Interfacial and Diffusive Electronic Energy Transfer from Molecular Monomers to Aggregates

Understanding microscopic mechanisms of electronic energy transfer and transport in organic semiconductors is crucial to their optoelectronic and photochemical applications. Yet the mechanisms are often strongly complicated by the entangled factors such as intermolecular coupling, structural fluctuations, energetic disorder and molecular aggregation. In particular, molecular aggregates reorganize the excitonic energy landscape and introduce additional pathways for energy relaxation. Here we identify a system where optically dark, H-like aggregates of boron dipyrromethene spontaneously formed in polymer films can function as energy sink for photoexcited monomers. By combining concentration-dependent and polarization-resolved ultrafast measurements, we identify two sequential energy migration processes: a relatively slow diffusive energy transport occurring through intermonomer exciton hopping, and a rapid (subps) interfacial monomer-to-aggregate energy transfer. Temperature-dependent measurements down to 1.7 K reveal that while intermonomer hopping is suppressed with decreasing temperature, interfacial energy transfer is instead accelerated.