Amin, Sharad
(2021)
The Supramolecular Chemistry of Amphiphilic Redox-Active Organic-Inorganic Hybrid Polyoxometalates.
PhD thesis, University of Nottingham.
Abstract
This thesis focuses on the synthesis and supramolecular assembly of Class II hybrid-POM amphiphiles, in which these molecules will be synthesized, characterized and their supramolecular assembly studied.
Chapter 2 explores the development and supramolecular assembly of a hybrid-POM amphiphile {W17C20} based on a lacunary Wells-Dawson polyphosphotungstate cluster hybridised to two hydrophobic Cn (n = 10, 12, 14, 16, 18, 20) chains through phosphonic acid linkers. This investigation focused on how ligand chain length affects the supramolecular assembly and the redox activity of the supramolecular assembly. Here, the longer chain lengths led to smaller, stable more discrete assemblies which demonstrate contrasting redox chemistry to the molecular species.
Chapter 3 studied the hierarchical self-assembly behavior of hybrid-POM amphiphile, where surface interactions and temperature stimuli were probed. Redox-active micelles based on {W17C20} were found to self-assemble into monolayers and retain redox-activity at glassy carbon electrode surfaces. Furthermore, based on the hydrophobicity of various carbon surfaces, micellar structures were found to transition upon deposition to other higher order structures such as fibers. Hierarchical selfassembly transition was also found when freezing aqueous micelle solutions of {W17Cn} which led to the formation of cylindrical micelles. The degree of order was correlated to the increasing chain length of the amphiphile. Furthermore, the cylindrical micelles could be isolated and acted as soft-templates for nanostructured metal oxide material.
Chapter 4 focused on the development of new hybrid linkers and ligands based on the hybridisation of diphosphonate molecules (methylene diphosphonate (MDP), imidodiphosphonate (IDP) and pyrophosphate (PP)) with lacunary Wells-Dawson cluster {W17}. We found that based on the electronic nature of the diphosphonate, the POM cluster could be photoactivated with varying degrees upon hybridisation which was tested through the photoreduction of the hybrid clusters in the presence of sacrificial electron donor DMF when irradiated with UV-Vis or visible light. This correlated to the electron withdrawing (EW) or electron donating (ED) effects imposed by the ligand on the POM. Here, MDP hybrid {W17MDP} was the most photoactivated towards oxidation of DMF under visible light irradiation and more easily reduced owing to MDPs lesser ED nature compared to the other hybrids. Furthermore, MDP type ligands could be modified to couple additional organic moieties to the POM.
Chapter 5 focused on the development of mixed-addenda hybrid organo-functionalized POMs. Here we found that the HOMO-LUMO energy levels of the POM could be further modulated and tuned with respected to organophosphate hybrids. Mo substitution (for W) into the hybrid clusters led to a positive shift in the reduction potentials with respect to it pure tungsten hybrid analogue. EPR spectroscopy also reported the localisation of the electrons on the Mo centers with respect to the first reductions. Furthermore, DFT calculations found that metal substitution lowers the HOMO-LUMO energy gap. Similarly, amphiphilic hybrid based on organophosphate mixed-metal cluster demonstrated contrasting surface confined electrochemistry to pure tungsten analogue.
Finally, chapter 6 focused on employing hybrid-POM amphiphiles as feedstock for spray-drying softtemplates for solid-state metal oxide particle fabrication. Here we found contrasting structures exhibited when processing plenary POM clusters compared to hybrid-POM amphiphiles; differences were found in the soft-template as well as in the post-calcination material. Mixed-metal hybrids were also employed to produce doped metal oxide particles. PXRD analysis showed transition from soft templates to mixed phases of WO3.
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