Stilbene Dimer Radical Cations in the Radiolyses of Stilbenes and 1,2,3,4-Tetraphenylcyclobutanes

The reaction of the stilbene radical cation formed by pulse radiolysis or γ-radiolyses is explained based on neutralization as well as the formation of a π-type stilbene dimer radical cation (π-St2+•), converting to the σ-type St2+• (σ-St2+•). The r-1,c-2,t-3,t-4-tetraphenylcyclobutane radical cation generated in a rigid matrix at 77 K which converted to σ-St2+• upon warming. Both r-1,c-2,t-3,t-4- and r-1,t-2,c-3,t-4-tetraphenylcyclobutane radical cations underwent photochemical cycloreversion to π-St2+• upon irradiation at wavelengths longer than 390 nm at 77 K, and converted to σ-St2+• upon warming. It is suggested that π-St2+• has overlapping arrangements of π-electrons, while σ-St2+• has radical and cation centers on the 1- and 4-positions of the C4 linkage.