A solvent-driven molecular spring

Abstract

A solvent-driven doubly threaded rotaxane dimer based on an amino-modified copillar[5]arene was prepared using bis(trifluoromethyl)phenyl isocyanate as stoppers. By comparison of proton NMR spectra of the rotaxane dimer and the control compound, the inclusion-induced shielding effects of the decyl protons of the dumbbell compound were estimated. From the crystal structures of previously reported analogous pillar[5]arene/alkane pseudorotaxanes, we know that four methylenes can be totally encapsulated in the pillar[5]arene cavity. When a pillar[5]arene is swaying along a guest with a long linear alkyl chain (more than four methylenes), its cavity statistically locates on the four methylenes whose protons showed relatively larger upfield shifts. Based on this, the length of the rotaxane dimer can be estimated. In CDCl₃, it was in a contracted state with a length of 31 Å. In DMSO-d₆, it was in a extended state with a length of 37 Å. Moreover, as the polarity of the solvent is changing, the length of the rotaxane dimer can change continuously as the contraction/stretching systems work in living organisms. Therefore, we can control the length of this molecular spring as needed.