The Study Of Energy Transfer In Photosynthesis Light-harvesting Systems

The two-dimensional third-order nonlinear response electronic spectroscopy(2DES) is a powerful research tool in probing micro structure and micro dynamics. It has been widely used in the study of energy transfer in photosynthesis light-harvesting systems. In this thesis, we will focus on the theoretically simulate the 2DES. Two typical photosynthesis light-harvesting systems will be studied by simulate the 2DES. One is the Fenna-Matthews-Olson(FMO) complex from green sulfur bacterium. The other one is the phycoerythrin 545(PE545) from Rhodomonas CS24.In Chapter one, we introduce the fundamental knowledge about the 2DES, mainly includes the light-matter interaction, perturbative expansion of the density matrix, nth-order response functions, linear absorption spectra and third-order nonlinear response function.In Chapter two, we derive the hierarchical equations of motion(HEOM). The basic idea of the HEOM approach is to mimic the system-bath interaction via an infinite hierarchy of auxiliary density matrices.In Chapter three, we investigate the system includes a subunit of FMO antenna complex and its reaction center(sink). At first, through fixing the site energies of bacteriochlorophyll molecules and the coupling strength between the BChls, we construct a Hamiltonian of the FMO-sink and its reduced models. Then, based on the Hamiltonian and by using the HEOM approach we investigate the evolutions of the populations of different BChls and the sink states. Thirdly, in oder to observe the coherence between different states in the processes of energy transfer, we calculate the 2DES of the FMO and its reduced models in different waiting times. Our results imply that for energy harvesting there may be some redundant chromophores in the FMO complex. This discovery may be helpful to find out some reduced structures for simulating photosynthesis.In Chapter four, we reproduce the 2DES of PE545 at ambient temperature(294 K) by using HEOM approach. The simulated spectra are in agreement with the experimental results reported in Wong et al.(Nat. Chem. 2012, 4, 396). The evolutions of cross peaks for rephasing spectra and diagonal peaks for nonrephasing spectra have also been plotted. The peaks oscillate with the population times, with frequencies, phases, and amplitudes of the oscillating curves also being qualitatively consistent with the experimental results.In the last chapter, we summarize our works in this thesis and outlook on the difficulties and research orientation when study the energy transfer in photosynthesis light harvesting systems.