Chemical reactions are all about making, breaking, and rearranging bonds. That process is relatively straight-forward if you consider only the reactants and products, but what happens during the reaction? In the Elles Group, we seek to answer this question at the molecular scale by studying the dynamics of solution-phase chemical reactions.
Our primary tool is a state-of-the-art ultrafast laser that delivers extremely short ( <35 fs) pulses of light. The laser pulses allow us to watch chemical reactions unfold on the timescale of atomic motion via time-resolved spectroscopy, where one laser pulse excites the sample, and a second, time-delayed pulse records changes in the spectrum as the system evolves. We can monitor the nuclear dynamics directly, or watch the kinetics of a reaction over longer timescales. Ultimately, we hope to use what we learn to selectively control the outcome of reactions.
Excited State Dynamics Of Molecular Switches: The first project looks at excited state dynamics of color-changing photoswitches. These molecules reversibly convert between two isomers with very different absorption spectra and are prime candidates for use in high-density data storage devices, such as 3-dimensional, many-layer DVDs. Our experiments follow the transformation from one isomer to another after the system absorbs a photon. Those ring-opening and ring-closing reactions involve up to three electronic states.
A key question that physical chemists want to answer is exactly how a system transfers from one electronic state to another, a process called non-adiabatic transition. Non-adiabatic transitions generally occur through conical intersections, where the potential energy surfaces cross. Conical intersections are a hot topic these days because they are ubiquitous in excited state reactions. We will use new excitation schemes to explore the reaction mechanisms of photo-switching and then apply what we learn to steer the reaction and thus improve conversion yields for ring-opening and closing.
Structure And Reactivity Of Weakly Interacting Species: A second area of study examines weak interactions between molecules in solution and seeks to understand how those interactions affect the reaction dynamics. For example, chlorine radicals form a non-covalent complex with benzene that increases the energy barrier for hydrogen-abstraction. The complex also is likely to change the dynamics along the reaction coordinate, but there are many details that are not well understood. How do solvent-solute interactions affect the energy and geometry of the transition state? Can we exploit those subtle changes as a way to control the outcome of a reaction? We explore these questions by observing halogen atom reactions in solution.
We are also interested in learning more about halogen bonding interactions. These halogen interactions, which typically are intermediate in strength between van der Waals forces and hydrogen-bonds, are important for controlled crystallization of new materials, supramolecular chemistry, and numerous biological systems. In addition to time-resolved laser measurements, we also use time-resolved x-ray absorption spectroscopy to obtain structural information on a sub-100ps timescale. The x-ray experiments are performed on a specialized beam-line at a synchrotron facility.