Theoretical study on electronic structure and optical properties of novel donor-acceptor conjugated copolymers derived from benzothiadiazole and benzoselenadiazole

One serious problem associated with luminescent polymers is the significant energy barrier for hole or electron injections and thus usually face charge injection and transport difficulties with the currently available cathode and anode materials. The incorporation of charge carriers is expected to improve the recombination of the charge carriers. In this contribution, we apply quantum-chemical techniques to investigate two carbazole-based donor-acceptor CT type copolymers, poly[4,7-(2,1,3-benzothiadiazole)-3,6-(N-(2-methyl)carbazole)] (PCzBTDZ) and poly[4,7-(2,1,3-benzoselenadiazole)-3,6-(N-(2-methyl)carbazole)) (PCzBSeDZ), in which the HOMO-LUMO gaps ΔH-L), the lowest excitation energies (Eg), ionization potentials (IP) and electron affinities (EA) are fine-tuned by the regular insertion of electron-accepting units, 2,1,3-benzothiadiazole (BTd) and 2,1,3-benzoselenadiazole (BSe), respectively. The results show that the alternated incorporation of electron-accepting moieties BTd and BSe significantly decrease the LUMO energy and thus enhance the EAs and consequently the electron-injection and transporting ability are greatly improved. Meanwhile, since carbazole has good hole transporting ability, the copolymers of a carbazolyl unit and BTd or BSe units possess high electron affinity and low ionic potential and achieve a relative balanced charge injection. In addition, due to the weak interactions between the two building blocks, the two copolymers have rather small energy gap and thus lead to a large bathochromic shift in absorption spectra compared with pristine polyfluorene and polycarbazoles.