Triplet-State Functional Knockouts Reveal Excitonic Structure in Photosynthetic Antenna Complexes

Congested spectroscopic signatures of excitonically coupled photosynthetic antenna complexes hinder the assignment of pigment-specific electronic properties, which are essential for modeling energy and electron transfer dynamics. Here, we explore the dynamics of the excitonic system in which one of the participating pigments is converted into the triplet state, which effectively removes this triplet-state pigment from the excitonic manifold. Thus, the excitonic system with the triplet-state pigment resembles a knockout mutant. The functional knockout strategy is tested using the Fenna-Matthews-Olson complex, for which we combine time-resolved circular dichroism (TRCD) and transient absorption spectroscopy with multiscale first-principles modeling. TRCD provides complementary structural information and is significantly more sensitive to changes in excitonic coupling than conventional transient absorption measurements. Quantitative interpretation of the time-resolved spectra is achieved by accounting for the Boltzmann distribution of the triplet-state population and electrochromic shifts induced by the triplet pigment on neighboring chromophores. Our results establish triplet-state spectroscopy, combined with polarizable multiscale modeling, as a powerful framework for disentangling excitonic interactions in complex light-harvesting systems.