Electrochemical overoxidation

Figure 4.13 illustrates several techniques for patterning PEDOT PSS via electrochemical overoxidation. Conversion of a simple plotter pen or a screen-printing station are relatively straightforward, the latter allowing print resolution of a few hundred micrometers. To demonstrate that the technique is limited only by the resolution of the printing method, 2 p,m lines (the smallest mask available) were patterned in a 200 nm thick film with the aid of a photoresist mask [29]. 

FIGURE 4.13 Electrochemical overoxidation techniques for use with polythiophenes, (a) A plotter pen modified to be used in an electrochemical cell, (b) Screen-printing electrochemical cell, (c) Photoresist patterned on a PEDOT PSS film protects areas of the film from overoxidation whereas the rest is patterned when wet by the liquid electrolyte. (Images from Tehrani, P., Remonen, T., Hennerdal, L.-O., Hall, J., Malmstrom, A., Leenders, L., Kugler, T., Robinson, N.D., Crispin, X., Fahlman, M., and Berggren, M., Smart Mater Struct 14, N21-N25. Copyright 2005, Institute of Physics Publishing. With permission.)... 

Tehrani, R, N.D. Robinson, T. Kugler, T. Remonen, L.-O. Hennerdal, J. Hall, A. Malmstrom, L. Leenders, and M. Berggren. 2005. Patterning polythiophene films using electrochemical overoxidation. Smart Mater Struct 14 N21-N25. 

A.. Zykwinska, W. Domagala, B. Pilawa, and M. Lapkowski. 2005. Electrochemical overoxidation of poly(3,4-ethylenedioxythiophene)— PEEXDT studied by means of in situ ESR spectroelectrochemistry. Electrochim Acta 50(7-8) 1625-1633. 

These facts are different demonstrations of the same event degradation reactions occur simultaneously with electropolymerization.49-59 These reactions had also been called overoxidation in the literature. The concept is well established in polymer science and consists of those reactions between the pristine polymer and the ambient that promote a deterioration of the original polymeric properties. The electrochemical consequence of a strong degradation is a passivation of the film through a decrease in the electrical conductivity that allows a lower current flow at the same potential than the pristine and nondegraded polymer film did. Passivation is also a well-established concept in the electrochemistry of oxide films or electropainting. 

Reactions with other nucleophiles follow a similar mechanism. For the reaction of Cl with poly(3-methylthiophene) in acetonitrile, the reaction stops at structure 5 (Scheme 2).128 A fully conjugated, Cl-substi-tuted product 6 can subsequently be obtained by electrochemical or chemical dehydrogenation.128 With Br and alcohols, the overoxidation... 

Hie electrochemical characteristics of overoxidation vary widely among polymers, solvents, and nucleophiles.129 Its rate depends on the degree of oxidation of the polymer (and therefore on the potential applied), and the concentration127 and reactivity of the nucleophile. Polypyrroles usually become overoxidized at lower potentials than polythiophenes because of their lower formal potentials for p-doping. In acetonitrile, the reactivity of the halides follows their nucleophilicity in aprotic solvents,... 

Electrochemical measurements on polyaniline (PANI) produce a picture of the charge storage mechanism of conducting polymers which differs fundamentally from that obtained using PTh or PPy. In the cyclic voltammetric experiment one observes at least two reversible waves in the potential range between —0.2 and -)-1.23 V vs SCE. Above -1-1.0 V the charging current tends to zero. Capacitive currents and overoxidation effects, as with PPy and PTh, do not occur The striking... 

Figure 3.80 Values of n, k and thickness L obtained t ia three parameter fits to the A, T and intensity data obtained during the growth of a polypyrrole film on a sputtered Pt electrode in N2-saturated I M NaCIO4/0,l M pyrrole. The potential was stepped from OV to 0.8 V vs, SCE for 15 s, and readings taken every 20 ms. Reprinted from tkarochimica Acia, 36. P,A. Christensen and A. Hamnett, In situ Spectroscopic Investigations of the Growth, Electrochemical Cycling and Overoxidation of Polypyrrolc in Aqueous Solution, pp. 1263-1286 (1991), with kind permission from Pergamon Press Ltd., Headington Hill Hall. Oxford 0X3 OBW, UK.

S. Descroix and F. Bedioui, Evaluation of the selectivity of overoxidized polypyrrole/superoxide dismutase based microsensor for the electrochemical measurement of superoxide anion in solution. 

Selective dehalogenation of halopyridines is an important industrial process for the same reason that reduction of carboxylic acids, esters, amides, and nitriles are also important. There is a dearth of selective oxidation technologies whether by conventional or electrochemical methods. Therefore, many intermediate oxidation stage products are made by overoxidation, i.e., overhalogenation, followed by selective reduction. 

C-H transformation of alkanes by SET is still a developing area of preparative organic chemistry. Generation of cr-radical cations from alkanes in solution requires strong oxidants, and is achieved by photochemical and electrochemical oxidation. Under these conditions even unstrained strained alkanes may be functionalized readily. The C-H substitution is selective if the hydrocarbon forms a radical cation with a definite structure and/or deprotonation from a certain C-H position of the radical cation dominates. Overoxidations are the most typical side reactions that lead to disubstituted alkanes. This can usually be avoided by running the reactions at low alkane conversions. 

These remarks must be balanced by some characteristic difficulties of using the electrochemical path. Sometimes, and in spite of tight potential control, two or more reactions take place at the same time and give not one product but a mixture. Correspondingly, overoxidation may occur the intended oxidation may continue to a further step by means of a chemical driving force outside the control of the potentiostat. 

Shi et al. [70] were the first to demonstrate the use of an air and moisture stable ionic liquid, [C4mim][PF,s], for the electrochemical synthesis of poly(thiophene), grown onto a platinum working electrode by potentiodynamic, constant potential or constant current techniques. The use of growth potentials between 1.7 and 1.9 V (vs. Ag/AgCl) reportedly gave smooth, blue-green electroactive films, whereas potentials above 2 V resulted in film destruction by overoxidation. 

The constructive and desired pathway towards the product competes with the electrochemical incineration. At high current densities the mineralization dominates. Therefore, lower current densities will be beneficial for a synthetic and nondestructive transformation. The compartment of electrochemical transformation caused by hydroxyl or methoxyl radicals can be estimated in the range of a few micrometers close to the BDD anode. Mass transport has to be efficient since the migration of products out of the electrochemical scene into bulk is crucial for avoiding the overoxidation. Control of both competing and critical processes will either cause failure (mineralization) or provide the opportunity for selective electroorganic synthesis (Fig. 2). 

Using electron spin resonance (ESR) spectroelectrochemistry, the effects of overoxidation on the properties of the polymer 160 were studied <2006MI2135>. Upon traversing of the potential boundary of electrochemical stability, a sharp drop in the number of free spins in the polymer was observed together with the changes in spectroscopic properties. 

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