Elucidating Ultrafast Photochemical And Photophysical Processes Of Natural And Model Light Harvesting Systems By Two-Dimensional Electronic Spectroscopy

Photosystem I (PSI), is a large natural light harvesting complex that drives oxygenic photosynthesis by conversion of the photon from sunlight into a long-lived charge separated state with near unity quantum efficiency. As PSI is one of nature’s most efficient energy converters, it has inspired decades of many researchers to unravel the primary photosynthetic mechanism. Because of the structural complexity of PSI ~300 tightly packed chlorophylls, there still remain unanswered questions regarding photochemical and photophysical mechanisms of PSI. Understanding the mechanisms of energy transfer and electron transfer in PSI is key for solving the puzzle of the high quantum efficiency of this complex and for achieving the improvement of artificial solar energy conversion systems. In this thesis, I have investigated 1) natural PSI complexes isolated from two of cyanobacteria and 2) BODIPY molecules, structurally simpler model light harvesting chromophore. To explore the ultrafast photoexcited dynamics of natural and model light harvesting systems, I use ultrafast two-dimensional electronic spectroscopy (2DES) and transient absorption spectroscopy (TA). This thesis presents 2DES studies of PSI complexes isolated from in different cyanobacterial species to gain further insights into ‘red’ chlorophylls. Red chlorophylls consist of a few groups of strongly coupled chlorophyll molecules that lie to lower energies than the reaction center, so the red chlorophylls must undergo uphill energy transfer. Through applying a global analysis to the 2DES spectra, I find that energy equilibration involving red chlorophylls occurs on two ultrafast timescales and by different pathways. Moreover I explore how ultrafast photophysical dynamics alter through non-covalent interactions and molecular substitutions comparing the halogenated and non-halogenated BODIPY dyes through applying 2DES and TA. Previous studies have shown that vibrational motion is important for the initial charge separation event of light harvesting complexes. In my work, I am investigating how derivatization effects vibrational motion strongly coupled to the electronic excitation, which can lead to insight into the design of artificial photosynthetic complexes based on BODIPY dyes. From these studies I can gain a further understanding of ultrafast energy transfer and charge separation in PSI complexes and gain additional insights into solvation dynamics and how non-covalent interactions act to alter the photophysical properties and solvation dynamics of molecules.