Quantum Dynamics Simulations Reveal Ultrafast and Coherent Charge Transfer in the Lhca4 Antenna of Photosystem I

Among the peripheral antenna complexes of Photosystem I, Lhca4 exhibits the most striking signatures of low-lying exciton states, such as red-shifted absorption and fluorescence bands. These so-called “red forms” arise from strong coupling between locally excited (LE) and charge-transfer (CT) states within the a603-a609 chlorophyll (Chl) dimer. We employ ML-MCTDH quantum dynamics simulations and first-principles calculated Hamiltonians to investigate the CT mechanism between the two Chls. Our simulations reveal ultrafast (∼50 fs) population transfer, finely modulated by the protein environment via the LE–CT energy gap. Our analysis reveals the adiabatic and coherent nature of the process, providing mechanistic insight into the CT process in Lhca4. The system relaxes toward a coherent exciton-CT mixture with partial charge separation, a low lying state that still supports bright emission and energy transfer.