Femtosecond pump probe spectroscopy of light harvesting complexes and Phthalocyanines

[摘要] ENGLISH ABSTRACT: The generation of ultrafast light pulses and the development of time resolvedspectroscopic techniques, such as the femtosecond pump probe spectroscopytechnique, have facilitated the study of ultrafast energy transfer in the photosyntheticsystems of green plants and photodynamic therapy drugs. It hasallowed the investigation of biological and chemical processes that take place onthe ultrafast timescale and has allowed us to obtain spectral and kinetic informationon energy transfer. In addition, it has allowed time resolved experimentsin which the transient absorption of species under investigation was observedand has elucidated molecular dynamics. In the present work this was done witha temporal resolution of approximately 200 fs and covering a pump-to-probedelay range of 300 fs to 2 ns.The main aims of this study were to characterise the femtosecond pump probespectroscopy system, to investigate the energy transfer in the natural lightharvesting complex II (LHC II) in view of future expansion to the study ofsynthesized arti cial functional light harvesting complexes andnally to studyultrafast processes in zinc phthalocyanine (ZnPc) systems.In photosynthetic organisms, LHC II is the most abundant light harvestingcomplex and it plays an important role in light harvesting and photoprotection.The light energy is absorbed by light harvesting complexes and transferred toa reaction centre (RC) in an ultrafast timescale.Phthalocyanines are a new class of photosensitiser used for photodynamictherapy. These drugs are used to treat small and super cial tumours. Theenergy transfer from the singlet excited state to the triplet excited state occurson an ultrafast timescale. However, recent work done on zinc phthalocyanine hasproved that the determination of the ultrafast component remains a challenge.Several ultrafast studies carried out on ZnPc in solvents have been not onlyunsuccessful to give a clear picture of the ultrafast dynamics but have alsoproduced divergent results.In this study, a characterisation of the femtosecond pump probe spectroscopysetup was done. The samples under investigation were probed by a white lightcontinuum. 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 generationis discussed in detail. Our femtosecond pump probe spectroscopy setup wasbenchmarked by using a well known dye, namely malachite green. In addition,the investigation of the transient absorption change of LHC II, an active componentin photosynthesis, as extracted from spinach leaves and the ultrafastdynamics of a promising photosensitiser ZnPc in dimethyl sulfoxide (DMSO) as well as in dimethyl formamide (DMF) was done. The spectral and dynamic resultsobtained using these three samples are described and exponentialts to theabsorbance decay curves used to estimate the timescales of the energy transferprocesses are presented. In this experiment, the dynamics and measured timeconstants related to the energy transfer between the different types of chlorophyllin LHC II was monitored, whereas with ZnPc, the dynamics and themeasured time constants associated with solvation dynamics and vibrationalrelaxation was examined.

[发布机构] Stellenbosch University