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.

Toxic singlet oxygen can be populated by the quenching of triplet states of chlorophyll (Chl). In photosynthetic light-harvesting complexes (LHCs), the gen- eration of singlet oxygen is prevented by a photoprotective mechanism based on an energy transfer from Chl triplets to carotenoids, which occurs via a Dexter mechanism (DET). The temperature dependence of the DET was studied in three selected LHCs by means of transient absorption spectroscopy. The emphasis was on a chlorophyll a-chlorophyll c2-peridinin-protein complex (acpPC) of Dinoflagel- late Amphidinium carterae. The results obtained from acpPC were compared with those for LHC-II from pea and chlorosomes of Chloroflexus aurantiacus. All three antennas exhibit high efficiency and fast rate of chlorophyll triplet quenching by carotenoids at room temperature, which prevents the accumulation of Chl triplets. The fast rate of quenching persists at low temperatures (≥77 K) in the case of LHC-II. However, the efficiency of the Chl triplets quenching is lower as proved by a detection of long-lived Chl triplets with a millisecond lifetime. These triplets were assigned to peripheral Chls that are not neighbouring with carotenoids active at 77 K. A similar population of long-lived Chl triplets was detected in the acpPC complex. In acpPC, the rate of the...

Chlorophyll molecules in their triplet excited state can react with the ground state oxygen, producing oxygen in a singlet excited state, which is very reactive and thus very harmful to the light-harvesting complex. Photosynthetic organisms employ carotenoids to prevent the damage by quenching both excited (singlet) states of oxygen and excited triplet states of chlorophyll. In this work, we use ns transient absorption spectroscopy and global analysis to study the dynamics of carotenoid and chlorophyll triplet states in two light-harvesting complexes of Amphidinium carterae, the Peridinin-Chlorophyll a-Protein complex (PCP) and the main light-harvesting complex (LHCP). It appears that at room temperature all triplets are transferred from chlorophylls to carotenoids within ~ 5 ns, providing a very efficient protection against formation of singlet oxygen. One carotenoid triplet with a lifetime of ~ 10.2 µs participating in the chlorophyll triplet quenching was observed in the PCP sample, while results from LHCP suggest that two carotenoid triplets with a similar lifetime of ~ 2.5 µs contribute to quenching of chlorophyll triplet states. The two carotenoid triplets are attributed to peridinin placed in a polar environment and peridinin placed in a non-polar environment in the LHCP complex.

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This thesis describes supramolecular approaches to porphyrin nanorings. Cyclic porphyrin arrays resemble natural light harvesting systems, and it is of interest to probe the photophysical effects of bending the porphyrin aromatic π-system. A general overview of the synthesis and photophysical properties of porphyrins and their arrays is carried out in Chapter 1. The electronic structure of porphyrins is examined, and how conformational effects in oligomers, such as inter-porphyrin torsional angle and backbone bending influence the π-conjugation pathway. The structures of light harvesting complexes are discussed. Chapter 2 describes the design and synthesis of a complementary 12-armed template designed to coordinate linear porphyrin oligomers in the correct conformation for cyclisation to give a cyclic porphyrin dodecamer. Chapter 3 demonstrates two approaches to a cyclic porphyrin dodecamer ring. Firstly, a classical templating approach using the 12-armed template is described. The limitations of this approach in the quest for larger nanorings are discussed. Vernier templating, which utilises a mismatch in the number of binding sites between a ligand and its receptor is introduced as a general strategy to the synthesis of large nanorings. This is demonstrated by the synthesis of cyclic dodecamer from a linear porphyrin tetramer and a hexadentate template via a figure-of-eight intermediate. The general utility of the Vernier method to large nanorings is explored in Chapter 4 with steps towards the synthesis of a cyclic tetracosamer, consisting of 24 porphyrin subunits. In preliminary experiments, an improved route to the cyclic porphyrin octamer is described. Finally, the photophysical properties of the nanoring series are explored in Chapter 5 as a function of size and conformation. Femtosecond photoluminescence spectroscopy shows that even in cyclic dodecamer, exciton delocalisation over the entire porphyrin backbone occurs on a sub-picosecond timescale, and parallels are drawn with the dynamics of natural light harvesting complexes.