| Abstract
| - We report the synthesis and voltamperometric reduction of 5H-benzo[b]carbazole-6,11-dione (BCD)and its 2-R-substituted derivatives (R = −OMe, −Me, −COMe, −CF3). The electrochemical behaviorof BCDs was compared to that of the 2-[(R-phenyl)amine]-1,4-naphthalenediones (PANs) previouslystudied. Like PANs, BCDs exhibit two reduction waves in acetonitrile. The first reduction step forthe BCDs represents formation of the radical anion, and the half-wave potential (E1/2) values forthis step are less negative than for that of the PANs. The second reduction wave, corresponding tothe formation of dianion hydroquinone, has E1/2 values that shift to more negative potentials. Agood linear Hammett−Zuman (E1/2 vs σp) relationship, similar to that for the PAN series, was alsoobtained for the BCDs. However, unlike the PANs, in the BCDs, the first reduction wave was moresusceptible to the effect of the substituent groups than was the second wave, suggesting that theordering of the two successive one-electron reductions in BCDs is opposite that in PANs. This isexplained by the fact that the electron delocalizations in the two systems are different; in the caseof BCDs there is an extra aromatic indole ring, which resists loss of its aromatic character. Theelectronic structures of BCD compounds were, therefore, investigated within the framework of thedensity functional theory, using the B3LYP hybrid functional with a double ζ split valence basisset. Our theoretical calculations show that the O1···H−N hydrogen bond, analogous to thatpreviously described for the PAN series, is not observed in the BCDs. Laplacians of the criticalpoints (∇2ρ) and the natural charges for the C−O bonds indicate that the first reduction wave forthe BCDs corresponds to the C4−O2 carbonyl, while in the PAN series the first one-electron transferoccurred at the C1−O1 carbonyl. Natural bond orbital analysis showed that, in all the BCDs, thelowest unoccupied molecular orbital (LUMO) is located at C4, whereas for the PANs, the LUMO isfound at C1. The good correlation between the LUMO energy values and the E1/2 potentials (waveI) established that the first one-electron addition takes place at the LUMO. Analysis of the moleculargeometry confirmed that, in both series of compounds, the effect of the substituent groups is mainlyon the C4−O2 carbonyl. These results explain the fact that reduction of the C4−O2 carbonyl(voltammetric wave II in the PANs and voltammetric wave I in the BCDs) is more susceptible tothe effect of the substituent groups than is reduction of the C1−O1 carbonyl (wave I in the PANsand wave II in the BCDs).
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