Theoretical studies on the electronic and optical properties of two blue-emitting fluorene–pyridine-based copolymers

Abstract

One serious problem associated with polyfluorene derivatives for light-emitting diodes is that they are usually much better at accepting and transporting holes than electrons. One way to achieve high electron affinity is the traditional structural modifications by the introduction of electron-withdrawing groups into π-conjugated systems. In this contribution, we apply density functional theory and the time-dependent density functional theory to investigate two series of alternating fluorene/pyridine oligomers and copolymers, namely, poly(2,7-(9,9-dimethylfluorene)- alt-3,5-pyridine) (PFPy) and poly(2,7-(9,9-dimethylfluorene-3,5-pyridine-)- alt-2,7-(9,9-dimethylfluorene)) (PFPyF) and gain a detailed understanding of the influence of pyridine units on the electronic and optical properties of fluorene derivatives. The outcomes show the incorporation of pyridine into the polyfluorene (PF) backbone resulted in a broadened energy gap and a blue shift of the absorption peaks. Most importantly, the LUMO energies of PFPy and PFPyF lower about 0.3 eV than that of their corresponding polyfluorene (PF), which directly results in the increasing of EAs than PF, indicating that the pyridine units have significantly improved the electron-accepting properties of the copolymers. In addition, the energy gap tends to broadening and the absorption peaks are gradually blue-shifted to shorter wavelengths with an increase in the pyridine content in the copolymers.