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

• Published in: Chinese journal of chemistry Vol. 81; no. 5
• Main Authors: Chongyang, Zeng, Ping, Hu, Biqin, Wang, Wenyan, Fang, Keqing, Zhao, Bertrand, Donnio, Donnio, Bertrand
• Format: Journal Article
• Language: English
• Published: Wiley-VCH Verlag 2023
• Subjects: Chemical Sciences; or physical chemistry; Organic chemistry; Theoretical and; multi-stimuli response; AIE; triazine; Suzuki cross-coupling reaction; Triphenylene; discotic liquid crystals
• ISSN: 1001-604X; 1614-7065
• DOI: 10.6023/A23010006

Electronic and optical properties of pi-conjugated materials are determined by their molecular structures and their supramolecular architectures. Discotic liquid crystals consist of -conjugated aromatic cores surrounded by various connected peripheral alkyl chains, which then self-organize into two-dimensional discotic columnar mesophases. They are of particular interest as they can be applied as charge carrier transport channels for optical and electronic thin-film devices. Order and dynamics are two unique features of discotic liquid crystalline semiconductors. Three star-shaped phenylene-bridged 1,3,5-triazine-triphenylene discotic liquid crystals TPPTn (n = 6, 8, 12) were synthesized by Suzuki-Miyaura cross-coupling reaction and characterized by 1 H and 13 C NMR, HRMS and EA. These TPPTn show good thermal stability by thermogravimetric analysis (TGA), with decomposition temperatures higher than 350 °C. UV-vis absorption and fluorescence measurements show that they possess outstanding fluorescence emission in solution, thin films and in solid state, and that the length of the alkoxy chains affects the fluorescence properties in films or solid state, due to slight changes of the molecular aggregation. TPPTn display aggregation-induced emission (AIE): the longer the alkoxy chains, the stronger the AIE; here, AIE effectively overcomes the disadvantage of aggregation caused quenching (ACQ) of pi-extended discotic liquid crystalline molecules. Further, TPPT6 exhibits a significant solvatochromism from blue to orange with increasing solvent polarity, due to intramolecular charge transfer (ICT), which thus provides a possibility for visual recognition of the solvent polarity. The fluorescence absolute quantum yield is up to 43%. Moreover, TPPT6 exhibits enhanced temperature-responsive fluorescence with increasing temperature in solution, which is different from the temperature quenching observed for traditional molecules; moreover, the fluorescence exhibits a reversible switching from green to blue with the increase/decrease of the temperature. Furthermore, TPPT6 has a clear acid-base stimulus-responsive fluorescence in the presence of CF3COOH/Et3N. 1 H NMR titration proves that the protonation of the triazine core is the main mechanism to acid discoloration. Based on the acid-base stimuli response phenomena, two kinds of fluorescent safety inks have been designed and demonstrated. DSC, POM and S/WAXS indicate that these starshaped molecules TPPTn have liquid crystal properties, self-organizing into columnar rectangular (Colrec) mesophase over large temperature range as wide as 278 °C. Trifluoroacetic acid (CF3COOH) modulates successfully the mesomorphic behavior of TPPTn by ionization of the triazine core. Finally, TPPT6 also displays electroluminescent properties, which can be used for the preparation of security inks and has been applied to green organic light-emitting diode (OLED), for which the luminous efficiency of the OLED reaches 7.41 lm/W. In conclusion, the star-shaped triphenylene-triazine TPPTn feature many potential applications as liquid crystalline semiconductors, fluorescent sensing, information encryption and organic light-emitting diodes.