Structure and electronic properties of mono and di-methyl substituted poly(p-phenylenebenzobisthiazole) oligomers: A computational study

The structural and electronic properties of mono and di-methyl substituted poly(p-phenylene benzobisthiazole) oligomers have been investigated theoretically employing quantum chemical calculations based on density functional theory (DFT) using the B3LYP functional. The neutral states geometries of these oligomers have been used to calculate the HOMO-LUMO gaps (Delta(H-L)), ionization potentials (IP) and electron affinities (EA). The lowest excitation energies (E-g) and the maximum absorption spectra have been studied using the TDDFT/B3LYP/6-31G(D) method. The cationic and anionic states of these oligomers have been optimized using the same methodology. The optimized lowest singlet excited-state geometries of oligomers have been used to calculate the emission spectra by using TDDFT method. The Delta(H-L)s, IPs, EAs, E(g)s, and absorption/emission spectra of the respective polymer have been obtained by extrapolating those of the oligomers to the inverse chain length equal to zero. From the reorganization energies, it can be seen that the electron transport energy values are smaller than the hole transport suggesting that these could be used as electron transport materials in light emitting diodes devices. Thus, these studies on the oligomers will help in rationalizing the properties of known polymers and to predict those of yet unknown ones for their utilization in electronic devices.