Micheal Alowakennu

Chemistry - 428

The potential of Ruthenium (Ru) and Iridium (Ir)-based systems as photocatalysts to affect a wide

range of organic transformations has been extensively studied. These species satisfy the general

criterion for a typical photocatalyst: good absorption cross-section, photostability, long lifetime,

high excited-state formation, reversible redox behavior, and tunable excited-state characteristics,

among others. However, the fact that these photocatalysts are made from rare and noble metals

raises legitimate questions on economic, environmental and scalability issues. Addressing such

issues, coupled with the quest for discovering unprecedented reactivity can be achieved by

shifting to the first-row transition series. A major drawback to the use of these first-row transition

metals is the ultrafast deactivation of the emissive charge-transfer states to lower-lying metal-

centered (MC) states,

which are usually characterized by structural distortions in the

chromophore. Recent studies have shown that these metal-centered ligand-field states can be

tuned and activated to participate in bimolecular chemistry.

Our group has focused on using

design strategies to understand the mechanism of reactivity associated with the metal-centered

state of these first-row congeners of Ru and Ir i.e., Iron (Fe) and Cobalt (Co), respectively. Although

isoelectronic to low-spin d

Fe(II), Co(III) presents a fundamentally different excited-state

dynamics. For example, Fe(II) had been documented to perform mild reductive chemistry from the

MC excited state,

but the relative stability of Co(II) versus Co(IV) could preferentially allow for

more potent, metal-centered oxidative photochemistry. To explore MC state reactivity in Co(III)

photosensitizers, my research will focus on synthetically tuning the ligand framework of selected

Co(III) systems using strong various bidentate and tridentate ligand scaffolds. Their potential to

be quenched will be probed based on their redox characteristics, and this will serve as a valuable

tool for catalyzing bimolecular coupling reactions that form the bedrock of organic synthesis.

References:

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catalyst design. Chem. Soc. Rev. 2016, 45, 5803−5820.

2. (a) McCusker,J. K. Electronic Structure in the Transition Metal Block and Its Implications for Light

Harvesting. Science 2019, 363, 484−488. (b) Zhang,W.; Alonso-Mori, R.; Bergmann, U.; Bressler, C.;

Chollet, M.; Galler, A.; Gawelda, W.; Hadt, R. G.; Hartsock, R. W.;Kroll, T.; Kjær, K. S.; Kubiek, K.;

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Charge and Spin Dynamics in Iron Coordination Complexes. Nature 2014, 509, 345−348.

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d-d Excited States of Ni(II) Complexes Relevant to Photoredox Catalysis: Spectroscopic

Identification and Mechanistic Implications. J. Am. Chem. Soc. 2020, 142, 5800−5810.

4. Woodhouse, M. D.; McCusker, J. K. Mechanistic Origin of Photoredox Catalysis Involving Iron(II)

Polypyridyl Chromophores. J. Am. Chem. Soc. 2020, 142, 16229−16233.