Charge Transfer in a Rotaxane Nanoelectronic Switch
Mechanically interlocked molecular architectures (MIMAs) are supramolecular assemblies that are made up of non-bonded molecular components. Rotaxanes and catenanes are members of this broad, topologically-complex, class of supramolecular assemblies. A rotaxane contains a long, dumbbell-shaped component that threads through at least one macrocyclic ring. A catenane contains two or more interlocked macrocyclic rings. Switchable rotaxanes and catenanes are designed such that a mechanical motion induced by an external stimulus results in a change in the system’s co-conformation. Switching may be induced electrochemically, photochemically, and by changes in pH, depending on the non-covalent interactions between the system’s subunits. Because of their switchable nature, these assemblies are promising candidates for nanoscale devices.
The most widely recognized example of MIMA-based electronic switch is the Stoddart-Heath-type [2]rotaxane. These bistable rotaxanes are able to act as simple nanoelectronic switches because their two co-conformations demonstrate different electrical conductance.
We investigated flow of charge in the two co-conformations of a bistable Stoddart-Heath-type [2]rotaxane. The system is made up of a tetracationic cyclobis-(paraquat-p-phenylene) (CBPQT4+) ring threaded by a dumbell that contains two π-electron rich binding sites: tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP). In the reduced state, the ring prefers to sit at the TTF site. Upon oxidation of the TTF group to a cationic state (TTF+ or TTF2+) by an electrochemical stimulus, the tetracationic ring is electrostatically repelled from the cationic TTF site to the neutral DNP site. The switch is in the “off,” or low conductance, state in the ground state co-conformation (GSCC) when the ring is centered at the TTF site, and switches to the “on,” or high conductance state, the metastable state co-conformation (MSCC), when the ring is centered at the DNP site.
We extended a statistical approach for quantifying transit time to a multi-electron time-dependent wavefunction, and used this approach to obtain statistical predictions of electron transit times in the switchable rotaxane. The timescale of charge movement from the TTF group to the DNP group was characterized in each co-conformation.
Partitioning this rotaxane into distinct spatial regions is complicated by the fact that the system contains mechanically interlocked subunits with a relative positioning that varies with co-conformation. To address these challenges, Mulliken population analysis is used to track the flow of electron density over time. This approach facilitates spatial region partitioning by allowing specific atoms to be assigned to a spatial region regardless of their relative positions.
Electron transfer times for each co-conformation are extracted for specific levels of statistical confidence. We find that charge transfer occurs more rapidly in the MSCC than the GSCC, consistent with the experimentally reported difference in electrical conductance. Path information revealed by this method offers information about the influence of ring position on the mechanism of electron transit.
This is the first reported application of this statistical approach for quantifying quantum particle transit to a switchable supramolecular assembly. These results show that this method can be used to explore the relative timescales of electronic charge transfer in switchable MIMAs, and could serve as a tool for designing nanoscale systems with suitable transport properties for applications in electronics and information storage.