Ultrafast Structural Reorganization and Charge Dynamics in a Photoionized Donor–Acceptor Biphenyl Molecule

Intramolecular charge transfer (CT) processes play a crucial role in fields such as energy conversion, molecular electronics, and biochemistry. Defined by the movement of electronic charge between donor and acceptor groups within a molecule, CT is inherently coupled with nuclear motions, making it essential for the understanding of energy flow and reaction mechanisms in molecular systems. Here, we employ attosecond pump-probe spectroscopy to explore the ultrafast dynamics of 4-amino-4′-nitrobiphenyl (ANB), a donor–π–acceptor molecule. Our experimental results in combination with time-dependent density functional theory and trajectory surface hopping simulations uncover a complex temporal evolution of CT spanning from a few to tens of femtoseconds. The process unfolds in distinct stages. An initial phase is marked by an ultrafast charge migration driven by a molecular planarization process, and it is followed by oscillations in the charge distribution that have a 2-fold origin: the electronic coherence induced by the broadband attosecond pulse and the coupling with nuclear motion. In ANB, these dynamics are exemplified by oscillations in charge density and structural parameters, such as dihedral angles and bond lengths, underscoring the interconnection between electronic and nuclear degrees of freedom.