In this CAREER proposal the PI proposes to further investigate how nature harnesses and controls light energy to meet the diverse demands of life processes using two-dimensional electronic spectroscopy (2DES) that has only recently become possible at optical frequencies. 2DES provides femtosecond time resolution, allowing the fastest processes to be captured, while containing the information derived from conventional nonlinear spectroscopies as a subset. With this tool, the PI will study the design principles of two key biological systems: the reaction center of photosystem II, and DNA. 2DES studies of photosystem II aim to unravel the complex sequence of energy and charge transfer that enable oxygenic photosynthesis, while in DNA, 2DES promises a detailed view of the mechanisms that furnish its genetic robustness. The educational component of this proposal aims to encourage students, educators, and the general public to appreciate the biophysical questions surrounding them and to help them make connections to the underlying physics. The PI will target introductory physics classes where hands-on demonstrations illustrating such connections will be used in an interactive teaching context. To help physics students appreciate biophysics, several experiments will be added to the advanced undergraduate laboratory, including an optical tweezers setup. The proposal also aims to promote a positive image of science through public education, and in particular to encourage women and minorities to enter careers in science through outreach activities and mentored undergraduate research. The proposed research will have impact on the design of artificial light-harvesting systems as well.
This project developed novel pulse-shaping-based multidimensional spectroscopy methods to enable ultrafast measurements of energy transfer and charge separation in biological systems. Pulse-shaping approaches simplify the implementation of multidimensional spectroscopies, facilitating their adoption by a growing number of research groups. The methods that were developed are broadly applicable to studies of both natural and artificial light-harvesting materials. The methods were applied to natural photosynthetic systems to improve our understanding of the design principles that provide rapid and efficient energy transfer and charge separation in photosynthesis. The multidimensional spectroscopy data were used to test and refine theoretical models of the electronic structure and relaxation processes in natural photosynthetic systems. Studies were also performed to understand how coupling between vibrational and electronic states is manifested in multidimensional spectra. It was found that specific vibrations strongly modulate multidimensional spectra, enabling the identification of particular vibrational modes that are strongly coupled to electronic transitions on the ground and excited state of single chromophores and multipigment photosynthetic complexes. In natural photosynthetic systems, where electronic coupling between the constituent pigments leads to delocalized excitations, it was found that the particular vibrations that are resonant with electronic energy gaps may play a key role in enhancing charge separation in photosynthesis. Vibrational-electronic resonances may be an important consideration for improving the design of artificial light-harvesting materials. The project also aimed to have a broad impact through a number of educational activities. In an effort to enhance the participation of women in physics the principal investigator chaired the organizing committee for two Conferences for Women in Physics (CUWiP) meetings at the University of Michigan, and was an invited speaker at CUWiP events at Ohio State University and MIT. These meetings reached hundreds of undergraduate women, educating them about possible careers in physics and building a network of women physicists across the US. The principal investigator also gave a variety of outreach seminars to the general public, undergraduates and graduate students and gave lab tours to Girl Scouts and high school student groups. The project supported the scientific training of graduate and undergraduate students. It also enhanced Biophysics education at the University of Michigan by supporting the development of several optics demonstrations and an optical tweezers laboratory.
