Electron–Nuclear Dynamics Accompanying Proton-Coupled Electron Transfer

Abstract

Although photoinduced proton-coupled electron transfer (PCET) plays an essential role in photosynthesis, a full understanding of the mechanism is still lacking due to the complex nonequilibrium dynamics arising from the strongly coupled electronic and nuclear degrees of freedom. Here we report the photoinduced PCET dynamics of a biomimetic model system investigated by means of transient IR and two-dimensional electronic−vibrational (2DEV) spectroscopies, IR spectroelectrochemistry (IRSEC), and calculations utilizing long-range-corrected hybrid density functionals. This collective experimental and theoretical effort provides a nuanced picture of the complicated dynamics and synergistic motions involved in photoinduced PCET. In particular, the evolution of the 2DEV line shape, which is highly sensitive to the mixing of vibronic states, is interpreted by accurate computational modeling of the charge separated state and is shown to represent a gradual change in electron density distribution associated with a dihedral twist that occurs on a 120 fs time scale.

Two-dimensional electronic–vibrational spectroscopy: Exploring the interplay of electrons and nuclei in excited state molecular dynamics

“…10). 29 In this example, a 120 fs rise in the CLS was observed [Fig. 10(a)], indicating an increasing electronic-nuclear correlation.…”

“…In addition, this electronic-nuclear correlation proved essential to understanding the solvent-dependent excited state proton transfer process of indigo carmine, 28 as well as the degree of concertedness in a biomimetic proton-coupled electron transfer (PCET) system. 29 In all these studies, it was clear that transient IR experiments alone lacked the detail necessary to map out potential surfaces or to understand the role of solvent.…”

“…This would be a particularly useful measure, for example, as it has been speculated that Herzberg-Teller coupling may be more effective in facilitating ET than the Franck-Condon coupling. 36 While the bulk of our work has centralized around natural 2,3,19,37 and biomimetic 29,38 photosynthetic systems, this represents only one avenue for the applicability of this technique. For example, Khalil and co-workers elegantly demonstrated how 2DEVS can also be a powerful tool to understand synthetic components utilized for solar conversion.…”

Multi PCET in symmetrically substituted benzimidazoles

“… 7–9 In our recent work, a low or barrierless pathway for PCET with the concerted process occurring in ∼100 fs has been identified using 2D electronic vibrational spectroscopy. 10 …”

“…recent work, a low or barrierless pathway for PCET with the concerted process occurring in ~100 fs has been identified using 2D electronic vibrational spectroscopy. 10 Upon electrooxidation of 1, a reversible one-electron coupled with one-proton transfer (E1PT) occurs. 2,11 This proton displacement between the phenol and the proximal benzimidazole nitrogen takes place in a well-defined and strong intramolecular hydrogen bond connecting both groups.…”