Bent Radical Cation Dimers from Bicyclooctene-Substituted Terthiophenes

Significance

The synthesis of bicyclooctene-substituted terthiophene 1, and the oxidation of this species to the corresponding radical cation are reported. Although the radical cation 1 ^+· did not dimerize in dilute (2.4 x 10^-4 M) dichloro­methane solution at temperatures as low as 180 K, more concentrated (1.2 x 10^-3 M) solutions displayed temperature-dependent changes in the UV-Vis and ESR spectra characteristic of π-dimer formation. In contrast, the fully substituted terthiophene 2 showed no propensity for dimerization in solution or in the solid state, due to the presence of bulky substituents on the central thiophene ring. In the solid-state structure of dimer (1 ^+·)₂ (depicted schematically above), the terthiophene units are bent substantially from planarity, in order to relieve steric interactions between substituents. Heating (1 ^+·)₂ in the solid state resulted in a new ESR signal assigned to a thermally excited triplet state, the first example of such an observation.