Substituent effects and photo-physical properties in polythiophene and its derivatives: A PBC-DFT study

Polythiophene (PT) and its derivatives (PTs) are the polymer-based materials with a π-conjugation framework. PT is a useful photoelectric material and can be used in the organic semiconductor devices, such as OLED, PLED, OFET and solar cells. Based on molecular structure and property relationship, PTs contain different substituents, such as electron-withdrawing or electron-donating groups, should have different degree of π-conjugation in the backbone of PT polymers as due to the steric and electronic effects. The energy gap (△Egap) between HOMO and LUMO levels is related to the π-conjugation in the PT polymer backbone. In this study, the PBC-DFT/B3LYP/6-31G(d) calculations were performed for the non-substituted PT, and 3- and 3,4-substituted PTs to investigate the stable geometries, electronic structures and photo-physical properties of PTs. For the 3-substituted PTs, the electron-donating and electron-withdrawing substituents were shown to decrease and increase the calculated △Egap, separately with respect to non-substituted PT. In addition to the electronic effect, the PBC-DFT calculation indicates that the steric effect may also play an important role in the 3,4-substituted PTs. The calculated △Egap for PTs are in good agreement with those of the experimental data.