Poly auto-doping

Ikenoue et al. were the first to offer an explanation of auto-doping [11]. They observed that poly(o>-(3-thienyl)alkanesulfonic acids) are extremely hygroscopic and thus films of the polymer contain moisture. In the presence of moisture, the acidic proton is solvated by water molecules. When water is removed by heating, the next most basic unit able to coordinate acidic protons is the polythiophene backbone itself as shown in Figure 20.11. Protonation of the backbone creates a polaronic residue on the polymer chain. This phenomenon is obviously not observed for the sodium salt, and hence the sulfonic acid form of the polymers exhibits a higher conductivity in its electrochemically neutral state than the corresponding sodium salt. 

The concept of acid-doping appears to be general [13-15]. Indeed, NMR analysis on oligothiophenes shows that protonation occurs at the a-position of the thienyl ring to yield species which have absorption spectra similar to auto-doped poly(o)-(3-thienyl)alk-anesulfonic acids) [16] (Figure 20.12). 

In the sulfonic acid form, poly( o-(3-thienyl)alkane-sulfonates) are auto-doped and exhibit conductivities of 5 X 10 —10 S cm [9]. The sulfonic acid derivatives prossessed considerable stability and exhibited an enhanced lifetime in their conductive state compared to conductive films obtained by oxidative doping using inorganic oxidants. Auto-doped materials can be further oxidized with oxidizing agents to increase their conductivity by an order of magnitude but the stability of the oxidized form is considerably diminished. 

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