Prof. Dr. Martina Havenith-Newen, Department of Physical Chemistry II, Ruhr University Bochum, Germany

"Caught in the act: Time-resolved THz spectroscopy puts the spotlight on the role of the solvent into photochemical reactions"

The development of time-resolved and surface-sensitive spectroscopies in the UV, optical, and IR frequency range have impacted our understanding of chemical reactions. Probing intermediates and transition states can now be probed experimentally and, in doing so, provide new perspectives on reaction pathways and dynamics. These techniques have so far mainly focused on the solutes, the reactants, and the products. Our goal is to put the spotlight on the solvent and visualize the correlated electron and solvent dynamics in electrocatalysis and photochemical reactions. While optical and IR spectroscopy are able to report on the electronic states and the Stokes shift and on the intramolecular modes, they provide less information on non-covalent and intermolecular interactions that are involved. We use THz spectroscopy as a most sensitive tool to probe intermolecular interactions such as hydrogen bonding and local solvation motifs [1,2]. Most notable, changes in these can be directly correlated to changes in solvation entropy and enthalpy, e.g., changes in free energy upon desolvation and resolvation [3]. Unlike conventional calorimetry, “THz calorimetry” which is based on spectroscopic observables, can be applied to inhomogeneous samples and can monitor the solvent response and the propagation of vibrational energy upon photoexcitation on a sec time scale by optical pump terahertz probe (OPTP) spectroscopy. Initial experiments revealed new insights into the solvent dynamics of prototype photoreactions [4,5].

References:
1. S. Pezzotti, W. Chen, F. Novelli, X. Yu, C. Hoberg, M. Havenith, Terahertz-calorimetry spotlights the role of water in biological processes, Nat. Rev. Chem., accepted (2025).

2. K. Mauelshagen, P. Schienbein, I. Kolling, G. Schwaab, D. Marx, M. Havenith, Random encounters dominate water-water interactions at supercritical conditions, Sci. Adv. 11, eadp8614 (2025).

3. S.S. Nalige, P. Galonska, P. Kelich, L. Sistemich, C. Herrmann, L. Vukovic, S. Kruss, M. Havenith, Fluorescence changes in carbon nanotube sensors correlate with THz absorption of hydration, Nat. Commun. 15, 6770 (2024).

4. C. Hoberg, J.J. Talbot, J. Shee, T. Ockelmann, D. Das Mahanta, F. Novelli, M. Head-Gordon, M. Havenith, Caught in the act: Real-time observation of the solvent response that promotes excited-state proton transfer in pyranine, Chem. Sci. 14, 4048–4058 (2023).

5. F. Novelli, K. Chen, A. Buchmann, T. Ockelmann, C. Hoberg, T. Head-Gordon, M. Havenith, The birth and evolution of solvated electrons in the water, PNAS 120, e2216480120 (2023).