Into the Rainbow: Kinetics of the Bacterial Reaction Center Using Broadband Two-Dimensional Electronic Spectroscopy

Abstract

Photosynthesis is a process by which light is converted to chemical energy, and which is responsible for nearly all life on Earth. A more complete understanding of the mechanisms behind photosynthesis, particularly the initial energy conversion steps, could inform the design of more efficient future photovoltaic devices. The bacterial reaction center (BRC) has been an important model system for understanding the mechanism of photosynthetic charge separation due to its well characterized structure and the distinct absorption features of its constituent pigments. Two-dimensional electronic spectroscopy (2DES) has recently emerged as a valuable tool for studying the electronic structure and dynamics of photosynthetic systems. 2DES can reveal electronic coupling and the ultrafast processes of energy transfer and charge separation. In this thesis, two-color and broadband 2DES are used to probe the electronic structure of the BRC, allowing us to identify the Qx exciton locations and characterize the charge separated intermediates across a wide spectral range spanning the Qy and Qx transitions. The spectral features and kinetics of the BRC at 77K are discussed qualitatively based on the results of a two-color 2DES experiment using a near-IR pump covering the BRC Qy transitions and visible probe covering the Qx band. Strong excited state absorption (ESA) features present in the Qx region were fit and subtracted out to reveal the underlying ground state bleach (GSB) structure. A rich network of cross peaks in the resulting spectra indicates the presence of a common ground state between the Qx and Qy transitions. It also strengthens our previous assignment of the energy of the upper exciton state of the special pair which has been highly controversial due to its weak transition strength. This work demonstrates that two-color 2DES can be used to provide valuable information about the electronic structure and coupling of the BRC by accessing regions in which different degrees of spectral overlap provide complementary information to one-color studies. A more quantitative analysis is performed using broadband 2DES data that extends the detection axis from the near-IR into the visible regime, covering both the Qy and Qx bands. The multiexcitation global kinetic analysis was adapted from earlier work by Niedringhaus et al. that compares models of charge separation in the Qy band. In this scheme, a kinetic model is constrained by prior knowledge of the system and used to fit both the 2DES and linear absorption spectra simultaneously. The spectral signature of the charge separated state is calculated by linear least squares and relevant physical parameters such as exciton energies are extracted from the optimized fit. This analysis was extended to model broadband data covering both the Qx and Qy bands, including the ESA contributions which had previously been neglected. Using this approach, the spectral signatures of the charge separation intermediates were characterized over the broadband detection axis. The major features of the 2DES data are replicated, although some discrepancies between the fit and measured data persist. Future work will consider additional approaches to account for excited state absorption and inhomogeneity to better reproduce the observed 2DES spectral features. The broadband 2DES data presented in this thesis, as well as future polarization-dependent 2DES studies, will provide an extensive data set for simulations that can test and refine exciton and charge separation models of the BRC.