EARLY TRANSITION METAL PHOTOCHEMISTRY

The novel photosensitizers developed in our lab find interesting applications in a variety of fields including photoredox catalysis, solar energy conversion, and solar fuels production. In addition, photochemical transformations in early transition metal complexes allow new avenues for C-C bond forming reactions and C-H bond activations that are not available under thermal conditions.

VISIBLE LIGHT PHOTOREDOX CATALYSIS

Photoinduced single-electron transfer reactions resulting in radical mechanisms provide access to new reactivity in organic synthesis. This approach is often complementary to classic polar or two-electron methodology. Due to the inability of many organic substrates to absorb visible light photosensitizers, often inorganic or organometallic compounds, are utilized to facilitate photocatalysis.

In the Milsmann lab, we showed for the first time that Earth-abundant early transition metal complexes with ligand-to-metal charge transfer (LMCT) character in the excited state are viable alternatives to the dominant precious metal chromophores in this field.

J. Am. Chem. Soc. 2016, 138, 13115

Nature Chemistry 2020, 12, 345

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MECHANISTIC STUDIES OF PHOTOREACTIONS

Photoredox catalytic transformations often exhibit complex radical mechanisms with several competing pathways.

In the Milsmann lab, we are particularly interested in the initial single-electron transfer step between our early transition metal photosensitizers and organic substrates. Through a combination of in-situ spectroscopic studies and isolation of reactive intermediates we aim to obtain a better understanding of the thermodynamic and kinetic aspects of photochemical reactions involving our photosensitizers.

J. Am. Chem. Soc. 2018, 140, 5934

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PHOTOINDUCED C-C BOND FORMATION

Photolytic metal-carbon bond homolysis reactions are well-established in organometallic chemistry and can result in interesting products with new carbon-carbon bonds. However, most reported transformations require activation with high-energy UV light.

Recently, we have been able to show that the incorporation of auxilliary ligands enabling LMCT transitions allows for photolysis of Zr-C bonds under visible light irradiation. Our photochemical approach allowed the isolation of an unprecedented zirconium cyclobutadienyl complex (see graphic) when the photolysis was conducted in the presence of diphenylacetylene. This reactivity is complementary to traditional pathways that result in zirconacyclopentadienyl formation from alkynes.

Organometallics 2018, 37, 4488

Chem. Commun. 2020, 56, 5397

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PHOTON UPCONVERSION

Photochemical upconversion by triplet-triplet annihilation (UC-TTA) allows the generation of high-energy photons from lower energy radiation. Transition metal chromophores are essential components of UC-TTA systems as they provide straightforward access to the triplet manifold of organic molecules through rapid intersystem crossing (ISC) followed by triplet-triplet energy transfer (TTET).

In a recent collaboration with the Castellano group, we were able to show that our LMCT zirconium photosensitizers can not only replace precious metal chromophores in UC-TTA systems, but outperform traditional ruthenium or iridium photosensitizers by enabling record-setting quantum efficiencies.

Chem. Sci. 2021, available online

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