Stephen Bradforth

Electron and Electronic Dynamics in Liquid Solutions

In the liquid phase, intermolecular interactions often control electronic structure, and therefore chemical reactivity and excited state photochemistry, but are complex to quantify. 
Photoelectron spectroscopy applied to liquid microjets has emerged as powerful way to access both solvent and solute electronic structure directly. Using liquid phase XPS, we have investigated the peculiar property liquid ammonia possesses at being able to stably support excess electrons, transitioning from an electrolyte to a metal. At low concentrations, electrons exist as localized solvated electrons and, with modest concentration increase, spin pair to form diamagnetic solutions, the basis for Birch’s reagent in organic synthesis. Further concentration increase yields a shiny golden liquid metal. Our experiments map out how the electronic band structure changes on approach to the metallic threshold. Complementary experiments have been conducted to explore metallic water solutions. 
 [2+2] cycloadditions and electrocyclic ring closings lead to highly strained cyclobutane structural motifs, often targets of interest in synthesis planning. Our efforts are to characterize such pathways from the perspective of high-throughput excited state reaction discovery.  The overall goal is to design a closed loop between theory and photochemical experiment at the level of individual observables – screening tens of substrate substituents and reaction conditions in a single excited state dynamics experiment. Hexafluorobenzene (HFB) will be described as an initial target for unravelling the excited state forces that guide an intramolecular 4π-electrocyclic ring closing. 
Background references:
1.Buttersack, T. et al. Photoelectron spectra of alkali metal–ammonia microjets: From blue electrolyte to bronze metal. Science 368, 1086–1091 (2020). 
2.Mason, P. E. et al. Spectroscopic evidence for a gold-coloured metallic water solution. Nature 595, 673–676 (2021).
3.Cox, J. M., Bain, M., Kellogg, M., Bradforth, S. E. & Lopez, S. A. Role of the Perfluoro Effect in the Selective Photochemical Isomerization of Hexafluorobenzene. J Am Chem Soc 143, 7002–7012 (2021).