Slip-Dependent Organization of Intermolecular Electronic Coupling in Isoxindigo-Based Oligomers

Abstract

Conjugated organic semiconductors provide opportunities to tune solid-state electronic properties through molecular and crystal engineering; however, establishing clear relationships between crystal packing and intermolecular electronic coupling remains challenging. Here, we investigate a series of isoxindigo-based oligomers (BFD, BFD2, and CCD) as structurally well-defined model systems to clarify how π-stack slip geometry is associated with intermolecular electronic coupling. Single-crystal analysis reveals that all compounds adopt brickwork-type packing motifs with distinct slip directions within the π-stacking arrangement. BFD and BFD2 form short-axis-slipped π-stacks with nearly coplanar molecular arrangements, whereas CCD adopts a long-axis-slipped configuration and exhibits qualitatively different intermolecular contact networks. Transfer-integral calculations reveal relatively strong short-range intermolecular coupling (∼100 meV) in BFD and BFD2 associated with short carbonyl-mediated contacts and favorable orbital-overlap geometry, while CCD exhibits weaker coupling (∼50 meV) due to the absence of such interactions. Although the calculated band-edge effective masses differ only moderately among the three systems, the spatial distributions of intermolecular electronic coupling vary significantly depending on the slip geometry. To describe these differences, we analyze the spatial distribution and anisotropy of intermolecular coupling, referred to here as coupling topology. Short-axis-slipped packing is associated with relatively strong and anisotropic intermolecular coupling, whereas long-axis-slipped packing exhibits a more spatially distributed coupling topology. These findings demonstrate that subtle differences in π-stack slip geometry are associated with distinct intermolecular coupling topologies in carbonyl-containing π-conjugated molecular crystals, highlighting slip geometry as an important structural feature associated with intermolecular electronic coupling.