Long-Lived Charge Separation in Single Crystals of an Electron Donor Covalently Linked to Four Acceptor Molecules

Crystalline donor–acceptor (D–A) systems serve as an excellent platform for studying CT exciton creation, migration, and dissociation into free charge carriers for solar energy conversion. Donor–acceptor cocrystals have been utilized to develop an understanding of CT exciton formation in ordered organic solids; however, the strong electronic coupling of the D and A units can sometimes limit charge separation lifetimes due to their close proximity. Covalent D–A systems that preorganize specific donor–acceptor structures can assist in engineering crystal morphologies that promote long-lived charge separation to overcome this limitation. Here we investigate photogenerated CT exciton formation in a single crystal of a 2,5,8,11-tetraphenylperylene (PerPh4) donor to which four identical naphthalene-(1,4:5,8)-bis(dicarboximide) (NDI) electron acceptors are covalently attached at the para positions of the PerPh4 phenyl groups to yield PerPh4-NDI4. X-ray crystallography shows that the four NDIs pack pairwise into two distinct motifs. Two NDI acceptors of one PerPh4-NDI4 are positioned over the PerPh4 donors of adjacent PerPh4-NDI4 molecules with the donor and acceptor π-systems having a large dihedral angle between them, while the other two NDIs of PerPh4-NDI4 form xylene-NDI van der Waals π-stacks with the corresponding NDIs in adjacent PerPh4-NDI4 molecules. Upon selective photoexcitation of PerPh4 in the single crystal, CT exciton formation occurs in <300 fs yielding electron–hole pairs that live for more than ∼16 μs. This demonstrates the effectiveness of covalently linked D–A systems for engineering single crystal structures that promote efficient and long-lived charge separation for solar energy conversion.