Interexciton nonradiative relaxation pathways in the peridinin-chlorophyll protein

Tilluck et al. reveal how excitation energy transfer in the peridinin-chlorophyll protein from marine dinoflagellates involves quantum coherent mixing of the excited states of the clustered chromophores in each protein domain. The results suggest that solar photons might be captured efficiently in photocatalytic devices and solar cells by delocalized nanomaterials. Excitation energy transfer can be unusually efficient and structurally robust when mediated by molecular excitons, which arise in a photosynthetic light-harvesting protein when delocalized excitations of the electronic chromophores are created upon absorption of light. Here we report results from two-dimensional electronic spectroscopy and electronic structure calculations, revealing that excitation energy is transferred between the peridinin and chlorophyll excitons in the peridinin-chlorophyll protein from marine dinoflagellates over a delocalized, two-step pathway. Upon absorption of light by the peridinins in the strong, mid-visible absorption band, excitation energy is trapped by the chlorophylls in less than 50 fs at room temperature. The overall process is slowed only by a few fs upon replacement of chlorophyll a with chlorophyll b. The key step in the pathway transfers excitation from higher-energy peridinin excitons with more extensive delocalization to the lowest-energy peridinin exciton, which is delocalized only over peridinin 614 and the neighboring chlorophyll.