Computational modelling of donor-acceptor-donor conjugated polymers based on benzothiadiazole

Since the interaction between alternating donors and acceptors results in a decreased in the band gap, a narrow band gap (<1.8 eV) will be expected in polymers containing donor-acceptor-donor (D-A-D) repeating units. It is possible to decrease the band gap as much as possible or to adjust HOMO (highest occupied molecular orbital)-LUMO (lowest unoccupied molecular orbital) levels by determining the proper donor (D) and acceptor (A) groups. In this study, quantum chemical calculations are performed using density functional theory (DFT) to investigate the HOMO-LUMO energy gap of benzothiadiazole based donor-acceptor-donor type conjugated polymers. The donors include thiophene, 3,4-ethylenedioxythiophene (EDOT) and 3,4-propylene dioxythiophene (ProDOT) units. These units are functionalized with methyl groups at the 3,4-, 2,3- and 3,3-positions of thiophene, EDOT, and ProDOT, respectively, in order to observe the effect of the alkyl side chains on the band gap of the polymers. The geometries of the monomers and oligomers were optimized using the semi-empirical PM6 method in the gas phase. Density functional theory single point calculations are carried out at the B3LYP/6-31G(d, p) level using optimized geometries. The energies (HOMO, LUMO) of the oligomers are obtained at the same level. The band gaps of the studied polymers are evaluated by extrapolating oligomers gaps to infinite chain lengths. The results indicate that calculated band gap values are in good agreement with the available experimental ones in the literature. The theoretical methods used in this study are encouraging for the modeling of similar donor-acceptor-donor type novel conjugated polymers. (C) 2018 Elsevier B.V. All rights reserved.