Polyoxometalate self-assembly: from molecules to hybrid materials

Molina Sánchez, Pedro (2013) Polyoxometalate self-assembly: from molecules to hybrid materials. PhD thesis, University of Glasgow.

Abstract

Polyoxometalates (POMs) are anionic oxides of the early transition metals in their highest oxidation state (i.e. V(V), Nb(V), Ta(V), Mo(VI) and W(VI)) which possess a wide structural diversity and an equally broad range of physical properties. Transition metal substituted polyoxometalates (TMSPs) are the subset of the POM family which members can be described as metal complexes of purely inorganic, highly nucleophilic, POM based ligands and oxophilic transition metals. TMSPs are usually synthesized in solution via multi-step reaction procedures; however the processes controlling their formation are poorly understood and usually described as "self-assembly". Hence, the design of synthetic methods aimed at producing a desired structure is based on a mixture of empirical observations and general considerations about the reactivities of both ligand and transition metal in solution. The judicious exploration of the synthetic parameter space (e.g. pH, added electrolytes, etc.) have resulted in the discovery of a high number of complexes, a process which contributes to a better understanding of the relationship between synthetic conditions and final structure. However, and despite the vast number of TMSPs reported in the past couple of decades, this connection between synthetic variables and the structure of the cluster is far from being fully established. In particular, the effect of high pH values of the reaction mixture and the presence of alkaline metal ions does not seem to have been fully explored and therefore a potentially high number of clusters may lay undiscovered. A section of this thesis is devoted to that exploration of the parameter space of TMSP synthesis involving four different lacunary POM ligands ({γ-SiW10}, {γ-GeW10}, {A-α-PW9} and {α-P2W15}) and late first-row transition metal ions. The synthetic variables scanned during this programme were primarily two: the increase of the pH, by means of the addition of several inorganic bases, and the addition of alkaline metal salts to the reaction mixtures. A number of novel TMSPs were thereby discovered which, thanks to their relevant structural features, represent a significant contribution to the final goal of rationally design pathways to produce these complex inorganic architectures.
The remarkable physical properties of POMs can be exploited by their integration into suitable composite materials. In particular, their ability to undergo several reversible electrochemical reductions whilst retaining their structural integrity, a property which is usually coupled to a pronounced change of optical absorbance, make these clusters ideal candidates for electrochromic applications. However, their performance in terms of optical contrast and switching time is typically lower than state-of-the-art electrochromic materials. A strategy employed to exploit and enhance the electrochromic response of POMs is to disperse them in an electrochromic organic polymer and hence produce an improved material in terms of its optical contrast. However, mostly classic POMs, with just one type of transition metal in their elemental composition and of relatively simple structure, have been examined for this application while only a small number of TMSPs have been evaluated for the same purpose. V(IV/V)-substituted POMs, for example, have not been used to fabricate this type of materials in spite of their electrochromic properties. A section of this thesis is devoted to the deposition and characterization of a number of TMSP-based hybrid films in terms of morphology, electrochemistry and electrochromic response. Electrodeposited films containing an electrochromic organic polymer and a dimeric V(V) TMSP, {V(V)6P4W30}, showed a higher optical contrast than films containing the monomeric equivalent, {V(V)3P2W15}, or POM-free films. This result constitutes a significant insight into the effect of the structure of the cluster on the electrochromic performance of POM-based hybrid materials.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Polyoxometalates, Inorganic synthesis, hybrid materials, thin films
• Subjects: Q Science > QD Chemistry
• Colleges/Schools: College of Science and Engineering > School of Chemistry
• Supervisor's Name: Cronin, Prof. Leroy
• Date of Award: 2013
• Embargo Date: 16 August 2017
• Depositing User: Mr. Pedro Molina-Sanchez
• Unique ID: glathesis:2013-4536
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Aug 2013 09:25
• Last Modified: 16 Aug 2016 08:00
• URI: https://theses.gla.ac.uk/id/eprint/4536