Quantum mechanical modeling of a tripodal [2]rotaxane and its binding to TiO₂

Recently, self-assembled functional nano-scale architectures that can respond to external stimulus have been the focus of considerable research. Such architectures hold promise for creating many useful nano machines, such as molecular sensors, molecular motors, and nano-scale information storage systems, by giving function to a surface. One such architecture, a tripodal [2]rotaxane, is known to display repeated folding and unfolding of the shaft in response to changes in its ionic state. It is challenging to describe this folding process theoretically because the system has many stable co-conformations due to the great flexibility of the two chemically independent molecules, an electron-rich crown-ether ring and a shaft containing two different electron-poor viologens along a polyether chain. Using computational modeling we have identified the most stable co-conformations in each charge state. The results show that the folding of the [2]rotaxane is strongly related to the binding site of the crown-ether ring on the shaft. The preferred binding site appears to correlate with the LUMO level rather than the HOMO level. The optimized structure of the [2]rotaxane when attached to a TiO₂ surface changes little from its gas phase structure.