Bistable redox switching

The redox-controlled mechanical switching in SAMs of disulfide-functionalized bistable TTF-DMN rotaxanes consisting of cyclophane 124+ and a dumbbell-shaped component containing TTF and DMN stations was also extensively investigated.49... 

A Redox and Chemically Controllable Bistable Neutral [2]Rotaxane 8.4.3.1 Electrochemical Switching... 

Non-periodic oscillations have been observed in the above system in CSTR. The visual oscillations are accompanied by oscillations in the redox potential and I. Oscillations are inhibited by lOj, Cl and NaHC03. These depend on [IO3] [arsenite] and the reciprocal of the residence time. There is an upper limit and a lower limit of [IO3 ] as well as [H3ASO3] in between which oscillations are observed. It is found that these occur in a narrow range of flow rate which is close to the region when the system can switch from the monostable state to a state of bistability [21(b)]. However, for periodic oscillations, different strategies were adopted by Epstein and co-workers [12]. 

Early in 1994, Kaifer, Stoddart, and coworkers reported the first bistable molecular shuttle (Figure 70) based on a [2]rotaxane driven by chemical and electrochemical stimuli. The [2]rotaxane comprised a CBPQ U+ ring threaded by an axle consisting of two binding sites, benzidine and biphenol units. Under redox reactions and the addition of acid or base, the CBPQT + ring moves back and forth along the axle. This molecnlar shuttle can be switched by two different mechanisms and is a good candidate for the construction of complex molecular machines. 

Stoddart et al. also incorporated a bistable [2]catenane (Figure 39) into macromolecules to form a side-chain poly[2]catenane (Figure 82). This poly[2]catenane could also behave as a molecular switch that can be addressed in an on or off state electrochemically by the virtue of the redox properties of the TTF moiety. Furthermore, the switching properties remain even in spherical aggregates. This stndy provides a good mechanically interlocked switchable polymeric scaffold for the construction of solid-state molecular electronic devices. 

For both rotaxane and catenane-based molecular machines, it is desirable to have recognition sites such that they can be easily controlled externally. Hence, it is preferable to build sites that are either redox-active or photo-active [144]. Catenanes can also be self-assembled [157]. An example of catenane assembled molecular motors is the electronically controllable bistable switch [158]. An intuitive way of looking at catenanes is to think of them as molecular equivalents of ball and socket and universal joints [153,159,160]. 

In contrast to the foregoing electrochromic solids, polymers especially have attracted interest since they eliminate the need for liquid electrolytes and offer bistable switching states. The electrochromic layer switches reversibly between an oxidized and a reduced state. This redox reaction is charge balanced by the release and incorporation of counter anions, which are either ionically bound to the electrochromic solid or exhibit a low mobility. Cations, usually lithium ions, therefore, have to move through the polymeric electrolyte to counteract the electron current. A general scheme of the redox reaction and layers employed is depicted in Figure 10.40 and Figure 10.41. 

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