The role of donor units in band gap engineering of donor-acceptor conjugated polymers

Most used 60 distinct electron-donating units have been modelled, analyzed, and compared using density functional theory (DFT) for tetramer structures in the form (D-B-A-B)₄ with fixed acceptor and bridge units, where D, A and B represents donor, acceptor and bridge, respectively. The frontier orbitals and reorganization energy of tetramers with alternating donor units were analyzed to assess their potential applicability in organic electronic applications. Key structural properties including dihedral angles between the acceptor, donor, and bridge units, bond order, and bond length alternation were found to significantly influence the frontier electronic energy levels affecting the planarity, conjugation and electron delocalization of polymer backbone. While extended conjugation and planar structures generally lower the band gap; the specific electronic impact of substituents, such as methoxy or fluorine groups, depend on their position and interaction within the conjugated system. Similarly, the incorporation of heavier heteroatoms, such as selenium, germanium or silicon, introduces steric and electronic effects that can either enhance or disrupt π-conjugation due to the change in the strength of donor unit. Additionally, substitution effects and morphological variations in donor units play a crucial role in defining the physical properties of D-A conjugated polymers. This study establishes a benchmark by providing essential insights into the band gap engineering and the molecular design of D-A copolymers by alternating donor units, thereby supporting significant advancements in organic electronic applications.