A theoretical and femtosecond spectroscopic investigation of energy transfer in photosynthetic complexes

The photosynthetic unit of plants and bacteria is an example of nature's supreme engineering. The application of the principles developed in nature to the design of synthetic solar energy conversion devices holds great promise. Photosynthetic energy transfer is achieved by chromophore-chromophore interactions. These interactions, as well as protein-chromophore interactions, cause substantial inhomogeneity of the pigments' site energies and coupling strengths described in the electronic Hamiltonian, deeming computation of the Hamiltonian challenging. Yet the Hamiltonian holds the key to determining the energy landscape and, therefore, the design of the complexes. In this dissertation, fundamental interactions between pigments in the photosynthetic apparatus are investigated and the electronic Hamiltonians of three photosynthetic proteins are computed.; To elucidate the energy transfer steps in Photosystem I (PSI) and the Fenna-Matthews-Olson (FMO) complex we use new methods in femtosecond photon echo spectroscopy and condensed-phase quantum dynamics. To obtain the Hamiltonian of the chlorophyll states in PSI, the semi-empirical INDO/S method is used to compute the in vivo excitation energies of and the couplings between the 96 non-equivalent chlorophylls in PSI. We developed modified Redfield/Forster theory to describe energy transfer between both weakly and strongly coupled pigments. By applying this method to both PSI and the FMO complex using the Master equation we provide insight on how the (bacterio)chlorophylls are optimized to generate high-efficiency photosynthetic antennas. Our energy transfer models for PSI and FMO are tested and refined by comparison to femtosecond photon echo data. One-color three pulse photon echo peak shift (3PEPS) experiments on PSI revealed timescales of energy transfer between pigments of similar excitation energy and two-color 3PEPS accessed couplings between pigments of different excitation energies. Two-dimensional electronic spectroscopy of the FMO complex revealed the step-wise exciton relaxation pathways. The photon echo data are well-simulated using our developed models.; The electronic excitation energies of the carotenoid peridinin in Peridinin-Chlorophyll-Protein are obtained using time-dependent density functional theory. We present theoretical confirmation of an intramolecular charge transfer (ICT) state in peridinin and show, using couplings calculated via the ab initio transition density cube method, that the ICT state enhances peridinin light-harvesting by increasing the coupling to chlorophyll. Similar ab initio calculations are also presented for the bacteriochlorophylls in the FMO complex.