Mechanistic Evaluation of Motion in Redox-Driven Rotaxanes Reveals Longer Linkers Hasten Forward Escapes and Hinder Backward Translations

Abstract

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.

Probing the Electrostatic Barrier of Tetrathiafulvalene Dications using a Tetra‐stable Donor–Acceptor [2]Rotaxane

“…Herein, we describe the synthesis and characterization of a tetra‐stable [2]rotaxane 1 ⋅4PF 6 (Figure ) in which the dumbbell contains four potential stations for CBPQT 4+ , namely an oxyphenylene (OP), a TTF, a monopyrrolo‐TTF (MPTTF), and a HQ station. The TTF and the MPTTF stations are located in the middle of the dumbbell and are connected by a triethylene glycol (TEG) chain in such a way that the pyrrole moiety of the MPTTF unit points toward the TTF station .…”

Wholly Synthetic Molecular Machines

“…Hence,r edox-responsive units, such as viologens, [71][72][73] tetrathiafulvalene derivatives, [74][75][76] ferrocenes, [77][78][79] naphthalened iimides, [80,81] and so forth, have been incorporated into av ariety of molecular machines. Hence,r edox-responsive units, such as viologens, [71][72][73] tetrathiafulvalene derivatives, [74][75][76] ferrocenes, [77][78][79] naphthalened iimides, [80,81] and so forth, have been incorporated into av ariety of molecular machines.…”

“…As an alternative to developing complicated waste-management circulatory systems, chemists have turned their attention, in the short term at least, to reactions that involvee ither only electron transfers or configurationali somerizationst hat avoid the generation of waste. Hence,r edox-responsive units, such as viologens, [71][72][73] tetrathiafulvalene derivatives, [74][75][76] ferrocenes, [77][78][79] naphthalened iimides, [80,81] and so forth, have been incorporated into av ariety of molecular machines. Cyclobis-(paraquat-p-phenylene)( CBPQT 4 + )f orms interpenetrating trisradicalt riscationic complexes with viologens under reducing conditions [82,83] that dissociate with the aid of strong Coulombic repulsion on subsequento xidation ( Figure 3b).…”

Towards Redox‐Driven Unidirectional Molecular Motion

“…[3] The catenane offers the possibility of both tetra-and penta-coordination, favoring coordination by two phenanthroline moietiesi nt he copper(I) state. [5] An alternative approacht or edox switching is to mimic the structuralc hanges observed during photochromic switching, [6] as shown for dithienylethene-baseds witches, in whichr ing opening or closing can be driven by oxidation. [4] Indeed,t he major effort over the last decades in redox-driven molecular motion have focused on rotaxane-based systems.…”

“…[4] Indeed,t he major effort over the last decades in redox-driven molecular motion have focused on rotaxane-based systems. [5] An alternative approacht or edox switching is to mimic the structuralc hanges observed during photochromic switching, [6] as shown for dithienylethene-baseds witches, in whichr ing opening or closing can be driven by oxidation. [7,8] The immobilization of dithienylethenes on conducting surfaces has also enabledr edox-driven bidirectional switching.…”