  Thiophene oligomers are an important class of organic materials for
photovoltaic applications, owing to their unique optoelectronic properties.
Recently it was suggested that incorporation of furan units to the thiophene
chains, maintaining the chain structure, namely thienylfuran linear oligomers,
can bring improvements to the final material. In this work, we present a
theoretical study of thiophene, furan and thienylfuran short chains, up to 4
units. Structural and electronic properties were obtained using Hartree-Fock
(HF) and Density Functional Theory (DFT) calculations plus beyond mean-field
methodologies, specifically Second-order M\"oller-Plesset perturbation theory
on HF (HF-MP2) and many-body perturbation theory by the G0W0 approximation on
DFT (G0W0@DFT). The optical properties were calculated on top of G0W0@DFT data
using the Bethe-Salpeter Equation. We investigate properties from the monomers
T and F to tetramers with different sequencing of units TT, TF or FF, always
bonded through the usual carbon atoms. As well known for the uniform
oligothiophene chains, also here for any TT sequencing we find a torsion angle
of $150^{\circ}$, while all other sequencing of units result planar, which can
be relevant for film producing. Also we find that the first optical transitions
for the oligomers reach a promising threshold of $\sim 3$ eV at the tetramer
length, combined with ionization potentials around $\sim 7$ eV, which confirms
the relevance of these organic compounds for photovoltaic applications.

Materials Science