Broadband Two-Dimensional Electronic Spectroscopy of Peridinin-Chlorophyll-a Protein: Identification of Excited State Coupling and Ultrafast Energy Transfer Channels out of the Peridinin S\(_{2}\) State

Abstract

The Peridinin Chlorophyll-a Protein (PCP) has been of significant interest to researchers due to its remarkably efficient excitation energy transfer between the carotenoid peridinin and Chlorophyll-a (Chl-a). It’s unique electronic properties, such as the proposed presence of an intramolecular charge transfer (ICT) state, have sparked significant debate in the field. Most recently, the rise of Chl-a signals at ultrafast timescales has caused researchers to reconsider excitation energy transfer pathways from peridinin on an ultrafast timescale. In this work, I apply two-dimensional electronic spectroscopy (2DES) to the PCP complex, using a broad bandwidth to excite a number of electronic transitions. I aim to use the enhanced spectral resolution of 2DES to clearly identify couplings and energy transfer channels present within the complex, in the hopes of shedding light on ultrafast energy transfer pathways present in the complex. I find the presence of clear cross peaks in the total/absorptive and the rephasing contribution to the signal between the S\(_{2}\) state of peridinin and both the Q\(_{x}\) and Q\(_{y}\) states of Chl-a, as well as between the Q\(_{x}\) and the Q\(_{y}\) states of Chl-a. I also find that the amplitude of the cross peaks increases with the progression of time within an ultrafast temporal range. I conclude the presence of electronic coupling between the S\(_{2}\) state of peridinin and both the Q\(_{x}\) and the Q\(_{y}\) states of Chl-a, as well as ultrafast energy transfer pathways observable within the 440 fs time frame considered in this study, with the S\(_{2}\) to Q\(_{x}\) energy transfer channel being the most rapid. Ultimately, I propose a pathway for ultrafast excitation energy transfer in the PCP complex.