Mesolysis of radical anions of tetra-, penta-, and hexaphenylethanes.

A central carbon–carbon (C–C) σ bond dissociation of polyphenylethane radical anions (Ph(n)E•-, n = 3–6), mesolysis, was investigated by the transient absorption measurement during pulse radiolysis of Ph(n)E in 2-methyltetrahydrofuran. The charge resonance (CR) band of 1,1,2,2-tetraphenylethane radical anion (1,1,2,2-Ph4E•-) was observed in the near-infrared region immediately after an electron pulse to be attributed to the intramolecular dimer radical anion. The CR band disappeared with simultaneous formation of two absorption bands at 330 and 460 nm corresponding to diphenylmethyl radical and diphenylmethyl anion, respectively, indicating the occurrence of the mesolysis in 1,1,2,2-Ph4E•-. During pulse radiolysis of 1,1,1,2,2,2-hexaphenylethane (Ph6E), an absorption band of triphenylmethyl radical was observed at 340 nm immediately after an electron pulse. It is suggested that one electron attachment to Ph6E is followed by the subsequent rapid C–C σ bond dissociation. Formation of intramolecular dimer radical anions in Ph(n)E•- such as 1,1,2-triphenylethane (Ph3E), 1,1,1,2-tetraphenylethane (1,1,1,2-Ph4E), and 1,1,1,2,2-pentaphenylethane (Ph5E) was also studied together with the subsequent mesolysis. The mesolysis of Ph(n)E•- is discussed in terms of charge delocalization in the intramolecular dimer radical anions and the central C–C σ bond as well as bond dissociation energy of the central C–C σ bond of Ph(n)E•-.