Unique Regioselective Binding of Permethylated β‐Cyclodextrin with Azobenzene Derivatives

The binding behavior of permethylated β‐cyclodextrin (PM‐β‐CD) upon complexation with 4‐hydroxyazobenzene (4‐HAB) and 4‐aminoazobenzene (4‐AAB) was investigated by X‐ray crystallography, circular dichroism, 2D NMR spectra, and isothermal titration calorimetry. In comparison to the reported β‐cyclodextrin (β‐CD) complexes with 4‐HAB and 4‐AAB, PM‐β‐CD complexes 3 and 4 present not only unique binding regioselectivity with azobenzene guests but also have distinct spatial arrangements, that is, β‐CDs arrange in a head‐to‐head manner, whereas PM‐β‐CDs arrange in a head‐to‐tail manner. The results obtained from circular dichroism and 2D NMR spectra indicate that the binding modes of complexes 3 and 4 in solution are in accordance with those in the solid state. Furthermore, the binding abilities and thermodynamic parameters of β‐CD and PM‐β‐CD upon complexation with azobenzene derivatives were discussed from the viewpoints of the flexibilities of the frameworks of the hosts and driving forces. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)