Most two-dimensional (2D) covalent organic frameworks (COFs) are non-fluorescent in the solid state even when they are constructed from emissive building blocks. The fluorescence quenching is usually attributed to non-irradiative rotation-related or π–π stacking-caused thermal energy dissipation process. Currently there is a lack of guiding principle on how to design fluorescent, solid-state material made of COF. Herein, we demonstrate that the eclipsed stacking structure of 2D COFs can be used to turn on, and tune, the solid-state photoluminescence from non-emissive building blocks by the restriction of intramolecular bond rotation via intralayer and interlayer hydrogen bonds among highly organized layers in the eclipse-stacked COFs. Our COFs serve as a platform whereby the size of the conjugated linkers and side-chain functionalities can be varied, rendering the emission colour-tuneable from blue to yellow and even white. This work provides a guide to design new solid-state emitters using COFs.

Covalent organic frameworks (COFs) represent a new type of crystalline porous materials that are covalently assembled from organic building blocks. Construction of functional COFs is, however, a difficult task because it has to meet simultaneously the requirements for crystallinity and functionality. We report herein a facile strategy for the direct construction of chiral-functionalized COFs from chiral building blocks. The key design is to use the rigid scaffold 4,4'-(1H-benzo[d]imidazole-4,7-diyl)dianiline (2) for attaching a variety of chiral moieties. As a first example, the chiral pyrrolidine-embedded building block (S)-4,4'-(2-(pyrrolidin-2-yl)-1H-benzo[d]imidazole-4,7-diyl)dianiline (3) was accordingly synthesized and applied for the successful construction of two chiral COFs, LZU-72 and LZU-76. Our experimental results further showed that these chiral COFs are structurally robust and highly active as heterogeneous organocatalysts.

Solid electrolytes (SEs) are milestones in the technology roadmaps for safe and high energy density batteries. The design of organic SEs is challenged by the need to have dynamic structural fluidity for ion motion. The presence of well-ordered one-dimensional (1D) channels and stability against phase transition in covalent organic frameworks (COFs) render them potential candidates for low-temperature SEs. Herein, we demonstrate two milestones using hydrazone COF as an SE: it achieves an ion conductivity of 10–5 S cm–1 at −40 °C with a Li+ transference number of 0.92 and also prevents the dissolution of small organic molecular electrode in all-solid-state batteries. Using 1,4-benzoquinone as the cathode, a lithium battery using hydrazone COF as a SE runs for 500 cycles at a steady current density of 500 mA g–1 at 20 °C. Considering that hydrazone COF is readily amenable to large-scale production and facile post-synthetic modification, its use in an all-solid-state battery is highly promising.

Two-dimensional (2D) covalent organic frameworks (COFs) are an emerging class of porous materials with potential for wide-ranging applications. Intense research efforts have been directed at tuning the structure and topology of COF, however the bandgap engineering of COF has received less attention, although it is a necessary step for developing the material for photovoltaic or photonic applications. Herein, we have developed an approach to narrow the bandgap of COFs by pairing triphenylamine and salicylideneaniline building units to construct an eclipsed stacked 2D COF. The ordered porous structure of 2D COF facilitates a unique moisture-triggered tautomerism. The combination of donor−acceptor charge transfer and tautomerization in the salicyclidineaniline unit imparts a large bandgap narrowing for the COF and turns it color to black. The synthesized COF with donor−acceptor dyad exhibits excellent nonlinear optical properties according to open aperture Z-scan measurements with 532 nm nanosecond laser pulses.

Two-dimensional covalent organic framework (COF) materials can serve as excellent candidates for gas storage due to their high density of periodically arranged pores and channels, which can be tethered with functional groups. However, post-functionalization tends to disturb the structure of the COF; thus, it is attractive to develop synthetic approaches that generate built-in functionalities. Herein, we develop a new strategy for the construction of 2D-COFs with built-in, unreacted periodic bonding networks by solventdirected divergent synthesis. Tetraphenylethane (TPE), which combines both π-rigidity for stacking and rotational flexibility, is selected as the central core for COF construction. By solvent control, two distinct COF structures could be constructed, arising from a [4 + 4] condensation pathway (TPE-COF-I) or an unusual [2 + 4] pathway (TPE-COF-II). TPE-COF-II contains unreacted linker units arranged around its pores and shows greatly enhanced carbon dioxide adsorption performance (23.2 wt %, 118.8 cm 3 g −1 at 1 atm, 273 K), which is among the best COF materials for CO 2 adsorption reported to date.