Simulation methods and models for state preparation, excitation energy transport, and relaxation in pigment-protein systems

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David Coker of Boston University is supported by an award from the Chemical Theory, Models and Computational Methods program in the Division of Chemistry. Coker and his research group develop methods to study how energy moves in molecular materials that are under strong light (photo-excited). Understanding how to manipulate where the energy goes in these materials as well as how quickly it gets there is central to designing new technologies that can harvest sunlight, using the sun's energy to conduct chemical reactions that produce fuels, to promote physical processes used in sensing and imaging applications, or to select and monitor the evolution of specific quantum states used in quantum computing. There are two components to these studies. The first objective is to understand how to manipulate chemical structures and compositions to enable process design and control. The second objective is to understand how light can be manipulated to select specific excited state processes and control their pathways. This research is resulting in new fundamental understanding for sustainable energy and information science applications, providing new computation methods and simulation tools to be used to design new materials and processes for these applications, and contributing significantly to highly skilled workforce development by training graduate student and post-docs and inspiring undergraduate and high school student through research experience programs. This project is developing a general first principles based simulation framework for exploring mechanisms of excitation energy transport (EET) and relaxation in light harvesting systems. The focus is on pigment-protein complexes that dissipate electronic excitation to down convert the energy of blue light absorbing antenna chromophores to the red for energy transport and transduction in harvesting networks and reaction centers. Ultrafast, nonlinear spectroscopy is the workhorse for experimental studies of these processes and theoretical modeling plays a critical role in disentangling mechanistic understanding from these experiments. The methods being developed go beyond the existing highly averaged empirical approaches that can give ambiguous mechanistic predictions. The new approach incorporates detailed statistical sampling of local inherent structures and fluctuations and parameterizes ensembles of model Hamiltonians using accurate first principles excited state calculations. These models are used in dissipative quantum dynamics calculations to directly compute nonlinear spectroscopy signals. These non-adiabatic dynamics methods are extended to incorporate high frequency quantum vibrational motions critical for dissipating large amounts of electronic energy quickly, promoting efficiency. Time dependent driving fields are also incorporated into these methods so they can treat the radiation – matter interaction beyond perturbation theory enabling the description of strong fields and complex sequences of overlapping light pulses that can be used to control excited state dynamics. Predictions are being benchmarked against detailed experiments.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.