Hybrid Macrocycles for Selective Binding and Sensing of Fluoride in Aqueous Solution

Synthesis
and anion binding properties of hybrid macrocycles containing
ammonium and hydrogen bond donor groups are reported. Receptor properties
were studied in a 10 mM MES buffer solution at pH 6.2, at which the
receptors carry two positive charges at the secondary amine groups.
Receptor 1 was found to bind fluoride with the highest
affinity (105 M–1) and selectivity among
the synthesized receptors. It was the only receptor that demonstrated
fluorescence increase upon addition of fluoride. Other titration experiments
with halides and oxyanions led to an anion-induced aggregation and
fluorescence quenching. The mechanism of the particular turn-on fluorescence
for fluoride was explained by the ability of receptor 1 to encapsulate several fluoride anions. Multiple anion coordination
resulted in the protonation of the tertiary amine group and subsequent
hindering of the PET process. 1H and 19F NMR
titrations, single-crystal X-ray structure of chloride complex, and
DFT calculation suggest that 1 can perfectly accommodate
two fluoride anions in the inner cavity but only one chloride, keeping
the second chloride in the outer coordination sphere. Thus, the importance
of size selectivity, which is reflected in a collective behavior of
molecules in an aqueous solution, represents a new strategy for the
design of highly selective probes for fluoride functioning in an aqueous
solution.