Consistent Model of Ultrafast Energy Transfer in Peridinin Chlorophyll-a Protein Using Two-Dimensional Electronic Spectroscopy and Förster Theory

This is the authors' abstract. We don't add key points for this paper.

Solar light harvesting begins with electronic energy transfer in structurally complex light-harvesting antennae such as the peridinin chlorophyll- a protein from dinoflagellate algae. Peridinin chlorophyll- a protein is composed of a unique combination of chlorophylls sensitized by carotenoids in a 4:1 ratio, and ultrafast spectroscopic methods have previously been utilized in elucidating their energy-transfer pathways and timescales. However, due to overlapping signals from various chromophores and competing pathways and timescales, a consistent model of intraprotein electronic energy transfer has been elusive. Here, we used a broad-band two-dimensional electronic spectroscopy, which alleviates the spectral congestion by dispersing excitation and detection wavelengths. Interchromophoric couplings appeared as cross peaks in two-dimensional electronic spectra, and these spectral features were observed between the peridinin S 2 states and chlorophyll- a Q x and Q y states. In addition, the inherently high time and frequency resolutions of two-dimensional electronic spectroscopy enabled accurate determination of the ultrafast energy-transfer dynamics. Kinetic analysis near the peridinin S 1 excited-state absorption, which forms in 24 fs after optical excitation, reveals an ultrafast energy-transfer pathway from the peridinin S 2 state to the chlorophyll- a Q x state, a hitherto unconfirmed pathway critical for fast interchromophoric transfer. We propose a model of ultrafast peridinin chlorophyll- a protein photophysics that includes (1) a conical intersection between peridinin S 2 and S 1 states to explain both the ultrafast peridinin S 1 formation and the residual peridinin S 2 population for energy transfer to chlorophyll- a, and (2) computationally and experimentally derived peridinin S 2 site energies that support the observed ultrafast peridinin S 2 to chlorophyll-a Q x energy transfer.