Approaches and challenges in the synthesis of three-dimensional covalent-organic frameworks

Abstract

Covalent organic frameworks, cross-linked crystalline polymers constructed from rigid organic precursors connected by covalent interactions, have emerged as a promising class of nanoporous materials owing to their highly desirable combination of attributes, including facile chemical tunability, structural diversity, and excellent stability. Despite the distinct advantages offered by three-dimensional covalent organic frameworks, research efforts have predominantly focused on the more synthetically-accessible, two-dimensional variants. Here we present an overview of synthetic approaches to yield three-dimensional covalent organic frameworks, identify synthetic obstacles that have hindered progress in the field and recently-employed methods to address them, and propose alternative techniques to circumvent these synthetic challenges. N anoporous materials have garnered tremendous interest in recent years owing to their specific and exceptional attributes, notably permanent porosity and large and accessible internal surface areas 1-5. Conventional nanoporous materials used commonly as adsorbents and heterogeneous catalysts and catalyst supports, such as zeolites and activated carbon, are based on inorganic building blocks; nevertheless, research interest in nanoporous materials bearing organic components has expanded rapidly 1,5,6. This is in part due to the exquisite structural and functional control such components provide and the flexibility they afford for designing materials specifically tailored towards the intended application 5. A large number of such nanoporous organic polymers have been fabricated in recent years, including polymers of intrinsic microporosity (PIMs) 5,6 , porous polymer networks (PPNs) 7 , and conjugated microporous polymers (CMPs) 8-10. These materials are uniformly amorphous and composed of multifunctional building blocks linked by covalent bonds, and many studies have demonstrated their synthesis and utility 7,11-13. Unfortunately, the amorphous nature of these materials can yield significant pore size dispersities, inhibiting their utilization in certain applications such as size-and shape-based gas separation and storage 2,11. Crystallinity, in contrast, is characterized by ordered structure and uniform porosity, ideal attributes for gas separation and storage, catalysis, and optoelectronic devices 2,14. Metal-organic frameworks (MOFs) are one such class of crystalline nanoporous materials that have been investigated extensively. MOFs are constructed from metal ions or clusters linked by organic

BODIPY-Decorated Nanoscale Covalent Organic Frameworks for Photodynamic Therapy

“…On the other hand, the bulky and active organic PSs, such as BODIPY herein, were sometimes hard to be safely anchored onto the COF framework by the bottom-up approach especially under the rigorous solvothermal conditions. In addition, the covalent decoration of PSs on the established COF framework by the existing PSM approach (Ma and Scott, 2018) might be limited by the stability and activity of the pre-embedded active precursors under the COF synthetic conditions (Xu et al., 2016), and also the incoming bulky functional moiety of PSs might lead to crystallinity decrease, structural transformation, and even structure collapse of the COF pristine, although some very impressive PSM examples have been reported very recently (Bunck and Dichtel, 2013, Chen et al., 2014, Daugherty et al., 2019, Dong et al., 2016, Guo et al., 2017a, Haase et al., 2018, Han et al., 2018, Huang et al., 2015a, Huang et al., 2015b, Li et al., 2018c, Li et al., 2019, Lohse et al., 2016, Lu et al., 2018, Mitra et al., 2017, Nagai et al., 2011, Qian et al., 2017, Qian et al., 2018a, Rager et al., 2017, Sun et al., 2017, Sun et al., 2018a, Sun et al., 2018b, Vardhan et al., 2019, Waller et al., 2016, Waller et al., 2018, Xu et al., 2014, Xu et al., 2015a, Xu et al., 2015b) (see Table S2 for details).…”

Redox-triggered switching in three-dimensional covalent organic frameworks

“…Theoretically, these unique advantages can minimize the interference between individual switches and also provide enough space for their conformational change, which will enable 3D COFs more suitable than 2D COFs for switching molecules in solid state. However, due to the synthetic difficulty and complicated structural determination, the construction of 3D COFs has been considered as a big challenge 45 . Since first reported by Yaghi in 2007 43 , only a handful (~50 examples) of 3D COFs have been announced 46 – 57 .…”

Direct quantification of surface barriers for mass transfer in nanoporous crystalline materials

“…he past decades saw the rapid development and utilization of nanoporous crystalline materials in a wide variety of scientific researches and industrial processes [1][2][3][4][5][6][7][8][9][10][11] . Typical applications are, but not limited to, catalysis [1][2][3][4] , separation [5][6][7] , and electrochemistry 8 .…”