Femtosecond pump probe spectroscopy of light harvesting complexes and Phthalocyanines

Ombinda-Lemboumba, Saturnin (2011-12)

Thesis (PhD)--Stellenbosch University, 2011.

Thesis

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.

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