Phenyl-calix[4]arene-Based Fluorescent Sensors: Cooperative Binding for Carboxylates

Tetrakis-(4-carbamoylphenyl)-substituted and tetrakis-(4-amidophenyl)-substituted calix[4]arenes as well
as the monomeric biphenylcarbamate have been synthesized as fluorescent receptors for anion sensing.
Their binding properties with various anions including F-, CH3COO-, Ph-COO-, and H2PO4- were
investigated by fluorescence titrations, Job plot experiments, 1H NMR spectroscopies, and ESI-MS
measurements. Importantly, we have found that calix[4]arene-based sensors exhibit greatly enhanced
binding affinity and selectivity toward carboxylates. The binding associations of tetrakis-(4-carbamoylphenyl)-substituted calix[4]arene for carboxylates are 1−2 orders of magnitude greater than those of
the monomeric biphenylcarbamate sensor. Such an enhancement in the binding affinity and selectivity is
attributed to the cooperative binding of the multiple ligating groups as revealed from the ab inito DFT
calculations. Although tetrakis-(4-amidophenyl)-substituted calix[4]arene exhibited relatively weaker
binding affinity toward anions, its superior binding selectivity for acetate ion over fluoride ion is evident.
Our results also suggest that carbamate functionality is a useful H-bond donor for hydrogen-bonding
interactions in molecular recognition and supramolecular chemistry.