Aromaticity of α-Oligothiophenes and Equivalent Oligothienoacenes.

The aromaticity and the degree of π-electronic delocalization have been theoretically investigated for α,α'-linked oligothiophenes containing three and five rings and for their fused analogs oligothienoacenes. By computing magnetic susceptibilities and (1)H NMR shieldings as well as current density maps, it is found that the fused oligomers are more aromatic than the corresponding nonfused partners. The increase of aromaticity with the size of the oligomer-even in the case of quinoidal forms-is also proven. The π-currents induced by an external magnetic field show that oligothienoacenes behave as single cycles since they present an intense diamagnetic current flowing around the whole molecular perimeter. In contrast, nonfused α-oligothiophenes exhibit diamagnetic currents localized over each thiophene ring. For the quinoidal oligomers, local diamagnetic π vortices appear around CC double bonds, indicating that the π electrons are rather localized as in conjugated, nonaromatic polyenes. For quinoidal nonathienoacene, it is however found that the electronic circulation around the ethylenic bonds tends to delocalize all over the carbon skeleton, indicating a more effective π-conjugation and some aromatic character.