No description available.

This work focuses on the design, synthesis and characterisation of novel ruthenium bipyridyl complexes and their use in dye sensitised solar cells (DSSCs). Four series of dyes have been synthesised with the general formula Ru(R-bpy)2L, where R = H, CO2Et, CO2H and L represents four different bidentate ligands; cyanodithioimidocarbonate (L1), ethyl xanthate (L2), 2,3-bis(2-cyanoethylthio)-6,7-bis(methylthio) tetrathiafulvalene (L3) and Cu(exoO2-cyclam) (L4). These have been chosen to investigate two key aims: firstly, the increase of the light-harvesting ability of the dye and secondly, the retardation of the recombination of the injected electron with the oxidised dye. Each complex was characterised using electrochemistry, absorption spectroscopy, spectroelectrochemistry and hybrid-DFT calculations. In addition the performance of the acid derivatives in a DSSC was also investigated using IPCE and IV measurements, as well as transient absorption spectroscopy. Two different S-donor ligands, L1 and L2, have been used to investigate the effect of these strongly electron-donating systems on the light-harvesting ability of the dye. Complexes utilising the di-anionic L1 were shown to exhibit significantly increased absorption range and higher extinction co-efficients, relative to the high efficiency dye N719. This dye series also showed a large degree of S-donor ligand character to the HOMO, deduced via spectroelectrochemical and computational studies. Despite these advantageous features the dye performed very poorly in a DSSC, which was attributed to fast recombination. This was a result of the cyano group of L1 coordinating to the TiO2 in addition to the acid groups of the bipyridine ligands. The complexes synthesised with the mono-anionic L2 on the other hand showed only a slightly increased lightharvesting ability relative to N719 and no significant ligand character to the HOMO. However, the performance of this dye in a DSSC was more promising, with efficiencies of up to ~ 2 % achieved. The control of the loss mechanism via recombination of the injected electron with the oxidised dye in a DSSC was investigated by incorporating redox-active ligands, L3 and L4. The series of dyes synthesised with L3 showed significant ligand character to the HOMO orbital, as deduced by spectroelectrochemical, emission and computational studies. Upon adsorption of the acid derivative to TiO2 an extremely long-lived chargeseparated state of 20 ms was observed via transient absorption spectroscopy. Despite this unique long-lived charge-separated state, the dye yielded extremely low DSSC efficiencies. This was attributed to the poor regeneration of the neutral dye by iodide, which in turn was thought to be the result of a stable intermediate formation between the dye cation and the iodide anion. The complexes synthesised with L4 showed the highest light-harvesting efficiency of all the series studied with a wide absorption range and large molar extinction co-efficients. Whilst the maximum efficiency of the dye in a DSSC was nearly 3 %, the performance was found to vary under prolonged irradiation. This was attributed to the degradation of the dye by either exchange of the counter ions with the electrolyte or loss of L4.

No description available.

No description available.

No description available.

No description available.

O presente trabalho versa sobre o desenvolvimento e a avaliação de três novas células espectroeletroquímicas para medidas de absorção na região do ultravioleta e visível, compatíveis com operação em fluxo, inclusive a técnica de FIA (Flow Injection Analysis). Características inerentes ao primeiro protótipo construído, tais como, geometria tubular, operação em fluxo ascendente e em condições de convecção forçada por vibração, separador ionomérico entre os eletrodos e detecção espectrofotométrica ex situ, permitiram a realização de estudos espectroeletroquímicos mesmo sob a decomposição paralela do solvente com intenso desprendimento de gases no eletrodo de trabalho. O desenvolvimento conjunto de eficiente dispositivo para remoção de bolhas possibilitou a obtenção de excelentes resultados espectrofotométricos. Abriu-se caminho para o estudo espectroeletroquímico de processos situados em regiões de potencial em que ocorre concomitante decomposição do solvente. Igualmente apropriado para a realização de estudos espectroeletroquímicos sob formação de gases, o segundo protótipo é de construção bastante semelhante ao primeiro, porém a elevada relação área do eletrodo de trabalho/volume de solução, conseguida principalmente pelo preenchimento da célula com fibras de carbono, foi capaz de promover eficiente conversão eletroquímica permitindo trabalho em condição de operação semi-coulométrica e conferindo à célula sensibilidade mais elevada que a do primeiro protótipo. O terceiro protótipo é do tipo camada delgada com caminho óptico longo, de aplicação restrita, pois, a regiões de potencial em que não há formação de gases. Características vantajosas frente às alternativas descritas na literatura são: uso de eletrodos de ouro planos (aproveitados de CDs regraváveis) de baixo custo e fácil construção, adaptação direta a cubatas convencionais de 1 cm de caminho óptico. Parâmetros experimentais como: potencial aplicado, concentração, vazão e volume injetado, foram investigados e avaliados com a finalidade de estabelecer as melhores condições de trabalho e funcionamento dos protótipos construídos visando posterior aplicação. / The present work discusses about the development and the evaluation of three new spectroelectrochemical cells for the UV-vis region, well suited for flow operation, including FIA (Flow Injection Analysis). Tubular geometry of the ionomeric separator of the electrode compartments, operation under ascending flow with convection increased by vibration and ex situ spectrophotometric detection after passage through an efficient debubbler granted excellent spectrophotometric results to the first prototype, even under intense gas evolution at the working electrode. This feature extends the spectroelectrochemical study to processes at extreme potential ranges. Equally appropriate for spectroelectrochemical studies under gas evolution, the second prototype presents geometry similar to the first one. However, filling of the tubular working electrode compartment with carbon fibers rendered higher ratio of electrode area/solution volume, promoting a more efficient electrochemical conversion, allowing semi-coulometric operation, and thus augmenting the sensitivity of the absorbance measurements. The third prototype is of the thin layer, long optical path type. Although not suitable for work under gas evolution, advantages of this cell in comparison with those described in the literature are: use of inexpensive flat gold electrodes, obtained from Au recordable CDs, installation in conventional cuvettes with an optical path of 1 cm, simple construction, perfect flow operation. Experimental parameters such as applied potential, analyte concentration flow rate and injected volume were inspected and analyzed for all assembled cells in a way that the best working conditions could be achieved.

Philosophiae Doctor - PhD / South African Carpobrotus species have been found to contain hydrolysable tannins,various flavonoids e.g. rutin and hyperoside, phytosterols and aromatic acids which have a diverse range of pharmacological properties including antimicrobial and, antioxidant activities. The main aim of the thesis was to determine the natural products in C. mellei and C. quadrifidus using chromatographic techniques and electrochemical analysis. The antioxidant activity of both Carpobrotus species was determined by using a superoxide dismutase (SOD) biosensor. ESI-LC-MS was used to separate and determine flavonoids in C. mellei and C. quadrifidus. 8 flavonoid compounds: catechin, epicatechin, epicatechin-epicatechin, coumarylquinic acid, isorhamnetin, quercetin-hexose (hyperoside), rutin and myricetin-deoxyhexose were identified. Cyclic and square wave voltammetry were used to detect flavonoids from C. mellei and C. quadrifidus. Catechin was detected in the ethyl acetate extract of C. mellei and C. quadrifidus. The oxidation potential of the plant extracts were observed at +150.6 mV to +1072.6 mV. The oxidation mechanism proceeds in sequential steps, related to the catechol moiety, -OH groups in C ring and the resorcinol group. The oxidation process of the catechol moiety involves a two electron - two proton reversible reaction and forms o-quinone. This occurs first at low potential and is a reversible reaction. The hydroxyl group in the C ring and resorcinol group oxidise there after and undergo an irreversible reaction. UV-vis and FTIR spectroscopy were used to confirm the presence of catechin in the ethyl acetate extract of both plants.UV-visible spectroelectrochemistry confirmed the oxidation process of catechin at constant potential. Since C. mellei and C. quadrifidus were confirmed to contain flavonoids by ESI-LC-MS and electrochemical analysis, the antioxidant activity was further investigated using a SOD biosensor. The superoxide dismutase (SOD) enzyme was immobilised with 1% Nafion on a platinum electrode. Detection limit and sensitivity of the SOD biosensor were found to be 0.03918 μmol L-1 and 1.44 μA(μmol L-1)-1, respectively. The results showed that C. mellei and C. quadrifidus have antioxidant activity, with relative antioxidant capacity (RAC) of 24% and 42%, respectively. May 2009

Philosophiae Doctor - PhD / South African Carpobrotus species have been found to contain hydrolysable tannins, various tlavonoids e.g. rutin and hyperoside, phytosterols and aromatic acids which have a diverse range of pharmacological properties including antimicrobial and, antioxidant activities. The main aim of the thesis was to determine the natural products in C. mellei and C. quadrifidus using chromatographic techniques and electrochemical analysis. The antioxidant activity of both Carpobrotus species was determined by using a superoxide dismutase (SOD) biosensor. ESI-LC-MS was used to separate and determine tlavonoids in C. mellei and C. quadrifidus. 8 tlavonoid compounds: catechin, epicatechin, epicatechin-epicatechin, coumarylquinic acid, isorhamnetin, quercetin-hexose (hyperoside), rutin and myricetin-deoxyhexose were identified. Cyclic and square wave voltammetry were used to detect tlavonoids from C. mellei and C. quadrifidus. Catechin was detected in the ethyl acetate extract of C. mellei and C. quadrifidus. The oxidation potential of the plant extracts were observed at +150.6 mV to +1072.6 mV. The oxidation mechanism proceeds in sequential steps, related to the catechol moiety, -OH groups in C ring and the resorcinol group. The oxidation process of the catechol moiety involves a two electron - two proton reversible reaction and forms o-quinone. This occurs first at low potential and is a reversible reaction. The hydroxyl group in the C ring and resorcinol group oxidise there after and undergo an irreversible reaction. UV-vis and FTIR spectroscopy were used to confirm the presence of catechin in the ethyl acetate extract of both plants. UV -visible spectroelectrochemistry confirmed the oxidation process of catechin at constant potential. Since C. mellei and C. quadrifidus were confirmed to contain flavonoids by ESI-LC-MS and electrochemical analysis, the antioxidant activity was further investigated using a SOD biosensor. The superoxide dismutase (SOD) enzyme was immobilised with 1% Nafion on a platinum electrode. Detection limit and sensitivity of the SOD biosensor were found to be 0.03918 umol L-' and 1.44 !lA (umol i.'):', respectively. The results showed that C. mellei and C. quadrifidus have antioxidant activity, with relative antioxidant capacity (RAC) of 24% and 42%, respectively.