An investigation on the electronic properties of thiadiazoloquinoxaline-based copolymers and the effect of donor-to-acceptor ratio on their properties

Three types of donor-acceptor conjugated copolymers were designed by choosing [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) as the acceptor and thiophene (Th-based copolymers), thieno[3,2-b]thiophene (TTh-based copolymers) or pyrrole (Py-based copolymers) as the donor unit. DFT method is employed to investigate the geometric and electronic structure of the designed copolymers. The theoretical calculations show that they all have a narrow band gap (⩽1.01 eV) due to their coplanar conformation, large intramolecular charge transfer and small bond length alternation. Topological analysis of electron density reveals that there exits intramolecular hydrogen bonds in Py-based copolymers, which is in favor of improvement of the coplanar conformation for them. The donor-to-acceptor (D-A) molar ratio of the studied copolymers has an important effect on their geometric and electronic properties. For one thing, the increase in D-A ratio leads to an increase in bridge length and intramolecular charge transfer. For another, the band gap, vertical ionization potential and vertical electron affinity of these copolymers except for TTh-based copolymers decrease with an increase in D-A ratio. In addition, n-doping is more favorable than p-doping for the proposed copolymers due to their large VEA (3.31-3.68 eV) and VIP (4.10-4.66 eV) values.