Development of Iridium Based Photosensitizer for Potent Photoreductants and Photocatalytic Application

Date

2020-08

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Abstract

This dissertation describes the development of iridium based organometallic photosensitizers which function as potent photoreductants for photoredox catalysis. Chapter 1 describes the basic concepts of photosensitizers in photocatalysis and strategies to modify the photophysical and electrochemical properties via ligand modification with different types of photosensitizers. Chapter 2 describes the first generation of bis-cyclometalated iridium photoreductants with electron-rich β-diketiminate (NacNac) ligands. The photophysical and electrochemical properties were compared to fac-Ir(ppy)3, known to be a ‘the-state-of-the-art’ photoreductant, and the calculated excited-state potentials (ESPs) show that our complexes have more reductive potential than fac-Ir(ppy)3. Stern-Volmer quenching experiments also support that the new complexes have higher ESP and reducing ability. Chapter 3 describes an expanded photocatalyst library with further modified NacNac ligands. Based on results from Chapter 1, we have prepared a dozen photosensitizers for the photocatalytic hydrodebromination of bromide substrate and we have screened to find the best-optimized conditions to activate the carbon-bromine bond to replace with a hydrogen atom. Chapter 4 is a deeper study of the hydrodehalogenation reaction with candidate photosensitizers from Chapter 3. Under modified and optimized reaction conditions, alkyl and aryl halides (X = Br, Cl, F) are tolerated in hydrodehalogenation and functionalization reactions proceeding through a radical-mediated reaction mechanism. Finally, Chapter 5 describes the second ligand modification strategy to elevate the excited-state energy (ET1) with alternative cyclometalating ligands which have been used for blue-emitting compounds. Replacing the cyclometalating ligands that we previously used with triazole or NHC-derviced cyclometalating ligands results in a highly destabilized LUMO energy level and record-high excited-state potentials for cyclometalated iridium photosensitizers.

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Keywords

Photoreductant, photoredox catalysis, photocatalyst

Citation

Portions of this document appear in: Shon, Jong-Hwa, and Thomas S. Teets. "Photocatalysis with Transition Metal Based Photosensitizers." Comments on Inorganic Chemistry 40, no. 2 (2020): 53-85; and in: Shon, Jong-Hwa, and Thomas S. Teets. "Molecular photosensitizers in energy research and catalysis: design principles and recent developments." ACS Energy Letters 4, no. 2 (2019): 558-566; and in: Shon, Jong-Hwa, and Thomas S. Teets. "Potent bis-cyclometalated iridium photoreductants with β-diketiminate ancillary ligands." Inorganic Chemistry 56, no. 24 (2017): 15295-15303; and in: Shon, Jong-Hwa, Steven Sittel, and Thomas S. Teets. "Synthesis and characterization of strong cyclometalated iridium photoreductants for application in photocatalytic aryl bromide hydrodebromination." ACS Catalysis 9, no. 9 (2019): 8646-8658.