The development of chemosensors for specific chemical species is emerging as a research area of considerable importance within the generalized field of supramolecular chemistry. [1] One of the more appealing approaches in this context involves the construction of optical sensors. [2±6] Such systems generally contain some combination of substraterecognition functionality (receptor) and optical-signaling capacity (chromophore), either directly linked [2,3,5] or appropriately associated in a noncovalent manner, [4] and are designed to permit the detection of substrates by bindinginduced changes in absorption or emission properties (termed colorimetric and fluorescent sensors, respectively). While the utility of these approaches are becoming increasingly appreciated in terms of both qualitative and quantitative analysis, [5] the number of colorimetric sensors available at present for anionic substrates remains quite limited. [6] Indeed, only a few systems are known that undergo color changes of sufficient magnitude that they can be used for the direct ªnaked-eyeº sensing of anions. [4b, 6b, c] Here we report the synthesis of a new class of covalently linked calix[4]pyrrole ± anthraquinone conjugates and show that they act as powerful naked-eye sensors for selected anions (namely, F À , Cl À , H 2 PO 4 À ) in dichloromethane.
The calix[4]pyrroles, [7] colorless macrocycles rich in pyrrole NH hydrogen bond donor functionality, are an easy-to-make class of uncharged anion receptors that show considerable promise in the area of anion sensing. In previous work we demonstrated that anion sensors could be generated from calix [4]pyrroles either by attaching a fluorescent reporter group to the basic tetrapyrrolic skeleton [7d] or through the use of a displacement process involving competition with paranitrophenolate anions. [4b] Unfortunately, we were unable to produce inherently colored calix [4]pyrrole derivatives that functioned as naked-eye anion sensors. Recently, however, we discovered that calix[4]pyrroles functionalized at the bpyrrolic positions [8] may be made using a modified Sonogashira coupling procedure. [9] By taking advantage of this approach as well as the current availability of the ethynylsubstituted calix[4]pyrrole 1, [8] we have now succeeded in preparing (in 73 % yield) an anthraquinone-functionalized system (2) that bears an appended chromophore directly [*] Prof.