Attochemistry Regulation of Charge Migration

Abstract

Charge migration (CM) is a coherent attosecond process that involves the movement of localized holes across a molecule. To determine the relationship between a molecule's structure and the CM dynamics it exhibits, we perform systematic studies of para-functionalized bromobenzene molecules (X-C₆H₄-R) using real-time time-dependent density functional theory. We initiate valence-electron dynamics by emulating rapid strong-field ionization leading to a localized hole on the bromine atom. The resulting CM, which takes on the order of 1 fs, occurs via an X localized → C₆H₄ delocalized → R localized mechanism. Interestingly, the hole contrast on the acceptor functional group increases with increasing electron-donating strength. This trend is well-described by the Hammett σ value of the group, which is a commonly used metric for quantifying the effect of functionalization on the chemical reactivity of benzene derivatives. These results suggest that simple attochemistry principles and a density-based picture can be used to predict and understand CM.

Figure 1

Effect of functional group on CM dynamics in bromobenzene derivatives. Panels (a–h) show the positive part of the time-dependent perpendicularly integrated hole densities following sudden ionization from the Br atom. As the electron-donating strength of the para group increases (a–h), the hole contrast on the end group increases.

Figure 2

Snapshot of the positive part of the hole density in 4-bromoaniline at 1.0 Å above the plane of the molecule immediately following ionization. The hole undergoes a localized → delocalized → localized charge migration process that takes 0.93 fs.

Figure 3

Hammett and hole contrast σ values for para-Br–C₆H₄–R for various functional groups. More negative values indicate stronger electron-donating strength/hole contrast. The data has been referenced to −Si(CH₃)₃ (see the text).