Exploring the electronic and photocatalytic properties of organic-inorganic hybrid polyoxometalates

This thesis encompasses the synthesis and characterisation of electro- and photoactive organic-inorganic hybrid polyoxometalates (POMs). Specifically, the use of aromatic organic groups, either as countercations (Class I), as covalently grafted moieties (Class II) or combination thereof, have been explored with regards to their abilities to fine-tune the inherent properties of the POM cluster. The ability to tune the properties of redox and photo-active polyoxometalates represents an important step in the formation of “designer” materials, where the physical and electronic properties of the system can be tailored towards bespoke applications. In Chapter 2, the combination of redox rich Wells-Dawson phosphotungstates with fluorescent dye-type π-extended benzathiadiazole (BTD) cations yield a family of Class I hybrids with different modes of quaternization (proton, methyl, acetic acid). Crystallographic studies on the protonated derivative showed that the planar BTDImH structure is perturbed by hydrogen bonding to the POM and the solvent. Measurements of the electronic properties of the materials in the solution and solid state revealed stark differences, suggesting that the properties of the material are a product of interactions between the two components in the solid state. It also demonstrated that the different quaternisation strategies resulted in different electrochemical behaviour due to the different protic environments provided. Finally, preliminary studies on the hybrid featuring BTDImH cations revealed that the compound is capable of acting as a carbon dioxide photoreduction catalyst, whereas the respective components showed no activity, testament to the synergic effects gained when the photoactive components are combined. Chapter 3 explores a new strategy for the covalent functionalisation of the Wells-Dawson phosphotungstate anion through the use of arylarsonic acids (Class II hybrids). Three different arylarsonic acids were explored bearing electron withdrawing (NO2), electron donating (NH2) and “neutral” (H) substituents in the para- position. Akin to their phenylphosphorus analogues, the arylarsonic hybrids were reduced at more positive potentials than the parent anion, and the redox potentials were tuneable based on the electronic nature of the rings. The perceived trends in the lowering of the LUMO energies was corroborated with DFT. The phenylarsonic hybrid was compared to phenylphosphonic and phenylsiloxane hybrids for the photoreduction of DMF with both UV-vis and visible light. The phenylarsonic hybrid was photosensitised towards visible light, and whilst the phenylphosphonic hybrid was more easily reduced, the phenylarsonic was more easily reoxidised with molecular oxygen. In Chapter 4, a combined approach is adopted in the synthesis of covalently functionalised polyoxometalate ionic liquids (POM-ILs) based on the Keggin anion. This work used the bulky trihexyltetradecylphosphonium cation (THTP) to generate ionic liquids from two covalently functionalised Keggins featuring phenylphosphonic and phenylsiloxane groups (Class I/II hybrids). Thermophysical techniques determined that the POM-ILs exhibited higher stability than their classical salt precursors and had a wider liquid range than the plenary POM-IL analogue, despite their physical similarities, the POM-ILs gave contrasting electrochemical properties due to the different nature of the linker atoms used to graft the phenyl ring to the POM. Remarkably, the phenylphosphonic derivative which is highly unstable as a classical salt showed drastic improvement in both its thermal and electrochemical stability due to the shrink-wrapping effect of the bulky cations

Influence of Chloride and Nitrate Anions on Copper Electrodeposition onto Au(111) from Deep Eutectic Solvents

Copper electrodeposition on Au(111) from deep eutectic solvents (DESs) type III was investigated employing cyclic voltammetry as well as chronoamperometry. It was further examined on Au(poly) using the electrochemical quartz crystal microbalance (EQCM). The employed DESs are mixtures of choline chloride (ChCl) or choline nitrate (ChNO$_{3}$) with ethylene glycol (EG) as hydrogen bond donor (HBD), each in a molar ratio of 1 : 2. CuCl, CuCl$_{2}$, or Cu(NO$_{3}$)$_{2}$ ⋅ 3H$_{2}$O were added as copper sources. Underpotential deposition (UPD) of Cu precedes bulk deposition in chloride as well as nitrate electrolytes. Cu deposition from Cu$^{+}$ in chloride media is observed as a one-electron reaction, whereas deposition from Cu$^{2+}$ occurs in two steps since Cu$^{+}$ is strongly stabilized by chloride. Cu$^{+}$ is less stabilized by nitrate and the beginning of bulk deposition in the nitrate-containing DES with Cu$^{2+}$ is shifted by several hundred mV to more positive potentials compared to the chloride DES. A diffusion-controlled, three-dimensional nucleation and growth mechanism is found by chronoamperometric measurements and analysis based on the model of Scharifker and Mostany

Organofunctionalized borotungstate polyoxometalates as tunable photocatalysts for oxidative dimerization of amines

Organofunctionalized borotungstate Keggin polyoxometalates, (nBu4N)3H[HBW11O39(P(O)Ph)2] (PBW11), (nBu4N)3H[HBW11O39(As(O)Ph)2] (AsBW11), and (nBu4N)4[HBW11O39(PhSiOSiPh)] (SiBW11), were synthesized and structurally characterized. Cyclic voltammetry showed that the electronic properties of the clusters are dependent on the nature of the appended main group atoms (P, As, or Si). The first reduction potentials were found to shift positively with respect to that of the unmodified parent species (nBu4N)5[BW12O40], with PBW11 showing the largest shift at +100 mV. All clusters were evaluated as photocatalysts for the oxidative dimerization of amines where the organophosphonate hybrid PBW11 was found to be the most active. This study demonstrates how organofunctionalization of polyoxometalates may be used to tune and improve their performance as photocatalysts for organic reactions

Electronic Structure and Photoactivity of Organoarsenic Hybrid Polyoxometalates

Organofunctionalization of polyoxometalates (POMs) allows the preparation of hybrid molecular systems with tunable electronic properties. Currently, there are only a handful of approaches that allow for the fine-tuning of POM frontier molecular orbitals in a predictable manner. Herein, we demonstrate a new functionalization method for the Wells−Dawson polyoxotungstate [P2W18O62]6−using arylarsonic acids which enables modulation of the redox and photochemical properties. Arylarsonic groups facilitate orbital mixing between the organic and inorganic moieties, and the nature of the organic substituents significantly impacts the redox potentials of the POM core. The photochemical response of the hybrid POMs correlates with their computed and experimentally estimated lowest unoccupied molecular orbital energies, and the arylarsonic hybrids are found to exhibit increased visible light photosensitivity comparable with that of arylphosphonic analogues. Arylarsonic hybridization offers a route to stable and tunable organic−inorganic hybrid systems for a range of redox and photochemical applications

A Cooperative Photoactive Class-I Hybrid Polyoxometalate With Benzothiadiazole–Imidazolium Cations

An organic–inorganic hybrid species based on the Wells–Dawson polyoxotungstate [P2W18O62]6− and novel fluorescent benzothiadiazole–imidazolium cations, [BTD-4,7-ImH]2+, has been synthesized. X-ray crystallographic analysis shows that the inorganic and organic components form a hydrogen-bonded superstructure and that the cations are revealed to be non-equivalent with varying degrees of rotation between the BTD and imidazolium rings due to competition between weak intra- and intermolecular interactions. The UV–vis diffuse reflectance spectra indicate that the hybrid has a band gap of 3.13 eV, while the solid-state fluorescence properties of the cation are quenched in the hybrid material, suggesting the existence of electron transfer between the inorganic and organic components. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies of the polyoxometalate (POM) and BTD-4,7-ImH precursors, estimated through UV–vis absorption spectroscopy and cyclic voltammetry, indicate that electron transfer from the BTD cations to the POM may occur in the excited state

Molecular redox species for next-generation batteries

This Tutorial Review describes how the development of dissolved redox-active molecules is beginning to unlock the potential of three of the most promising ‘next-generation’ battery technologies – lithium–air, lithium–sulfur and redox-flow batteries. Redox-active molecules act as mediators in lithium–air and lithium–sulfur batteries, shuttling charge between electrodes and substrate systems and improving cell performance. In contrast, they act as the charge-storing components in flow batteries. However, in each case the performance of the molecular species is strongly linked to their solubility, electrochemical and chemical stability, and redox potentials. Herein we describe key examples of the use of redox-active molecules in each of these battery technologies and discuss the challenges and opportunities presented by the development and use of redox-active molecules in these applications. We conclude by issuing a “call to arms” to our colleagues within the wider chemical community, whose synthetic, computational, and analytical skills can potentially make invaluable contributions to the development of next-generation batteries and help to unlock of world of potential energy-storage applications

2021 roadmap on lithium sulfur batteries

Abstract: Batteries that extend performance beyond the intrinsic limits of Li-ion batteries are among the most important developments required to continue the revolution promised by electrochemical devices. Of these next-generation batteries, lithium sulfur (Li–S) chemistry is among the most commercially mature, with cells offering a substantial increase in gravimetric energy density, reduced costs and improved safety prospects. However, there remain outstanding issues to advance the commercial prospects of the technology and benefit from the economies of scale felt by Li-ion cells, including improving both the rate performance and longevity of cells. To address these challenges, the Faraday Institution, the UK’s independent institute for electrochemical energy storage science and technology, launched the Lithium Sulfur Technology Accelerator (LiSTAR) programme in October 2019. This Roadmap, authored by researchers and partners of the LiSTAR programme, is intended to highlight the outstanding issues that must be addressed and provide an insight into the pathways towards solving them adopted by the LiSTAR consortium. In compiling this Roadmap we hope to aid the development of the wider Li–S research community, providing a guide for academia, industry, government and funding agencies in this important and rapidly developing research space

Exploring the electronic and photocatalytic properties of organic-inorganic hybrid polyoxometalates

This thesis encompasses the synthesis and characterisation of electro- and photoactive organic-inorganic hybrid polyoxometalates (POMs). Specifically, the use of aromatic organic groups, either as countercations (Class I), as covalently grafted moieties (Class II) or combination thereof, have been explored with regards to their abilities to fine-tune the inherent properties of the POM cluster. The ability to tune the properties of redox and photo-active polyoxometalates represents an important step in the formation of “designer” materials, where the physical and electronic properties of the system can be tailored towards bespoke applications. In Chapter 2, the combination of redox rich Wells-Dawson phosphotungstates with fluorescent dye-type π-extended benzathiadiazole (BTD) cations yield a family of Class I hybrids with different modes of quaternization (proton, methyl, acetic acid). Crystallographic studies on the protonated derivative showed that the planar BTDImH structure is perturbed by hydrogen bonding to the POM and the solvent. Measurements of the electronic properties of the materials in the solution and solid state revealed stark differences, suggesting that the properties of the material are a product of interactions between the two components in the solid state. It also demonstrated that the different quaternisation strategies resulted in different electrochemical behaviour due to the different protic environments provided. Finally, preliminary studies on the hybrid featuring BTDImH cations revealed that the compound is capable of acting as a carbon dioxide photoreduction catalyst, whereas the respective components showed no activity, testament to the synergic effects gained when the photoactive components are combined. Chapter 3 explores a new strategy for the covalent functionalisation of the Wells-Dawson phosphotungstate anion through the use of arylarsonic acids (Class II hybrids). Three different arylarsonic acids were explored bearing electron withdrawing (NO2), electron donating (NH2) and “neutral” (H) substituents in the para- position. Akin to their phenylphosphorus analogues, the arylarsonic hybrids were reduced at more positive potentials than the parent anion, and the redox potentials were tuneable based on the electronic nature of the rings. The perceived trends in the lowering of the LUMO energies was corroborated with DFT. The phenylarsonic hybrid was compared to phenylphosphonic and phenylsiloxane hybrids for the photoreduction of DMF with both UV-vis and visible light. The phenylarsonic hybrid was photosensitised towards visible light, and whilst the phenylphosphonic hybrid was more easily reduced, the phenylarsonic was more easily reoxidised with molecular oxygen. In Chapter 4, a combined approach is adopted in the synthesis of covalently functionalised polyoxometalate ionic liquids (POM-ILs) based on the Keggin anion. This work used the bulky trihexyltetradecylphosphonium cation (THTP) to generate ionic liquids from two covalently functionalised Keggins featuring phenylphosphonic and phenylsiloxane groups (Class I/II hybrids). Thermophysical techniques determined that the POM-ILs exhibited higher stability than their classical salt precursors and had a wider liquid range than the plenary POM-IL analogue, despite their physical similarities, the POM-ILs gave contrasting electrochemical properties due to the different nature of the linker atoms used to graft the phenyl ring to the POM. Remarkably, the phenylphosphonic derivative which is highly unstable as a classical salt showed drastic improvement in both its thermal and electrochemical stability due to the shrink-wrapping effect of the bulky cations