Vibronic Landscape of Excitons in Photosynthetic Antenna

Abstract: Light-harvesting and excitation energy transfer in photosynthesis generally involve chlorophyll-molecules, maintained by their host proteins at short distances from each other, this resulting in excitonic coupling. The transfer of excitation energy to the reaction centers consists of exciton migration and relaxation within and between photosynthetic proteins. The dynamics of this process depends on the vibrational modes resonant with the energy gaps between the participating excited states. The precise structure and vibrational landscape of excitons is thus essential knowledge to understand the amazing efficiency of photosynthesis. In this work, we characterize the vibrational properties of excitons in light-harvesting proteins from purple photosynthetic bacteria, which remarkably unveil on how many bacteriochlorophylls they reside and in which proportions. Vibrational spectra obtained from bacteriochlorophylls in proteins generally contain additional vibronic contributions when compared to isolated pigments, opening additional pathways for vibrationally-assisted excitation energy transfer. In contrast, the absence of new vibronic contributions in the spectra of chlorophyll -containing photosynthetic proteins above 100 cm-1 suggests that in oxygenic photosynthesis, vibrationally-assisted excitation energy transfers occurs through vibrational modes of chlorophyll molecules in equilibrium configuration.