Photoinduced Charge Transfer at Discrete Molecular Interfaces in Cocrystals

Precise construction of molecular heterostructures in organic donor-acceptor (D-A) cocrystals is crucial for understanding charge transfer (CT) dynamics and developing high-performance optoelectronic materials. Although cocrystals with densely packed D-A arrays have been widely investigated, discrete heterojunctions at the molecular scale have been scarcely explored. Herein, we demonstrate an approach to create what we have referred to as discrete molecular interfaces in D-A cocrystals employing a tetracationic naphthalenediimide-based macrocycle (NBox⁴⁺) and an electron-rich guest pyrene (Pyr). The large cavity of NBox⁴⁺ reduces its host-guest binding affinity with pyrene in solution but facilitates the formation of A-D-A discrete molecular interfaces in the NBox·Pyr cocrystal. The NBox·Pyr cocrystal exhibits a 20 nm red shift in UV-vis absorption and a ∼0.1 eV lower CT state energy compared to the NPy·Pyr cocrystal, formed from Pyr and the monomeric analogue (NPy²⁺) of NBox⁴⁺, which features conventional 1D alternating D-A stacks. Femtosecond transient absorption microscopy reveals a shorter CT state lifetime of 203 ps in NBox·Pyr (versus 1083 ps in NPy·Pyr), indicating faster charge recombination as a result of stronger electronic coupling at the discrete molecular interfaces in the NBox·Pyr cocrystal. This research highlights the critical role of discrete molecular interfaces in tailoring CT interactions and excited-state dynamics in the solid state, offering a powerful and versatile strategy for designing optoelectronic materials with molecular-level spatial resolution.