Synthesis, single crystal X-ray structure and optical properties of 3,4-dimethyl-dithieno[2,3-b : 3 ',2 '-d]thiophene-7,7-dioxide

The synthesis of the title compound (4), starting from commercial 3-bromo-4-methylthiophene, is described. The single crystal packing mode of 4, as well as the absorption and photoluminescence properties in solution and in the solid state are reported and analyzed in relation to those of the isomeric 3,5-dimethyl-dithieno[3,2-b:2′,3′-d]thiophene-4,4-dioxide (7). The different reactivities of 4 and 7 towards bromination are analyzed in the light of the results of ab initio B3LYP/6-31G* calculations on both compounds.