Excited-state dynamics of photochemical processes are determined by the interactions of a chromophore system with its environment. Controlling these dynamics will enable new applications in chemistry and materials science, ranging from solar light harvesting to photocatalysis. Detailed interactions between the excited states of the materials and their environment can only be understood from observations on the timescale of electronic and nuclear motion. I will discuss some aspects of my work on visualizing femtosecond excited-state dynamics. Biology offers a wealth of systems in which well-defined nanoscale environments control excited-state dynamics. The Fenna-Matthews-Olson (FMO) complex from green sulfur bacteria consists of a set of bacteriochlorophyll chromophores held in specific orientations by a dynamic protein scaffold. Using single-shot 2D electronic spectroscopy measurements, I will discuss how redox changes in the protein environment affect the flow of energy through the complex on the ultrafast timescale. I identify the specific protein structural elements that allow the complex to tune energy transfer in response to changing conditions, thus protecting more fragile photosynthetic machinery downstream. In addition, I investigate an open question regarding the role of quantum coherences in energy transfer through FMO. My data show a clear correlation between the existence of quantum coherences, particularly those involving excited electronic states, with faster, more efficient energy transfer through the complex. These results have implications for the long-standing debate regarding the coherences’ origin. Finally, I will discuss measurements of ultrafast dynamics of photoprotection in cyanobacteria in vivo. We seek to identify spectroscopically the short-lived intermediates in the photoprotective process to assign the molecular mechanism used by these organisms. Ultimately, we would like to directly image femtosecond dynamics on the nanoscale, thus enabling us to visualize the flow of energy through systems and materials when the structures are not already known. To tackle this challenge, I am working to develop Optical Resonance Imaging, which uses tilted ultrafast laser pulses to map space to time, thus overcoming the diffraction-limit for imaging in a way analogous to MRI. I will discuss the concepts and approach to this technique to generate widefield superresolution images of ultrafast excited-state dynamics.

Visualizing chemical dynamics to control energy transfer

2019 Fall National Meeting

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