Development of High-Throughput Screening Techniques of Iridium(III) Photocatalysts for Visible-Light Driven Chemical Transformations
Creators: name: Mdluli, Velabo
Publisher: name: Carnegie Mellon University
Year: 2022
Subjects: subject: 39901 Environmental Chemistry (incl. Atmospheric Chemistry)
subject: FOS: Chemical sciences

description: Visible light-driven chemical transformations are essential for the continued development of sustainable, efficient, and selective reactions for producing solar fuels and high value organic compounds. To develop solar-based systems that can compete with fossil-fuel-based technology, scientists need to identify highly efficient and long-lasting light harvesting materials. This search can be accelerated using high-throughput experimentation (HTE) methods. In this thesis, over a thousand structurally diverse [Ir(C^N)<sub>2</sub>(N^N)]+ complexes, where C^N is a cyclometalating ligand and N^N is an ancillary 1,2-diimine ligand, were in-situ synthesized in 96-well plates. Colorimetric techniques were then developed for the real-time monitoring of the catalytic activity of iridium (III) complexes in custom-built photoreactors. The first technique is a rapid assay that utilizes the colorimetric changes of a high-contrast indicator dye, coumarin 6. The bright-green luminescence of the dye can be quenched by protons liberated by oxidation of tertiary amines mediated by the photoexcited [Ir(C^N)<sub>2</sub>(N^N)]+ complex. A resulting [Ir(C^N)<sub>2</sub>(N^N)]<sup>0</sup> can then reduce an aryl bromide to form the highly reactive aryl radical intermediate. The rate of this reaction was dictated by the molecular structure of both coordinating ligands. Relative reaction rate constants determined using this method correlated closely with 19F NMR measurements obtained using a fluorinated substrate. A simple model that expressed the rate constant as a product of a single “strength” parameter assigned to each of the 72 ligands accounted for the 1152 measured rate constants. The best performing complexes exhibited much higher reactivity than the benchmark photocatalysts commonly used in photoredox transformations. From the large library of catalysts, photocatalysts were chosen to test in chemoselective debrominations. A second technique was developed to monitor catalytic activity via the color changes of methyl viologen dichloride upon undergoing single electron transfer (SET) processes in parallelized reactions run in the custom-built photoreactors. Such a system helped us rapidly identify photocatalysts that are good for the oxidative quenching pathway of [Ir(C^N)<sub>2</sub>(N^N)]+ complexes. This catalytic route made it possible to utilize carbon-neutral sacrificial electron donor sources, such as oxalic acid, for the reduction of water to evolve hydrogen and the reduction of sulfoxides to sulfides. Focusing on the oxidative quenching pathway enabled us to design high-performing photocatalysts that survived the detrimental N^N ligand dissociation event, common with the [Ir(C^N)<sub>2</sub>(N^N)]+ photocatalysts. This was demonstrated by the photoactivity of a [Ir(C^N)<sub>2</sub>Cl(solvent)]+ complex in a photocatalyst/water/cobaloxime system. Therefore, judicious design of ligands to prepare the iridium (III) photocatalysts for a particular catalytic route can ensure highly efficient photocatalytic systems.