Large-Amplitude Charge Relocation in a Vibrationally Excited Molecular Crystal Probed by Femtosecond X-ray Diffraction

Intra- and intermolecular charge transfer is governed by an interplay of electronic and nuclear degrees of freedom, with an inherent many-body character due to long-range electric interactions. In most cases, the spatiotemporal dynamics of charge are not understood at the molecular level, calling for experimental probes of time-dependent charge distributions. Here, we apply femtosecond X-ray powder diffraction to map transient charge distributions of polycrystalline 4-(diisopropylamino)benzonitrile (DIABN), a prototypical dipolar chromophore. Femtosecond excitation of the C≡N stretching vibration in the electronic ground state and subsequent vibrational relaxation induce oscillatory intramolecular charge relocations of large spatial amplitude, driven by small-amplitude displacements of the crystal lattice at low frequency. The crystalline environment, electrically coupled via intermolecular dipole-dipole interactions, exhibits charge oscillations of opposite phase to minimize the transient electrostatic energy. Such soft-mode behavior has a direct impact on the macroscopic electric polarization of crystalline molecular materials.