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Porous molecular crystals (PMCs) are a class of nanoporous materials composed of discrete, typically organic molecules held together by noncovalent interactions. While they parallel crystalline polymeric porous materialszeolites, metal–organic frameworks (MOFs), and covalent organic frameworks (COFs)in many of their characteristics, they are also distinctly solution- and vapor-processable, offering better possibilities for incorporation into devices and open unexplored frontiers in the creation of porous liquids. In this Spotlight, we will review and examine the examples of emerging applications of PMCs in gas storage and separations, analytical chemistry, the development of porous liquids, and other fields. [ABSTRACT FROM AUTHOR]

The physisorption-based separation of CO2 from CO is an energy-efficient yet challenging approach to obtain pure CO. We have applied an intrinsically porous molecular crystal (PMC), assembled from the readily synthesized macrocyclic cyclotetrabenzoin acetate to the problem of CO2/CO separation. This nanoporous material possesses both kinetic (micropore) and thermodynamic (CO2–π and CO2–carbonyl interactions) factors needed for selective CO2 capture. The moderate CO2–adsorbent interaction allows easy regeneration needed for pressure swing adsorption (PSA). Both trace (1%) and bulk (50%) amounts of CO2 can be removed from CO2/CO gas mixtures in PSA processes. [ABSTRACT FROM AUTHOR]

Metal alkoxides, such as NaOt-Bu or Ti(OBu)4, can initiate acyl exchange within complex ester libraries. Reactive distillation of such dynamic combinatorial libraries (DCLs) isolates the most volatile ester at the expense of the less volatile library members that share a constituent with it. This process can be iteratively repeated to yield up to four industrially relevant esters as pure products from a single reaction setup. An algorithm has been developed to predict reactive distillation products in DCLs of as many as 121 members. [ABSTRACT FROM AUTHOR]

Among the growing collection of molecular systems under consideration for nanoscale device applications, mechanically interlocked compounds derived from electrochemically switchable bistable [2]rotaxanes and [2]catenanes show great promise. These systems demonstrate dynamic, relative movements between their components, such as shuttling and circumrotation, enabling them to serve as stimuli-responsive switches operated via reversible, electrochemical oxidation−reduction rather than through the addition of chemical reagents. Investigations into these systems have been intense for a number of years, yet limitations associated with their synthesis have hindered incorporation of their mechanical bonds into more complex architectures and functional materials. We have recently addressed this challenge by developing new template-directed synthetic protocols, operating under both kinetic and thermodynamic control, for the preparation of bistable rotaxanes and catenanes. These methodologies are compatible with the molecular recognition between the π-electron-accepting cyclobis(paraquat-p-phenylene) (CBPQT4+) host and complementary π-electron-donating guests. The procedures that operate under kinetic control rely on mild chemical transformations to attach bulky stoppering groups or perform macrocyclizations without disrupting the host−guest binding of the rotaxane or catenane precursors. Alternatively, the protocols that operate under thermodynamic control utilize a reversible ring-opening reaction of the CBPQT4+ ring, providing a pathway for two cyclic starting materials to thread one another to form more thermodynamically stable catenaned products. These complementary pathways generate bistable rotaxanes and catenanes in high yields, simplify mechanical bond formation in these systems, and eliminate the requirement that the mechanical bonds be introduced into the molecular structure in the final step of the synthesis. These new methods have already been put into practice to prepare previously unavailable rotaxane architectures and novel complex materials. Furthermore, the potential for utilizing mechanically interlocked architectures as device components capable of information storage, the delivery of therapeutic agents, or other desirable functions has increased significantly as a result of the development of these improved synthetic protocols. [ABSTRACT FROM AUTHOR]

The article describes an alternative synthetic approach to donor-acceptor rotaxanes, including previously inaccessible [3]- and [4]rotaxanes. Their synthesis depends on the efficient stoppering of their pseudorotaxane precursors. The click reaction has been known for its high regioselectivity, tolerance of sensitive functional groups, mild reaction conditions and excellent yields. It occurs at room temperature requiring only the addition of catalytic amounts of copper sulfate and ascorbic acid.

Porphyrins have been used frequently to construct supramolecular assemblies. In contrast, noncovalent ensembles derived from expanded porphyrins, larger congeners of naturally occurring tetrapyrrole macrocycles, are all but unknown. Here we report a series of expanded porphyrin-anion supramolecular assemblies. These systems display unique environmentally responsive behavior. Addition of polar organic solvents or common anions to the ensembles leads to either a visible color change, a change in the fluorescence emission features, or differences in solubility. The actual response, which could be followed easily by the naked eye, was found to depend on the specifics of the assembly, as well as the choice of analyte. Using the ensembles of this study, it proved possible to differentiate between common solvents, such as diethyl ether, THF, ethyl acetate, acetone, alcohol, acetonitrile, DMF, and DMSO, identify complex solvent systems, as well as distinguish between the fluoride, chloride, bromide, nitrate, and sulfate anions.

A series of 11 cross-conjugated cruciform fluorophores based on a benzobisimidazole nucleus has been synthesized and characterized. Like in their previously reported benzobisoxazole counterparts, the HOMOs of these new fluorophores are localized along the vertical bisethynylbenzene axes, while their LUMOs remain relatively delocalized across the molecule, except in cruciforms substituted with electron-withdrawing groups along the vertical axis. Benzobisimidazole cruciforms exhibit a pronounced response to deprotonation in their UV/vis absorption and emission spectra, but their response to protonation is significantly attenuated.

Atmospherically stable porous frameworks and materials are interesting for heterogeneous solid-gas applications. One motivation is the direct and selective uptake of pollutant/hazardous gases, where the material produces a measurable response in the presence of the analyte. In this report, we present a combined experimental and theoretical rationalization for the piezochromic response of a robust and porous molecular crystal built from an extensively fluorinated trispyrazole. The electronic response of the material is directly determined by analyte uptake, which provokes a subtle lattice contraction and an observable bathochromic shift in the optical absorption onset. Selectivity for fluorinated absorbates is demonstrated, and toluene is also found to crystallize within the pore. Furthermore, we demonstrate the application of electronic structure calculations to predict a physicochemical response, providing the foundations for the design of electronically tunable porous solids with the chemical properties required for development of novel gas-uptake media.

CONSPECTUS: Cross-conjugated molecular cruciforms are intriguing platforms for optoelectronic applications. Their two intersecting π-conjugated arms allow independent modulation of the molecules' HOMO and LUMO levels and guarantee a well-defined optical response to analyte binding. In addition, the rigid cross-conjugated geometries of these molecules allow their organization in two- and three-dimensional space with long-range order, making them convenient precursors for the transition from solution-based to the more practical solid-state- and surface-based devices. Not surprisingly, a number of molecular cruciform classes have been explored because of these appealing properties. These include tetrakis(arylethynyl)benzenes, tetrastyrylbenzenes, distyrylbis(arylethynyl)benzenes, tetraalkynylethenes, biphenyl-based "swivel" cruciforms, and benzobisoxazole-based cruciforms. In this Account, we summarize our group's work on benzobisoxazole molecular cruciforms. The heterocyclic central core of these molecules forces their HOMOs to localize along the vertical bisethynylbenzene axis; the HOMO localization switches to the horizontal benzobisoxazole axis only in cases when that axis bears electron-rich 4-(N,N-dimethylamino)phenyl substituents and the vertical axis does not. In contrast, the LUMOs are generally delocalized across the entire molecule, and their localization occurs only in cruciforms with donor-acceptor substitution. Such spatially isolated frontier molecular orbitals (FMOs) of the benzobisoxazole cruciforms make their response to protonation very predictable. Benzobisoxazole cruciforms are highly solvatochromic, and their fluorescence quantum yields reach 80% in nonpolar solvents. Solutions of cruciforms in different solvents change emission colors upon addition of carboxylic and boronic acid analytes. These changes are highly sensitive to the analyte structure, and the emission color responses permit qualitative discrimination among structurally closely related species. In self-assembled complexes with boronic acids, benzobisoxazole fluorophores switch their analyte preferences and become responsive to Lewis basic species: phenoxides, amines, ureas, and small organic and inorganic anions. These sensing complexes allow the decoupling of the sensor's two functions: a nonfluorescent boronic acid does the chemistry through the exchange of its labile B-O bonds for other nucleophiles, and it can be optimized for solubility and analyte specificity; the benzobisoxazole fluorophore senses the electronic changes on the boron and reports them to the operator through changes in its emission colors, allowing this sensing element to be kept constant across a broad range of analytes. We have recently expanded our studies to benzimidazole-based "half-cruciforms", which are L-shaped rigid fluorophores that maintain most of the spatial separation of FMOs observed in benzobisoxazole cruciforms. Unlike benzobisoxazoles, benzimidazoles are acidic on account of their polar N-H bonds, and this feature allows them to respond to a broader range of pH values than their benzobisoxazole counterparts. The deprotonated benzimidazolate anions maintain their fluorescence, which makes them promising candidates for incorporation into solid-state sensing materials known as zeolithic imidazolate frameworks.

Metal alkoxides, such as NaOt-Bu or Ti(OBu)4, can initiate acyl exchange within complex ester libraries. Reactive distillation of such dynamic combinatorial libraries (DCLs) isolates the most volatile ester at the expense of the less volatile library members that share a constituent with it. This process can be iteratively repeated to yield up to four industrially relevant esters as pure products from a single reaction setup. An algorithm has been developed to predict reactive distillation products in DCLs of as many as 121 members.

Dynamic libraries of [n × n] imine components spontaneously simplify during a slow oxidation reaction to produce only n discrete products. The selectivity of this self-sorting process is a consequence of different oxidation rates for various imines, while the dynamic nature of the library enables self-sorting to proceed with high efficiency.