Connect four: A macrotricyclic receptor equipped with two convergent calix[4]pyrrole units features unprecedented binding properties towards two ion pairs. A highly cooperative binding process is observed for pairs of tetrabutylammonium (TBA) salts. MTOA=methyltrioctylammonium.
![Highly Cooperative Binding of Ion-Pair Dimers and Ion Quartets by a Bis(calix[4]pyrrole) Macrotricyclic Receptor](https://www.iciq.org/wp-content/uploads/2014/03/Highly-Cooperative-Binding_Pau20Angew.gif)