Date of Award
Doctor of Philosophy (PhD)
Chemistry & Biochemistry
David M. Jonas
Joel D. Eaves
Robert P. Parson
Steven T. Cundiff
Niels H. Damrauer
Development of efficient light-harvesting technologies hinges on our understanding of the fundamental physics of light-harvesting in both natural and artificial systems. This work addresses the following topics, i.) the mechanism underlying the remarkably efficient electronic energy transfer in natural light harvesting antennas, ii.) a femtosecond time-resolved photonumeric technique to quantitatively characterize transient chemical species.
A non-adiabatic model for photosynthetic energy transfer in light harvesting antennas is proposed. Light harvesting antennas use a set of closely spaced pigment molecules held in a controlled relative geometry by a protein. It is shown that in the Fenna-Matthews-Olson (FMO) antenna protein, the antenna found in green sulfur bacteria, the excited state electronic energy gaps are resonant with a quantum of vibrational energy on its pigment, bacteriochlorophyll a. Through a dimer model loosely based on FMO, it is shown that such a resonance leads to an unavoidable nested non-adiabatic energy funnel on the excited states of photosynthetic antennas. The non-adiabatic model presented here leads to enhanced vibrational oscillations on the ground electronic state of these antennas, the 2D spectroscopic signatures and oscillation frequencies of which are consistent with all the reported 2D signatures of long-lived oscillations, including the ones that are not explained by prior models of excited state electronic energy transfer. Extensions that account for both resonant and near-resonant pigment vibrations suggest that photosynthetic energy transfer presents a novel design in which electronic energy transfer proceeds non-adiabatically through clusters of vibrations with frequencies distributed around electronic energy gaps.
The latter part of the thesis presents absolute measurements of femtosecond pump-probe signal strength. The experiments demonstrate quantitative time-resolved measurement of absolute number of excited state molecules. Based on these measurements, an all-optical technique that simultaneously determines concentration and extinction coefficient of an unknown sample is presented. Unlike prior such analytical techniques, the present photonumeric method does not require any sample isolation, physical handling or in situ calibrant. In principle, the experimental and theoretical framework developed allows extensions towards characterization of transient chemical species.
Tiwari, Vivek, "Non-Adiabatic Mechanism for Photosynthetic Energy Transfer and All-Optical Determination of Concentration using Femtosecond Lasers" (2014). Chemistry & Biochemistry Graduate Theses & Dissertations. 122.