Exciton mobility control throughsub−Åpacking modifications in molecular crystals

Exciton mobility in \ensuremath{\pi} stacks of organic chromophores is shown to be highly sensitive to the interference between long-range Coulombic coupling and a short-range coupling due to wave function overlap. A destructive interference, which leads to a compromised exciton bandwidth, can be converted to constructive interference (and an enhanced bandwidth) upon sub-Angstrom transverse displacements between neighboring chromophores. The feasibility of the control scheme is demonstrated theoretically on a derivative of terrylene, where the exciton is essentially immobile despite strong Coulombic coupling. A transverse slip of only 0.5 \AA{} along either the short or the long molecular axis boosts the exciton velocity to $2\ifmmode\times\else\texttimes\fi{}{10}^{4} \mathrm{m}/\mathrm{s}$. Changes in the mobility are correlated to changes in the absorption spectrum, allowing the latter to be used as a screen for high mobility aggregates.