The 'blue box' (cyclobis(paraquat-p-phenylene) or CBPQT(4+)), developed by Stoddart and colleagues, forms effective charge transfer complexes with a variety of electron-rich species and has been used to support the formation of a wide range of interlocked structures. However, little effort seems to have been devoted to generalizing the blue box concept. We describe a new flexible tetracationic macrocycle-a 'Texas-sized' molecular box. This positively charged receptor is capable of binding the mono-terephthalate anion, forming pseudorotaxanes. These pseudorotaxanes self-assemble into short supramolecular pseudo-oligorotaxanes in solution and more extended pseudo-polyrotaxanes in the solid state. The supramolecular oligomers formed in solution are environmentally responsive; they undergo deaggregation as the overall concentration of the cationic and anionic constituents is reduced, the temperature is increased, or the protonation state of the threading mono-terephthalate anion is changed.

Over the last two decades, researchers have focused on the synthesis and development of mechanically interlocked molecules (MIMs). The intramolecular motion of mechanical bonds and the ability to induce this effect with the choice of the proper external stimuli has prompted the development of macromolecular systems that possess the ability to "perform work" at the molecular level. Currently, researchers are working to incorporate interlocked species into complex structural systems, such as molecular frameworks and nanoparticles, and to create ever more elegant noncovalent architectures. This effort provides an incentive to generate new building blocks for the construction of MIMs. In this Account, we describe progress in the development of a new cationic building block inspired by the "blue box" of Stoddart and collaborators. The blue box (cylcobis(paraquat-p-phenylene) or CBPQT(4+)) is a tetracationic, electron-deficient macrocycle widely recognized for its role in the construction of MIMs. This venerable receptor displays a high affinity for a variety of π-donor guests, and researchers have used them to construct a wide range of molecular and supramolecular structures, including rotaxanes, catenanes, pseudorotaxanes, polypseudorotaxanes, pseudo[n]polyrotaxanes, and electrochemically switchable molecules. To date, several synthetic analogues of the basic CBPQT(4+) structure have been reported, including systems containing biphenylene linkers and chiral tetracationic cyclophanes. However, researchers have not yet fully generalized the promise of the blue box. In this Account, we chronicle the development of a larger, more flexible tetracationic macrocycle, referred to as the "Texas-sized" molecular box. To highlight its relatively increased size and to distinguish it from CBPQT(4+), we have chosen to color this new receptor burnt orange. The Texas-sized box (cyclo[2](2,6-di(1H-imidazol-1-yl)pyridine)[2](1,4-dimethylenebenzene), 1(4+)·4PF(6)(-)) acts as a dynamic molecular receptor that displays an ability to adjust its shape and conformation to accommodate anionic guests of different size and charge within its central core. The use of different guests can favor different binding modes and promote the formation of different macromolecular aggregates. Furthermore, the proper selection of the guest allows for the "turning on" or "turning off" of molecular threading and can be used to produce new kinds of threaded species. This dynamic behavior is a special feature of the Texas-sized molecular box, as is its ability to stabilize a range of polypseudorotaxanes, rotaxane-containing metal-organic frameworks (MORFs), and rotaxane-based supramolecular organic frameworks (RSOFs).

Interfacial solar evaporation is considered to be a promising technology to treat brine with high energy transfer efficiency and a minimized carbon footprint. However, salt accumulation on solar evaporators during the brine treatment process has limited their widespread application. Herein, a hierarchically designed salt‐resistant solar evaporator is demonstrated, featuring confined Na+ with salt‐resistant ability based on the Donnan effect. The high chemical potential of confined Na+ leads to the Donnan distribution equilibrium, which minimizes the amount of the salt ions diffusing into the water supply layer and therefore fundamentally avoids salt accumulation. With this hierarchical design, the solar evaporator enables stable evaporation from high‐salinity brine (15 wt% NaCl) with a solar‐to‐vapor efficiency of 80% under 1 sun irradiation over a long period of time. Therefore, it provides an alternative and promising pathway for solar water treatment of high salinity brine.