Atomic-Scale Imaging Reveals Polar‑π Interactions in Two-Dimensional Molecular Superlattices

Controlling coassembly of synthetic oligomers into binary
superlattices
at the atomic level is challenging. We report a strategy for programming
polar-π interactions in oligomeric peptoids, a class of sequence-defined
peptidomimetics, facilitating the formation of homogeneous two-dimensional
(2D) superlattices. N-2-phenylethyl and N-(2-perfluorophenyl)ethyl side chains, similar in size, but with
contrasting electrostatic characteristics, were introduced at defined
sequence positions to generate favorable dipolar aromatic interactions.
The resulting nanosheets exhibit different crystal motifs depending
on the side chain interactions: systems containing only one type of
aromatic side chain form a parallel V-shaped motif driven by π–π
interactions, whereas a combination of both types of aromatic side
chains, either within one backbone or through the coassembly of two
distinct peptoids, adopt an antiparallel V-shaped superlattice with
higher thermal stability, driven by polar-π interactions. Cryogenic
transmission electron microscopy directly resolved the packing arrangement
of perfluorophenyl and phenyl rings in individual nanosheet superlattices,
confirming that intermolecular polar-π interaction dominates
the superlattice motifs and increases lattice stability. Molecular
dynamics simulations and density functional theory calculations further
substantiate the energetic favorability of polar-π interactions
over π–π interactions, rationalizing the formation
of homogeneous superlattices with enhanced thermal stability. Our
discoveries establish a design principle for binary coassembly using
sequence-defined oligomers, which enables control over unit cell geometry,
lattice stability, and molecular registration through aromatic side
chain polarization and sequence control. This ability to program atomic-scale
binary superlattices opens new avenues for designing functional 2D
soft materials.