David Osborn

A long-term goal of the US Department of Energy is to control transformations between different forms of energy.  A recurring need is to store solar energy for use at a later time and potentially in a different location.  Chemical batteries have the potential to be a low-cost, easily scalable solution.  One way to build a chemical battery is using reversable photoswitches—molecules that photoisomerize to a less stable isomer—with catalytic conversion back to the starting isomer.  These systems have been explored for decades and are termed MOlecular Solar Thermal energy storage (MOST).  Photoisomerization of the C7H8 isomer norbornadiene to its less-stable isomer quadricyclane is the most widely studied system, although typically norbornadiene is decorated with substituents that red-shift its absorption spectrum to overlap with the solar spectrum at the surface of the Earth.

Although practical applications of MOST systems will likely require liquid- or solid-state systems, we are pursuing fundamental insights from detailed studies of the gas-phase photochemistry of norbornadiene.  In this talk, I’ll provide a brief overview of the challenges of probing complex chemically reacting systems and present a new instrument for Time-Resolved PhotoElectron PhotoIon COincidence spectroscopy (TR-PEPICO) that addresses these challenges with highly multiplexed data.  We excite norbornadiene at 248 and 193 nm, observing photoisomerization, a reverse Diels-Alder product channel producing c-C5H6 + HCCH, and several minor product channels.  Surprisingly, the photoisomerization pathway does NOT produce quadricyclane at either wavelength.  Furthermore, we provide evidence of unusually slow time-resolved collision-induced intersystem crossing that we assign to at least one triplet state in the non-adiabatic decay from the initially excited state.  Together, these experimental data provide a new target for theoretical exploration of the simplest member of the norbornadiene MOST family and emphasize the value of highly multiplexed experiments.