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.