Coherences of Bacteriochlorophyll a Uncovered Using 3D-Electronic Spectroscopy.

Mapping the multidimensional energy landscape of photosynthetic systems is crucial for understanding their high efficiencies. Multidimensional coherent spectroscopy is well suited to this task but has difficulty distinguishing between vibrational and electronic degrees of freedom. In pigment-protein complexes, energy differences between vibrations within a single electronic manifold are similar to differences between electronic states, leading to ambiguous assignments of spectral features and diverging physical interpretations. An important control experiment is that of the pigment monomer, but previous attempts using multidimensional coherent spectroscopy lacked the sensitivity to capture the relevant spectroscopic signatures. Here we apply a variety of methods to rapidly acquire 3D electronic-vibrational spectra in seconds, leading to a mapping of the vibrational states of Bacteriochlorophyll a (BChl a) in solution. Using this information, we can distinguish features of proteins containing BChl a from the monomer subunit and show that many of the previously reported contentious spectral signatures are vibrations of individual pigments.