Two-Dimensional Electronic Spectroscopy of the D1-D2-cyt b559 Photosystem II Reaction Center Complex

We present two-dimensional electronic spectra of isolated D1-D2-cyt b559 reaction centers (PSII RC) at 77 K. We decompose the data into two-dimensional decay-associated spectra (2D DAS) to reveal the excitation- and detection-dependent spectral signatures associated with energy and charge transfer within the PSII RC. We interpret the 2D DAS in the context of an exciton model and proposed pathways for charge separation. The 2D DAS reveal a high degree of variation in the kinetics as a function of excitation wavelength. We observe rapid ∼50−150 fs energy equilibration throughout the PSII RC and a heterogeneous distribution of time scales associated with charge transfer, ranging from ∼1−3 ps. We observe 6−8 ps energy transfer from states absorbing near 670 nm, as well as ∼40−60 ps processes consistent with secondary charge transfer from a radical pair intermediate or charge transfer from a trap state.