First- and Second-Order Topological States in Two-Dimensional Noncovalent Molecular Chiral Crystals

Topological band physics has been extensively investigated in inorganic solid-state materials with a bonding structure. While covalent organic frameworks or metal-organic frameworks have garnered significant research interest, the high-order topological states in two-dimensional noncovalent molecular crystals remain largely uncharted. Here we investigated noncovalent molecular chiral crystals assembled from achiral molecules using first-principles calculations and tight-binding model analysis. The rotated achiral molecules introduce chiral enantiomers, promoting topological chiral states in the vicinity of conduction band edges. The structural chirality in the rotated monolayer crystal is described by the rotating Kekulé model, which possesses an opposite Berry curvature. Therefore, we further obtain the topological kink states of parallel propagating channels at the neighboring boundary of the two chiral enantiomers. In addition, this model breaks the chiral symmetry, thereby accounting for the origin of the higher-order topological corner states in the valence band. We simultaneously identify valley topological gapless edge states and second-order topological corner states, representing a significant step toward the development of first- and second-order topological insulators in noncovalent molecular chiral crystals. This work sheds light on intriguing higher-order topological states in noncovalent molecular crystals.