The electronic structure and optical properties of carbazole-based conjugated oligomers and polymers: A theoretical investigation

Abstract

The application of polyfluorenes (PFs) in polymeric light-emitting diodes (PLEDs) has been hampered because of the charge injection difficulties and the troublesome formation of a tailed emission band at long wavelengths (>500 nm) during device fabrication and operation, leading to both a color instability and reduced efficiency. The polycarbazoles have been proved to efficiently suppress the keto defect emission. In this contribution, we apply quantum-chemical techniques to investigate poly( N-methyl-2,7-carbazolediyl) (PCz), and its copolymers poly( N-methyl-2,7-carbazolediyl-alt-2,5-thiophene) (PCzT) and poly( N-methyl-2,7-carbazoleethynylene) (PCzE), and gain a detailed understanding of the influence of carbazole units and the introduction of different charge carriers on the electronic and optical properties. The electronic properties of the neutral molecules, HOMO–LUMO gaps ( Δ H–L), in addition to IPs and EAs, are studied using B3LYP functional. The lowest excitation energies ( E gs) and the maximal absorption wavelength λ abs are studied employing the time dependent density functional theory (TDDFT). The calculated results show that the HOMO energies lift about 0.3 eV and thus the IPs decrease about 0.3 eV in all the carbazole-based polymers, suggesting the significant improved hole-accepting and transporting abilities. More important, by introducing the charge carriers thiophene ring and ethynylene, the LUMO energies in PCzT and PCzE decrease around 0.4 and 0.6 eV, respectively, which contributes to the decreasing EAs and the consequent improved electron-accepting and transporting properties. In addition, the energy gap tends to narrow and the absorption and emission peaks are gradually red-shifted to longer wavelengths with an increase in chain planarity in the copolymers.