New article in J. Phys. Chem. Letters

We investigate the molecular origin of the low-energy “red forms” in the Lhca4 light-harvesting antenna of Photosystem I, which play a key role in regulating energy flow in plant photosynthesis. The study combines first-principles QM/MM-based Hamiltonians with advanced quantum dynamics simulations using the ML-MCTDH approach to get insight into the ultrafast excited-state dynamics in the a603–a609 chlorophyll dimer. The simulations reveal an ultrafast (~50 fs) and coherent population transfer from locally excited states to a charge-transfer state, strongly modulated by the protein environment through the exciton–CT energy gap. The results demonstrate that the red state of Lhca4 is a coherent exciton–charge-transfer mixture with partial charge separation, capable of maintaining bright emission and supporting energy transfer. These findings provide new mechanistic insight into the origin of red forms in Photosystem I and highlight the role of coherent charge-transfer processes in tuning the functionality of natural light-harvesting complexes.