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.
Highlights
The drive to achieve the miniaturization of electronic devices beyond the limits that “top-down” conventional lithographic techniques can provide has led1 to the research and development of an array of “bottom-up” protocols
One approach invented and implemented by Hawker et al.2 relies on the use of nanoscale lithographic templates made possible by the self-assembly of block copolymers into highly ordered hexagonal and rectilinear arrays. Another approach created and developed by Mirkin et al.3 relies on the use of atomic force microscopy (AFM) tips within the context of what has become known as dip-pen nanolithography (DPN), which when used as massively parallel arrays of “ink”-coated tips can deliver soft organic substrates onto hard surfaces with remarkable degrees of order and complexity
Fujita et al.5b have demonstrated the use of self-assembled nanocages in between a gold substrate and a scanning tunneling microscopy (STM) tip, which act as hosts for a relatively small number of π-stacked organic guests
Summary
The drive to achieve the miniaturization of electronic devices beyond the limits that “top-down” conventional lithographic techniques can provide has led to the research and development of an array of “bottom-up” protocols. One approach invented and implemented by Hawker et al. relies on the use of nanoscale lithographic templates made possible by the self-assembly of block copolymers into highly ordered hexagonal and rectilinear arrays. Another approach created and developed by Mirkin et al. relies on the use of atomic force microscopy (AFM) tips within the context of what has become known as dip-pen nanolithography (DPN), which when used as massively parallel arrays of “ink”-coated tips can deliver soft organic substrates onto hard surfaces with remarkable degrees of order and complexity. In collaboration with Heath, we have relied on fabrication of nanoelectronic devices with two-
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