Nonlinear Optical Studies of Natural and Artificial Light Harvesting Antennae

Womick, Jordan

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March 22, 2019

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Womick, Jordan
- Affiliation: College of Arts and Sciences, Department of Chemistry

Abstract

This dissertation investigates the influence of nuclear motion on energy transfer processes in natural and artificial light harvesting antennae. In natural systems, light harvesting antennae increase the efficiency of photosynthesis by absorbing light and transferring electronic excitations to the reaction center. These antennae are highly optimized through evolution, and it is estimated that energy transfers to the reaction center through these structures with exceptional efficiency, motivating their investigation. Light harvesting proteins derived from cyanobacteria and red algae enable the study of the simplest possible energy transfer network: pigment dimers composed of one donor and one acceptor. Double walled cylindrical molecular aggregates, which are inspired by the chlorosome of green sulfur bacteria, are used as models for the investigation of many-body coherent electronic relaxation dynamics. Evidence of electronic relaxation is gained through pulsed laser experiments. Many processes like energy transfer, solvation, and the dephasing of coherence take place on the tens to hundreds of femtosecond timescale. Femtosecond time resolved experiments investigate the initial events of these processes using a variety of advanced spectroscopic methods. Additional insight into electronic relaxation dynamics is gained by varying aspects of the radiation fields (e.g., frequency, electric field polarizations). In all cases, the interpretation of experimental data is aided by comparison to theoretical calculations. Chapters 1 - 3 introduce the systems, theory, and experimental methods used in this dissertation. Chapters 4 - 8 present a series of studies on light harvesting antennae from cyanobacteria and red algae. Advanced spectroscopies in chapters 4 - 7 lead to a unified treatment of electronic and nuclear relaxation dynamics using a vibronic exciton model originally derived for the treatment of organic semiconductor crystals. Chapter 8 studies the complex pigment network of the highest energy absorber in the light harvesting complex of red algae. Chapters 9 - 10 investigate the presence and origin of an elusive coherent relaxation process (i.e., coherence transfer) in cylindrical molecular aggregates.

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