In recent years, action-detected ultrafast spectroscopies have gained popularity offering distinct advantages over their coherently detected counterparts, such as spatially resolved and operando measurements with high sensitivity. However, there are also fundamental limitations connected to the process of signal generation in action-detected experiments. Here we perform fluorescence-detected two-dimensional electronic spectroscopy (F-2DES) of the light-harvesting II (LH2) complex from purple bacteria. We demonstrate that the B800-B850 energy transfer process in LH2 is weak but observable in F-2DES, unlike in coherently detected 2DES where the energy transfer is visible with 100% contrast. We explain the weak signatures using a disordered excitonic model that accounts for experimental conditions. We further derive a general formula for the presence of excited-state signals in multichromophoric aggregates, dependent on the aggregate geometry, size, excitonic coupling and disorder. We find that the prominence of excited-state dynamics in action-detected spectroscopy offers a unique probe of excitonic delocalization in multichromophoric systems.

Center Algatech Institute of Microbiology Czech Academy of Sciences 37901 Třeboň Czech Republic

Department of Materials Science and Engineering University of Michigan 2300 Hayward St Ann Arbor Michigan 48109 2136 United States

Department of Physics University of Michigan 450 Church St Ann Arbor Michigan 48109 United States

Department of Physics University of Ottawa 150 Louis Pasteur Pvt Ottawa ON Canada K1N 6N5

Faculty of Mathematics and Physics Charles University Ke Karlovu 5 121 16 Praha 2 Czech Republic

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