The association of a concave macrocyclic compound to one or multiple fluorophores is an appealing strategy for the design of chemosensors. Indeed, as with biological systems, a cavity-based selectivity can be expected with such fluorescent receptors. Examples of calix[6]arene-based systems using this strategy are rare in the literature, and to our knowledge, no examples of fluorescent receptors that can bind organic contact ion pairs have been reported. This report describes the straightforward synthesis of fluorescent calix[6]arene-based receptors 4a and 4b bearing three pyrenyl subunits and the study of their binding properties toward anions and ammonium salts using different spectroscopies. It was found that receptor 4a exhibits a remarkable selectivity for the sulfate anion in DMSO, enabling its selective sensing by fluorescence spectroscopy. In CDCl3, the receptor is able to bind ammonium ions efficiently only in association with the sulfate anion. Interestingly, this cooperative binding of ammonium sulfate salts was also evidenced in a protic environment. Finally, a cavity-based selectivity in terms of size and shape of the guest was observed with both receptors 4a and 4b, opening interesting perspectives on the elaboration of fluorescent cavity-based systems for the selective sensing of biologically relevant ammonium salts such as neurotransmitters.

The ipso-nitration of calix[6]arene-based molecular receptors is a important synthetic pathway for the elaboration of more sophisticated systems. This reaction has been studied for a variety of capped calixarenes, and a general trend for the regioselective nitration of three aromatic units out of six in moderate to high yield has been observed. This selectivity is, in part, attributed to the electronic connection between the protonated cap at the small rim and the reactive sites at the large rim. In addition, this work highlights the fact that subtle conformational properties can drastically influence the outcome of this reaction.

The construction of self-assembled receptors based on flexible concave subunits is a challenging task and constitutes an interesting approach to mimic binding processes occurring in biological systems. The receptors studied herein are based on flexible calix[6]arene skeletons bearing three (or more) acid-base functionalities at their narrow rim. When complementary, they self-assemble in a tail-to-tail manner to give a diabolo-like complex, provided that each calixarene subunit hosts a guest. The allosterically-driven multi-recognition pattern is highly selective and leads to stable quaternary adducts. In order to evaluate the scope of this system, various polyamino and polyacidic calix[6]arenes have been studied. It is shown that modifications of the nature of the wide rim substituents do not alter the efficiency of the quaternary self-assembling process, even with the more flexible macrocycles that lack tBu substituents. On the contrary, the replacement of the latter by smaller groups led to receptors with broader scope, as larger guests such as tryptamine and dopamine derivatives were stabilized in the cavities. Implementation of extra-functionalities at the narrow rim were revealed also to be of high interest. Indeed, it is shown that secondary interactions take place between the two calix-subunits when they present additional and complementary functions such as carboxylate and ureido moieties. The ureido arms are also capable of binding the counter anion Cl(-) of the ammonium guest, thus leading to a quinternary neutral complex. Such remarkable behavior is due to the versatility of the calix[6]arene platform, which allows the implementation of a high number of functions, leading to multiple non-covalent attractive interactions, whereas the macrocycle remains flexible, thus allowing induced-fit processes to occur.