Synthesis of a Molecular Tube in Dimethyl Sulfoxide and Its Inclusion Complexation Behavior with Poly(ethylene oxide-ran-propylene oxide)

Abstract

A molecular tube was synthesized by cross-linking α-cyclodextrins (α-CDs) of a polyrotaxane with epichlorohydrin in dimethyl sulfoxide (DMSO). The obtained molecular tube was characterized with 1H and 13C NMR, size-exclusion chromatography, and UV−vis absorption titration with iodine. The use of DMSO afforded greater cross-linking efficiency than that with the conventional aqueous NaOH. The inclusion−dissociation behavior of the molecular tube and α-CD with poly[(ethylene oxide)-ran-(propylene oxide)] (PEOrPO) having a central azobenzene moiety was investigated using induced circular dichroism measurements. The PEOrPO with azobenzene was included in the tube cavity, resulting in the exhibition of induced circular dichroism. This result suggests the stable inclusion of propyrene oxide units with a molecular tube for the first time. Higher temperature and higher molecular weight of PEOrPO yielded lesser amounts of the inclusion complexes with the molecular tube. The enthalpy change of inclusion complexation of the molecular tube with the PEOrPO-Az was calculated to be −3.6 kJ mol−1 per PO unit, which is similar to that with a linear alkyl chain.