Photochemical Sciences Ph.D. Dissertations

Title

Natural and Artificial Flavin-Based Catalysis

Date of Award

2013

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Ksenija Glusac, Dr

Second Advisor

Moira van Staaden, Dr (Committee Member)

Third Advisor

Alexander Tarnovsky, Dr (Committee Member)

Fourth Advisor

Mikhail Zamkov, Dr (Committee Member)

Abstract

Artificial water splitting becomes an essential element for solar energy conversion, since the obtained energy can be stored in the form of hydrogen and oxygen gas. There are numerous studies on the development of efficient water oxidation catalysts based on transition metal complexes. One of the obstacles in using them in real large-scale applications is their high cost. Hence, the development of efficient, cheap and made from earth-abundant elements is critical for successful implementation of the solar fuel cells. Nature controls a tremendous amount of oxidation reactions that are catalyzed by fully organic compounds. For example, in the bioluminescence reaction the oxygen transfer process is catalyzed by a flavin derivative. In particular, our group is attempting to study the reverse of the bioluminescence reaction, which is the oxidation of water to oxygen using the oxidized form of flavin. Inspired by organocatalysts present in biological systems we designed and study a fully organic catalyst N(5)-ethylflavinium ion (Et-Fl+) and its pseudobase Et-FlOH as the potential water oxidation catalysts. Initially our studies have been focused on Et-FlOH as a possible water oxidation catalyst using the electrochemical and chemical oxidation experiments. Two previously published reports suggested the two-electron oxidation of Et-FlOH leads to the formation of Et-Fl+ which release the hydroxyl cation. Based on our results we conclude that this is not the case and Et-FlOH is not capable of oxidizing water itself. However, the cyclic voltammogram of Et-Fl+ showed interesting catalytic properties, which triggered us to further investigate. It was shown that Et-Fl+ performs catalytic water oxidation at a high overpotential ~ +1.9V vs NHE selectively on carbon and platinum electrodes. We carried out several different experiments such as bulk electrolysis coupled with oxygen sensing, rotating disk electrode experiments, spectroelectrochemistry to study electrochemical and catalytic performance of Et-Fl+ along with identifying the key intermediates in the catalytic cycle. We found that Et-Fl+ exhibits a modest TON=13. Density function theory (DFT) calculations revealed the mechanistic insights and the role of the working electrode in the catalysis. The essential O-O bond-forming step is proposed to occur by the coupling of two oxygen-centered radicals, one of which is attached to the flavin moiety and the other one which is attached to the electrode surface. Another interesting side of the bioluminescence reaction is the light production reaction. It is believed that hydroxyflavin is the emitter in the bioluminescence reaction and is responsible for the light production. However, outside of the protein this compound no longer emits light. Such a behavior triggered us to study the mechanism of the fast excited state deactivation. As a model compound we chose Et-FlOH. Using pump-probe spectroscopy we found that Et-FlOH exhibits a short-lived excited state. Several possible mechanisms for the fast excited deactivation have been considered and explored. The DFT calculations suggested that the existence of conical intersection. Ab initio calculations were performed on a reduced model of Et-FlOH to identify the nuclear coordinates responsible for the fast excited state deactivation. Based on our findings we concluded that there are three main nuclear modes involved, where the main one is associated with the distortion of the ring.

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