Publicação
Magnetic properties of first-row transition metal compounds with different nuclearities
| Resumo: | Spin crossover is a phenomenon, exhibited by transition metal complexes with appropriate spin pairing and ligand field energies, which shows great promise for the design of high-density information storage devices and molecular switches and sensors. The final application of the spin crossover device is determined by the spin transition profile. In the present work, several spin crossover candidate compounds with different nuclearities are presented. New materials were also fabricated based on a known spin crossover molecule. In chapter 2.1, several FeIII mononuclear related compounds, of the [Fe(5-X-salEen)2]Y type, were synthesised with varying halogens as substituents and most of these exhibited a gradual spin crossover, when studied by SQUID magnetometry. Different magnetic behaviour was observed for different solvates and compounds with a different ionic pair, as had been also noted in the literature. Density Functional Theory calculations suggested that the halogen electronic effect is not noticeable for the fluorine substituted compounds. The effect of ligand rigidity was also shown to affect the spin crossover phenomenon as one hexadentate compound was synthesised and exhibited no spin transition in the studied temperature range. In chapter 2.2, binuclear compounds with hydrazide ligands of the [Fe2(L)3] type, were also synthesised and studied by SQUID magnetometry. While none of the compounds exhibited any kind of spin transition, an antiferromagnetic coupling between both FeIII centres was observed for all compounds and later confirmed with Density Functional Theory calculations. Chapter 2.3 addresses an FeII cage that was designed and its synthesis attempted, however, the current available characterisation methods do not confirm the synthesis of the proposed cage. The isolated purple powder did not show spin crossover, but it exhibited different coupling effects between the Fe centres, depending on the temperature. Chapter 2.4 contains the fabrication of materials from spin crossover complexes based on [Fe(5-I-salEen)2]ClO4. Nano-sized structures were synthesised, from a known spin crossover compound, in a PEG matrix. Their size was studied by both NANO-flex measurements and AFM imaging, however, the results were not in agreement, as the particles observed by AFM probably corresponded to aggregates and the particles were destroyed when the sample for the NANO-flex measurements was prepared, hence, one can only conclude they are indeed in the nano-scale. Magnetometry measurements of these particles showed a gradual spin crossover behaviour which is expected for this kind of structures. Although no magnetic behaviour was determined, a few nano-rods were synthesised from the same compound and observed by AFM imaging. In a different approach, gold nanoparticles were coated with a spin crossover candidate compound, as suggested by UV-Vis spectroscopy and AFM imaging, however it was not possible to determine the magnetic behaviour of the resulting particles. Finally, in chapter 2.5, one can find a thermal analysis by Differential Scanning Calorimetry that was conducted on previously synthesised spin crossover compounds. Sharp peaks were observed at the same temperatures as the abrupt spin transitions, however, nothing is observed when the transition was gradual. |
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| Autores principais: | Martins, Frederico de Lacerda Ferreira dos Santos |
| Assunto: | Materiais magnéticos Transição de spin Nanopartículas Complexos de metais de transição Gaiolas moleculares Teses de mestrado - 2018 |
| Ano: | 2018 |
| País: | Portugal |
| Tipo de documento: | dissertação de mestrado |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Lisboa |
| Idioma: | inglês |
| Origem: | Repositório da Universidade de Lisboa |
| Resumo: | Spin crossover is a phenomenon, exhibited by transition metal complexes with appropriate spin pairing and ligand field energies, which shows great promise for the design of high-density information storage devices and molecular switches and sensors. The final application of the spin crossover device is determined by the spin transition profile. In the present work, several spin crossover candidate compounds with different nuclearities are presented. New materials were also fabricated based on a known spin crossover molecule. In chapter 2.1, several FeIII mononuclear related compounds, of the [Fe(5-X-salEen)2]Y type, were synthesised with varying halogens as substituents and most of these exhibited a gradual spin crossover, when studied by SQUID magnetometry. Different magnetic behaviour was observed for different solvates and compounds with a different ionic pair, as had been also noted in the literature. Density Functional Theory calculations suggested that the halogen electronic effect is not noticeable for the fluorine substituted compounds. The effect of ligand rigidity was also shown to affect the spin crossover phenomenon as one hexadentate compound was synthesised and exhibited no spin transition in the studied temperature range. In chapter 2.2, binuclear compounds with hydrazide ligands of the [Fe2(L)3] type, were also synthesised and studied by SQUID magnetometry. While none of the compounds exhibited any kind of spin transition, an antiferromagnetic coupling between both FeIII centres was observed for all compounds and later confirmed with Density Functional Theory calculations. Chapter 2.3 addresses an FeII cage that was designed and its synthesis attempted, however, the current available characterisation methods do not confirm the synthesis of the proposed cage. The isolated purple powder did not show spin crossover, but it exhibited different coupling effects between the Fe centres, depending on the temperature. Chapter 2.4 contains the fabrication of materials from spin crossover complexes based on [Fe(5-I-salEen)2]ClO4. Nano-sized structures were synthesised, from a known spin crossover compound, in a PEG matrix. Their size was studied by both NANO-flex measurements and AFM imaging, however, the results were not in agreement, as the particles observed by AFM probably corresponded to aggregates and the particles were destroyed when the sample for the NANO-flex measurements was prepared, hence, one can only conclude they are indeed in the nano-scale. Magnetometry measurements of these particles showed a gradual spin crossover behaviour which is expected for this kind of structures. Although no magnetic behaviour was determined, a few nano-rods were synthesised from the same compound and observed by AFM imaging. In a different approach, gold nanoparticles were coated with a spin crossover candidate compound, as suggested by UV-Vis spectroscopy and AFM imaging, however it was not possible to determine the magnetic behaviour of the resulting particles. Finally, in chapter 2.5, one can find a thermal analysis by Differential Scanning Calorimetry that was conducted on previously synthesised spin crossover compounds. Sharp peaks were observed at the same temperatures as the abrupt spin transitions, however, nothing is observed when the transition was gradual. |
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