Manifestation of Hydrogen Bonding and Exciton Delocalization on the Absorption and Two-Dimensional Electronic Spectra of Chlorosomes

Chlorosomes are supramolecular aggregates that contain
thousands
of bacteriochlorophyll molecules. They perform the most efficient
ultrafast excitation energy transfer of all natural light-harvesting
complexes. Their broad absorption band optimizes light capture. In
this study, we identify the microscopic sources of the disorder causing
the spectral width and reveal how it affects the excited state properties
and the optical response of the system. We combine molecular dynamics,
quantum chemical calculations, and response function calculations
to achieve this goal. The predicted linear and two-dimensional electronic
spectra are found to compare well with experimental data reproducing
all key spectral features. Our analysis of the microscopic model reveals
the interplay of static and dynamic disorder from the molecular perspective.
We find that hydrogen bonding motifs are essential for a correct description
of the spectral line shape. Furthermore, we find that exciton delocalization
over tens to hundreds of molecules is consistent with the two-dimensional
electronic spectra.