Synthesis and optoelectronic properties of luminescent poly(p- phenylenevinylene) derivatives containing electron-transporting 1,3,4-oxadiazole groups

Three random copolymers (P1-P3) comprising phenylenevinylene and electron-transporting aromatic 1,3,4-oxadiazole segments (11, 18, 28 mol %, respectively) were prepared by Gilch polymerization to investigate the influence of oxadiazole content on their photophysical, electrochemical, and electroluminescent properties. For comparative study, homopolymer poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-p-phenylenevinylene] (P0) was also prepared by the same process. The polymers (P0-P3) are soluble in common organic solvents and thermally stable up to 410 °C under a nitrogen atmosphere. Their optical properties were investigated by absorption and photoluminescence spectroscopy. The optical results reveal that the aromatic 1,3,4-oxadiazole chromophores in P1-P3 suppress the intermolecular interactions. The HOMO and LUMO levels of these polymers were estimated from their cyclic voltammograms. The HOMO levels of P0-P3 are very similar (-5.02 to -5.03 eV), whereas their LUMO levels decrease readily with increasing oxadiazole content (-2.7, -3.08, -3.11, and -3.19 eV, respectively). Therefore, the electron affinity of the poly(p-phenylenevinylene) chain can be gradually enhanced by incorporating 1,3,4-oxadiazole segments. Among the polymers, P1 (11 mol % 1,3,4-oxadiazole) shows the best EL performance (maximal luminance: 3490 cd/m², maximal current efficiency: 0.1 cd/A). Further increase in oxadiazole content results in micro-phase separation that leads to performance deterioration.