Theoretical investigation on π-dimer formation in oligothiophenes

The electronic structure and stability of oligothiophene pairs (both neutral and positively charged) up to quaterthiophenes are investigated by ab initio calculations which explicitely taken electron-electron correlation into account. The results indicate that the stability of π-dimers increases with the oligomer size in good agreement with experimental data and that the intermolecular interaction can be strong enough to allow for an intermolecular charge separation. Absorption spectra in the visible to ultra-violet (vis-UV) region, commonly used to characterize charged defects in oligomers and polymers as well as π-dimers, was simulated by a semiempirical technique and the results were found very consistent with the available experimental data of these systems.