pH-responsive rotaxanes and polypseudorotaxanes

In this study, a series of mechanically interlocked molecules like polypseudorotaxanes, rotaxanes and pseudorotaxanes have been synthesized via CB6 catalyzed 1,3-dipolar cycloaddition using diazide and dialkyne monomers, which contain long dodecyl and short propyl aliphatic spacers. To reach these novel interlocked molecules, first appropriate monomeric units were designed and synthesized. These monomeric units were diazido and dialkyne functionalized, propyl and dodecyl spacers containing, diamine salts. These monomers were fully characterized by spectroscopic techniques like 1H, 13C-NMR and FT-IR and elemental analysis. After the preparation of the monomers, polypseudorotaxanes were synthesized via CB6 catalyzed 1,3-dipolar cycloaddition. The polymer formation proceeded through step-growth polymerization in the presence of CB6. The reaction was followed by 1H-NMR spectroscopy easily, because the appearance of a diagnostic peak at 6.5 ppm indicated the formation of triazole ring, which joins the monomers. The polypseudorotaxane was also characterized by spectroscopic techniques like 1H, 13C-NMR and FT-IR and matrix assisted laser desorption time-of-flight mass spectrometry (MALDI-TOF MS). It produced a maximum mass at around 15600 Da which corresponds to about six repeating units that is basically 12 CB6s threaded triazoles. The experimental results reveal that this polypseudorotaxane behaves as a pH-driven polymeric switch. Thus, when amine groups are protonated at an appropriate pH, CB6s are located on the triazole rings due to ion–dipole interaction, whereas at high pH they move onto the hydrophobic aliphatic spacer rather than slipping off the polypseudorotaxane. After the synthesis of the polypseudorotaxanes, a series of rotaxanes and pseudorotaxanes have also been synthesized using the already prepared dialkyne and diazide monomers. Rotaxanation was also carried out via a 1, 3-dipolar cycloaddition reaction catalyzed by CB6. Among them, a bistable CB6-based [3]rotaxane synthesized through CB6 catalyzed 1,3-dipolar cycloaddition contains two recognition sites and behaves as a reversible molecular switch. It exhibits conformational changes caused by the movement of rings under base, acid and heat stimuli from one location to the other.