Electronic structure of push-pull molecules based on thiophene oligomers

Thiophene oligomers (alpha-alpha' linkages) have been intensively studied in the past few years. In particular, sexithiophenes have been synthesized with a high degree of purity and good crystallinity, allowing the fabrication of reliable field effect transistors. When donor-acceptor terminal groups are attached to thiophene oligomers, ail anomalous behavior of the fluorescence as a function of the number of rings is reported. There are experimental evidences of charge transfer suppression for oligomers containing more than four rings. In this work, we have investigated the electronic structure of these systems. Geometrical optimizations and spectroscopic features of both ground and excited states were obtained through the use of sophisticated semiempirical methods. Our results have shown that charge transfer suppression can be explained in terms of the evolution of the electronic spectrum.