title: Functionally rigid bistable [2]rotaxanes.
abstract: Two-station [2]rotaxanes in the shape of a degenerate naphthalene (NP) shuttle and a nondegenerate monopyrrolotetrathiafulvalene (MPTTF)/NP redox-controllable switch have been synthesized and characterized in solution. Their dumbbell-shaped components are composed of polyether chains interrupted along their lengths by (i) two pi-electron-rich stations-two NP moieties or a MPTTF unit and a NP moiety-with (ii) a rigid arylethynyl or butadiynyl spacer situated between the two stations and terminated by (iii) flexibly tethered hydrophobic stoppers at each end of the dumbbells. This modification was investigated as a means to simplify both molecular structure and switching function previously observed in related bistable [2]rotaxanes with flexible spacers between their stations and incorporating a cyclobis(paraquat-p-phenylene) (CBPQT4+) ring. The nondegenerate MPTTF-NP switch was isolated as near isomer-free bistable [2]rotaxane. Utilization of MPTTF removes the cis/trans isomerization that characterizes the tetrathiafulvalene (TTF) parent core structure. Furthermore, only one translational isomer is observed (> 95 < 5), surprisingly across a wide temperature range (198-323 K), meaning that the CBPQT4+ ring component resides, to all intents and purposes, predominantly on the MPTTF unit in the ground state. As a consequence of these two effects, the assignment of NMR and UV-vis data is more simplified as compared to previous donor-acceptor bistable [2]rotaxanes. This development has not only allowed for much better control over the position of the ring component in the ground state but also for control over the location of the CBPQT4+ ring during solution-state switching experiments, triggered either chemically (1H NMR) or electrochemically (cyclic voltammetry). In this instance, the use of the rigid spacer defines an unambiguous distance of 1.5 nm over which the ring moves between the MPTTF and NP units. The degenerate NP/NP [2]rotaxane was used to investigate the shuttling barrier by dynamic 1H NMR spectroscopy for the movement of the CBPQT4+ ring across the new rigid spacer. It is evident from these measurements that the rigid spacer poses a much lower barrier to the 1.0 nm movement of the CBPQT4+ ring from one station to another as compared with previous systems-a finding that is thought to be a result of the combination of fewer favorable interactions between the spacer and the CBPQT4+ ring and a relatively unimpeded path between the two NP stations. This example augers well for exploiting rigidity during the development of well-defined bistable [2]rotaxanes, which are unencumbered by the excesses of structural conformations that have characterized the first generations of molecular switches based on the donor-acceptor recognition motif.
authors.affiliation: Department of Physics and Chemistry, University of Southern Denmark (Odense University), Campusvej 55, DK-5230, Odense M, Denmark.
authors.forename: Sune
authors.initials: S
authors.lastname: Nygaard
authors.forename: Ken C-F
authors.initials: KC
authors.lastname: Leung
authors.forename: Ivan
authors.initials: I
authors.lastname: Aprahamian
authors.forename: Taichi
authors.initials: T
authors.lastname: Ikeda
authors.forename: Sourav
authors.initials: S
authors.lastname: Saha
authors.forename: Bo W
authors.initials: BW
authors.lastname: Laursen
authors.forename: Soo-Young
authors.initials: SY
authors.lastname: Kim
authors.forename: Stinne W
authors.initials: SW
authors.lastname: Hansen
authors.forename: Paul C
authors.initials: PC
authors.lastname: Stein
authors.forename: Amar H
authors.initials: AH
authors.lastname: Flood
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart
authors.forename: Jan O
authors.initials: JO
authors.lastname: Jeppesen