Electron Transfer and Switching in Rigid [2]Rotaxanes Adsorbed on TiO2 Nanoparticles

Bistable [2]rotaxanes have been attached through a bulky tripodal linker to the surface of titanium dioxide nanoparticles and studied by cyclic voltammetry and spectroelectrochemical methods. The axle component in the [2]rotaxane contains two viologen sites, V1 and V2, interconnected by a rigid terphenylene bridge. In their parent dication states, V12+ and V22+ can both accommodate a crown ether ring, C, but are not equivalent in terms of their affinity towards C and have different electrochemical reduction potentials. The geometry and size of the tripodal linker help to maintain a perpendicular [2]rotaxane orientation at the surface and to avoid unwanted side-to-side interactions. When the rigid [2]rotaxane or its corresponding axle are adsorbed on a TiO2 nanoparticle, viologen V22+ is reduced at significantly more negative potentials (−0.3 V) than in flexible analogues that contain aliphatic bridges between V1 and V2. These overpotentials are analysed in terms of electron-transfer rates and a donor–bridge–acceptor (D–B–A) formalism, in which D is the doubly reduced viologen, V10, adjacent to the TiO2 surface (TiO2–V10), B is the terphenylene bridge and A is viologen V22+. We have also found that, in contrast with earlier findings in solution, no molecular shuttling occurs in rigid [2]rotaxane adsorbed at the surface. The observations were explained by the relative position of the viologen stations within the electrical double layer, screening of V22+ by the counterions and high capacity of the medium, which reduces the mobility of the crown ether. The results are useful in transposing of solution-based molecular switches to the interface or in the design and understanding of the properties of systems comprising electroactive and/or interlocked molecules adsorbed at the nanostructured TiO2 surface.