Speaker
Description
Proton-coupled electron transfer (PCET) is a fundamental class of photochemical reactions in which the absorption of light triggers both charge transfer and a change in hydrogen bonding on the excited state potential surface in molecules where a hydrogen donor and acceptor group are in close proximity to one another. The distinct proton and electron transfer steps may occur sequentially or concomitantly and the process itself as well as the nature of the resulting product species are often intimately linked to the coupling of electronic and nuclear degrees of freedom [1, 2]. A full characterization of a PCET reaction involves distinguishing which step happens first, how much partial charge is transferred during each step, and which nuclear motions are necessary to complete the reaction.
In order to answer these questions, a femtosecond probe sensitive to both the electronic and nuclear structure is required. Time-resolved X-ray photoelectron spectroscopy (TR-XPS), now possible at the current generation X-ray free electron lasers, represents an ideal probe for such dynamics because it combines the ultrafast time resolution and inherent element-selectivity of femtosecond X-ray pulses with the sensitivity of core-level XPS to the local chemical environment of specific sites within the system.
In this talk, I will present results from a TR-XPS measurement on the prototypical PCET system 2-(2′-hydroxyphenyl) benzothiazole in the gas phase performed at the Maloja endstation of SwissFEL’s soft X-ray beamline. By combining experimental observations at the donor and acceptor sites with state-of-the-art core-level XPS calculations, the charge transfer and proton transfer steps are unambiguously distinguished and the partial charges involved in the reaction are extracted. By analyzing the nuclear wavepacket dynamics in the excited state, the nuclear modes active in the PCET process are identified. Finally, the formation of two distinct and long-lived product states via a conical intersection is observed by tracking the chemical shifts as the molecule undergoes its proceeding nonadiabatic dynamics.
[1] S. Lochbrunner et al., The Journal of Physical Chemistry A, 107, 49, 10580–10590 (2003).
[2] S. Pijeau et al., The Journal of Physical Chemistry A, 121, 24, 4595-4605 (2017).
| Scientific Topics | Chemistry |
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