Development of Two-dimensional Stark Spectroscopy for the Investigation of Photosynthetic Charge Separation

Abstract

Charge transfer reactions are critical for the efficient function of photosynthetic enzymes. With growing energy demand, understanding the design principles of natural photosynthetic systems is important to aid efforts in developing sustainable energy sources that do not add to the carbon-dioxide burden of the atmosphere. Photosystem II is particularly interesting because it is an ideal model for artificial photovoltaic devices for energy applications: it is efficient, stabilizes the energized state for useful times, and is resilient to photo-damage. Despite decades of study, the mechanism of primary charge separation in this system is still under debate, primarily because the charge-transfer intermediates involved in these reactions do not have strong spectral signatures and are extremely short-lived.

I have developed a novel spectroscopy method called two-dimensional electronic Stark spectroscopy (2DESS) for the study of fast processes involving the movement of charge in photosynthetic proteins. It combines the high sensitivity of Stark spectroscopy to charge-transfer reactions and the high temporal and spectral resolution of two-dimensional electronic spectroscopy. In collaboration with Darius Abramavicius at Vilnius University in Lithuania, I simulated a charge-transfer dimer system similar to the ``special-pair" chlorophylls found in PSII RC thought to be involved in the primary charge-separation process in this system. Based on these simulations, I demonstrated that the 2DESS and Stark spectra for CT states in the PSII do not follow typical Liptay models. There is also evidence to suspect that the parameters used to model the PSII is incorrect. I then demonstrated the experimental technique on an organic polymer often used for photovoltaic applications, observing first-derivative lineshapes consistent with predictions. Following this demonstration, I observed spectral signatures consistent with charge-separation of the PSII RC. Work is underway to extend the simulations to a more complete model system, as well as utilize the experimentally-obtained data to verify proposed models of charge-separation in the PSII RC.

In combination with other spectroscopy techniques, 2DESS will allow us to obtain a complete description of the initial charge-separation kinetics in photosystem II and may suggest ways to mimic its extraordinary efficiency. We expect this technique to be applicable to other systems such as organic photovoltaics, in which the role of CT states is unclear or is hard to trace.