Stable Cross-Conjugated Tetrathiophene Diradical.

A tetracyano quinoidal tetrathiophene, having a central bi(thieno[3,4-c]pyrrole-4,6-dione) acceptor, has been studied. The recovered aromaticity of the thiophenes produces a diradical species with cross-conjugation between the inter-dicyano and inter-dione acceptor paths. A diradical character of y0 =0.61 and a singlet-triplet gap of -2.76 kcal mol-1 were determined. Competition between the two cross-conjugated paths enhances the disjointed character of the SOMOs and results in the confinement of the diradical to the molecular center, enabling a thermodynamic diradical stabilization featuring a half-life of 262 hours. Cross-conjugation effects have been also addressed in the anionic species (up to a radical trianion).