Molecular Engineering of Interlayer Exciton Delocalization in 2D Perovskites

In recent years, significant progress has been made in improving the stability, photocurrent efficiency and charge transport properties of 2D hybrid perovskites, making them increasingly relevant for optoelectronic devices. Although these layered systems are typically considered quantum wells due to carrier confinement, an emerging strategy is to generate new perovskite functionalities with π-conjugated electroactive cores as spacer molecules, which introduce electronic coupling between the inorganic metal-halide and organic sublattices. Realizing these functionalities requires an understanding of how this coupling is achieved and how it affects exciton behavior. Using first-principles modeling and single-crystal optical spectroscopy, we find that the linker length (C_x, where x = 2 or 4) controls the inorganic-organic electronic coupling and, therefore, the exciton properties of pyrene-alkylammonium (Pyr-C_x)-based electroactive 2D perovskites. Whereas both (Pyr-C₂)₂PbI₄ and (Pyr-C₄)₂PbI₄ incorporate the π-conjugated core, only the latter has electroactive characteristics, as the longer linker length (x = 4) allows favorable π-π stacking that, together with energy alignment of organic and inorganic orbitals, results in interlayer organic-inorganic hybridization. This tailored hybrid coupling induces substantial exciton "leakage" through multiple PbI₄^2- layers, enabling efficient interlayer exciton transport. By contrast, due to a type-I band alignment and orthogonal orientation of the π-systems with respect to the PbI₄^2- layers in (Pyr-C₂)₂PbI₄, the interlayer hybridization is lost, resulting in traditional quantum well properties. This study reveals new molecular engineering design principles to control excitons in 2D perovskites, emphasizing the importance of active π-core orientation and energetic band alignment─marking a critical step toward harnessing active organic cations in perovskite optoelectronics.