ИСТИНА

The ability of indole units to bind anions through hydrogen bonds is well-known. Also, grafting of several indole groups onto a platform, such as calixarenes, can provide a selective anion recognition by the hosts due to specific mutual orientation of the H-bond donors. These turn conformationally rigid oligoindolic assemblies to be very attractive anion binding sites for construction of calixarene-based multitopic hosts and sensors assuming the indole fluorescence.1 It is known already for many decades that 3-substituted indoles undergo acid-promoted dimerization leading, after oxidation of indolylindolines, to highly fluorescent and rigid 2,2’-bisindoles.2 Here we applied this synthetic approach to introduce 2,2’-bisindole bridges to narrow rims of cone calix[4]arenes and 1,3-alternate calix[4]crown-ethers. Simple keeping of 0.01 M solutions of bis(indole)calix[4]arenes (or thiacalix[4]arenes) in neat trifluoroacetic acid for 2 days lead almost quantitatively to indolylindoline-bridged products which can be fully characterized by NMR. Further treating with slight excess of 2,3-dichloro-5,6-dicyanobenzoquinone in dioxane gave target mono- and bis-bridged calixarenes in moderate to high yield. The hosts obtained are multitopic ones as they have 1) rigidified bis-indole bridging moieties those fluorescence is affected by anion complexation and/or conformational motions of calixarene cores; 2) pre-organized cation-binding sites formed by two amide groups, and 3) crown-ether binding sites (in cases of calix[4]crown-derivatives) that possess highly specific ionophore ability depending on the loop size and calixarene type (classical or thiacalixarene). Anion- and cation-binding abilities of bisindole-bridged calixarenes and of starting bis(indole)calix[4]arenes were probed by fluorometric titrations. The results obtained are discussed in terms of novel switchable supramolecular systems design.