Carbazole-Based Colorimetric Anion Sensors

Endowed with strong carbazole chromophore and fluorophore as well as with powerful and convergent hydrogen bond donors, 1,8-diaminocarbazoles are amongst the most attractive and synthetically versatile building blocks for the construction of anion receptors, sensors and transporters. Aiming at the development of carbazole-based colorimetric anion sensors, we describe here the synthesis of 1,8-diaminocarbazoles substituted with strongly electron-withdrawing substituents: 3,6-dicyano and 3,6-dinitro. Both of these precursors were subsequently converted into model diamide receptors. Anion binding studies revealed that the new receptors show greatly enhanced anion affinity, but also significantly increased acidity. We found also that rear substitution of 1,8-diamidocarbazole with two nitro groups shifts its absorption spectrum to the visible region and converts receptor into colorimetric anion sensor. The new sensor produces vivid changes in colour and fluorescence upon addition of basic anions in wet DMSO, but is poorly selective: due to its greatly enhanced acidity, for most anions the dominant receptor-anion interaction is proton transfer and, accordingly, similar changes in colour can be observed for every basic anion. The highly acidic and strongly binding receptors developed in this study may find applications in organocatalysis or in pH-switchable anion transport through lipophilic membranes.