Electrochemical oxidation, memory

Matsumura and co-workers reported a memory effect of chirality in the electrochemical oxidation of 95 to give 96, although the enantioselectivity was modest (Scheme 3.25). The reaction is assumed to proceed via carbenium ion intermediate Q.46 The mechanism for asymmetric induction is not clear. A possible mechanism involves chiral acid (95)-mediated deracemization of racemic 96 produced by the electrochemical oxidation of 95. However, this suggestion may be eliminated based on the finding that treatment of racemic 96 with 95 in methanol containing 5% formic acid did not produce optically active 96. 

Eurukawa and co-workers [81] state that PANI is an interesting material because it is not only an ECP but is also a good material to use as an electrode of a secondary battery with aqueous or non-aqueous electrolytes. PANI polymerised from aniline in an aqueous acid solution is converted to several forms with different electrical properties by acid/base treatments and oxidation/reduction. The as-polymerised form gives high electrical conductivity ( 5 S/cm). It becomes insulating when treated with an aqueous alkaline solution or is reduced electrochemically in an aqneons acid solution. Reduced-alkali-treated PANI is also insulating and is unstable in air its colour changes from white to blue upon exposure to air. PANI doped with electrolyte anions is obtained by electrochemical oxidation [82]. It was found in this work to be a new conductivity form (o = 5.8 S/cm). Recently, a secondary lithium battery with a reduced alkali pellet as the cathode, and non-aqueous electrolytes has been developed as a power source of memory back up and a maintenance-free power source combined with a solar battery. 

In the memory of chirality via carbenium ion chemistry, we reported that when N-o-phenylbenzoylated oxazoline and thiazoUne derivatives were electrochemically oxidized, optically active products (83 % and 91 % enantiomeric excess (ee), respecfively) were obtained (Scheme 3) [4, 5]. 

Wanyoike GN, Matsumura Y, Kuriyama M, Onomura O (2010) Memory of chirality in the electrochemical oxidation of thiazolidine-4-carboxylic acid derivatives. Heterocycles 80 1177-1185. doi 10.3987/COM-09-S(S)101... 

Launay et al have shown that following electrochemical oxidation, many DTE systems undergo a thermal isomerisation from one photoisomer to the cation radical of the other isomeric form. This opens up the possibility of electrochromism with memory via thermal transformations of the oxidised switch, as a reduction step does not return the molecule to its initial state but to its photoisomer. The electronic properties of 5-thienyl susbstituents on the DTE switch does exert a degree of control over these characteristics because these groups can modify the oxidation potential of the dithienyl moiety and therefore provide the trigger required to unlock the thermally-derived forms (Scheme 12). 

A very important electrochemical phenomenon, which is not well understood, is the so-called memory effect. This means that the charging/discharging response of a conducting polymer film depends on the history of previous electrochemical events. Thus, the first voltammetric cycle obtained after the electroactive film has been held in its neutral state differs markedly in shape and peak position from subsequent ones [126]. Obviously, the waiting time in the neutral state of the system is the main factor determining the extent of a relaxation process. During this waiting time, which extends over several decades of time (1-10 s), the polymer slowly relaxes into an equilibrium state. (Fig. 13) After relaxation, the first oxidation wave of the polymer appears at more... 

Fig. 5. A Schematic representation of a locking molecular memory. B A locking molecular memory based on photochemical switching and electrochemical locking. C Cyclic voltammograms of the electrochemically inactive open -state 8 and the electrochemically active closed -state 9, which can be reversibly oxidized to the quinoid locked -state 10. Recorded in THF with Bu4NC104 (0.1 M) and a potential scan rate of 100 mV s-1. D Absorbance spectra of the three states of the lockable molecular memory system. C and D are adapted from [25] with permission...

The retardation observed in the oxidation process when the polymer was previously polarized at high cathodic potentials for long periods of time, reported as a memory effect by Villeret and Nechtschein [174], was partially quantified by Oden and Nechtschein [167,168,175]. A complete description of these memory effects, based on the electrochemically stimulated conformational relaxation (ESCR) model, has been provided by Otero et al. [176-178]. The knowledge and control of those conformational relaxation processes are essential from a technological point of view. 

Types of Electrochromic Materials. Electrochromic materials are of three basic types (2). In a given electrolyte solution, type I materials are soluble in both the reduced and oxidized (redox) states. Type II materials are soluble in one redox state, but form a solid film on the surface of an electrode following electron transfer. Electrochromic polymers are examples of type III materials, where both redox states are solids, generally studied as thin films on electrode surfaces. For types II and III, once the redox state has been switched, no further charge injection is needed to retain the new electrochromic state, and such systems are said to have optical memory . In contrast, for type I electrochromic materials, diffusion of the soluble electrochemically-generated product material away from the electrode occurs and it is necessary to keep current flowing until the whole solution has been electrolyzed. 

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