Anion recognition by anthracene appended ortho-aminomethylphenylboronic acid: a new PET-based sensing mechanism

Abstract

Interactions of anthracene appended ortho-aminomethylphenylboronic acid 1 with 20 organic and inorganic anions have been studied by fluorescence, ¹H and ¹¹B NMR titrations in DMSO. Carboxylate, phosphate and sulphate anions form hydrogen bonded complexes with the B(OH)₂ group producing a strong turn-off fluorescence response attributed to the enhanced photoinduced electron transfer (PET) resulting from the removal of the intramolecular hydrogen bond between boronic acid and the amino groups of 1. The preferable hydrogen bonding interaction of 1 with a dihydrogen phosphate anion, which typically coordinates directly to the B(III) atom, is confirmed by DFT structural optimization. The logarithms of the binding constants for the majority of anions correlate linearly with the pK_a of their conjugate acids, but sulfate and hydrogen oxalate show strong positive deviations. Anomalously strong binding of hydrogen oxalate is attributed to the formation of the covalent cyclic ester. In the series of monoanions of dicarboxylic acids HOOC–R–COO⁻, the monoanion of malonic acid induces a strong opposite turn-on effect, demonstrating the ability of 1 to act as a ditopic receptor through the simultaneous binding of basic and acid groups of the guest to the boronic acid and amino groups of the receptor, respectively. The results of this study demonstrate the possibility of successful manipulation of the efficiency of PET produced by the anthracene attached aminomethyl group by the guest hydrogen bonding in addition to well-known protonation and metal coordination effects.