A study on the preparation of supramolecular and polymeric materials containing lanthanide ions and their luminescent or catalytic properties

Abstract

Lanthanide(III) ions are of great interest due to their characteristic optical properties such as narrow emission and long lifetime originated from intraconfigurational f-f transition. In this study, lanthanide(III) ions were utilized as a luminescence center in organogelators after coordination with aromatic molecules, showing enhanced luminescence and color change during gelation in organic solvents. A lanthanide(III) ion was also incorporated in the binding sites of a molecularly imprinted polymer as a signal transducer in luminescence sensing. When a lanthanide(III) ion was doped in microporous organic polymer, gas adsorption and catalytic activity of the polymer were enhanced. Firstly, Eu(III) and Tb(III) complexes (Eu2 and Tb2) were prepared with phenanthroline derivatives, which formed organogels in n-decane. The organogels of Eu2 and Tb2 showed red and green emissions, respectively, when excited at 330 nm. TEM images of dry gels of Eu2 and Tb2 showed entangled fiber network structures with fiber diameters ranging from 20 to 80 nm. Because of the structural similarity of Eu2 and Tb2, they formed stable mixed gels of different compositions in n-decane. The emission spectrum of a mixed gel showed emission peaks from Eu(III) at 595 and 618 nm and from Tb(III) at 496 and 551 nm when excited at 330 nm. A broad emission was observed around 470 nm that was attributed primarily to the ligands of Tb2. At a ratio of Eu2 / Tb2 of 1 : 19 by weight, the mixed gel exhibited white luminescence. The 1931 CIE color coordinates of the mixed gel were x = 0.36 and y = 0.30 in the white region. The TEM image of the dry gel revealed entangled fibers with diameters ranging from 20 to 80 nm without evidence of self-sorting. Trimetallic organogelators, Eu4 and Tb4, were prepared by coordination of Eu2 and Tb2 with two 1,3-diketone groups. Eu4 showed three transitions of Eu(III) ion in red region and a ligand-centered emission but Tb4 showed only a broad emission in greenish blue region from the ligands. Eu4 and Tb4 also formed a homogeneous mixed gel in n-decane, showing a whitish luminescence at the ratio of 3 : 1 by weight. Furthermore, Eu4 showed film-forming property. The complex was fabricated as a not only thin film by spin casting but also free-standing film by melt casting without losing the red luminescence. A Er(III) complex, Er4 was also synthesized and fabricated as fiber network embedded in cross-linked EGDMA film. The film showed an emission in NIR region ranging from 1450 to 1650 nm, potentially utilized in telecommunication. Secondly, it was demonstrated that the molecularly imprinted system bearing the Eu(III) ions could be used for the direct detection of chromophoric organic molecules. A molecularly imprinted polymer containing Eu(III) ions in binding cavities (MIP-Eu) was prepared by polymerization of a complex of non-chromophoric monomer (3-allylpentane-2,4-dione) and the Eu(III) ion. Picloram was used as a template molecule, which is a widely used chlorinated herbicide persistent in water or soil. The ability of MIP-Eu to recognize the template was investigated by photoluminescence spectroscopy. It was able to detect the template molecules captured in the cavities directly by observing the sensitized luminescence of the Eu(III) ions. The emission peak intensities of the Eu(III) ions at 594 and 616 nm intensified with the increase in the picrolam concentration. The specific recognition ability of MIP-Eu was also investigated for the template against its structural analogs such as dicamba and 2-amino-4,6-dichloropyrimidine-5-carboxaldehyde by photoluminescence spectroscopy. Lastly, a Yb(III)-incorporated microporous polymer (Yb-ADA) was synthesized and its gas adsorption property and catalytic activity were studied. The adamantane-based porous polymer (ADA) was obtained from an ethynyl-functionalized adamantane derivative and 2,5-dibromoterephthalic acid through Sonogashira-Hagihara cross-coupling. ADA had carboxyl groups which were used for Yb(III) coordination under basic condition. The Brunauer-Emmett-Teller (BET) surface area of ADA was 970 m2 g-1. As Yb(III) ions were incorporated into ADA, the surface area of the polymer (Yb-ADA) was reduced to 885 m2 g-1. However, Yb-ADA exhibited a significantly enhanced CO2 and H2 adsorption capacities despite the reduction of the surface area. The CO2 uptakes of ADA and Yb-ADA were 1.56 and 2.36 mmol g-1 at 298 K, respectively. The H2 uptake of ADA also increased after coordination with Yb(III) from 1.15 to 1.40 wt% at 77 K. Yb-ADA showed a high catalytic activity in the acetalization of 4-bromobenzaldehyde and furfural with trimethylorthoformate and could be reused after recovery without severe loss of activity.