Anthracene-linked calix[4]pyrroles: fluorescent chemosensors for anions

Abstract

Fluorescent anthracene-calix[4]pyrrole conjugates 2, 4 and 5 have been synthesized; these receptors can detect the presence of anions (e.g. F 2 , Cl 2 , H 2 PO 4 2 ) by a significant quenching of their fluorescence.The selective detection of guest species by redox 1 or photoactive 2 host molecules is a challenging area of supramolecular chemistry that has attracted increasing interest in the last few years. In spite of this, optical sensors for anions are still relatively rare. [3][4][5][6][7] We have recently discovered that calix[4]pyrroles, a class of molecule known for over a century, are effective anion binding agents in solution and in the solid state. [8][9][10] Our first attempt at using the calixpyrrole moiety as a sensor involved attaching redox active ferrocene moieties to the calixpyrrole skeleton via amide linkages and studying perturbations in the ferrocene/ferrocenium redox couple on addition of anionic guests. 11 Substantial changes occurred in the electrochemical properties of these materials but these changes were unpredictable and difficult to rationalize. We therefore decided to investigate whether calix [4]pyrrole, when combined with a fluorescent reporter group, would act as an optical sensor for anions. Here we report the synthesis a series of compounds 2, 4 and 5 that contain a calix[4]pyrrole anion binding site linked to a fluorescent anthracene moiety.In order to minimise the distance between the anion binding site and the fluorophore (and also provide a conjugated bond pathway between them), a new calix[4]pyrrole-monoacid precursor 1 was prepared using a modification of the procedure used previously to prepare a homologous calix[4]pyrrole ester derivative. 12 Specifically, meso-octamethylcalix [4]pyrrole 13 was dissolved with stirring in dry THF under an argon atmosphere, and the flask cooled to 278 °C. A solution of BuLi in hexanes (4.0 equiv.) was added dropwise to the calix[4]pyrrole solution and, after 30 min, solid CO 2 was added. The reaction was stirred for a further 3 h at 278 °C and then allowed to warm to room temperature. The solvent was removed and the residue was suspended in water. The suspension was filtered to remove unreacted meso-octamethylcalix[4]pyrrole and the solution was acidified with perchloric acid, whereupon calix[4]pyrrole acid 1 precipitated out as a white powder. Recrystallisation from acetone-water by slow evaporation produced colourless crystals of the mono-acid 1 in 6.8% yield. The anthracene-linked calix[4]pyrrole 2 was prepared by activation of calix[4]pyrrole acid 1 with DCC and hydroxybenzotriazole hydrate (HOBt) in DMF followed by treatment with commercially available 1-aminoanthracene. Purification by column chromatography afforded compound 2 in 34% yield (Scheme 1).Compound 4 was prepared by coupling calixpyrrole monoacid 3 12,14 with 1 equiv. of 1-aminoanthracene using the benzotriazol-1-yloxytris(dimethylamino)phosphonium hexafluorophosphate (BOP) amide coupling reagent (1.1 equiv.) in DMF in the presence of excess Et 3 N. After removal of DMF,...

An Internal Fluorescent Probe Based on Anthracene to Evaluate Cation–Anion Interactions in Imidazolium Salts

“…[44,45] For example, this phenomenon is caused by addition of inorganic salts containing halides and other anions. [46] Indeed, numerous recognition systems based on anthracene derivatives have been reported, [43,44,47] usually involving the introduction of a functional group at the anthracence backbone, which can interact with a specific anion, thereby modifying the emission intensity. In the reported systems, cationanion interactions play an important role because electron transfer from the donor (anion) to the excited acceptor (the anthracene-containing cation) is strongly influenced by minor changes in the structure of the anthracene moiety.…”

Exploring the Dynamics of Calix[4]pyrrole: Effect of Solvent and Fluorine Substitution

“…Many of them have been carried out in an effort to generate chemical sensors capable of detecting anionic analytes through changes in redox [25] or optical properties. [1,20,[26][27][28][29] Such efforts have produced CPbased systems capable of effecting the colorimetric or fluorescence detection of ions in organic solvents [17,20,21,29] and biological fluids. [1] Efforts have also been devoted to the synthesis of more efficient and specific CP-type receptors.…”

Syntheses of Calix[4]Pyrroles by Amberlyst-15 Catalyzed Cyclocondensations of Pyrrole with Selected Ketones

“…Calix [4]pyrroles are conformationally flexible macrocycles [6,7] of significant importance due to their binding under different conditions with anions [8], neutral substrates [9] and metal ions [10]. The complexation behavior of calix [4]pyrroles with anions and cations has been widely studied using fluorescent [11], colorimetric [12] and electrochemical signaling [13] devices. They find interesting applications as raw materials for transformation into novel calix [4]pyridines and calix [4]pyridinopyrroles [14], as new solid supports capable of separating anion mixtures [15], in optical recognition of organic vapors [16] and as biologically active species [17].…”