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Aug 3, 2024, 10:59:19 AM8/3/24

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Crystal size effect is of vital importance in materials science by exerting significant influence on various properties of materials and furthermore their functions. Crystal size effect of covalent organic frameworks (COFs) has never been reported because their controllable synthesis is difficult, despite their promising properties have been exhibited in many aspects. Here, we report the diverse crystal size effects of two representative COFs based on the successful realization of crystal-size-controlled synthesis. For LZU-111 with rigid spiral channels, size effect reflects in pore surface area by influencing the pore integrity, while for flexible COF-300 with straight channels, crystal size controls structural flexibility by altering the number of repeating units, which eventually changes sorption selectivity. With the understanding and insight of the structure-property correlation not only at microscale but also at mesoscale for COFs, this research will push the COF field step forward to a significant advancement in practical applications.

Crystal size represents an important element of control, beyond chemical compositions, for manipulating the physicochemical properties of matter at mesoscale in materials science1,2,3,4. For example, the crystal size and morphology of zeolites closely relates to the effectiveness of industrial catalysis5. For metal-organic frameworks (MOFs), besides having critical impacts on sorption, catalysis, photoelectric property, etc.6,7, crystal size effect is still one of the most challenging topics in controlling framework flexibility towards smart material8,9,10,11,12,13,14,15,16,17. As a class of porous crystalline material, covalent organic frameworks (COFs) represents one of the most promising porous materials with a wealth of applications, such as sorption, catalysis, optoelectronics, sensors, drug delivery, etc.18,19,20,21,22,23,24,25. Although great advances have been witnessed in the synthesis and utilization of COFs in the past decade, the crystal size of COFs can hardly be controlled over the wide range from nanometre to micrometre because of the crystallization problem of covalent crystals26,27. Until recently, we reported the growth of large single-crystal COFs and initially realized the crystal-size-control by a modulation approach27. However, further understanding of crystal-size-dependent properties of COFs is still in blank. Here, we successfully apply and optimize our crystal-size-controlled synthetic method to different types of COFs and report the diverse crystal size effects of COFs.

With the control of synthetic precision, LZU-111 and COF-300 are selected as a counter pair in this work to study the crystal size effect (Fig. 1). Although these two COFs are both constructed with tetrahedral nodes (Fig. 1a), they possess different structures with different types of channels and framework motilities. Determined by single-crystal X-ray diffraction (SXRD)27, LZU-111 crystallizes in the hexagonal system with a threefold interlocked lonsdaleite topology (lon-b-c3, Fig. 1b), while COF-300 crystallizes in the tetragonal system with a sevenfold interpenetrated diamond topology (dia-c7, Fig. 1c). LZU-111 has three-dimensional (3D) spiral channels and a rigid framework (Fig. 1d) that guest molecules (e.g. N2, 1,4-dioxane) hardly induced its structure transformation27. In contrast, COF-300 possesses one-dimensional (1D) straight channels in a flexible framework which can adapt itself upon interacting with guest molecules to form contracted or expanded phases (Fig. 1e)27,28,29,30. These characteristic structural correlations and differences between two representative COFs motivate us to perform a comparative study with the aim of investigating the diversiform manifestations of crystal size effects in COF systems, which is vital to provide hints for their real applications but still unknown until now.

With the crystal-size-controlled synthesis, the manufacturing of COFs is more than the regulation of constructional/functional building blocks32,33,34,35,36,37 and post-synthetic modification38 at atomic/molecular level, and also beyond the chemical/physical processing for nano-scaled powder39,40,41,42,43,44. The assembly of COFs now can be precisely achieved at multiple scales45 from microscopic to mesoscopic level in a bottom-up way. Therefore, the different properties of two COFs with variable crystal sizes are influenced by both intrinsic and extrinsic factors. From the microscopic view, each adamantane-like cage in a subnet of LZU-111 possesses two types of conformations, as shown in Fig. 1b, the chair and the boat forms, driving three lonsdaleite subnets to interlock each other without overlapping by translating to three directions with different offsets. This interpenetration mode prevents the relative sliding between subnets and even limits the deformation of adamantane-like cages, which finally results in a relatively rigid lon-b-c3 framework of LZU-111 containing spiral channels throughout the framework (Fig. 1d). On the contrary, all the adamantane-like cages in subnets of COF-300 possess solely the chair conformation (Fig. 1c), which is beneficial for seven diamond subnets interweaving to each other along the c axis with the same displacement. As a result, the flexible dia-c7 COF-300 with 1D straight channels formed (Fig. 1e). Upon the adsorption of specific guest molecules, transformation between contracted and expanded structures occurs readily in COF-300 with such a flexible interpenetration mode, since the adamantane-like cages can conform themselves uniformly along a and b axes without colliding with each other.

With the intrinsic characteristics of both architectures and pore structures, the crystal size at mesoscopic level exerts multiple effects on different COFs, resulting in featured characterization data and diverse adsorption behaviours. With fewer repeating units for small crystals of LZU-111, integrated 3D spiral channels could hardly form and distribute throughout the framework (Fig. 5a). Meanwhile, pore blocking occurs easily within the fragmentary pores containing defects. Consequently, nano-sized LZU-111 only adsorbed a small amount of gas molecules (N2 and Ar) with an unconsolidated interparticle adsorption, leading to low overall gas uptakes. On the contrary, the 3D channels in larger crystals of LZU-111 with many more unit cells extend to all directions, possessing high pore integrity and less defects, which is evidenced by a series of characterizations (PXRD, SSNMR, 129Xe NMR, etc.). Therefore, larger crystals of LZU-111 can provide more open space and more accessible sites to adsorb gases, giving rise to much higher gas uptakes.

In conclusion, with the controllability at mesoscale in growing COFs crystals with desirable sizes range from hundreds of nanometre to tens of micrometre, we have demonstrated that crystal size had dramatical effect on the crystallinity, structural anisotropy, etc. and further controlled sorption behaviour and the structural flexibility of different COFs. Our data displays that, not only sorption capacity is enhancing with the crystal size increasing in rigid COF, but also sorption selectivity can be regulated by adjusting crystal size in flexible COF. These results imply that crystal size engineering of COFs will offer a promising approach to fabricating high-performance adsorbents and catalysts. It is foreseeable that crystal size effect will also express in photoelectric and sensing COFs and further influence their optical and electrical performances. Therefore, our ability to understand and take advantage of diverse crystal size effect will shed a light on the practical application of COFs in the future.

The data generated and analysed during the current study are included in this published article and its Supplementary Information, which are available from the corresponding author on reasonable request.

We thank P.-F. Wei, Y.-T. Chen in Lanzhou University, C. Lin in Peking University and L. Long in ShanghaiTech University for their assistance during gas adsorption measurements. We thank H. Lyu for his assistance on drawing figures. T. Ma thanks for the support of Peking University Boya Postdoctoral Fellowship. The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (Nos. 21632004, 21527803, 21871009, 21621061 and 21522105).

T.M., J.S., Y.-B.Z. and W.W. conceived the idea. T.M. synthesized all the COF samples and conducted the characterization and analyses. L.W. and Y.-B.Z. collected the gas/vapour adsorption isotherms of COF-300. L.L. and X.W. took the SEM images. S.Y. and L.X. provided important advises on crystal growth and paper writing. J.N. and H.X. collected the SSNMR spectroscopy. T.M., J.S., Y.-B.Z. and W.W. drafted the manuscript and all the authors commended and revised jointly on it.

Peer review information Nature Communications thanks Rahul Banerjee and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Author Contributions Q.Y. and Z.-W.S. carried out the experiments, J.L. constructed the model, X.H. interpreted the EBSD results, L.X. supervised the sample selection, J.S. designed the project, J.L. and E.M. wrote the paper. All authors contributed to the discussions.