Modulating the Single-Molecule Magnet, Magnetocaloric and Luminescent Behavior in Metallacrowns.

Abstract

The first part of this thesis focuses on the study of single-molecule magnets (SMMs), which have potential uses in high-density magnetic data storage. A new family of [M4Ln2(shi3-)4(Hshi2-)2(H2shi-)2(C5H5N)4(CH3OH)x(H2O)x] complexes (M = GaIII, FeIII; Ln = GdIII, TbIII, DyIII, ErIII, YIII0.9DyIII0.1) were prepared in order to investigate the effect of 3d and 4f magnetic interactions on slow magnetic relaxation behavior. It was found the antiferromagnetic 3d-4f coupling had adverse effects on slow magnetic relaxation. Furthermore, the dynamic magnetic behavior in the Ga4Dy2 analogue was elucidated, with two relaxation processes being attributed to the decoupled and excited ferromagnetic states. The magnetocaloric effect (MCE) is a phenomenon which holds promise for low-temperature refrigeration applications. Iron(III), an inexpensive, isotropic S = 5/2 ion, was selected to develop efficient low-temperature magnetic refrigerants. An investigation of FeIII(X)3[9-MCFeIIIN(shi)-3] compounds (X = acetate or benzoate) revealed that inter- and intramolecular magnetic interactions could be tuned to achieve greater MCE behavior. The acetate complex exhibited a -ΔSm value of -15.4 J kg-1 K-1 (T = 3 K, ΔH = 7 T), which is comparable to higher nuclearity FeIII clusters. Extensive antiferromagnetic intermolecular interactions resulted in a smaller MCE in the benzoate derivative and an analogous FeIII2(isopthalate)3[9-MCFeIIIN(shi)-3]2 dimer compound. These studies show that rational design and control of magnetic interactions may be employed to develop high performance MCE materials. LnIII(benzoate)4[12-MCGaIIIN(shi)-4](pyridinium+) complexes (LnIII = SmIII, EuIII, GdIII, TbIII, DyIII, HoIII, ErIII, TmIII, YbIII) were found to be capable of sensitizing both visible and NIR emitting LnIII ions. Efficient energy transfer from the ligand T1 state to the emitting state on the LnIII led to the observation of remarkable luminescent behavior. In particular, solid state quantum yields for the YbIII and ErIII analogues (5.88% and 4.4·10-2%, respectively) are greater than any reported in the literature. This system presents a highly efficient and modular platform on which to develop practical bio-imaging agents. The work presented in this thesis demonstrates that physical properties can be tuned through systematic ligand and metal substitution in metallacrown coordination complexes. These results have given new insight towards the understanding of single-molecule magnets, MCE materials and luminescent lanthanide complexes.