Dissecting Porosity in Molecular Crystals: Influence of Geometry, Hydrogen Bonding, and [π···π] Stacking on the Solid-State Packing of Fluorinated Aromatics.

Porous molecular crystals are an emerging class of porous materials that is unique in being built from discrete molecules rather than being polymeric in nature. In this study, we examined the effects of molecular structure of the precursors on the formation of porous solid-state structures with a series of 16 rigid aromatics. The majority of these precursors possess pyrazole groups capable of hydrogen bonding, as well as electron-rich aromatics and electron-poor tetrafluorobenzene rings. These precursors were prepared using a combination of Pd- and Cu-catalyzed cross-couplings, careful manipulations of protecting groups on the nitrogen atoms, and solvothermal syntheses. Our study varied the geometry and dimensions of precursors, as well as the presence of groups capable of hydrogen bonding and [π···π] stacking. Thirteen derivatives were crystallographically characterized, and four of them were found to be porous with surface areas between 283 and 1821 m2 g-1. Common to these four porous structures were (a) rigid trigonal geometry, (b) [π···π] stacking of electron-poor tetrafluorobenzenes with electron-rich pyrazoles or tetrazoles, and

Cyclotetrabenzoin: Facile Synthesis of a Shape-Persistent Molecular Square and Its Assembly into Hydrogen-Bonded Nanotubes.

Cyanide-catalyzed benzoin condensation of terephthaldehyde produces a cyclic tetramer, which we propose to name cyclotetrabenzoin. Cyclotetrabenzoin is a square-shaped macrocycle ornamented with four α-hydroxyketone functionalities pointing away from the central cavity, the dimensions of which are 6.9×6.9 Å. In the solid state, these functional groups extensively hydrogen bond, resulting in a microporous three-dimensional organic framework with one-dimensional nanotube channels. This material exhibits permanent-albeit low-porosity, with a Langmuir surface area of 52 m(2) g(-1) . Cyclotetrabenzoin's easy and inexpensive synthesis and purification may inspire the creation of other shape-persistent macrocycles and porous molecular crystals by benzoin condensation.

Benzobisimidazole cruciform fluorophores.

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.

Benzobisoxazole cruciforms as fluorescent sensors.

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.

L-shaped benzimidazole fluorophores: synthesis, characterization and optical response to bases, acids and anions.

Nine L-shaped benzimidazole fluorophores have been synthesized, computationally evaluated and spectroscopically characterized. These "half-cruciform" fluorophores respond to bases, acids and anions through changes in fluorescence that vary from moderate to dramatic.