3D Covalent Organic Framework as a Metastable Intermediate in the Formation of a Double-Stranded Helical Covalent Polymer

Abstract

The design and development of intricate artificial architectures have been pursued for decades. Helical covalent polymer (HCP) was recently reported as an unexpected topology that consists of chiral 1D polymers assembled through weak hydrogen bonds from achiral building blocks. However, many questions remained about the formation, driving force, and the single-handedness observed in each crystal. In this work, we reveal a metastable, racemic, fully covalently cross-linked, 3D covalent organic framework (COF) as an intermediate in the early stage of polymerization, which slowly converts into single-handed HCP double helices through partial fragmentation and self-sorting with the aid of a series of hydrogen bonding. Our work provides an intriguing example where weak noncovalent bonds serve as the determining factor of the overall product structure and facilitate the formation of a sophisticated polymeric architecture.

Dynamic Entwined Topology in Helical Covalent Polymers Dictated by Competing Supramolecular Interactions

“…Subsequently, the reaction was heated at 120 °C for 30 days. The formation of HCP−Na follows an intermediate srs‐phase, as previously reported (Figure S1) [20] . Notably, the srs‐phase persists more when using alternative solvent systems, such as 1 : 2 acetonitrile/mesitylene or boric acid as the boron source (Table S1).…”

“…The dynamic nature of the bonds enables error correction as a thermodynamic control to favor the formation of structures that represent thermodynamic minimums. There is great interest in further understanding the factors that lead to certain structures being more thermodynamically favorable and the equilibrium between polymetric phases [17–22] . Additionally, it has been demonstrated that minor changes to the structure, such as larger counter‐ions or the introduction of steric interactions, can change the interpenetration, stacking between layers, and topology [23–27] .…”

“…There is great interest in further understanding the factors that lead to certain structures being more thermodynamically favorable and the equilibrium between polymetric phases. [17][18][19][20][21][22] Additionally, it has been demonstrated that minor changes to the structure, such as larger counter-ions or the introduction of steric interactions, can change the interpenetration, stacking between layers, and topology. [23][24][25][26][27] Elucidating the crystal structures of the polymers through single-crystal XRD (SCXRD) or electron diffraction techniques enabled the detailed analysis of polymeric structures at the atomic level.…”

“…The formation of HCPÀ Na follows an intermediate srs-phase, as previously reported (Figure S1). [20] Notably, the srs-phase persists more when using alternative solvent systems, such as 1 : 2 acetonitrile/ mesitylene or boric acid as the boron source (Table S1). This persistence is attributed to the difference in the reaction kinetics and the availability of HHTP in solution.…”

Highly Crystalline Helical Covalent Organic Frameworks

“…2D COFs constructed by monomers with varied geometries and symmetries adopt different topological networks such as hexagonal, rhombic, Kagome, etc. In addition, there have been reports about helical COFs whose structures are more complex and unique, which expands the properties and functionalities of COF materials. − As we know, the properties and potential applications of COFs are dependent not only on their intralayer linkage and network topology but also on the interlayer interactions and stacking mode. Herein, we report a highly crystalline helical COF material with a rod-like morphology, termed helical-COF TP‑Py , in which neighboring layers stack in a rotational manner with respect to one another, leading to a helical morphology clearly discernible under scanning electron microscopy (SEM) observation.…”