Quantum Interference in Real-Time Electron-Dynamics: Gaining Insights from Time-Dependent Configuration Interaction Simulations

Femtosecond electron dynamics based on time-dependent configuration interaction (TDCI) is a numerically rigorous approach for quantitative modeling of electron-injection across molecular junctions. Our simulations of cyanobenzene thiolates---para- and meta-linked to an acceptor gold atom---corroborate aromatic resonance stabilization effects and show donor states \emph{conjugating} with the benzene $\pi$-network to exhibit superior electron-injection dynamics across the para-linked isomer compared to the meta counterpart. For a \emph{non-conjugating} initial state, we find electron-injection through the meta-channel to stem from non-resonant quantum mechanical tunneling. Furthermore, we demonstrate quantum interference to drive para- vs. meta- selectivity in the coherent evolution of superposed $\pi$(CN)- and $\sigma$(NC-C)-type wavepackets. Analyses reveal that in the para-linked molecule, $\sigma$, and $\pi$ MOs localized at the donor terminal are \emph{in-phase} leading to constructive interference of electron density distribution while phase-flip of one of the MOs in the meta-linked molecule results in destructive interference. The findings reported here suggest that \emph{a priori} detection of orbital phase-flip and quantum coherence conditions can aid in molecular device design strategies.