Radialene with DT Rings

The cyclopentanone analogs 32c-e show two pairs of two-electron redox waves corresponding to the redox processes of the four DT groups in the CV (Table 8.6). The first redox potentials of 32c-e are found to be lower than those of the corresponding [4]radialenes 30c-e (Table 8.5) despite the insertion of 

30c - e. Taking into account of the fact that antiaromaticity of cyclopentadienone and cyclobutadiene of the central ring contribute to the teracationic species of and 32, respectively, the lowering of the redox potentials of 41 can be attributed to the lower antiaromaticity of cyclopentadienenone compared to that of cyclobutadiene. 

In contrast to this behavior, [5]radialenes 33b and 33e exhibit notable redox processes, that is, a four-electron transfer occurring simultaneously. Detailed digital simulation analysis assuming Nernstian response in 33e led to the conclusion that the apparent one-step four-electron redox wave involved two redox couples of 33e/33e and 33 33e estimated at —0.074 and —0.066 V (vs Fc/Fc ), respectively. In fact, the CV of 33a having two electron-donating methyl groups clearly exhibits two pairs of two-electron-reversible redox waves. These results suggest that [5]radialenes are subject to a two-step two-electron transfer and sequentially converted into the dication 33 and subsequently into the tetracation 33.  

The detailed electronic structure was elucidated by spectroelectrochemistry. The absorption bands in electronic spectra of 33a and 33e are basically similar. The absorption of 33a at longer wavelength = 993 nm), however, appears more clearly than that in 33e = 883 nm). The spectrum of 33a  

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