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Bilirubin is a degradation product of haem, which is constantly formed in all mammals. Increased levels of bilirubin in humans lead to jaundice, a condition that is very common during the first days after birth. This neonatal jaundice can routinely be treated by phototherapy without any serious side effects. During this treatment, bilirubin undergoes a photoreaction to isomers that can be excreted. The most efficient photoreaction is the isomerisation around a double bond (Z-E-isomerisation), which results in more soluble photoproducts. The work presented in this thesis shows results of a femtosecond optical spectroscopy study, combined with quantum-mechanical investigations, of the mechanism of isomerisation of bilirubin. The spectroscopic research was conducted with bilirubin in organic solvents, and in buffer complexed by human serum albumin. This albumin complex is present in the blood, and has thus medical importance. Quantum-chemical calculations (CASSCF) on a bilirubin model were used to explain experimental results. The fluorescence decay observed with femtosecond spectroscopy shows an ultrafast component (~120 fs), which is explained by exciton localisation, followed by processes with a lifetime of about 1-3 ps. These are interpreted as the formation of a twisted intermediate, which decays with a lifetime of 10-15 ps back to the ground state, as observed by absorption spectroscopy. CASSCF calculations, in combination with the experimental results, suggest the ca. 1-3 ps components to be relaxation to the twisted S1 minimum, followed by the crossing of a barrier, from where further relaxation takes place through a conical intersection back to the ground state. Time-dependent DFT calculations were utilised to analyse the absorption spectrum of bilirubin. Good agreement with the measured spectrum was achieved, and low-lying states were observed, that need further investigation. The theoretically obtained CD spectrum provides direct evidence that bilirubin preferentially binds to human serum albumin in the enantiomeric P-form at neutral pH. / Bilirubin är en nedbrytningsprodukt av hem som ständigt bildas hos alla däggdjur. En förhöjd bilirubinkoncentration i den mänskliga kroppen kan leda till gulsot, något som är mycket vanligt under de första dagarna efter födelsen (neonatal gulsot). Fototerapi används rutinmässigt som säker behandlingsmetod, under vilken bilirubin genomgår en fotoreaktion till en isomer som kan utsöndras. Den mest effektiva fotoreaktionen är en Z-Eisomerisation, vilken leder till lösligare fotoprodukter. Arbetet som presenteras i denna avhandling visar resultaten av en kombinerad femtosekund optisk-spektroskopisk och kvantmekanisk undersökning av mekanismen bakom bilirubins isomerisation. Den spektroskopiska studien genomfördes med bilirubin, löst i organiska lösningsmedel och i buffert i komplex med humant serumalbumin. Detta albuminkomplex finns i blodet, och är därför av medicinskt intresse. Kvantmekanistiska CASSCFberäkningar på en bilirubinmodell användes för att förklara de experimentella resultaten. Det uppmätta fluorescence sönderfallet visar ultrasnabba komponenter (~120 fs). Dessa tolkas som excitonlokalisering, som följs av bildandet av ett vridet intermediat med en hastighetskonstant på ca. 1 ps-1(beroende på lösningsmedlet). Absorptionsmätningar visar att detta intermediat sönderfaller tillbaka till grundtillståndet med en livstid på 10-15 ps. CASSCF beräkningar, i kombination med de experimentella resultaten, tyder på att sönderfallet med livslängden på ca. 1 ps är en relaxation till det vridna S1-tillståndet. Reaktionsvägen därifrån antas passera en barriär till en konisk genomskärning, som möjliggör snabb relaxation till grundtillståndet. Tidsberoende DFT-beräkningar användes för att analysera bilirubins absorptionsspektrum, vilket gav bra överensstämmelse med uppmätta data. Dessutom hittades ett tillstånd med låg excitationsenergi, som kräver ytterligare studier. Med hjälp av det beräknade CD-spectret kunde det visas att bilirubin binder till albumin i P-formen vid neutralt pH.

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Thesis (PhD)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The initial charge transfer from dye molecules' excited states to the conduction band of a semiconductor, after absorption of visible light by the former, is critical to the performance of Dye sensitized Solar Cells (DSC). In a ZnO-based DSC sensitized by the organic indoline derivative D149, the dynamics associated with charge transfer are investigated with femtosecond transient absorption spectroscopy. The time-resolved measurement of the photo-initiated processes reveal electron transfer rates corresponding to excited state lifetimes of 100s of fs, consistent with previously measured high absorbed photon to current conversion efficiencies. The photo-electrode measured as an isolated system shows decay times of bound electrons in excited states of the dye to be ~150 fs and shows the subsequent emergence of absorption bands of the oxidized molecules. When the I-/I-3 redox couple is added to the system, these excited state lifetimes change and are found to be dependent on the cation in the electrolytic solution. Small cations like Li+ reduce the excited state lifetime to sub-100 fs, whilst larger cations like the organic tetrabutylammonium result in longer lifetimes of 240 fs. The action of the electrolyte can be observed by the reduced lifetime of the oxidized dye molecules' absorption bands. The effect of operating parameters and changes in the production protocol of the DSC on the primary charge injection are also investigated and reported on. / AFRIKAANSE OPSOMMING: Die aanvanklike ladingsoordrag vanuit kleurstofmolekules' opgewekte toestande tot in die leidingsband van 'n halfgeleier, na absorpsie van sigbare lig deur eersgenoemde, is van kritiese belang vir die uitset van halfgeleier-gebaseerde sonkragselle wat met kleurstowwe vir absorpsie verhoging, gebind is. In hierdie werk word hierdie proses en verwante fotodinamika in die geval van 'n ZnO sonkragsel gekleur met indolien D149 ondersoek d.m.v femtosekonde-tydopgelosde absorpsiespektroskopie. Hierdie metings onthul elektron-oordragstempos wat ooreenstem met lewenstye van opgewekte toestande in die orde van 100 fs. Hierdie is met voorheen-bepaalde hoë foton-tot-stroom omskakelingsdoeltreffendheid ooreenkomstig. Die foto-elektrode, as geïsoleerde sisteem beskou, toon afvalstye van gebonde elektrone in opgewekte toestande van ~150 fs, en die gevolglike opkoms van absorpsie deur geoksideerde molekules word waargeneem. As die I-/I-3 redoks oplossing tot die sisteem bygevoeg word, verander die opgewekte toestande se afvalstye en toon 'n katioon-afhanklikheid. Klein katioone soos Li+ verkort die afvalstye tot onder 100 fs, terwyl groter katioone soos die organiese tetra-butielammonium langer afvalstye (240 fs) tot gevolg het. Die werking van die elektrolitiese oplossing kan waargeneem word deur die verkorte lewenstyd van die absorpsiebande wat aan die geoksideerde molekules toegeken is. Die uitwerking van operasionele parameter asook veranderinge in die produksie protokol op die primêre ladingsoordrag word ondersoek en verslag daarop word gelewer.

The design of dyes for NiO-based dye-sensitized solar cells (DSSCs) has drawn attention owing to their potential applications in photocatalysis and because they are indispensable for the development of tandem dye-sensitized solar cells. The understanding of the electron transfer mechanisms and dynamics is beneficial to guide further dye design and further improve the performance of photocathode in solar cells and solar fuel devices. Time-resolved spectroscopy techniques, especially femtosecond and nanosecond transient absorption spectroscopy, supply sufficient resolution to get insights into the charge transfer processes in p-type dye sensitized solar cell and solar fuel devices. In paper I-V, several kinds of novel organic “push-pull” and inorganic charge transfer dyes for sensitization of p-type NiO, were systematically investigated by time-resolved spectroscopy, and photo-induced charge transfer dynamics of the organic/inorganic dyes were summarized. The excited state and reduced state intermediates were investigated in solution phase as references to confirm the charge injection and recombination on the NiO surface. The charge recombination kinetics is remarkably heterogeneous in some cases occurring on time scales spanning at least six orders of magnitude even for the same dye. In this thesis, we also proposed a novel concept of solid state p-type dye sensitized solar cells (p-ssDSSCs) for the first time (paper VI), using an organic dye P1 as sensitizer on mesoporous NiO and phenyl-C61-butyric acid methyl ester (PCBM) as electron conductor. Femtosecond and nanosecond transient absorption spectroscopy gave evidence for sub-ps hole injection from excited P1 to NiO, followed by electron transfer from P1●- to PCBM. The p-ssDSSCs device showed an impressive 620 mV open circuit photovoltage. Chapter 6 (paper VII) covers the study of electron transfer mechanisms in a covalently linked dye-catalyst (PB-2) sensitized NiO photocathode, towards hydrogen producing solar fuel devices. Hole injection from excited dye (PB-2*) into NiO VB takes place on dual time scales, and the reduced PB-2 (PB-2●-) formed then donates an electron to the catalyst unit.  The subsequent regeneration efficiency of PB-2 by the catalyst unit (the efficiency of catalyst reduction) is determined to ca. 70%.

In this thesis, we investigate the nature of charge carrier generation, relaxation and recombination in a range of lead halide perovskites. We focus on understanding whether the photophysical behaviour of these perovskite materials is like that of highly-ordered inorganic crystalline semiconductors (exhibiting ballistic charge transport) or disordered molecular semiconductors (exhibiting strong electron-phonon coupling and highly localised excited states) and how we can tune these photophysical properties with inorganic and organic additives. We find that the fundamental photophysical properties of lead halide perovskites, such as charge carrier relaxation and recombination, arise from the lead halide lattice rather than the choice of A-site cation. We show that while the choice of A-site cation does not affect these photophysical properties directly, it can have a significant impact on the structure of the lead halide lattice and therefore affect these photophysical properties indirectly. We demonstrate that lead halide perovskites fabricated from particular inorganic and organic A-site cation combinations exhibit low parasitic trap densities and enhanced carrier interactions. Furthering our understanding of how the photophysical properties of these materials can be controlled through chemical composition is extremely important for the future design of highly efficient solar cells and light emitting diodes.

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