The first electronically stabilized phenalenyl radical: Effect of substituents on solution chemistry and solid-state structure

We report an investigation into the effect of oxygen and sulfur substitution on the dimerization of phenalenyl-based neutral radicals; these substituents are expected to electronically stabilize the free radical while sterically interfering with dimerization. The 1,3,4,9-tetramethoxyphenalenyl (7a) and 1-ethoxy-3,4,9-trimethoxyphenalenyl (7b) radicals are prepared by chemical reduction of the cation salts, but the crystals that are isolated from these solutions are shown to be composed of alkoxy-substituted peropyrenes by X-ray crystallography. Crystallization of 1,9-dithiophenalenyl (8) gives rise to a pi-dimer, and this is the first example of a radical based on a single phenalenyl unit that has been stabilized against sigma-dimerization in the solid state by electronic effects rather than by the presence of sterically bulky substituents.