Selective recognition of bisphenol S isomers in water by β-cyclodextrin

The two isomers 4,4´-dihydroxydiphenyl sulphone and 2,4´-dihydroxydiphenyl sulphone (bisphenol S, BPS) form inclusion complexes with β-CD in solution, in the gas phase, and in the solid state. The complexes 4,4´-BPS@β-CD and 2,4´-BPS@β-CD were studied in solution by 1D and 2D NMR experiments. Calorimetric investigation by ITC evidenced that the β-CD host shows a greater affinity for 4,4´-BPS, with a 4,4´-BPS/2,4´-BPS selectivity ratio of 6.3. In the gas phase, CID MS experiments indicate that 4,4´-BPS has a higher kinetic barrier to escape from the β-CD cavity than the isomeric 2,4´-BPS. In the solid state, the 4,4´-BPS@β-CD and 2,4´-BPS@β-CD complexes form head-to-head dimers constituted by two β-CD macrocycles, with each β-CD hosting one BPS guest. The dimers of β-CD are sealed by H-bonding interactions involving exclusively the secondary OH groups. Finally, the formation of inclusion complexes between 4,4´-BPS or 2,4´-BPS and β-CD was also followed by FT-IR, DSC, and TGA analysis.