A new study has uncovered a cruciform-shaped molecule that can discriminate between a range of structurally similar carboxylic and organoboronic acids. It does this by binding to the compounds and emitting a different fluorescent signal for each one. The research also showed how to analyze phenols through substituent displacement of boronic acids, with the sensors being based on small cross-shaped molecules built around a central benzobisoxazole nucleus.

The study, which was published in the journal Chemical Science [Lim et al. Chem. Sci. (2011) DOI: 10.1039/c1sc00610j], took up this difficult analytical challenge on fluorescent sensoring by basing it only on observing emission colors. At an early stage, it was also realized that benzobisoxazole cruciforms could operate as sensors due to previous work that explored cruciforms where one arm is intentionally produced electron-poor, while the other is made electron-rich. With this polarization inevitably comes the localization of highest occupied molecular orbital (HOMO) along the electron-rich arm and the lowest unoccupied molecular orbital (LUMO) along the electron-poor branch.
Researcher Ognjen Miljanic from the University of Houston, pointed out “Separating these orbitals on different parts of the cruciform meant that analyte binding would inevitably affect only one of these orbitals – thus invariably changing the HOMO–LUMO gap.” This gap has a significant influence on fluorescence properties, allowing the scientists to modulate the fluorescence of the molecules through straightforward protonation of the basic pyridyl and dimethylamino groups located along each arm.
They then noted that this optical response is dependent on the kind of acid being used to induce protonation, allowing an identification of minute structural differences among closely related acids. With these shifts in emission colors with carboxylic acids, they then examined boronic acids and phenols – and found a great ability to discriminate among very similar compounds.
Although boronic acids are not fluorescent, they do bind to phenols to generate esters, changing the electronic properties of the boron atom, which is then reported via the fluorescent cruciform. This important detection principle means that diverging from a single receptor species, a range of additives and chemical reactions could be used to adapt to extremely varied analytes, while maintaining the fluorescent response at a constant level.
Possible applications from the study could lie in routine quality control for detecting small qualitative differences in compounds that have closely related structures, such as those from counterfeiting or decomposition. For instance, carboxylic acids and phenols are common constituents of pharmaceuticals, trace ingredients in alcoholic drinks, and are in a great deal of food additives.