Porous organic materials offer vast future opportunities

In light of the surging research on porous organic materials, we herein discuss the key issues of their porous structures, surface properties, and end functions. We also present an outlook on emerging opportunities, new applications, and data science-assisted materials discovery.

the pore interconnectivity, reducing structural tortuosity remains another grand challenge of porous membranes transporting guest molecules across.
Among the many properties, interfacial properties, which are primarily determined by surface functional groups, govern the wettability to liquids, binding affinity to molecules, and hence, the end function of molecular permeability and adsorption selectivity. Therefore, tuning surface functionalities is critical for optimizing the performances of porous organic materials in separation, gas storage, and biological applications. For instance, in gas separation membranes, pore surfaces decorated with functional groups can selectively interact with certain gases. One can design surface functional groups in the molecular blocks that build up the porous matrix. These functional groups become, at some point, inherent to the organic matrix. Alternatively, one can introduce surface functional groups through post-processing, that is, functionalizing the porous organic materials with moieties to modify the surface chemistries. Chemical modifications under controlled conditions of time, temperature, and pressure add functional groups to porous organic materials; however, caution must be exercised when post-treating porous organic materials with methods such as thermal annealing. Polymers have specific thermal stability and operating temperature ranges, and treating them at temperatures outside the suitable ranges will irreversibly alter their molecular structures or even degrade them. In this respect, strategies that specifically change the surface functional groups are of paramount importance for accurate tunability. Notably, functional groups of tunable compositions, densities, and spatial distributions can open new applications of porous organic materials. This goal is potentially achievable by borrowing strategies from other communities. For example, to control the composition of surface functionalities of graphene electrocatalysts, spatial confinement directs the formation of planar N-dopants (pyridinic-and pyrrolic-nitrogen) over nonplanar quaternary N-dopants 5 .
Emerging opportunities and applications Thanks to precise control over the structure, property, and function, porous organic materials have found great use in separation, filtration, and storage, and the applications are poised to expand in the ensuing decades. Although porous organic materials are versatile, their applicability under extreme conditions is still limited, for example, under extremely high or low temperatures, extremely high tension, and exposure to highenergy radiation that are associated with space exploration. Developing new porous organic materials, especially those with uncompromised thermal stability, mechanical strength, and processability, challenges future research on porous organic materials. Exploiting existing high-performance engineering polymers, such as polyimide, in a porous form, represents a straightforward yet effective strategy 6,7 . The innovation of chemistry to synthesize high-performance porous polymers will be an enormous opportunity for polymer chemists. Furthermore, from a processing perspective, ensuring controllability over the porous structures is required and can be challenging.
One breakthrough in porous organic materials is porous organic frameworks, including covalent organic frameworks (COFs) and metal-organic frameworks (MOFs) 8 . Both COFs and MOFs have distinct organic building blocks. Recently their organic building blocks have embraced novel properties. For example, redox-active benzoquinone-based COFs 9 have revolutionized the conventional notions that porous organic materials are poor electrical conductors and inert for electrochemical energy storage. These conductive porous materials provide both high capacities and rate capability, which are often mutually exclusive for conventional electrode materials. The success will motivate future investigations and expand the applications of porous organic materials. One can envision future organic porous materials as separators for safe and long-lasting batteries, catalyst supports for extraordinary reactant diffusion and product selectivity, and gas separation membranes with outstanding selectivity and permeability.
Highly controlled structures enable systematic mechanistic investigations. Porous organic materials with uniform pore sizes or distinct functionalities minimize the number of variables and simplify the models for computational simulations. For example, in capacitive energy storage, molecular dynamics can simulate the interplays among pore widths, capacitances (or capacities), and rate capabilities 10 . Unfortunately, the lack of porous organic materials with uniform pores and homogeneous functional groups hinders experimental verifications of the simulations. Electrically conductive porous organic frameworks, with their highly uniform pore sizes and functionalities, are emerging electrochemical materials that can potentially enable such mechanistic studies. Pioneering work on charge-storage mechanisms of COFs 9 and MOFs 11 are recent examples. Highcarbon-yield porous organic materials can function as templates to prepare porous carbon powders 12 , films 13 , and fibers 14 . Notably, porous carbon fibers from porous organic materials have shown outstanding electrical and ion conductivities 14 , as well as capability of hosting guest materials and allowing for fast charging and discharging 4 .

Emerging materials development by computation and data science
Computational tools such as molecular simulations have always been indispensable to propel the development of porous organic materials. Emerging methods such as machine learning and data science are powerful to accelerate the discovery, design, synthesis, processing, and evaluation of new porous organic materials. The research of porous organic materials has accumulated countless data and information in the literature, but most of them are intuitional or empirical. Manually extracting, sorting, and analyzing the massive data become increasingly challenging. Machine learning and data science, however, can help collect and analyze enormous combinations of materials metrics, including compositions, morphologies, pore sizes/volumes, and surface chemistries. They can assist the identifications of porous organic The advancement of porous organic materials will benefit from interdisciplinary efforts. Joint endeavors by theorists and experimentalists from both science and engineering fields will accelerate the identification of preferable structures, properties, and functionalities for targeted applications (Fig. 2). Extensive and frequent exchanges of ideas and knowledge are most preferred for effective cooperation. The alliance of researchers with diverse backgrounds and specialties are mutually beneficial: Theorists provide insightful guidelines to reduce the burdens of materials synthesis and structure optimization, while experimentalists return with first-hand experimental data for model refinement; Scientists offer fundamental knowledge for materials synthesis and processing, whereas engineers repay with mass production and performance evaluation of newly developed porous organic materials. Theorists design the structures, predict the properties, and simulate the functionalities of porous organic materials, which can be refined with input from experimentalists. Exprimentalists synthesize molecules with tailored structures, characterize the properties, and evaluate the functionalities of porous organic materials, with the input from theorists to revise the experimental protocols. The efficient information and experience exchanges between theorists and experimentalists will expedite the development of porous organic materials.