Off-pathway synthesis of micron-thick, oriented 2D covalent organic framework films

Abstract

Despite the advancements in the field of 2D polymerization, the synthesis of high-quality films of oriented 2D covalent organic frameworks (2D COFs) remains a longstanding challenge. Herein, we describe a method for preparing robust, large-area, porous 2D COF films with near-perfect face-on orientation via amorphous covalent adaptable network (CAN) intermediates. Generated by solution casting, the kinetically trapped CANs undergo an unusual spontaneous alignment in response to the tensile stresses emerging during the evaporation of the solvent. A subsequent amorphous-to-crystalline transformation proceeding under solvothermal conditions converts the 3D oriented networks into porous, free-standing 2D COF films. This protocol is general and suitable for a broad range of building units and network topologies, constituting a convenient synthetic tool for assembling high-quality, oriented, robust 2D COFs. The advent of reticular chemistry has enabled the rational design and synthesis of crystalline two-dimensional polymers referred to as 2D covalent-organic frameworks (COFs)^1–3. These materials are composed of topologically planar, ordered polymeric networks that stack in the third dimension, giving rise to 1D void channels ideally accessible only from directions perpendicular to the covalently linked 2D structure^4–6. In principle, this architecture, facilitates both efficient charge and mass transport along the stacked columns and the pore channels, respectively, rendering these materials appealing for applications ranging from energy storage and conversion to high-efficiency separation, catalysis, and sensing^7–12. However, 2D polymerization in solution is often compromised by the irreversible aggregation of the growing oligomers leading to the precipitation of isotropic polycrystalline powders, thus precluding the growth of adequately sized single crystals (Fig. 1a)^13–15. Moreover, the intrinsic thermosetting behavior of 2D COFs limits their post-synthetic processability¹⁶, leaving the bottom-up synthesis of oriented 2D COF films as the exclusive option for leveraging the anisotropic properties of these materials for their technological applications. Current syntheses of oriented 2D COF films exploit various interfacial interactions inducing the confinement and pre-organization of the precursors to balance the entropic penalty associated with the formation of an oriented 2D network^17,18. As a consequence of this spatial confinement, the 2D polymerizations carried at liquid-liquid^19–21, liquid-air^22–24, liquid-solid^25–29, or vapor-solid³⁰ interfaces yield ultrathin films, i.e. sub- to several-nanometers thick oriented fragile materials, which need to be supported on a solid substrate for any further manipulation (Fig. 1b). Meanwhile, colloidal printing methods provide thicker but poorly oriented and often discontinuous materials^31,32. Therefore, none of these methods can yield large area robust, free-standing oriented 2D COFs films. Yet, mechanical stability is critical for the practical applications of these materials, and therefore, the development of a general synthetic strategy providing micron-thick oriented 2D COF films remains a fundamental challenge. Here, we report a widely applicable method for the preparation of free-standing, micron-thick, highly oriented, and crystalline films of imine-linked 2D COFs by convenient a solvent processing method. Capitalizing on the pathway complexity of dynamic polycondensation, our strategy disentangles the orientation and crystallization processes. In the first step, we impart an orientation to the film by leveraging the spontaneous alignment of 3D covalent adaptable networks (CANs) in response to self-developing tensile stresses. In the second step, we introduce crystallinity by converting 3D CANs into 2D COFs in the solid-state, while preserving their face-on orientation. Most notably, by circumventing the necessity of interfacial confinement of reactants, the fabrication of robust, micron-thick, and large-area films by simple solution casting and subsequent solvothermal annealing becomes possible. The quality and thickness of the films enabled their structural characterization by conventional laboratory x-ray diffraction. We demonstrate that this strategy can be extended to various molecular building blocks, highlighting its general applicability.