Attochemistry Principles of Charge Migration

Attosecond charge migration (CM) is a coherent process where an electron density hole moves across a molecule in a particle-like manner. Despite intense experimental and theoretical interest, many questions remain about how charge migration can be modulated by systematic variation in the chemical structure of a molecule, and how this can be understood mechanistically. Building on our previous studies of halogen effects and bonding, we present systematic time-dependent density-functional theory (TDDFT) simulations of CM in para-functionalized bromobenzene derivatives (Br-C₆H₄-R). We observe enhanced hole contrast for the strong electron donors (e.g., R = N(CH₃)₂, -NH₂), and reduced contrast for electron acceptors (e.g., -CF₃). The trend is quantitatively described by the Hammett sigma value of the group, which is a metric describing the chemical reactivity of benzene derivatives. We also present results demonstrating how the initial hole location affects CM, and how more complex molecular geometries can result in steering of CM. Collectively, our observations demonstrate that simple attochemistry principles and a density-based picture are useful tools for predicting and understanding CM, without resorting to an ambiguous interpretation in terms of a complicated beating of many states.