Nonporous Adaptive Crystals of Pillararenes

• Published in: Accounts of chemical research 2018-09, Vol.51 (9), p.2064-2072
• Main Authors: Jie, Kecheng, Zhou, Yujuan, Li, Errui, Huang, Feihe
• Format: Article
• Language: English
• ISSN: 0001-4842; 1520-4898
• DOI: 10.1021/acs.accounts.8b00255

Conspectus Porous materials with high surface areas have drawn more and more attention in recent years because of their wide applications in physical adsorption and energy-efficient adsorptive separation processes. Most of the reported porous materials are macromolecular porous materials, such as zeolites, metal–organic frameworks (MOFs), or porous coordination polymers (PCPs), and porous organic polymers (POPs) or covalent organic frameworks (COFs), in which the building blocks are linked together by covalent or coordinative bonds. These materials are barely soluble and thus are not solution-processable. Furthermore, the relatively low chemical, moisture, and thermal stability of most MOFs and COFs cannot be neglected. On the other hand, molecular porous materials such as porous organic cages (POCs), which have been developed very recently, also show promising applications in adsorption and separation processes. They can be soluble in organic solvents, making them solution-processable materials. However, they are usually sensitive to acid/base and humid environments since most of them are based on dynamic covalent bonding. These macromolecular and molecular porous materials usually have two similar features: high Brunauer–Emmett–Teller (BET) surface areas and rigid pore structures, which are stable during adsorption and separation processes. In this Account, we describe a novel class of solid materials for adsorption and separation, nonporous adaptive crystals (NACs), which function at the supramolecular level. They are nonporous in the initial crystalline state, but the intrinsic or extrinsic porosity of the crystals along with a crystal structure transformation is induced by preferable guest molecules. Unlike solvent-induced crystal polymorphism phenomena of common organic crystals that occur at the solid–liquid phase, NACs capture vaporized guests at the solid–gas phase. Upon removal of guest molecules, the crystal structure transforms back to the original nonporous structure. Here we focus on the discussion of pillararene-based NACs for adsorption and separation and the crystal structure transformations from the initial nonporous crystalline state to new guest-loaded structures during the adsorption and separation processes. Single-crystal X-ray diffraction, powder X-ray diffraction, gas chromatography, and solution NMR spectroscopy are the main techniques to verify the adsorption and separation processes and the structural transformations. Compared w