Slow Anion Exchange, Conformational Equilibria, and Fluorescent Sensing in Venus Flytrap Aminopyridinium-Based Anion Hosts

The synthesis, anion binding, and conformational properties of a series of 3-aminopyridinium-based, tripodal, tricationic hosts for anions are described. Slow anion and conformational exchange on the
1H NMR time scale at low temperature, coupled with NMR titration, results in a high level of understanding
of the anion-binding properties of the compounds, particularly with respect to significant conformational
change resulting from induced fit complexation. Peak selectivity for halides, particularly Cl-, is observed.
The approach has been extended to dipodal and tripodal podands based on 3-aminopyridinium “arms”
containing photoactive anthracenyl moieties. The 1,3,5-tripodal host shows a remarkable selectivity for
acetate over other anions, in contrast to the analogous unsubstituted tris(3-aminopyridinium) analogue,
despite the fact that low-temperature 1H NMR experiments reveal a total of four acetate-binding
conformations. Photodimerization of anthracene units results in the formation of potential fluorescent anion
sensors.