Neutral Acyclic Anion Receptor with Thiadiazole Spacer: Halide Binding Study and Halide-Directed Self-Assembly in the Solid State

A halide binding study of a newly synthesized neutral acyclic receptor LH2 with a thiadiazole spacer has been methodically performed both in solution and in the solid state. Crystal structure analysis of the halide complexes elucidate the fact that fluoride forms an unusual 1:1 hyrogen-bonded complex with monodeprotonated receptor, whereas in the case of other congeners, such as chloride and bromide, the receptor binds two halide anions along with formation of a halide-bridged 1D polymeric chain network by participation of N–H···X– and aromatic C–H···X– hydrogen-bonding (where X = Cl and Br) interactions. The presence of a rigid thiadiazole spacer presumably opens up enough space for capturing two halide anions by a single receptor molecule, where the coordinated −NH protons are pointed in the same direction with respect to the spacer and eventually favor formation of halide (Cl– and Br–) induced polymeric architecture, although no obvious chloride- or bromide-directed polymeric assembly is found in solution. A significant red shift of 243 nm in the absorption spectra of LH2 was solely observed in the presence of excess fluoride anion, which enables LH2 as an efficient colorimetric sensor for optical detection of fluoride anion (yellow to blue). Furthermore, spectroscopic titration experiments with increasing equivalents of fluoride anion suggest formation of a H-bonded complex with subsequent stepwise deprotonation of two N–H groups, which can be visually monitored by a change in color from yellow to blue via pink.