Speaker
Description
We have set up a novel Optical Pump Terahertz Probe (OPTP) Spectrometer, which enables monitoring of the solvent response and the propagation of vibrational energy upon photoexcitation. We have reached a sensitivity that allows us to follow with ps time resolution the steps from photoexcitation to the formation of intermediates and the subsequent energy transfer into the bulk solvent (1). Here, we will present the results of monitoring two prototype photoreactions:
Real-time observation of the solvent response that promotes excited-state proton transfer: The solvent response of pyranine and two derivatives after photoexcitation has been observed in the time range of from sub-ps to 300 ps. We map the effect of the reaction on the surrounding solvent to reveal details that were partly or even fully hidden when focusing solely on observables tagged to the photoacid or its conjugate base (2). The rather surprising observation of an oscillation in the signal within the first 5 ps could be assigned, with the help of theory, to vibrational beatings, which, when undamped, correspond to energy transfer between the photoexcited chromophore and the water solvent. Fragment-localized modes were identified that make and break hydrogen bonds between the chromophore and solvent subsystems, which facilitates proton transfer. The time scales for this vibrational energy transfer could be deduced to be < 0.5 ps. We conclude that efficient energy transfer stops the oscillatory mode within the first cycle (0.5 ps) and corresponds to the onset of proton transfer.
The birth and evolution of solvated electrons in the water: The solvated electron is a fundamental reaction intermediate in physical, chemical, and biological processes. Using OPTP, we follow the birth and time evolution of the solvated electron via state-of-the-art, solvent-sensitive kinetic terahertz spectroscopy, and molecular simulations (3). In the first, we observe a spectroscopic signature attributed to the delocalized electron, followed by the onset of a solvent perturbation. While initially, the electron is delocalized, the solvated electron converges to an average value of within. Due to the high experimental sensitivity, we can observe the spectroscopic signature of the localized electron, which is long-lasting (> 250 ps). While the water network rearranges to accommodate an anion or cation, usually causing an entropic penalty for creating a cavity for the solvated electron, we observe a weakening of the hydrogen bond network by the localized electron, which correlates with an increase in entropy. This stabilizes the electron in the water network.
References
(1) C. Hoberg, P. Balzerowski, M. Havenith, Integration of a rapid scanning technique into THz time-domain spectrometers for nonlinear THz spectroscopy measurements, AIP Adv. 9, 035348 (2019).
(2) 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).
(3) 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).
