Revealing packing structure in benzimidazole-containing polyimides: Anti-parallel offset π-π stacking

Aromatic polyimides (PIs) exhibit excellent mechanical and thermal properties due to the charge transfer (CT) interactions. The introduction of benzimidazole with hydrogen interactions is a notably effective strategy for yielding exceptional superheat resistance and high thermal dimensional stability of PIs. Decoupling the stacking behavior under CT interactions and hydrogen interactions is crucial for elucidating the behavior of PIs macromolecules. Herein, the packing structure of PIs guided by hydrogen interactions was visualized using three plausible model compounds and theoretical simulations. The phthalimide fragment exhibited a preference for anti-parallel offset stack π-π stacking, while the phenylbenzimidazole fragment formed a shoulder-to-shoulder arrangement, resulting in the adoption of a Mixed Layer Packing (MPL) conformation, which is not the common Preferred Layer Packing (PLP) conformation. The CT interactions of phthalimide served as the primary stacking driving force, rather than the hydrogen interaction of benzimidazole. The intrinsic nature of packing behavior was that the minima and maxima parts of the electron density deviated from the midperpendicular of benzene and succinimide, where the LOL-π map, ESP map and SAPT calculations provided insights into the microscopic electronic structure. Finally, experimental PI fibers was prepared and their XRD pattern further confirmed the presence of an anti-parallel offset π-π stack conformation.