Photochemical Sciences Ph.D. Dissertations

Title

Iminium Based Electrocaralysts for Water Oxidation and Organic Photohydrides for Proton Reduction

Date of Award

2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Ksenija Glusac (Advisor)

Second Advisor

Ray Larsen (Other)

Third Advisor

John Cable (Committee Member)

Fourth Advisor

R. Marshall Wilson (Committee Member)

Abstract

Earth-abundant catalysts for water splitting process are needed to facilitate the large-scale implementation of solar fuel cells. Most molecular model catalysts for oxygen and hydrogen evolution are made of transition metals. The approach used in our labs involves fully organic, bio-inspired catalysts for water splitting. While such model catalysts are likely easy to be prepared and inexpensive, the challenge is to control the reactivity of organic compounds and avoid catalyst degradation due to undesired chemical reactions. Our recent findings on simple flavin-based iminium ion (Et-Fl+) which facilitates the electrocatalytic water oxidation postulated that the catalysis occurs in cooperation with electrode surface oxides. Since a fully molecular system facilitates spectroscopic studies of the catalytic process, we are interested in developing such a system that consists of covalently-linked iminium ions. The mechanism of operation in this type of a catalyst is proposed in four major steps: (i) pseudobase formation via a reaction of flavinium ions with water; (ii) proton-coupled electron transfer (PCET) of pseudobases to generate alkoxyl radicals; (iii) coupling of alkoxyl radicals to generate the peroxide intermediate; (iv) oxidation of the peroxide to release molecular oxygen and regenerate the catalyst. Although the pseudobase formation step is common, the possibility of PCET of the formed pseudobase to generate alkoxy radical is not known.

Therefore, we evaluate here the thermodynamic possibility of this process using two iminium-based pseudobases: 2,7-dimethyl-9-hydroxy-9-phenyl-10-tolyl-9,10-dihydroacridine (AcrOH) and 6-phenylphenanthridinol (PheOH). The comparative study reveals the importance of having the redox active –N center closer to –OH functionality in order to drive the PCET process. Pourbaix diagrams constructed using pKa values and standard reduction potentials show that only PheOH has a wide and mild range (pH= 2.8 – 13.3) of pH values in which both the alcohol and alkoxy radical can coexist in order to facilitate the coupled process.

Furthermore, in an attempt to identify the functional groups responsible for catalytic oxidation from Et-Fl+ ion, a comparative research was conducted with a simple iminium ion derivative N-methyl-9-phenylacridinium perchlorate (Acr+). Although, some similarities were found in electrochemical behavior of Et-Fl+ and Acr+, the catalytic water oxidation was not facilitated by Acr+.

In addition, an excited state hydride release from an organic hydride is proposed to reduce the protons. The proposed photohydride, 10-methyl-9-phenyl-9, 10-dihydroacridine (PhAcrH) was oxidized to PhAcr+, with 52% conversion, while only 2.5% of hydrogen is liberated. The reduction of the solvent (CH3CN/H2O mixture) was suggested as the reason for low yield of H2. The hydride transfer mechanism was identified as stepwise electron/hydrogen atom transfer process originating from the triplet excited state.

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