Laser, camera, action: Ultrafast ring opening of thiophenone tracked by time-resolved XUV photoelectron spectroscopy

Light-induced ring opening reactions, which form the basis of important biological processes such as vitamin D synthesis and are also touted as promising candidates for the development of molecular switches, have intrigued the photochemistry community for decades. In recent years, new time-resolved techniques have emerged to investigate these processes with unprecedented temporal and spatial resolution, making it possible to visualize electronic changes and the movement of atoms in the molecule at each step of a chemical reaction. 
Using time-resolved extreme ultraviolet (XUV) photoelectron spectroscopy at the LDM end-station of the FERMI free-electron laser (FEL) along with high-level electronic structure and molecular dynamics calculations, an international collaboration led by Prof. Daniel Rolles and Shashank Pathak from Kansas State University, Prof. Mike Ashfold from the University of Bristol, and Prof. Basile Curchod and Lea-Maria Ibele from Durham University succeeded in unravelling the dynamics of such a reaction in a prototypical heterocyclic molecule, thiophenone, along the full photochemical cycle – from photoexcitation, ring opening, all the way through to the subsequent ground state dynamics, and spanning a range of tens of femtoseconds (1 femtosecond = 10^-15seconds) to hundreds of picoseconds (1 picosecond = 10^-12seconds). 
Five-membered heterocyclic molecules (e.g. thiophenes, furans, pyrroles) are common building blocks for functionalized materials, polymers, optoelectronic devices, and feature in the synthesis of biologically active compounds. Electrocyclic ring-opening reactions are promising candidates for building molecular switches and serve as benchmark systems for fundamental, light-driven biochemical reactions. The excited-state dynamics and subsequent deactivation pathways are crucial for understanding, designing and applying such functional materials.
Combining the established time-resolved photoelectron spectroscopy (TRPES) technique with the high-brightness, high-photon-energy and narrow-bandwidth XUV pulses provided by the seeded FERMI FEL, the collaboration was able to significantly extend the range and versatility of traditional TRPES, tracing the entire reaction pathway from the instant of photoexcitation, through multiple conical intersections, all the way to back to the ground (electronic) state molecule. Total energy must be conserved, of course, so these ground state molecules are necessarily formed with substantial internal (vibrational) excitation. One key finding from the present study is the extent to which these highly vibrationally excited ground state species interconvert between different isomeric structures en route to the final products.  
Figure 1(a) shows a schematic of the UV-photoinduced ring-opening process in thiophenone, wherein the time-evolving photoproducts are probed (by ionization) using an XUV pulse from the FERMI FEL. The electronic and geometric changes during the reaction are visualized by recording electron energy spectra as a function of the time delay between the UV pump and the FEL probe pulses (shown in Figure 1(b)). For each molecule, the reaction is initiated by abosrption of a UV photon, and the time-delayed photoelectron spectra provide snapshots of the on-going reaction. This yields a 'molecular movie', that shows how the electronic structure and the arrangement of the nuclei within the molecule evolve with time. The movie can then be compared to cutting-edge quantum-chemical calculations that further enrich the experimental measurements, enable extrapolation of the molecular evolution to times beyond those probed in the experiment, and allow development of more general rules that can help guide the visualization of similar photoinduced isomerization reactions that occur in nature or are used in technology. 
 

Figure 1.    (a) Schematic of the UV excitation, ring opening and photoionization of thiophenone and its isomerization products. (b) Photoelectron kinetic energy spectra of UV-excited thiophenone as a function of the delay time between UV and FEL pulses. Negative delay times correspond to the FEL pulse preceding the UV pulse and positive delay times to the UV pulse preceding the FEL pulse, while the colour represents the normalized photoelectron intensity.
 

This research was conducted by the following research team:

Daniel Rolles¹, Shashank Pathak, Jan Tross¹,Basile Curchod², Lea M. Ibele²,Michael N. R. Ashfold³,Carlo Callegari⁴,Oksana Plekan⁴,Michele Di Fraia⁴,Kevin C. Prince⁴,Alexander Demidovich⁴, Luca Giannessi⁴,Rebecca Boll⁵,Benjamin Erk⁶, Richard J. Squibb⁷, Raimund Feifel⁷,Ruaridh Forbes⁸,Christopher S. Hansen⁹,David M. P. Holland¹⁰,Rebecca A. Ingle¹¹,Robert Mason¹²,Arnaud Rouzée¹³

¹ Department of Physics, Kansas State University, Manhattan, USA
² Department of Chemistry, Durham University, Durham, UK
³ School of Chemistry, University of Bristol, Bristol, UK.
⁴ Elettra – Sincrotrone Trieste S.C.p.A., Trieste, Italy
⁵ European XFEL, Schenefeld, Germany.
⁶ Deutsches Elektronen-Synchrotron, Hamburg,Germany
⁷ Department of Physics, University of Gothenburg, Gothenburg, Sweden
⁸ Stanford PULSE Institute, SLAC National Accelerator Laboratory, Menlo Park, CA, USA.
⁹ School of Chemistry, University of New South Wales, Sydney, Australia.
¹⁰ Daresbury Laboratory, Warrington, UK
¹¹ Department of Chemistry, University College London, London, UK
¹² Department of Chemistry, University of Oxford, Oxford, UK
¹³ Max-Born-Institut, Berlin, Germany

Contact persons:

Carlo Callegari