Aromaticity of alpha-Oligothiophenes and EquivalentOligothienoacenes

The aromaticity and the degree of π-electronic delocalization have been theoreticallyinvestigated for R,R′-linked oligothiophenes containing three and five rings and for their fusedanalogs oligothienoacenes. By computing magnetic susceptibilities and 1H NMR shieldings aswell as current density maps, it is found that the fused oligomers are more aromatic than thecorresponding nonfused partners. The increase of aromaticity with the size of the oligomersevenin the case of quinoidal formssis also proven. The π-currents induced by an external magneticfield show that oligothienoacenes behave as single cycles since they present an intensediamagnetic current flowing around the whole molecular perimeter. In contrast, nonfusedR-oligothiophenes exhibit diamagnetic currents localized over each thiophene ring. For thequinoidal oligomers, local diamagnetic π vortices appear around CC double bonds, indicatingthat the π electrons are rather localized as in conjugated, nonaromatic polyenes. For quinoidalnonathienoacene, it is however found that the electronic circulation around the ethylenic bondstends to delocalize all over the carbon skeleton, indicating a more effective π-conjugation andsome aromatic character.