A new generation of 1,8-diaminocarbazole building blocks for the construction of fluorescent anion receptors

We describe the synthesis of a new generation of 1,8-diaminocarbazole building blocks for the construction of anion receptors and fluorescent sensors. These new building blocks feature mildly electron-withdrawing ester substituents at positions 3 and 6 of the carbazole core, which improve anion affinities and significantly enhance solubilities, without compromising fluorescent response. To demonstrate the advantages of the new building blocks, three of them were converted into model diamide receptors R1-R3. The resulting ester-substituted receptors showed greatly improved solubilities and fluorescent response in comparison to their 3,6-dichloro-substituted predecessors, while retaining very high affinity and selectivity for oxyanions, particularly dihydrogen phosphate, even in partially aqueous solutions. In view of these promising results and the known synthetic versatility of primary amines, we envisage that the new building blocks will be useful for the construction of various classes of fluorescent anion receptors with improved solubility, affinity, and fluorescent response.