Novel photoluminescent chiral liquid crystalline oligomers containing lanthanide ions

Abstract

Novel photoluminescent chiral liquid crystalline oligomers with lanthanide ions (Ln-LCOs) exhibiting excellent liquid crystalline properties and unique fluorescence properties were prepared using poly(methylhydrogeno)siloxane, cholesteric LC M₁, carboxyl-containing LC M₂ and anhydrous lanthanide chlorides. The chemical structures and liquid-crystalline behaviors of Ln-LCOs were characterized by various experimental techniques. The introduction of low content of lanthanide ions endowed the oligomers with significant luminescence properties. All the oligomers were chiral. The lanthanide ions did not change the liquid crystalline textures of the oligomers which were confirmed by X-ray diffraction. Fourier transform infrared imaging study indicated that the lanthanide ions were evenly distributed in the oligomers, which avoided the fluorescence quenching caused by the aggregation of lanthanide ions. A binary complex model with lanthanide ions acting as central ions and chiral liquid crystalline oligomers acting as ligands was established to express the interaction and distribution of the components. The Ln-LCOs showed reversible mesomorphic phase transitions, wide mesophase temperature ranges, and high thermal stabilities. The thermogravimetric analysis results displayed that the decomposition temperatures (5% weight loss) of all Ln-LCOs were greater than 300 °C. The Ln-LCOs can emit red light or green light when being excited. Luminescence intensities of the Ln-LCOs gradually increased with an increase of lanthanide ions from 1 to 4 mol%. The temperature dependence of luminescent intensity was studied, where the fluorescence intensities of Ln-LCOs decreased monotonically with an increase of temperature in liquid crystalline phases.