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Photosensitized production of singlet oxygen is used in the treatment of cancer by photodynamic therapy (PDT), in which a photosensitizer is administered and excited locally in the tumour tissue, generating singlet oxygen to destroy, selectively, malignant cells. The use of two-photon excitation (2PE) in PDT has significant advantages in achieving enhanced spatial selectivity and greater depth of penetration through tissue. This is driving the development of new photosensitizers with high 2-photon cross-sections, but progress is inhibited by the difficulty of determining, in vitro, the efficiency of singlet oxygen production in response to TPE. In conventional experiments, the extremely high photon flux needed to achieve two-photon absorption is created by focusing a pulsed laser beam into a spot of about 1 μm in diameter. This minuscule (femtolitre) excitation volume makes the detection of two-photon-induced singlet oxygen generation extremely challenging. This thesis describes the development of an optofluidic system for the measurement of singlet oxygen quantum yields under 2PE, using hollow-core photonic crystal fibre (HC-PCF). HC-PCF allows the guidance of light in a well-defined fundamental mode within the hollow core, sustaining 2PE over long path lengths. When the fibre core is filled with photosensitizer solution, the intense light-matter interaction over a long path length enables ultra-sensitive detection of singlet oxygen. In the present work, the detection was achieved using a singlet oxygen-specific fluorescent probe, singlet oxygen sensor green (SOSG), employing a two-colour pump-probe methodology, with an 800-nm pump beam (2PE of photosensitizer) and a 488-nm probe beam (excitation of fluorescent probe) co-coupled into the hollow core of the fibre. Using this approach, it was possible to detect quantitatively singlet oxygen produced by 2PE of sub-micromolar concentrations ofphotosensitizer in an aqueous solution. The efficiency of singlet oxygen generation for a variety of PSs, including aluminium (III) phthalocyanine chloride tetrasulfonate chloride (AlPcS4), Meso-tetrakis (4-sulfonatophenyl) tetrasulfonate porphyrin (TPPS4), Protoporphyrin IX (PPIX), Zinc phthalocyanine tetrasulfonate (ZnPcS4), Chlorin E6 (CE6), Flavin Mononucleotide (FMN) and Hypericin (Hyp), was measured relative to a reference standard, Rose Bengal (RB). The parameter obtained directly from these measurements was the product of the cross-section for two-photon absorption (2PA) and the singlet oxygen quantum yield. To extract the value of the 1O2 quantum yield, the 2PA cross-section was measured in separate, cuvette-based experiments. 2PA cross-sections were determined by measuring the two-photon brightness (product of 2PA cross-section and fluorescence quantum yield) of the PS relative to a standard fluorescent dye. To extract the value of the 2PA cross-section, the fluorescence quantum yield was measured, under one-photon excitation. For many of the PSs, this was the first time that the 2PA cross-section had been determined in biologically relevant, aqueous conditions. In some cases, aggregation effects were seen, where the 2PA cross-section value depended on the concentration of the PS. Singlet oxygen quantum yields were also measured under one-photon excitation, in cuvette experiments, using SOSG, to provide reference values for comparison with those measured under 2PE in HC-PCF. These experiments also revealed the effects of aggregation on singlet oxygen quantum yields. The work presented in this thesis has demonstrated, for the first time, that the unique optofluidic properties of hollow-core photonic crystal fibre enable singlet oxygen quantum yields of two-photon-excited photosensitizers to be measured, under conditions relevant to clinical application, in aqueous solution and using low pulse-energy excitation.

Photovoltaic technologies for solar energy conversion are promising routes to renewable energy generation. A major difficulty in improving photovoltaic energy conversion efficiency lies in the spectral mismatch between the energy distribution of photons in the incident solar spectrum and the bandgap of semiconductor materials. Luminescent downshifting (LDS) layers and luminescent solar concentrators (LSCs) offer a passive approach to utilise the short-wavelength portion of the solar spectrum by absorbing short-wavelength photons and re-emitting at longer wavelengths, where the external quantum efficiency of the photovoltaic device is high. β-Diketonate-based europium(III) complexes are promising candidates for use as spectral converters due to their high quantum yields, extended absorption window, large Stokes shift between the absorption and emission spectra and narrow emission bands in the red region. Two Eu(III) complexes have been synthesised, and their photophysical properties investigated: Eu(tta)3DPEPO and Eu(hfac)3DPEPO, where tta is 2-Thenoyltrifluoroacetone, hfac is hexafluoroacetylacetone, and DPEPO is bis[2-(diphenylphosphino)phenyl]ether oxide. The photophysical properties of the complexes were examined in the solution phase and in polymethylmethacrylate (PMMA) films. Both complexes exhibited favourable photophysical properties with total quantum yields in the range of 0.75 to 0.85. Co-doping europium complexes with light-harvesting organic chromophores were considered as a practical approach to cut the cost of the metal used and to extend the harvesting window in the UV region. The use of Förster resonance energy transfer to enhance ultraviolet excitation of the Eu(III) complexes in PMMA films co-doped with organic molecules was investigated. The absorption spectra of the chosen donors, DPEPO, indole, biphenyl, and p-terphenyl, complement those of the complexes, making them useful in harvesting high-energy photons in the 250-320 nm region. Examination of the excitation spectra of the europium complexes in the presence and absence of the donors showed that, in each case, an enhancement in UV excitation of the Eu(III) complex was observed, except for Eu(hfac)3DPEPO co-doped with indole. The energy transfer efficiencies of the different donors were determined. DPEPO had the highest energy transfer efficiency for each complex, 33% and 38% for Eu(tta)3DPEPO and Eu(hfac)3DPEPO, respectively. Preliminary solar testing experiments of Eu(tta)3DPEPO and Eu(tta)3DPEPO co-doped with DPEPO were conducted to evaluate their practical performance as spectral converters on c-Si solar cells. Results showed an improvement in the short-circuit current density and the external quantum yield of the device. The role of an optically active host material in LDS layers and LSCs applications was considered in terms of their photophysical properties. The photoactive nature of Poly(N-vinylcarbazole) (PVK) and its unique photophysical properties were discussed. The use of PVK as an active host for Eu(hfac)3DPEPO and Eu(tta)3DPEPO was investigated. It was determined that energy transfer to the complexes occurs from the carbazole monomer excited state, not from the emitting excimer species. Thus, in PVK doped with the europium complexes, each complex acts as a trap for the migrating monomer excitons. Energy transfer from PVK to Eu(hfac)3DPEPO was found to occur with an efficiency of ~15% and appeared to be independent of complex concentration over the range of 2.5 to 16 wt%. However, absorption by the PVK matrix competes with direct excitation of the complex, resulting in an overall depletion in excitation intensity rather than the desired enhancement. It was predicted that if the energy transfer efficiency could be increased to 50% ere would be a substantial enhancement of the excitation intensity over the majority of the wavelength range. In contrast, for Eu(tta)3DPEPO, the use of PVK as a matrix was found to give a beneficial enhancement in excitation intensity. The energy transfer efficiency from PVK to the complex was found to be ~20%, similar to that found for energy transfer in Eu(hfac)3DPEPO. The effect of varying the PVK matrix concentration on energy transfer to Eu(tta)3DPEPO was investigated. The effect of adding co-dopant organic chromophores to Eu(tta)3DPEPO in the PVK matrix was also investigated. A multistep energy transfer process can be proposed to explain the enhancement in the excitation intensity by DPEPO and biphenyl. In summary, the thesis reports the synthesis and photophysical characterisations of two europium complexes in the solution phase and PMMA for spectral converters applications. Co-doping the europium complexes with organic chromophores was investigated, and energy transfer process efficiencies were determined and analysed. The most efficient system was coupled to a c-Si solar cell for solar testing and evaluation. The role of an optically active polymer in transferring energy to the europium complexes and to the Eu(tta)3DPEPO co-doped with organic molecules was discussed, and energy transfer processes were proposed and analysed.