Dynamical Localization Limiting the Coherent Transport Range of Excitons in Organic Crystals

Exciton or energy transport in organic crystals is commonly described by a series of incoherent hoppings. This picture is no longer valid if the transport range is on the order of the exciton coherent (or delocalization) size. However, coherent effects are often neglected because the exciton wave function generally localizes to a few molecules within an ultrafast time scale (<1 ps) after photoexcitation. Here, by using time-resolved photoemission spectroscopy and nanometer-thick zinc phthalocyanine crystals, we are able to observe a transition from the coherent to incoherent transport regime while the exciton coherent size is decreasing as a function of time. During the transition, a distinct phonon mode is excited, which suggests that the electron-vibrational interaction localizes the exciton and reduces its coherent size. It is anticipated that the coherent transport range can be increased by controlling the electron-vibrational coupling. An enhanced coherent transport range can be advantageous in applications such as organic photovoltaics.