Regulation of pseudo-polyrotaxane formation between α-cyclodextrins and azobenzene-terminated poly(ethylene glycol)

Regulation of pseudo-polyrotaxane formation between alpha-cyclodextrins (alpha-CDs) and azobenzene(Az)-terminated poly(ethylene glycol) (PEG) was studied in terms of the photoisomerization of the terminal Az groups. pseudo-Polyrotaxane formation was analyzed by a change in the transmittance at 550 nm in water. It was found that the rate of transmittance change was decreased with an increase in the cis isomer content in Az-PEG. The cis-isomerization effect was quantitatively evaluated, assuming that Az-PEG having two cis-isomerized Az groups does not participate in the pseudo-polyrotaxane formation. From this analysis, it is suggested that pseudo-polyrotaxane formation between alpha-CDs and PEG was hindered when the both terminal Az groups were isomerized to cis isomer. The content of Az-PEG in the pseudo-polyrotaxane aggregate and the content of cis-isomerized Az-PEG in the supernatant were measured by H-1 NMR and UV-Vis absorption spectrum, respectively, and the obtained results supported our suggestion. It is concluded that the terminal Az groups act as a gate to regulate the pseudo-polyrotaxane formation.