ABSTRACT
It would be an oversimplification to assume that these simple coupling and electrontransfer reactions reflect all steps occurring in front of the electrode. In addition, parallel reactions such as homogeneous comproportionations (D2+ + M ~ M+ + o+·) autocatalytically produce radical cations of the lower oligomers [34]. Other possible side reactions are, for example, (reversible) dimerizations without elimination of protons or ,8-linkage reactions. Scheme 4 provides a corresponding representation of the oligomerization reactions in solution. Thus, Cava et al. could show that during oligomerization in solution branched ,B, ,B-coupling steps compete with the linear a, a-coupling [35]. In addition, their results reveal that the elimination of protons after the couipling of the radical cations may be slow. In this case, a charged a-complex exists as a short-lived intermediate. In the case of the pyrrole oligomerization (dimer~tetramer), the deprotonation step is significantly slower than the radical~radical coupling step [28a], indicating that the stability of the a-dimer increases as a function of chain length. Very recent data provide evidence that the positively charged a-dimer of the thiophene octamer does not undergo a deprotonation step as long as the electrode potential does not exceed the redox potential of the octathiophene dication.
