Generally, the band gap is estimated as the difference between the HOMO and the LUMO level energies (Eg = DEHOMO–LUMO) on the ground singlet state. The oligomer geometries and their corresponding band gap calculations were performed using DFT/B3LYP method with 3-21G⁄ and 6-31G⁄ basis sets. In our case, the band gap of (PCzBT)n (n = 1–4) oligomers is listed in Table 2. By using the linear extrapolation technique [27], it can be seen from Fig. 4, that this value decreases with increasing the chain length from monomer to quatermer. Moreover, the theoretical data resulting from the two considered bases are very close and no significant changes are noticed when going from the 3-21G⁄ to the 631G⁄ basis set calculations.