In this award, funded by the Chemical Structure, Dynamics and Mechanisms Program of the Chemistry Division, Professor Stephen Bradforth of the University of Southern California and graduate students will study chemical reactions dynamics in water. Novel femtosecond electronic and photoelectron spectroscopies will be employed to unravel ionization, dissociation and proton-coupled electron transfer reaction pathways in water with photochemistry in the phenol family of molecules serving as model systems. The hydrogen atom dissociation and ionization dynamics of phenol and thiophenol, bearing many similarities in reaction dynamics to that of excited-state water but with additional surface-hopping dynamics, will be mapped from gas phase to bulk aqueous phase by new dispersed transient absorption experiments with 30 fs time resolution. These will be complemented by the construction of a new liquid time-resolved photoelectron spectrometer, capable of probing evolution along the reaction coordinate via the photoelectron energy and angular distribution. Initial targets for the liquid-jet spectrometer will be the steady-state angular distributions from inorganic anions of different symmetries and time-resolving phenol excited state chemistry.
While this research project focuses specifically on the aqueous phenol system, it will lead to insights into the interplay between solvent motions and solute reactivity that directly impact how most chemists and biochemists design and/or model chemical transformations. The photochemistries of heterocyclic aromatics, including the DNA bases, have related excited state pathways to the phenolic chromophores in solution. Phenol itself is a direct model for tyrosine side chain in proteins where tyrosyl radicals are often significant intermediates in electron transfer activity. A new generation of physical science students will gain vital training in new spectroscopic probes of matter as well as designing and analyzing experiments at the cutting edge of the field. These skills will prepare them for careers in the vital US science and technology sector as well as academic research. Graduate and undergraduate students will have the opportunity to learn from each other and collaborate with international scientists. The Bradforth research group will also participate in outreach efforts at USC to provide hands-on energy-conversion themed summer internships for community-college students serving the largely Hispanic population in South Eastern Los Angeles County. Broad participation of different groups, and collaboration amongst those participants, will continue to be emphasized at all levels of involvement in the project
Our project provides a fundamental and detailed view of how chemical reactivity occurs in water. Reactivity and the electronic structure of a solute are experimentally studied using two techniques that employ femtosecond lasers: electronic absorption and photoelectron spectroscopy. These are used to reveal how the surrounding liquid behaves and modifies the chemical event taking place in its midst. In this project two classes of reactivity were explored: (1) Photo-initiated hydrogen atom ejection reactions in the phenol family of molecules, and (2) Ionization of molecules important to radiation damage of biological tissue (water, phenols, nucleobases, ribose sugars, phosphate and DNA). In the first class, our experimental work was able to resolve multiple re-crossings of the chemical transition state in a bond-breaking reaction. Specifically, we followed hydrogen ejection from a thio-substituted phenol and measured how the energy released after breaking the S-H bond was distributed into solute and solvent degrees of freedom. In the parent phenol, the breaking of the O-H bond takes six-orders of magnitude longer in time, and this reactivity was observed to change from H-atom ejection to electron ejection in water. For the second class, using photoelectron spectroscopy we measured the energies necessary to ionize DNA and its component parts. This was the first time such measurements had been under physiologically relevant conditions, namely aqueous pH-controlled solutions. Another key aspect of this project was the development and construction of a new liquid jet photoelectron spectrometer at USC – a novel instrument to measure the electron orbitals of the solute via the energies with which each electron is bound and their 3D shapes in space. This instrument was completed during the grant period and is now being used to time-resolve reaction dynamics in water. We were able to confirm that the angular distributions in 3D space of electrons emerging from a liquid are not randomized by scattering but preserve information about the orbital from which they were ejected. Our project aims at uncovering the fundamentals under-pinning all chemistry in aqueous solution. However, there are direct applications. For example, a new optical device was designed and tested with a commercial partner in the performance of our project. Further, we have a better understanding of the primary processes taking place in water and biological tissue exposed to ionizing radiation, such as that occurring in cancer radiation therapy and in the nuclear fuel cycle. There are in fact direct synergies between this project’s outcomes and parallel research in Bradforth’s lab in developing improved cancer treatments using nanoparticle-based radiation therapy. The performance of this research involved 5 undergraduate and 5 graduate students as well as a postdoctoral scholar. The project involved three international collaborations (UK, Germany, Czech Republic). A USC PhD student participated in experimental work in Berlin and we hosted five international students in Los Angeles. An additional dimension to the project was a direct outreach to the community colleges in Los Angeles County. Each summer, summer interns were hosted in Bradforth’s lab from Cerritos Community College and the principal investigator made a presentation about energy research at L.A. Trade Tech College.
