Rational design and modelling of f-block molecular nanomagnets
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2016
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07-03-2016
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Abstract
The project developed in this thesis is to provide a suitable framework to further develop an emerging research area in nanoscience that deals with the chemical design of molecular systems exhibiting interesting magnetic phenomena. In particular, it focuses on the setting up, application and benchmarking of a theoretical and computational framework for the inexpensive rationalisation and prediction of new f-element molecular nanomagnets. These systems are amongst the most complex entities in molecular magnetism, exhibiting slow relaxation of the magnetisation and magnetic hysteresis at low temperatures. They have been proposed as promising candidates for the development of high-density magnetic memories, magnetic refrigeration, quantum computing devices and for several applications in molecular spintronics.
The first part of this thesis describes the development of the theoretical framework. The main tool of this project, the computational package SIMPRE, is presented in Chapter 2, providing some practical information about its organisation and usage. Chapter 3 consists in a preliminary attempt to rationalise the conditions that a lanthanoid-based magnetic molecule needs to satisfy to act as a single-ion magnet (SIM) using the original point charge electrostatic model. The inherent drawbacks of the PCEM are overcome in Chapter 4, where two effective electrostatic approaches that include covalent effects, the Radial Effective Charge (REC) and the Lone Pair Effective Charge (LPEC) models, are presented.
The second part deals with the application of these effective electrostatic models to a series of lanthanide homoleptic coordination
compounds. In Chapter 5, a simultaneous systematic study of four isostructural families coordinated by halogen atoms using spectroscopic information permits an estimation of the REC parameters by relating them to chemical concepts such as Pauling electronegativity and the coordination number. This simplifies the task of obtaining an initial set of CFPs for phenomenological fittings. Chapter 6 extends the application of the REC model to systems coordinated by oxygen-donor ligands. These include five octa-coordinated polyoxometalate (POM) complexes with interest in molecular magnetism and quantum computing and a high-symmetry oxydiacetate family, where the capabilities of the model are benchmarked owing to the available photoluminiscence spectra. Furthermore, the model is used to interpret the properties of two layered dysprosium hydroxides and played a key role in the rational design of the first example of metal-organic framework having lanthanide SIMs as nodes. In Chapter 7, the LPEC model is used to model the magnetic and spectroscopic properties of the first SIM reported in the literature, based on phthalocyaninato ligands, where the electron density is not pointing directly to the lanthanoid. Two complexes coordinated to pyrazolyl-based ligands are also analysed in terms of the REC model on that chapter, which completes the trilogy presented in this part of the thesis.
The final part of the work is centred on the current challenges of the developed framework. Chapter 8 discusses the capabilities of both the program and the REC model to deal with the understanding of uranium SIMs, and introduces the use of the CONDON package to complement the limitations of the first. A second challenge –the modelling of magnetic molecular anisotropy– is addressed in Chapter 9 using an organometallic SIM as model system. Subsequently, the REC model is used to investigate the treatment of heteroleptic complexes coordinated by oxygen and nitrogen atoms. The determination of the chemical structure at different temperatures allows to quantify, for the first time, the influence of the thermal evolution of the molecular structure in the electronic structure and magnetic anisotropy. Finally, Chapter 11 provides a few insights for the rational design of molecular spin qubits based on mononuclear lanthanide and uranium complexes. Through two examples, the chapter also describes the main features that make POM complexes particularly interesting for quantum information purposes.