Correlated Protein Environments Drive Quantum Coherence Lifetimes in Photosynthetic Pigment-Protein Complexes

Early reports of long-lived quantum beating signals in photosynthetic pigmentprotein complexes were interpreted to suggest that electronic coherence benefits from protection by the protein, but many subsequent studies have suggested instead that vibrational or vibronic contributions are responsible for the observed signals. Here, we devised two 2D-spectroscopy methods to observe how each exciton is perturbed by its nuclear environment in a photosynthetic complex. The first approach simultaneously monitors each exciton's energy fluctuations over time to obtain its time-dependent electronic-nuclear interactions. The second method isolates evidence of coupled interexcitonic environmental motions. The techniques are validated with Nile Blue A and subsequently used on the Fenna-Matthews-Olson (FMO) complex. The FMO data reveal that each exciton experiences nearly identical spectral motion after excitation and that spectral motion of one excited exciton induces similar motion on unpopulated neighboring excitonic states. These synchronized and correlated spectral dynamics prolong coherences in the FMO complex after femtosecond excitation.