Ultrafast dynamics in a molecular photoswitch

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.

Figure 1: Schematic of the QC ⇄ NBD interconversion.

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.

Experimentally measured TRPES spectra as a function of the time delay between the UV pump and the FEL probe pulses are shown in Figure 2a. The narrow and long-lived features assigned to the 3p and 3s Rydberg states are responsible for the slower decay mechanism in QC. A concurrent spectrally broad but short-lived structure extending from the Rydberg states to approximately 7 eV binding energy (BE) indicates the existence of a rapid (<100 fs) deexcitation pathway leading to the formation of vibrationally hot photoproducts (VHPs). The pronounced negative signal is due to the depletion of the QC ground state (GS).

Figure 2: Experimentally measured TRPES spectra of UV-excited QC. Negative delays correspond to the FEL pulse preceding the UV excitation pulse. (b) Populations of the electronic states as a function of time, where S_n ≥ 2 shows the cumulative population of the excited states other than S₁. The fraction of molecules with NBD-like geometry (defined as r_CC > 2.0 Å) is also presented.

Obtained TRPES data were interpreted with the help of state-of-the-art computer simulations and reveal that the photodynamics of excited QC are strongly dependent on the initial excited state. Figure 2b shows the time-dependent populations for the ground (S₀) and first five singlet excited S₁ (3s Rydberg), S₃ and S₄ (3p Rydberg), S₂ and S₅ (mixed 3p_x/valence) electron states identified as being active in the dynamics probed by the present experiment. Dynamics initiated on the S₂ state are labelled fast valence, shown in the top panel, while dynamics initiated on the S₃ and S₄ states are labelled slow Rydberg and shown in the bottom panel. It appears that the S₁/S₀ conical intersection has a strong influence on the NBD/QC branching ratio in the ground state, since the fraction of NBD approaches for the slow Rydberg the same value as the fast valence pathway.

This combined study discovered faster than previously identified pathways leading to the interconversion, which may help designing new ways of controlling the outcome and efficiency of this important class of photoreactions.

Kurtis D. Borne¹, Joseph C. Cooper², Michael N. R. Ashfold³, Julien Bachmann⁴, Surjendu Bhattacharyya¹, Rebecca Boll⁵, Matteo Bonanomi^6,7, Michael Bosch⁴, Carlo Callegari⁸, Martin Centurion⁹, Marcello Coreno^8,10, Basile F. E. Curchod³, Miltcho B. Danailov⁸, Alexander Demidovich⁸, Michele Di Fraia⁸, Benjamin Erk¹¹, Davide Faccialà⁶, Raimund Feifel¹², Ruaridh J. G. Forbes¹³, Christopher S. Hansen¹⁴, David M. P. Holland¹⁵, Rebecca A. Ingle¹⁶, Roland Lindh¹⁷, Lingyu Ma¹⁸, Henry G. McGhee¹⁶, Sri Bhavya Muvva⁹, Joao Pedro Figueira Nunes⁹, Asami Odate¹⁸, Shashank Pathak¹, Oksana Plekan⁸, Kevin C. Prince⁸, Primoz Rebernik⁸, Arnaud Rouzée¹⁹, Artem Rudenko¹, Alberto Simoncig⁸, Richard J. Squibb¹², Anbu Selvam Venkatachalam¹, Caterina Vozzi⁶, Peter M. Weber¹⁸, Adam Kirrander² & Daniel Rolles¹

¹ J.R. Macdonald Laboratory, Department of Physics, Kansas State University, Manhattan, KS, USA.
² Physical and Theoretical Chemistry Laboratory, Department of Chemistry, University of Oxford, Oxford, UK.
³ School of Chemistry, Cantocks Close, University of Bristol, Bristol, UK.
⁴ Chemistry of Thin Film Materials, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany.
⁵ European XFEL, Schenefeld, Germany.
⁶ Istituto di Fotonica e Nanotecnologie (CNR-IFN), CNR, Milano, Italy.
⁷ Dipartimento di Fisica, Politecnico di Milano, Milano, Italy.
⁸ Elettra – Sincrotrone Trieste S.C.p.A., Trieste, Italy.
⁹ Department of Physics and Astronomy, University of Nebraska–Lincoln, Lincoln, NE, USA.
¹⁰ Istituto di Struttura della Materia (ISM-CNR), CNR, Trieste,Italy.
¹¹ Deutsches Elektronen-Synchrotron DESY, Hamburg, Germany.
¹² Department of Physics, University of Gothenburg, Gothenburg, Sweden.
¹³ Linac Coherent Light Source, SLAC National Accelerator Laboratory, Menlo Park, CA, USA.
¹⁴ School of Chemistry, University of New South Wales, Sydney, New South Wales, Australia.
¹⁵ Daresbury Laboratory, Warrington, UK.
¹⁶ Department of Chemistry, University College London, London, UK.
¹⁷ Department of Chemistry - BMC, Uppsala University, Uppsala, Sweden.
¹⁸ Department of Chemistry, Brown University, Providence, RI, USA.
¹⁹ Max-Born-Institut, Berlin, Germany.

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