Substituent Effect on Bithiophene-Bipyridine Organic Conjugated Systems: Theoretical Investigation

Organic pi-conjugated compounds have attracted the interest of many researchers world wide due to their stability, high conductivity and intriguing optoelectrical properties. In this study, theoretical calculations were carried out to investigate the effect of electron-withdrawing and electron-donating groups at a different position on electronic properties, geometry and photophysical parameters of bithiophene and bipyridine ring linked by a single bond using density functional theory (DFT) and time dependent DFT (TD-DFT). The calculated energy gap (Eg) of the studied co-oligomers with substituents decreased with substituent strength. The reduction in the value of ɳ (1.54eV to 1.8eV) of the substituted co-oligomers compared with 1.95eV of unsubstituted co-oligomer implies lower resistance to distortion which enhances electron-withdrawing. The theoretical maximum absorption (λ-max) values for substituted compounds range between 335.4nm to 404.2nm compared with 318.2nm of an unsubstituted compound. Hence, the study results show that the substitute groups' nature, species, and position affect both the electronic and absorption properties without any significant difference in the geometry of the bithiophene-bipyridine co-oligomer. The mutual comparison of substituted and unsubstituted compounds shows a decrease in energy gap (Eg) and significantly reflected bathochromic shift for all substituted compounds, especially compound with NO2 substituent, indicating that the HOMO and LUMO level of the studied compounds were well controlled by the attached substituents groups. Hence, it is expected to be useful for opto electronic material design.