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 towards a coherent exciton-CT mixture with partial charge separation, a low lying state that still supports bright emission and energy transfer.