New cyclic oligothiophenes and their co-oligomers derivatives of molecular engineering interest

• The stability of ring forms of bridged oligo(6)biheterocycles is evidenced.
• Bridged cyclic co-oligo(3)(bithiophene-bipyrrole) exhibit a very small energy gap.
• Macrocyclic species good candidates for molecular engineering applications.

The electronic properties of several series of bridged oligomers and their co-oligomers derivatives of five-membered biheterocycles (bithiophene, bipyrrole, bifuran bridged by CO or CS group) in their linear and cyclic structures (planar rings and crowns), have been studied using Density Functional Theory (DFT) calculations. The stability of ring forms of bridged oligo(6)biheterocycles is evidenced. Moreover, when replacing the CO bridging group by the CS one, in all oligomers the HOMO–LUMO energy gap is reduced, due mainly to the strongest stabilization of the LUMO. This energy gap is further reduced with the introduction of bridged bithiophene or bridged bifuran in the cyclic chain of bridged bipyrrole, thus suggesting an improvement of the conductivity properties of such species. The CS bridged co-oligo(3)(bithiophene-bipyrrole) and CS bridged co-oligo(3)(bifuran-bipyrrole) seems to be promising candidates for molecular engineering applications.