Exciton mobility control through sub-Å packing modifications in molecular crystals

Exciton mobility in 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 angstrom along either the short or the long molecular axis boosts the exciton velocity to 2 x 10(4) m/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.