thin film and even in solid state, and the length of alkoxy chain affects a little the fluorescence properties of the film or solid state by affecting the molecular aggregation. TPPTn have the aggregation-induced emission (AIE) effect: longer the alkoxy chain, more obvious the AIE phenomenon, this AIE effectively overcomes the disadvantage of aggregation caused quenching (ACQ) of p-extended discotic liquid crystalline molecules. Further, TPPT6 exhibits significant solvatochromism from blue to orange with the increasing of solvent polarity, due to the intramolecular charge transfer (ICT), which provides possibility for visual recognition of solvent polarity. The fluorescent emission absolute quantum yield is up to 43%. What is more, TPPT6 exhibits a temperature-responsive fluorescence enhancement with increasing temperature in solution, which is different from the temperature quenching of traditional molecules, and the fluorescence exhibits a reversible switching from green to blue with increasing/decreasing of temperature. TPPT6 also has obvious acid-base stimuli- responsive fluorescent response by CF3COOH and Et3N. 1H NMR titration proves that the protonation of triazine core is the mechanism to acid discoloration. Based on the acid-base stimuli response phenomena, two kinds of fluorescent safety inks have been designed and demonstrated. Differential scanning calorimetry (DSC), polarizing microscope (POM) and X-ray diffraction (XRD) indicate that these star-shaped molecules TPPTn have liquid crystal properties, self-organizing into columnar rectangular (Colrec) mesophase with mesophase temperature range as wide as 278.. Trifluoroacetic acid (CF3COOH) modulates successfully the mesomorphic behavior of TPPTn by ionizing the triazine core. In addition, TPPT6 also displays electroluminescent properties, which has been applied to a green organic light-emitting diode (OLED), and the luminous efficiency of OLED reaches 7.41 lm/W. The star-shaped triphenylene-triazine TPPTn have potential applications as liquid crystalline semiconductors, fluorescence sensing, information encryption and organic light-emitting diodes.