Electrochemical and fluorescent properties of alternating copolymers of 9,9-dioctylfluorene and oxadiazole as blue electroluminescent and electron transport materials

The properties of alternating copolymers of 9,9-dioctylfluorene and oxadiazole (F/Ox copolymers) have been studied by cyclic voltammetry (CV), photoluminescence (PL) and electroluminescence (EL). The copolymers contained oxadiazole units symmetrically dispersed in the main chain between one, P(F-1-alt-Ox), three, P(F-3-alt-Ox), or four, P(F-4-alt-Ox), fluorene units, as well as asymmetrically distributed in the chain, P(F-2-as-Ox). CV studies revealed that all the copolymers, except P(F-1-alt-Ox), had high electrochemical stabilities, and exhibited both reversible n- and p-doping processes. Meanwhile P(F-1-alt-Ox) only showed a stable and reversible n-doping process. The CV data also showed that the LUMO levels for the copolymers are close to the work functions of cathode materials such as Ca and Mg, indicating a favourable electron transport property of the copolymers. The PL spectra of the copolymers show a stable blue light-emitting behaviour in the solid state upon annealing under inert gas at temperatures from 40 to 150degreesC. However, a broad peak appeared at 510 nm when the samples were annealed at 120degreesC in the presence of oxygen. EL studies of a bi-layer light-emitting diode (LED) device produced from P(F-3-alt-Ox) demonstrated stable spectra with features similar to those of the PL spectra. The results of this study suggest that F/Ox copolymers are good candidates for use as electron transport layers and blue light-emitting materials in bi-layer LED devices.