Induced Fit Interanion Discrimination by Binding-Induced Excimer Formation

The synthesis, photophysical, and anion-binding properties of a series of di-, tri-, and tetrapodal
anion-binding hosts based on aminopyridinium units with pyrenyl reporter groups are described. The ditopic
mesitylene-derived calix[4]arene-based host 4 binds strongly to dicarboxylates, particularly malonate, in a
2:1 anion:host ratio but is essentially nonemissive in the presence of all anions except chloride because of
intramolecular quenching by the pyridinium units. Addition of chloride results in a conformational change,
giving an initial increase in emission assigned to intramolecular excimer formation. Further chloride addition
also results in an increase in the intensity of the pyrenyl monomer emission as chloride binding reduces
the acceptor ability of the pyridinium groups. This behavior is not exhibited by control compounds 5 and 6,
which lack the ditopic geometry and calixarene spacer unit; however, tripodal 6 forms 1:2 anion:host
complexes with a range of anions.