Theoretical study of the size confinement effect in linear π-conjugated oligomers

Linear π-conjugated oligomers represent model systems suitable for studying the size confinement effect in their spectroscopic response. In our study, based on Frenkel exciton models, we demonstrate that the excitonic approach successfully describes the experimentally observed size-dependent red shift of the optical spectra in thiophene- and benzene-based oligomer series. In addition, we will show that such an approach gives an interpretation of the excitation delocalization in such monodisperse linear conjugated oligomers in terms of the parameters of Frenkel exciton models. In our approach, these parameters are the oligomer length, on-site excitation energies, and the nearest-neighbor coupling strength. These parameters are obtained by fitting the curve, which represents the wave length value of the theoretical absorption line as a function of the oligomer length, to the experimental data. The definition of the structure of the repeat unit in the model description of oligomers is discussed.