Seminar

Professor Felix Castellano of North Carolina State University

Tuesday, April 25, 2023 - 10:45am
Neville 3

"Photoactivation Platforms Enabling Catalysis and NMR Hyperpolarization"

Transition metal-based photosensitizers are of significant value for promoting numerous light-activated processes and chemical transformations relying on excited state electron and energy transfer reactions. Historically, the most extensively used inorganic photosensitizers have been confined to nd6 (n = 3, 4, or 5) electron configurations and their associated metal-to-ligand charge transfer (MLCT) excited states. This presentation will discuss the unique and unexpected photophysics discovered in several distinct transition-metal-containing photosensitizers derived from the earth-abundant metal ions Cu(I) and Cr(III). Cu(I)-based MLCT excited states, as well as intramolecularly sensitized ligand-field excited states in light-harvesting Cr(III) chromophores, will be described. The ultrafast dynamics of these novel chromophores probed in the UV, Vis, and near-IR, along with conventional static, dynamic, and temperature-dependent photoluminescence spectroscopy, will also be detailed.

Despite its enormous utility in structural characterization, nuclear magnetic resonance (NMR) spectroscopy is inherently limited by low spin polarization. One method to address the low polarization is para-hydrogen (p-H2) induced polarization (PHIP) which uses the singlet spin isomer of H2 to generate disparate nuclear spin populations, amplifying the associated NMR signals. This presentation will describe the first examples of visible-light photosensitized PHIP using transition metal triplet sensitizers in concert with in-situ photochemical NMR spectroscopy, termed Trip-PHIP. The discovery of light-activated hydrogenation enables direct and timed control over the hyperpolarization of target substrates, critical for identifying short-lived intermediates. Trip-PHIP represents a facile experimental means for probing triplet-sensitized light activation in transition metal catalysts possessing low-lying triplet ligand-field states, informing mechanistic insight of potentially tremendous value in chemical catalysis.