Control and Characterization of Electron Transfer with Vibrational Excitations
Zheng Ma, Ph.D. candidate
David Beratan, advisor
Wednesday, January 31, 2018 - 9:00am to 10:00am
Location: French Family Science Center 3232
Avery, Meg

The interactions between electronic dynamics and the molecular vibrations in a donor-bridge-acceptor (DBA) structure lie at the core of electron transfer (ET) reactions mechanisms. Aiming to control and characterize ET reactions via vibrational excitations of molecular modes, in this thesis, we discuss three aspects of the interplay between ET and nuclear vibrations and the charge transfer in transition metal complex compounds.

First, a theoretical framework is established to explore how transient infra-red (IR) excitation perturbs ET kinetics and dynamics in DBA systems. Recent experiments find that IR excitation can change ET rates and can even change the relative dominance of ET and competing reactions. A comprehensive theoretical framework is formulated to describe IR-perturbed nonadiabatic ET reactions, including the effects of IR-induced non- equilibrium initial state populations and IR-perturbed bridge-mediated couplings. We find that these effects can produce either rate slowing or acceleration, depending on structural and energetic features of the bridge. The framework is used to interpret the origins of the observed rate slowing of charge separation and to predict rate acceleration for charge recombination in a DBA structure, and to describe the microscopic origins.

Second, a non-equilibrium molecular dynamics (NEqMD) computational methodology is employed to explore how the molecular underpinnings of how vibrational excitation may influence non-adiabatic electron-transfer. NEqMD combines classical molecular dynamics simulations with nonequilibrium semiclassical initial conditions to simulate the dynamics of vibrationally excited molecules. We combine NEqMD with electronic structure computations of bridge-mediated donor-acceptor couplings to probe IR effects on electron transfer in two molecular species, DMA-GC-Anth and PP-BCN-CA. In DMA-GC-Anth, the simulations find that IR excitation of NH2 scissoring motion, and subsequent intramolecular vibrational energy redistribution (IVR) do not significantly alter the mean-squared DA coupling interaction. This finding is consistent with earlier static system analysis. In PP-BCN-CA, IR excitation of the bridging C=N bond changes the bridge-mediated coupling for charge separation and recombination by ~ 30 - 40%. These methods provide an approach to exploring out of equilibrium molecular dynamics may impact charge transfer processes at the molecular scale.

In addition, we explore the feasibility of using transient 2D-IR spectroscopy for examining elastic and inelastic ET pathway in DBA systems. Two pulse sequences were analyzed by simulating the transient 2D-IR spectrum based on a 3-state 2-mode model that is used for describing the ET processes in DBA systems. We were able to identify the signature 2D-IR spectral pattern for elastic and inelastic ET pathways. The influence of ET parameters and the vibrational parameters used in the DBA ET system model on the detectability of elastic/inelastic signatures are discussed. It was found that a DBA system with reasonably large electronic coupling, a reaction free-energy that is on the same energy scale as vibrational frequency of reaction coordinate and electron-phonon couplings that produces typical reaction coordinate Huang-Rhys factor value of 0.1-2.5 is favorable for observing 2D-IR signals associated with both elastic and inelastic ET pathways. We also propose to use waiting- time dynamics to estimate the rate of electron leaving donor and the rate of electron arriving at vibronic states on acceptor surface.

Finally, quantum chemistry characterization of charge transfer reactions in Rhenium complex compounds and the IR-induced rate modulation mechanism are discussed. Electronic structure calculations and normal mode analysis indicates that in rhenium-centered complex compounds, IR-induced donor-acceptor energy gap change modulates the charge transfer rates.