The vibronic landscape of excitons in photosynthetic antennae

Light harvesting and excitation energy transfer in photosynthesis generally involve chlorophyll molecules held by their host proteins at short distances from each other, resulting in excitonic coupling. The transfer of excitation energy to the reaction centres consists of exciton migration and relaxation within and between photosynthetic proteins. The dynamics of this process depend on excitonic coupling between the pigments, on the energy gaps and reorganisation energies of the excited states involved, on the disorder imposed by the proteins, and on the vibrational modes resonant with these gaps. The precise structures and vibrational landscapes of excitons are thus essential knowledge for understanding the amazing efficiency of photosynthesis. In this work, we characterise the vibrational properties of excitons in light-harvesting proteins from purple photosynthetic bacteria, which indicate on how many bacteriochlorophylls they reside and suggest in which proportions. Vibrational spectra obtained from bacteriochlorophylls in proteins generally contain additional vibronic contributions when compared to isolated pigments, which could open additional pathways for vibrationally assisted excitation energy transfer. In contrast, the absence of new vibronic contributions in the spectra of the chlorophyll-containing proteins studied here above 100 cm^-1 suggests that in these complexes, vibrationally assisted excitation energy transfer occurs through the vibrational modes of chlorophyll molecules in an equilibrium configuration.