Mechanistic understanding of the translational movements in molecular switches is essential for designing machine-like prototypes capable of following set pathways of motion. To this end, we demonstrated that increasing the station-to-station distance will speed up the linear movements forward and slow down the movements backward in a homologous series of bistable rotaxanes. Four redox-active rotaxanes, which drove a cyclobis(paraquat-p-phenylene) (CBPQT(4+)) mobile ring between a tetrathiafulvalene (TTF) station and an oxyphenylene station, were synthesized with only variations to the lengths of the glycol linker connecting the two stations (n = 5, 8, 11, and 23 atoms). We undertook the first mechanistic study of the full cycle of motion in this class of molecular switch using cyclic voltammetry. The kinetics parameters (k, ΔG(‡)) of switching were determined at different temperatures to provide activation enthalpies (ΔH(‡)) and entropies (ΔS(‡)). Longer glycol linkers led to modest increases in the forward escape (t(1/2) = 60 to <7 ms). The rate-limiting step involves movement of the tetracationic CBPQT(4+) ring away from the singly oxidized TTF(+) unit by overcoming one of the thiomethyl (SMe) speed bumps before proceeding on to the secondary oxyphenylene station. Upon reduction, however, the return translational movement of the CBPQT(4+) ring from the oxyphenylene station back to the neutral TTF station was slowed considerably by the longer linkers (t(1/2) = 1.4 to >69 s); though not because of a diffusive walk. The reduced rate of motion backward depended on folded structures that were only present with longer linkers.

An active molecular plasmonics system is demonstrated where a supramolecular chromophore generated in a host-guest binding event couples with the localized surface plasmon resonance (LSPR) arising from gold nanodisc gratings. This coupling was achieved by wavelength-matching the chromophore and the LSPR with the laser excitation, thus giving rise to surface-enhanced resonance Raman scattering (SERRS). The chromophore is a broad charge-transfer (CT) band centered at 865 nm (epsilon = 3500 M(-1) cm(-1)) generated by the complexation of cyclobis(paraquat-p-phenylene) (CBPQT(4+)) and the guest molecule tetrathiafulvalene (TTF). The substrates consist of sub-1-microm gold nanodisc arrays which display dimension-tunable plasmon wavelengths (600-1000 nm). The vibrational spectra of the complex arising from SERRS (lambda(exc) = 785 nm) were generated by irradiating an array (lambda(LSPR) = 765 nm) through the solution to give a chromophore-specific signature with the intensities surface enhanced by approximately 10(5). Surface adsorption of the empty and complexed CBPQT(4+) is also implicated in bringing the chromophore into the electric field arising from the surface-localized plasmon. In a titration experiment, the SERRS effect was then used to verify the role of resonance in turning on the spectrum and to accurately quantify the binding between surface-adsorbed CBPQT(4+) and TTF. The use of a nonpatterned gold substrate as well as a color mismatched complex did not show the enhancement, thus validating that spectral overlap between the chromophore and plasmon resonance is key for resonance surface enhancement. Simulations of the electric fields of the arrays are consistent with interdisc plasmon coupling and the observed enhancement factors. The creation of a responsive plasmonic device upon the addition of the guest molecule and the subsequent coupling of the CT chromophore to the plasmon presents favorable opportunities for applications in molecular sensing and active molecular plasmonics.