Switching mechanism, counterions

K. Martfnez-Mayorga, M. C. Pitman, A. Grossfield, S. E. Feller, and M. F. Brown,/. Am. Chem. Soc., 128, 16502 (2006). Retinal Counterion Switch Mechanism in Vision Evaluated by Molecular Simulations. 

A rather neat example of the manipulation of film electroneutrality maintenance mechanism is provided by the redox behavior of poly(xylylviologen) (PXV) films, in which the nominal counterion is poly(styrenesulfonate) (PSS). In this case, the counterion is effectively immobile, and the question arises as to whether the PXV and PSS ion charges compensate each other or are separately compensated by (other) electrolyte ions. This will clearly have implications for the possible sources and sinks of ionic charge available to satisfy electroneutrality upon PXV redox switching, and the EQCM is ideally placed to make the distinction. It was found that... 

In an effort to produce polymer films with improved mechanical properties in aqueous solutions, pyrrole was electrochemically polymerized in the presence of surfactants such as sodium dodecylsulfate and sodium dodecylbenzenesulfonate. In the case of dodecylsulfate anion, the polymers switched between a transmissive yellow in the neutral state to violet when partially oxidized and brown when fully oxidized [97]. When electropolymerized in the presence of dodecylbenzenesulfonate anion, the polymer films switched between a transmissive yellow when neutral to dark blue when oxidized [98]. In both cases, the polymers showed an improved electrochemical stability over polymer films produced with other inorganic counterions. This has been attributed to the binding of the surfactant dopant within the film during charge compensation since there is less mechanical distortion of the polymer film. 

Redox processes of conducting polymers combined with the doping of counterions can affect their features directly to switch the chemical, optical, electrical, magnetic, mechanical, and ionic properties [6]. The following applications are representative ones. 

Like rotaxanes, catenanes are mechanically interlocked molecules. However, instead of interlocking one ring shaped macrocycle and a dumbbell shape, catenanes consist of interlocked macrocycles. The number of macrocycles contained in a catenane is indicated by the numeral that precedes it. Catenanes have bistable and multistable forms and a switchable, bistable [2]catenane is commonly exploited in nanotechnology and molecular electronics because its behavior can be controlled by electrochemical processes [89]. Collier et al. was the first to demonstrate the electroactivity of interlocked catenanes [90]. The authors affixed phospholipid counterions to a monolayer of [2]catenanes and then sandwiched this system between two electrodes. This work resulted in a molecular switching device that opened at a positive potential of 2 V and closed at a negative potential of 2 V. 

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