A Comparative Study of Coherent and Fluorescence-Detected 2D Electronic Spectroscopies of a Multichromophore Photosynthetic System

Coherent two-dimensional electronic spectroscopy (C-2DES) is a powerful tool for resolving excitonic structure and dynamics in light-harvesting complexes with dense energy-level manifolds. In contrast, action-detected 2DES (A-2DES) has seen limited use on such systems, and its capacity to reveal electronic structure and dynamics remains unclear. Here we compare C-2DES and fluorescence-detected 2DES (F-2DES) on the major plant light-harvesting complex (LHCII) at 78 K. C-2DES clearly resolves excitonic features and their spectral evolution, mapping energy-transfer pathways. F-2DES shows strong cross-peaks, but these are largely time-invariant and dominated by incoherent mixing, obscuring the dynamics. C-2DES is therefore preferred for resolving ultrafast energy transfer in light-harvesting complexes and other large interacting molecular networks. Combined, the two measurements place a lower bound on the size of the chromophore domain contributing to incoherent mixing: C-2DES resolves the population-transfer dynamics, while their absence above the F-2DES noise floor constrains the minimum size of the annihilation domain through the combinatorial model. We obtain lower limits of 12 and 9 chromophores at the analyzed peaks, consistent with an energy-transfer domain extending over a substantial fraction of the LHCII trimer.