Theoretical studies on the electronic and optical properties of two thiophene-fluorene based π-conjugated copolymers

The low PL quantum efficiency, typically 1-3%, in solid film, limits the application of polythiophene and derivatives (PTs) in PLEDs. The six-member aromatic rings polyfluorenes (PFs) with higher PL efficiencies have been introduced into the backbone of PTs, in an effort to develop highly efficient, desirable charge carrier transporting and low energy gap thiophene-fluorene based light-emitting polymers. In this contribution, quantum-chemical techniques are employed to study two fluorene-thiophene incorporated π-conjugated polymers, namely, poly((5,5-E-α-(2-thienyl)methylene)-2-thiopheneacetonitrile)-alt-2,7-(9,9-dimethylfluorene) (PFTCNVT) and poly((5,5″-(3′,4′-dimethyl-2,2′;5′,2″-terthiophene1′,1′-dioxide))-alt-2,7-(9,9-dimethylfluorene)) (PFTORT). Density functional theory (DFT) and time-dependent DFT approaches are employed to study the neutral molecules, positive and negative ions, the IPs and EAs, HOMO-LUMO gaps (ΔH-L), as well as the lowest excitation energies (Egs). It is interesting to note that the two copolymers PFTCNVT and PFTORT are superior to the properties of pristine polyfluorene (PF) and polythiophene (PT). In addition to the improved PL efficiency, they still presented lower energy-gap comparable to PTs. Furthermore, the LUMO energies lower about 1.4 eV and thus the EAs increase around 1.4 eV in PFTCNVT and PFTORT compared with PFs, suggesting the significant improved electron-accepting and transporting abilities in the two copolymers. These properties can be explained by the presence of more electron-accepting thiophene units in the repeated unit of the copolymers and the more planar conformations in the two copolymers under study.