The construction and operation of interlocked molecular machines often rely on the mutual recognition of different building blocks through a range of non-covalent interactions. Researchers have now shown that the versatility of bipyridinium systems can be increased by taking advantage of the complexes formed between their radical cations; with this approach they have been able to make electrochemically switchable bi- and tristable rotaxanes.
Although it may seem counter-intuitive, the attraction between positively charged radical ions offers a new approach to driving controlled motion in molecular machines.
The tendency for viologen radical cations to dimerize has been harnessed to establish a recognition motif based on their ability to form extremely strong inclusion complexes with cyclobis(paraquat-p-phenylene) in its diradical dicationic redox state. This previously unreported complex involving three bipyridinium cation radicals increases the versatility of host–guest chemistry, extending its practice beyond the traditional reliance on neutral and charged guests and hosts. In particular, transporting the concept of radical dimerization into the field of mechanically interlocked molecules introduces a higher level of control within molecular switches and machines. Herein, we report that bistable and tristable rotaxanes can be switched by altering electrochemical potentials. In a tristable rotaxane composed of a cyclobis(paraquat-p-phenylene) ring and a dumbbell with tetrathiafulvalene, dioxynaphthalene and bipyridinium recognition sites, the position of the ring can be switched. On oxidation, it moves from the tetrathiafulvalene to the dioxynaphthalene, and on reduction, to the bipyridinium radical cation, provided the ring is also reduced simultaneously to the diradical dication.