Two kinds of interlocked supramolecular complexes that display stimulus-responsive assembly and disassembly have been described. One is a pseudorotaxane driven by hydrogen-bonding interactions between rings 2a and 2b and rods 1a and 1b. The rods contain a binding site for the ring as well as a stimulus-responsive diazo group, both of which are conformationally constrained in parallel by connecting them to a rigid xanthene skeleton. The trans isomer of 1a bearing a rigid binding site cannot form the pseudorotaxanes with the rings 2a and 2b because the neighboring diazophenyl group sterically shields the binding site. However, when trans-1a was converted to the corresponding cis-1a by UV light, the pseudorotaxanes are immediately formed with association constants of 70 ± 10 M-1 and (1.1 ± 0.1) × 103 M-1 for 2a and 2b, respectively, in CDCl3 at 24 ± 1°C. The pseudorotaxanes are completely disassembled into their molecular component when heated at 80-85°C for 20 min. The assembly and disassembly processes can be reversibly cycled by repeating irradiation and heating alternatively. In the case of the rod 1b that possesses a flexible binding site, both cis and trans isomers can form the corresponding pseudorotaxanes with association constants of (2.0 ± 0.3) × 10 2 M-1 for 2a and trans-1b and of (7.4 ± 0.5) × 102 M-1 for 2a and cis-1b in CDCl3 at 24 ± 1°C. In this system, therefore, external stimuli can modulate the relative distribution of the pseudorotaxane and its components. Finally, the work was extended to the construction of a kinetically more stable molecular machine based on a rotaxane-like complex 10-11 between a metallocycle 11 and a dumbbell 10. In this system, the complex and its components showed separate sets of the signals, not the averaged, in 1H NMR spectroscopy as expected by the increased kinetic stability.
All Science Journal Classification (ASJC) codes
- Organic Chemistry