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Multifunctional Magnets Through Rational Design

Final Report Summary - FUNCTIONAL MAGNETS (Multifunctional Magnets Through Rational Design)

The aim of this project was to develop multifunctional magnetic framework materials as a means of elucidating chemical and physical change in porous materials. Through the use of organic radical ligands and metalloligands, new framework materials would be created that allow us to investigate physical processes such as guest sorption and structural changes through perturbation of the magnetic properties of the materials. Chemical change could also be tracked through turn-on/turn-off switching of radicals either trapped in pores or included as part of the structure of the framework. This could give insights into the tailored design and construction of magnetic materials with desirable magnetic properties for switching and sensing applications and also into how magnetic species could be utilised for devices such as memory storage and spintronics.

The execution of the project adopted varied synthetic approaches including the synthesis of radical ligands for clusters and the creation of paramagnetic or fluorescent porous framework materials for guest detection.

▪ Radicals and ligands
The synthesis of nitronyl nitroxide radicals follows literature precedent and progresses through the reduction of 2,3-dimethyl-2,3-dinitrobutane, condensation with an aldehyde and then oxidation over PbO2 or NaIO4 to give strongly-coloured radicals. A number of new radicals were envisaged with multiple chelation sites to allow direct overlap of paramagnetic metals orbitals and the radicals, leading to exceptionally strong magnetic couplings. A number of α,β-dinitro compounds were tested along with a range of aldehydes with multiple coordinating groups.

▪ Frameworks and coordination polymers
The synthetic techniques used in framework and coordination polymer synthesis were varied but mainly centred on slow diffusion techniques and solvothermal crystallisation. The former allows the growth of crystals from solution mixtures that would normally lead to instant precipitated powders by markedly decreasing the rate of reaction. The latter method involves heating solvents to high temperatures, often above the boiling point of the solvent, in high-pressure reactors with reactants, increasing the solubility of the reactants and products and allowing larger crystals to form. Slow decomposition of dimethylformamide under sealed high-temperature conditions allows for the gradual production of dimethylamine which deprotonates carboxylate ligands to form frameworks with metal ions at a controlled rate. The MOFs formed are then activated by exchanging DMF from the pores with a volatile, non-coordinating solvent such as dichloromethane and then removing this under vacuum.

▪ Magnetic characterisation
Magnetic susceptibility measurements were performed in conjunction with collaborators in Sydney and Dublin and utilised new techniques such as time-dependent magnetometry for tracking structural changes through modification of the magnetic properties of paramagnetic metal ions. In addition to magnetometry, EPR was carried out on solid and solutions to identify precursor and transient species in the formation of coordination materials.