  Photoinduced charge-transfer excitations are key to understand the primary
processes of natural photosynthesis and for designing photovoltaic and
photocatalytic devices. In this paper, we use Bacteriochlorophyll dimers
extracted from the light harvesting apparatus and reaction center of a
photosynthetic purple bacterium as model systems to study such excitations
using first-principles numerical simulation methods. We distinguish four
different regimes of intermolecular coupling, ranging from very weakly coupled
to strongly coupled, and identify the factors that determine the energy and
character of charge-transfer excitations in each case. We also construct an
artificial dimer to systematically study the effects of intermolecular distance
and orientation on charge-transfer excitations, as well as the impact of
molecular vibrations on these excitations. Our results provide design rules for
tailoring charge-transfer excitations in Bacteriochloropylls and related
photoactive molecules, and highlight the importance of including
charge-transfer excitations in accurate models of the excited-state structure
and dynamics of Bacteriochlorophyll aggregates.
