Design and study of the properties of lanthanide molecular architectures

Abstract

Lanthanides are important elements for modern technology such as permanent magnets, electric car batteries, lasers, phosphors, medical imaging agents, etc. Their rich coordination chemistry, photophysical and magnetic properties allow researchers to develop better materials fulfilling the future demand. A brief introduction to lanthanides and their properties is included in Chapter 1. To contribute in this field, this thesis aims to design new lanthanide molecular complexes and supramolecular structures with intriguing properties such as photoluminescence, triboluminescence, high pressure luminescence and single­molecule-magnetism. Triboluminescence (TL) is a light emitting phenomenon induced upon mechanical stress on the materials. Although this process was discovered more than a hundred years, its mechanism is still scarcely known; therefore, more examples for its understanding is vital for potential application such as stress sensing and anticounterfeit labels. In Chapter 2, a series of lanthanide(III) β-diketonate complexes were synthesized for studying the induction of triboluminescence. The emitting species of the complexes were proven to be consistent between triboluminescence and photoluminescence. Correlation of crystal symmetry, lattice packing and gas discharge with triboexcitation found that either non­centrosymmetric or centrosymmetic compounds can be TL active. Finally, a new triboluminescence material [Eu(dbm)4TMP] was discovered with impressive triboluminescence visible under daylight. In Chapter 3, triboluminescence was studied by using lanthanide(III) halogenated-β­diketonate complexes. Two centrosymmetric coordination compounds [Eu(pp-dbm­Cl2)3phen] and [Eu(mm-dbm-Cl2)3phen] were found with triboluminescence and aggregation-induced emission. Their close crystal packing with extensive π···π, C-H···π and C-H···Cl-C interactions may allow TL, rarely for centrosymmetric compounds. For another series [Eu(dbm-X2)3bipy] (X = Cl, Br, F, I), it was found that the size and position of halogen atoms influence the crystal packing and triboluminescence. High pressure was applied to the materials by the diamond anvil cell, inducing phase shifting, peak shifting and broadening in luminescence spectrum of the complex. The results indicate the triboluminescence of the EuIII complexes occur at low pressure and is a fracture-induced process. This work can improve our understanding in triboluminescence and explore the possibility of lanthanide complexes applied as a pressure-stress senor for the first time. The multinuclear lanthanide(III) complexes also arise some interest in the field of single-molecule magnetism (SMM). Although the hydrolytic synthesis of lanthanide(III) clusters is well-established, it is scarcely studied along the whole LnIII series. In Chapter 4, tetranuclear cubane-like clusters, [Ln4(µ3-OH)4(µ­tfa)4(hfa)4(phen)4] (1-9, Ln = La-Dy (except Pm), tfa = trifluoroacetate, hfa = hexafluoroacetylacetonate), and dinuclear clusters, [Ln2(µ-OH)2(hfa)4(phen)2] (10-16, Ln = Tb-Lu), were synthesized and characterized, revealing the size of the LnIII ions determines the type of clusters formed. Furthermore, 8-Tb4 and 9-Dy4 cubanes are ferromagnetically coupled and exhibit unexpectedly good SMM behavior. The effective energy barrier Ueff of 9-Dy4 was determined to be 67.0 K under a zero-dc field. Additionally, 6-Eu4 and 8-Tb4 are emissive at room temperature and their photophysical properties were studied. The final types of complexes are coordination-driven lanthanide supramolecular cages, which can be potentially used in catalysis, chiral sensing and optical devices. However, most LnIII cages are not luminescent and thermodynamically stable. For improvement, in Chapter 5, β-diketonate chelating groups, which are known for good EuIII sensitization and providing ionic nature for the coordination bonds, were employed to construct C3 ligands to form face-directed M4L4 tetrahedral supramolecular cages. Flexible and rigid ligands were synthesized. Upon complexation with EuIII ion, the former resulted in mononuclear chelates, and the latter gave the tetranuclear (cage) and trinuclear (triangle) complexes. For the rigid ligand with a trifluoromethyl moiety, the EuIII cage and triangle were shown to be luminescent with 28% and 24% quantum yield respectively.