Monitoring ultrafast evolution of electronic structure within photosynthetic antenna complexes using multidimensional spectroscopies

Photosynthetic antenna complexes capture solar radiation and transfer the resultant excitation energy to neighboring reaction centers. The quantum efficiency of the excitation energy transfer process in these systems can approach unity at low light intensities. The underlying physical mechanisms that govern the efficiency of this process are not understood. Spectroscopic experiments have revealed that the relaxation process in individual antenna complexes contains a coherent component, whereby the excitation maintains phase information while relaxing. The coherent signatures are manifested as oscillations in signal intensities which are suppressed following photoexcitation due to interaction of the chromophores with the surrounding environment. An understanding of what molecular mechanisms govern the dynamics in these systems could inform design principles for synthetic devices that replicate the efficient dynamics.;The work in this thesis explores the complex underlying physical mechanisms governing the dephasing and relaxation dynamics through the development of spectroscopic and theoretical methodologies. Multidimensional spectroscopic methodologies allow for the elucidation of the mechanism of relaxation and provides insights into high order-correlations present in the protein bath. The time scales of the relaxation dynamics disclose information about the surrounding protein environment, revealing that proteins motions correlate energetic fluctuations following photoexcitation. The rate of dephasing of different coherences can be understood as arising from the interplay of excitonic mixing and spatial proximity of the chromophores. A higher-order multidimensional method is explored to reveal aspects of non-Gaussian dynamics of solvation and vibronic coupling. A nonlinear solvation model is developed to explore the increased sensitivity to molecular details provided by multidimensional methods and can explain what physical parameters lead to non-Gaussian dynamics. Control of the polarization of the excitation pulses allows for different aspects of the relaxation dynamics to be selectively probed. The coherent contributions to the relaxation can selectively be detected revealing the time scales of the dephasing dynamics. Signatures of dynamic localization and supertansfer can be revealed through chiral nonlinear methodologies.