Characterization of the Quinoid Structure for the 2,2‘-Bithiophene and 2,2‘,5‘,2‘‘-Terthiophene Dications

We report the results of a detailed ab initio study about the molecular structure of 2,2‘-bithiophene and 2,2‘,5‘,2‘‘-terthiophene dications. The molecular geometry and conformational behavior of the quinoid structure obtained for 2,2‘-bithiophene dication are compared with the benzoid structure of noncharged 2,2‘-bithiophene. Calculations on 2,2‘,5‘,2‘‘-terthiophene dication indicate that with the increasing number of thiophene rings the quinoid structure disappears whereas the benzoid structure gradually develops. Finally, the relative stabilies between the different minima found for the 2,2‘-bithiophene dication ( anti ↔ syn ) and the 2,2‘,5‘,2‘‘-terthiophene dication ( anti − anti ↔ syn − anti ↔ syn − syn ) are analyzed.