In spite of wide-ranging previous studies on synthetic macrocycles, the installation of open-close functions into the frameworks remains a challenge. We present a new polyaromatic macrocycle capable of switching between open and closed forms in response to external stimuli, namely, base and acid. The macrocycle, which is prepared in three steps, has a well-defined hydrophobic cavity with a length of around 1 nm, surrounded by four pH-responsive acridinium panels. The open and closed structures were confirmed by single-crystal X-ray analysis. The cylindrical cavity can bind long hydrophilic molecules up to 2.7 nm in length in neutral water and then release the bound guests through a reversible open-to-closed structural change upon simple addition of base.

In spite of wide-ranging previous studies on synthetic macrocycles,the installation of open-close functions into the frameworks remains ac hallenge.W ep resent an ew polyaromatic macrocycle capable of switching between open and closed forms in response to external stimuli, namely,base and acid. The macrocycle,which is prepared in three steps,has aw ell-defined hydrophobic cavity with al ength of around 1nm, surrounded by four pH-responsive acridinium panels. The open and closed structures were confirmed by singlecrystal X-raya nalysis.T he cylindrical cavity can bind long hydrophilic molecules up to 2.7 nm in length in neutral water and then release the bound guests through areversible open-toclosed structural change upon simple addition of base.