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The use of the 3,4-difluoro-1H-pyrrole as a building block for the preparation of octamethyloctafluorocalix[4]pyrrole and 2,3-di(3‘,4‘-difluoropyrrol-2‘yl)quinoxaline is described. These latter two entities act as neutral anion receptors and were found to bind anions such as fluoride, chloride, or dihydrogen phosphate with an enhanced affinity compared to their non-fluorinated congeners as judged from 1H NMR, 19F NMR, and fluorescence emission spectroscopic analyses. The increase in affinity was especially high in case of chloride and dihydrogen phosphate anion, with the 2,3-di(3‘,4‘-difluoropyrrol-2‘-yl)quinoxaline system, in particular, displaying an affinity for H2PO4 - that was improved by 3 orders of magnitude as compared to its non-fluorinated congener. This improvement in the affinity for the dihydrogen phosphate is accompanied by change of color from pale yellow to orange, thus allowing the use of such compounds as naked-eye sensors for phosphate anion. In the case of the octafluorocalix[4]pyrrole system X-ray diffraction analyses revealed the presence of four different macrocyclic conformations in the solid state, as well as close intermolecular contacts mediated by apparent CF- -HN hydrogen bonds.

The dramatic and distinctive color changes induced upon the addition of fluoride, chloride, or dihydrogen phosphate ions (A−) to solutions of calix[4]pyrroles bearing anthraquinone moieties attached through a conjugating ethyndiyl linker (see picture) in dichloromethane allow the presence of these anionic substrates to be determined by simple visual means.

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Abstract:The synthesis and properties of a number of functionalized calix[4]pyrroles are described. To date, two generalized preparative approaches have been pursued. The first involves modifying the basic pyrrole-plus-ketone synthesis of calix[4]pyrrole by using modified precursors or by co-condensing more than one ketone with pyrrole. The second approach relies on the reaction of a pre-formed calix[4]pyrrole with an electrophile. In both cases, the resulting species can be subject to further manipulation. In this way a range of structures, including ones bearing ancillary recognition subunits, electro-or photochemical reporter groups, and/or water solubilizing substituents may be obtained. Solid supports bearing calix[4]pyrroles may also be produced in this way.