Calix[4]pyrroles: versatile molecular containers with ion transport, recognition, and molecular switching functions

Over the last two decades, calix[4]pyrroles have attracted considerable attention as molecular containers. Used in this capacity, they have been exploited as strong and selective receptors and as extractants for both anions and ion pairs. More recently, calix[4]pyrroles have found application as carriers, systems capable of transporting ions and ion pairs across lipophilic membranes. The use of calix[4]pyrroles as building blocks for the preparation of stimulus-responsive material has also been demonstrated. In this latter context, as well as others, the conformational switching, from 1,3-alternate to cone upon anion binding has been exploited to control both structure and substrate binding. In this Review, we describe recent results involving the use of calix[4]pyrrole systems for ion transport. Also summarised is work devoted to the formation of higher order supramolecular aggregates and studies of their response to external stimuli. Taken together, these examples serve to highlight new uses of calix[4]pyrroles as molecular containers.