Radiaannulene

Cross-Conjugated Systems in Acetylene-Extended Radiaannulene Frameworks... 

In recent years, several groups have reported on a series of radiaannulenes incorporating TTF or DT rings. Radiaannulenes are hybrid frameworks composed of a cross-conjugated radialene and a hnearly conjugated dehydroannulene [78-81]. These compounds possess a so-called prearomaticity that is, the ability of the compound to form aromatized mesomeric 14jt-electron system as the result of oxidation or reduction (Scheme 8.12). 

Zhao and coworkers [82,83] have also reported on the dibenzo-annulated radiaannulene compounds 87 and 88 containing DT rings at their terminal positions (Figure 8.25). From the X-ray crystallographic analyses, the central radiaannulenes units in both 87 and 88 adopt almost planar structures. In addition, the bond length alternation (BLA) indexes of 87 and 88 are found to be 0.165 and 0.155 A, respectively, owing to a moderate it-delocahzation of the central cyclic... 

Scheme 8.12 Redox process of radiaannulene frameworks containing redox-active substituents.

Recently, a hybrid radiaannulene 90 containing electron-donating TTFs and electron-accepting TEE moieties has been prepared by Nielsen and coworkers [86]. This system can be considered as a two-stage Weitz-type redox system, which leads to the aromatic 144t-annulene structure by either oxidation or reduction. Thus, CV analysis clearly showed three reversible oxidation waves corresponding to the formation of three redox states (+1/+2/+4) and showed two reversible reduction waves corresponding to two redox states (—1/—2). From the... 

CycUc cross-conjugated bridges can result in electronic communication not conveyed by related acyclic bridges. Thus, the first oxidation of each TTF unit in the cyclic compound 29 (Figure 9.5) was split by 0.09 V, while the TTF units in compound 30, incorporating a TEE spacer, acted as independent redox centers and were oxidized at the same potential [22]. The singly oxidized species of 29 showed a characteristic MV absorption at 2257 nm. The cyclic core of 29 is called a radiaannulene as it has both endocyclic double bonds (as in aimulenes) and... 

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