Theory of time-resolved non-linear optical spectra of pigment-protein complexes
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In green sulfur bacteria the FMO protein connects the outer light-harvesting complex, the chlorosomes, with the reaction center complex . The flow of excitation energy can be measured with time-resolved non-linear optical spectroscopy. In the present work a structure-based description of time-resolved pump-probe spectra of the FMO protein is offered.
Applying a reduced density matrix approach to describe exciton dynamics in the PPC, exciton relaxation is calculted in secular- and Markov-approximation. A key quantity for exciton relaxation is the spectral density of the vibrational coupling between the pigments and the protein. Its intermolecular contributions are obtained from a classical normal mode analysis (NMA) of the PPC combined with quantum chemical/electrostatic calculation of optical transition energies and excitonic couplings . Intramolecular contributions, obtained from a quantum chemical normal mode analysis in the literature, are added to the intermolecular parts of the spectral density .
Our calculations show that the modulation of optical transition energies of the pigments is the most important contribution to the intermolecular part to the spectral density and that also the intramolecuar modulations of transition energies are crucial for the dissipation of excess energy of excitons. Exciton relaxation, which occurs on a sub-pico second to pico second time scale, can qualitatively be understood.
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