title: Radically enhanced molecular switches.
abstract: The mechanism governing the redox-stimulated switching behavior of a tristable [2]rotaxane consisting of a cyclobis(paraquat-p-phenylene) (CBPQT(4+)) ring encircling a dumbbell, containing tetrathiafulvalene (TTF) and 1,5-dioxynaphthalene (DNP) recognition units which are separated from each other along a polyether chain carrying 2,6-diisopropylphenyl stoppers by a 4,4'-bipyridinium (BIPY(2+)) unit, is described. The BIPY(2+) unit acts to increase the lifetime of the metastable state coconformation (MSCC) significantly by restricting the shuttling motion of the CBPQT(4+) ring to such an extent that the MSCC can be isolated in the solid state and is stable for weeks on end. As controls, the redox-induced mechanism of switching of two bistable [2]rotaxanes and one bistable [2]catenane composed of CBPQT(4+) rings encircling dumbbells or macrocyclic polyethers, respectively, that contain a BIPY(2+) unit with either a TTF or DNP unit, is investigated. Variable scan-rate cyclic voltammetry and digital simulations of the tristable and bistable [2]rotaxanes and [2]catenane reveal a mechanism which involves a bisradical state coconformation (BRCC) in which only one of the BIPY(•+) units in the CBPQT(2(•+)) ring is oxidized to the BIPY(2+) dication. This observation of the BRCC was further confirmed by theoretical calculations as well as by X-ray crystallography of the [2]catenane in its bisradical tetracationic redox state. It is evident that the incorporation of a kinetic barrier between the donor recognition units in the tristable [2]rotaxane can prolong the lifetime and stability of the MSCC, an observation which augurs well for the development of nonvolatile molecular flash memory devices.
authors.affiliation: Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA.
authors.forename: Albert C
authors.initials: AC
authors.lastname: Fahrenbach
authors.forename: Zhixue
authors.initials: Z
authors.lastname: Zhu
authors.forename: Dennis
authors.initials: D
authors.lastname: Cao
authors.forename: Wei-Guang
authors.initials: WG
authors.lastname: Liu
authors.forename: Hao
authors.initials: H
authors.lastname: Li
authors.forename: Sanjeev K
authors.initials: SK
authors.lastname: Dey
authors.forename: Subhadeep
authors.initials: S
authors.lastname: Basu
authors.forename: Ali
authors.initials: A
authors.lastname: Trabolsi
authors.forename: Youssry Y
authors.initials: YY
authors.lastname: Botros
authors.forename: William A
authors.initials: WA
authors.lastname: Goddard
authors.forename: J Fraser
authors.initials: JF
authors.lastname: Stoddart