Charge-transfer complexes with various organic

Fig. 6.37 Modulation of initial work function of various polymers and their response to formation of charge-transfer complexes with organic vapors (adapted from Josowicz, 1988)...

Charge-transfer complexes of variously functionalized, both electron-rich and electron-poor, [3]radialenes have attracted attention because of their magnetic properties [7] and as potential organic metals [17, 18]. Most of the [3]radialenes reported so far have been studied with respect to their oxidation and reduction chemistry (see the reviews [7, 8] and some examples mentioned in this chapter). With appropriate substitution patterns, the range of redox properties extends from the most electron-rich [3]radialene 31 to the very electron-poor radialene 24a, a strong oxidant [21]. The first one-electron reduction step of 18 (X = Y = CN) [16], 19, and 20 [17] occurs at potentials similar to TCNQ, and at potentials similar to chloranil for other derivatives of 18 (Scheme 4.3). 

Various redox compounds that fulfil catalyst characteristics have been investigated in systems with recycling of NAD by electrocatalytic methods. Quinones, formed either by oxidation of carbon surfaces [143, 145] or adsorbed to the electrode surface [146, 147], phenazines [148, 149], phenoxazine derivatives such as Meldola Blue [182], 9-naphthoyl-Nile Blue [151, 152] and l,2-benzophenoxazine-7-one [153], and also the organic conducting salt N-methyl phenazinium tetracyanoquinodimethanide (TTF TCNQ") [154, 155], ferricinium ions [156, 157] and hexacyanoferrat(IIl) ions [158, 159] can act as catalysts for the electrochemical oxidation of NADH. It is assumed that in corresponding electron-transfer reactions a charge-transfer complex between the immobilized mediator and NADH is formed. The intermediate reduced redox mediator will be reoxidized electrochemically. Most systems mentioned, however, suffer from poor electrode stabilities. 

Adsorption of pyridine, a rigid molecule able to attain various orientations with respect to the metal surface, evidently remains the most studied process of adsorption of organic molecules at the electrodes. Pyridine replaces water molecules at the electrode surface the number of removed H2O molecules depends on the orientation of pyridine molecules with respect to the electrode surface. This process is associated with the partial charge transfer across the electrode surface. Using the SERS method, Ma and Wu have studied the pyridine-iodine charge-transfer complex at the Ag electrode [152]. Yang et al. 

Finally it should be remembered that all electrically conducting organic materials are due to the various dopant charge-transfer complexes, and coming back to the roots—in the 60s—polymer CT-complexes are found to be electrically conducting materials with up to lO S/cm [6]. 

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