Cyclic Oligothiophenes: Novel Organic Materials and Models for Polythiophene. A Theoretical Study

Cyclic oligothiophenes (CnT, n = 6−30, even only) in syn- and anti-conformations are studied theoretically
at the B3LYP/6-31G(d) level of theory. Strain energies, ionization potentials, HOMO−LUMO gaps,
bond length alternations, NICS values, and IR and Raman spectra have been studied. The properties of
anti-conformers change systematically with increasing ring size and were studied in detail in neutral,
radical cation, and dication forms. In syn-conformation, the oligomers lose their nearly planar ring shape
and bend significantly for n > 14, and thus properties cannot be related to ring size. The HOMO−LUMO gap in C14T-syn is even lower than polythiopehene. IR and Raman spectra calculated at the
B3LYP/6-31G(d) level are used to differentiate syn- from anti-conformations. The properties of cyclic
oligomers are compared to those of the linear system, and cyclic oligothiophenes are revealed as good
models for polythiophene. To assist the experimental study of known cyclic oligomers having dibutyl
substituents on alternate thiophene rings, the corresponding dimethyl-substituted oligomers are also studied.
The experimentally evaluated HOMO−LUMO gaps for alternately dibutyl-substituted cyclic oligomers
match the calculated values; however, they are significantly higher than those of the unsubstituted
analogues.