Reversible control of assembly and disassembly of interlocked supermolecules

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) x 10(3) M(-1) for 2a and 2b, respectively, in CDCl3 at 24 +/- 1 degrees C. The pseudorotaxanes are completely disassembled into their molecular component when heated at 80-85 degrees 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) x 10(2) M(-1) for 2a and trans-1b and of (7.4 +/- 0.5) x 10(2) M(-1) for 2a and cis-1b in CDCl3 at 24 +/- 1 degrees 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.