Importance and Nature of Short-Range Excitonic Interactions in Light Harvesting Complexes and Organic Semiconductors

The
singlet excitonic coupling between many pairs of chromophores
is evaluated in three different light harvesting complexes (LHCs)
and two organic semiconductors (amorphous and crystalline). This large
database of structures is used to assess the relative importance of
short-range (exchange, overlap, orbital) and long-range (Coulombic)
excitonic coupling. We find that Mulliken atomic transition charges
can introduce systematic errors in the Coulombic coupling and that
the dipole–dipole interaction fails to capture the true Coulombic
coupling even at intermolecular distances of up to 50 Å. The
non-Coulombic short-range contribution to the excitonic coupling is
found to represent up to ∼70% of the total value for molecules
in close contact, while, as expected, it is found to be negligible
for dimers not in close contact. For the face-to-face dimers considered
here, the sign of the short-range interaction is found to correlate
with the sign of the Coulombic coupling, i.e. reinforcing it when
it is already strong. We conclude that for molecules in van der Waals
contact the inclusion of short-range effects is essential for a quantitative
description of the exciton dynamics.