Molecules that undergo photoinduced isomerization reactions and are capable of storing the absorbed light as chemical energy, releasing it as thermal energy on demand, are referred to as molecular solar thermal energy storage (MOST) or solar thermal fuels (STF). An ideal model system for such technologically important applications is the photoswitchable pair of isomers quadricyclane (QC, a highly strained multicyclic hydrocarbon), and its lower-energy isomer norbornadiene (NBD). The isomers, shown in Figure 1, interconvert upon photoabsorption in the deep ultraviolet (UV) range. An experiment performed at FERMI sheds new light on the mechanism of the reverse interconversion, QC → NBD, which is of both fundamental photochemical interest and practical importance since it represents the undesired UV-induced photoreversion process in MOST systems based on the QC/NBD pair.
Using time-resolved photoelectron spectroscopy (TRPES) with extreme ultraviolet (XUV) probe pulses at the Low Density Matter end-station of the seeded FEL FERMI, along with non-adiabatic molecular dynamics simulations, an international collaboration led by Prof. Daniel Rolles and Dr. Kurtis D. Borne from Kansas State University, Prof. Adam Kirrander from the University of Oxford, and Prof. Caterina Vozzi from Politecnico di Milano succeeded in tracking the two competing pathways by which electronically excited quadricyclane molecules relax to the electronic ground state.
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Image: Schematic of the QC ⇄ NBD interconversion.