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The excited state electronic structures of several biologically important chromophores were studied by Stark spectroscopy. The extent of charge redistribution upon excitation to the lowest excited states of the oxidized and semiquinone forms of flavin adenine dinucleotide (FAD) bound to the light activated DNA repair enzyme DNA photolyase have been studied previously by this technique. This work focuses on the catalytically active form, the two-electron reduced anion. To facilitate analysis of this experiment, the Stark spectra of a simple flavin derivative that is soluble in organic solvents was measured. The results of the analysis of these data are in agreement with previously a published linear dichroism experiment that found the absorption spectrum of flavins in this redox state arises from two distinct electonic transitions in the visible/near-ultraviolet spectral range, a fact that has not been incorporated into the analysis of many ultrafast spectroscopic experiments of reduced anionic flavins/flavoproteins. The difference dipole moment of the second, more intense, transition was found to be about twice as large as that of the lowest energy transition. With the aid of ab initio calculations, the directions of these dipole moments in the molecular frame were assigned. For both transitions, it was found that negative charge density is shifted toward the xylene ring of the flavin upon excitation. Another important consideration for the correct analysis of the photolyase spectra is the possibility of contamination by small amounts of the antenna chromophore, which also has absorption intensity in the near-ultraviolet region. We chose to study the cofactor for E. coli photolyase, 5,10-methenyltetrahydrofolate, and its photodecomposition product, 5,10-methylenetetrahydrofolate. The difference dipole moments for the lowest energy transitions of both of these chromophores were found to be quite large, ranging from 9-12 D fc and lying primarily along the transition dipole moment. Additionally, the difference polarizability of both chromophores was large, on the order of 200-300 Å3 fc2 . The Stark spectra of reduced anionic FAD in photolyase agrees well with the findings of the experiments on flavin in organic solvent; the magnitude of the difference dipole moments in both cases match within experimental error. While the direction of the difference dipole moment for the lowest transition is also the same in both cases, that of the second transition is changed in the protein matrix. The assignment of these vectors in the molecular frame shows that the two dipole moments are coincident for the cofactor bound to photolyase. This finding, where electron density is shifted toward the point of the flavin ring closes to the DNA lesion bound to the enzyme, is strong evidence that direct electron transfer takes place from the isoalloxazine ring of FAD to the DNA substrate in the catalytic cycle. The usefulness of Stark spectroscopy in investigating photoinduced charge redistribution was also shown for the donor-π-acceptor flavin dyad, azobenzylflavin (ABFL). The difference dipole moment was found to be 22 D, an approximately three-fold increase from the largest difference dipole moment found in naturally occurring flavins. This extensive charge redistribution corresponds to a large hyperpolarizability of the chromophore that suggests that ABFL may be useful in nonlinear optical applications. Transient absorption was used to supplement these experiments by monitoring the decay kinetics of ABFL after excitation. It was found that ABFL undergoes ultrafast charge recombination within 6 ps after excitation, leading to depopulation of the charge separated state before useful work can be performed for applications requiring electron transfer. These studies provide the ground work for rational design of other ABFL-like derivatives for use in a variety of applications. / Chemistry

In this dissertation, the laser spectroscopic methods, second harmonic generation (SHG) and ultrafast transient absorption, have been employed to study the reactions and dynamics in two different types of materials, namely, silver nanoparticles and micro-sized ultrathin crystalline oligoacenes. These two materials, although both are in small dimensions, represent two distinct types of systems with divergent characteristics: 1) systems in which interactions at the surface/interface are dominant, and 2) systems in which bulk interactions are dominant. Silver nanoparticles are an important member of the class of noble metal nanoparticles, and possess unique optical and chemical properties due to their ultrafine size and high surface-to-volume ratio. Strong SHG signal has been observed from silver nanoparticles dispersed in aqueous colloidal solution, in which the SHG signal is enhanced due to a resonance with the localized surface plasmon of silver nanoparticles. Further experiments proved that the SHG signal predominantly originates from the particle surface, in full agreement with the intrinsically interface-sensitive properties of SHG. With the surface origin of the signal now well established, SHG can be used to probe the adsorption and reactions of thiol molecules at the nanoparticle surface in situ and in real time. It is experimentally demonstrated that the free energy change, activation energy, as well as adsorption density of the reactions of a variety of neutral and anionic thiols at the particle surface can be measured by means of SHG. The reaction mechanisms at the molecular level have been deduced, and the neutral vs anionic thiols are found to exhibit qualitatively different reaction mechanisms that reflect the effect of their molecular interactions with the particle surface. Oligoacenes, such as pentacene and hexacene, constitute a family of organic semiconductors that exhibit remarkable optoelectronic properties. In contrast to the nanoparticles in which surface interactions are dominant, as the sizes of materials become larger, the bulk characteristics become more deterministic. Therefore, polarized linear absorption and transient absorption spectroscopies have been applied to study the excitonic properties of crystalline pentacene and the mechanism of singlet fission in crystalline hexacene, respectively. The polarized absorption spectra of crystalline pentacene have been obtained by measuring transmitted light normal to the ab herringbone plane of micro-sized ultrathin single crystals. The significant deviations between the spectral line shapes polarized along the b-axis and orthogonal to the b-axis provide detailed information on the anisotropic mixing nature of the Frenkel/charge-transfer excitons responsible for the pronounced Davydov splitting between the lowest-energy singlet states. Additionally, both singlet and triplet Davydov splittings were also observed from the linear and transient absorption experiments in micrometer-sized ultrathin hexacene single crystals. A two-step process of anisotropic singlet fission was uncovered from the kinetic data, in which singlet fission at different rates were deduced along the a- and b-axes. Both the spectral and kinetic features indicate that singlet fission in crystalline hexacene is an anisotropic and charge-transfer mediated many-molecule process. / Chemistry

Thesis (MSc)--Stellenbosch University, 2014. / ENGLISH ABSTRACT: The demand for renewable energy sources has grown out of the humanity’s increasing need for electricity as well as depleting fossil fuel reserves. Organic-dye sensitised solar cells were developed as a green, cost-effective alternative to the market-dominating silicon solar cell technology. The field of photovoltaic devices and organic-DSSCs is interesting because we want to develop better, more efficient cells at lower costs using environmentally friendly materials. By studying the fundamental physics and chemistry processes occurring during and after the interaction of light with these devices, we create a window into the mechanism of photosynthesis. Our DSSCs were prepared by sensitisation of highly porous ZnO with different indoline dyes containing the same chromophore, but different alkyl chain lengths bonded to one of two carboxyl anchors as: DN91 (1 C) < DN216 (5 C) < DN285 (10 C). The role of the dye molecules is to absorb photons and donate electrons to the ZnO which acts as the charge acceptor, at the dye|ZnO interface. Through photoelectrochemical characterisation it was found that the structure of the dyes has an effect on the maximum current (JSC) produced by the cells: the shorter the alkyl chain, the higher the JSC. This macroscopic investigation was complimented by microscopic measurements in the form of transient absorption spectroscopy. This allows us to follow, in real time, the photoinduced oxidation of the dye and its regeneration occurring through desired and undesired pathways. It was found that the injection efficiencies of the dye molecules were directly responsible for the trend in the short circuit currents. / AFRIKAANSE OPSOMMING: Die aanvraag na die ontwikkeling van herwinbare energie bronne spruit voort uit die voorsienbare uitputting van fossiel brandstof bronne sowel as die groeiende behoefte om aan die mensdom se elektrisiteit behoeftes te voldoen. Kleurstof gesensitiseerde sonselle is ontwikkel as ’n groen, koste-effektiewe alternatief tot die silikon sonsel tegnologie wat die mark domineer. Die fotovoltaïse toestel veld, spesifiek organiese kleurstof gesensitiseerde sonselle is interessant omdat daar ruimte bestaan vir die ontwikkeling van beter meer effektiewe selle in terme van vervaardigings koste en prosesse wat omgewingsvriendelik is. Deur die fundamentele fisika en chemiese prosesse wat plaas vind tydens en na lig interaksie met hierdie selle te bestudeer gee dit insig oor die werkingsmeganisme van fotosintese. Ons kleurstof gesensitiseerde sonselle is voorberei deur sensitasie van hoogs poreuse ZnO met verskillende indolien kleurstowwe wat dieselfde kromofoor bevat wat met verskillende alkiel ketting lengtes verbind is aan een van twee karboksiel ankers as: DN91 (1 C) < DN216 (5 C) < DN285 (10 C). Die rol van die kleurstof molekules is om fotone te absorbeer en elektrone te doneer aan die ZnO wat as die lading akseptor dien by die kleurstof|ZnO intervlak. Deur fotoelektrochemiese karakterisasie is bevind dat die struktuur van die kleurstof ’n effek het op die maksimum stroom (JSC) wat die selle produseer: hoe korter die die akiel ketting, hoe hoër die JSC. Hierdie makroskopiese ondersoek is voltooi deur mikroskopiese metings in die vorm van tydopgelosde absorpsiespektroskopie. Dit laat ons toe om die fotogeinduseerde oksidasie asook regenerasie van die kleurstof te volg soos wat dit plaas vind deur gewenste sowel as ongewenste roetes. Dit is bevind dat die inspuitings effektiwiteit van die kleurstof molekules direk verantwoordelik is vir die waarneembare trajek in die kortsluitings stroom.

Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: The generation of ultrafast light pulses and the development of time resolved spectroscopic techniques, such as the femtosecond pump probe spectroscopy technique, have facilitated the study of ultrafast energy transfer in the photosynthetic systems of green plants and photodynamic therapy drugs. It has allowed the investigation of biological and chemical processes that take place on the ultrafast timescale and has allowed us to obtain spectral and kinetic information on energy transfer. In addition, it has allowed time resolved experiments in which the transient absorption of species under investigation was observed and has elucidated molecular dynamics. In the present work this was done with a temporal resolution of approximately 200 fs and covering a pump-to-probe delay range of 300 fs to 2 ns. The main aims of this study were to characterise the femtosecond pump probe spectroscopy system, to investigate the energy transfer in the natural light harvesting complex II (LHC II) in view of future expansion to the study of synthesized arti cial functional light harvesting complexes and nally to study ultrafast processes in zinc phthalocyanine (ZnPc) systems. In photosynthetic organisms, LHC II is the most abundant light harvesting complex and it plays an important role in light harvesting and photoprotection. The light energy is absorbed by light harvesting complexes and transferred to a reaction centre (RC) in an ultrafast timescale. Phthalocyanines are a new class of photosensitiser used for photodynamic therapy. These drugs are used to treat small and super cial tumours. The energy transfer from the singlet excited state to the triplet excited state occurs on an ultrafast timescale. However, recent work done on zinc phthalocyanine has proved that the determination of the ultrafast component remains a challenge. Several ultrafast studies carried out on ZnPc in solvents have been not only unsuccessful to give a clear picture of the ultrafast dynamics but have also produced divergent results. In this study, a characterisation of the femtosecond pump probe spectroscopy setup was done. The samples under investigation were probed by a white light continuum. The generation of the white light continuum introduced chirp, which in uenced the temporal evolution of the transient absorption results. The technique used to correct the chirp introduced by white light generation is discussed in detail. Our femtosecond pump probe spectroscopy setup was benchmarked by using a well known dye, namely malachite green. In addition, the investigation of the transient absorption change of LHC II, an active component in photosynthesis, as extracted from spinach leaves and the ultrafast dynamics of a promising photosensitiser ZnPc in dimethyl sulfoxide (DMSO) as well as in dimethyl formamide (DMF) was done. The spectral and dynamic results obtained using these three samples are described and exponential ts to the absorbance decay curves used to estimate the timescales of the energy transfer processes are presented. In this experiment, the dynamics and measured time constants related to the energy transfer between the different types of chlorophyll in LHC II was monitored, whereas with ZnPc, the dynamics and the measured time constants associated with solvation dynamics and vibrational relaxation was examined. / AFRIKAANSE OPSOMMING: Die vorming van ultravinnige lig pulse en die ontwikkeling van tyd opgelosde spektroskopiese tegnieke, soos die femtosekonde pomp proef spektroskopie tegniek, het die studie van ultravinnige energie oordrag in fotosintetiese stelsels van groen plante en chemiese prosesse gefasiliteer, wat kan plaasvind op die ultravinnige tyd skaal en laat dit toe om spektrale en kinetiese informasie oor die energie oordrag te kan bepaal. Dit het ook dit moontlik gemaak om tyd opgelosde eksperimente te kan doen waarin ons veranderlike absorbsie van die monster kon ondersoek en die molekulere dinamika kon ontrafel. In hierdie werk is dit gedoen met n tyd resolusie van omtrent 200 fs termyl 'n pomp-tot-proef tydvertraging van 300 fs tot 2 ns gebruik is. Die hoof doelwitte van hierdie werk was om 'n femtosekonde pomp proef spektroskopie stelsel te karakteriseer, die energie oordrag in die natuurlike ligoes kompleks II te ondersoek met die toekomstige uitbreiding van die studie na sintetiese lig-oes komplekse as oogmerk en laastens om ultravinnige prosesse in Sink Ftalosianiene stelsels te ondersoek. In fotosintetiese organismes, is lig oes kompleks II die mees volop lig oes kompleks en speel 'n belangrike rol in lig oes en foto skerming. Die lig energie word geabsorbeer deur lig oes komplekse en dan oorgedra na reaksie middelpunte in 'n ultravinnige tydskaal. Ftalosianiene is 'n nuwe klas fotosensiteerder wat gebruik word in fotodinamiese terapie. Hierdie dwelms word gebruik om klein en oppervlakkige gewasse te behandel. Die energie oordrag van die opgewekte singlet tot die triplet toestand vind plaas op die ultravinnige tydskaal. Onlangse navorsingswerke het getoon dat die bepaling van die ultravinnige komponent 'n uitdaging bly. Verskeie vorige ondersoeke is gedoen op Sink Ftalosianiene in verskeie oplosmiddels, en nie net het hierdie studies nie 'n helder prentjie verskaf van die ultravinnige dinamika nie, maar het ook divergerende resultate opgelewer. In hierdie werk word 'n karakterisering van die femtosekonde pomp proef spektroskopie stelsel gedoen. Die monsters is ondersoek met 'n wit lig kontinuum proef. Die vorming van die wit lig kontinuum het tjirp veroorsaak, wat die tyd evolusie van die veranderlike absorbsie resultate beinvloed het. Die tegniek wat gebruik was om die tjirp te korregeer word bespreek. Ons femtosekonde pomp proef spektroskopie stelsel is gestandardiseer deur die welbekende kleurstof malachiet groen. Ons het ook die veranderlike absorbsie van lig oes kompleks II ondersoek, 'n aktiewe komponent in fotosintese, soos dit onttrek is uit spinasie blare, asook die ultravinnige dinamika van die belowende photosensitizer Sink Ftalosianiene in DMSO asook DMF. Die spektrale en dinamiese resultate verkry vanaf hierdie drie monsters word beskryf en eksponensiele passings aan die absorbsie verval kurwes is gebruik om die tydskale van die energie oordrag prosesse te skat. In hierdie eksperiment is dinamika en gemete tydkonstantes waargeneem wat toegeskryf kan word aan die energie oordrag tussen verskillende soorte chloro l in lig oes kompleks II. In die Sink Ftalosianien eksperimente is dinamika en gemete tydkonstantes waargeneem wat toegeskryf kan word aan solverings dinamika asook vibrasionele ontspanning.

This thesis improves our understanding of the charge-transfer dynamics in organic materials employed in dye-sensitized and nanotube-thiophene solar cells. For the purpose of this work, a femtosecond transient absorption spectroscopy setup was built. Additionally, microsecond transient absorption spectroscopy was utilised to explore dynamics on a longer time-scale. In the first study, the dependence of dye regeneration and charge collection on the pore- filling fraction (PFF) in solid-state dye-sensitized solar cells (DSSCs) is investigated. It is shown that while complete hole transfer with PFFs as low as ~30% can be achieved, improvements beyond this PFF are assigned to a stepwise increase in the charge-collection efficiency in agreement with percolation theory. It is further predicted that the chargecollection efficiency saturates at a PFF of ~82%. The study is followed by an investigation of three novel hole-transporting materials for DSSCs with slightly varying HOMO levels to systematically explore the possibility of reducing the loss-in-potential and thus improving the device efficiency. It is shown that despite one new HTM showing a 100% hole-transfer yield, all devices based on the new HTMs performed worse than those incorporating spiro-OMeTAD. Furthermore, it is demonstrated that the design of the HTM has an additional impact on the electronic density of states present at the TiO₂ electrode surface, and hence influences not only hole- but also electron-transfer from the sensitizer. Finally, a study on a polymer-single-walled carbon nanotube (SWNT) molecular junction is presented. Results from femtosecond spectroscopic techniques show that the polymer poly(3-hexylthiophene) (P3HT) is able to transfer charges to the SWNT within 430 fs. Addition of excess P3HT polymer leads to long-lived free charges making these materials a viable option for solar cells.

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%.