Immobilised Transition Metal Photosensitisers: Robust, Sustainable, Solar-Driven Systems for Water Purification

The aim of this work is to produce a point-of-use water disinfecting technology that harnesses the power of sunlight to disinfect water through generation of singlet oxygen by metal complex photosensitisers attached to solid supports. To that end a number of Ru(II) and Cu(I) complex photosensitisers based on bipyridyl-type ligands have been synthesised and immobilised onto various supports. The compounds were tested with regards to their singlet oxygen generating abilities and bactericidal efficiencies against Staphylococcus aureus and Escherichia coli. Chapter 1 contains an introduction to the topic of singlet oxygen photosensitisation, photodynamic inactivation of bacteria, and a literature survey of the field of solar purification of water using supported singlet oxygen photosensitisers. Chapter 2 discusses the main synthetic work: synthesis of a wide range of Ru(II) and Cu(I) complexes with polypyridyl ligands, followed by spectroscopic characterisation using UV-Vis, steady-state and time-resolved fluorescence spectroscopy. The complexes’ immobilisation onto solid supports is described and the resulting immobilised compounds were also characterised using UV-Vis, Infrared and fluorescence spectroscopy. The ruthenium complex [Ru(bpy)2(bpy-silatrane)]Cl2 (RuBS) was synthesised and covalently grafted onto chromatography silica (40-60 mesh), SBA-15 and MCM-41 with surface coverages of 38, 50 and >80 µmol g-1 respectively. A number of other Ru(II) polypyridyl complexes were synthesised with the aim of improving singlet oxygen quantum yield. Tetrasodium Ruthenium tris(bathophenantholine disulfonate) (RuBPS) was ionically bound onto the surface of Amberlite® IRA900 with a surface loading of 1.20 µmol g-1. To PDMS were covalently grafted two Ru(II) polypyridyl complexes. This was performed by two methods: amination with 3-ii amino(propyl triethoxysilane) (APTES) and 3-mercapto(propyl trimethoxysilane) (MPTS). A number of Cu(I) diamine diphosphine complexes ({Cu(NN)(PP)}+) were also synthesised with optimisation of singlet oxygen quantum yield and absorption in the visible spectrum being evaluated. Of all of the synthesised complexes the known compound [Cu(2,9-dimethyl-1,10- phenanthroline)(xantphos)]tfpb was dry-loaded onto chromatography grade silica to produce the compound CuXD-CS, with surface loading of 11 µmol g-1 of the active complex. Chapter 3 describes the quantification of singlet oxygen production by the synthesised compound, both in solution and when grafted onto solid supports. The singlet oxygen quantum yields of the following Ru(II) complexes was measured: RuBS (ΦΔ= 0.55 ±0.03), RuBMS (ΦΔ= 0.60 ±0.04) and RuPS (ΦΔ= 0.88 ±0.03); along with the Cu(I) complexes {Cu(dmp)(xant)}+ (ΦΔ= 0.30 ±0.04), {Cu(BC)(xant)}+ (ΦΔ= 0.41 ±0.03), {Cu(BCS)(xant)}+ (ΦΔ= 0.42 ±0.08), {Cu(dmp)(DPEPhos)}+ (ΦΔ= 0.40 ±0.09). In Chapter 4 the singlet-oxygen based bactericidal activity of two of the solid supportimmobilised complexes was tested on S. aureus and E. coli with varying degrees of success following an introduction into the cell wall structure of Gram-positive and Gram-negative bacteria. RuBS covalently grafted onto chromatography silica (RuBS-CS) was unable to reduce colonies of S. aureus and E. coli after 2 hours of illumination by 455 nm light (2.5 mWcm-2). The complex [Cu(2,9-dimethyl-1,10-phenanthroline)(xantphos)]tfpb also showed no photodynamic bactericidal activity on its own. However, the same Cu(I) complex dry-loaded onto chromatography silica, CuXD-CS, reduced colonies of S. aureus and E. coli by 99.9999% (6-log10) after 2 and 3 hours of illumination by 405 nm light (14.3 mW cm-2) respectively. Chapter 5 includes all synthesis and characterisation data of the complexes presented in this work.