Multidimensional Spectroscopy of Photosynthetic Complexes
Experiments using two-dimensional (2D) electronic spectroscopy to investigate the structure-function relationships that give rise to photosynthetic energy transfer within pigment protein complexes are presented and discussed in this dissertation. 2D electronic spectroscopy using ultrafast laser pulses throughout the visible regime was applied to study excitation energy transfer in the major light harvesting complex of photosystem II (LHCII) and the reaction center from purple bacteria. These experiments elucidated information about the excited state structure and the energy transfer timescales within these complexes. All-parallel 2D spectroscopy was used to monitor the energy transfer dynamics in LHCII and reveals previously unobserved sub-100 fs energy transfer between the chlorophyll-b (Chl-b) and chlorophyll-a (Chl-a) bands and within the Chl-a band. Reproducing these results with simulations led to improvements in the values of the uncoupled transition energies of the chlorophyll in the working Hamiltonian of LHCII. The delocalized excited states observed in the experimental and theoretical results were found to increase the range of optimal angles for energy transfer from LHCII to neighboring pigment-protein complexes, as opposed to the case of a single, isolated donor excited state. Polarized 2D spectroscopy experiments reported here identified previously unresolved excitation energy transfer steps in LHCII. These results were used to determine the angle between
transition dipole moments of the donor and acceptor. A new method was developed to use the angle between transition dipole moments to find the uncoupled transition energies of the chlorophyll, previously the major unknown for an accurate electronic Hamiltonian. This method was applied to LHCII. Quantum coherence, or a long-lived superposition of excited states, was observed in LHCII using a second polarization sequence. The observable timescales of coherence was determined to be 700-900 fs, which illustrates that quantum coherence lasts longer than many energy transfer steps. The potential contribution of coherence to the robustness of photosynthetic energy transfer to the rugged energy landscape and to temperature variations is discussed. Experiments on the B band of the bacterial reaction center were able to isolate the previously inseparable two peaks and observe energy transfer between these two excited states. A new extension of 2D spectroscopy, two-color 2D spectroscopy, was demonstrated for
examining the interactions between two spectrally separate chromophores. Using this approach, energy was found to transfer from the carotenoid to the bacteriochlorophyll both via S1 and via Qx in the bacterial reaction center in an approximately 2:1 ratio, and within about 750 fs.