Covalent organic frameworks (COFs) prevail as diverse forms of solids in bulk, e.g., powders, monoliths, thin films, etc. These architectures constructed from a similar net design at the molecular to framework level differ in their hierarchical assembly laid upon the nano-to mesoscale structures. However, even after a decade and a half of research, the primary focus of the field lies on the covalent reticular structure, and the chemistry of COFs in the nano regime is often overlooked. The COF nanostructures (nanosheets, nanofibers, and nanospheres) are quite distinct from the bulk forms and often influence the properties of their bulk counterparts. Although the reticular chemistry of strong directional covalent bonds generates these nanostructures from molecular building blocks, divergent chemical and supramolecular interactions assemble these nanomotifs into the macroscale solids. We believe that the synthetic design can be extended beyond the molecule → framework to the nano → meso → macroscale hierarchy in COFs. In this review, we take a closer look at this "supramolecular nanosynthesis" and discuss how the properties of the nanostructures reciprocate in the bulk forms.

Empowered by crystalline ordered structures and homogeneous fabrication techniques, covalent organic frameworks (COFs) have been realized with uniform morphologies and isotropic properties. However, such homogeneity often hinders various surface-dependent properties observed in asymmetric nanostructures. The challenge remains to induce heterogeneity in COFs by creating an asymmetric superstructure such as a Janus thin film. In this regard, we propose a versatile yet straightforward interfacial layer-grafting strategy to fabricate free-standing Janus-type COF-graphene thin films. Herein, two-dimensional graphene sheets were utilized as the suitable grafter due to the possibility of noncovalent interactions between the layers. The versatility of the approach was demonstrated by fabricating two distinct Janus-type films, with the COF surface interwoven with nanofibers and nanospheres. The Janus-type films showcase opposing surface morphologies originating from graphene sheets and COF nanofibers or nanospheres, preserving the porosity (552−600 m 2 g −1 ). The unique surface chemistries of the constituent layers further endow the films with orthogonal mechanical properties, as confirmed by the nanoindentation technique. Interestingly, the graphene sheets favor the Janus-type assembly of COF nanofibers over the nanospheres. This is reflected in the better nanomechanical properties of COF fiber -graphene films (E graphene = 300−1200 MPa; E COF = 15−60 MPa) compared to the COF spheregraphene films (E graphene = 11−14 MPa; E COF = 2−5 MPa). These results indicate a direct relationship between the mechanical properties and homo/heterogeneity of Janus-type COF films.

The development of synthetic routes for the formation of robust porous organic polymers (POPs) with well‐defined nanoscale morphology is fundamentally significant for their practical applications. The thermodynamic characteristics that arise from reversible covalent bonding impart intrinsic chemical instability in the polymers, thereby impeding their overall potential. Herein, a unique strategy is reported to overcome the stability issue by designing robust imidazole‐linked POPs via tandem reversible/irreversible bond formation. Incorporating inherent rigidity into the secondary building units leads to robust microporous polymeric nanostructures with hollow‐spherical morphologies. An in‐depth analysis by extensive solid‐state NMR (1D and 2D) study on 1H, 13C, and 14N nuclei elucidates the bonding and reveals the high purity of the newly designed imidazole‐based POPs. The nitrogen‐rich polymeric nanostructures are further used as metal‐free electrocatalysts for water splitting. In particular, the rigid POPs show excellent catalytic activity toward the oxygen evolution reaction (OER) with long‐term durability. Among them, the most efficient OER electrocatalyst (TAT‐TFBE) requires 314 mV of overpotential to drive 10 mA cm−2 current density, demonstrating its superiority over state‐of‐the‐art catalysts (RuO2 and IrO2).

Covalent organic nanotubes (CONTs) are one-dimensional porous frameworks constructed from organic building blocks via dynamic covalent chemistry. CONTs are synthesized as insoluble powder that restricts their potential applications. The judicious selection of 2,2′-bipyridine-5,5′-dicarbaldehyde and tetraaminotriptycene as building blocks for TAT-BPy CONTs has led to constructing flexible yet robust and self-standing fabric up to 3 μm thickness. The TAT-BPy CONTs and TAT-BPy CONT fabric have been characterized by solid-state one-dimensional (1D) 13C CP-MAS, two-dimensional (2D) 13C–1H correlation NMR, 2D 1H–1H DQ–SQ NMR, and 2D 14N–1H correlation NMR spectroscopy. The mechanism of fabric formation has been established by using high-resolution transmission electron microscopy and scanning electron microscopy techniques. The as-synthesized viscoelastic TAT-BPy CONT fabric exhibits high mechanical strength with a reduced modulus (E r) of 8 (±3) GPa and hardness (H) of 0.6 (±0.3) GPa. Interestingly, the viscoelastic fabric shows time-dependent elastic depth recovery up to 50–70%.