Schwartz, Benjamin J.

Research in the Schwartz group is aimed a building a molecular-level understanding of chemical reactivity in complex environments by studying condensed-phase chemical reaction dynamics using both experimental and theoretical techniques: femtosecond laser spectroscopies and mixed quantum/classical molecular dynamics simulations. Projects fall into two main areas: studies of how solvent motions control the rates and even the choice of products in solution-phase chemical reactions, and investigations into the electronic structure and optoelectronic behavior of semiconducting conjugated polymers.

Our first main area centers on solvent effects on chemical reactivity. We use femtosecond pump-probe spectroscopies to experimentally monitor the course of solution-phase chemical reactions in real time, allowing us to “watch” the motions of solvent molecules responding to chemical changes of reacting solutes, monitor the flow of energy between reacting species and the solvent, and identify the motion of electrons during charge transfer reactions. These experiments are accompanied by computer simulations, which make use of sophisticated algorithms allowing for quantum dynamics in the absence of the Born-Oppenheimer approximation (see Figure 1). Projects range from studies of model systems such as solvated metal anions or solvated electrons to investigations of large organic molecules with complex photochemistry. Simulations are done in close connection to the femtosecond experiments, so that experimental results drive new simulations and vice-versa, providing students in the group with an opportunity to do both experimental and theoretical work.

Our second main area of research is focused on the electronic structure of conjugated polymers, which have enormous commercial potential for use in light-emitting diodes, displays and photodetectors. Upon photoexcitation, the electrons and holes created in semiconducting polymers interact with their environment, leading to relaxation processes on multiple time scales; many of the important dynamical issues are qualitatively similar to the solution phase reactions discussed above. One of the main focuses in our group is the study of how excitations on neighboring conjugated polymer chains interact. By combining information from femtosecond and steady-state spectroscopies, scanning force microscopy, and the behavior of polymer light-emitting or photovoltaic devices, we can identify and potentially eliminate undesirable electronic properties. Projects include: studies of energy transfer and lasing behavior in inorganic/conjugated polymer composite materials (see Figure 2); studies of the electronic processes that take place in conjugated polymer/fullerene bulk heterojunction solar cells; and characterization of polymer-metal electrode interfaces using non-linear spectroscopies such as second harmonic generation. This work provides students the opportunity to learn fundamental photophysics, polymer processing techniques, and semiconductor device construction.