Evidence of Heteroepitaxy and Solid Solutions in Lattice Matched Ternary Covalent Organic Framework Systems

Covalent organic frameworks (COFs) are crystalline, porous materials with the possibility for broad applications, but their structural diversity remains constrained by simple net topologies, limiting functional versatility. To address this challenge, we developed a strategy for incorporating linkers with normally mismatched geometries, exemplified by a [4-c + 2-c + 3-c] system incorporating pentagonal motifs for 2D tiling. Central to this approach is the derivation of a length ratio parameter, α, which provides a quantitative guide for evaluating the compatibility of linkers in ternary systems. Investigating a model system with close to ideal α, we demonstrate that precise size matching enables the formation of localized solid solutions and heteroepitaxial interfaces, as seen by transmission electron microscopy. These findings showcase a pathway for expanding the structural and functional complexity of COFs, opening new avenues for tailored material design.