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Author: Fingerhut, Benjamin P. × Publisher: Melville, NY : American Institute of Physics ×

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    On the role of non-diagonal system-environment interactions in bridge-mediated electron transfer
    (Melville, NY : American Institute of Physics, 2020) Acharyya, Nirmalendu; Ovcharenko, Roman; Fingerhut, Benjamin P.
    Bridge-mediated electron transfer (ET) between a donor and an acceptor is prototypical for the description of numerous most important ET scenarios. While multi-step ET and the interplay of sequential and direct superexchange transfer pathways in the donor-bridge-acceptor (D-B-A) model are increasingly understood, the influence of off-diagonal system-bath interactions on the transfer dynamics is less explored. Off-diagonal interactions account for the dependence of the ET coupling elements on nuclear coordinates (non-Condon effects) and are typically neglected. Here, we numerically investigate with quasi-adiabatic propagator path integral simulations the impact of off-diagonal system-environment interactions on the transfer dynamics for a wide range of scenarios in the D-B-A model. We demonstrate that off-diagonal system-environment interactions can have profound impact on the bridge-mediated ET dynamics. In the considered scenarios, the dynamics itself does not allow for a rigorous assignment of the underlying transfer mechanism. Furthermore, we demonstrate how off-diagonal system-environment interaction mediates anomalous localization by preventing long-time depopulation of the bridge B and how coherent transfer dynamics between donor D and acceptor A can be facilitated. The arising non-exponential short-time dynamics and coherent oscillations are interpreted within an equivalent Hamiltonian representation of a primary reaction coordinate model that reveals how the complex vibronic interplay of vibrational and electronic degrees of freedom underlying the non-Condon effects can impose donor-to-acceptor coherence transfer on short timescales. © 2020 Author(s).
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    Ultrafast phosphate hydration dynamics in bulk H2O
    (Melville, NY : American Institute of Physics, 2015) Costard, Rene; Tyborski, Tobias; Fingerhut, Benjamin P.; Elsaesser, Thomas
    Phosphate vibrations serve as local probes of hydrogen bonding and structural fluctuations of hydration shells around ions. Interactions of H2PO4− ions and their aqueous environment are studied combining femtosecond 2D infrared spectroscopy, ab-initio calculations, and hybrid quantum-classical molecular dynamics (MD) simulations. Two-dimensional infrared spectra of the symmetric (𝜈𝑆(PO−2)) and asymmetric (𝜈𝐴𝑆(PO−2)) PO−2 stretching vibrations display nearly homogeneous lineshapes and pronounced anharmonic couplings between the two modes and with the δ(P-(OH)2) bending modes. The frequency-time correlation function derived from the 2D spectra consists of a predominant 50 fs decay and a weak constant component accounting for a residual inhomogeneous broadening. MD simulations show that the fluctuating electric field of the aqueous environment induces strong fluctuations of the 𝜈𝑆(PO−2) and 𝜈𝐴𝑆(PO−2) transition frequencies with larger frequency excursions for 𝜈𝐴𝑆(PO−2). The calculated frequency-time correlation function is in good agreement with the experiment. The 𝜈(PO−2) frequencies are mainly determined by polarization contributions induced by electrostatic phosphate-water interactions. H2PO4−/H2O cluster calculations reveal substantial frequency shifts and mode mixing with increasing hydration. Predicted phosphate-water hydrogen bond (HB) lifetimes have values on the order of 10 ps, substantially longer than water-water HB lifetimes. The ultrafast phosphate-water interactions observed here are in marked contrast to hydration dynamics of phospholipids where a quasi-static inhomogeneous broadening of phosphate vibrations suggests minor structural fluctuations of interfacial water.