No abstract

Most synthetic materials that show molecular-scale porosity consist of one-, two- or three-dimensional networks. Porous metal-organic frameworks in particular have attracted a lot of recent attention. By contrast, discrete molecules tend to pack efficiently in the solid state, leaving as little empty space as possible, which leads to non-porous materials. This Perspective discusses recent developments with discrete organic molecules that are porous in the solid state. Such molecules, which may be either crystalline or amorphous, can be categorized as either intrinsically porous (containing permanent covalent cavities) or extrinsically porous (inefficiently packed). We focus on the possible advantages of organic molecules over inorganic or hybrid systems in terms of molecular solubility, choice of components and functionalities, and structural mobility and responsiveness in non-covalent extended solids. We also highlight the potential for 'undiscovered' porous systems among the large number of cage-like organic molecules that are already known.

A series of tetrahedrally linked conjugated microporous polymer networks were prepared using a variety of bond-forming chemistries including Sonogashira-Hagihara coupling, Yamamoto coupling, thermal alkyne condensation, and "click" chemistry. These thermally stable polymers exhibit high surface areas (up to 3200 m 2 /g) and adsorb up to 2.34 wt % hydrogen by mass at 1.13 bar/77 K and 7.59 wt % carbon dioxide by mass at 1.13 bar/298 K.

Nitrogen-rich carbon nitride (CN x , x >/= 1) network materials have been produced as disordered structures by a variety of precursor-based methods, many that involve solid-state thermolysis at or above 500 degrees C. One popular precursor building block is the triazine unit (C 3N 3), and most postulated amorphous CN x network structures are based on cross-linked triazine units. Since hydrogen is most often observed in the product, these materials are usually more appropriately described as CN x H y materials. Results from recent carbon nitride studies using larger conjugated heptazine (C 6N 7) precursors and from rigorous structural investigations of triazine to heptazine thermal conversion processes have prompted a reexamination of likely local structures present in amorphous carbon nitride networks formed by triazine thermolysis reactions. In the present study, the formation and local structure of a CN x H y material formed via the rapid and exothermic decomposition of a reactive triazine precursor, C 3N 3(NHCl) 3, was examined by byproduct gas mass spectrometry, NMR and IR spectroscopy, base hydrolysis, and crystallographic analysis. The combined results clearly indicate that the moderate-temperature ( approximately 400 degrees C) self-sustaining decomposition of trichloromelamine results in ring fragmentation and reorganization into a CN x H y product that contains predominantly larger heptazine-like structural building blocks. These results may have applicability to many other disordered carbon nitride materials that are formed via triazine thermolysis. It also provides clearer and more accurate structural guidance in the use of these carbon nitrides as photoactive materials or coordination supports for metal and nonmetal species.

The scope for obtaining extremely high surface areas in porous solids has broadened dramatically in the last ten years. Materials such as metal‐organic frameworks, covalent organic frameworks, and organic polymers can now be prepared with measured surface areas the range 4500 to 6000 square meters per gram. This highlight charts the rapid growth in this area and points to potential applications as well as research challenges to be addressed. We also argue that surface area should be considered as just one property among many when considering practical applications, and that factors such as stability, cost, life‐cycle analysis and synthetic ‘added‐value’ may often dominate the prospects for supplanting more established sorbents such as activated carbons.