Synthetic Approaches to Light-Harvesting Materials Containing Earth-Abundant Metals

Abstract

In this thesis, entitled Synthetic Approaches to Light-Harvesting Materials Containing Earth-Abundant Metals, a crystal engineering approach is taken to investigate two kinds of materials, namely coordination polymers based on Cu(I) photosensitive metallo-linkers available from dye-sensitised solar cell research, as well as phosphonate-stabilised titanium-oxo clusters. The resulting materials were characterised using single crystal X-ray diffraction, their photo- and electrochemical properties examined, and structure-property relationships elucidated. Moreover, the inherent synthetic opportunities, but also limitations towards novel materials were investigated. Chapter 1 provides a summary of available literature, which illustrates the need for light-harvesting materials, the necessity to base those on earth-abundant metals, and the background of the materials discussed in this work. The first part of this work is concerned with the incorporation of Cu(I) photosensitisers as metallo-linkers into coordination polymers. In Chapter 2, the synthesis, structural characterisation of five isostructural 2D coordination polymers based on a simple bis(diimine)copper(I) complex is presented. It provides the first investigation of the effect of immobilisation of such Cu(I) photosensitisers into extended structures with respect to photo- and electrochemical properties. In Chapter 3, the influence of ligand modifications onto stability of the copper(I) complexes during solvothermal synthesis is elucidated, with respect to extension of the back-bone and steric encumbrance of the metal centre. While no heterometal-containing extended structures could be obtained, valuable information for more refined strategic approaches with respect to future work was obtained. The second part of this work is focussed on exploring the synthetic robustness of a phosphonate-stabilised titanium-oxo cluster with a Ti6P2 skeleton, with respect to the introduction of different functional groups and their influence on in situ reactivity of the system, as well as the optical band gap. In Chapter 4, the conditions to functionalise the cluster with various aminobenzoic acids are established and their influence on the clusters optical band gap discussed. Moreover, it is demonstrated that the corresponding Schiff bases are formed in situ by the addition of aldehydes. By utilising o-aminobenzoic acid, the cluster core is halide-doped and Schiff bases reduced. The proximity between amine and carboxylic acid as well as the influence of chirality onto the systems was further investigated with alanines. In Chapter 5, the Ti6P2 motif's tendency to form and band gap tunability is further demonstrated with the aid of carboxylic acid azo- and coumarin-dyes, as well as extension of the pi-system with respect to previously reported clusters. The anthraquinone-2-carboxylate cluster demonstrates redox non-innocence and viable mechanisms for the in situ reduction are elucidated. Moreover, a quantitative relationship between the electronic properties of substituents in terms of Hammett constants and the titanium-oxo cluster's optical band gap is established. In Chapter 6, a heteroleptic Cu(I) photosensitiser is anchored onto the Ti6P2 motif as a model compound for photosensitiser-TiO2 composite materials. This is achieved both by first attaching an ancillary ligand and then the Cu(I) centre, as well as by direct cluster formation from the preformed Cu(I) complex. Steady-state photophysical and electrochemical features of the heteroleptic Cu(I) on the cluster are retained with respect to its ester. Chapter 7 provides overall conclusions of this work and the details of experimental procedures are presented in Chapter 8. The bibliography of this thesis is listed in Chapter 9. Additional information is found in the appendix. Crystallographic information files for each structure, FTIR spectra not shown in main body, and EDX spectra are provided on the attached CD.