Triphenylene Discotic Pd(II) Metallomesogens Based on Triazole Ligands Derived from the Click Reaction

Two palladium complexes (Pd[DiTP-triazole]2Cl2 and Pd[TrTP-triazole]2Cl2) bearing a 1,2,3-triazole segment as ligands were synthesized. The 1,2,3-triazole moiety in Pd[DiTP-triazole]2Cl2 was synthesized by the Cu(I)-catalyzed azide–alkyne cycloaddition reaction. Meanwhile, the 1,2,3-triazole in Pd[TrTP-triazole]2Cl2 were synthesized through the Click reaction involving an internal alkyne and an organic azide. These two complexes feature soft spacers of comparable length between the triphenylene cores and the 1,2,3-triazole units, which are designed to enhance the molecular symmetry and improve the liquid crystalline properties. The structures of both complexes were fully characterized, with a specific focus on their confirmation through single-crystal X-ray diffraction (XRD) analysis of two model molecules, Pd[DiB-triazole]2Cl2 and Pd[TrB-triazole]2Cl2. Additional techniques, such as differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and powder XRD, were used to confirm the room temperature discotic liquid crystal (DLC) properties of all palladium complexes. It was found that both complexes Pd[DiTP-triazole]2Cl2 and Pd[TrTP-triazole]2Cl2 exhibit hexagonal columnar mesophases at room temperature. Significantly, the temperature ranges of these mesophases were substantially extended, showing increases of 70 and 56 °C, respectively, compared to their ligands. Additionally, upon excitation at 279 nm, all the ligands and complexes emit ultraviolet to visible blue light as for the critical role of the triphenylene cores.