First-principles studies of the dynamics of [2]rotaxane molecular switches

Realization of controlled binary switching in individual molecules is of fundamental importance for nanoscale electronics where the use of molecular components promises the flexibility of engineering performance through controlled organic synthesis. The active component of the [2]rotaxane molecule consists of a cyclobis-(paraquat-p-phenylene) ring-shaped structure [(CBPQT(4+))(PF(6)(-))(4)], proposed to switch between two stations, tetrathiafulvalene (TTF) and 1,5-dioxynapthalene (DNP), that lie along a common molecular backbone. However, there are still several open questions regarding their operation and performance, particularly in a device geometry. In this work, the switching speed of crossbar array devices based on [2]rotaxane arrays is studied with first principles density functional theory (DFT). The energetics of a likely configurational pathway for the CBPQT-ring shuttling along the molecular backbone between stations is computed and analyzed, as are ionization potentials and electrostatic screening properties. From these quantities, a new switching mechanism is identified. The applied bias at the cathode alters the energy landscape, making the OFF-state configuration energetically unfavorable relative to the ON-state without involving charging, as previously suggested. (1) For a crossbar memory array of reasonable size, the calculations predict that the switching speed is dominated by the shuttling time of the CBPQT-ring, which is estimated to be a few microseconds. The applicability of this technology is discussed in light of this result.