Thermodynamic forecasting of mechanically interlocked switches

Abstract

Mechanically interlocked molecular (MIM) switches in the form of bistable [2]rotaxanes and [2]catenanes have proven to be—when incorporated in molecular electronic devices (MEDs) and in nanoelectromechanical systems (NEMS)—a realistic and viable alternative to the silicon chip density challenge. Structural modifications and chemical environment can have a large impact on the relaxation thermodynamics of the molecular motions, such as translation and circumrotation in bistable rotaxanes and catenanes responsible for the operation of devices based on MIMs. The effects of structural modifications on the difference in free energy (ΔG^o) for the equilibrium processes in switchable MIMs can be predicted by considering, firstly, the interactions present in their precursor pseudorotaxanes. By employing isothermal titration microcalorimetry (ITC) to investigate the thermodynamic parameters governing pseudorotaxane formation for a series of monosubstituted, acceptor host cyclophanes with various donor guests, in conjunction with X-ray crystallographic data, an obvious link between the noncovalent bonding interactions in pseudorotaxanes and MIMs that survive following the formation of the mechanical bond can be identified. It follows that the changes (ΔΔG^o values) in the difference of free energy during the formation of different pseudorotaxanes can subsequently be extrapolated to predict ΔG^o values for the thermodynamics associated with switching in analogous MIM switches, employing the same donor–acceptor recognition components. In this manner, a systematic and predictive thermodynamic approach to designing and tuning switchable MIMs and MIM-based materials has been established. Additionally, these thermodynamic relationships are reminiscent of the long forgotten concept of the ‘parachor’ as a molecular descriptor with respect to the additivity of physical properties in chemical systems dealing specifically with quantitative structure property-activity relationships (QSPR/QSAR).