Structure-Function Relationships in Chlorophyll a Derivatives for Artificial Photosynthesis

Replication of the key processes in photosynthesis for the generation of solar energy conversion technologies has principally involved the synthesis of materials capable of efficiently collecting light energy, transferring energy, separating charges, and directing charge transport to catalytic centers for fuel generation. The high efficiencies and rates of these primary natural photosynthetic steps rationalize their application as ideal models for the solar energy conversion, often referred to as artificial photosynthesis. These biomimetic approaches to photosynthesis have led to the development and physical characterization of molecules designed to replicate the above processes, while aiming to elucidate how structure and function relate. Artificial photosynthetic systems have encompassed a wide variety of pigments, including molecules derived from chromophores found in the natural reaction center and in the antenna proteins: carotenoids, chlorophylls, pheophytins, and quinones. This thesis focuses on a series of semi-synthetic chlorophyll a derivatives, where chlorophyll was extracted from algae and then chemically modified to append functional groups that allow them to be incorporated into molecules to target specific photophysical properties. Steady-state and time-resolved spectroscopy were employed to determine rates and efficiencies of photodriven energy and electron transfer, while advanced X-ray, NMR, and optical experiments were used to resolve solution-phase structures. Additionally, chlorophyll derivatives were designed to mimic the supramolecular architectures established by specific natural protein environments through intermolecular forces, including pi-pi interactions, hydrogen-bonding, and metal-ligand coordination, to self-assemble chlorophyll molecules in solution. These self-assembled architectures induce and/or enhance photodriven functionalities that are distinctly different than the photophysical properties of the corresponding monomers. Such assembly methods minimize synthetic efforts and exert structural and photophysical control to generate tunable, discrete supramolecular systems, which are imperative for converting solar energy into electricity or fuels. The fundamental molecular level understanding gained in the presented covalent and self-assembled structure-function chlorophyll relationships described enable key guidance for the fabrication of economically viable artificial photosynthetic devices that utilize earth-abundant robust materials with similar photophysical properties.