Indole-5-carboxylic acid films

We have previously considered [8,9] the redox behaviour and film structure of indole-5-carboxylic acid trimer (ICA trimer) films formed by the electrooxidation of indole-5-carboxylic acid (ICA, Fig. 11.2). We have shown that electrooxidation involves the production and deposition of an asymmetric cyclic trimer (Fig. 11.3) onto the electrode surface 

We propose that this trimer forms from coupling of the 3,3 -dimer radical cation and a monomer radical cation. (The electron density in the monomer radical cation is largest at the 3-position. The electron density in the dimer radical cation is largest in the 2-position.) The trimer is initially formed near the electrode surface, where it deposits onto the electrode. Further growth then occurs by adsorption and cyclisation on the surface 

Ring-disc studies have confirmed [11] that for ICA between 7 and 9 electrons are passed and between 6 and 8 protons are evolved in the production of these films. This shows that the trimer is deposited in its oxidised form, with the incorporation of an anion from the electrolyte in order to preserve electroneutrality. 

ICA trimer films were produced by the electropolymerisation of 100 mM ICA in background electrolyte at + 1.46 V at the RDE, rotating at 4 Hz. Under these conditions [8,9], electrooxidation and film production occurs under steady-state current conditions at close to 100% current efficiency, and the film is expected to consist almost entirely of free trimer. The electrooxidation reaction was terminated after passing a charge of 173 mC. From equation (11.1) and (11.23) we calculate that this corresponds to 25 mC of redox charge or approximately 250 nmoles of deposited trimer. 

On cycling, peaks start to appear (Fig. 11.4(b)), and after several days of repeated oxidation and reduction, redox peaks dominate the vol-tammogram (Fig. 11.4(c)). That these redox peaks involve cation insertion and ejection can be demonstrated by immersing this conditioned film in 0.1 M NaCI04. The resulting LSV data (Fig. 11.4(d)) show an almost complete loss of redox activity. 

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Although much less so than pyrrole polymers, indole polymers are beginning to be synthesized and studied as new materials. Electropolymerized films of indole-5-carboxylic acid are well-suited for the fabrication of micro pH sensors and they have been used to measure ascorbate and NADH levels. The three novel pyrroloindoles shown have been electrochemically polymerized, and the polymeric pyrrolocarbazole has similar physical properties to polyaniline. 

A conducting, polymeric film of poly(indole-5-carboxylic acid) has been employed for covalent immobilization of tyrosinase, which retains catalytic activity and catalyzes oxidation of catechol to the quinone <2006MI41>. Poly(l-vinylpyrrole), polyfl-vinylindole), and some methyl-substituted compounds of poly(l-vinylindole) are of potential interest as photorefractive materials with a relatively low glass-transition temperature and requiring a lower quantity of plasticizer in the final photorefractive blend <2001MI253>. Polymers of 5,6-dihydroxyindoles fall within the peculiar class of pigments known as eumelanins and their chemistry has been reviewed <2005AHC(89)1>. 

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