Carbonyl groups as molecular valves to regulate chloride binding to squaramides.

Environment-sensitive binding of anions to synthetic receptors is important for the functional mimicry of ion channels. We describe new squaramide-based chloride ion receptors whose anion binding cavity can be opened and closed by using carbonyl groups as valves. In nonpolar solvents, the carbonyls preclude chloride binding via intramolecular hydrogen bonding with the squaramide NHs. In polar solvents, disruption of the intramolecular hydrogen bonds reorients the carbonyl groups and opens the anion-binding cavity.