Ultrafast photo-driven charge transfer exciton dynamics in mixed-stack pyrene-perylenediimide single co-crystals

Electron donor-acceptor co-crystals are receiving increasing interest because of their many useful optoelectronic properties. While the steady-state properties of many different co-crystals have been characterized, very few studies have addressed how crystal morphology affects the dynamics of charge transfer (CT) exciton formation, migration, and decay, which are often critical to their performance in device structures. Here we show that co-crystallization of a pyrene (Pyr) electron donor with either N,N′-bis(2,6-diisopropylphenyl)- or N,N′-bis(3′-pentyl)-perylene-3,4:9,10-bis(dicarboximide) (diisoPDI or C5PDI) electron acceptors, respectively, yields mixed π-stacked Pyr-diisoPDI or Pyr-C5PDI donor-acceptor co-crystals. Femtosecond transient absorption microscopy is used to determine the CT exciton dynamics in these single crystals. Fitting the data to a one-dimensional charge transfer CT exciton diffusion model reveals a diffusion constant that is two orders of magnitude higher in the Pyr-diisoPDI co-crystal compared to the Pyr-C5PDI co-crystal. By correlating the co-crystal structures to their distinct excited-state dynamics, the effects of each mixed stacked structure on the exciton dynamics and the mechanisms of CT exciton diffusion are elucidated.