University of Kansas
Monday, November 28, 2011
We use transient absorption spectroscopy to study the chemical reaction dynamics of photochromic molecular switches. The molecular switches reversibly convert between two isomers that have very different properties. One laser pulse excites the molecule, and then a second pulse records the changing optical absorption spectrum as the system evolves. The change is quite dramatic because one isomer is transparent to visible light, whereas the other one is strongly absorbing. More importantly, the transformation from one species to the other involves multiple electronic states of the molecule. The transitions from one electronic state to another violate the Born-Oppenheimer approximation (the separation of electronic and nuclear degrees of freedom), which is a fundamental simplification in the description of chemical dynamics. In order to probe the role of these non-adiabatic transitions, we use one- and two-photon excitation schemes to access different initial excited states. We also study the role of a competing photophysical pathway in which the molecule undergoes efficient intersystem crossing to a long-lived triplet state. Triplet formation may explain the limited durability of molecular switches, which typically survive for only 100-10,000 cycles.