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.