Photochromic molecular switches are compounds that change color upon optical excitation. The color change is a response to the making, breaking, and rearranging of bonds in the molecule. We use ultrafast spectroscopy to probe changes in the molecular structure and to reveal new information about the fundamental reaction dynamics of molecules in solution. In particular, we examine the dynamics of molecules in highly excited electronic states (above S1). We use both transient absorption and resonance Raman measurements to monitor the dynamics of molecules that are excited using a pair of time-delayed laser pulses. Beyond simply watching the reaction unfold, sequential excitation with two independent photons allows us to control the dynamics of the molecule and influence the outcome of the reaction. For example, selectively varying the relative time delay and wavelength for secondary excitation allows the molecule to sample different reaction channels. These experiments probe the potential energy surfaces that determine the motions of the atoms and provide unique insight on the dynamics of molecules in highly excited electronic states, an important frontier in chemical reaction dynamics.


Chris Elles (University of Kansas)